Monday, March 15, 1993 Basaltic Achondrites 8:30 - 12:00 a.m. Room A Chair(s): T. L. Grove J. J. Papike Ganguly J.* Tazzoli V. Fe2+-Mg Interdiffusion in Orthopyroxene: Constraints from Cation Ordering and Structural Data and Implications for Cooling Rates of Meteorites Orthopyroxene crystals in a number of meteorites exhibit compositional zoning of Fe and Mg, which provide an important constraint on their cooling rates. However, attempts to model the cooling rate of these crystals from Fe-Mg zoning profiles suffer from the lack of any measured or theoretically well- constrained Fe-Mg interdiffusion data in OPx. It has been assumed that Fe-Mg interdiffusion in OPx is only slightly slower than that in olivine. The purpose of this paper is to (1) calculate the Fe-Mg interdiffusion coefficient (D(Fe-Mg)) in OPx from the available data on the kinetics and thermodynamics of intracrystalline Fe-Mg fractionation and (2) provide analytical formulation relating cooling rate to the length of the diffusion zone across the interface of the overgrowth of a mineral on itself with application to Mg diffusion profile across OPx overgrowth on OPx in certain mesosiderites. Mittlefehldt D. W.* Igneous Fractionation and Subsolidus Equilibration of Diogenite Meteorites Diogenites are coarse-grained orthopyroxenite breccias of remarkably uniform major element composition. Most diogenites contain homogeneous pyroxene fragments up to 5 cm across of Wo(sub)2En(sub)74Fs(sub)24 composition. Common minor constituents are chromite, olivine, troilite, and metal, while silica, plagioclase, merrillite, and diopside are trace phases. Diogenites are generally believed to be cumulates from the eucrite parent body, although their relationship with eucrites remains obscure. It has been suggested that some diogenites are residues after partial melting. I have performed EMPA and INAA for major, minor, and trace elements on most diogenites, concentrating on coarse-grained mineral and lithic clasts in order to elucidate their igneous formation and subsequent metamorphic history. Papike J. J.* Spilde M. N. Fowler G. W. Shearer C. K. Orthopyroxenes as Recorders of Diogenite Petrogenesis: Major and Minor Element Systematics As a part of our research to better understand magmatic processes in the Eucrite Parent Body we have initiated an ambitious program of study of major, minor, and trace elements in orthopyroxene from diogenites. This paper reports preliminary results for major and minor elements in orthopyroxenes for a suite of 13 diogenites: Aioun El Atrouss, ALH 84001, ALH A 77256, EET 87530, Ellemeet, Garland, Ibbenburen, Johnstown, Manegoan, Peckelsheim, Roda, Shalka, and Tatahouine. A companion paper by Shearer et al. (this volume) reports new trace element data for ALH 84001, ALH A 77256, Ibbenburen, and Tatahouine. We have presently collected over 800 high quality pyroxene microprobe analyses for Si, Al, Ca, Na, Mn, Fe, Mg, Cr, and Ti. The chemical systematics observed for these orthopyroxenes reflect original magmatic mineral/melt partitioning plus later trapped liquid/mineral equilibration, subsolidus exsolution, and mineral/mineral metamorphic reactions. We have therefore avoided, at this point, any attempt to use statistical analysis to group (e.g. factor or cluster analysis) these orthopyroxenes chemically. Shearer C. K.* Papike J. J. Layne G. D. Orthopyroxenes as Recorders of Diogenite Petrogenesis: Trace Element Systematics Eucrite, howardite, and diogenite members of the achondrites are considered, by many, to be genetically related. Therefore, each provide a piece of the puzzle for reconstructing magmatic processes on the eucrite parent body (EPB). The relationship between eucrites and diogenites can be viewed within the context of two distinctly different models: (1) fractional crystallization and (2) partial melting. In fractional crystallization models, eucrites and diogenites represent a complementary continuum of planetary fractional crystallization products in which the diogenites represent crystal accumulations during the crystallization of eucritic magmas at shallow to deep levels in the EPB. Alternatively, experimental studies may be interpreted as indicating eucrites represent peritectic partial melts of a primitive, chondritic EPB mantle. Within this type of model, the diogenites are also generally considered to be cumulates; however, their petrogenetic relationship to the eucrites is less clear. Sack et al. proposed that the olivine diogenites represent residua from the partial melting events that produced eucritic liquids. Initial trace element studies of orthopyroxene (OPX) are consistent with this model. However, this trace element modeling of the olivine diogenites is nonunique. As a further test of these models, we (1) analyzed OPX from cumulate diogenites to compare with the olivine diogenite data, (2) improved ion microprobe analytical techniques for the analysis of elements critical to our interpretations, and (3) selected more relevant Kds for opx-eucritic melt. Phinney W. C.* Lindstrom D. J. Mittlefehldt D. W. Martinez R. R. Post-Igneous Redistribution of Components in Eucrites Evidence for redistribution of Fe and Mg in plagioclase of lunar and terrestrial samples during high-temperature metamorphism or very slow cooling has been described previously. Redistribution of REEs in plagioclase of terrestrial anorthosites has also been demonstrated, whereas lunar anorthosites do not appear to exhibit this effect. Evidence for reequilibration between plagioclase and melts for some eucritic meteorites has also been provided on the basis of the extraordinary partition coefficients that are required to account for the contents of REEs and other components in separated plagioclases and their collateral whole rock eucrites. Isotopic data show REE reequilibration between plagioclase, but not pyroxene, and phosphate in the meteorite Ibitira. Because analyses of separated phases may be compromised by contaminants that could not be eliminated during separation, it is desirable to analyze individual mineral grains for their trace elements and utilize appropriate partition coefficients to test the reasonableness of calculated melts. Although microbeam analyses provide an excellent means for such analyses, sensitivity or interference problems may limit the range of elements and accuracy for such analyses. Therefore, we opted for another means of analysis. Stewart B. Papanastassiou D. A.* Capo R. C. Wasserburg G. J. Fine Resolution Chronology Based on Initial 87Sr/86Sr It has been recognized that small variations in initial 87Sr/86Sr(Sr(sub)I) can provide a fine-scale relative chronology for the chemical fractionation of materials with low Rb/Sr from parent reservoirs with high Rb/Sr. Similarly, SrI, as determined for low Rb/Sr phases in meteorites, may permit a fine- resolution chronology of the recrystallization or metamorphism of planetary materials. For the establishment of a primitive 87Sr/86Sr chronology, it is important to search for samples with extremely low Rb/Sr for which the measured 87Sr/86Sr is below BABI, in which case the primitive nature of the Sr can be directly established. Using the measured Rb/Sr to calculate an initial 87Sr/86Sr can introduce substantial uncertainty if the Rb-Sr systems are disturbed. We report 87Sr/86Sr in plagioclase from silicate pebbles from the Vaca Muerita mesosiderite on which we have reported l47Sm-l43Nd and l42Nd correlations. For the purpose of cross calibration with our previous work we have performed extensive new measurements on Angra dos Reis and on anorthite from Moore County, which have very low Rb/Sr and primitive 87Sr/86Sr. Shukolyukov A.* Lugmair G. W. 60Fe and the Evolution of Eucrites We have recently presented evidence for the existence of live 60Fe in the early solar system in the form of 2.4 to 50 sigma unit (1 part in 10^4) relative excesses of ^60Ni (^60Ni) measured in samples from the eucrite Chervony Kut (CK). These isotopic excesses have been produced by the decay of the now extinct short-lived radionuclide ^60Fe (T1/2 = 1.5 Ma). Because CK originates from a planetesimal that was totally molten and its high Fe/Ni ratio is due to a planet-wide Fe-Ni fractionation during metal-silicate segregation, the presence of ^60Ni* indicates the large scale abundance of ^60Fe in the early solar system and its presence during differentiation of this planetesimal. The observed variable ^60Ni excesses in CK can only be understood if some ^60Fe was still alive when the meteorite material had solidified on the eucrite parent body. From a correlation of ^60Ni* with Fe/Ni ratios in several samples we obtain a ^60Fe/^56Fe ratio of (3.9+-0.6) x 10^-9 and an initial ^60Ni of 3.2+-0.9 sigma units at the time of crystallization of CK. Studies on samples from another eucrite, Juvinas (JUV), clearly confirm the evidence for the existence of live ^60Fe in the eucrite parent body at the time of planet differentiation. However, after contamination with isotopically normal Ni is removed from JUV samples, the data indicate that this meteorite has cooled more slowly and has solidified at least 5 Ma after CK. The time between core formation on the eucrite parent body and CK crystallization is 1 to 4 Ma. Estimates based on a -10 Ma time interval between CK solidification and formation of the earliest condensates in the solar system followed by rapid accretion of planetary bodies indicate that the decay of ^60Fe could produce sufficient heat to melt these planetesimals. If ^26Al was present on a planetary scale as ^60Fe and at abundances close to values observed in Allende inclusions then melting of small early formed planets is inevitable. Collinson D. W.* Morden S. J. Remanent Magnetism of HED Meteorites--Implications for Their Evolution and Ancient Magnetic Fields The magnetic properties of extraterrestrial materials, in particular natural remanent magnetization (NRM), is a potentially useful study for detecting ancient Solar System magnetic fields and for elucidating meteorite evolutionary processes. Results reported here are for howardites Kapoeta, Petersburg, Le Teilleul, and EET87503, eucrites Sioux County and Millbillillie, and diogenites Shalka and Johnstown. Significant features of their magnetism are within-sample inhomogeneity of NRM directions in several of the meteorites and within-sample uniformity of axes of anisotropy of magnetic susceptibility. Both these phenomena bear on the meteorites' evolution and the timing of the magnetisation process. Jurewicz A. J. G. Jones J. H.* Weber E. T. Mittlefehldt D. W. Partial Melting of Ordinary Chondrites: Lost City (H) and St. Severin (LL) Eucrites and diogenites are examples of asteroidal basalts and orthopyroxenites, respectively. As they are found intermingled in howardites, which are inferred to be regolith breccias, eucrites and diogenites are thought to be genetically related. But the details of this relationship and of their individual origins remain controversial. Delaney J. S.* Boesenberg J. S. Fe/Mn Constraint on Precursors of Basaltic Achondrites Most achondritic meteorites have Fe/Mn ratios that are lower than those of carbonaceous chondrites and of course are lower than the solar system abundance ratio of these elements. Models of the origin of achondritic assemblages must therefore account for these ratios. Fe/Mn ratios are suggested to be distinctive for samples from each achondrite parent body and for the Earth and Moon, but the correspondence between the Fe/Mn systematics of achondrites and chondritic precursors is unclear. Most models of achondrite genesis involve magmatic differentiation of chondritic precursors. The Fe/Mn difference between achondrites and chondrites is particularly significant since Fe and Mn are geochemically similar elements with similiar partitioning behavior in familiar magmatic systems and are generally coupled during crystal-liquid fractionation. In contrast, however, Mn is more volatile than Fe in a nebular setting. Variation of Fe/Mn ratios based on the relative volatility of these elements in the early nebula provides a constraint for models by which the basaltic achondrites (with Fe/Mn ratios ~ 25-50) are derived from mixtures of nebular components that were enriched in volatile components such as Mn. However, such volatile enriched components have not been identified in chondrites. When the abundance in achondrites of elements of similiar volatility is examined, anomalies appear. For example, Na is massively depleted in basaltic achondrites when compared to Mn. These anomalies might be explained using current models but the alternative hypothesis, that Fe/Mn ratio is controlled not by nebular volatility constraints, but by planetary differentiation, should be explored. Grove T. L.* Petrologic Constraints on the Surface Processes on Asteroid 4 Vesta and on Excavation Depths of Diogenite Fragments The eucrite-howardite-diogenite meteorite groups are thought to be related by magmatic processes. Asteroid 4 Vesta has been proposed as the parent body for the these basaltic achondrite meteorites. The similarity of the planetesimal's surface composition to eucrite and diogenite meteorites and the large size of the asteroid (r = 250 km) make it an attractive source, but its position in the asteroid belt far from the known resonances from which meteorites originate make a relation between Vesta and eucrite-howardite-diogenite group problematic. It has been proposed that diogenites are low-Ca pyroxene-rich cumulates that crystallized from a magnesian parent (identified in howardite breccias), and this crystallization process led to evolved eucrite derivative magmas. This eucrite-diogenite genetic relationship places constraints on the physical conditions under which crystallization occurred. Elevated pressure melting experiments on magnesian eucrite parent compositions show that the minimum pressure at which pyroxene crystallization could lead to the observed compositions of main series eucrites is 500 bars, equivalent to a depth of 135 km in a 4 Vesta-sized eucrite parent body. Therefore, the observation of diogenite on the surface of 4 Vesta requires a postcrystallization process that excavates diogenite cumulate from depth. The discovery of diogenite asteroidal fragments is consistent with an impact event on 4 Vesta that penetrated the deep interior of this planetesimal. Binzel R. P.* The Asteroid-Meteorite Connection: Forging a New Link to Vesta as the Parent Body for Basaltic Achondrite (HED) Meteorites Asteroid 4 Vesta has been at the center of the debate over the identity of the howardite-eucrite-diogenite (HED) parent body since the early 1970s. Despite its unique (among the 500 largest asteroids) compositional match to HED meteorites, substantial dynamical difficulties in delivering fragments from Vesta to the Earth have precluded any conclusive HED parent body link. These dynamical difficulties arise because Vesta's orbital location is far from known resonances. Consequently, it has been argued as dynamically improbable that meteoroid-sized (~1 km) fragments could be excavated from Vesta with sufficient velocities to reach the resonances. Through new astronomical observations, numerous small (4-7 km) asteroids between Vesta and the 3:1 resonance have been discovered to have eucrite and diogenite compositions. Based on similar orbital elements to Vesta, all of these new asteroids are probably large impact fragments excavated from Vesta. Their current orbits imply ejection velocities in excess of 700 m/sec. Smaller (~1 km) fragments can therefore be expected to have been ejected with velocities >1 km/sec, sufficient to reach the 3:1 and nu6 resonances. Thus it now appears to be dynamically viable for Vesta to be linked as the HED parent body. Jurewicz A. J. G.* McKay G. A. Angrite LEW87051: Are the Olivines Pheno's or Xeno's? A Continuing Story This study presents the results of an independent petrologic look at an olivine in LEW87051, and uses an isothermal, composition-independent constant source diffusion model (as a first approximation) to explain the observed zoning. This model gives the apparent residence time (or equilibration time) of the large olivines as <10 days. This zoning does not necessarily contradict the model of McKay et al (1991), which models the bulk crystal using fractional crystallization from a finite melt. Rather, a two-stage or complex thermal history is inferred. Monday, March 15, 1993 The Geology of Venus: A Tribute of Academician Valery Leonidovich Barsukov 8:30 - 12:00 a.m. Room B Chair(s): J. W. Head III R. S. Saunders Saunders R. S.* Stofan E. R. Plaut J. J. Senske D. A. Magellan at Venus: Summary of Science Findings This paper gives a brief summary of the principal science findings of the Magellan mission and plans for future data acquisition. This includes high- resolution gravity data from a nearly circular orbit and atmospheric drag and occultation experiments. The Magellan science results represent the combined effort of more than 100 Magellan investigators and their students and colleagues. More complete discussions can be found in the August and October, 1992 issues of the Journal of Geophysical Research, Planets. The Magellan mission's scientific objectives were (1) to provide a global characterization of landforms and tectonic features; (2) to distinguish and understand impact processes; (3) to define and explain erosion, deposition, and chemical processes; and (4) to model the interior density distribution. All but the last objective, which requires new global gravity data, have been accomplished or we have acquired the data that are required to accomplish them. Senske D. A.* Stofan E. R. Bindschadler D. L. Smrekar S. E. Volcanic Rises on Venus: Geology, Formation, and Sequence of Evolution Large centers of volcanism on Venus are concentrated primarily in the equatorial region of the planet and are associated with regional topographic rises. Analysis of both radar images and geophysical data suggest that these uplands are sites of mantle upwelling. Magellan radar imaging provides a globally contiguous dataset from which the geology of these regions is evaluated and compared. In addition, high-resolution gravity data currently being collected provide a basis to assess the relationship between these uplands and processes in the planet's interior. Studies of the geology of the three largest volcanic highlands (Beta Regio, Atla Regio, Western Eistla Regio) show them to be distinct, having a range of volcanic and tectonic characteristics. In addition to these large areas, a number of smaller uplands are identified and are being analyzed [Bell Regio, Imdr Regio, Dione Regio (Ushas Innini and Hathor Montes), Themis Regio]. To understand better the mechanisms by which these volcanic rises form and evolve, we assess their geologic and geophysical characteristics. Turcotte D. L.* Is There Uniformitarian or Catastrophic Tectonics on Venus? The distribution and modification of craters on Venus favors a near global, volcanic resurfacing event about 500 m.y. ago. Such an event indicates that the tectonic evolution of Venus has been catastrophic rather than uniformitarian. Phillips R. J.* Hansen V. L. Venus Magmatic and Tectonic Evolution Two years beyond the initial mapping by the Magellan spacecraft, hypotheses for the magmatic and tectonic evolution of Venus have become refined and focused. We present our view of these processes, attempting to synthesize aspects of a model for the tectonic and magmatic behavior of the planet. The ideas presented should be taken collectively as a hypothesis subject to further testing. The quintessence of our model is that shear and buoyancy forces in the upper boundary layer of mantle convection give rise to a spatially and temporally complex pattern of strain in a one-plate venusian lithosphere and modulate the timing and occurrence of magmatism on a global basis. Hansen V. L.* Phillips R. J. Ishtar Deformed Belts: Evidence for Deformation from Below? The mountain belts of Ishtar Terra are unique on Venus. Models for their formation include mantle upwelling, mantle downwelling, and horizontal convergence. The present forms of these models are too simple to predict surface strain, topography, or gravity. More-detailed models will require specific constraints as imposed by geologic relations. In order to develop specific constraints for geodynamic models we examine the geology of Ishtar Terra as viewed in Magellan SAR imagery in an attempt to interpret regional surface strain patterns. In this paper we present geologic and structural relations that lead us to postulate that Ishtar deformed belts result from shear forces within the mantle acting on the lithosphere, and not by horizontal forces from colliding plates. We propose that the surface strains result from differential strain and displacement of domains within the upper mantle, and that further analysis of Ishtar deformation may allow us to identify individual domains within the mantle, and to constrain displacement trajectories between domains. Marchenkov K. I.* Saunders R. S. Banerdt W. B. Geophysical Models of Western Aphrodite-Niobe Region: Venus In terms of its mechanical parameters (such as mass, mean radius, and mean density) Venus is very similar to the Earth. But the tectonic regimes of Venus and Earth are quite different. Unlike the Earth, where the effects of mantle convection are manifest on the surface in midoceanic ridges (spreading zones) and trenches (subduction zones), a global system of ridges rifts and subduction zones is not observed on Venus. There is no evidence of Earthlike plate tectonics and Venus may be a one-plate (or no-plate) planet. Magellan radar data confirm this point of view. One possible consequence of this conclusion is that Venus's convection is confined beneath a thick buoyant lithosphere. In thermal models of Venus, a thick venusian crust overlies a convecting mantle. The style of convection in Venus is an open issue (e.g., two layer or whole mantle convection. But two layer convection provides high temperature at the crust-mantle boundary. Additionally, the geochemical data show that the upper mantle of Venus is apparently depleted. Grosfils E. B.* Head J. W. III Spatially Extensive Uniform Stress Fields on Venus Inferred from Radial Dike Swarm Geometries: The Aphrodite Terra Example The high resolution and near global coverage of Magellan radar images is facilitating attempts to systematically investigate the stresses that have deformed the venusian crust. Here we continue earlier efforts to utilize ~170 large, radially lineated structures interpreted as dike swarms to assess the orientation of the regional maximum horizontal compressive stress (MHCS) that existed in their vicinities during emplacement. Examination of swarms near the equator reveals a link to broad-scale regional structures, such as Aphrodite Terra, across distances in excess of 1000 km, suggesting the existence of first-order stress fields that affect areas of more than 10^6 km^2 in a uniform fashion. Focusing further upon the Aphrodite Terra region, the MHCS field in the surrounding lowlands inferred from radial swarms is oriented approximately normal to the slope of the highland topography. This stress configuration appears, at a simple level, to be incompatible with that expected during either upwelling or downwelling construction of the highlands. In addition, the relatively undeformed geometry of the radial structures within the highlands implies that these dike swarm features formed more recently than their highly deformed surroundings. We therefore conclude that the differential stresses that existed during emplacement of the dike swarms within and adjacent to the Aphrodite Terra highlands are related to the gravitational relaxation of pre-existing topography. Stofan E. R.* Hamilton V. E. Cotugno K. Parga and Hecate Chasmata, Venus: Structure, Volcanism and Models of Formation Linear zones of deformation in the equatorial zone of Venus characterized by extension and volcanism have previously been identified in Pioneer Venus and Arecibo data. Two of these zones, Parga and Hecate Chasmata, are seen in Magellan data to be characterized by systems of fractures along which lie numerous coronae and coronalike features. The relationship between tectonic deformation, corona formation, and volcanism in Parga and Hecate Chasmata is examined using Magellan image and altimetry data. We examine several hypotheses for the origin of these zones, including extension and upwelling and delamination or subduction. In a companion abstract, we examine a section of Hecate Chasma in detail. Basilevsky A. T.* Estimation of Age of Dali-Ganis Rifting and Associated Volcanic Activity, Venus This paper deals with the estimation of age for the Dali and Ganis Chasma rift zones and their associated volcanism. One approach is based on photogeologic analysis of stratigraphic relations of rifting and rift-associated volcanism with impact craters that have associated features indicative of their age. The features are radar-dark and parabolic, and they are believed to be mantles of debris derived from fallout of the craters' ejecta [1]. They are thought to be among the youngest features on the venusian surface, so their "parent" craters must also be very young, evidently among the youngest 10% of Venus' crater population. If the average crater retention age of the Venus surface is about 500 million years, the craters with the radar-dark, parabolic features are evidently not older than about 50 million years. Robinson C. A.* Subduction on the Margins of Coronae on Venus: Evidence from Radiothermal Emissivity Measurements Retrograde subduction has been suggested to occur at three coronae on Venus: Latona, Artemis, and Eithinoha. Using the mineralogical arguments of Klose et al. (1992) to explain surface emissivity, a study of radiothermal emissivity of Venus coronae has shown that emissivity changes associated with Latona, Artemis and Ceres imply the same crustal movements predicted by the subduction model of Klose et al. Brown C. D.* Grimm R. E. Flexure and the Role of Inplane Force Around Coronae on Venus Although flexural topography around large coronae has been fit well by elastic plate models using large applied bending moments, an alternative mechanism for flexure on Venus may be inplane force. If coronae are formed by mantle plumes, as the upwelling spreads out beneath the outlying lithosphere it will couple to the base of the plate due to the lack of a low-viscosity zone. The resulting shear tractions can be treated as a compressive inplane force. Alternatively, the hot, expanding corona edge may apply a compressive force to the surrounding plate. We fit an elastic plate bending equation with inplane force (N) and zero applied moment (M) to Artemis (N = -8.1 x 10^13 N/m) and Latona (N = -1.9 x 10^13 N/m) topography and obtain fits as good as those with bending moments alone. By approximating the inplane force inferred from gravity data at Artemis, and considering the compressive load limit of a 5 K/km inelastic rheology, we find that Artemis requires an applied moment of 1.2 x 10^17 N for a more reasonable inplane force of -4.0 x 10^13 N/m. This moment may be provided by the partially compensated ridge of Artemis. Since coronae ridges sometimes exhibit flexure on both sides and are relatively narrow, it may be valid to assume that they are incompletely compensated. If so, convective shear coupling may be the cause of much of the flexure around coronae. Burt J. D.* Head J. W. III Buoyant Subduction on Venus: Implications for Subduction Around Coronae Potentially low lithospheric densities, caused by high Venus surface and perhaps mantle temperatures, could inhibit the development of negative buoyancy-driven subduction and a global system of plate tectonics/crustal recycling on that planet. No evidence for a global plate tectonic system has been found so far; however, specific features strongly resembling terrestrial subduction zones in planform and topographic cross section have been described, including trenches around large coronae and chasmata in eastern Aphrodite Terra. The cause for the absence, or an altered expression, of plate tectonics on Venus remains to be found. Slab buoyancy may play a role in this difference, with higher lithospheric temperatures and a tendency toward positive buoyancy acting to oppose the descent of slabs and favoring underthrusting instead. This study seeks to explore the effect of slab buoyancy on subduction and to define the conditions that would lead to underthrusting versus those allowing the formation of trenches and self- perpetuating subduction. Applying a finite element code to assess the effects of buoyant forces on slabs subducting into a viscous mantle, we find that mantle flow induced by horizontal motion of the convergent lithosphere greatly influences subduction angle, while buoyancy forces produce a lesser effect. Induced mantle flow tends to decrease subduction angle to near an underthrusting position when the subducting lithosphere converges on a stationary overriding lithosphere. When the overriding lithosphere is in motion,as in the case of an expanding corona, subduction angles are expected to increase. Monday, March 15, 1993 Solar System Origins 8:30 - 1:30 a.m. Room C Chair(s): E. H. Levy T. Stepinski Moore M. Ferrante R. Hudson R. Tanabe T. Nuth J.* Catalytic Crystallization of Ices by Small Silicate Smokes at Temperatures Less Than 20 K Samples of methanol and water ices condensed from the vapor onto aluminum substrates at low temperatures (below ~80 K) form amorphous ices; annealing at temperatures in excess of 140-155K is usually required to convert such amorphous samples to crystalline ices. However, we have found that when either methanol or water vapor is deposited onto aluminum substrates that have been coated with a thin (0.1-0.5 mm) layer of amorphous silicate smoke, the ices condense in crystalline form. We believe that crystalline ice forms as the result of energy liberated at the ice/silicate interface, perhaps due to weak bonding of the ice at defect sites on the grains and the very high surface to volume ratio and defect density of these smokes. Annealing of amorphous water ice mixed with more volatile components such as methane, carbon monoxide, etc., has been suggested as an efficient way to produce clatherates in the outer solar nebula and thus explain the volatile content of comets and icy satellites of the outer planets. This hypothesis may need to be reexamined if amorphous ice does not form on cold silicate grains. Cameron A. G. W.* The Giant Impact Produced a Precipitated Moon The author's current simulations of giant impacts on the proto-Earth show the development of large hot rock vapor atmospheres. The Balbus-Hawley mechanism will pump mass and angular momentum outwards in the equatorial plane; upon cooling and expansion the rock vapor will condense refractory material beyond the Roche distance, where it is available for lunar formation. Humayun M.* Clayton R. N. Potassium Isotope Cosmochemistry, Volatile Depletion and the Origin of the Earth We report the first results obtained by our techniques for the precise and accurate determination of the isotopic composition of potassium to constrain the mechanism of volatile element depletion in the formation of the Earth, Moon, and meteorites. Our measurements of delta41K for 6 chondrites and 10 terrestrial rocks attained an average precision of the individual measurement of +/-0.4 permil (2 sigma; +/-0.2 permil/ a.m.u.) and yield a net chondrite-Earth difference unresolved at the 99% confidence limit, DELTA41K = -0.32+/-0.35 permil (3 sigma). This sets a firm upper limit of 1.3+/-1.4 % Rayleigh evaporation of terrestrial potassium (using alpha = square root(41/39)), compared with an observed ~85% chemical depletion of K relative to C1 chondrites. Similar conclusions are reached for the SNC meteorites, Shergotty and Zagami, for 15495 (lunar mare gabbro), and for the eucrite Juvinas. Our conclusion is that direct evaporation of volatile elements from planets (e.g., from silicate vapor atmospheres following giant impact, e.g., Cameron and Benz, 1991) can be ruled out, and the cause of volatile loss must be sought elsewhere, e.g., nebular processes. Our present findings do not support the conclusions of Hinton et al. (1987, 1988), the discrepancy to be resolved at a later date. We also find lunar soil 64801, 641K = +4.99+/-0.53 permil, to be distinctly heavy in accord with Garner et al. (1975). Stevenson D. J.* Volatile Loss from Accreting Icy Protoplanets A large self-gravitating body does not easily lose significant mass because the escape velocity is much larger than the sound speed of atmosphere-forming species under ambient thermal conditions. The most significant exceptions to this are giant impacts or impact jetting by fast-moving projectiles [McKinnon, Geophys. Res. Lett. 16, 1237(1989)]. A very small object (e.g. a comet) also does not easily lose significant volatile mass upon formation because the energy release associated with its accretion is so small. (It can however lose a great deal of mass if it is subsequently moved closer to the Sun.) I argue that there is an intermediate mass range (corresponding to bodies with radii of ~300-800 km) for which the ambient steady-state mass loss is a maximum. By ambient, I mean those conditions pertaining to the formation region of the body. By steady state, I mean to exclude infrequent traumas (giant impacts). The existence of a preferred intermediate mass arises through the competition of growing gravitational containment and growing energy release by accretion; it corresponds typically to GM/(Rc(sub)s^2)~2 to 4, where M is the protoplanet mass of radius R, and cs is the sound speed. Several factors determine the amount of volatile loss in this vulnerable zone during accretion but in general the loss is a substantial fraction of the volatiles, sometimes approaching 100%. The principal implication is that bodies larger than a few hundred kilometers in radius will not have a "primitive" (i.e. cometary) composition. This is relevant for understanding Triton, Pluto, Charon, and perhaps Chiron. Wasilewski P. J.* Faris J. L. O'Bryan M. V. Magnetic Record in Chondrite Meteorites In this paper we summarize what we know about the magnetic record in chondrite meteorites based on new data and previously published results. Strips from thin slabs of chondrite meteorites were cut into near-cubical subsamples (several mm on edge) numbering ~60 to ~120 per meteorite. A common orientation was assigned to each subsample from a given meteorite in order to ensure that we could discover the vector makeup of the bulk meteorite. The new dataset includes Shaw (L7), Roy, (L5/6), Claytonville (L5), Plainview (H5), Etter (H5), Leoville (C3V), and Allende (C3V). In addition to these new results, literature data of sufficient detail, e.g., Bjurbole (L4), ALH 769 (L6), Abee (E4), Allende (C3V), and Olivenza (L5), is considered. Stepinski T. F. Reyes-Ruiz M.* Magnetically Controlled Solar Nebula It is widely believed that a primordial solar nebula, the precursor of the Sun and its planetary system, could be best described in terms of an accretion disk. Such an accretion disk is thought to be turbulent, and it is usually imagined that turbulent viscosity alone provides a torque responsible for the structure and the evolution of the nebula. However, it was found that an MHD dynamo operating in a turbulent nebula can contemporaneously produce magnetic fields capable of significantly altering or even dominating the total torque. Thus, it seems that no model of a viscous solar nebula is complete without taking magnetic fields into consideration. We present calculations that reveal basic aspects of the influence of magnetic fields on the structure of the viscous solar nebula. Dolginov A. Z. Stepinski T. F.* Are Cosmic Rays Effective for Ionization of the Solar Nebula? Most models of the presolar nebula predict that beyond a certain maximum radius R{rm max} sim 1 A.U. the gas temperature is too low to cause thermal ionization of any important gas constituent. In the nebular regions beyond R(sub)max significant levels of electrical conductivity require some nonthermal ionization sources. Two such sources have been identified and considered in the literature; these are cosmic rays and radioactive nuclei 26Al and 40K. The effectiveness of those nonthermal ionization sources is crucial for our understanding of the hydromagnetic state of the nebula. It has been shown that dynamically significant nebular magnetic fields cannot arise as the result of the compression of interstellar magnetic field during the nebula collapse, but have to be contemporaneously regenerated by the dynamo mechanism. Models of nebular dynamos showed that ionization levels due to in situ radioactive nuclei alone are generally too low for a nebular dynamo to operate. The ionization rate (per H atom) due to cosmic rays is given by chi(sub)cr = 10^-17 exp {-l/l(sub)0}, where l ~ rho S is the column density of mass traversed by cosmic rays from the nebular surface to the given point inside the nebula, l(sub)0 ~ 100 g cm^-2 is a characteristic attenuation length, and S is the length of the path traveled by cosmic rays. Nebular disk surfaces are defined at +-h from the midplane, where h is the disk half-thickness. Thus, in the parts of the nebula that are too cool for thermal ionization to occur, directly penetrating cosmic rays may provide levels of ionization that are sufficient for maintaining a nebular dynamo, provided that the surface density at those regions is not much larger than l(sub)0. In this paper we argue that the effectiveness of cosmic rays to ionize the bulk of the nebular gas may be further impaired by the influence of the magnetic field on the propagation of cosmic rays. Levy E. H.* Ruzmaikina T. V. Possible Dust Contamination of the Early Solar System Measurements carried out over more than twenty years indicate a deficiency of neutrinos emitted from the deep interior of the Sun in comparison with the neutrino flux expected from canonical solar models. The early measurements were sensitive only to high-energy neutrinos emitted from ^8B on a minor branch of the energy-producing nuclear reactions in the solar interior. Thus these measurement were not widely considered to be a definitive test of solar physics. However the more recent measurements, which are sensitive to lower energy neutrinos, produced primarily by the p-p reaction on the main energy- producing branch of the solar nuclear reactions, pose a far more significant mystery in physics. One possibility is that the Sun's interior opacity is lower than expected due to a paucity of elements. This paper discusses the possibility that the Sun formed from material less abundant in heavy elements than usually believed, and the subsequent contamination due to the settling of surrounding dust brought the abundance of heavy elements--in the protoplanetary nebula, and in the Sun's convective envelope--up to the currently observed value. Cassen P.* Why Convective Heat Transport in the Solar Nebula was Inefficient The radial distributions of the effective temperatures of circumstellar disks associated with pre-main sequence (T Tauri) stars are relatively well constrained by ground-based and spacecraft infrared photometry and radio continuum observations. If the mechanisms by which energy is transported vertically in the disks are understood, these data can be used to constrain models of the thermal structure and evolution of solar the nebula. Several studies of the evolution of the solar nebula have included the calculation of the vertical transport of heat by convection. Such calculations rely on a mixing length theory of transport and some assumption regarding the vertical distribution of internal dissipation. In all cases, the results of these calculations indicate that transport by radiation dominates that by convection, even when the nebula is convectively unstable. We present here a simple argument that demonstrates the generality (and limits) of this result, regardless of the details of mixing length theory or the precise distribution of internal heating. It is based on the idea that the radiative gradient in an optically thick nebula generally does not greatly exceed the adiabatic gradient. Boss A. P.* Midplane Temperatures in the Solar Nebula Cosmochemical analyses of meteorites imply that maximum temperatures in the inner solar nebula were on the order of 1300 K, yet standard viscous accretion disk models predict much lower midplane temperatures (about 300 K at 2 AU to 3 AU) in a minimum mass nebula. A second-order accurate radiative hydrodynamics code has been used to construct models of the late-phase solar nebula appropriate for low-mass star formation (10^-6 to 10^-5 solar masses per year). For a minimum mass (0.02 solar mass) nebula and a solar-mass protostar, the new models show that compressional heating due to mass accretion onto the nebula and subsequent vertical contraction of the nebula are sufficient to lead to midplane temperatures Tm > 1400 K at 1 AU and Tm > 1000 K at 2.5 AU. Weidenschilling S. J. Ruzmaikina T. V.* Coagulation of Grains in Static and Collapsing Protostellar Clouds COAGULATION OF GRAINS IN STATIC AND COLLAPSING PROTOSTELLAR CLOUDS S.J. Weidenschilling (Planetary Science Institute) and T.V. Ruzmaikina (Lunar and Planetary Laboratory, Univ. of Arizona, Tucson, and Schmidt Inst. of Earth Physics, Moscow)The wavelength dependence of extinction in the diffuse interstellar medium implies that it is produced by particles of dominant size ~10-5 cm. There is some indication that in the cores of dense molecular clouds, submicrometer grains can coagulate to form larger particles; this process is probably driven by turbulence. The most primitive meteorites (carbonaceous chondrites) are composed of particles with a bimodal size distribution with peaks near 1 micrometer (matrix) and 1 mm (chondrules). Cameron suggested that grains could coagulate during collapse of the presolar cloud, with the short free- fall time (~10^5 yr) offset by higher densities and turbulent velocities. Models for chondrule formation that involve processing of presolar material by chemical reactions or through an accretion shock during infall assume that aggregates of the requisite mass could form before or during collapse. Most evaluations of grain aggregation have been simple comparisons of collision timescales with cloud lifetimes and free-fall times; only Cameron computed actual size distributions of aggregates. The effectiveness of coagulation during collapse has been disputed; it appears to depend on specific assumptions. Here we report the first results of detailed numerical modeling of spatial and temporal variations of particle sizes in presolar clouds, both static and collapsing. Malhotra R.* On the Delivery of Planetesimals to a Protoplanet in the Solar Nebula A planetesimal moving in the solar nebula experiences an aerodynamic drag that causes its orbit to circularize and shrink. However, resonant perturbations from a protoplanet interior to the planetesimal's orbit can counteract both the orbital decay and the damping of the eccentricity: the planetesimal can be captured into an orbital resonance and its eccentricity pumped up to an equilibrium value. The resonance trapping phenomenon is sensitive to several factors and we elaborate on these below. Orbital resonances form (partial) barriers to the transport of planetesimals into the feeding zone of the protoplanet. Here we report on a systematic study of the idealized system of a single protoplanet of mass much larger than the masses of the planetesimals, comparing relevant analytic estimates with numerical simulations. Our principal conclusion is that in a gas-rich environment, the bulk of the solid material delivered to the feeding zone of the protoplanet is in the form of quite small bodies. Wetherill G. W.* Variety in Planetary Systems Observation of circumstellar disks, regular satellite systems of outer planets, and planet-sized objects orbiting pulsars support the supposition that formation of planetary systems is a robust, rather than a fragile, by-product of the formation and evolution of stars. The extent to which these systems may be expected to resemble one another and our solar system, either in overall structure or in detail, remains uncertain. When the full range of possible stellar masses, disk masses, and initial specific angular momenta are considered, the possible variety of planetary configurations is very large. Monday, March 15, 1993 Impact Cratering and Shock Metamorphism 8:30 - 12:00 a.m. Room D Chair(s): M. B. Boslough R. Bottomley Holsapple K. A.* The Size of Complex Craters Lunar craters larger than about 15 km and terrestrial craters larger than about 3 km in diameter presumably underwent gravity-driven, "late- stage" collapse that modified an initial transient bowl-shaped "simple" crater into the flat-floored complex craters observed. These same mechanisms were operative for the larger craters on other solar system bodies, at a threshold size inversely proportional to gravity. This paper presents a new look at the scaling relations for these complex craters. Takata T.* Ahrens T. J. SPH Modelling of Energy Partitioning During Impacts on Venus Impact cratering of the venusian planetary surface by meteorites was investigated numerically using the Smoothed Particle Hydrodynamics (SPH) method. Venus presently has a dense atmosphere. Vigorous transfer of energy between impacting meteorites, the planetary surface, and the atmosphere is expected during impact events. We have concentrated our investigation on the effects of the atmosphere on energy partitioning and the flow of ejecta and gas. Schmidt R. M.* Pressure Versus Drag Effects on Crater Size The topic of atmospheric effects on crater formation is very complex because it includes not only pressure effects on excavation, but also drag effects on ejecta placement. Experiments have to be designed very carefully to allow isolation of the two phenomena. Historically, numerous investigators (Johnson et al., 1969; Herr, 1971; Schultz and Gault, 1979; Holsapple, 1980; and Schultz, 1992) have shown an influence of atmospheric pressure. However none have identified the scaling that correctly isolates pressure from drag effects. On-going work (Housen and Schmidt, 1990; Housen,et al., 1992; and Schmidt et al., 1992) in explosive cratering has produced scaling paradigms for deeply buried explosive charges where drag effects are negligible. Here it was found that increased pressure caused significant induced strength effects that impeded crater excavation. The effect is more pronounced with increasing burial depth and less pronounced with increased yield. Barnouin O. S.* Schultz P. H. Behavior of Vortices Generated by an Advancing Ejecta Curtain in Theory, in the Laboratory, and on Mars Several papers assess the interaction between an atmosphere and advancing ejecta to assess possible atmospheric processes affecting ejecta emplacement. Ejecta travel through an atmosphere in two modes: larger ejecta blocks follow ballistic trajectories unhindered by the atmosphere; finer ejecta are entrained in a turbulent basal cloud, which develops as the advancing ejecta curtain generates strong atmospheric winds. Laboratory experiments reveal that this cloud of fine ejecta produce ramparts, flow lobes, or radial scouring that superposes larger ballistic ejecta emplaced earlier. Martian, venusian, and terrestrial ejecta facies can be interpreted in terms of processes observed in the laboratory with appropriate first-order corrections for scaling. A continuum model of the atmospheric flow around an advancing inclined plate simulated and reproduced some of the complex flow patterns observed in front and at the top of the curtain. Here we consider improvements to the model to compare quantitatively the approximate position of ejecta deposition (i.e., run-out distance) with laboratory experiments and martian ejecta facies. Crawford D. A.* Schultz P. H. Macroscopic Electric Charge Separation During Hypervelocity Impacts: Potential Implications for Planetary Paleomagnetism The production of transient magnetic fields by hypervelocity meteoroid impact has been proposed to possibly explain the presence of paleomagnetic fields in certain lunar samples as well as across broader areas of the lunar surface. In an effort to understand the lunar magnetic record, continued experiments at the NASA Ames Vertical Gun Range allow characterizing magnetic fields produced by ~5 km/s impacts of 0.32-0.64 cm projectiles over a broad range of impact angles and projectile/target compositions. From such studies, another phenomenon has emerged, macroscopic electric charge separation, which may have importance for the magnetic state of solid-body surfaces. Adushkin and Soloviev observed this phenomenon during explosive cratering experiments but the magnetic consequences of macroscopic electric charge separation (as opposed to plasma production) during explosion and impact cratering have not, to our knowledge, been explored before now. It is straightforward to show that magnetic field production due to this process may scale as a weakly increasing function of impactor kinetic energy although more work is needed to precisely assess the scaling dependence. Fiske P. S.* Gratz A. J. Nellis W. J. NMR Spectroscopy of Experimentally Shocked Single Crystal Quartz: A Reexamination of the NMR Shock Barometer Cygan et al. report a broadening of the 29Si NMR peak for synthetic quartz powders with increasing shock pressure, which they propose as a shock wave barometer for natural systems. We expand on these results by studying single- crystal quartz shocked to 12 and 33 GPa using the 6.5-m two-stage light-gas gun at Lawrence Livermore National Laboratories. Our NMR results differ substantially from those of Cygan et al. and suggest that the proposed shock wave barometer may require refinement. The difference in results between this study and that of Cygan et al. is most likely caused by different starting materials (single crystal vs. powder) and different shock loading histories. NMR results from single crystal studies may be more applicable to natural systems. Boslough M. G.* Cygan R. T. Kirkpatrick R. J. 29Si NMR Spectroscopy of Naturally Shocked Quartz from Meteor Crater, Arizona: Correlation to Keiffer's Classification Scheme We have applied solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy to five naturally shocked Coconino Sandstone samples from Meteor Crater, Arizona, with the goal of examining possible correlations between NMR spectral characteristics and shock level. This work follows our observation of a strong correlation between the width of a 29Si resonance and peak shock pressure for experimentally shocked quartz powders. The peak width increase is due to the shock-induced formation of amorphous silica, which increases as a function of shock pressure over the range that we studied (7.5 to 22 GPa). The Coconino Sandstone spectra are in excellent agreement with the classification scheme of Kieffer in terms of presence and approximate abundances of quartz, coesite, stishovite, and glass. We also observe a new resonance in two moderately shocked samples that we have tentatively identified with silicon in tetrahedra with one hydroxyl group in a densified form of amorphous silica. Spray J. G.* Delimitation of Terrestrial Impact Craters via Pseudotachylitic Rock Distribution The determination of the shape and size of terrestrial impact craters is problematic, yet is critical for understanding cratering mechanics and for scaling bolide mass, volume, and impact velocity with crater size and target response. The problem is particularly difficult in older geological terrains (e.g., Precambrian) that are more likely to have suffered postimpact deformation and hence distortion of the original structure and/or where weathering may have partly removed or obscured its original shape. Traditionally, a number of features are used to assist us in determining the shape and size of an impact structure. These include (a) the occurrence of faults, especially those disposed concentrically relative to the crater: the outermost ring faults being interpreted as indicating a viable minimum diameter and (b) the development of so-called breccias, some of which are also associated with faults (e.g., the Sudbury Breccia developed within the target rocks of the Sudbury Structure of Ontario, Canada). "Breccia" is not a satisfactory term because a number of breccia types exist at impact sites (e.g., fall-back breccias and in situ brecciated target material). Schultz P. H.* Bunch T. E. Koeberl C. Collins W. Further Analyses of Rio Cuarto Impact Glass Initial analyses of the geologic setting, petrology, and geochemistry of glasses recovered from within and around the elongate Rio Cuarto (RC) craters in Argentina focused on selected samples in order to document the general similarity with impactites around other terrestrial impact craters and to establish their origin. Continued analysis has surveyed the diversity in compositions for a range of samples, examined further evidence for temperature and pressure history, and compared the results with experimentally fused loess from oblique hypervelocity impacts. These new results not only firmly establish their impact origin but provide new insight on the impact process. Bottomley R. J.* York D. Grieve R. A. F. Age of Popigai Impact Event Using the 40Ar-39Ar Method The Popigai impact structure of central Siberia is the largest known impact crater in the Commonwealth of Independent States with an original diameter of some 100 km. The age of the crater is constrained by the existing stratigraphy to a period between 5-65 Ma. Attempts to date the impact event using conventional K-Ar on whole-rock samples and fission-track dating on glasses yield a spread of ages between 30 and 45 Ma. Argon step-heating analyses of several whole-rock samples performed with the Argon Laserprobe at the University of Toronto indicated an age of impact of about 36 Ma. However, a more recently reported 40Ar-49Ar result on glass separated from a suevite sample gave a 65 Ma age and raised the possibility that Popigai was involved with the K/T boundary event. We have pursued further analyses at the University of Toronto on a broader spectrum of Popigai samples. These results confirm an age of about 36 Ma for the formation of this crater, and indicate that Popigai was not associated with the K/T boundary event. Brandt D.* Reimold W. U. A Structural and Petrographic Investigation of the Pretoria Saltpan Impact Structure The Pretoria Saltpan crater is located in the southern portion of the Bushveld Igneous Complex, some 40 km north-northwest of Pretoria, South Africa. The near-circular structure of 1.13 km diameter exhibits a well-preserved, uptilted granite rim. Granitic breccia overlies Karroo sediment in places, indicating a post-Karroo age for the cratering event. The coincidence of the spatial occurrence of the crater with respect to various alkaline and ultramafic intrusives has been the main argument put forward against an impact origin for the structure. Detailed mapping of the crater rim exposures and the crater environs was carried out and revealed many occurrences of intrusives in the whole region. Structural analysis along the rim revealed the presence of typical impact crater-related structures. Comparative petrographic and chemical studies of crater-related and non-related intrusives showed close similarities between these sample suites. Koeberl C.* Shirey S. B. Osmium Isotopes in Ivory Coast Tektites: Confirmation of a Meteoritic Component and Rhenium Depletion We used the sensitive negative thermal ionization mass spectrometry method for the measurement of concentrations and isotopic ratios of osmium and rhenium in four Ivory Coast tektites. These tektites have crustal major- and trace-element composition, as well as large negative Nd (-20) and positive Sr (+260 to +300), which are characteristic for old continental crust. We have found Os concentrations ranging from 0.09 to 0.30 ppb, clearly much higher than average crustal values, l87Os/l86Os ratios of about 1.2-1.7, and low l87Re/l860s ratios. These results show unambiguously the existence of a meteoritic component (on the order of 0.06%) in the Ivory Coast tektites. Low Re abundances are the result of fractionation of Re during the impact. Monday, March 15, 1993 Solar, Cosmic Ray, and Dynamical Studies This session is dedicated to Professor James R. Arnold in honor of his seventieth birthday. 1:30 - 5:00 p.m. Room A Chair(s): D. Black R. C. Reedy Garrison D. H.* Rao M. N. Bogard D. D. Reedy R. C. SCR 21Ne and 38Ar in Lunar Rock 68815: The Solar Proton Energy Spectrum over the Past 2 Myr We determined concentration profiles of 21Ne, 22Ne, and 38Ar produced by solar protons as a function of depth in oriented lunar rock 68815. A comparison with model predictions indicate a solar proton flux J(4 pi; E >10 MeV) of 100-125 p/cm^2/s and a rigidity R(sub)o of 85-100 MV, assuming an erosion rate of 1-2 mm/Myr. These results for 68815 and similar results on 61016 define the integrated solar proton energy spectrum at the Moon over the past ~2 Myr. Rao M. N.* Garrison D. H. Bogard D. D. SCR Neon and Argon in Kapoeta Feldspar: Evidence for an Active Ancient Sun From etched feldspar size fractions of Kapoeta, we determine a significant excess of cosmogenic 21Ne and 38Ar over that produced by galactic cosmic rays. This excess component is attributed to early production by energetic solar protons and suggests that the energetic proton flux from the ancient Sun was several hundred times more intense than that of the contemporary Sun. Wieler R.* Baur H. Signer P. A Long-Term Change of the Ar/Kr/Xe Fractionation in the Solar Corpuscular Radiation Solar noble gases in an ilmenite separate from breccia 79035 (antiquity >1 Ga) were analyzed by closed system stepped etching (CSSE). All five gases show the familiar two-component structure: first solar-wind (SW) gases are released, followed by gases from solar energetic particles (SEP). Element patterns in 79035 are similar to those of 71501 ilmenite (antiquity <100 Ma). SW-He-Ne were partly lost, but SEP-He-Ne-Ar are retained (nearly) unfractionated. Constant Ar/Kr/Xe ratios indicate that ilmenites contain an unfractionated sample of the heavy SW-SEP noble gases. Ar/Kr/Xe ratios in the solar corpuscular radiation are, however, different from "solar system" values, whereby the Kr/Xe difference in 79035 is about twice as large as in 71501. We propose that Xe is less fractionated than Kr and Ar, though its first ionisation potential (FIP) is higher than the "cutoff" at 11.5 eV, above which all elements in SEP are usually assumed to be depleted by a roughly constant factor. SW-Ne may be isotopically slightly heavier in the ancient SW trapped by 79035, as proposed earlier. Reedy R. C.* Masarik J. Nishiizumi K. Arnold J. R. Finkel R. C. Caffee M. W. Southon J. Jull A. J. T. Donahue D. J. Cosmogenic-Radionuclide Profiles in Knyahinya: New Measurements and Models Cosmic-ray-produced nuclides measured in samples taken from known locations on a big slab of the large (R = 45 cm) L5 chondrite Knyahinya provide good depth- vs.-concentration profiles to develop and test models for the production of cosmogenic nuclides in meteorites. We report new profiles for Be10, Al26, Cl36 in metallic and nonmagnetic phases of eight documented samples from Knyahinya and for C14 in bulk samples from seven Knyahinya samples. These new measured profiles are very similar to profiles calculated with particle fluxes from the LAHET Monte Carlo production and transport code system and with cross sections for major reactions. Arnold J. R.* Kohl C. P. Nishiizumi K. Caffee M. W. Finkel R. C. Southon J. R. Measurements of Cosmogenic Nuclides in Lunar Rock 64455 Eleven samples were ground from the glass-coated surface of lunar rock 64455,82 with an average depth resolution of 50 micrometers and were measured for 10Be, 26Al, and 36Cl using AMS (accelerator mass spectrometry). Results show no evidence of SCR (solar cosmic ray) effects. The flat cosmogenic nuclide profiles and activity levels are consistent with a 2-My exposure history for the rock and a sample location on the bottom of the rock. These AMS measurements are some of the most precise ever obtained for these three nuclides. This precision and the demonstrated fine depth resolution will enable us to conduct a number of detailed studies of depth effects in lunar and meteoritic samples, including investigating SCR effects in the surface exposed top of the glass coating of 64455 and possibly in the underlying rock. This work is underway. Graf Th.* Niedermann S. Marti K. A Calibration of the Production Rate Ratio P21/P26 by Low Energy Secondary Neutrons: Identification of Ne Spallation Components at the 10(6) Atoms/g Level in Terrestrial Samples The spallation ratio (22Ne/21Ne)(sub)c from Si was determined as 1.243+-0.022 in a terrestrial quartz sample. We carried out a calibration of the in situ production rate ratio P21/P26 in quartz samples for which 10Be and 26Al production rates were previously measured. A ratio P21/P26 of 0.67+-0.12 is obtained. Loeken Th.* Scherer P. Schultz L. Noble Gases in Twenty Yamato H-Chondrites: Comparison with Allan Hills Chondrites and Modern Falls Concentration and isotopic composition of noble gases have been measured in 20 H chondrites found on the Yamato Mountains ice fields in Antarctica. The distribution of exposure ages as well as of radiogenic 4-He contents is similar to that of H chondrites collected at the Allan Hills site. Furthermore, a comparison of the noble gas record of Antarctic H chondrites and finds or falls from non-Antarctic areas gives no support to the suggestion that Antarctic H chondrites and modern falls derive from differing interplanetary meteorite populations. Nishiizumi K.* Arnold J. R. Caffee M. W. Finkel R. C. Southon J. R. Nagai H. Honda M. Imamura M. Kobayashi K. Sharma P. Exposure Ages of Carbonaceous Chondrites-I The recent exposure histories of carbonaceous chondrites have been investigated using cosmogenic radionuclides. Our results may indicate a clustering of exposure ages of C1 and C2 chondrites into two peaks, 0.2 My and 0.6 My, perhaps implying two collisional events of Earth-crossing parent bodies. Dodd R. T.* Lipschutz M. E. A CM Chondrite Cluster and CM Streams An elongate year-day concentration of CM falls between 1921 and 1969 is inconsistent with a random flux of CM meteoroids and suggests that most or all such meteorites, and perhaps the Kaidun C-E chondrite breccia, sampled streams of meteoroids in nearly circular, Earth-like orbits. Herzog G. F.* Souzis A. E. Xue S. Klein J. Juenemann D. Middleton R. 26Al-26Mg Ages of Iron Meteorites An exposure age for an iron meteorite can be calculated from measurements of a radioactive nuclide and a stable nuclide that are produced by similar sets of nuclear reactions, provided that the stable nuclide is present with low initial abundance. The standard methods rely on either ^40K (t(sub)1/2 = 1.26 Gy), ^39K, and ^41K or on a shorter-lived radionuclide and a stable, noble-gas isotope. Widely used pairs of this type include ^36Cl/^36Ar and ^26Al/^21Ne. Other pairs may serve the purpose, for iron meteorites contain many stable isotopes besides those of K and the noble gases that are produced partly by cosmic rays. We consider here the calculation of exposure ages, t(sub)26, from measurements of ^26Al (t(sub)1/2 = 0.7 My) and (stable) ^26Mg. Ages based on ^26Al/^26Mg ratios, like those based on ^36Cl/^36Ar ratios, are "buffered" against changes in relative production rates due to shielding because decay of the radioactive nuclide accounts for a good part of the inventory of the stable nuclide. Asphaug E.* Melosh H. J. Ryan E. Ejecting Basaltic Achondrites from Vesta: Hydrodynamical Impact Models Vesta is a large (~570 km diameter) asteroid whose crust is mostly basaltic. Spectral heterogeneity suggests a sizable olivine feature that may be explained as impact excavation (exposure of sub-crustal material). The spectral data probably show a localized feature ~200 km in diameter or a diffuse feature ~400 km in diameter. Lightcurve irregularities suggest heterogeneity on a similar scale. This heterogeneity may represent the crater bowl, the extent of its ejecta deposit, or indeed something unrelated to cratering. In any case drawing direct inferences about the state of Vesta's surface on the basis of these observations involves substantial speculation. Williams D. R.* Wetherill G. Equilibrium Models of Mass Distribution and Collisional Lifetimes of Asteroids An understanding of the steady-state distribution expected in the present-day asteroid belt is important to our understanding of the collisional evolution of the asteroids and their physical properties. We have extended earlier work to show that, in the absence of gravity, a simple power law distribution as a function of mass with constant exponent will give an equilibrium distribution of asteroids for all bodies much smaller than the largest asteroids. This result holds for realistic fragmentation mechanisms and is independent of the physical properties of the asteroids. Inclusion of the effects of gravity on disruption and fragmentation of asteroids precludes an analytic solution to this problem, and rules out a simple power law distribution. We are currently calculating numerical solutions in order to determine the expected steady- state mass distribution in the asteroid belt. Zook H. A.* Grun E. Baguhl M. Balogh A. Bame S. J. Fechtig H. Forsyth R. Hanner M. S. Horanyi M. Kissel J. Lindblad B.-A. Linkert D. Linkert G. Mann I. McDonnell J. A. M. Morfill G. E. Phillips J. L. Polanskey C. Schwehm G. Siddique N. Staubach P. Svestka J. Taylor A. On Dust Emissions from the Jovian System As described by Grun et al., the dust impact detector on the Ulysses spacecraft detected a totally unexpected series of dust streams in the outer solar system near the orbit of Jupiter. Five considerations lead us to believe that the dust streams emanate from the jovian system itself: (1) The dust streams only occur within about 1 AU of the jovian system, with the strongest stream being the one closest to Jupiter (about 550 RJ away). (2) The direction from which they arrive is never far from the line-of-sight direction to Jupiter. (3) The time period between streams is about 28 (+/-3) days. (4) The impact velocities are very high--mostly around 40 km/s. (5) We can think of no cometary, asteroidal, or interstellar source that could give rise to the above four phenomena; such streams have never before been detected. Monday, March 15, 1993 Venus Volcanism 1:30 - 5:00 p.m. Room B Chair(s): B. Campbell J. B. Garvin Kauhanen K.* Tectonics of Neyterkob Corona on Venus Neyterkob double corona (50 degrees N 202 degrees) presents an area of corona-related interfering tectonic patterns that are formed in different phases of evolution of the corona and modified by regional stresses. Analysis of the patterns can tell us something about coronal formation. Tectonic features form distinct units on topographic depressions, slopes, and volcanic flows extending over one radius of the corona. A remarkable number of compressional features was found near the rim and related to interaction between adjacent coronae. Radial extension was mainly observed on a peculiar northeast-southwest-trending high-crossing the corona. Concentric fractures were found to the east partly connected to the lithospheric flexure. Tectonic features indicate movements of volcanic activity and modification of the area by more regional stresses. Garvin J. B.* Williams R. S. Morphometric Comparison of Icelandic Lava Shield Volcanoes Versus Selected Venusian Edifices Shield volcanos are common landforms on the silicate planets of the inner solar system, and a wide variety have recently been documented on Venus by means of Magellan observations. Head and colleagues have compiled a comprehensive global census of discrete volcanos on Venus, with a diameter cutoff for equidimensional, shieldlike varieties of ~ 25 km. Aubele and colleagues have documented the distribution of clusters of small shieldlike volcanos on Venus ("shield fields"). Most of the apparently conical landforms that define the shield fields are less than 25 km in diameter, and Garvin and Williams have argued that many of these volcanos [called "domes" by Aubele and Crumpler] are analogous to Icelandic lava shield volcanos on the basis of their inferred morphology. In this report, we emphasize our recently completed morphometric survey and analysis of three representative Icelandic lava shields -- the classic Skjaldbreidur edifice, the low-relief Lambahraun feature, and the monogenetic Sandfellshaed shield -- as a basis for comparison with representative venusian edifices (>60 km in diameter). Our detailed morphometric measurements of a representative and well-studied set of Icelandic volcanos permits us to make comparisons with our measurements of a reasonable subset of shieldlike edifices on Venus on the basis of Magellan global radar altimetry (i.e., GxDR). Our study has been restricted to venusian features larger than ~60 km in diameter, on the basis of the minimum intrinsic spatial resolution (8 km) of the Magellan radar altimetry data. Finally, in order to examine the implications of landform scaling from terrestrial simple and composite shields to larger venusian varieties, we have considered the morphometry of the subaerial component of Mauna Loa, a type locality for a composite shield edifice on Earth. Wilson L.* Pinkerton H. Head J. W. Magee Roberts K. A Classification Scheme for the Morphology of Lava Flow Fields Analysis of the processes controlling the advance of lava flows shows that, if no other factors intervene, thermal constraints will act to limit the maximum length of a flow being fed at a given volume or mass effusion rate from a vent. These constraints can be characterised through the Gratz number, which takes on a large value at the vent and decreases down flow. Early application of this principle showed that, despite the many subtleties of modes of heat loss from flows, motion apparently ceases when the Gratz number has decreased to a value close to 300. Recent analyses of flow units from the 1983-86 Pu'u 'O'o eruption of Kilauea and of other, more silicic lava flow units confirm this finding. Campbell B. A.* Zimbelman J. R. Lava Flows on Venus: Analysis of Motion and Cooling Effusive volcanism is a major mechanism for resurfacing on Venus. The large edifice complexes are marked by lava flow fields that extend hundreds of kilometers from their apparent source vents. Earlier work on the emplacement of lava predicted few major differences in flow length between Venus and the Earth (for similar magmas) based primarily on assessment of the relative rates of heat loss to their respective atmospheres. This analysis, combined with the obvious presence of very long lava flows on Venus, has led to suggestions that basalt magmas on Venus may be of generally lower viscosity or that magma effusion rates exceed terrestrial norms. If correct, these hypotheses might imply systematically different chemical and dynamic characteristics for volcanic processes on Venus, and a somewhat catastrophic history for many of the large edifices and flow fields. In this abstract, we reexamine the mechanics of magma motion and cooling, and discuss some of the issues involved in analyzing flows on Venus. Zimbelman J. R.* Campbell B. A. Kousoum J. Lampkin D. J. Numerical Simulation of Lava Flows; Applications to the Terrestrial Planets Lava flows are the visible expression of the extrusion of volcanic materials on a variety of planetary surfaces. A computer program described by Ishihara et al. appears to be well suited for application to different environments, and we have undertaken tests to evaluate their approach. Our results are somewhat mixed; the program does reproduce reasonable lava flow behavior in many situations, but we have encountered some conditions common to planetary environments for which the current program is inadequate. Here we present our initial efforts to identify the `parameter space' for reasonable numerical simulations of lava flows. Sakimoto S. E. H.* Zuber M. T. Venus Pancake Dome Formation: Morphologic Effects of a Cooling-induced Variable Viscosity During Emplacement The distinctive steep-sided "pancake" domes discovered in the Magellan images of Venus have morphologies that suggest formation by a single continuous emplacement of a high-viscosity magma. A resemblance of the venusian domes to much smaller terrestrial rhyolite and dacite volcanic domes has prompted some authors to suggest that the domes on Venus also have high silica compositions and thus high viscosities. However, viscosity is a function of crystallinity as well as silica content in a magma, and thus increases as a result of magmatic cooling. To investigate the effect of a cooling-induced viscosity increase on dome morphology, we are modeling the domes as radial viscous gravity currents that cool during emplacement. Bulmer M. H.* Guest J. E. Michaels G. Saunders S. Scalloped Margin Domes: What are the Processes Responsible and How do they Operate? Studies of scalloped margin domes (SMD) indicate the scallops are the result of slope failure. SMDs have similar but smaller average diameters (26.5 km) than unmodified domes (29.8 km), and the majority plot at altitudes ranging from 0.5 to 4.7 km, relative to the mean planetary diameter. A range of morphological types exist from those least modified to those that show heavy modification. Of the 200 SMDs examined, 33 have clearly discernible debris aprons. Examination and comparison of debris aprons with mass movement features on the Moon, Mars, and in subaerial and submarine environments on Earth1 using H/L against area (km^2), suggests there are three main types of failure; debris avalanche, slumps, and debris flow. Bussey D. B. J.* Sorensen S.-A. Guest J. E. The Origin of Venusian Channels: Modelling of Thermal Erosion by Lava Magellan imagery has revealed that channels, apparently volcanic in origin, are abundant on the surface of Venus. There has been much debate about the origin of these channels. Are they the result of erosional (either thermal or mechanical) or constructional processes? Komatsu G.* Baker V. R. Meander Properties of Venusian Channels Venusian lava channels have meander dimensions that relate to their mode of formation. Their meander properties generally follow terrestrial river trendsof wavelength (L) to width (W) ratios, suggesting an equilibrium adjustment of channel form. Slightly higher L/W for many venusian channels in comparison to terrestrial rivers may relate to nonaqueous flow processes. The unusually low L/W values for some venusian and lunar sinuous rilles probably indicate modification of original meander patterns by lava-erosional channel widening. Weitz C. M.* Basilevsky A. T. Geology and Radiophysical Properties of the Venera and Vega Landing Sites We have produced geologic maps for all seven Venera and Vega landing sites. Because the radius of error for each landing site is 150 km, we have mapped the entire landing circle centered about the estimated landing point. We found correlations between the Magellan imagery and the lander TV panoramas and geochemical measurements made at the landing site. Radiophysical properties, including backscatter cross sections, altimetry, emissivity, and rms slopes, were determined for each landing circle. After mapping the geology of the landing circles, we mapped the regional geology of the seven sites and determined the geologic history of these large regions. Fegley B. Jr.* Lodders K. The Rate of Chemical Weathering of Pyrite on the Surface of Venus We report results of an experimental study of the chemical weathering of pyrite (FeS2) under Venus-like conditions. This work, which extends the earlier study by Fegley and Treiman, is part of a long-range research program to experimentally measure the rates of thermochemical gas-solid reactions important in the atmospheric-lithospheric sulfur cycle on Venus. The objectives of this research are (1) to measure the kinetics of thermochemical gas-solid reactions responsible for both the production (e.g., anhydrite formation) and destruction (e.g., pyrrhotite oxidation) of sulfur-bearing minerals on the surface of Venus and (2) to incorporate these and other constraints into holistic models of the chemical interactions between the atmosphere and surface of Venus. Straub D. W.* The Role of CO2 in Weathering Reactions and the Presence of S2 on Venus: Implications for the Pyrite Stability Field The stability of pyrite has become the focal point of study since regions of low radio thermal emissivity were detected during the Pioneer Venus and Magellan radar mapping missions of Venus. Thermodynamic calculations and experimental studies have been performed assuming CO2 is the major gas species in the venusian atmosphere. The purpose of the present study, however, is to demonstrate that CO2 is a nonreactive gas and that a minor species, S2, is the most important gas in determining whether pyrite is stable in the high radar reflectivity regions. Monday, March 15, 1993 Manson: The Hole and Shocking Story 1:30 - 5:30 p.m. Room C Chair(s): R. Anderson E. M. Shoemaker Shoemaker E. M. Roddy D. J.* Anderson R. R. Research Program on the Manson Impact Crater, Iowa At or near the end of the Cretaceous, at least two large impact events occurred in the western hemisphere. One of these impacts formed the 180-km diameter Chicxulub crater on the northern tip of the Yucatan peninsula. Another formed the 35-km diameter Manson structure in Western Iowa. Many lines of evidence, including the chemical and isotopic composition of glass found in the K/T boundary layer at sites in the Gulf of Mexico and Caribbean region, the regional variation in thickness and the size of spherules in the K/T boundary layer, and evidence for disturbance and deposition of beds at and near the boundary by giant waves in the Gulf of Mexico, point to Chicxulub as a major source of K/T boundary material including a lower layer of clay at K/T boundary sites in western North America. The size, abundance, and mineral and lithic composition of shocked grains from an upper K/T boundary found at Western North American sites, on the other hand, point to Manson as a possible source crater. More than one impact seems to be indicated by the K/T boundary stratigraphy in Western North America and present constraints on the age of the Manson structure suggest that it, as well as Chicxulub, may be a K/T boundary crater. Anderson R. R.* Witzke B. J. Hartung J. B. Shoemaker E. M. Roddy D. J. Descriptions and Preliminary Interpretations of Cores Recovered from the Manson Impact Structure (Iowa) A core-drilling program initiated by the Iowa Geological Survey Bureau and U.S. Geological Survey in 1991 and 1992 collected 12 cores totaling over 1200 m from the Manson Impact Structure, a probable K-T boundary structure located in north-central Iowa. Cores were recovered from each of the major structural terranes, with two cores (M-3 and M-4) from the Terrace Terrane, four cores (M-2, M-2A, M-6, and M-9) from the Crater Moat, and six cores (M-1, M-5, M-7, M-8, M-10, and M-11) from the Central Peak. These supplemented two central peak cores (1-A and 2-A) drilled in 1953. The cores penetrated five major impact lithologies including (1) sedimentary clast breccia (SCB), a polymictic breccia interpreted as a postimpact debris flow, dominated by clasts of Cretaceous marine shale, lesser numbers of Paleozoic carbonate, sandstone, and shale clasts, minor Proterozoic red clastic clasts, and rare clasts of igneous and metamorphic rocks and impact melt rock; these clasts are in a medium gray, calcareous, sandy, silty, shale matrix; (2) impact ejecta, an inverted sequence of Proterozoic red clastics and Paleozoic carbonates apparently emplaced as an overturned ejecta flap; (3) central peak crystalline rocks (CPC), Proterozoic gneisses and granites uplifted from the crystalline basement to form the Central Peak; (4) crystalline clast breccia with a sandy matrix (CCB-S), a polymictic breccia composed of clasts of Proterozoic gneisses and granites displaying abundant planar deformation features (PDFs) and other impact deformation features in a matrix of sand- to silt-sized crystalline rock fragments and mineral grains from disaggregated crystalline rock; and (5) crystalline clast breccia with a melt matrix (CCB-M), a rock composed of clasts similar to the sandy matrix breccia in a dominantly isotropic matrix that displays regions of flow banding, interpreted as originally melt in some areas, subsequently crystallized to varying degrees. This unit is the impact melt layer. Roddy D. J.* Shoemaker E. M. Anderson R. R. The Manson Impact Crater: Estimation of the Energy of Formation, Possible Size of the Impacting Asteroid or Comet, and Ejecta Volume and Mass A new research program dedicated to the study of the Manson impact event has helped improve definition of this eroded crater and its central peak(s), terraced rim, internal deposits, and dimensions (see other Manson papers this conference volume). The dimensional data, especially the average crater dlameter, are of particular interest in that this information is essential to the estimation of the kinetic energy of crater formation and the size of the projectile. These data in turn are important to studies of regional and global effects of the Manson impact event and to mass balance studies of ejecta in the K/T boundary in western North America. Keiswetter D. A.* Black R. Steeples D. W. Anderson R. R. High-Resolution Seismic Reflection Survey at the Manson Crater, Iowa Approximately 17.4 km of high-resolution reflection data were acquired along an east-west radius of the Manson Impact Structure (MIS) to delineate the shallow (upper 300 m) subsurface structural configuration. The geometry of the shallow structure is poorly known due to a 30-90 m thick Pleistocene till cover. The resolution of the new seismic data is roughly 5-10 times that of existing Vibroseis data. Data quality varies rapidly along the line from exceptional to poor, due primarily to velocity variations associated with the geological complexity of the area. Preliminary results indicate subsurface structural blocks previously envisioned to be several hundreds of meters in size are actually an order of magnitude smaller and more complex. A seismogram-by-seismogram analysis is necessary to confidently identify intricate stratigraphic and structural relationships seen on preliminary CDP sections, as numerous faults, diffractions, and complicated reflection patterns create potential pitfalls. Plescia J. B.* Gravity Investigation of the Manson Impact Structure, Iowa The Manson crater, of probable Cretaceous/Tertiary age, is located in northwestern Iowa (center at 42 degrees 34.44' N; 94 degrees 33.60' W), a general review of the data available on the structure is presented in J. B. Hartung et al. A seismic reflection profile along an east west line across the crater and drill hole data indicate a crater about 35 km in diameter having the classic form for an impact crater, an uplifted central peak composed of uplifted Proterozoic crystalline bedrock, surrounded by a "moat" filled with impact produced breccia and a ring graben zone composed of tilted fault blocks of the Proterozoic and Paleozoic country rocks. The structure has been significantly eroded. This geologic structure would be expected to produce a significant gravity signature and study of that signature would shed additional light on the details of the crater structure. A gravity study was undertaken to better resolve the crustal structure. Steiner M. B.* Shoemaker E. M. Two-Polarity Magnetization in the Manson Impact Breccia Preliminary paleomagnetic study of the impact breccia matrix and clasts has produced surprising results: nearly antipodal normal and reversed polarity magnetic vectors are observed in different portions of the core. Near-antipodal magnetizations within a segment of matrix, and within individual samples rule out core inversion as the explanation of the dual polarity. In both the dense and the sandy matrix breccias, the magnetizations of clasts and matrix within the same core segment are identical; this negative "conglomerate test" indicates that magnetization originated after impact. Zeitler P. K.* Kunk M. J. Age and Thermochronology of K-Feldspars from the Manson Impact Structure As a contribution to the effort to obtain a precise age for the Manson Impact Structure, we are approaching the problem from a thermochronological perspective, with the goal of extracting an age from 40Ar/39Ar age-spectrum analysis of partially overprinted K-feldspars taken from granitoid clasts. We find that shocked feldspars from Manson generally show a strong overprint in their age spectra, with more than 50% of each spectrum being reset. The reset portions of the age spectra correspond to gas lost from very small diffusion domains, and a characteristic of the Manson samples is the very large range in apparent diffusion dimensions that they display, with the smallest domains being some 400 times smaller than the largest domains. It is also noteworthy that the small domains comprise a substantial portion of the volume of the feldspars (50% or more). These observations are consistent with the extreme shock experienced by these samples. In detail, the spectra we have measured to date are saddle-shaped and show minimum ages of between 67 and 72 Ma, which we interpret to be maximum estimates for the age of the impact. In the case of one sample (M1-678.3; K-feldspar from a large syenite block located well below the apparent melt-matrix breccia in the M1 borehole), isotope-correlation analysis suggests the presence of a nonatmospheric trapped Ar component (40Ar/36Ar of 660 +/- 40), and an age of about 65.3 +/- 0.5 Ma (2 sigma). Our interpretation of our results is that the shock of impact greatly reduced the diffusion-domain sizes of our samples, making them susceptible to significant Ar loss during heating associated with impact. It appears that while our feldspars were partially open to Ar loss, they equilibrated with a nonatmospheric Ar component, probably related to impact-related degassing of old basement around the impact site. Kunk M. J.* Snee L. W. French B. M. Harlan S. S. McGee J. J. Preliminary 40Ar/39Ar Age Spectrum and Laser Probe Dating of the M1 core of the Manson Impact Structure, Iowa: A K-T Boundary Crater Candidate A preliminary 40Ar/39Ar age spectrum and laser probe dating results from new drill core from the 35-km-diameter Manson Impact Structure (MIS), Iowa, indicate a reasonable possibility that the MIS is a Cretaceous-Tertiary (K-T) boundary impact event. Several different types of samples from a melt-matrix breccia, a unit of apparent crater fill intersected by the M1 core, have been analyzed. 40Ar/39Ar results from these samples indicate a maximum age for the MIS of about 65.4 +- 0.4(2 sigma) Ma. Petrographic analyses of the samples indicate a high probability that all the dated samples from the melt-matrix breccia contain relict grains that were not entirely melted or degassed at the time of impact, suggesting that the actual age of the MIS could be somewhat younger than our preliminary results indicate. The results are consistent with a previously published age estimate of shocked microcline from the MIS central uplift of 65.7 +- 1.0 Ma. Izett G. A.* Reynolds R. L. Rosenbaum J. G. Nishi J. M. A Discontinuous Melt Sheet in the Manson Impact Structure Petrologic studies of core recovered from holes drilled in the Manson, Iowa, buried impact structure may unravel the thermal history of the crater-fill debris. We made a cursory examination of about 200 m of core recovered from the M-1 bore hole, which was the first of 12 holes drilled as part of a cooperative drilling program between the U.S. Geological Survey and the Iowa Geological Survey Bureau. The M-1 core hole is about 6 km northeast of the center of the impact structure, apparently on the flank of its central peak. We developed a working hypothesis that a 30-m-thick breccia unit within a 53- m-thick unit previously termed the "crystalline clast breccia with glassy matrix" is part of a discontinuous melt sheet in the crater-fill impact debris. The 30-m-thick breccia unit reached temperatures sufficient to partially melt some small breccia clasts and convert the fine-grained breccia matrix into a silicate melt that cooled to a greenish-black, flinty, microcrystalline rock. Bell M. S.* Reagan M. K. Anderson R. R. Foster C. T. Jr. Petrography and Preliminary Interpretations of the Crystalline Breccias from the Manson M-1 Core The M-1 core was drilled on the eastern edge of the central uplift within the Manson Impact Structure in Iowa. The lower 107.9 m (106.4 to 214.3 m below ground surface) of the core consists of crystalline breccias. Twelve intervals of thin sections from this core have been studied for preliminary discussion. The breccias are divided into three units by matrix size and abundance. Unit 1 is characterized by a high volume fraction of matrix (ave. 0.54), and a decreasing proportion of matrix with depth. This matrix (106.4 to 147 m) is nearly isotropic and consists of grains < 0.005 to < 0.02 mm in length. The matrix between 112 and 146 meters depth consists of a crystalline intergrowth of felsic and opaque minerals with or without chlorite. This was the hottest section of the core after impact, and may have undergone high temperature metamorphic recrystallization. Unit 2 is transitional between units 1 and 3 (147 - 161 m), and is delineated by a rapid increase in grain size to.01-.04 mm and a decrease in matrix abundance to 10%. Unit 3 (161 to 214.3) has a coarse, often porous matrix, whose abundance changes from about 10% at the top to about 2% at the base. Grain sizes range from 0.01-0.1 mm over this interval and coarsen with depth. Changes in the character of the matrix as well as the changes in clast lithology and abundance outlined below suggest that unit 3 is in-situ brecciated basement with injected melt and shale fragments; unit 1 is a crater veneer deposit consisting of transported basement materials and unit 2 is a mixed zone between units 1 and 3. Short N. M.* Gold D. P. Petrography of Shock Features in the 1953 Manson 2-A Drill Core Drilling of Nx core in late 1953 into an anomalous zone of disturbed rocks northwest of Manson, Iowa, disclosed the presence of extensive breccias, including crystalline rocks brought to the surface from depths of 4 km or more. Hole 2-A penetrated breccias dominated by leucocratic igneous and metamorphic lithologies, later interpreted to be part of a general ringed peak complex within a 35-km-wide impact structure produced about 65 Ma ago. Proof of this origin was given in 1966 by NMS through recognition of shock metamorphic features in 2-A materials during a cursory examination of samples provided by R. A. Hoppin, University of Iowa. A detailed study of this material now underway has revealed that most breccia clasts in 2-A show abundant and varied evidence of shock damage, including extensive planar deformation features (PDF) in quartz, K-feldspar, plagioclase, and a pyroxene and varying degrees of isotropization and incipient melting in feldspars. Crossey L. J.* McCarville P. Post-Impact Alteration of the Manson Impact Structure Core materials from the Manson impact site (Manson, Iowa; Fig. 1) are examined in order to evaluate postimpact alteration processes. Diagenetic interpretation of postimpact events is based on petrologic, mineralogic, and geochemical investigation of core materials; including target strata, disturbed and disrupted strata, ejecta, breccias, microbreccias, and impact melt. The diagenetic study utilizes research cores obtained by the continental scientific drilling project (CSDP) at the Manson structure, as well as core and cuttings of related materials (Alvina Luebke core, Manson 1-A, Manson 2-A, Amoco Eisheid #1 Deep Petroleum Test Well). Samples include impactites (breccias, microbreccias, and melt material), crater fill material (sedimentary clast breccias), disturbed and disrupted target rocks, and reference target material (Amoco Eisheid #1 materials). The study of multiple cores will permit development of a regional picture of post-impact thermal history. The specific objectives are: (1) provide a detailed description of authigenic and alteration mineralogy from diverse lithologies encountered in research drill cores at the Manson impact structure and (2) identify and relate significant postimpact mineral alteration to postimpact thermal regime (extent and duration). Results will provide mineralogical and geochemical constraints on models for postimpact processes (including infilling of the crater depression; cooling and hydrothermal alteration of melt rocks; and subsequent long- term, low-temperature alteration of target rocks, breccias, and melt rocks). Preliminary petrologic and X-ray diffraction examination of fracture linings and void fillings from research core M1 indicate the presence of quartz, chlorite, mixed-layer clays, gypsum/anhydrite, calcite, and Koeberl C.* Anderson R. R. Hartung J. B. Reimold W. U. Manson Impact Structure, Iowa: First Geochemical Results from Drill Core M-1 The Manson Impact Structure is a large complex impact crater centered ca. 5 km north of the town of Manson, Iowa. It is the largest intact impact structure recognized in the United States (35 km in diameter). Its 40Ar/39Ar age is indistinguishable from that of the Cretaceous- Tertiary (K-T) boundary. The Manson structure may be one element of the events at the K-T boundary. The crater is completely covered by Quaternary glacial sedimentary deposits that are normally underlain by Cretaceous clastic sediments and flat-lying carbonate sediments of Phanerozoic age, as well as Proterozoic red clastic, metamorphic, volcanic, and plutonic rock sequences. The study of a reflection seismic profile, provided by Amoco, was critical in interpreting the structure. In the 35-km-diameter zone that marks the extension of the crater, the normal rock sequence is disturbed due to the impact, and at the center of the structure granitic basement rocks are present that have been uplifted from about 4 km depth. Blum J. D.* Chamberlain C. P. Hingston M. J. Koeberl C. Strontium and Oxygen Isotope Study of M-1, M-3 and M-4 Drill Core Samples from the Manson Impact Structure, Iowa: Comparison with Haitian K-T Impact Glasses Strontium and oxygen isotope analyses were performed on eight samples from the M-1, M-3, and M-4 cores recently drilled at the Manson impact structure. The samples were three clastic sedimentary rocks (of probable Cretaceous age) that occurred as clasts within the sedimentary clast breccia, two samples of crystalline rock breccia matrix, and three samples of dolomite and limestone. The 87Sr/86Sr (corrected to 65 Ma) ratios were much higher than those in impact glasses from the Haitian Cretaceous-Tertiary (K-T) boundary. Isotope mixing calculations demonstrate that neither the silicate or carbonate rocks analysed from the Manson crater, or mixtures of these rocks, are appropriate source materials for the Haitian impact glasses. However, the 87Sr/86Sr(65Ma) ratio and delta 18O value of the Ca-rich Haitian glasses are well reproduced by mixtures of Si-rich Haitian glass with platform carbonate of K-T age. Tuesday, March 16, 1993 Primitive Achondrites 8:30 - 12:00 a.m. Room A Chair(s): D. Bogard O. Eugster McCoy T. J.* Keil K. Clayton R. N. Mayeda T. K. Classificational Parameters for Acapulcoites and Lodranites: The Cases of FRO 90011, EET 84302 and ALH A81187/84190 Acapulcoites and lodranites probably sample a common parent body, which has experienced a range of partial melting. We present classificational parameters that allow acapulcoites-lodranites to be distinguished from other groups of meteorites, as well as from each other. Petrography can complement oxygen isotopic compositions in separating these meteorites from other groups of stony-irons and primitive achondrites, while petrographic properties alone distinguish acapulcoites from lodranites. Acapulcoites differ from lodranites in having smaller grain sizes, abundant Fe,Ni-FeS as micrometer-sized veins and plagioclase that escaped melting. We have applied these criteria to three new members of the group. FRO 9001l is a typical lodranite; EET 84302 is intermediate in many properties between acapulcoites and lodranites; and ALH A81187/84190 are paired meteorites and are the first low-FeO acapulcoites. These meteorites provide a wider spectrum of samples from the acapulcoite-lodranite parent body and suggests that this body may have had a complex structure. Davis A. M.* Prinz M. Weisberg M. K. Trace Element Distributions in Primitive Achondrites The primitive achondrites have approximately chondritic bulk chemical composition but achondritic textures. Clayton et al. [1992] showed that nine of these meteorites, the acapulcoites and the lodranites, have similar oxygen isotopic compositions. The acapulcoites appear to be highly metamorphosed, but undifferentiated, meteorites of chondritic composition, whereas the lodranites appear to have lost a feldspathic partial melt. In order to learn more about metamorphic processes and partial melt removal, we have measured the trace- element compositions of constituent phases of a number of primitive achondrites by ion microprobe. We have analyzed two acapulcoites, Acapulco and ALH81261 (paired with ALH77081), and three lodranites, Lodran, LEW88280, and MAC88177. In addition, we analyzed LEW88663, which has the bulk composition, mineral chemistry, and oxygen isotopic composition of L-chondrites, but is metal-free and has an achondritic texture; and Divnoe, a plagioclase-poor, olivine-rich primitive achondrite with an oxygen isotopic composition similar to that of the group IAB iron meteorites. These meteorites show a variety of REE patterns in their constituent phases, and there are systematic differences between acapulcoites and lodranites that are consistent with removal of a LREE- and Eu-enriched melt that is apparently responsible for the low plagioclase content of lodranites. Bogard D. D.* Garrison D. H. McCoy T. J. Keil K. 39Ar-40Ar Ages of Acapulcoites and Lodranites: Evidence for Early Parent Body Heating New age dating of acapulcoites (Acapulco, Monument Draw) and lodranites (Gibson) allow us to reconstruct the history of their parent body in a chronological framework. These meteorites originated on a common parent body ~4.55 Ga ago. Non-collisional heating caused partial melting, with lodranites having been heated slightly higher than acapulcoites. This parent body cooled to the Ar closure temperature at ~4.51 Ga for acapulcoites and ~4.48 Ga for lodranites. The difference in ages is consistent with the higher temperature and longer cooling time for lodranites. Cooling probably occurred at a few to tens of degrees C/Ma, with slower cooling rates at lower temperatures. Kim Y.* Marti K. Isotopic Signatures and Distribution of Nitrogen, and Trapped and Radiogenic Xenon in the Acapulco and FRO90011 Meteorites Acapulco metal and silicate show distinct N isotopic signatures. Trapped heavy noble gases are carried by "magnetic" opx, and radiogenic 129Xe excesses are observed in phosphate and in minor surficial phases on metal grains. N and Xe isotopic signatures in FRO90011 do not agree with those observed in Acapulco. Eugster O.* Weigel A. Xe-Q in Lodranites and a Hint for Xe-L. FRO90011 Another Lodranite? The Lodran achondrite contains about one-quarter metallic Fe/Ni, two-thirds olivine and pyroxene, some troilite, plus minor phases. In a previous study we demonstrated that Lodran and three other lodranites--LEW88280, Yamato 791491, and MAC88177--yield the same cosmic-ray exposure age of a few million years, suggesting that they originate from the same parent body. In the present work we show that the mineral phases of Lodran contain large concentrations of planetary-type but no solar-type trapped noble gases. Surprisingly, the highest concentrations were observed in the Fe/Ni phase (e.g., 1520 x 10^-12 cm^3 STP/g 132Xe). A large fraction of the trapped gas is released between 1200 degrees C and 1400 degrees C. The Xe isotopic pattern is similar to that of XeQ. The 1400 degree C fraction of the FP/Ni phase shows excesses of 124Xe, 126Xe, and 128Xe similar to Xe-L (presolar Xe enriched in the light isotopes) that has, until now, only been observed in combination with Xe-H (presolar Xe enriched in the heavy isotopes). Boynton W. V.* Hill D. H. Trace-Element Abundances in Several New Ureilites Four new ureilites are analyzed for trace-element abundances. Frontier Mountain (FRO) 90054 is an augite-rich ureilite and has high REE abundances with a pattern expected of augite. FRO 90036 and Acfer 277 have REE patterns similar to the V-shape pattern of other ureilites. Nuevo Mercurio (b) has very high REE abundances, but they look like they are due to terrestrial alteration. The siderophile-element pattern of these ureilites are similar to those of known ureilites. Goodrich C. A.* Lugmair G. W. Stalking the LREE-enriched Component in Ureilites Ureilites contain a LREE-enriched component whose origin and identity are unknown, but which may have been introduced into 4.55 Ga olivine + pyroxene assemblages at various times in the period 4.55->3.74 Ga. This component is volumetrically minor, inhomogeneously distributed, and can be removed with concentrated HNO3, which suggests that it may be contained in minor, interstitial phases not previously recognized in ureilites. There is evidence, however, that not all of this component is leachable. Whole rock samples of Kenna, Novo Urei, and ALHA77257 form a ^143Nd/^144Nd-^147Sm/^144Nd line with a slope corresponding to an age of 3.74 Ga. Although this line could be a mixing line, with one end-member being the LREE-enriched component and the other being a 4.55 Ga olivine + pyroxene assemblage, the observation that a pyroxene separate from Kenna plots on the line suggests that the LREE-enriched component at least partially equilibrated with the olivine and pyroxene and that the line is an isochron. We performed a leaching experiment on Kenna, using various acids and concentrations, in an attempt to isolate the LREE- enriched component and differentiate between possible hosts for it, and to determine whether all of it is leachable. Our results suggest that most of the LREE-enriched component is surface-sited, rather than contained in a discrete mineral. However, some of it is not readily leachable because it equilibrated with Kenna olivine + pyroxene at 3.79+-0.05 Ga. Russell S. S.* Arden J. W. Franchi I. A. Pillinger C. T. A Carbon and Nitrogen Isotope Study of Carbonaceous Vein Material in Ureilite Meteorites The ureilite meteorite group is known to be rich in carbon in the form of graphite/diamond veins that are associated with planetary type noble gases. This paper reports preliminary data from a systematic study of the carbon and nitrogen isotopic composition of this carbonaceous vein material. A previous study focused on the whole-rock signatures and reported that the carbon inventory appeared to be dominated by the graphitic/diamond intergrowths, whereas the nitrogen was clearly composed of several distinct components, including one that that was isotopically light, possibly associated with the carbonaceous material. Recent studies have demonstrated that diamonds in the solar system formed in many different environments. C and N measurements from ureilitic diamond made in a similar way would be a useful addition to this overall study. The methods used for isolating diamonds of possible presolar origin from primitive meteorites are equally applicable to the processing of carbon-bearing components in the ureilite meteorite group so that their stable isotopic composition can be determined. Casanova I.* McCoy T. J. Keil K. Metal-rich Meteorites from the Aubrite Parent Body We studied three metal-rich meteorites-Mt. Egerton, Horse Creek, and LEW 88055-and suggest that they formed in the aubrite parent body. LEW 85369 and 88631 may also have a common origin, but these rocks have not yet been studied in detail. This body was probably heated to about 1600 C by a very strong heat source. While molten, metal agglomerated into sizeable nodules that never segregated efficiently to form a core, but were trapped in the silicate mantle. Different clasts and lithologies in aubrites solidified and cooled under local equilibrium conditions of oxygen fugacity, and with different thermal histories. Impacts mixed clasts from throughout the parent body, creating the typical aubrite breccias. Takeda H.* Saiki K. Otsuki M. Hiroi T. A New Antarctic Meteorite With Chromite, Orthopyroxene and Metal With Reference to a Formation Model of S Asteroids Mineralogy of a unique meteorite from Antarctica, EET84302, and the related primitive achondrites has been studied to gain better understanding on the relationship between S asteroids and this meteorite group. EET84302 consists of a metal-rich (M) region with metal filling interstices of silicate grains of orthopyroxene (Opx), olivine Fa8, plagioclase An24, and augite (Aug), a chromite-rich (C) region with major chromite and Opx, and a silicate-rich (S) region with mainly Opx. This is the first example of chromite-rich achondrite. Distribution of the three areas (C, M, S) are in line with a proposed model of primitive achondrites and S asteroids. Petaev M. I.* Opaque-rich Lithology in the Divnoe Achondrite: Petrology and Origin An opaque-rich lithology (ORL) makes up to ~9 vol% of the Divnoe achondrite. It is characterized by enrichment of troilite and pyroxene compared to bulk Divnoe, the presence of tiny remnants of olivine in low-Ca pyroxene and low minor-element concentrations in pyroxene. The ORL was formed by local reaction between gaseous sulfur and olivine in the Divnoe. Tuesday, March 16, 1993 Venus Resurfacing and Tectonics 8:30 - 12:00 a.m. Room B Chair(s): R. R. Herrick E. M. Parmentier Thompson T. W.* Magellan Flight Team Magellan Mission Progress Report The Magellan spacecraft was launched from Cape Kennedy on May 4, 1989, and was inserted into orbit around Venus on August 10, 1990. The Magellan spacecraft carries a radar instrument that makes synthetic aperture radar (SAR) images of the surface, measures the altitude of the venusian surface directly below the spacecraft, and obtains radiometric observations of the surface. Radar and radiometric observations of the venusian surface commenced on September 15, 1990, and continued until September 15, 1992. Gravity observations began on September 24, 1992, and will continue until late May 1993. The radar observations have produced SAR images and surface topography for 99% of the surface. These radar observations support the objective of improving the knowledge of the geological history of Venus by analysis of surface morphology and the processes that control them. The gravity observations that are being conducted now support the Magellan objective of improving the knowledge of the geophysics of Venus, principally its density distribution and dynamics. Also, Magellan has generated more digital planetary image data than all previous planetary missions. Simpson R. A.* Tyler G. L. Maurer M. J. Holmann E. Wong P. B. Scattering by Venus' Surface During 1992 Magellan altimetry echoes were processed to yield scattering functions sigma(sub)0(phi) over near-nadir angles phi <= 10 degrees from some 2000 Venus orbits. SAR image strips acquired simultaneously at oblique angles were also reduced to give scattering functions but over a few degrees of incidence angle in the range 15 <= phi <= 45 degrees. By sorting, the altimetry and SAR results can be combined to give composite scattering functions for much of Venus' surface. The data in hand should allow definition of such composite functions for 70-80% of Venus at resolutions of 20 km-- approximately one million separate functions. Izenberg N. R.* Arvidson R. E. Phillips R. J. A First-Order Model for Impact Crater Degradation on Venus A first-order impact crater aging model is presented based on observations of the global crater population of Venus. The total population consists of 879 craters found over the ~98% of the planet that has been mapped by the Magellan spacecraft during the first three cycles of its mission. The model is based upon three primary aspects of venusian impact craters: 1) extended ejecta deposits (EEDs); 2) crater rims and continuous ejecta deposits; and 3) crater interiors and floors. Strom R. G.* Parabolic Features and the Erosion Rate on Venus The impact cratering record on Venus consists of 919 craters covering 98% of the surface. These craters are remarkably well preserved, and most show pristine structures including fresh ejecta blankets. Only 35 craters (3.8%) have had their ejecta blankets embayed by lava and most of these occur in the Atla-Beta Regio region; an area thought to be recently active. Parabolic features are associated with 66 of the 919 craters. These craters range in size from 6 to 105 km diameter. The parabolic features are thought to be the result of the deposition of fine-grained ejecta by winds in the dense venusian atmosphere. The deposits cover about 9% of the surface and none appear to be embayed by younger volcanic materials. However, there appears to be a paucity of these deposits in the Atla-Beta Regio region, and this may be due to the more recent volcanism in this area of Venus. Since parabolic features are probably finegrain, wind-deposited ejecta, then all impact craters on Venus probably had these deposits at some time in the past. The older deposits have probably been either eroded or buried by eolian processes. Therefore, the present population of these features is probably associated with the most recent impact craters on the planet. Furthermore, the size/frequency distribution of craters with parabolic features is virtually identical to that of the total crater population. This suggests that there has been little loss of small parabolic features compared to large ones, otherwise there should be a significant and systematic paucity of craters with parabolic features with decreasing size compared to the total crater population. Whatever is erasing the parabolic features apparently does so uniformly regardless of the areal extent of the deposit. The lifetime of parabolic features and the eolian erosion rate on Venus can be estimated from the average age of the surface and the present population of parabolic features. Kaula W. M.* Implications of Crater Distributions on Venus The horizontal locations of craters on Venus are consistent with randomness. However, (1) randomness does not make crater counts useless for age indications; (2) consistency does not imply necessity or optimality; and (3) horizontal location is not the only reference frame against which to test models. Re (1), the apparent smallness of resurfacing areas means that a region on the order of 1% of the planet with a typical number of craters, 5- 15, will have a range of feature ages of several 100 My. Re (2), models of resurfacing somewhat similar to Earth's can be found that are also consistent and more optimal than random, i.e., resurfacing occurring in clusters that arise and die away in time intervals on the order of 50 My. These agree with the observation that there are more areas of high crater density, and fewer of moderate density, than optimal for random. Re (3), 799 crater elevations were tested; there are more at low elevations and fewer at high elevations than optimal for random, i.e., 54.6% below the median. Only one of 40 random sets of 799 was as extreme. Malin M. C. Grimm R. E.* Herrick R. R. Tectonic Resurfacing of Venus Magellan observations of impact crater distribution and morphology on Venus have yielded two key observations: craters are randomly distributed around the planet and most are unmodified. Global geologic mapping may be used to test alternative hypotheses for crater obliteration and resurfacing. For now, we present one preliminary result using an automated mapping procedure. We find that faulted terrains correspond roughly to RMS slopes of 2.5 degrees and greater. Such terrains cover nearly 60% of the planet but contain only 40% of craters with D > 30 km. There are fewer craters in the tectonized zones because they are being destroyed. The distribution of crater preservation states supports this conclusion. Therefore craters are clearly not randomly distributed with respect to tectonized vs. untectonized terrain, in spite of a random spatial distribution. This implies that resurfacing is widespread, recurrent, and operating at a variety of scales. These requirements are in good agreement with the emerging picture of global tectonics on Venus. Direct coupling to mantle convection results in pervasively distributed deformation, but in patterns coherent over length scales of several hundred kilometers-- precisely the size of the necessary resurfacing "patch." Reorganization of mantle convective patterns can assure spatial and temporal variability. Tectonic resurfacing is therefore a natural consequence of standard geophysical models and is geologically straightforward. Solomon S. C.* A Tectonic Resurfacing Model for Venus Two remarkable aspects of the population of impact craters on Venus are that craters of all sizes are indistinguishable from a random population and that the vast majority of craters have not been significantly modified by tectonic strain or by volcanic flows external to the crater rim, despite evidence from Magellan images that volcanic and tectonic features are widespread on Venus. One interpretation of these observations is that most of the surface dates from the end of a catastrophic global resurfacing event that ceased about 500 My ago, and that the small fraction of craters volcanically embayed or modified by deformation indicates that volcanic and tectonic activity subsequent to that time has been at much lower levels. An alternative model, in which resurfacing occurs episodically in patches a few hundred kilometers in extent and there is a wider spectrum of surface ages, also appears to be consistent with the characteristics of impact craters on Venus. A number of potential mechanisms for catastrophic resurfacing of Venus have been proposed, ranging from geologically sudden convective destabilization of the global lithosphere to strongly time-dependent heat flux and melt generation in the underlying mantle. In most of these geophysical models, resurfacing occurs implicitly or explicitly by volcanism. We explore here the hypothesis that, at least in the geologically recent history of Venus, the primary resurfacing mechanism has been tectonic deformation rather than volcanism. We show how such a hypothesis provides at least as good an explanation of a wide range of observations as do volcanic resurfacing models. Finally, we explore the implications of the tectonic resurfacing hypothesis for the controversy over the recent resurfacing history of the planet. Solomatov V. S.* Stevenson D. J. Differentiation of Magma Oceans and the Thickness of the Depleted Layer on Venus Various arguments suggest that Venus probably has no asthenosphere and it is likely that beneath the crust there is a highly depleted and highly viscous mantle layer that was probably formed in the early history of the planet when it was partially or completely molten [Kaula, 1990; Head et al., 1992; Parmentier and Hess, 1992]. Models of crystallization of magma oceans suggest that just after crystallization of a hypothetical magma ocean, the internal structure of Venus consists of a crust up to about 70 km thick, a depleted layer up to about 500 km, and an enriched lower layer that probably consists of an undepleted "lower mantle" and heavy enriched accumulates near the core- mantle boundary. Parmentier E. M.* Hess P. C. Sotin C. Mixing of a Chemically Buoyant Layer at the Top of a Thermally Convecting Fluid: Implications for Mantle Dynamics with Application to Venus Partial melting to generate the crust of a planet can create a buoyant residual layer at the top of the mantle that may have important implications for episodic planetary evolution. However, the rate of mixing of such a chemically buoyant layer with a thermally convecting mantle is an important unresolved question. Except for a few laboratory and numerical studies designed to address questions related to convection in the Earth's mantle, previous studies have generally treated only the mixing of passive tracers. The inhibiting role of chemical buoyancy on mixing is intuitively obvious but not fully understood quantitatively. In this study we examine the dynamics of an intrinsically buoyant fluid layer at the top of a deeper, thermally convecting, infinite Prandtl number fluid that is heated from below. Gilmore M. S.* Head J. W. III The Formation and Evolution of Alpha and Tellus Tesserae on Venus Although tesserae comprise approximately 10% of the surface of Venus, the nature of their formation and evolution is not well understood. One important clue to elucidate this problem is tessera boundaries, which are of two types: Type 1 are generally embayed by plains; Type II boundaries are characterized by being linear at the 100-km scale and often associated with steep scarps or tectonic features. Previous study of a distinctive Type II boundary at Alpha Regio was pursued in order to characterize and assess the implications for this style in general. A model of gravitational relaxation of the tessera block was presented to explain the specific style of Type II feature seen at western Alpha; however, this model cannot account for the full range of interior structures of Alpha, namely widespread compressional features overprinted by extensional features and volcanism. In order to explain the features of Alpha we have considered a three-stage model involving (1) mantle downwelling and compression of the lithosphere, (2) delamination of the thickened tessera root, and (3) gravitational relaxation and extension of the tessera plateau. We compare this to the characteristics of Tellus Regio in order to test its broader application. Raitala J.* Chocolate Tablet Aspects of Cytherean Meshkenet Tessera Meshkenet Tessera structures were mapped from Magellan data and several resemblances to chocolate tablet boudinage were found. The complex fault sets display polyphase tectonic sequences of a few main deformation phases. Shear and tension have contributed to the areal deformation. Main faults cut the 1600-km-long Meshkenet Tessera highland into barlike blocks that have ridge and groove patterns oriented along or at high angles to the faults. The first approach to the surface block deformation is an assumption of initial parallel shear faulting followed by a chocolate tablet boudinage. Major faults (a), which cut Meshkenet Tessera into rectangular blocks, have been active repetitively, while two progressive or superposed boudinage set formations (b, c) have taken place at high angles during the relaxational or flattening type deformation of the area. Tuesday, March 16, 1993 Chicxulub, KT Boundary, and Other Impact Ejecta 8:30 - 12:00 a.m. Room C Chair(s): B. Schuraytz B. Bohor Hildebrand A. R.* Gregoire D. C. Attrep M. Jr. Claeys P. Thompson C. M. Boynton W. V. Trace-Element Composition of Chicxulub Crater Melt Rock, K/T Tektites and Yucatan Basement The Cretaceous/Tertiary (K/T) boundary Chicxulub impact is the best preserved large impact in the geologic record. The Chicxulub crater has been buried with no apparent erosion of its intracrater deposits and its ejecta blanket is known and is well preserved at hundreds of localities globally. Although most of the molten material ejected from the crater has been largely altered, a few localities still preserve tektite glass. Availability of intra- and extracrater impact products as well as plausible matches to the targeted rocks allows the comparison of compositions of the different classes of impact products to those of the impacted lithologies. Urrutia-Fucugauchi J.* Marin L. E. Sharpton V. L. Quezada J. M. Reverse Polarity Magnetized Melt Rocks from the Chicxulub Impact Structure, Yucatan Peninsula, Mexico We report further paleomagnetic data for core samples of melt rock recovered in the Petroleos Mexicanos (PEMEX) exploratory wells within the Chicxulub structure, northern Yucatan Peninsula, Mexico. A previous report by Sharpton et al. (1992) has shown that the rocks studied contain high iridium levels and shocked breccia clasts, and an 40Ar-39Ar age of 65.2 +- 0.4 Ma. The geomagnetic polarity determined for two samples is reverse (R) and was correlated with chron 29R that includes the K/T boundary. Our present analysis is based on two samples from each of three clasts of the melt rock from PEMEX well Y6-N17 (1295 to 1299 m.b.s.l.). This study concentrates on the vectorial nature and stability of the remanence (NRM), the magnetic mineralogy and remanence carriers (i.e., the reliability and origin of the record), and on the implications (correlation with expected paleolatitude and polarity). Sharpton V. L.* Burke K. Hall S. Lee S. Marin L. E. Suarez G. Quezada J. M. Uruttia J. Chicxulub Impact Basin: Gravity Characteristics and Implications for Basin Morphology and Deep Structure Reprocessed Bouguer (onshore) and Free Air (offshore) gravity data over Northern Yucatan reveal at least three concentric rings associated with the KT-aged Chicxulub Impact Basin. The positions of these rings follow the 'root 2' spacing rule derived empirically from analysis of multiring basins on other planets, indicating that these rings probably correspond to now-buried topographic basin rings. A forward model along a radial transect from southeast margin of the structure indicates that the gravity signature of Chicxulub is compatible with this interpretation. We estimate the basin rim diameter to be 204 +- 16 km; the central peak ring diameter is 104 +- 6 km. An intermediate, incipient ring is suggested in the gravity data at a diameter of 150 +- 16 km. These gravity data further indicate that the weak concentric expression in the northwest quadrant may be caused by the interference of a linear gravity high extending roughly north-south through the basin. We suspect this feature is part of the pre-impact deep structural fabric of Yucatan, either developed during the opening of the Gulf of Mexico, or inherited from previous geological activity. The prerift position of Yucatan against the Florida Escarpment and south of the Suwannee-Wiggins Suture Zone indicates that the basement under Chicxulub is (~500 Ma) Pan-African Terrane. Consequently, it seems that the recently reported 550 Ma U-Pb ages of zircons from the upper member of the KT boundary layer in the Western U.S. implicate Chicxulub rather than Manson in the formation of this layer. Carey S.* Sigurdsson H. D'Hondt S. Espindola J. M. Stratigraphy and Sedimentology of the K/T Boundary Deposit in Haiti The K/T boundary sequence is exposed in uplifted carbonate sediments of the southwest peninsula of Haiti. It is found at 15 localities within the Beloc formation, a sequence of limestone and marls interpreted as a monoclinal nappe structure thrust to the north. This tectonic deformation has affected the K/T boundary deposit to varying degrees. In some cases the less competent K/T deposit has acted as a slip plane leading to extensive shearing of the boundary layer, as well as duplication of the section. The presence of glassy tektites, shocked quartz, and an Ir anomaly directly link the deposit to a bolide impact. Stratigraphic and sedimentological features of the tripartite sequence indicate that it was formed by deposition from ballistic fallout of coarse tektites, emplacement of particle gravity flows, and fine-grained fallout of widely dispersed impact ejecta. Bohor B. F.* Glass B. P. Betterton W. J. K/T Spherules from Haiti and Wyoming: Origin, Diagenesis, and Similarity to Some Microtektites Spherules with relict glass cores in the K/T boundary bed of Haiti allow for a comparison of these bodies with hollow goyazite shells in the K/T boundary claystone of Wyoming and with younger microtektites of the Ivory Coast strewn field. Samples of the Haitian beds from undisturbed sections at Beloc, as determined by Jehanno et al., contain both hollow shells and relict glass cores rimmed by palagonite that has been partially converted to smectite. These palagonite rims developed from hydration zones formed when hot, splash-form droplets of andesitic impact glass were deposited into water. Mutual collisions between these droplets in the ejecta curtain may have formed point-source stresses on their surfaces. Initiation of hydration would be facilitated at these surface stress points and propagated radially into the glass. The inner surface of these merged hemispherical fronts appears mammillary, which is reflected as scalloping in Haitian relict glass cores. Glass B. P.* Bohor B. F. Betterton W. J. Cretaceous-Tertiary Boundary Spherules and Cenozoic Microtektites: Similarities and Differences Bohor and Betterton pointed out that the K-T spherules can be divided into three groups. Their Type 1 spherules appear to be found in or adjacent to North America, particularly the Western Interior and in Haiti and Mexico. The Type 1 spherules occur in the lower part of the K- T boundary clay below an Ir anomaly. It is the Type 1 spherules that are most similar to microtektites. The discovery of K-T boundary spherules in Beloc, Haiti, and Mimbral, Mexico, with residual tektitelike glass cores supports the hypothesis that the Type 1 spherules are diagenetically altered microtektites. How similar are the Type 1 K-T boundary spherules to previously described Cenozoic microtektites and in what ways are they different? Premo W. R.* Izett G. A. U-Pb Provenance Ages of Shocked Zircons from the K-T Boundary, Raton Basin, Colorado U-Pb isotopic systematics from analyses of single zircons identify at least two provenance ages, 575 Ma and 330 Ma, for zircons from the impact layer of the K-T boundary, Raton Basin, Colorado. These data are a preliminary confirmation of results reported from the same layer. The zircon provenance ages provide a unique signature for identification of the source crater since igneous rocks of these ages (or sedimentary rocks derived from them) must characterize part of the impact stratigraphy. Espindola J. M.* Carey S. Sigurdsson H. Modelling of Dispersal and Deposition of Impact Glass Spherules from the Cretaceous-Tertiary Boundary Deposit The dispersal of glass spherules or tektites from a bolide impact with the Earth is modelled as ballistic trajectories in standard atmosphere. Ballistic dispersal of Cretaceous-Tertiary boundary impact glass spherules found in Haiti and Mimbral, Mexico,requires a fireball radius in excess of 50 km but less than 100 km to account for the observed distribution. Bostwick J. A.* Kyte F. T. Impact Mineralogy and Chemistry of the Cretaceous-Tertiary Boundary at DSDP Site 576 We have identified the K/T boundary in pelagic clay sediments from cores at DSDP Site 576 in the western North Pacific (32 degrees 21.4'N, 164 degrees 16.5'E). Detailed geochemical and trace mineralogical analyses of this boundary section are in progress and initial results indicate similarities and differences relative to the only other clay core investigated in detail; DSDP Site 596, a locality in the western South Pacific. Peak Ir concentrations of 13 ng/g in DSDP Hole 576B are virtually identical with those observed in the South Pacific, but in the North Pacific this peak is much narrower and the integrated Ir fluence of ~85 ng cm^-2 is ~4 times lower (320 in Hole 596). Of the 34 elements measured, only Ir and Cr were found to have anomalous concentrations in K/T boundary samples. Chamberlain C. P.* Blum J. D. Koeberl C. Oxygen Isotopes as Tracers of Tektite Source Rocks: An Example from the Ivory Coast Tektites and Lake Bosumtwi Crater Oxygen isotope studies of tektites and impact glasses provide an important tool to help in identifying the target lithologies for terrestrial impacts, including the K-T boundary impact. However, such studies may be complicated by modification of the original oxygen isotope values of some source rocks during the tektite formation process either by vapor fractionation or incorporation of meteoric water. To further investigate the relationship between the oxygen isotopic composition of tektites and their source rocks we have studied Ivory Coast tektites and samples of impact glasses and bedrock lithologies from the Bosumtwi Crater in Ghana-which is widely believed to be the source crater for the Ivory Coast tektites. Our preliminary results suggest that the phyllites and metagraywackes from the Bosumtwi Crater were the predominant source materials for the impact glasses and tektites and that no significant oxygen isotope modification (<=1 permil delta 18O) took place during impact melting. This contrasts with previous studies of moldavites and Australasian tektites and their sedimentary source materials, which suggests a 4-5 permil lowering of delta 18O due to meteoric water incorporation during impact melting. Claeys P.* Casier J.-G. Microtektite-like Glass Spherules in Late Devonian (367 Ma) Shales Glass spherules are found closely associated with the Frasnian-Famennain (FF) boundary in two sections, Senzeille and Hony, located in the Dinant basin in the South of Belgium. These spherules are interpreted based on their chemistry and low water content as impact-produced microtektite- like glass. Craters of Late Devonian ages include the Siljan Ring (Sweden), Charlevoix (Quebec), and Taihu Lake (China). The occurrence of microtektite-like glass closely associated with the FF mass extinction is intriguing. The relationships between the impact and the extinction event deserve further study. Mittlefehldt D. W.* See T. H. Scott E. R. D. Siderophile Element Fractionation in Meteor Crater Impact Glasses and Metallic Spherules Meteor Crater, Arizona provides an opportunity to study, in detail, elemental fractionation processes occurring during impacts through the study of target rocks, meteorite projectile, and several types of impact products. We have performed EMPA and INAA on target rocks, two types of impact glass and metallic spherules from Meteor Crater. Using literature data for the well studied Canyon Diablo iron, we can show that different siderophile element fractionations affected the impact glasses than affected the metallic spherules. The impact glasses primarily lost Au, while the metallic spherules lost Fe relative to other siderophile elements. Tuesday, March 16, 1993 Remote Sensing/Space Weathering 8:30 - 12:00 a.m. Room D Chair(s): B. Clark B. W. Hapke Goguen J. D.* A Test of the Applicability of Independent Scattering to High Albedo Planetary Regoliths We show that "independent scattering" is a useful approximation for high- albedo particles whose size and packing density are similar to typical particles in the lunar regolith. Laboratory measurements of the intensity and linear polarization of light scattered from a laboratory sample of glass spheres of known size and composition are compared to radiative transfer calculations of the same observable quantities. Mie scattering is integrated over the size distribution of the particles to determine the mean phase and polarization functions, or phase matrix, of the particles. Assuming that the particles scatter independently, the "doubling method" is used to RIGOROUSLY calculate multiple scattering in an optically thick layer of these anisotropically scattering particles. All of the major features "predicted" in the calculated intensity (double peaks at small phase angles) and polarization (negative branch at small phase angles, large positive peak near 20 degrees phase, and small polarization elsewhere) are observed in the laboratory measurements, with good quantitative agreement indicated at phase angles <90 degrees. Even though the particles are supported by physical contact with each other in the sample, as are the particles in planetary regoliths, the independent scattering calculation yields a good approximation to both the intensity and polarization. The physical parameters input to the calculation are only the size distribution of the particles and their complex index of refraction (composition). Significant advantages of this approach are that the phase matrix is calculated from basic physical principles and both the intensity and polarization are determined simultaneously. This model may have broad applications to the interpretation of photometry, spectroscopy, and polarimetry of the icy regoliths of high-albedo satellites. Nelson R. M.* Hapke B. W. Smythe W. D. Gharakanian V. Herrera P. The Coherent Backscattering Opposition Effect We have measured the opposition effect, the non-linear surge in reflectance seen in particulate materials as phase angle approaches zero degrees, in a suite of materials of varying particle size and reflectance. These samples were illuminated by linearly and circularly polarized monochromatic radiation at two wavelengths, 0.442 micrometers and 0.633 micrometers. By measuring the linear and circular polarization ratios for each sample we have found that in highly reflective materials the behavior of the reflected radiation is consistent with the coherent backscattering process that has recently been proposed to explain the opposition surge that is seen in such media. The size and width of the coherent backscattering opposition peak vary as a function of reflectance of the sample. Shepard M. K.* Guinness E. A. Arvidson R. E. The Roughness of the Martian Surface: A Scale Dependent Model In the coming decade, several lander missions to Mars are planned (e.g., MESUR Pathfinder, MESUR). One of the dangers facing planners of these missions is the rough topography observed at both Viking Lander sites. Both landing sitesare ubiquitously covered with meter-scale boulders. Objects of this size pose obvious threats to soft landers, especially at Mars where the distance from Earth causes prohibitive time lags between the transmission of commands and feedback from the spacecraft. An obvious solution is to scout for a "smooth" site prior to the landing. However, the best resolutions realizable on current and future missions (i.e., Mars Observer) are on the order of several meters. Even at this scale, boulders of 1-2 meters in size are unresolvable. Additionally, the amount of time and spacecraft resources required to search even a small area of the planet are unrealistic given other mission objectives. An alternative is to determine the "roughness" of the surface at a subpixel scale using bidirectional reflectance observations. Much larger areas of the planet can be searched and much of the search can easily be automated. Brackett R. A.* Arvidson R. E. Compositional and Textural Information from the Dual Inversion of Visible, Near and Thermal Infrared Remotely Sensed Data A technique is presented that allows extraction of compositional and textural information from visible, near, and thermal infrared remotely sensed data. Using a library of both emissivity and reflectance spectra, endmember abundances and endmember thermal inertias are extracted from AVIRIS (Airborne Visible and Infrared Imaging Spectrometer) and TIMS (Thermal Infrared Mapping Spectrometer) data over Lunar Crater Volcanic Field, Nevada, using a dual inversion. The inversion technique is motivated by upcoming Mars Observer data and the need for separation of composition and texture parameters from subpixel mixtures of bedrock and dust. Salisbury J. W. Wald A.* D'Aria D. M. Thermal Infrared Remote Sensing and Kirchhoff's Law: I Laboratory Measurements With the near-term prospect of thermal infrared spectroscopic measurements of martian surface materials by Mars Observer, it has become important to understand the emissivity behavior of materials, especially particulate materials. Perhaps the most fundamental question to be answered is the extent to which such materials follow Kirchhoff's Law under less than ideal conditions. Henderson B. G.* Jakosky B. M. Near-Surface Temperature Gradients and Their Effects on Thermal-Infrared Emission Spectra of Particulate Planetary Surfaces The infrared energy emitted from a planetary surface is generated within a finite depth determined by the material's absorption skin depth. This parameter varies significantly with wavelength in the infrared, but has an average value of around 50 m for most geologic materials. In solid rock, heat transfer is efficient enough so that this 50-m zone of the near surface from which the radiation emanates will be more or less isothermal. In particulate materials, however, heat transfer is more complicated and occurs via a combination of mechanisms, including solid conduction within grains and across grain contacts, conduction through the interstitial gas, and thermal radiation within individual particles and across the void spaces in between grains. On planets with substantial atmospheres, the gas component dominates the heat transfer and tends to mitigate near-surface thermal gradients. However, on airless bodies, the gas component is absent and heat transfer occurs via solid conduction and radiation. If the particles are small relative to the average absorption skin depth, then the top 50-100 m or so of the surface will be cooled by radiation to space allowing the creation of significant near-surface thermal gradients. In those regions of the spectrum where the absorption coefficient is low, the emission will come from the deeper, warmer parts of the medium, whereas in regions of high absorption, the emission will emanate from shallower, cooler parts of the medium. The resulting emission spectrum will show noncompositional features as a result of the thermal structure in the material. Lucey P. G. Domergue-Schmidt N.* Henderson B. G. Jakosky B. First Results from a Laboratory Facility for the Measurement of Emission Spectra Under Simulated Planetary Conditions We have developed a laboratory spectroscopic facility for the measurement of emission spectra under simulated planetary conditions. Spectral measurements are made from 6 to 13 micrometers with a scanning grating monochromator equipped with a HgCdTl detector. An environment chamber in service in Hawaii for several years in which we can control the temperature from 77 K to 500 K, the pressure from 10-5 torr to 2 atm, has been equipped with a 77 K or 273 K cold shield. The shield serves to minimize light reflected off the sample and to aid in development of thermal gradients for obtaining spectra under conditions simulating the thermal environment of airless bodies. Samples are placed in small cups on a temperature-controlled substrate allowing measurements of emission due to heating from below by the substrate, or from illumination from a solar simulation source. Hapke B.* Why is the Moon Dark? This paper reports the results of attempts to model the spectral properties of the lunar regolith as consisting of crystalline rocks and submicroscopic metallic iron and glass, produced by a process involving vapor phase differentiation. The models differ in the location of the iron. Allen C. C.* Morris R. V. Lauer H. V. Jr. McKay D. S. Effects of Microscopic Iron Metal on the Reflectance Spectra of Glass and Minerals Maturity, which is a measure of the time lunar soils have been in the upper ~1 mm of the surface, is correlated with changes in the soils' optical properties. Mature soil is darker and somewhat redder than immature soil of the same composition, and features in the reflectance spectrum become more subdued with increasing maturity. The maturation process also involves accumulation of agglutinitic glass and submicroscopic metallic iron particles. We have produced submicrometer iron metal blebs on the surfaces and within glass and mineral grains by the method of high-temperature reduction. This technique has permitted the determination of optical changes in samples much simpler than lunar soil, and offers a useful tool with which to investigate the maturation process. Pieters C. M.* Fischer E. M. Rode O. D. Basu A. Optical Effects of Space Weathering on Lunar Soils and the Role of the Finest Fraction Lunar soils represent a cumulative product of space weathering and as such they exhibit distinct optical alteration. In comparison to the optical properties of similar rock powders, lunar soils are darker, have weaker diagnostic absorption bands, and exhibit a characteristic red continuum slope between 0.3 and 4.5 micrometers. Well-developed (mature) lunar soils have a mean grain size of about 60 m and may contain more than 50% agglutinates (complex, glass-welded aggregates), which were previously believed to be the principal carriers of optical alteration resulting from space weathering. A detailed analysis of the spectral reflectance properties of the bulk soil, size separates, and agglutinate separates of several Apollo and Luna soils has been undertaken to evaluate the validity of the agglutinate paradigm for optical effects of space weathering. The data and results are summarized here, which indicate that the finest fraction of natural lunar soils, rather than larger complex agglutinates, carries the principal effects of optical alteration. Fanale F. P.* Clark B. E. Chondrites, S Asteroids, and "Space Weathering": Thumping Noises from the Coffin? Most of the spectral characteristics of ordinary chondrites and S asteroids in the visible and infrared can be reduced to three numerical values. These values represent the depth of the absorption band resulting from octahedrally coordinated Fe2+, the reflectance at 0.56 micrometers, and the slope of the continuum (as measured according to convention). By plotting these three characteristics, it is possible to immediately compare the spectral characteristics of large numbers of ordinary chondrites and S asteroids. Commonality of spectral characteristics between these populations can thus be evaluated on the basis of overlap in position on three two-coordinate systems: albedo vs. band depth, band depth vs slope, and slope vs. albedo. In order to establish identity, members of the two populations must overlap on all three of these independent parameter spaces. In this coordinate system, spectra of 23 ordinary chondrites (representing .111 metamorphic grades), and 39 S asteroids were compared. It was found that there was no overlap between the two populations in terms of the slope vs. band depth parameters, nor were most chondrites identical to the S asteroids with respect to the other criteria. Gaffey M. J.* Bell J. F. Brown R. H. Burbine T. H. Piatek J. L. Reed K. L. Chaky D. A. Spectral Evidence of Size Dependent Space Weathering Processes on Asteroid Surfaces Most compositional characterizations of the minor planets are derived from analysis of visible and near-infrared reflectance spectra. However, such spectra are derived from light that has only interacted with a very thin surface layer. Although regolith processes are assumed to mix all near-surface lithologic units into this layer, it has been proposed that space weathering processes can alter this surface layer to obscure the spectral signature of the bedrock lithology. It has been proposed that these spectral alteration processes are much less pronounced on asteroid surfaces than on the lunar surface, but the possibility of major spectral alteration of asteroidal optical surfaces has been invoked to reconcile S-asteroids with ordinary chondrites. The reflectance spectra of a large subset of the S-asteroid population have been analyzed in a systematic investigation of the mineralogical diversity within the S-class. In this sample, absorption band depth is a strong function of asteroid diameter. The S-asteroid band depths are relatively constant for objects larger than 100 km and increase linearly by factor of two toward smaller sizes (~40 km). Although the S-asteroid surface materials includes a diverse variety of silicate assemblages, ranging from dunites to basalts, all compositional subtypes of the S-asteroids conform to this trend. The A-, R-, and V-type asteroids, which are primarily silicate assemblages (as opposed to the metal-silicate mixtures of most S-asteroids) follow a parallel but displaced trend. Some sort of textural or regolith equilibrium appears to have been attained in the optical surfaces of asteroids larger than about 100 km diameter but not on bodies below this size. Tuesday, March 16, 1993 Martian Meteorites and Volatiles: A Session Honoring the Service of Don Bogard as PM&G Discipline Scientist 1:30 - 5:00 p.m. Room A Chair(s): D. S. Burnett J. Longhi Jagoutz E.* Luck J. M. Ben Othman D. Wanke H. Os Isotopes in SNC Meteorites and Their Implications to the Early Evolution of Mars and Earth A new development on the measurement of the Os isotopic composition by mass spectrometry using negative ions opened a wide field of applications. The Re- Os systematic provides time information on the differentiation of the noble metals. The noble metals are strongly partitioned into metal and sulphide phases, but also the generation of silicate melts might fractionate the Re-Os system. Compared to the other isotopic systems, which are mainly dating the fractionation of the alkalis and alkali earth elements, the Re-Os system is expected to disclose entirely new information to the geochemistry. Especially the differentiation and early evolution of the planets such as the formation of the core will be elucidated with this method. Wasylenki L. E.* Jones J. H. Le L. Jurewicz A. J. G. Equilibrium and Fractional Crystallization of a Primitive Shergottite Composition The shergottites are a subset of the SNC meteorites, which are believed to have come from Mars. Petrologic studies of these basaltic meteorites suggest that the most primitive shergottite composition is the groundmass of EETA79001 (A lithology). This inference is based on (i) its high Mg# relative to the other shergottites; (ii) normative olivine; and (iii) the lack of cumulate crystals as found in most SNC meteorites. However, the relationship between this primitive composition and the compositions of the other, more evolved shergottites cannot be discovered through petrologic investigations alone. Therefore, liquidus phase relations were experimentally determined for a synthetic EETA79001A groundmass composition at 1 bar and QFM in order to find whether the bulk compositions of the other shergottites could be derived by simple equilibrium or fractional crystallization of the EETA79001A groundmass composition. Ford D. J.* Rutherford M. J. Primitive SNC Parent Magmas and Crystallization: Low PH20 Experiments SNC meteorites are generally believed to present one of the best opportunities to study the composition and petrogenesis of Mars magmas. The crystallization ages, noble gas content, oxygen isotopic composition, and shocked minerals of the meteorites are consistent with a martian origin. The samples range from dunite to clinopyroxenite to microgabbro. Efforts by researchers to determine parental magmas for the more primitive SNC meteorites have been complicated by crystal accumulation and possible melt segregation and removal. This has resulted in a range of parent magma estimates, although all appear to be Fe- rich and Al-poor. One major objective of this project is to refine the Chassigny parent magma estimate by forcing olivine + clinopyroxene saturation upon the proposed melt composition. EETA 79001 magma compositions are also being investigated to determine the parent magma and the origin of the coarse- grained olivine and orthopyroxene megacrysts. Low-pressure experiments with small but finite PH20 are being utilized to facilitate equilibrium, and to simulate the H20 indicated for these magmas. Treiman A. H.* The Parent Magma of the Nakhla (SNC) Meteorite: Reconciliation of Composition Estimates from Magmatic Inclusions and Element Partitioning The composition of the parent magma of the Nakhla meteorite has been difficult to determine because it is a cumulate rock, enriched in olivine and augite relative to a basalt magma. A parent magma composition is estimated here from electron microprobe area analyses of magmatic inclusions in olivine. This composition is consistent with an independent estimate based on the same inclusions, and with chemical equilibria with the cores of Nakhla's augites (recalculated after Longhi and Pan, 1989). This composition reconciles most of the previous estimates of Nakhla's magma composition, and obviates the need for complex magmatic processes. Inconsistency between this composition and those calculated by Treiman (1986) suggests that magma flowed through and crystallized into Nakhla as it cooled. McKay G.* Le L. Wagstaff J. The Nakhla Parent Melt: REE Partition Coefficients and Clues to Major Element Composition Nakhla is one of the SNC meteorites generally believed to be of martian origin. It is a medium-grained augite-olivine cumulate with a variolitic groundmass of sodic plagioclase, alkali feldspar, and Fe-rich pyroxenes and olivine. One of the major tasks in deciphering Nakhla's petrogenesis is determining the composition of its parent melt. Gaining an understanding of the composition and petrogenesis of this parent melt may help unravel Nakhla's relationship to the other SNCs, and provide clues to martian petrogenesis in general. Our experimental partitioning studies provide new information that helps constrain both the major and trace element composition of the Nakhla parent melt. Wadhwa M.* Crozaz G. Rare Earth Elements in Individual Minerals in Shergottites Shergottites (i.e., Shergotty, Zagami, EETA79001, ALHA77005, and LEW88516) are an important set of achondrites because they comprise the majority of the SNC group of meteorites (nine, in total, known to us), which are likely to be samples of the planet Mars. Study of these meteorites may therefore provide valuable information about petrogenetic processes on a large planetary body other than Earth. Rare earth element distributions between various mineral phases have been found to be useful in geochemically modelling the petrogenesis of various rock types (terrestrial and meteoritic). However, with the exception of a few ion microprobe studies and analyses of mineral separates, there has previously not been any comprehensive effort to characterize and directly compare REE in individual minerals in each of the five known shergottites. We have, therefore, made ion microprobe analyses on thin sections of each of the shergottites. Minerals analysed were pyroxenes (pigeonite and augite), maskelynite, and whitlockite. McCoy T. J.* Keil K. Taylor G. J. The Dregs of Crystallization in Zagami The Zagami shergottite is a basaltic meteorite that formed when a phenocryst-bearing lava flow was emplaced at or near the surface of Mars. Recently, a centimeter-sized olivine-rich lithology has been identified in Zagami by Mossbauer spectroscopy. Olivine is extremely rare in shergottites, particularly in Zagami and Shergotty, where it occurs only as minute grains. We report petrologic and microprobe studies of this olivine-rich lithology. This material represents the last few percent of melt and is highly enriched in phosphates, opaques, and mesostases, all of which are late-stage crystallization products. Phosphates replaced augite as a phenocryst phase when the magma became saturated in P. This late-stage melt also includes a fayalite-bearing, multiphase intergrowth that crystallized after the melt became too rich in iron to crystallize pigeonite. We can now reconstruct the entire crystallization history of the Zagami shergottite from a deep-seated magma chamber to crystallization of the final few percent of melt in a near-surface dike or thick flow. Small pockets (tens of micrometers) of late-stage melt pockets are ubiquitous but volumetrically minor in Zagami. We do not know the physical relationship between these areas and the centimeter-sized olivine-rich material described here. It is possible that these small pockets were mobile, forming larger areas. Perhaps inspection of the entire hand specimen of Zagami would clarify this relationship. Chen J. H.* Wasserburg G. J. LEW88516 and SNC Meteorites We report on the isotopic composition of Pb and the concentration of Pb, U, and Th in a sample of LEW88516 (LEW). LEW was a 13-g stone recovered from Antarctica and was classified as a new member of the Shergottite group. This work was undertaken with the hope that LEW might yield new inforrnation to elucidate further the origin and evolution of Shergottites and other SNC meteorites. We have previously studied U-Th-Pb in other Shergottites, namely Shergotty, Zagarni, EETA79001, and ALHA77005 as well as Nakhla. The results indicate that the initial leads of these Shergottites were well defined, were distinct from each other, and have high ^204Pb/^206Pb (from 0.0652 to 0.0739). These leads evolved in different reservoirs over most of solar system history in a low ^238U/^204Pb(mu) ~5 environment. The U-Th-Pb isotopic systems are quite regular, which unambiguously indicate an event of U-Th-Pb fractionation at 200 m.y. in accord with Shih et al. The details of the data arrays are complex. The young age is in general agreement with some of the ages obtained by other methods, but precise concordance between the different methods is not established. Becker R. H.* Pepin R. O. Nitrogen and Noble Gases in a Glass Sample from LEW88516 The Antarctic meteorite LEW88516 has been classified as a member of the SNC group of meteorites, specifically a shergottite. It is reported to be remarkably similar in mineralogy, petrogenesis, and chemistry to the previously known ALH77005 shergottite, with both being compositionally distinct from other shergottites. LEW88516 shows pervasive shock features and has been found to contain glass veins attributable to a shock origin. In an effort to determine whether the glass in LEW88516 contains any of the isotopically-heavy trapped nitrogen component observed in EETA 79001 glass, as well as the related high-40Ar/36Ar and high-129Xe/132Xe components, we undertook an analysis of an 11.9 mg glass sample (LEW88516,4) provided to us by H. Y. McSween, Jr. as part of a consortium study of this meteorite. Grady M. M.* Wright I. P. Franchi I. A. Pillinger C. T. Nitrates in SNCs: Implications for the Nitrogen Cycle on Mars Nitrogen is the second most abundant constituent of the martian atmosphere, after CO2, present at a level of ca. 2.7%. Several authors have hypothesised that earlier in the planet's history, nitrogen was more abundant, but has been removed by processes such as exospheric loss from the atmosphere. However, an alternative sink for atmospheric nitrogen is the regolith; model calculations have predicted that, via the formation of NOx, HNO2, and HNO3 in the lower layers of the martian atmosphere, the regolith might trap nitrite and nitrate anions, leading to the build-up of involatile nitrates. Integrated over 4.5 x 10^9 yr, such a mechanism would contribute the equivalent of a layer of nitrates up to 0.3 cm thick distributed across the martian surface. Features in thermal emission spectra of the surface of Mars have been interpreted tentatively as emanating from various anions (carbonates, bicarbonates, sulphates, etc.), and the presence of nitrates has also been addressed as a possibility. The identification of carbonates in SNC meteorites has allowed inferences to be drawn concerning the composition and evolution of the martian atmosphere in terms of its carbon isotope systematics; if nitrites, nitrates, or other nitrogen-bearing salts could be isolated from SNCs, similar conclusions might be possible for an analogous nitrogen cycle. Wright I. P.* Douglas C. Pillinger C. T. The Carbon Components in SNC Meteorites of Feldspathic Harzburgite Composition Two meteorites collected in Antarctica, ALH A77005 and LEW 88516, have characteristics that link them to the shergottite group of SNC meteorites. Essentially, ALH A77005 and LEW 88516 are feldspathic harzburgites, as they comprise roughly equal quantities of olivine and pyroxene, with an additional few percent of feldspar that has subsequently been converted to maskelynite by shock. The meteorites represent samples of a cumulate rock that is itself composed of two different lithologies: In one, large pyroxenes poikilitically enclose olivine crystals, while the other consists of interstitial areas made up of pyroxene, olivine, maskelynite, whitlockite, troilite, ilmenite, and chlorapatite. It has been proposed that meteorites such as ALH A77005 (and LEW 88516) are relict samples of the source peridotite from which the other shergottites formed. If this is the case, it should be informative to study in detail the carbon components present within these samples in order to make comparisons with data from other shergottites. Although not plutonic in origin, and therefore not sampling a truly deep source, analyses of ALH A77005 and LEW 88516 should assist with attempts to define the bulk carbon isotopic composition of Mars. This has been assessed previously through analyses of carbon of presumed magmatic origin in other SNC meteorites, but the carbon isotopic compositions obtained seem to be at variance with what might be expected. It is important to constrain the carbon isotopic composition of Mars as well as possible so that models of atmospheric evolution, based on carbon isotopic data, can yield the most reliable results. Karlsson H. R.* Clayton R. N. Mayeda T. K. Jull A. J. T. Gibson E. K. Martian Carbon Dioxide: Clues from Isotopes in SNC Meteorites Attempts to unravel the origin and evolution of the atmosphere and hydrosphere on Mars from isotopic data have been hampered by the impreciseness of the measurements made by the Viking Lander and by Earth-based telescopes. The SNC meteorites, which are possibly pieces of the martian surface, offer a unique opportunity to obtain more precise estimates of the planet's volatile inventory and isotopic composition. Recently, we reported results on oxygen isotopes of water extracted by pyrolysis from samples of Shergotty, Zagami, Nakhla, Chassigny, Lafayette, and EETA-79001. Now we describe complementary results on the stable isotopic composition of carbon dioxide extracted simultaneously from those same samples. We will also report on 14C abundances obtained by accelerator mass spectrometry for some of these CO2 samples. Watson L. L.* Hutcheon I. D. Epstein S. Stolper E. M. High D/H Ratios of Water in Magmatic Amphiboles in Chassigny: Possible Constraints on the Isotopic Composition of Magmatic Water on Mars We have measured the D/H ratios of kaersutitic amphiboles contained in magmatic inclusions in the SNC meteorite Chassigny using the ion microprobe. A lower limit on the delta D(sub)SMOW of the amphiboles is +1420 +/- 47 per mil. Assuming Chassigny comes from Mars and the amphiboles have not been subject to alteration after their crystallization, this result implies either that recycling of D-enriched martian atmosphere-derived waters into the planetary interior has taken place, or that the primordial hydrogen isotopic composition of the interior of Mars differs significantly from that of the Earth (delta D(sub)SMOW ~ 0 per mil). In addition, the measurements indicate that the amphiboles contain <0.3 wt% water. This is much lower than published estimates, and indicates a less hydrous Chassigny parent magma than previously suggested. Tuesday, March 16, 1993 Venus Gravity from Magellan and Mars Geophysics 1:30 - 5:00 p.m. Room B Chair(s): W. Sjogren B. Bills Sjogren W. L.* Konopliv A. S. Borderies N. Batchelder M. Heirath J. Wimberly R. N. Venus Gravity: New Magellan Low Altitude Data Acquisition of a new high-quality gravity dataset has begun, which presently covers one third of the venusian longitude. Better spatial resolution is obtained from a 60th degree and order spherical harmonic solution. Plans for aerobraking in May 1993 into a near-circular orbit will provide excellent data for higher-latitude regions. Smrekar S. E.* Parmentier E. M. Response of the Topography and Gravity Field on Venus to Mantle Upwelling Beneath a Chemical Boundary Layer The long wavelength correlation of the gravity and topography and the large apparent depths of compensation (~150-300 km) for large highland regions on Venus require significant differences between the interior structure of Earth and Venus. The morphology, geologic history, and large apparent depths of compensation for many highlands have been interpreted to indicate areas of mantle upwelling. A large apparent depth of compensation at a mantle upwelling is generally interpreted to indicate the base of the thermal boundary layer of convection. A boundary layer thickness of 150-300 km implies that the interior of Venus is presently much colder than Earth and thus tectonically less active. The recent Magellan mission has provided contradictory evidence regarding the present level of tectonic activity on Venus, prompting considerable debate. In this study, we investigate the possibility that a chemical boundary layer acts together with a thermal boundary layer to produce large apparent depths of compensation, or, equivalently, large geoid-to- topography-ratios (GTRs). The crust of a planet forms through partial melting of mantle material. Both the melt and the residuum are lower in density than unmelted (or undepleted) mantle. In the absence of vigorous plate tectonics, a thick layer of buoyant residuum, or depleted mantle, may collect beneath the lithosphere. In this scenario, the thermal lithosphere need not be usually thick and cold to match the GTRs. Cooling of the depleted layer may lead to overturn of the upper mantle and episodic resurfacing with time scales on the order of 300-500 MY, consistent with the resurfacing age of Venus. Buck W. R.* Can Weak Crust Explain the Correlation of Geoid and Topography on Venus? The effect on geoid and topography of low viscosity crust overlying a steady- state convecting mantle is estimated under the assumption that the shear between crust and mantle does not alter the mantle flow. The weak crustal layer can change the sign of the geoid to topography ratio (admittance). The positive long wavelength admittance for Venus is consistent with a weak crust overlying a mantle with a viscosity that increases strongly with depth. Moresi L.* Effective Elastic Thickness of the Venusian Lithosphere with Lateral Viscosity Variations in the Mantle Both the Earth and Venus have a convecting mantle at the top of which is a relatively strong, mechanical boundary layer. The surface topography and gravity signals that result from the convection within the viscous mantle are modified by the elastic properties of this lithospheric boundary layer. In particular the ability of the lithosphere to support loads and transmit stresses from below is a function of the wavelength of the load--the lithosphere is strong to loading at shorter wavelengths. As a consequence it is usual to expect that long wavelength topography cannot be supported by the mechanical strength of the lithosphere and must be compensated--isostatically or dynamically--within the uppermost mantle or the crust. The flexural rigidity of the lithosphere can therefore be determined by estimating the greatest wavelength at which uncompensated surface topography can be supported, usually by measuring the admittance as a function of wavelength. In fact this procedure for determining the elastic thickness relies upon being able to distinguish topography with underlying support from that supported by the brittle lithosphere on the basis of their each having a characteristic value of the admittance. However, in the presence of lateral viscosity variations in the mantle, it is possible for topography to be generated that is NOT compensated by density anomalies in the underlying mantle at the same wavelength. Although this effect is not likely to be important for the Earth, on Venus, where the high surface temperatures would be expected to give a weaker lithosphere, lateral viscosity variations in the mantle can give a misleadingly large apparent elastic thickness for the lithosphere. Simons M.* Hager B. H. Solomon S. C. Geoid, Topography, and Convection-driven Crustal Deformation on Venus High-resolution Magellan images and altimetry of Venus reveal a wide range of styles and scales of surface deformation that cannot readily be explained within the classical terrestrial plate tectonic paradigm. The high correlation of long-wavelength topography and gravity and the large apparent depths of compensation suggest that Venus lacks an upper-mantle low-viscosity zone. A key difference between Earth and Venus may be the degree of coupling between the convecting mantle and the overlying lithosphere. Mantle flow should then have recognizable signatures in the relationships between the observed surface topography, crustal deformation, and the gravity field. Therefore, comparison of model results with observational data can help to constrain such parameters as crustal and thermal boundary layer thicknesses as well as the character of mantle flow below different venusian features. We explore in this paper the effects of this coupling by means of a finite element modelling technique. Smith D. E.* Lerch F. J. Nerem R. S. Zuber M. T. Patel G. B. Fricke S. K. Lemoine F. G. GMM-1: A 50th Degree and Order Gravitational Field Model for Mars Knowledge of the gravitational field, in combination with surface topography, provides one of the principal means of inferring the internal structure of a planetary body. The highest-resolution gravitational field for Mars published thus far was derived from Doppler tracking data from the Mariner 9 and Viking 1 and 2 spacecraft and is complete to degree and order 18 corresponding to a half-wavelength resolution of approximately 600 km. This field, which is characterized by a spatial resolution that is slightly better than that of the highest-resolution (16 x 16) topographic model, has been utilized extensively in analyses of the state of stress and isostatic compensation of the martian lithosphere. However, the resolution and quality of current gravity and topographic fields are such that the origin and evolution of even the major physiographic features on Mars, such as the hemispheric dichotomy and Tharsis rise, are not well understood. Bills B. G.* Frey H. V. Kiefer W. S. Nerem R. S. Zuber M. T. Spectral Analysis of the Gravity and Topography of Mars New spherical harmonic models of the gravity and topography of Mars place important constraints on the structure and dynamics of the interior. The gravity and topography models are significantly phase coherent for harmonic degrees n <= 30 (wavelengths >= 700 km). Loss of coherence below that wavelength is presumably due to inadequacies of the models, rather than a change in behavior of the planet. The gravity/topography admittance reveals two very different spectral domains: for n > 4 a simple Airy compensation model, with mean depth of ~100 km, faithfully represents the observed pattern; for degrees 2 and 3, the effective compensation depths are 1400 and 550 km respectively, strongly arguing for dynamic compensation at those wavelengths. Kiefer W.* Bills B. Frey H. Nerem S. Roark J. Zuber M. An Inversion of Geoid and Topography for Mantle and Crustal Structure on Mars A number of prior studies have analyzed martian gravity anomalies and topography in terms of isostasy and flexure of the crust and lithosphere. Other studies have emphasized the role of mantle convection in producing gravity anomalies and topography in some regions of Mars. In this study, we invert geoid and topography observations for simultaneous estimates of density anomalies in the crust and mantle of Mars. In performing this study, we make use of a recent degree 50 spherical harmonic expansion of the martian gravity field (GMM-1) and a corresponding resolution expansion of the USGS Mars topography model. However, our analysis is restricted to harmonic degrees up to L = 25, which are better determined than the higher harmonics. This provides a half-wavelength horizontal resolution of 425 km. Frey H.* Bills B. G. Kiefer W. S. Nerem R. S. Roark J. H. Zuber M. T. New Mars Free-Air and Bouguer Gravity: Correlation with Topography, Geology, and Large Impact Basins We compare free-air and Bouguer gravity anomalies from a 50 x 50 field (MGM-635), derived at the Goddard Space Flight Center, with global topography, geology, and the distribution of large impact basins. The free-air gravity anomalies were derived from re-analysis of Viking Orbiter and Mariner 9 tracking data and have a spatial resolution of 250-300 km. Bouguer anomalies were calculated using a 50 x 50 expansion of the current Mars topography and the GSFC degree 50 geoid as the equipotential reference surface. Rotational flattening was removed using a moment of inertia of 0.365 and the corrections from Table B2 of Sleep and Phillips. Crustal density and mean density were assumed to be 2.9 and 3.93 gm/cm^3. Lemoine F. G.* Smith D. E. Fricke S. K. McCarthy J. J. A Simultaneous Estimation of the Mass of Mars and Its Natural Satellites, Phobos and Deimos, from the Orbital Perturbations on the Mariner 9, Viking 1, and Viking 2 Orbiters The natural satellites of Mars, Phobos and Deimos, caused perturbations on the orbits of the Mariner 9 and the Viking spacecraft that have been used to estimate the satellite masses. The Viking spacecraft were specifically targeted to make close flybys (within a few hundred kilometers) of Phobos in February 1977 and of Deimos in October 1977. These close encounters have been used to estimate the moons' gravitational constant, GM (the universal constant of gravitation multiplied by the satellite mass). However, the Viking and Mariner 9 spacecraft made numerous flybys of Phobos and Deimos at distances of a few thousand kilometers. The tracking data from these more "distant" encounters have been processed to estimate the masses of Mars, Phobos, and Deimos. Stevenson D. J.* Expectations for the Martian Core Magnetic Field In the traditional view of planetary magnetism, a planet either has a core dynamo ( Earth, Jupiter, Saturn, Uranus, Neptune, maybe Mercury) or does not (Mars, Venus, Moon...). I argue that this view is simplistic in two respects. First, mantle convection in terrestrial planets is invariably at high enough Rayleigh number that it is time variable; this leads to the intermittent arrival of mantle "cold fingers" at the core-mantle boundary, promoting at least local core convection and dynamo action even when the planetary core is stably stratified on average. Thus, I predict an intermittent dynamo regime in addition to the simple dynamo-on (Earth) and dynamo-off regimes. Second, the mantle convection-driven horizontal temperature gradients just below the core- mantle boundary can lead to unstable flows that will convert thermoelectric or electrochemical toroidal fields into externally detectable poloidal fields, even when a dynamo is not possible. It is likely that Mars possesses an interesting core magnetic field of the latter kind, complex but with a dipole that might be approximately aligned with the rotation axis and a surface field of a few to tens of gammas. Dolginov A. Z.* Magnetic Fields and Nonuniform Structures of the Moon Direct magnetic measurements performed by space probes demonstrated the existence of small-scale, stationary surface magnetic fields on the Moon [1]. The magnetic field averaged over a region of ~ 600 km was found to be no larger than ~10^-5 G, that of the regions ~100-200 km is ~ 10^-5- 10^-4 G, and that of ~10-100 km reaches ~10^-3 G. Investigations of certain lunar rocks reveal stable residual magnetization, which could have been acquired during the crystallization of the rocks in the presence of the outer magnetic field, provided such a field existed at the initial stage of lunar evolution. Estimates show that 4 10^9 yr ago the field was small, then it rose to, 1.3 G at the beginning of 3.9 10^9 yr with a subsequent exponential decrease during the period of 3.9 109 yr to 3.2 10^9 yr ago. Small-scale fields have been explained by some authors as due to mechanical impacts produced by meteors. The theory of this effect is not elaborated in detail. This can in no way explain the paleomagnetic data. These data are commonly explained as a result of the dynamo action in the liquid lunar core. This action can cease for various causes, for example, when the core temperature decreases. To obtain the field ~10^-2-1 G on the surface, the field on the core boundary must be larger than ~10 G if the core radius was less than ~400-500 km. Should this field be the result of the dynamo, the convective velocity in the lunar core have to be much larger than that in the Earth's core. The motion can amplify the field only if the kinetic energy density is larger than the magnetic energy density, i.e., if rho u^2 > B^2/4 pi, where rho is the matter density, u is the velocity of the medium, and B is the magnetic field. Hence u must be larger than 1-10 cm/s. Motions of this kind should lead to rapid smoothing of the temperature in the core, and the temperature gradient will drop below its adiabatic value. It switches off the convection. The gravitational separation of the heavy and light fractions cannot support convection of the necessary intensity in the considered case. Gravitational energy release in the process of the lunar core formation has been estimated as ~10^l5 erg/s. The energy needed for the large- scale magnetic field generation is no less than 10^17 erg/s if the field is about 1 G . This is a serious problem of the dynamo hypothesis. One of the alternatives is the assumption that the lunar field was induced by the Earth. This hypothesis requires the assumption that the Earth used to be much closer to the Moon (3-4) 10^9 yr ago ( at a distance ~5- 10 Earth radii) than today and the Earth's field was significant larger (10-50 G). There is no evidence for these values. Tuesday, March 16, 1993 Large Impact Events: Theory and Observations 1:30 - 3:15 p.m. Room C Chair(s): P. H. Warren O'Keefe J. D.* Ahrens T. J. Dynamics of Large-scale Impacts on Venus and Earth Large-scale impacts are a key aspect of the accretion and growth of the planets, the evolution of their atmospheres, and the viability of their life forms. We have performed an extensive series of numerical calculations that examined the mechanics of impacts over a broad range of conditions and are now extending these to account for the effects of the planetary atmosphere. We have examined the effects of large-scale impacts in which the trapping and compression of an atmosphere during impact is a significant factor in the transfer of energy to the atmosphere. Ivanov B. A.* Ford P. G. The Depths of the Largest Impact Craters on Venus The largest impact craters on Venus may be used as witnesses of various geological processes within the venusian crust. We seek to continue the task of constructing a database for the further investigation of large craters on Venus, hoping to find evidence of crater relaxation that might constrain the thickness and thermal gradient of the crust. The current work concentrates on 27 impact craters with diameters (d) larger than 70 km, i.e., large enough that the footprint of the Magellan altimeter has a good chance of sampling the true crater bottom. All altimeter echoes from points located within (d/2)+70 km from the crater center have been inspected. Chadwick D. J.* Schaber G. G. A Two-Stage (Turbulent-Drainage) Mechanism for Emplacement of Impact Crater Outflows on Venus Several emplacement mechanisms have been proposed for the unique flow features associated with 402 of the 912 impact craters found on Venus to date. Studies based on rheological models and laboratory results have suggested two separate depositional flow mechanisms: a turbulent emplacement and a lavalike emplacement. Schultz proposed a progressive transition from the turbulent to the lavalike mechanism during deceleration of some turbulent flows. He suggested that these composite flows were emplaced prior to the deposition of the normal ejecta. Ahrens T. J.* Giant Impact-induced Atmospheric Blow-Off How are the present atmospheres of the Earth, Venus, and Mars related to the earliest (primordial) atmospheres that surrounded these planets? Recent modeling of the accretion of the planets from a swarm of planetesimals orbiting the early Sun demonstrates that smaller planetesimals gained specific kinetic energy at the expense of the larger planetesimals. As a result, runaway impact accretion and growth occurred for marginally larger objects such that in ~10^5 years some of the 10^24 g objects grew to 10^26 g. Mutual impact of lunar- and Mars- sized objects resulted in a series of "large body" impacts during the latter stages of accretion. Assuming the composition of the planetesimals are similar to the primitive meteorites, Lange and Ahrens showed that after the Earth and Venus accreted to about 0.1 of their present mass, the volatiles (H2, CO2, SO2, NH3, CH4 + noble gases) within the later impacting planetesimals became vaporized and produced the giant primordial atmospheres overlying a magma ocean, which remained during the completion of the accretion. The termination of the coaccretion of an atmosphere results from at least three different mechanicsm: Vickery A. M.* Numerical Modeling of Impact Erosion of Atmospheres: Preliminary Results It is clear from the great diversity of atmospheres among the terrestrial planets that their formation and evolution must have depended on a balance among a number of different processes. One of these processes is atmospheric erosion by impacts, which may have been particularly effective on Mars. The reason is that geomorphic evidence on Mars suggests that this planet had, early in its history, a dense enough atmosphere to sustain active precipitation over geologically significant periods of time. Analytic calculations indicate that neither the projectile entering the atmosphere nor the main crater ejecta can cause the loss of significant amounts of atmosphere. The vapor plume that is formed, however, expands rapidly as its internal energy is converted into kinetic energy, and may blow off the overlying atmosphere. A model of this part of the impact/atmosphere interaction predicts Mars could have lost a substantial early atmosphere by impact erosion alone. Although our more detailed calculations, which took into account the anisotropy of the atmosphere with respect to zenith angle, show that the process isn't quite as effective, they still indicate the probability of substantial atmospheric loss from Mars. In this abstract, I discuss the first results from 2-D hydrocode runs. These include two runs that make most of the same simplifying approximations as the analytic models, in order to compare tha analytic and numerical results directly, and one run (as yet incomplete) that models the full impact. Warren P. H.* Limits on Differentiation of Melt "Sheets" from Basin-scale Lunar Impacts In the literature on cratering, one frequently finds allusions to the "sheet" of impact melt that supposedly forms in the aftermath of any large impact. The presumption seems to be that either (1) most of the impact melt is formed virtually at the surface, or else (2) most of the melt inevitably collects into a near-surface, roughly sill-shaped "sheet," soon after the impact. However, in basin-scale impacts on the mature Moon, neither of these scenarios appears likely. Differentiation of impact melts on the Moon is hampered by similarity in density vs. the porous lunar crust, and by the adverse melt/"displaced" matter ratio that results from the lunar g. BASIN-SCALE LUNAR IMPACT MELTS: Warren P. H. PRISTINE NONMARE ROCKS: Warren P. H. Therriault A. M.* Reid A. M. Reimold W. U. Origin of the Vredefort Structure, South Africa: Impact Model A model is presented for the evolution of the Vredefort structure, based on reasoned constraints on the original size of the Vredefort structure from observational data and comparison with other terrestrial impact craters. The models for complex craters (ring and multiring basins) of Croft, Grieve and co-workers, and Schultz and co-workers were used to reconstruct the Vredefort impact event, using a final crater diameter of 300 km, as estimated by Therriault et al. Tuesday, March 16, 1993 Galileo Earth/Moon Results 3:15 - 5:00 p.m. Room C Chair(s): A. S. McEwen Greeley R.* Belton M. J. S. Head J. W. McEwen A. S. Pieters C. M. Neukum G. Becker T. L. Fischer E. M. Kadel S. D. Robinson M. S. Sullivan R. J. Sunshine J. M. Williams D. A. Galileo Imaging Results from the Second Earth-Moon Flyby: Lunar Maria and Related Units The second flyby of the Earth-Moon System by Galileo occurred on December 7, 1992, on its trajectory toward Jupiter. The flyby took the spacecraft over the lunar north polar region from the dark farside and continued across the illuminated nearside. This provided the first opportunity to observe northern and northeastern limb regions with a modern, multispectral imaging system with high spatial resolution (up to 1.1 km/pixel). Scientific objectives included compositional assessment of previously uncharacterized mare regions, study of various light plains materials, and assessment of dark mantle deposits (DMD) and dark halo craters (DHC). Color composite images were prepared from ratios of Galileo SSI filter data (0.76/0.41 --> red; 0.76/0.99 --> green; 0.41/0.76 --> blue) and used for preliminary comparison of units. The 0.41/0.76 ratio has been empirically correlated to Ti content of mare soils (blue is relatively high, red is relatively low). The relative strengths of the ferrous 1-micrometer absorption in mafic minerals can be compared using the 0.76/0.99 ratio. In addition, relative ages of units analyzed spectrally were determined from crater statistics using Lunar Orbiter images following the techniques of Neukum et al. Mare deposits analyzed include Mare Humboldtianum, central and eastern Mare Frigoris, Mare Crisium and other deposits in the Crisium Basin, and isolated mare patches on the northeastern lunar limb. Preliminary results show a diversity of 0.41/0.76-micrometer signatures, implying a wide range of titanium contents. Some light plains units are similar to units found at the Apollo 16 site; others may be ancient mare materials. Dark mantle deposits (DMD) analyzed also are variable. Head J. W.* Belton M. Greeley R. Pieters C. Fischer E. Sunshine J. Klaasen K. McEwen A. Becker T. Neukum G. Oberst J. Pilcher C. Plutchak J. Robinson M. Johnson T. Williams D. Kadel S. Sullivan R. Antonenko I. Bridges N. Galileo Imaging Team Lunar Impact Basins: New Data for the Nearside Northern High Latitudes and Eastern Limb from the Second Galileo Flyby During the December 1992 Galileo Earth/Moon encounter the northern half of the nearside, the eastern limb, and parts of the western farside of the Moon were illuminated and in view, a geometry that was complementary to the first lunar encounter in December 1990, which obtained images of the western limb and eastern farside. The Galileo Solid State Imaging system (SSI) obtained multispectral images for these regions during the second encounter and color ratio composite images were compiled using combinations of band ratios chosen on the basis of telescopic spectra and laboratory spectra of lunar samples. Ratios of images taken at 0.41 and 0.76 micrometers are sensitive to changes in the slope in the visible portion of the spectrum, and ratios of 0.99 and 0.76 micrometers relate to the strength of near-infrared absorptions due to iron-rich mafic minerals (0.76/0.99 ratio) such as olivine and pyroxene. esults of the analyses of the compositional diversity of the crust, maria, and Copernican craters are presented elsewhere. Carlson R. W. Kieffer H. H.* Baines K. H. Becker K. J. Danielson G. E. Edwards K. Fanale F. P. Forsythe J. Gaddis L. R. Granahan J. C. Hui J. Johnson T. V. Lopes-Gautier R. Kamp L. W. Matson D. L. McCord T. B. Mehlman R. Ocampo A. C. Soderblom L. A. Smythe W. D. Torson J. Weissman P. R. Preliminary Report of Lunar Observations by the Near-Infrared Mapping Spectrometer (NIMS) During the Second Galileo Earth-Moon Encounter The Galileo encounter with the Earth-Moon system on December 8, 1992, provided a unique opportunity to observe the Moon. Galileo's closest approach to the Moon was at an altitude of about 110,000 km above an area at about latitude 60N, longitude 60E. During the 12 hours surrounding the time of lunar closest approach, 12 observational sequences were executed by the near-infrared mapping spectrometer (NIMS), many in coordination with other Galileo instruments. These NIMS observations provided nearly complete coverage of the illuminated crescent from phase angles of 123 degrees to 14 degrees. This phase-angle coverage with nearly constant illumination is not possible from Earth; it is expected to provide substantial new information on the nature of the lunar photometric function over wavelength. As of January 1993, most of the analysis has focused on the highest resolution data (55 km/pixel). Pieters C. M.* Belton M. Head J. W. Greeley R. McEwen A. Fischer E. M. Sunshine J. M. Klaasen K. Plutchak J. Neukum G. Johnson T. V. SSI Team Compositional Diversity of the Lunar North Pole: Preliminary Analyses of Galileo SSI Data In December 1992 the Galileo spacecraft passed through the Earth-Moon system for its final gravity assist to Jupiter. The SSI camera obtained several six- color mosaics of the lunar north polar region and the sunlit nearside and eastem limb at ~1.3 km/pixel. Initial analyses have concentrated on the north polar areas to assess the composition of the crust in that region. Shown in the figures are representative six-color calibrated SSI spectra (typically 5 x 5 pixels). Photometric corrections have not yet been applied, and all spectra are scaled to unity at 0.56 m. The data were first calibrated relative to MS2, a standard area in Mare Serenitatis (~18.7 degrees N, 21.5 degrees E), and the top four plots of highlands, highland craters, maria, and mare craters are displayed relative to MS2. SSI spectra of areas measured with telescopic data (mare MTl/MS2 and mare crater MSA/MS2) agree well with previous data, confirming that the calibration procedures and SSI data are spectrally accurate. The bottom three plots of craters/Sun have been calibrated to reflectance using previously obtained telescopic spectra of Apollo 1 6/MS2 (shown with Highlands/MS2) and laboratory spectra of mature Apollo 16 soil (shown for reference with the Highland Craters/Sun). Although some variations in these spectra mimic previously observed spectra of lunar terrains, several characteristics are unusual. Familiar and unfamiliar properties are observed in these northern latitudes and both types merit further investigation in their geologic context. McEwen A. S.* Greeley R. Head J. W. Pieters C. M. Fischer E. M. Johnson T. V. Neukum G. Galileo SSI team Galileo SSI Lunar Observations: Copernican Craters and Soils The Galileo spacecraft completed its first Earth-Moon flyby (EM1) in December 1990 and its second flyby (EM2) in December 1992. Copernican-age craters are among the most prominent features seen in the SSI (Solid-State Imaging) multispectral images of the Moon. The interiors, rays, and continuous ejecta deposits of these youngest craters stand out as the brightest features in images of albedo and visible/1-micron color ratios (except where impact melts are abundant). Crater colors and albedos (away from impact melts) are correlated with their geologic emplacement ages as determined from counts of superposed craters; these age-color relations can be used to estimate the emplacement age (time since impact event) for many Copernican-age craters on the near and far sides of the Moon. Greenberg R. Belton M. DeJong E. Ingersoll A. Klaasen K. Geissler P.* Moersch J. Thompson W. R. Galileo Imaging Team Earth Imaging Results from Galileo's Second Encounter The recent flyby of the Galileo spacecraft en route to Jupiter contributes a unique perspective to our view of our home planet. Imaging activities conducted during the second Earth encounter provide an important opportunity to assess new methods and approaches on familiar territory. These include unique multispectral observations, low light-level imaging (searches for aurorae, lightning and artificial lights on the nightside) and experiments with multiple exposure times to extend the effective radiometric resolution and dynamic range of the camera system. Tuesday, March 16, 1993 Martian Geomorphology 1:30 - 5:00 p.m. Room D Chair(s): R. A. Craddock J. Grant Leonard G. J.* Tanaka K. L. Hellas Basin, Mars: Formation by Oblique Impact Hellas, a 2,000-km-diameter, roughly circular multiring impact basin in the southern highlands of Mars, has a pronounced southeastern lobe of rim material that extends for some 1500 km. This lobe and a system of ridges concentric to the southern part of the basin (including part of the lobe) has been interpreted to be formed by an oblique impact that was inclined in the direction of the lobe. Our preliminary geologic mapping of the Hellas region (lat -20 degrees to -65 degrees, long 250 degrees to 320 degrees) at 1:5,000,000 scale provides additional supporting evidence for this hypothesis, including a symmetric distribution of basin ejecta and volcanic centers across the inferred impact trend. Furthermore, measurements of relief indicate that the downrange ejecta may be about twice as thick as other parts. Ching D.* Taylor G. J. Mouginis-Mark P. Bruno B. C. Fractal Dimensions of Rampart Impact Craters on Mars Ejecta blanket morphologies of martian rampart craters may yield important clues to the atmospheric densities during impact, and the nature of target materials (e.g., hard rock, fine-grained sediments, presence of volatiles). In general, the morphologies of such craters suggest emplacement by a fluidized, ground-hugging flow instead of ballistic emplacement by dry ejecta. We have quantitatively characterized the shape of the margins of the ejecta blankets of 15 rampart craters using fractal geometry. Our preliminary results suggest that the craters are fractals and are self-similar over scales of ~0.1 km to 30 km. Fractal dimensions (a measure of the extent to which a line fills a plane) ranges from 1.06 to 1.31. No correlations of fractal dimension with target type, elevation, or crater size were observed, though the database is small. The range in fractal dimension and lack of correlation may be due to a complex interplay of target properties (grain size, volatile content), atmospheric pressure, and crater size. The mere fact that the ejecta margins are fractals, however, indicates that viscosity and yield strength of the ejecta were at least as low as those of basalts, because silicic lava flows are not generally fractals. Barlow N. G.* Increased Depth-Diameter Ratios in the Medusae Fossae Formation Deposits of Mars Depth-to-diameter ratios for fresh impact craters on Mars are commonly cited as approximately 0.2 for simple craters and 0.1 for complex craters. Recent computation of depth-diameter ratios in the Amazonis-Memnonia region of Mars indicates that craters within the Medusae Fossae Formation deposits found in this region display greater depth-diameter ratios than expected for both simple and complex craters. Grant J. A.* Schultz P. H. Martian Crater Degradation by Eolian Processes: Analogy with the Rio Cuarto Crater Field, Argentina Numerous degraded and rimless craters occur across broad areas of the martian surface that are mantled by thick, unconformable deposits. These regions include Arabia, Mesogaea, Electris, Tempe, the interior and surface to the northwest of Isidis Basin, southern Ismenius Lacus, and the polar layered terrains. Occurrence of the deposits and low regional thermal inertias indicate that at least some accumulated fine-grained sediment (effective particle diameters of 0.1-0.5 mm or coarse silt to medium sand) to a thickness of hundreds to thousands of meters. Most unconformable deposits experienced some eolian modification that may be recent in some locales. Despite the presence of these deposits, simple eolian deposition appears incapable of creating the numerous degraded and rimless craters occurring within their limits. Nevertheless, terrestrial analyses of the Rio Cuarto craters formed into loessoid deposits demonstrate that eolian redistribution of fine-grained sediment in and around craters produces degraded morphologies that are analogous to some found in mantled regions on Mars. Greeley R.* Anderson F. S. Blumberg D. Lo E. Xu P. Sand Transport on Mars: Preliminary Results from Models Most studies of active aeolian processes on Mars have focused on dust, i.e., particles ~1 micrometer in diameter that are transported in suspension by wind. The presence of sand dunes on Mars indicates that larger grains (approximately >60 micromters, transported primarily in saltation) are also present. Although indirect evidence suggests that some dunes may be active, definitive evidence is lacking. Nonetheless, numerous studies demonstrate that sand is substantially easier to transport by wind than dust, and it is reasonable to infer that sand transportation in saltation occurs under present martian conditions. In order to assess potential source regions, transportation pathways, and sites of deposition for sand on Mars, an iterative sand transport algorithm was developed that is based on the Mars General Circulation Model of Pollack et al. The results of the dust transport model are then compared with observed surface features, such as dune field locations observed on images, and surficial deposits as inferred from Viking IRTM observations. Preliminary results suggest that the north polar dune fields in the vicinity of 270 degrees W, 70 degrees N originated from weathered polar layered plains centered at 280 degrees W, 85 degrees N, and that Thaumasia Fossae, southern Hellas Planitia, and the area west of Hellespontus Montes are sand depositional sites. Examples of transportation "corridors" include a westward pathway in the latitudinal band 35 degrees N to 45 degrees N, and a pathway southward from Solis Planum to Thaumasia Fossae, among others. Christensen P. R. Malin M. C.* A Simple Model of Clastic Sediments on Mars In preparation for the start of Mars Observer operations at Mars later this year, Viking Infrared Thermal Mapper (IRTM) observations have been synthesized into a simple but geologically coherent conceptual model for use in establishing targets for coordinated Thermal Emission Spectrometer (TES) and Mars Observer Camera (MOC) observations. The model is based on three assumptions that are, at best, only partly true: that albedo is a measure of the presence or absence of dust; that thermal inertia is a measure of the weighted average particle size; and that rock abundance is a measure of the statistical thickness of fine sediment (i.e., that the observed areal abundance of rock reflects the whole or partial burial of rocks). Using this model, it is possible to show that, on the scale of 30 km, mantles of wind- transportable sediment (dust and sand) are at most about 1 m thick, and that on a global average such mantles are about 35 cm thick. 3.8 X 10^19 cm^3 of such sediment covers Mars equatorward of +/-60 degrees latitude. Using the model, interpretation of digital maps of IRTM data focus attention not only on areas where dust is the primary sediment (e.g., Arabia Terra), but also on areas where sand is the primary sediment (e.g., eastern Valles Marineris) and where rocks and other coarse materials are abundant (e.g., eastern Kasei Vallis). Craddock R. A.* Crumpler L. S. Aubele J. C. Geologic History of Central Chryse Planitia and the Viking 1 Landing Site, Mars 1:500,000-scale geologic mapping was undertaken to synthesize the broad-scale geology of Chryse Planitia with the local geology of the Viking 1 landing site. The geology of MTMs 20047 and 25047 has been presented previously. As part of the goals for the Mars Geologic Mapping program, the rational and scientific objectives for a return mission to Chryse Planitia and the Viking 1 Lander have also been presented. However, in mapping central Chryse Planitia our principle objective was to determine the depositional and erosional history of the Chryse Planitia basin. Tuesday, March 16, 1993 Chicxulub, KT Boundary, and Other Impact Ejecta Poster Session 7:00 - 9:00 p.m. LPI Ortiz Aleman C. O. Pilkington M. Hildebrand A. R. Roest W. R. Grieve R. A. F. Keating P. Modelling the Gravity and Magnetic Field Anomalies of the Chicxulub Crater The ~180-km-diameter Chicxulub crater lies buried by ~1 km of sediment on the northwestern corner of the Yucatan Peninsula, Mexico. Geophysical, stratigraphic, and petrologic evidence support an impact origin for the structure and biostratigraphy suggests that a K/T age is possible for the impact. The crater's location is in agreement with constraints derived from proximal K/T impact-wave and ejecta deposits and its melt-rock is similar in composition to the K/T tektites. Radiometric dating of the melt rock reveals an age identical to that of the K/T tektites. The impact that produced the Chicxulub crater probably produced the K/T extinctions and understanding the now-buried crater will provide constraints on the impact's lethal effects. The outstanding preservation of the crater, the availability of detailed gravity and magnetic data sets, and the two-component target of carbonate/evaporites overlying silicate basement allow application of geophysical modeling techniques to explore the crater under most favorable circumstances. We have found that the main features of the gravity and magnetic field anomalies may be produced by the crater lithologies. Bohor B. F. Betterton W. J. Arroyo El Mimbral, Mexico, K/T Unit: Origin as Debris Flow/Turbidite, Not a Tsunami Deposit Coarse, spherule-bearing, clastic units have been discovered at 10 marine sites that span the K/T boundary in northeastern Mexico. We examined one of the best exposed sites in Arroyo el Mimbral, northwest of Tampico. The Mimbral outcrop displays a layered clastic unit up to 3 m thick enclosed by marly limestones of the Mendez (Latest Maastrichian) and Velasco (Earliest Danian) Formations. At its thickest point, this channelized clastic unit is comprised of three subunits: 1) a basal, poorly sorted, ungraded calcareous spherule bed 1 m thick containing relict impact glass and shocked mineral grains, 2) a massive set of laminated calcite-cemented sandstones up to 2 m thick with plant debris at its base, 3) capped by a thin (up to 20 cm) set of rippled sandstone layers separated by silty mudstone drapes containing a small (921 pg/g) iridium anomaly. This tripartite clastic unit is conformably overlain by marls of the Velasco Formation. We also visited the La Lajilla site east of Ciudad Victoria; its stratigraphy is similar to Mimbral's, but its clastic beds are thinner and less extensive laterally. Claeys P. Alvarez W. Smit J. Hildebrand A. R. Montanari A. KT Boundary Impact Glasses from the Gulf of Mexico Region Cretaceous-Tertiary boundary (KTB) tektite glasses occur at several sites around the Gulf of Mexico. Contrary to the rumor among KTB workers, glass fragments have been found by several researchers in the base of the spherule bed at Arroyo el Mimbral in northeast Mexico. The presence of green, red, and transparent glass fragments at Mimbral only demonstrates that the Mimbral glass is not a laboratory contamination by Beloc glass. The chemistry and ages of the glass are consistent with an origin from the Chixculub impact crater in Yucatan. No evidence supports a volcanic origin for the KTB glasses. Pope K. O. Ocampo A. C. Baines K. H. Ivanov B. A. Global Blackout Following the K/T Chicxulub Impact: Results of Impact and Atmospheric Modeling Several recent studies have suggested that shock decomposition of anhydrite (CaSO4) target rocks during the K/T Chicxulub impact would have ejected tremendous amounts of sulfur gas into the stratosphere. One of the many potential biospheric effects of this sulfur gas is the generation of a sulfuric acid (H2S04) aerosol layer capable of causing darkness and severe disruption of photosynthesis for periods of years. In this paper we report the preliminary results of our modeling of shock pressures within the anhydrites and of light attenuation by the H2S04 aerosol cloud. These models indicate that earlier studies overestimated the amount of sulfur gas produced, but that more than enough was produced to extend global blackout conditions 4-6 times longer than the ~3-month predictions for silicate dust alone. Tyburczy J. A. Ahrens T. J. Impact-induced Devolatilization of CaSO4 Anhydrite and Implications for K-T Extinctions: Preliminary Results The recent suggestions that the target area for the K-T bolide may have been a sulfate-rich evaporite and that the resulting sulfuric acid-rich aerosol was responsible for the subsequent cooling of the Earth and the resulting biological extinctions have prompted us to experimentally examine the impact- induced devolatilization of the sulfate minerals anhydrite (CaSO4) and gypsum (CaSO4 x 2H2O). Here we report preliminary results for anhydrite. Up to 42 GPa peak shock pressure, little or no devolatilization occurs, consistent with chemical thermodynamic calculations. Calculation of the influence of the partial pressure of gas species on impact-induced devolatilization suggest that an even greater amount of sulfur than that proposed by Brett could have been released to the atmosphere by an impact into a sulfate-rich layer. Chen G. Ahrens T. J. Shock-induced Reaction in Chicxulub Target Materials (CaSO4 and SiO2) and Their Relation to Extinctions The global platinum-element-rich layer, presence of shocked quartz grains (in some cases with stishovite), and the observation of a tektite-rich layer precisely at the K-T boundary are the three major arguments for the extinction bolide impact hypothesis of Alvarez et al.. Tektites (spherules) from Beloc in Haiti and Mimbral in Mexico received particular interest because of their geological proximity to the Chicxulub impact structure, which is a leading candidate for at least one of the K-T impact craters. Although in most localities the original glass has weathered to clay minerals, some shock-induced glass is found in outcrops and drill cores, which is used for 38Ar/39Ar dating. The glassy tektites have been found to be chemically similar and coeval at 65.0 Ma with Chicxulub melt rock. Boundy-Sanders S. Q. Hervig R. L. Minor and Trace Element Composition and Age of Yukon Probable-Microtektites Major, minor, and trace element composition of the candidate microtektites from Yukon Territory suggest a possible impact site of hydrothermally altered limestone and sand or chert, or possibly a carbonatite. Their REE/chondrite curve is similar in character to, but higher than, shale composites of North America, Australia, and Europe. Relative to these same composites, the Yukon droplets are enriched in Y, F, S, Sr, P, Mn, Mg, and Ca. They are depleted in Rb, Li, Th, Nb, Ti, K, Na, Fe, Si, and Al. Biostratigraphic constraints on the droplets indicate they are Middle to Late Devonian, more likely Middle Devonian, in age. Tuesday, March 16, 1993 Impact Cratering and Shock Metamorphism Poster Session 7:00 - 9:00 p.m. LPI Colwell J. E. Power-Law Confusion: You Say Incremental, I Say Differential Power-law distributions are commonly used to describe the frequency of occurrences of crater diameters, stellar masses, ring particle sizes, planetesimal sizes, and meteoriod masses, to name just a few. The distributions are simple, and this simplicity has led to a number of misstatements in the literature about the kind of power law that is being used: differential, cumulative, or incremental. Although differential and cumulative power laws are mathematically trivial, it is a hybrid incremental distribution that is often used, and the relationship between the incremental distribution and the differential or cumulative distributions is not trivial. In many cases the slope of an incremental power law will be nearly identical to the slope of the cumulative power-law of the same distribution, not the differential slope. I argue for a consistent usage of these terms and against the oft-made implicit claim that incremental and differential distributions are indistinguishable. Benz W. Ashpaug E. Explicit 3D Continuum Fracture Modeling with Smooth Particle Hydrodynamics Impact phenomena shaped our solar system. From the accretion of the planetesimals 1.6 billion years ago to the comparatively recent spallations of meteorites from their parent bodies, which take them to Earth, this ceaseless process has left no bit of solid matter untouched. Coombs C. R. Atkinson D. R. Watts A. J. Wagner J. W. Allbrooks M. K. Hennessy C. J. Modelling Hypervelocity Impacts into Aluminum Structures Based on LDEF Data Realizing and understanding the effects of the near-Earth space environment on a spacecraft during its mission lifetime is becoming more important with the regeneration of America's space program. Included among these potential effects are erosion and surface degradation due to atomic oxygen impingement, ultraviolet exposure embrittlement, and delamination, pitting, cratering, and ring formation due to micrometeoroid and debris impacts. These effects may occur synergistically and may alter the spacecraft materials enough to modify the resultant crater, star crack, and/or perforation. This study concentrates on modeling the effects of micrometeoroid and debris hypervelocity impacts into aluminum materials (6061-T6). Space debris exists in all sizes, and has the possibility of growing into a potentially catastrophic problem, particularly since self-collisions between particles can rapidly escalate the numbers of small impactors. We have examined the morphologies of the Long Duration Exposure Facility (LDEF) impact craters and the relationship between the observed impact damage on LDEF versus the existing models for both the natural (micrometeoroid) and manmade (debris) environments in order to better define these environments. Housen K. Simulation of Collisional Fragmentation with Explosives For practical reasons, experimental studies of collisional fragmentation must at times rely on explosives to fragment a target body. For example, Housen et al. (1991) described experiments in which spheres were fragmented in a pressurized atmosphere. Explosives were used because impacts could not be performed in the pressure chamber. Explosives can also be used to study targets much larger than those that can be disrupted by conventional light-gas guns, thereby allowing size- and rate-effects to be investigated. The purpose of this study is to determine the charge burial depth required to simulate various aspects of collisions. EXPLOSIVE FRAGMENATION; K. Housen Schultz P. H. Gault D. E. Impactor Control of Central Peak and Peak-Ring Formation The relation between the depth and diameter of excavation for impacts typically is assumed to be proportional. Such an assumption is consistent with the constant aspect ratio (diameter: depth) observed for simple craters found in a wide range of planetary settings and crater-scaling laws derived from laboratory experiments. Although complex craters exhibit evidence for floor uplift and rim collapse of a transient profile, they are typically thought to resemble initially smaller, simple craters. At large scales, however, early- time processes consume a greater fraction of crater growth and the assumption of late-time equivalence of energy release as a point source becomes inappropriate. We propose instead that crater diameter, depth, and impactor penetration represent separable dependent variables that underscore the fundamental difference between impact and point-source explosion excavation process. An important consequence of this perspective is that central pits, peaks, and rings may represent contrasting target responses to impactor penetration and could provide an important indicator of impactor dimensions. Evans N. J. Ahrens T. J. Shahinpoor M. Anderson W. W. Projectile-Target Mixing in Melted Ejecta Formed During a Hypervelocity Impact Cratering Event Tektites contain little to no projectile contamination while, in contrast, some distal ejecta deposits can be relatively projectile rich (e.g., the Cretaceous-Tertiary (K-T) boundary clay). This compositional difference motivated an experimental study of hypervelocity target-projectile mixing processes. We hope to scale up the results from these experiments and apply them to terrestrial impact structures like the Chicxulub Crater, Yucutan, Mexico, the leading contender as the site for the impact that caused the mass extinction that marks the K-T boundary. Shock decomposition of the +500-m thickness of anhydrite, or greater thickness of limestone, in the target rocks at Chicxulub may have been a critical mechanism for either global cooling via SO3, and subsequently H2SO4, formation or, possibly, global warming via increased CO2 formation. Understanding target-projectile mixing processes during hypervelocity impact may permit more accurate estimates of the amount of potentially toxic, target-derived material reaching stratospheric heights. Cintala M. J. Grieve R. A. F. Differential Scaling: Implications for Central Structures in Large Lunar Craters The change in morphology of central structures with crater size is an important topic in cratering studies. The amount of stratigraphic uplift (u(sub)s) in craters is a quantity that is also highly relevant to geophysics, sample studies, and remote sensing. In the case of terrestrial craters for which subsurface data exist, the value of u(sub)s can be obtained directly. Given the depth from the original target surface (d(sub)p) to the top of the central structure and an estimate of the depth of impact melting (d(sub)m), u(sub)s can be estimated for a lunar crater by subtracting d(sub)p from d(sub)m. A plot of these values for the terrestrial case yields a well-defined trend with some scatter due to erosion and difficulty in fixing the original sizes of these craters. The lunar values derived from the melt calculations and the topographic data fall to the upper part of, but well within, the terrestrial range. The degree of agreement between the two datasets indicates that d(sub)m might play an important role in influencing the amount of uplift in large impact events. Furthermore, using d(sub)m as the minimum depth of origin of the central structure implies that central peaks can be derived from substantial depths. The top of the central peak in a crater the size of Copernicus (96 km), for example, could have originated at a depth of at least 10 km. Watts A. J. Atkinson D. R. Rieco S. R. Brandvold J. B. Lapin S. L. Coombs C. R. A Fresh Look at Crater Scaling Laws for Normal and Oblique Hypervelocity Impacts With the concomitant increase in the amount of man-made debris and an ever- increasing use of space satellites, the issue of accidental collisions with particles becomes more severe. While the natural micrometeoroid population is unavoidable and assumed constant, continued launches increase the debris population at a steady rate. Debris currently includes items ranging in size from micrometers to meters that originated from spent satellites and rocket cases. To understand and model these environments, impact damage in the form of craters and perforations must be analyzed. Returned spacecraft materials such as those from LDEF and Solar Max have provided such a testbed. From these space-aged samples various impact parameters (i.e., particle size, particle and target material, particle shape, relative impact speed, etc.) may be determined. These types of analyses require the use of generic analytic scaling laws that can adequately describe the impact effects. Currently, most existing analytic scaling laws are little more than curve-fits to limited data and are not based on physics, and thus are not generically applicable over a wide range of impact parameters. During this study, we have generated a series of physics-based scaling laws for normal and oblique crater and perforation formation into two types of materials: aluminum and Teflon. Yanagisawa M. Itoi T. Antipodal Fragment Velocities for Porous and Weak Targets at Catastrophic Impacts Mortar, porous alumina, and sand targets, which were spherical in shape, 11-15 cm in diameter, were impacted normally by plastic (polycarbonate) projectiles of nearly 1 g in mass at velocities about 6 km/s. Fragment velocity at the antipole of impact site (antipodal velocity, V(sub)a), for each experiment, was obtained from two Flash X-ray images recorded prior to and at a predetermined delayed time after the impact event. It has been revealed that the velocities for the same E/M(sub)t (impact energy divided by target mass) depend strongly on target material, and differ about an order of magnitude between the sand and basalt. Reimold W. U. Le Roux F. G. Koeberl C. Shirey S. B. Kalkkop Crater, Eastern Cape--A New Impact Crater in South Africa In 1992, Reimold et al. suggested that the 640-m-diameter Kalkkop crater, located at 32 degrees 43' S and 24 degrees 34' E in the Eastern Cape Province of South Africa, could possibly be of impact origin. This idea was based on the circular appearance of this structure, its regional uniqueness, complete lack of recent igneous activity in the region, and descriptions of drillcore indicating that the crater is not underlain by a salt dome and is partially filled with a breccia layer of a thickness that would agree with the dimensions expected for an impact structure of this size. Unfortunately the old drillcore was no longer available for detailed study, and in the absence of sufficient surface exposure only drilling could provide the evidence needed to solve the problem of the origin of Kalkkop. For this reason and to study the crater fill from a paleoevironmental point of view, the Geological Survey of South Africa decided to sponsor a new research drilling project at the Kalkkop site. We are now able to present first petrographic and isotopic results from the Kalkkop drillcore studies that confirm, without doubt, that this crater is of impact origin. Storzer D. Koeberl C. Reimold W. U. The Age of the Pretoria Saltpan Impact Crater, South Africa The Pretoria Saltpan impact crater, situated about 40 km NNW of Pretoria, South Africa, has a diameter of about 1.13 km. The structure was formed in 2.05 Ga Nebo granite of the Bushveld Complex. The impact origin of the crater was recently established by the discovery of characteristic shock-metamorphic features in breccias found in drill cores at depths >90 m. Impact glass fragments were recovered by standard magnetic separation techniques and handpicking from the melt breccias. As no reliable crater age was known so far, several hundred submillimeter-sized glass fragments were studied for fission tracks. The results show that the Saltpan impact crater has an age of 220+-52 ka, which is in agreement with field geological observations. Wong A. M. Reid A. M. Hall S. A. Sharpton V. L. Characterization of the Marquez Dome Buried Impact Crater Using Gravity and Magnetic Data The buried impact crater, Marquez Dome, located in Leon County in east central Texas, is an approximately 15-km-diameter structure whose central uplift is now partially exposed due to headward erosion of the post-impact cover. The central uplift is approximately 3 km in diameter and the rocks within it have been uplifted more than 1200 m above their regional level. The crater rim remains buried and previous attempts to determine its location have had to rely on seismic reflection data and geologic well logs. These attempts have been somewhat successful in mapping the extent of the disturbed zone around Marquez Dome, but more limited in their ability to image the shallow buried rim. In an attempt to define accurately the whole Marquez Dome structure and assist in the selection of drilling sites, a geophysical investigation involving gravity and magnetic data over the central uplift and the surrounding area has been undertaken. Xue S. Herzog G. F. Hall G. S. Klein J. Middleton R. Juenemann D. Stable Ni Isotopes and 10Be and 26Al in Metallic Spheroids from Meteor Crater, Arizona The Canyon Diablo spheroids, which are found around Meteor Crater, Arizona, are nickel-enriched objects with diameters from <0.1 to several millimeters. Previous studies have suggested that the enrichment of nickel resulted either from shock-melting of S-rich areas followed by solidification of the liquids under strongly nonequilibrium conditions at rapid cooling rates during flight outward from the crater or from the selective oxidation of iron. Isotopic studies are an effective tool for constraining the degree of open-system evaporation experienced by a system. The purpose of this study was to see whether Ni isotopes had been fractionated by volatilization during spheroid formation; the degree of fractionation observed was quite small. In addition, the cosmogenic nuclides 10Be and 26Al were measured to try to estimate the depths in the parent meteorite from which the spheroids came. The low activities measured indicate either deep burial in the parent or losses of Be and Al during spheroid formation. Tonks W. B. Melosh H. J. Core Formation by Giant Impacts: Conditions for Intact Melt Region Formation Among the many effects of high-speed, giant impacts is widescale melting that can potentially trigger catastrophic core formation. If the projectile is sufficiently large, the melt pools to form an intact melt region. The dense phase then segregates from the melt, forming a density anomaly at the melt region's base. If the anomaly produces a differential stress larger than a certain minimum, it overcomes the mantle's long-term elastic strength and rapidly forms a core. We previously showed that giant impacts effectively trigger core formation in silicate bodies by the time they grow to the mass of Mercury and in icy bodies by the time they grow larger than Triton (Tonks and Melosh 1991, 1992 a,b). In order for this process to be viable, an intact melt region must be formed. We now examine in more detail than previously published conditions under which this occurs. Hartmann W. K. Gaskell R. W. Confirmation of Saturation Equilibrium Conditions in Crater Populations We have continued work on realistic numerical models of cratered surfaces, as first reported at last year's LPSC. We confirm the saturation equilibrium level with a new, independent test. One of us (RWG) has developed a realistic computer simulation of a cratered surface. The model starts with a smooth surface or fractal topography, and adds primary craters according to the cumulative (or log- incremental) power law with exponent -1.83, as observed on lunar maria and martian plains. Each crater has an ejecta blanket with the volume of the crater, feathering out to a distance of 4 crater radii. Chyba C. F. Collisions of Small Spacewatch Asteroids with the Earth Rabinowitz et al. have reported the discovery, with the Spacewatch Telescope, of eight Earth-approaching objects smaller than 100 m in diameter. I have calculated the probability and velocity of collision with Earth for each of these objects, using Opik's equations, including terms for Earth's eccentricity. Using a code we have successfully employed to model the Tunguska and Revelstoke atmospheric explosions, I have simulated the entry of these objects into Earth's atmosphere, assuming iron, stony, carbonaceous, and cometary compositions. The smallest of these objects (with tens to hundreds of kilotons of kinetic energy) pose a substantial threat at the surface only if they are iron objects. (An object is taken to "pose a substantial threat" if it either craters the ground, or explodes in the atmosphere with sufficient energy at low enough altitude to create an overpressure at the surface capable of felling trees or destroying buildings.) Larger objects, with energies greater than about a megaton, devastate the surface regardless of whether they are of iron, stony, or carbonaceous composition. Iron objects crater the ground, whereas stony and carbonaceous objects explode in the atmosphere low enough to fell trees and damage buildings over thousands of square kilometers. Spacewatch objects, if presumed to be of carbonaceous composition, are as dangerous as their stony counterparts, as the former objects' lower yield strengths (and hence, higher explosion altitudes) are roughly compensated for by their larger masses (as derived from their lower albedos for a given observed magnitude). Although comets are intrinsically less dangerous than asteroids, the 90-m-diameter Spacewatch objects devastate hundreds of square kilometers at the surface even if cometary. Melosh H. J. Artemjeva N. A. Golub A. P. Nemchinov I. V. Shuvalov V. V. Trubetskaya I. A. Remote Visual Detection of Impacts on the Lunar Surface Collisions of small meteoroids or comets with the terrestrial planets and their satellites are an important factor in the evolution of the solar system. Moderate size impacts on the Earth may even present a hazard to human civilization. Further information about the composition and size-velocity distribution of such objects is thus of great interest. We propose a novel method of remotely observing impacts on the airless Moon that may extend the present database on meteoroids down to 1 m in diameter. Meteorites or comets of radius r(sub)0 ~ 1-100 m are burnt away or dispersed in the atmospheres of the Earth and Venus. However, when such objects strike the Moon they deposit their energy in a small initial volume, forming a plasma plume whose visible and infrared radiation may be visible from the Earth. Wieczorek M. A. Mendell W. W. Degradation Sequence of Young Lunar Craters from Orbital Infrared Survey Using new software, nighttime thermal maps of the lunar surface have been generated from data obtained by the Apollo 17 Infrared Scanning Radiometer (ISR) in lunar orbit. Most of the thermal anomalies observed in the maps correspond to fresh lunar craters because blocks on the lunar surface maintain a thermal contrast relative to surrounding soil during the lunar night. Craters of Erastosthenian age and older - relatively young by lunar standards - have developed soil covers that make them almost indistinguishable from their surroundings in the thermal data. Thermal images of Copernican age craters show various stages of a degradation process, allowing the craters to be ranked by age. The ISR data should yield insights into lunar surface evolution as well as a more detailed understanding of the bombardment history after formation of the great mare basins. Tuesday, March 16, 1993 Differentiated Meteorites Poster Session 7:00 - 9:00 p.m. LPI Drake M. J. Owen T. Swindle T. Musselwhite D. Noble Gas Evidence of an Aqueous Reservoir Near the Surface of Mars More Recently than 1.3 Ga Considerable evidence points to a martian origin of the SNC meteorites. One of these meteorites, Nakhla, contains a leachable component that has an elevated ^l29Xe/^l32Xe ratio relative to its ^84Kr/^l32Xe ratio when compared to the approximately linear array defined by Chassigny, most shergottites, and lithology C of EETA 79001. This array is thought to be a mixing line between martian mantle and martian atmosphere. The leachable component probably consists in part of iddingsite, an alteration product produced by interaction of olivine with aqueous fluid at temperatures lower than 150 degrees C. The radiogenic Xe component may represent a distinct reservoir in the Martian crust or mantle. More plausibly, it is martian atmosphere, fractionated by solution in liquid water and by interaction with sediment. The crystallization age of Nakhla is 1.3 Ga. Its low shock state suggests that it was ejected from near the surface of Mars. Liquid water is required for the formation of iddingsite. These observations provide further evidence for the near surface existence of aqueous fluids more recently than 1.3. Wentworth S. J. Gooding J. L. Weathering Features and Secondary Minerals in Antarctic Shergottites ALHA77005 and LEW88516 The shergottite meteorites Allan Hills, Antarctica, A77005 (ALHA77005) and Lewis Cliffs, Antarctica, 88516 (LEW88516) are known to be closely similar in their igneous petrological and geochemical characteristics. Comparative SEM/EDS studies suggest greater contrast in their contents of secondary minerals; aqueous alteration appears to be more diverse in ALHA77005 than in LEW88516. Fusion crusts of both meteorites bear sulfates of Antarctic origin. The interior of LEW88516 contains traces of Na- and Ca-sulfates and a S,Cl-bearing iron silicate "rust"; the interior of ALHA77005 is distinguished by K,Fe-sulfate, a discrete low- Al silicate clay, free silica, and an unusual Mg, Fe-phosphate. Agerkvist D. P. Vistisen L. Mossbauer Spectroscopy of the SNC Meteorite Zagami We have performed Mossbauer spectroscopy on two different pieces of the meteorit belonging to the group of SNC meteorites. In one of the samples we found a subst of olivine intergrown with one kind of pyroxene, and also another kind of pyroxe the pyroxene in the other sample we examined. Both samples showed less than 1% o silicate phase. Fisher D. S. Burns R. G. Cronstedtite and Iron Sulfide Mineralogy of CM-type Carbonaceous Chondrites from Cryogenic Mossbauer Spectra Determinations of oxidation states and the crystal chemistry of iron- bearing minerals in CM meteorites by Mossbauer spectroscopy are complicated by thermally induced electron hopping in cronstedtite and by ill-defined contributions from the hydrous iron sulphide phase believed to be tochilinite. Mossbauer spectral measurements at 30 K of several cronstedtite and tochilinite specimens have enabled modal proportions of these minerals, as well as Fe3+/Fe2+ ratios, to be determined quantitatively for a suite of CM-type carbonaceous chondrites that included Murchison, Murray, Cold Bokkeveld, ALH 83100 and LEW 90500. Brearley A. J. Carbonaceous Chondrite Clasts in the Kapoeta Howardite A petrographic and mineralogical study of a number of carbonaceous chondrite clasts in the Kapoeta howardite has been carried out. Most of the clasts have mineralogical and chemical properties that link them to the CM carbonaceous chondrites. Some clasts contain chondrules that often have well-developed fine-grained rims, but many have been extensively brecciated. PCP-rich objects are common and pentlandite and pyrrhotite also occur. Calcite has also been found. The remainder of the clasts are extremely fine-grained and appear to be closely related to CI carbonaceous chondrites. In these clasts magnetite framboids are common and fine-grained sulfides and magnetite occur disseminated throughout the matrix. Nazarov M. Brandstatter F. Kurat G. Carbonaceous Xenoliths from the Eravan Howardite Preliminary studies of the Erevan howardite showed that the meteorite is a polymict breccia. Here we report on our study of CM-type carbonaceous xenoliths. All of these clasts are enriched in tochilinite and carbonate inclusions as compared to CM chondrites. They also contain a new P-rich sulphide beside pentlandite. The P-rich sulphide represents a new type of P-bearing phase. It indicates a chalcophile behaviour of P under certain nebular conditions. Buchanan P. C. Zolensky M. E. Reid A. M. Barrett R. A. EET87513 Clast N: A CM2 Fragment in an HED Polymict Breccia Xenoliths of material resembling carbonaceous chondrites have been found in several HED polymict breccias (e.g., Kapoeta, Bholghati). Most workers have concluded that these clasts are related to CM2 meteorites on the basis of texture, bulk composition, and mineralogy. The present study reports data on clast N, a carbonaceous chondrite fragment from the howardite EET87513, large enough (~4 x 5 mm on the surface of the slab from which it was separated) to extract bulk samples for INAA and oxygen isotope analysis and to provide a thin section for electron microprobe, SEM, and TEM analysis. Preliminary data for this clast have been reported in Buchanan et al. and Zolensky et al. INAA was performed at Oregon State University by Y.-G. Liu and R. A. Schmitt and bulk oxygen isotopic composition was determined at the University of Chicago by T. K. Mayeda and R. N. Clayton. These data confirm that EET87513 clast N is a fragment of CM2 material. Petaev M. I. Lamellar Olivine in the Divnoe Achondrite: Evidence for High-Pressure Exsolution? The olivine-rich Divnoe achondrite contains numerous large olivine grains that have a lamellar or banded appearance in backscattered electron images, caused by minor compositional differences. One such grain, viewed in transmitted light, displays a system of lamellae with the same orientation and scale as the compositional banding. The only process known to produce such structure and chemical variability in olivine grains is high-pressure transformations between alpha, beta and gamma olivines, but in other meteorites and in experimental products the structure is ~100 times finer than the Divnoe lamella. Grady M. M. Pillinger C. T. EUROMET Ureilite Consortium: A Preliminary Report on Carbon and Nitrogen Geochemistry The first Euromet expedition to the Frontier Mountain in Antarctica in December 1990 recovered two ureilites, FRO 90036 (34.6 g) and FRO 90054 (17.5 g). Preliminary classification indicated that the specimens had very different textures and mineral chemistries, and hence were not paired. A third ureilite, Acfer 277 (41.0 g) has also recently been returned from the Sahara. Due to the small sample sizes of the meteorites, and the unusual mineralogy of FRO 90054, a consortium was established to ensure the most effective study of these samples; this abstract reports on the carbon and nitrogen stable isotope geochemistry of two of the three ureilites issued to the consortium. Petaev M. I. Clarke R. S. Jr. Olsen E. J. Jarosewich E. Davis A. M. Steele I. M. Lipschutz M. E. Wang M.-S. Clayton R. N. Mayeda T. K. Wood J. A. Chaunskij: The Most Highly Metamorphosed, Shock-modified and Metal-rich Mesosiderite The 1990 g Chaunskij meteorite was found in 1985 and classified as an anomalous ungrouped iron. It contains ~10 vol% mono- and polymineralic troilite-phosphate-silicate inclusions, micrometers to centimeters in size. In Petaev et al. (1992) we proposed its affinity with mesosiderites; here we present mineralogical, chemical, and isotopic data establishing that Chaunskij is the most highly metamorphosed, shock-modified and metal-rich of the mesosiderites. The most striking manifestation of metamorphism in Chaunskij is the presence in it of a cordierite-bearing assemblage substituting for basalt lithology. Tuesday, March 16, 1993 Techniques and Experimental Studies Poster Session 7:00 - 9:00 p.m. LPI Kubicki J. D. Stolper E. M. Evaporation Kinetics of Mg2SiO4 Crystals and Melts from Molecular Dynamics Simulations Computer simulations based on the molecular dynamics (MD) technique have been used to study the mechanisms and kinetics of free evaporation from crystalline and molten forsterite (i.e., Mg2SiO4) on an atomic level. The interatomic potential employed for these simulations reproduces the energetics of bonding in forsterite and in gas-phase MgO and SiO2 reasonably accurately. Results of the simulation include predicted evaporation rates, diffusion rates, and reaction mechanisms for Mg2SiO4(s or l)-->2Mg(g) + 2O(g) + SiO2(g). Borizov A. Palme H. Spettel B. The Solubility of Gold in Silicate Melts: First Results The effects of oxygen fugacity and temperature on the solubility of Au in silicate melts were determined. Pd-Au alloys were equilibrated with silicate of anorthite-diopside eutectic composition at different T-fO2 conditions. The behaviour of Au was found to be similar to that of Pd reported recently. Au solubilities for alloys with 30 to 40 atom% Au decrease at 1400 degrees C from 12 ppm in air to 160 ppb at a log fO2 = -8.7. The slope of the log(Me-solubility) vs. log(fO2) curve is close to 1/4 for Au and the simultaneously determined Pd, suggesting a formal valence of Au and Pd of 1+. Near the IW buffer Pd and Au solubilities become even less dependent on fO2, perhaps reflecting the presence of some metallic Au and Pd. Boesenberg J. S. Delaney J. S. Preliminary Results of Mn Partitioning Experiments on Murchison Analogues INTRODUCTION: Eucrites, howardites, and diogenites have Fe/Mn ratios between 30 and 45, while carbonaceous chondrites have much higher values between 90 and 150. Stolper first showed that basaltic achondrites could evolve from a precursor chondritic material through simple partial melting. These experiments indicated that chondritic material heated to temperatures near 1180 degrees C with a fugacity of one log unit below the iron-wustite buffer curve, produced a eucritic mineralogy that contained olivine, pigeonite, plagioclase, spinel, glass, and metal. The partial melting experiments of Jurewicz et al. on anhydrous Murchison and Allende also showed that HED compositions were produceable at temperatures between 1130 degrees C and 1325 degrees C with fugacities below and above the iron-wustite buffer curve. However, the MnO abundances of Jurewicz et al. were too low to produce suitable Fe/Mn ratios for HEDs. We present below our results of partial melting experiments on Murchison analogues that involved temperatures between 1180 degrees C and 1580 degrees C and fugacities below the iron-wustite buffer curve. Our experiments resulted in MnO abundances nearly twice that of Jurewicz et al. and indicate that the production of basaltic achondrite-like Fe/Mn ratios from precursor chondritic material are possible. Jurewicz S. R. Jones J. H. Experimental Partitioning of Zr, Ti, and Nb Between Silicate Liquid and a Complex Noble Metal Alloy and the Partitioning of Ti Between Perovskite and Platinum Metal Experiments were performed to determine the partitioning behavior of Zr, Nb, and Ti between silicate liquid and: 1) pure Pt-metal in an Fe-Free system and 2) a Os-Pt-Ru-Ir complex alloy, both at approximate solar nebula conditions. The results suggest that neither system produces a Zr partition coefficient large enough to explain observed "percent" levels in some RPMNs, while Nb and Ti partition coefficients suggest that these elements should be abundant in noble metal nuggets. Lithophile elements in RPMNs are probably not the result of silicate/metal equilibrium at solar nebula conditions. Sutton S. R. Delaney J. Bajt S. Rivers M. L. Smith J. V. Microanalysis of Iron Oxidation State in Iron Oxides Using X-ray Absorption Near Edge Structure (XANES) An exploratory application of X-ray absorption near edge structure (XANES) analysis using the synchrotron X-ray microprobe was undertaken to obtain Fe XANES spectra on individual submillimeter grains in conventional polished sections. The experiments concentrated on determinations of Fe valence in a suite of iron oxide minerals for which independent estimates of the iron speciation could be made by electron microprobe analysis and X-ray diffraction. Tuesday, March 16, 1993 Terrestrial Studies Poster Session 7:00 - 9:00 p.m. LPI Liu Y.-G. Schmitt R. A. Earth's Partial Pressure of CO2 Over the Past 120 Ma; Evidence from Ce Anomalies in the Deep (>600 m) Pacific Ocean, 1 We have found that Ce serves as a chemical tracer of paleo-oceanic redox conditions. It has been shown that the unoxidized and soluble Ce3+ in modern sea water exhibits a negative anomaly relative to the other soluble REE3+. We derived an expression of soluble Ce3+ in sea water that was 1900X greater than the average observed Ce in 600-5000 m Pacific sea water. Since Ce(CO3) + and Ce(CO3)2 complexes greatly exceed the Ce(PO4) complexes in sea water, we have followed formulations using carbonate complexes and have found that the calculated Ce and observed concentrations in the deep 600-5000 m Pacific Ocean agree within the uncertainties of the thermodynamic data. As expected, the calculated Ce concentrations are a strong function of pH and found to be lesser functions of C03 activities. Niedermann S. Eugster O. Hofmann B. Thalmann Ch. Reimold W. U. Dating Native Gold by Noble Gas Analyses Our recent work on He, Ne, and Ar in Alpine gold samples has demonstrated that gold is extremely retentive for He and could thus, in principle, be used for U/Th-4He dating. For vein-type gold from Brusson, Northern Italy, we derived a U/Th-4He age of 36 Ma, in agreement with the K-Ar formation age of associated muscovites and biotites. However, in placer gold from the Napf area, Central Switzerland, we observed large excesses of both 4He and radiogenic 40Ar (40Ar(sub)rad, defined as 40Ar - 295.5 36Ar). The gas release systematics indicate two distinct noble gas components, one of which is released below about 800 degrees C and the other one at the melting point of gold (1064 degrees C). We now present results of He and Xe measurements in a 1-g placer gold sample from the river Kruempelgraben, as well as He and Ar data for Brusson vein-type gold and for gold from the Lily Gold Mine, South Africa. We calculate reasonable U/Th-4He as well as U-Xe ages based on those gases that are released at <=800 degrees C. Probably the low-temperature components represent in situ-produced radiogenic He and fission Xe, whereas the gases evolving when gold melts have been trapped during gold formation. Therefore only the low-temperature components are relevant for dating purposes. Kurosawa M. Yurimoto H. Sueno S. Water in Earth's Mantle: Hydrogen Analysis of Mantle Olivine, Pyroxenes and Garnet Using the SIMS Hydrogen (or water) in the Earth's interior plays a key role in the evolution and dynamics of the planet. However, the abundance and the existence form of the hydrogen have scarcely been clear in practice. Hydrogen in the mantle was incorporated in the interior during the formation of the Earth. The incorporated hydrogen was hardly possible to concentrate locally inside the Earth considering its high mobility and high reactivity. The hydrogen, preferably, could be distributed homogeneously over the mantle and the core by the subsequent physical and chemical processes. Therefore, hydrogen in the mantle could be present in the form of trace hydrogen in nominally anhydrous mantle minerals. In this study, we have determined the hydrogen and the other trace elements in mantle olivines, orthopyroxenes, clinopyroxenes, and garnets using secondary ion mass spectrometry (SIMS) for elucidating(1) the exact hydrogen content, (2) the correlation between hydrogen and other trace elements, (3) the dependence of hydrogen content on depth, and (4) the dependence of the whole rock water content on depth. Snyder G. A. Jerde E. A. Taylor L. A. Halliday A. N. Sobolev V. N. Sobolev N. V. Clayton R. N. Mayeda T. K. Deines P. Primary Differentiation in the Early Earth: Nd and Sr Isotopic Evidence from Diamondiferous Eclogites for Both Old Depleted and Old Enriched Mantle, Yakutia, Siberia Ancient, stable, continental cratons possess thick, subcontinental-lithosphere mantle "keels" that favor particularly the emplacement of diamondiferous kimberlites, and included peridotites and eclogites. These refractory mantle samples of the roots provide hard constraints on the theories of formation, growth, and evolution of these cratons. Xenoliths containing only primary garnet and clinopyroxene (eglogites), although rare in most kimberlites, can retain the geochemical signatures of their parent protoliths (e.g., subducted oceanic crust, ancient mantle), thus offering the opportunity to address mantle processes that may have taken place at earlier times in the Earth's history. In fact, it has been postulated that some eclogites are residues from the accretion of the early Earth. Nd and Sr isotopic data are presented that may be interpreted as evidence of an early (>4 Ga) mantle differentiation event. Tuesday, March 16, 1993 Cosmic Ray Effects, Ion Implantation, and Orbits Poster Session 7:00 - 9:00 p.m. LPI Masarik J. Reedy R. Effects of Bulk Composition on Production Rates of Cosmogenic Nuclides in Meteorites The bulk chemical composition of meteorites has been suggested as a main factor influencing the production of cosmogenic nuclides. Numerical simulations with Los Alamos Monte Carlo production and transport codes were done for Ne22/Ne21 ratios and Ar38 production rates in meteorites with a wide range of compositions. The calculations show that an enhanced flux of low- energy secondary particles in metal-rich phases is the essential key for the explanation of experimentally observed differences in nuclide production processes in various meteorite classes. Wacker J. F. Aluminum-26 Activities in Meteorites We report 26Al activities for 686 Antarctic meteorites and 46 non- Antarctic meteorites. The data set includes new results for 253 Antarctic and 5 non-Antarctic meteorites. Most of the Antarctic specimens were collected from the Allan Hills region, but specimens from other regions have been measured as well. The non-Antarctic specimens include freshly fallen meteorites, in which short-lived radionuclides (e.g., 22Na, 54Mn, 57Co, etc.) were also measured. The data have been analyzed for terrestrial age distributions, identification of samples with unusual activities, pairing of specimens, and comparison of Antarctic and non-Antarctic activity distributions. Michlovich E. Elmore D. Vogt S. Lipschutz M. Masarik J. Reedy R. C. 26Al Production Profile and Model Comparisons in Canyon Diablo The large preatmospheric size of the Canyon Diablo meteorite, a radius of about 15 m, makes it suitable for systematic studies of cosmogenic nuclide production rates of iron objects in a 2-pi geometry. To reconstruct the exposure history of the meteoroid, Heymann et al. investigated several fragments recovered from known geographic locations for their shock features and cosmogenic noble gases. They applied the Signer-Nier noble gas production rate model to establish the preatmospheric depth of the specimens in the meteoroid. Cosmic ray exposure ages suggested a multi-episodic irradiation, with 170 or 540 Ma being inferred for most of the samples studied while two anomalous specimens indicated a possible third exposure age at 940 Ma. Be-10 and Cl-36 have been measured in a number of these same samples by accelerator mass spectrometry (AMS), with use being made of the preatmospheric depths determined in Heymann et al., to construct production profiles. The present study extends the cosmogenic radionuclide data to Al-26 and compares the results with both the production rate model of Reedy and Arnold and production rates determined from the cross sections used by the Reedy-Arnold model (for the major nuclear reactions making Al-26) in combination with differential fluxes calculated using the Los Alamos High Energy Transport (LAHET) Code System. Model calculations for Be-10 and Cl-36 have also been obtained and will be presented. All AMS measurements were made at the PRIME Lab facility at Purdue University. Shima M. Okada A. Nagao K. The Chondrite Mihoneseki--New Observed Fall On December 10, 1992, 21:00 hours Japanese standard time, a stone weighing 6.385 kg, struck the two-storied house in Mihonoseki-machi, Yatsuka-gun, Shimane-ken, Japan, 35 degrees 34.1' N., 133 degrees 13.2'E. By petrographical and mineralogical examination and rare gas analyses, the meteorite was classified as L6 chondrite. The chondrite is rather small preatmospheric size, (22Ne/21Ne = 1.180, extremely low 60Co activity) and shocked feature is not distinct. Cosmic-ray exposure ages obtained from 3He, 21Ne, and 38Ar are 61 m.y., and 40K-40Ar age is 4.41 b.y. Pedroni A. Abundance and Composition of Solar Kr in the H3-H6 Chondrite Acfer111 He/Ne, Ne/Ar, and Ar/Kr abundance ratios of solar gases extracted by stepped heating, stepped oxidation, and stepped etching of lunar and meteoritic regoliths are significantly lower than ratios measured directly or predicted by model estimates. Of these, the differences in the He/Ne and Ne/Ar ratios are explained to be owing to diffusive fractionation losses from the host minerals. In contrast, it remains controversial if the Ar/Kr and Kr/Xe ratios were fractionated prior to or after the implantation of the gases into the minerals. In the H3-H6 chondritic regolith breccia Acfer 111, measured He/Ne and Ne/Ar ratios appear to be of nearly unfractionated solar composition. The Ar/Kr ratio of Acfer 111 might thus be also unfractionated. We examined by stepped etching a metal sample of Acfer 111 and obtained an average solar 36Ar/84Kr = 3150 + 300, which is in agreement with the model predictions. The isotopic composition of solar Kr was observed to change in the course of the etching in a way very similar to that reported for lunar ilmenites by the Zurich group. This can be interpreted as a change of the mixing ratio of solar wind (SW) and solar energetic particles (SEP). The isotopic composition of the SEP component obtained from our Kr data, however, is distinct from that reported by the Zurich group. Sugiura N. Futagami T. Nagai S. Implantation of Nitrogen: Effects of Hydrogen and Implantation Energy To solve the questions on solar nitrogen in lunar soils, i.e., variation in isotopic composition and apparently high retentivity compared with rare gases, nitrogen implantation experiments were conducted. At the Meteoritical Society Meeting in Copenhagen, we presented the results of stepped combustion of implanted nitrogen in ilmenite and olivine. The degassing behavior of nitrogen (and also Ar) was quite different from that observed in the case of lunar soils). Extraction temperatures are higher (>1100 C for ilmenite and 1500 C for olivine) than that for lunar soils). Both nitrogen and Ar seem to be retained at the same efficiency. Therefore, additional experiments were conducted to make degassing behavior of nitrogen closer to that observed in the case of lunar soils. Ponganis K. V. Graf Th. Marti K. Low-Energy Ion Implantation: Large Mass Fractionation of Argon Trapped argon acquired by low-energy implantation (<=100 eV) into solids is strongly mass fractionated (>=3%/amu). This has potential implications for the origin and evolution of terrestrial planet atmospheres. Benoit P. H. Sears D. W. G. Meteorites from Recent Amor-type Orbits Observations of observed falls (including three photographed falls) have shown that most meteorites derive from meteoroids in orbits similar to those of Earth-crossing (Apollo) asteroids with perihelia close to 1 AU. We report the discovery of a recent meteorite shower in Antarctica, the members of which have very high natural thermoluminescence levels. It is apparent from these data that (1) the shower has been on Earth only a short time (terrestrial age ~1000 years) and (2) the meteorite probably came to Earth through rapid (<10^5 years) evolution from an orbit with perihelion >1.1 AU, similar to Amor asteroids. Only a very small number of meteorites, including a few modern falls, appear to have had similar orbital histories. Jackson A. A. Zook H. A. Orbital Evolution of Dust from Comet Schwassmann-Wachmann 1: A Case of One-to-One Resonance Trapping In a recent study, we have modeled the orbital evolution of dust particles released from comets and asteroids in the solar system. The source bodies were either asteroids inside Jupiter's orbit or comets from the Jupiter family of comets. However there are other dust producing parent bodies in the solar system of interest, one of these is comet P/Schwassman-Wachmann 1. Since comet Schwassman-Wachmann 1 has an orbit outside of Jupiter's orbit, is an active dust producer and has low eccentricity, dust particle evolution from it is of interest. We report on a particular 2 micron radius particle that captured into a 1 to 1 mean motion resonance orbit with Saturn. Tuesday, March 16, 1993 Remote Sensing/Space Weathering Poster Session 7:00 - 9:00 p.m. LPI Clark B. E. Fanale F. P. Robinson M. S. Simulation of Possible Regolith Optical Alteration Effects on Carbonaceous Chondrite Meteorites As the spectral reflectance search continues for links between meteorites and their parent body asteroids, the effects of optical surface alteration processes need to be considered. We present the results of an experimental simulation of the melting and recrystallization that occurs to a carbonaceous chondrite meteorite regolith powder upon heating. As done for the ordinary chondrite meteorites we show the effects of possible parent body regolith alteration processes on reflectance spectra of carbonaceous chondrites (CCs). For this study six CCs of different mineralogical classes were obtained from the Antarctic Meteorite Collection: two CM meteorites, two CO meteorites, one CK, and one CV. Each sample was ground with a ceramic mortar and pestle to powders with maximum grain sizes of 180 and 90 micrometers. The reflectance spectra of these powders were measured at RELAB (Brown University) from 0.3 to 2.5 micrometers. Following comminution, the 90-micrometer grain size was melted in a nitrogen controlled-atmosphere fusion furnace at an approximate temperature of 1700 C. The fused sample was immediately held above a flow of nitrogen at 0 C for quenching. Following melting and recrystallization, the samples were reground to powders and the reflectance spectra were remeasured. Mustard J. F. Sunshine J. M. Pieters C. M. Hoppin A. Pratt S. F. From Minerals to Rocks: Toward Modeling Lithologies with Remote Sensing High spectral resolution imaging spectroscopy will play an important role in future planetary missions. Sophisticated approaches will be needed to unravel subtle, superimposed spectral features typically of natural systems, and to maximize the science return of these instruments. Carefully controlled laboratory investigations using homogeneous mineral separates have demonstrated that variations due to solid solution, changes in modal abundances, and the effects of particle size are well understood from a physical basis. In many cases, these variations can be modeled quantitatively using photometric models, mixing approaches, and deconvolution procedures. However, relative to the spectra of individual mineral components, reflectance spectra of rocks and natural surfaces exhibit a reduced spectral contrast. In addition, soils or regolith, which are likely to dominate any natural planetary surface, exhibit spectral properties that have some similarities to the parent materials, but due to weathering and alteration, differences remain that cannot yet be fully recreated in the laboratory or through mixture modeling. A significant challenge is therefore to integrate modeling approaches to derive both lithologic determinations and include the effects of alteration. We are currently conducting laboratory investigations in lithologic modeling to expand upon the basic results of previous analyses with our initial goal to more closely match physical state of natural systems. The effects of alteration are to be considered separately. Pieters C. M. Mustard J. F. Pratt S. F. Sunshine J. M. Hoppin A. Visible-Infrared Properties of Controlled Laboratory Soils Almost all surfaces available for remote observation consist of particulate materials, or soils. The distribution of mean particle sizes depend on the original material and physical and chemical processes that have acted on the surface over time. It is well known that the optical and infrared spectral properties of materials depends on the particle size. There has been little detailed study, however, of natural soils, namely particulate materials with a range of particle sizes. Current models for intimate mixing typically use an average particle size in calculations and are most successful when the particle size is constrained by known sieve tractions. Reported here are preliminary results of a study in which soils were prepared with a known composition and range of particle sizes. This discussion presents the overall visible-to-infrared properties of these synthetic soils and evaluates the mid- infrared properties. A companion paper discusses the optical properties and information extraction from modeling lithologic mixtures. Wilson L. Parfitt E. A. Formation of Perched Lava Ponds on Basaltic Volcanoes: Interaction Between Cooling Rate and Flow Geometry Allows Estimation of Lava Effusion Rates Perched lava ponds are infrequent but distinctive topographic features formed during some basaltic eruptions. Two such ponds, each 150 m in diameter, formed during the 1968 eruption at Napau Crater and the 1974 eruption of Mauna Ulu, both on Kilauea Volcano, Hawaii. Each one formed where a channelised, high-volume flux lava flow encountered a sharp reduction of slope: the flow spread out radially and stalled, forming a well-defined terminal levee enclosing a nearly circular lava pond. We describe a model of how cooling limits the motion of lava spreading radially into a pond and compare this with the case of a channelised flow. The difference in geometry has a major effect, such that the size of a pond is a good indicator of the volume flux of the lava forming it. Lateral spreading on distal shallow slopes is a major factor limiting the lengths of lava flows. Tuesday, March 16, 1993 Solar Systems Origin Poster Session 7:00 - 9:00 p.m. LPI Ruzmaikina T. V. Khatuncev I. V. Konkina T. V. Formation of the Low-Mass Solar Nebula The reconstruction of the solar nebula by adding lost volatiles (mostly H and He composition results in the minimum mass about 0.01 to 0.07 solar masses and angu g cm^2/s. Opaque for the thermal IR-radiation circumstellar disks around many T between lO^-2 and 1O^-1 solar masses and radii between 10 and 10^2 AU. The angul disks ranges from l0^5l to 10^52 g cm^2/s. Even more extended disks have a compa if they have a low mass. Thus a keplerian disk about HL Tau, whose mass is estim mass, has sn angular momentum of about 10^53 g cm^2/s, provided that its radius remarkably less if only the internal region of the radius 10^2 AU is a keplerian possibility assumes that the velocity distribution in the HL Tau envelope should nonaxially symmetric infall, instead of keplerian rotation. Angular momentum of 10^53 g cm^2/s could be typical for a dense core of solar mass (or a central reg in the molecular cloud) if rotation of the core has been braked before the start collapse. Therefore, a value of the order of 10^52 g cm^2/s is a reasonably low momentum of presolar nebula. Sears W. D. Diffusive Redistribution of Water Vapor in the Solar Nebula Revisited Stevenson and Lunine presented a model for enhancing the abundance of solid material in the region of the solar nebula at the water condensation point. This was used to provide a means to produce a much more rapid formation of Jupiter than the standard solar nebula models. However they underestimated the drag induced Sunward radial drift of the planetesimals of interest. Re- analysis reveals that these particles would spread over the inner solar system and might influence the formation of the asteroids. Boss A. P. Myhill E. A. Initiating Solar System Formation Through Stellar Shock Waves Isotopic anomalies in presolar grains and other meteoritical components require nucleosynthesis in stellar interiors, condensation into dust grains in stellar envelopes, transport of the grains through the interstellar medium by stellar outflows, and finally injection of the grains into the presolar nebula. The proximity of the presolar cloud to these energetic stellar events suggests that a shock wave from a stellar outflow might have initiated the collapse of an otherwise stable presolar cloud. We have begun to study the interactions of stellar shock waves with thermally supported, dense molecular cloud cores, using a three spatial dimension (3D) radiative hydrodynamics code. Supernova shock waves have been shown by others to destroy quiescent clouds, so we are trying to determine if the much smaller shock speeds found in, e.g., asymptotic giant branch (AGB) star winds, are strong enough to initiate collapse in an otherwise stable, rotating, solar-mass cloud core, without leading to destruction of the cloud. Malcuit R. J. Winters R. R. Tidal Regime of Intact Planetoid Capture Model for the Earth-Moon System: Does It Relate to the Archean Sedimentary Rock Record? Regardless of one's favorite model for the origin of the Earth-Moon system (fission, coformation, tidal capture, giant impact) the early history of lunar orbital evolution would produce significant thermal and Earth and ocean tidal effects on the primitive Earth. Three of the above lunar origin models (fission, coformation, giant impact) feature a circular orbit that undergoes a progressive increase in orbital radius from the time of origin to the present time. In contrast, a tidal capture model places the Moon in an elliptical orbit undergoing progressive circularization from the time of capture (for model purposes about 3.9 b.y. ago) for at least a few 10^8 years following the capture event. Once the orbit is circularized, the subsequent tidal history for a tidal capture scenario is similar to that for other models of lunar origin and features a progressive increase in orbital radius to the current state of the lunar orbit. This elliptical orbit phase, if it occurred, should have left a distinctive signature in the terrestrial and lunar rock records. Depositional events would be associated with terrestrial shorelines characterized by abnormally high, but progressively decreasing, ocean tidal amplitudes and ranges associated with such an orbital evolution. Tuesday, March 16, 1993 Venus Craters Poster Session 7:00 - 9:00 p.m. LPI Ivanov M. A. Basilevsky A. T. Density of Impact Craters on Tessera, Venus After the plains, tessera is the most abundant terrain on Venus. It occupies about 10% of Venus' surface, forming the continentlike blocks and small islands above the adjacent plains. Tessera is a result of tectonic deformations of some precursor terrain. However, the nature of that precursor, as well as the causes and mechanisms of its deformations, are under debate. Any models considering tessera terrain involve estimation of tessera age, either relative or absolute. It is well known that the important information on the age of a planetary surface comes from impact crater statistics. The Magellan global overview of Venus with improved resolution provides an opportunity to gather data on impact craters in amounts large enough for statistically reliable estimations of crater density for different terrains. Our study of impact crater density on tesserae compared to the surrounding terrains has a goal to determine whether it is higher, lower, or the same, and to interpret it in terms of the tessera age and processes involved. Wood C. A. Tam W. Morphologic Classes of Impact Basins on Venus An independent survey of 60% of Venus has resulted in the detection of 35 impact basins and associated transitional rings. Contrary to previous studies we identify central peak basins, as well as peak ring basins. But no unambiguous multi-ring basins have been detected. A new class of crateriform - expanded peak structure - has been noticed, which is transitional in diameter, but apparently not in structure, between central peak and peak ring basins. Wichman R. W. Elevation and Igneous Crater Modification on Venus: Implications for Magmatic Volatile Content Although most impact craters on Venus preserve nearly pristine crater rim and ejecta features, a small number of craters have been identified showing clear evidence of either igneous intrusion emplacement (floor-fracturing) beneath the crater floor or of volcanically embayed exterior ejecta deposits. Since the volcanically embayed craters consistently occur at higher elevations than the identified floor-fractured craters, I propose that igneous crater modification on Venus is elevation dependent. This abstract describes how regional variations in magmatic neutral buoyancy could produce such elevation dependent crater modification and considers the implications for typical magmatic volatile contents on Venus. Wichman R. W. Schultz P. H. Large Floor-fractured Craters and Isostatic Crater Modification: Implications for Lithospheric Thickness on Venus Several of the largest craters on Venus, including Mead, Meitner, and Isabella, exhibit well-developed floor fracture patterns combining a central set of radial features with a peripheral set of concentric fractures. This pattern strongly resembles the fracture patterns observed in the largest floor-fractured craters on the Moon (eg. Humboldt, Gauss, Petavius). Although most lunar floor-fractured craters apparently reflect crater modification by igneous intrusions and volcanism, we propose that the fractures in these larger craters represent domical flexure events in response to post-impact isostatic uplift. Since the extent of uplift and surface failure in this model depends on both the size of the basin cavity and the local lithospheric thickness, this interpretation also provides a means for constraining lithospheric thicknesses on Venus. Based on the apparent onset diameter of isostatic crater modification, we derive lithospheric thickness estimates for the Moon of ~80-100 km, and for Venus of ~50-70 km. Spudis P. D. Sharpton V. L. Impact Basins on Venus and Some Interplanetary Comparisons Impact is one of the many processes that have shaped the surface of Venus. The largest impact craters, basins, are important features affecting the evolution of the terrestrial planets. Because Venus has an atmosphere, a gravity similar to Earth's, and a surface target with a high geothermal gradient, venusian basins provide an important comparative set of data to test our ideas about basin-forming impacts and their geological effects on the evolution of the crusts of the terrestrial planets. Speidel D. H. The "Missing" Impact Craters on Venus The size-frequency pattern of the 842 impact craters of Venus measured to date can be well described (across four standard deviation units) as a single log normal distribution with a mean crater diameter of 14.5 km. This result was predicted in 1991 on examination of the initial Magellan analysis. If this observed distribution is close to the real distribution, the "missing" 90% of the small craters and the "anomalous" lack of surface splotches may thus be neither missing nor anomalous. Schultz P. H. Searching for Ancient Venus The cratering record on Venus provides one of the few available remote chronometers for establishing relative age. Because the dense atmosphere shields the surface from smaller impactors, the most statistically significant fraction of the cratering record is incomplete at best and indeterminate at worst. Larger craters represent survivors of entry but occur too infrequently for delineating statistically significant ages on a local scale. This contribution reconsiders processes affecting the statistical cratering record and argues that the globally averaged age approaches 2-3by with isolated relict surfaces dating back to 3-4by. Johnson J. R. Baker V. R. Radar Properties of Several Fluidized Ejecta Blankets on Venus Magellan SAR imagery, altimetry, and radiometry are being analyzed to characterize the radar properties of the fluidized ejecta blankets (FEBs) that are associated with over 40% of the impact craters on Venus. The FEB flows and plains units surrounding the craters Isabella (175 km), Addams (90 km), Seymore (65 km), and a crater located at 4 degrees S, 155.5 degrees E (70 km) are examined here using the MIT-produced ARCDR and GxDR data. Individual orbital footprints obtained from the ARCDRs have been classified according to their dominant simple geologic unit (e.g., plains, FEB flows). This permits average values of reflectivity (corrected for diffuse scattering), rms meter- scale slopes, emissivity, and SAR backscatter to be calculated for each unit. GxDR images provide a means of visualizing the spatial relations between the various data sets. Variability of radar properties within the FEBs and relative to surrounding regions has implications concerning the genesis and possible emplacement mechanisms of fluidized ejecta. Herrick R. R. The Three Ages of Venus: A Hypothesis Based on the Cratering Record The elevation range of 6051.9 - 6053.9 km on Venus is deficient in craters but has a high proportion of embayed and tectonically deformed craters. On the basis of this data and previous work, I propose that Venus has experienced three distinct geologic ages: pseudo-plate tectonics until 1 - 2 Ga, volcanic flooding of low-lying areas, and currently hot spot tectonics. Alexopoulos J. S. McKinnon W. B. Morphology of Large Impact Craters and Basins on Venus: Implications for Ring Formation A nearly complete examination of the Magellan radar data for the venusian surface reveals 72 unequivocal peak-ring craters and 4 larger structures that we interpret to be multiringed. This report updates our earlier studies and that of the Magellan team. The general morphology of peak-ring craters, decreasing ring-diameter ratio trends with increasing crater diameter, and the general size-morphology progression from complex central-peak crater to peak- ring crater on Venus and the terrestrial planets suggest similar processes of peak-ring formation. Observations are consistent with a model of dynamic collapse, downward and outward, of an unstable central peak to form a ring. We interpret the four larger ringed structures (Klenova, Lise Meitner, Mead, and Isabella) to be morphologically similar to the Orientale Basin on the Moon, and thus true multiringed basins. Tuesday, March 16, 1993 Venus Geology and Geophysics: Global Synthesis Poster Session 7:00 - 9:00 p.m. LPI Tormanen T. Complex Ridged Terrain-Related Ridge Belts on Venus: Global Distribution and Classification Fifty-six features were identified in a global survey of complex ridged terrain (CRT)-related ridge belts. Nonrandom aerial distribution was observed with all of the CRT-related ridge belts located to the north of 20 degrees S and increasing in number toward the north. In the equatorial highlands the only concentration of CRT-related ridge belts occurs along north and northeast Ovda Regio and western Thetis Regio. Major areas of CRT devoid of related ridge belts (Beta , Phoebe, Alpha Regiones, and northern Lada Terra) are regions dominated by rifting, fractures, coronae, and volcanic features. A noticeable concentration of ridge belts is located within a region 20 degrees S-80 degrees N,0 degrees-150 degrees E. Three classes of CRT-related ridge belts were defined: (1) ridge belts directly in contact with CRT margins, (2) ridge belts located apart from the CRT boundary, but whose shape and strike are affected by CRT, and (3) ridge belts terminating against a margin of CRT. There does not appear to be any relation between ridge belt class and type of CRT margin. Some of the class 2 and 3 belts of the 20 degree S-80 degree N,0 degree-150 E region seem to be continuations of adjacent elongated blocks of CRT and could reflect the hypothesized basement of tesseralike material. Majority of class 1 and 2 ridge belts within this region parallel north or northeast boundaries of large CRT plateaus or arclike arrangements of tesserae. These relationships show that this region has been dominated by compressional stresses oriented perpendicular to the CRT boundaries, in north-south/northeast-southwest directio Manley C. R. Venusian "Pancake" Domes: Insights from Terrestrial Voluminous Silicic Lavas and Thermal Modeling The so-called "pancake" domes, and several other volcanoes on Venus, appear to represent large extrusions of silicic lava. Similar voluminous rhyolite lava flows, often associated with mantle plumes, are known on Earth. Venus' high ambient temperature, and insulation by the dome's brecciated carapace, both act to prolong cooling of a dome's interior, allowing for episodic lava input over an extended period of time. Field relations and aspect ratios of terrestrial voluminous rhyolite lavas imply continuous, nonepisodic growth, reflecting tapping of a large volume of dry, anatectic silicic magma. Petrogenetically, the venusian domes may be analogous to chains of small domes on Earth, which represent "leakage" of evolved material from magma bodies fractionating from much more mafic liquids. Magee Roberts K. Head J. W. Large-scale Volcanism Associated with Coronae on Venus The formation and evolution of coronae on Venus are thought to be the result of mantle upwellings against the crust and lithosphere and subsequent gravitational relaxation. A variety of other features on Venus have been linked to processes associated with mantle upwelling, including shield volcanoes on large regional rises such as Beta, Atla, and Western Eistla Regiones and extensive flow fields such as Mylitta and Kaiwan Fluctus near the Lada Terra/Lavinia Planitia boundary. Of these features, coronae appear to possess the smallest amounts of associated volcanism, although volcanism associated with coronae has only been qualitatively examined. An initial survey of coronae based on recent Magellan data indicated that only 9% of all coronae are associated with substantial amounts of volcanism, including interior calderas or edifices greater than 50 km in diameter and extensive, exterior radial flow fields. Sixty-eight percent of all coronae were found to have lesser amounts of volcanism, including interior flooding and associated volcanic domes and small shields; the remaining coronae were considered deficient in associated volcanism. It is possible that coronae are related to mantle plumes or diapirs that are lower in volume or in partial melt than those associated with the large shields or flow fields. Regional tectonics or variations in local crustal and thermal structure may also be significant in determining the amount of volcanism produced from an upwelling. It is also possible that flow fields associated with some coronae are sheetlike in nature and may not be readily identified. If coronae are associated with volcanic flow fields, then they may be a significant contributor to plains formation on Venus, as they number over 300 and are widely distributed across the planet. As a continuation of our analysis of large-scale volcanism on Venus, we have reexamined the known population of coronae and assessed quantitatively the scale of volcanism associated with them. In particular, we have examined the percentage of coronae associated with volcanic flow fields (i.e., a collection of digitate or sheetlike lava flows extending from the corona interior or annulus), the range in scale of these flow fields, the variations in diameter, structure, and stratigraphy of coronae with flow fields, and the global distribution of coronae associated with flow fields. Raitala J. Tormanen T. Kauhanen K. Tokkonen T. Ridge Belt-related Scarps and Troughs: Compressional Crustal Bending on Venus Cytherean ridge arcs resulted in crustal shortening in places with either the strongest compression or weakest surface layers. Arcuate scarps on or close to the edge of the ridge arcs were compressional fronts formed by overthrust over or subduction of adjacent lowland.These ridge arcs became wider due to the propagation of new ridge formation close to previous one(s). As this process repeated itself with time, the crust became thicker and the foreland was bent either due to the excess load, overthrust, or subduction event. Price M. Suppe J. Some Deformation Trends and Topographic Characteristics of Tesserae on Venus Mapping of dominant trends of deformation and topographic analysis of tesserae reveal that (1) consistent patterns of deformation exist between individual tesserae; (2) regional deformation associated with coronae and rifts commonly overprints tesserae fabrics; (3) normalized area frequency plots of tesserae display a strongly bimodal distribution between -70 and 70 degrees latitude; and (4) the relief at tessera margins is relatively constant and independent of tessera elevation. We are continuing to expand these observations and test their implications for the formation of tesserae and for global models of deformation on Venus. Parfitt E. A. Head J. W. Formation and Evolution of Radial Fracture Systems on Venus A survey of ~90% of the surface of Venus using Magellan data has been carried out to locate all radial fracture systems and to assess their association with other features such as volcanic edifices and coronae. Squyres et al. and Stofan et al. have discussed the association of radial fracture features in relation to coronae features; our approach was to assess the associations of all of the fracture systems. These fracture systems have two broad types of form--some fracture systems are associated with updomed topography, radiate from a point, and have relatively uniform fracture lengths while others have a wider range of fracture lengths and radiate from the outer edge of a central caldera. Squyres et al. and Stofan et al. have interpreted both types of feature as reflecting tectonic fracturing resulting from uplift of the surface as a mantle plume impinges upon the crust. While it is true that a number of features are related to uplift and that such uplift will induce stresses consistent with radial fracturing we explore the possibility that these fractures are not exclusively of tectonic origin. Purely tectonic fracturing will tend to generate a few main fractures/faults along which most of the stresses due to uplift will be accommodated leading to the triple-junction form common for terrestrial updoming. Though this type of feature is observed on Venus (e.g., feature located at 34S86), the majority of radial fracture systems display much more intensive fracturing than this through a full 360 degrees, this is difficult to explain by purely tectonic processes. The association of many of the fractures with radial lava flows leads us to interpret these fractures as reflecting dike emplacement, the form of the fractures being consistent with primarily vertical propagation from the head of a mantle plume. In the case of the second type of fracture system (those radiating from a central caldera) an even stronger case can be made that the fractures are not of tectonic origin. These features are not as commonly associated with updoming of the surface and where they are, the fractures extend out well beyond the edge of the topographic rise--an observation that is not consistent with the fractures being of tectonic uplift origin. Furthermore, the fractures have a distribution of lengths (many short, fewer long) that is characteristic of dike swarms, and show direct associations with calderas and lava flows consistent with a volcanic origin. In addition, the longest fractures have a radial pattern only close to the center of the system but bend with distance to align themselves with the regional stress field. This behaviour is very difficult to explain on purely tectonic grounds, but is a pattern commonly seen for terrestrial dikes. For these reasons we argue that many, if not the majority, of radial fracture systems found on Venus are the surface reflection of dike swarms, those associated with positive topography reflecting vertical emplacement and those radiating from calderas reflecting lateral propagation. Sahuaro Astronomy Research Class Lockwood J. F. Ellison M. Evergreen High School Research Class Johnson J. Kamatsu G. Distribution of Small Volcanic Cones on Venus by Size and Elevation: Implication for Differential Deposition of Volcanic Features It appears that volcanic features are not evenly distributed on the surface of Venus. Head et. al. theorizes that the sparcity of volcanic features in the lowlands may be due to an altitude dependant inhibition of volatile exsolution and the resulting production of neutral buoyancy zones sufficient to form magma reservoirs and favoring flood lavas at lower elevation. The astronomy research classes of Evergreen and Sahuaro High Schools surveyed a cross- section of different elevation topography to investigate size and distribution of small volcanic cones by elevation. Keddie S. T. Head J. W. The Distribution of Large Volcanoes on Venus as a Function of Height and Altitude Theory predicts that the slower cooling of lava flows on Venus should result in lava flows that are typically 20% longer than their terrestrial counterparts, and that the development of neutral buoyancy zones (NBZ) on Venus may be strongly influenced by altitude-controlled variations in surface pressure. Observations that support these predictions would include relatively low heights for Venus volcanoes (discussed with pre-Magellan data), and an increase in both the number and development of large edifices with increasing basal altitude. We present the results of an analysis of the height and altitude distribution of 123 large (diameter >100 km) volcanoes made using Magellan image and altimetry data and we use these results to begin to test the predications of the above theories. Ivanov M. Head J. W. Tessera Terrain on Venus: Global Characterization from Magellan Data Tessera terrain is characterized by relatively high elevations and complex tectonic patterns; analysis of Venera 15/16 data showed that large (up to thousands of kilometers across) and small (up to hundreds of kilometers across) occurrences of tesserae are widespread and nonrandomly distributed and make up about 10-15% of the surface of Venus north of ~30 degrees N. In a previous analysis, we used the Magellan Cycle 1 and 2 data to map the global distribution of tesserae on the basis of: (1) complex deformational patterns (two or more trends), (2) relatively high radar backscatter, and (3) relatively high elevation. Here we report on the quantitative aspects of tesserae areal, size, and shape distribution, and on the characterisitcs and distribution of tesserae boundaries. Experiments on volcanic flooding of tessera and implications for tessera presence beneath the plains and analysis of the distribution of impact craters on tesserae and the plains are reported elsewhere. Ernst R. E. Head J. W. Parfitt E. Wilson L. Grosfils E. Giant Radiating Dyke Swarms on Earth and Venus On Earth, giant radiating dyke swarms are usually preserved as fan-shaped fragments that have been dismembered from their original configuration by subsequent plate tectonic rifting events. Analysis of the largest fragments and consideration of their original configuration has led to the idea that many swarms are plume related, and that dyke swarms radiate away from plume centers. Magellan radar data reveal abundant intact giant radiating swarms on Venus that are similar in scale and pattern to those on Earth. The absence of intense weathering and plate tectonic processes on Venus accounts for the preservation of the primary radiating patterns. It is characteristic of both Earth and Venus that giant radiating dikes are emplaced laterally for distances of at least 2000 km away from plume centers. At distances beyond the influence of the plume on both Earth and Venus, the radiating dyke pattern is often swept into a linear pattern aligned with the regional stress field. There is tremendous potential synergism between the characterization and analysis of terrestrial dyke swarms (where significant erosion has revealed their structure and emplacement directions at depth) and the giant swarms of Venus (where the the complete circumferential structure is preserved, and the surface fracture systems above near-surface dikes and the nature of the central source regions are revealed). In this study, we report on the characteristics of radial dyke swarms on Earth and Venus and draw some preliminary comparisons from the two perspectives. Bilotti F. Connors C. Suppe J. Global Organization of Tectonic Deformation on Venus The geographic organization of surface deformation on Venus as on Earth is a key to understanding the global tectonic system. To date we have mapped the distribution of three unambiguous tectonic land forms on Venus: (1) linear foldbelts analogous to those at plate margins of the Earth, (2) linear rift zones, analogous to continental rifts on the Earth, and (3) distributed plains deformation in the form of wrinkle ridges and extensional faults and fractures. The linear foldbelts are the dominant structural style in the northern hemisphere; Ninety percent of the planet's foldbelts lie above the equator. In contrast, compressive deformation in the southern hemisphere is dominated by two large, sweeping patterns of wrinkle ridges. The two hemispheres are divided by an equatorial region that is largely covered by rift zones and several large tessera blocks. A tectonic model of generally poleward convergence of the northern hemisphere explains the distribution of foldbelts and rift zones. In our model, a northern hemispherical plate (or system of plates) moves poleward and deforms along discrete, predominately longitudinal bands. Tuesday, March 16, 1993 Venus Volcanism and Geology Poster Session 7:00 - 9:00 p.m. LPI Plaut J. J. Magellan Vertical Polarization Observations The Magellan high-gain radar antenna system was designed to transmit and receive signals in a pure linear polarization state. The nominal mapping configuration placed this linear polarization direction parallel to the surface of Venus, providing SAR image data in the HH polarization (horizontal transmit and receive) and radiothermal emission data in the H (horizontal- receive only) polarization. During Magellan's extended mission (cycles 2 and 3), two brief experiments were conducted in which the spacecraft was rotated 90 degrees along the axis of the antenna boresight, producing SAR data in the VV polarization and emission data in the V polarization. This study will focus on the SAR results from the first experiment, which included portions of the highly reflective Beta Regio highlands. Senske D. A. Rifting at Devana Chasma, Venus: Structure and Estimation of the Effective Thickness of the Elastic Lithosphere To understand the relationship between extension and sites on Venus interpreted to be associated with mantle upwelling, the characteristics of the northern part of Devana Chasma in Beta Regio, are examined. The structure of this rift is compared to that of terrestrial continental rifts. To ascertain the degree to which the lithosphere at Beta might be thinned, estimates of lithospheric thickness are calculated using a plate flexure model. These values are compared to those determined for other parts of the planet. Moore H. J. Plaut J. J. Parker T. J. Relief of Some Small Landforms on Venus Image displacements in the radar images that are caused by the relief of landforms provide several methods of estimating this relief: (1) monoscopic measurements of foreshortening of landforms that are symmetrical in the plane of the look-direction of the radar (includes radial symmetry), (2) stereoscopic measurements of parallax in same-side image pairs (cycles 1 and 3), and (3) measurements of parallax in opposite-side image pairs (cycles 1-2 and/or 2-3). Success in methods 2 and 3 (especially 3) depends on identifying conjugate image points in the two images. Lancaster M. G. Guest J. E. Roberts K. M. Sheet Flow Fields on Venus A survey of flow fields with areas greater than 50,000 km^2 has revealed fields with sheetlike morphology. These sheet flow fields are distinguished by their relatively uniform backscatter, lack of internal flow structure such as well-defined lava streams or channels, absence of flow lobes, and irregular boundaries. Internal flow boundaries are essentially absent in these fields, and as such they cannot usually be divided into separate stratigraphic units or eruptive episodes. This is unlike other flow fields with more digitate morphologies, which are made up of discrete flow lobes, and can usually be divided into several episodes of flow emplacement. Five sheet flow fields have been studied so far, and other candidates have been found. A flow field associated with Lauma Dorsa is taken as an example and examined in detail here. Keep M. Hansen V. L. Structural Mapping of Maxwell Montes Four sets of structures have been mapped in the western and southern portions of Maxwell Montes. An early north-trending set of penetrative lineaments is cut by dominant, spaced ridges and paired valleys that trend northwest. To the south the ridges and valleys splay and graben form in the valleys. The spaced ridges and graben are cut by northeast-trending graben. The northwest-trending graben formed synchronously with or slightly later than the spaced ridges. Formation of the northeast-trending graben may have overlapped with that of the northwest-trending graben, but occurred in a spatially distinct area (regions of 2 degrees slope). Graben formation, with northwest-southeast extension, may be related to gravity-sliding. Individually and collectively these structures are too small to support the immense topography of Maxwell, and are interpreted as parasitic features above a larger mass that supports the mountain belt. Helgerud M. B. Zimbelman J. R. Emplacement of Multiple Flow Units on Very Shallow Slopes, East Kawelu Planitia Flow Field, Venus Lobate flows with variable radar brightness occur at several locations on Venus. Here we present preliminary mapping results for a lava flow field on the eastern side of Kawelu Planitia, centered at approximately 41 degrees N, 251 degrees E. Numerous individual flows of varying brightness can be traced for up to 400 km at this location. Magellan image F-MIDRP.40N251;1 contains two major flow complexes as well as a distinct minor flow field associated with a volcanic dome. The southern flow field changes from radar dark to radar bright in a convex arc from west to east. The southern lobe of unit E disappears under a broad leveed channel (41.3 degrees N, 250.1 degrees E) in unit 5, only to reappear south of the unit 5 boundary. Hamilton V. E. Stofan E. R. Morphology and Models for the Evolution of Eastern Hecate Chasma, Venus Hecate Chasma is a deep trough characterized by a chainlike concentration of coronae and coronalike features trending approximately southwest-northeast between approximately 200 and 260 degrees east longitude (terminating at Beta Regio). The section of Hecate in which we have concentrated our study is centered at 15N, 249, where the trough is especially well defined. Nearby, a smaller chain of eight coronae lies along a minor trough parallel to the general trend of the greater chain. The trough itself is unusual in this area because it has a highly asymmetric profile. Using Magellan radar and topography data, we have examined the morphology of this area in order to assess the tectonic and volcanic history of the area. After examining the most important types of features (linear, arcuate, and circular) in eastern Hecate, we present two possible models of origin. A companion abstract presents an overview of the Hecate and Parga linear deformation zones. Ghail R. C. Pargo Chasma and its Relationship to Global Tectonics I have produced a detailed 1:10 million scale map of Pargo Chasma and the surrounding area from preliminary U.S.G.S. controlled mosaiced image maps of Venus constructed from Magellan data. In view of the problems highlighted above in relation to the efforts already made at identifying a particular set of features, I have mapped the region purely on the basis of the geomorphology visible in the Magellan data without any attempt at identifying a particular set or class of features. Thus the map produced distinguishes between areas of different brightness and texture. This has the advantage of highlighting the tectonic fabric of Pargo Chasma and clearly illustrates the close interrelationship between individual coronae and the surrounding tectonic belts. The difficulty faced in attempting to identify individual tectonic features within this belt is apparent. A useful, though much simplified, classification scheme has been identified from the geomorphological map: the ejecta deposits associated with impact craters may be identified by their bright, rough appearance; radiating bright (rough) and dark (smooth) flows locate the presence of volcanic vents, while corona may be identified by their bright lineated annuli. Crumpler L. S. Head J. W. Aubele J. C. Regional Mantle Upwelling on Venus: The Beta-Atla-Themis Anomaly and Correlation with Global Tectonic Patterns The morphology and global distribution of volcanic centers and their association with other geological characteristics offers significant insight into the global patterns of geology, tectonic style, thermal state, and interior dynamics of Venus. Magellan data permit the detailed geological interpretation necessary to address questions about interior dynamics of Venus particularly as they reflect relatively physical, chemical, and thermal conditions of the interior. This paper focuses on the distribution of anomalous concentrations of volcanic centers on Venus and regional patterns of tectonic deformation as it may relate to the identification of global internal anomalies, including mantle dynamic, petrological, or thermal patterns. Crown D. A. Stofan E. R. Plaut J. J. Volcanism in Southern Guinevere Planitia, Venus: Regional Volcanic History and Morphology of Volcanic Domes Guinevere Planitia is a low-lying region located between the highlands of Beta Regio and Eistla Regio. Analyses of Pioneer Venus, Goldstone, and Arecibo radar data suggested that the surface of Guinevere Planitia is dominated by volcanism, primarily in the form of bright, dark, and mottled plains units. Also identified in this region was the Beta-Eistla Deformation Zone, composed of ovoids and discontinuous segments of lineament belts that have been embayed by the surrounding plains. The resolution of Magellan SAR images allows detailed investigations of the volcanic deposits found in the area in order to determine the types of eruptive activity that have occurred and to constrain the regional volcanic history. Analyses of an area of southern Guinevere Planitia between 0 degrees-25 degrees N and 300 degrees-330 degrees E indicate the presence of a wide variety of volcanic landforms, including large shield volcanoes, widespread plains, lava flow fields, and small domes, cones, and shields as well as coronae and other circular structures that have associated volcanic deposits. Campbell B. A. Rogers P. G. Geology and Surface Characteristics of Bell Regio, Venus Bell Regio is a broad topographic rise located at approximately 30 degrees N, 45 degrees E, which extends 1500 km in the N-S direction and is characterized by extensive volcanism. In this work, the geology and surface characteristics of Bell Regio are examined in order to understand the geologic evolution of the region and the surface characteristics of the major volcanic units. Relationships between Magellan SAR backscatter values and altimeter-derived rms slope data are analyzed, and terrestrial SAR data are used to suggest possible surface morphologies. Basilevsky A. T. Weitz C. M. Regional Geology of the Venera Landing Sites: Tentative Results of Photogeologic Mapping The regional geology of the five Venera landing sites, where geochemical measurements and TV observations on the venusian surface were made, was studied based on the photogeologic analysis of the Magellan C1-MIDRP imagery for the large area that we will call the Venera region (38 degrees N to 22.6 degrees S and 268 degrees to 344 degrees E). The results of the analysis were compiled in the form of a synoptical geologic map at about 1:10 M scale. MIT-processed Magellan altimetry was also used for the analysis. Carlson R. W. Baines K. H. Girard M. Kamp L. W. Drossart P. Encrenaz T. Taylor F. W. Galileo/NIMS Near-Infrared Thermal Imagery of the Surface of Venus Numerous highland and lowland features are observed in multispectral imagery acquired at ~50 km spatial resolution by the Near-Infrared Mapping Spectrometer (NIMS) on board the Galileo spacecraft in February 1990. Specifically, such features are observed at 1.18 micrometers, a wavelength particularly sensitive to thermal emission from the hot, lower atmosphere (<10 km) and surface, and show up particularly well when this image is "de-clouded" using a simultaneously acquired 2.3-micrometer image of the upper, cloudy atmosphere. Due to the steep atmospheric temperature gradient (approximately 8 degrees per km), hot lowland areas appear relatively bright, while cooler, highland areas appear dark (due to the steep atmospheric temperature gradient, approximately 8 degrees per km, surface temperatures span approximately 100 K over the 13-km range of surface altitudes observed in this image). Tuesday, March 16, 1993 Venus: Volcanos and Weathering Poster Session 7:00 - 9:00 p.m. LPI Sugita S. Matsui T. Are Plinian Type Eruptions Possible on Venus? Calculations of dynamics of eruption plumes in the venusian atmosphere reveal the following results: (1) Conditions for a convective eruption column are very limited on Venus with required magma temperatures higher than 1100 K. Otherwise pyroclastic flows form. (2) Once a convective eruption column is established, it may extend as high as about the tropopause (~60 km). (3) Critical eruption velocities for a convective eruption column as a function of temperature and gas content of magma may provide a useful tool for estimating the interior condition of Venus, including its volatile inventory. Kirk R. L. Separation of Topographic and Intrinsic Backscatter Variations in Biscopic Radar Images: A "Magic Airbrush" Shaded-relief maps portraying landforms as they would appear in the absence of variations in the intrinsic brightness of the surface are a venerable and extremely useful tool in planetary geology. Such maps have traditionally been produced by a highly labor intensive manual process. Skilled cartographer- artists develop detailed mental images of landforms by meticulous scrutiny of all available data, and are able to use an airbrush and electric eraser to draw these images on a map. This process becomes increasingly time-consuming or even impossible if--as is true for radar data in general and Magellan data in particular--the effects on image brightness of varying scattering properties greatly outweigh those of slope variations. Because of the difficulty of interpreting relief in the Magellan images, the airbrush technique is being used only to remove obvious artifacts from low-resolution, shaded-relief images computed digitally from altimetric data. The purpose of this abstract is to describe a novel and surprisingly simple digital- processing technique that can be applied to pairs of radar images to produce shaded-relief-like results at the full image resolution. These shaded-relief images can be used not only as base maps, but to improve the accuracy of quantitative topographic mapping by radarclinometry and stereoanalysis. Fagents S. A. Wilson L. Vulcanian Explosive Eruptions: A Mechanism for Localised Pyroclast Dispersal on Venus A model of the mechanisms involved in transient (vulcanian-type) explosive eruptions has enabled us to place constraints on the velocities of blocks ejected during such events on Earth as well as on the excess pressure and concentration of gas in the vent prior to the onset of the explosion. This model, suitably modified, can predict the results of similar eruptions occurring under the differing venusian environmental conditions. It is found that the much higher atmospheric pressure (~100 bars at the mean planetary radius) dominates the form of the resulting deposit in two main ways: (1) by inhibiting the expansion out of the vent of overpressured gas, hence reducing ejecta velocity, and (2) by retarding the flight of ejected blocks via drag effects. Thus, it is expected that such deposits will typically extend to ~200 m on Venus as compared to several kilometers for documented terrestrial deposits. Bullock M. A. Grinspoon D. H. Head J. W. Venus Resurfacing Rates: Constraints Provided by 3-D Monte Carlo Simulations We have implemented a 3-D Monte Carlo model that simulates the evolving surface of Venus under the influence of a flux of impacting objects and a variety of styles of volcanic resurfacing. For given rates of impact events and resurfacing, the model predicts the size-frequency and areal distributions of surviving impact craters as a function of time. The number of craters partially modified by volcanic events is also calculated as the surface evolves. We find that a constant, global resurfacing rate of approximately 0.4 km^3/yr is required to explain the observed distributions of both the entire crater population, and the population of craters partially modified by volcanic processes. Burl M. C. Fayyad U. M. Smyth P. Aubele J. C. Crumpler L. S. A Pattern Recognition System for Locating Small Volcanoes in Magellan SAR Images of Venus The Magellan dataset constitutes an example of the large volumes of data that today's instruments can collect, providing more detail of Venus than was previously available from Pioneer Venus, Venera 15/16, or groundbased radar observations put together. However, data analysis technology has not kept pace with data collection and storage technology. Due to the sheer size of the data, complete and comprehensive scientific analysis of such large volumes of image data is no longer feasible without the use of computational aids. We report on our progress toward developing a pattern recognition system for aiding in the detection and cataloging of small-scale natural features in large collections of images. Combining classical image processing, machine learning, and a graphical user interface, we are initially targeting the detection of the "small-shield" volcanos (less than 15 km in diameter) that constitute the most abundant visible geologic feature in the more that 30,000 synthetic aperture radar (SAR) images of the surface of Venus. Our eventual goal is to provide a general, trainable tool for locating small-scale features where scientists specify what to look for simply by providing examples and attributes of interest to measure. This contrasts with the traditional approach of developing problem-specific programs for detecting specific patterns. We report on our approach and initial results in the specific context of locating small volcanos. Bindschadler D. L. Schubert G. Venus' Center of Mass-Center of Figure Displacement and Implications Earth, Moon, Mars, and Venus all have centers of mass (C.M.) that are displaced from their centers of figure (C.F.) by amounts that range from ~340 m (Venus) to ~2.5 km (Mars). These offsets have all been calculated from the first-degree terms in spherical harmonic expansions of topography. We describe an alternate method for calculating C.M.-C.F. offsets directly from a global topographic dataset and apply it to Venus. Using Magellan altimetry, we find that Venus' C.F. is displaced approximately 280 m from its C.M. in the direction of Western Aphrodite Terra (4.4 degrees S, 135.8 degrees E). We investigate several simple models for this offset and find that it is most consistent with thickened crust in Ovda and Thetis Regiones (which constitute most of W. Aphrodite). The location of the C.F. offset also places constraints on the degree of crustal thickening in Western Ishtar Terra and/or this highland's mode of origin. We favor a model in which offset due to thick crust in Western Ishtar Terra is balanced by an opposing offset due to cold, downwelling mantle material beneath the highland. Straub D. W. Burns R. G. The Stability of Oxyamphiboles: Existence of Ferric-bearing Minerals Under the Reducing Conditions on the Surface of Venus An enigma of venusian mineralogy is the suggestion that ferric-bearing minerals exist under the reducing conditions of the venusian atmosphere. Several ferric oxide and silicate minerals have been proposed to occur on Venus, including metastable hematite, laihunite or ferrifayalite, ferric- bearing tephroite, oxyamphiboles, and oxybiotites. Heating experiments performed on these ferric-bearing minerals under temperature-oxygen fugacity conditions existing on Venus suggest that only oxyamphiboles and oxybiotites may be stable on the venusian surface. Burns R. G. Straub D. W. Venus Mountain-Top Mineralogy: Misconceptions About Pyrite as the High Radar-Reflecting Phase Altitude-dependent, high radar-reflectivity surfaces on Venus are observed on most mountainous volcanic terranes above a planetary radius of about 6054 km. However, high radar-reflectivity areas also occur at lower altitudes in some impact craters and plain terranes. Pynte (FeS2) is commonly believed to be responsible for the high radar reflectivities at high elevations on Venus, on account of large dielectric constants measured for sulfide-bearing rocks that were erroneously attributed to pynte instead of pyrrhotite. Pentlandite-pyrrhotite assemblages may be responsible for high reflectivities associated with impact craters on the venusian surface, by analogy with Fe-Ni sulfide deposits occulting in terrestrial astroblemes. Mixed-valence Fe2+-Fe3+ silicates, including oxyhornblende, oxybiotite, and ilvaite, may contribute to high radar- reflecting surfaces on mountaintops of Venus. Kucinskas A. B. Borderies N. J. Turcotte D. L. Spectral Study of Venus Global Topography and Geoid from Magellan and PVO Data We have conducted an analysis of newly available global harmonic models for topography and geoid on Venus. We find that the power spectral density for Venus topography has a power - law dependence on wave-number characteristic of Brown Noise, similar to what is found for the Earth. However, the Venus topography spectrum presents a rollover at lower degree (l=3) than is observed for the Earth spectrum and has smaller amplitudes than that of the Earth's. The Venus geoid also obeys a power-law relationship, at least for small values of l, but with a smaller slope and more power (for l>3) than the Earth geoid. Kaula W. M. Megaplumes on Venus The geoid and topography heights of Atla Regio and Beta Regio, both peaks and slopes, are explicable as steady-state plumes, if nonlinear viscosity is taken into account. Strongly constrained by the data are an effective plume depth of about 700 km, with a temperature anomaly thereat of 30 degrees, leading to more than 400 degrees at the plumehead. Also well constrained is the combination QV/S^4(sub)0 = (volume flow rate) x viscosity/(plume radius)^4, about 11 Pa/m/sec. The topographic slopes dh/ds constrain the combination Q/A, where A is the thickness of the spreading layer, since the slope varies inversely with velocity. The geoid slopes dN/ds require enhancement of the deeper flow, as expected from nonlinear viscosity. The Beta data are best fit by Q = 500 m^3/sec and A=140 km; the Atla, by Q = 440 m^3/sec and A = 260 km. The dynamic contribution to the topographic slope is minor. Johnson C. L. Sandwell D. T. Estimates of Lithospheric Thickness on Venus Magellan altimetry data have revealed many examples of topographic flexure on Venus. Modeling of flexural features is of interest as it provides information on spatial (and for the earth, temporal) variations in lithospheric thickness. Lithospheric thickness may be determined solely from modeling topographic flexure or by combining gravity and topography data. On Venus even the highest resolution gravity is insufficient for modeling all but the very longest wavelength flexural features, so we rely heavily on altimetry data for information about lithospheric thickness. Sandwell and Schubert modeled flexure around four coronae and found lithospheric thicknesses h, in the range 35-70 km. Studies of several more flexural features suggests that these are typical of Aphrodite Terra and other chasmata regions on Venus. However, lithospheric thicknesses associated with other regions are in the range 15-30 km. McKenzie et al. noted that part of Aphrodite Terra appeared similar in planform and morphology to the subduction zones of the East Indies on Earth. Other flexure studies using Magellan data have looked at smaller coronae (h = 5-30 km), and rifts (h = 8-20 km). It can be seen that the range of thicknesses suggested by studies to date is extremely large and it is difficult to establish whether their mean is in agreement with that predicted by heat flow scaling arguments (h ~ 18 km). Here we present results from a global study of flexure on Venus with particular emphasis on the variation in our results with different tectonic settings. Wednesday, March 17, 1993 Interplanetary Dust: Laboratory Studies and Results from Spacecraft 8:30 - 12:00 a.m. Room A Chair(s): J. A. M. McDonnell G. Blanford Stephan T.* Klock W. Jessberger E. K. Rulle H. Zehnpfenning J. Multielement Analysis of Interplanetary Dust Particles Using TOF-SIMS Sections of three stratospheric particles, U2015G1, W7029*A27, and L2005P9, respectively, were analyzed with TOF-SIMS (Time-Of-Flight Secondary-Ion-Mass-Spectrometry) continuing our efforts to investigate the element distribution in IDPs with high lateral resolution (~0.2 micrometers), to examine possible atmospheric contamination effects, and to further explore the abilities of this technique for element analysis of small samples. The samples, previously investigated with SXRF (synchrotron X-ray fluorescence analysis), are highly enriched in Br (Br/Fe: 59xCI, 9.2xCI, and 116xCI, respectively). U2015G1 is the IDP with by far the highest Zn/Fe-ratio (81xCI) ever reported in chondritic particles. Clemett S. J.* Maechling C. R. Zare R. N. Swan P. D. Walker R. M. Measurement of Polycyclic Aromatic Hydrocarbon (PAHs) in Interplanetary Dust Particles We report the first definitive measurements of specific organic molecules (PAHs) in interplanetary dust particles (IDPs). An improved version of the microbeam-two-step laser mass spectrometer (microliter^2 MS) described previously was used for the analysis. Two IDPs gave similar mass spectra showing an abundance of PAHs. Control samples, including particles of probable terrestrial origin from the same stratospheric collector, gave either null results or quite different spectra. We conclude that the PAHs are probably indigenous to the IDPs and are not terrestrial contaminants. Keller L. P.* Thomas K. L. McKay D. S. Carbon Abundances, Major Element Chemistry, and Mineralogy of Hydrated Interplanetary Dust Particles Hydrated interplanetary dust particles (IDPs) comprise a major fraction of the interplanetary dust particles collected in the stratosphere. While much is known about the mineralogy and chemistry of hydrated IDPs, little is known about the C abundance in this class of IDPs, the nature of the C-bearing phases, and how the C abundance is related to other physical properties of hydrated IDPs. Here we report bulk compositional data (including C and O) for 11 hydrated IDPs that were subsequently examined in the TEM to determine their mineralogy and mineral chemistry. Our analysis indicates that these hydrated IDPs are strongly enriched in C relative to the most C-rich meteorites. The average abundance of C in these hydrated IDPs is 4X CI chondrite values. Thomas K. L.* Keller L. P. Blanford G. E. McKay D. S. Cometary Interplanetary Dust Particles? An Update on Carbon in Anhydrous IDPs Chondritic anhydrous interplanetary dust particles (IDPs) are widely considered to be the most pristine samples available for the study of the early solar system because of their primitive mineralogy, chemistry, and isotopic characteristics. Previously, we quantitatively analyzed anhydrous IDPs for light elements and found that these particles have significantly higher bulk carbon abundances than known chondritic meteorites. We have also identified a relationship between carbon abundance and silicate mineralogy that, in general, shows that particles dominated by pyroxenes have a higher carbon abundance than those dominated by olivines. Particles containing equal amounts of olivine and pyroxene show a range of carbon contents and can be grouped with either the pyroxene- or olivine-dominated particles based on their carbon abundance. We have suggested that high carbon pyroxene-rich IDPs are derived from cometary sources. Bradley J. P.* Unequilibrated, Equilibrated, and Reduced Aggregates in Anhydrous Interplanetary Dust Particles Track-rich anhydrous IDPs are probably the most primitive IDPs because (1) they have escaped significant postaccretional alteration, (2) they exhibit evidence of (nebular) gas phase reactions, (3) their mineralogy is similar to comet Halley's dust, and (4) some of them exhibit cometlike IR spectral characteristics. However, basic questions about the mineralogy and petrography of anhydrous IDPs remain unanswered because they contain aggregated components that can be heterogeneous on a scale of nanometers. In some IDPs, aggregates account for >75% of the volume of the particle. The aggregates have been systematically examined using an analytical electron microscope (AEM), which provides probe-forming optics and (X-ray and electron) spectrometers necessary to analyze individual nanometer-sized grains. The AEM results reveal at least three mineralogically distinct classes of aggregates in anhydrous IDPs, with mineralogies reflecting significantly different formation/aggregation environments. Flynn G. J.* Sutton S. R. Bajt S. Trace Element Contents of Chondritic Cosmic Dust: Volatile Enrichments, Thermal Alterations, and the Possibility of Contamination We have measured trace element abundances in 51 chondritic interplanetary dust particles (IDPs) by Synchrotron X-Ray Fluorescence (SXRF). The data allow us to determine an average composition of chondritic IDPs and to examine the questions of volatile loss during the heating pulse experienced on atmospheric entry and possible element addition due to contamination during atmospheric entry, stratospheric residence, and curation. Rietmeijer F. J. M.* Micrometeorite Dynamic Pyrometamorphism: Observation of a Thermal Gradient in Iron-Nickel Sulfide Dynamic pyrometamorphism describes the mineralogical changes in a micrometeorite that occur in response to flash-heating during atmospheric entry. Mineral reactions during this event will be endothermic and act as local sinks for thermal energy, which delays a uniform temperature distribution in decelerating micrometeorites. The most common pyrometamorphic reaction is formation of magnetite nanocrystal decorations on Mg,Fe-silicate grains and magnetite or (partial) maghemite rims. Constituent silicates also respond to this thermal event and show formation of olivine-maghemite symplectites and growth of garnet and partial laihunitisation. In a continued effort to constrain dynamic pyrometamorphism of unmelted interplanetary dust particles (IDPs), I determined the mineralogical composition, and Fe,Ni- sulfide chemistry in the sulfide-rich particles L2005C39, L2005E40, and L2006A28 using a JEOL 2000FX analytical electron microscope equipped with a TN5500 energy dispersive spectrometer for in situ microanalysis. Sulfide compositions in these three IDPs are distinctly different from those in silicate-rich particle L2005T13, which might suggest a delicate balance of the fO2/fS2 ratios during dynamic pyrometamorphism. Nier A. O.* Schlutter D. J. Helium in Interplanetary Dust Particles Helium and neon were extracted from fragments of individual stratosphere- collected interplanetary dust particles (IDPs) by subjecting them to increasing temperature by applying short-duration pulses of power in increasing amounts to the ovens containing the fragments. The experiment was designed to see whether differences in release temperatures could be observed that might provide clues as to the asteroidal or cometary origin of the particles. Variations were observed that show promise for elucidating the problem. Brownlee D. E.* Joswiak D. J. Love S. G. Nier A. O. Schlutter D. J. Bradley J. P. Identification of Cometary and Asteroidal Particles in Stratospheric IDP Collections We have determined He release temperature curves for a specially processed set of 5-micrometer to 15-micrometer stratospheric IDPs whose masses, densities, and compositions have been accurately measured. The He release temperature in combination with atmospheric entry calculations yields a most probable entry velocity for each particle and association with either an asteroidal (low- velocity) or cometary (high-velocity) origin. We find that over half the 5-15- micrometer IDPs have entry velocities consistent with asteroidal origin and that at least 20% have cometary origins. A few of the asteroidal particles are porous aggregates and it appears that there may be close material similarities among some primitive asteroids and comets. In the processing of individual 5- micrometer IDPs and determination of entry velocities we preserve a few dozen microtome slices that can be used for a variety of detailed TEM, IR, and ion probe studies. These procedures provide laboratory samples that can be generically associated with asteroids and comets and are in a sense a limited sample return mission from these primitive bodies. Presper T.* Kurat G. Koeberl C. Palme H. Maurette M. Elemental Depletions in Antarctic Micrometeorites and Arctic Cosmic Spherules: Comparison and Relationships Antarctic micrometeorites (MMs) and Arctic cosmic spherules (CSs) have bulk compositions comparable to those of chondritic meteorites. However, abundances of Na, Ca, Mn, Ni, Co, and S are commonly lower in MMs and CSs as compared to chondrites. Our SEM, EMP, and INAA studies suggest that these elemental depletions in unmelted MMs are likely to be due to leaching of soluble components from the MMs in the upper atmosphere and the melt ice water. Depletions in CSs appear to be mainly due to volatilization during melting in the atmosphere or to sampling bias during aggregate formation or parent rock break- up. Barrett R. A.* Zolensky M. E. Bernhard R. Mineralogy of Chondritic Interplanetary Dust Particle Impact Residues from LDEF A detailed structural and compositional analysis of several impactor residues was performed using transmission electron microscopy, energy dispersive spectroscopy, and electron diffraction. Residues from the interior of several craters in gold surfaces were removed with a tungsten needle, mounted in EMBED-812 epoxy, and ultramicrotomed. The presence in these residues of equilibrated ferromagnesian minerals, recrystallization textures, glass, and melted metal and sulfide bodies decorating grain boundaries is indicative of varying degrees of shock metamorphism in all impact residues we have characterized. Bernhard R. P.* See T. H. Horz F. Composition and Modal Frequencies of Hypervelocity Particles <1 mm in Diameter in Low-Earth Orbit We have continued to systematically analyze the projectile residues associated with hypervelocity impact features on the surfaces of the "Chemistry of Micrometeoroid Experiment" (CME) that was in low-Earth orbit (LEO) for 5.7 years aboard the Long Duration Exposure Facility (LDEF). Details of the CME instrument are presented in Horz et al. (1991). Last year we reported on the systematic analysis, via SEM-EDX methods, of all craters >20 micrometers in diameter (i.e., 199) on the collectors from LDEF's trailing edge (location A03; 0.85 m^2 of high-purity gold), and ~200 craters >75 micrometers in diameter from the aluminum 1100 (>99% aluminum) substrates from the forward- facing surfaces. This latter group represents <20% of all craters on the A11 collectors, because the particle flux is substantially higher for the forward- facing orientations relative to the trailing-edge surfaces. To date, we have analyzed some 600 craters on the A11 aluminum collectors, yet this report summarizes only 400 features, because not all of the results have been completely reduced and entered into our database. McDonnell J. A. M.* Resolving LDEF's Flux Distribution: Orbital (Debris?) and Natural Meteoroid Populations A consistent methodology for the collation of data from both penetration and perforation experiments and from data in the Meteoroid and Debris Special Investigator Group (M-D SI G) database has led to the derivation of the average impact flux over LDEF's exposure history 1984-1990. Data are first presented for LDEF's N, S, E, W, and Space faces ("offset" by 8 degrees and "tilted" by 1 degree respectively). A model fit is derived for ballistic limits of penetration from 1 m to l mm of aluminum target, corresponding to impactor masses from 10^-18 kg ( for rho(sub)p = 2 g/cm^3) to l0^-10 kg (for rho(sub)p = 1 g/cm^3). A second-order harmonic function is fitted to the N, S, E, and W fluxes to establish the angular distribution at regular size intervals; this fit is then used to provide "corrected" data corresponding to fluxes applicable to true N, S, E, W, and Space directions for a LEO 28.5 degree inclination orbit at a mean altitude of 465 km. Wednesday, March 17, 1993 Martian Surface Mineralogy and Spectroscopy 8:30 - 12:00 a.m. Room B Chair(s): J. Bishop R. V. Morris Bell J. F. III* Mustard J. F. A Comparison of Telescopic and Phobos-2 ISM Spectra of Mars in the Short-Wave Near-Infrared (0.76-1.02 micrometers) Recent analyses of near-IR (0.76-3.16 micrometers) Mars surface reflectance spectra obtained by the Phobos 2 ISM instrument during early 1989 have revealed the presence of substantial variability in surface spectral properties. Strong absorption features seen in the 0.85-1.05-micrometer region are up to 10-15% deep relative to the local continuum and have been interpreted as evidence of Fe2+ and Fe3+ -bearing minerals (pyroxenes and iron oxides, respectively). Though these observed band depths are comparable to those seen in laboratory reflectance spectra, they are up to three times larger than most previously reported band depths for Mars spectra at these wavelengths. Thus, we have posed the following questions: (1) Do the ISM data accurately represent the spectral behavior of the martian surface in the near- IR? If so, are the differences between the ISM spectra and decades of previous groundbased observations a result of (a) differences in spatial resolution, or (b) changes in the spectral character of the martian surface and/or atmosphere with time? If not, are the differences due to calibration errors in the groundbased data, the ISM spectra, or both? (2) If these large band depths are indeed real, what are the implications for Mars surface mineralogy and past/present weathering environments? Mustard J. F.* Murchie S. L. Erard S. Composition of Weakly Altered Martian Crust The mineralogic and chemical composition of weakly altered crust remains an unresolved question for Mars. Dark regions hold clues to the composition since they are thought to comprise surface exposures of weakly altered crustal materials. Understanding the in situ composition of relatively pristine crustal rocks in greater detail is important for investigating basic volcanic processes. Also, this will provide additional constaints on the chemical pathways by which pristine rocks are altered to produce the observed ferric iron-bearing assemblages and inferred clay silicate, sulphate, and magnetic oxide phases. Reflectance spectra of dark regions obtained with the ISM instrument are being used to determine the basic mineralogy of weakly altered crust for a variety of regions on Mars. The first results of this analysis are presented here. Miller J. S.* Singer R. B. Analysis of Pyroxene Absorptions Observed in Martian Dark Regions We have investigated the mineralogy of the martian dark regions by analysis of the pyroxene Fe2+ absorption band near 1.0 micrometer in a set of VIS/NIR reflectance spectral images. The data used for these analyses were selected from a larger set, extending from 0.44 to 1.02 micrometers, obtained during the close 1988 opposition and covering substantially all of Mars south of 40 degrees N. This dataset is being used in regional mapping of spectral parameters related to surface mineralogy. Martian dark regions are of interest in reconstructing the geologic history because they contain exposures of unaltered or little-altered basaltic crustal material; mapping differences in composition among dark regions could reveal regional or temporal variation in magmatic activity or mantle source composition. Two types of dark regions are seen, with the pyroxene band present and absent; where present, the inferred composition is in the range pigeonite-augite to very high-Fe, low-Ca pyroxene, with a two-pyroxene mixture possible. Sabol D. E. Jr.* Bell J. F. III Adams J. B. Detectability of Crystalline Ferric and Ferrous Minerals on Mars Telescopic and spacecraft spectroscopic and geochemical data have been used to constrain the surface mineralogy of Mars and to yield clues about past and present Mars surface weathering/alteration scenarios. Based primarily on their visible to near-IR reflectance properties, several terrestrial iron-bearing minerals have been either identified on Mars or proposed as Mars spectral analogues. Among these are crystalline hematite, pyroxenes, and poorly crystalline materials like nonophase hematite and palagonite. Other iron-bearing minerals including (but not limited to) nontronite, magnetite, jarosite, and goethite, have been proposed as Mars surface constituents based on Viking Lander measurements or geochemical modeling. If present on Mars, these materials probably appear as spectral mixtures at the coarse spatial resolution of remotely sensed data. The detectability of any of these components must be evaluated relative to the other (background) components with which they occur. The primarily goal of this study is to determine how much of any given mineral would have to be present for it to be detectable in remotely-sensed data. Here, the detectability of iron-bearing minerals ("target spectra") was evaluated using detection threshold analysis (DTA), an analytical technique based on spectral mixture analysis. Murchie S.* Mustard J. Erard S. Geissler P. Singer R. Variations in the Fe Mineralogy of Bright Martian Soil Bright regions on Mars are interpreted as "soil" derived by chemical alteration of crustal rocks, whose main pigmentary component is ferric oxide or oxyhydroxide. The mineralogy and mineralogic variability of ferric iron are important evidence for the evolution of martian soil: mineralogy of ferric phases is sensitive to chemical conditions in their genetic environments, and the spatial distributions of different ferric phases would record a history of both chemical environments and physical mixing. Reflectance spectroscopic studies provide several types of evidence that discriminate possible pigmentary phases, including the position of a crystal field absorption near 0.9 micrometers and position and strengths of absorptions in the UV-visible wavelength region. Recent telescopic spectra and laboratory measurements of Mars soil analogues suggest that spectral features of bright soil can be explained based on a single pigmentary phase, hematite, occurring in both "nanophase" and more crystalline forms. Here we report a systematic investigation of martian bright regions using ISM imaging spectrometer data, in which we examined spatial variations in the position and shape of the ~0.9- micrometer absorption. We found both local and regional heterogeneities that indicate differences in Fe mineralogy. These results demonstrate that bright soils do not represent a single lithology that has been homogenized by eolian mixing, and suggest that weathering of soils in different geologic settings has followed different physical and chemical pathways. Farrand W. H.* Singer R. B. A Comparison of the Visible and Near Infrared Reflectance of Hydrovolcanic Palagonite Tuffs and Martian Weathered Soils The visible and near-infrared reflectance of martian weathered soils, abundant in the bright regions, have been shown to resemble certain examples of the X- ray amorphous mineraloid palagonite. To date, most comparisons between terrestrial palagonites and martian telescopic spectra have been done using palagonites that were formed under ambient semi-arid weathering conditions on Hawaiian volcanoes such as Mauna Kea. Here we examine palagonites associated with the tephra deposits that make up tuff rings and tuff cones. Tuff rings and tuff cones result from hydrovolcanic activity, defined as the interaction of magmas (in this instance, of basaltic composition) with surface or near- surface water. Tuff rings and tuff cones can contain variable amounts of country rock and juvenile crystals, but their primary constituent is fine- grained sideromelane (basaltic glass). Sideromelane alters to palagonite. Unlike the aforementioned Hawaiian palagonites, the alteration of sideromelane to palagonite seems to occur relatively rapidly in tuff cones, on the order of days to months. In comparing the spectra of hydrovolcanically produced palagonites with telescopic spectra of Mars, one is struck by their similarity in the VNIR and dissimilarity in the SWlR. Bishop J. L.* Pieters C. M. Burns R. G. Reflectance Spectra of Sulfate- and Carbonate-bearing Fe3+-doped Montmorillonites as Mars Soil Analogs Ferric smectites and ferrihydrite may be common alteration products of igneous lithologies on Mars, and experiments involving montmorillonite enriched with Fe3+ support the likelihood of ferric smectites on Mars. Mossbauer spectroscopy has been used to identify ferrihydrite [Fe4(O,OH,H2O)12] as the primary ferric material in Fe3+-doped montmorillonite. Ferrihydrite is especially interesting due to its role as a precursor in the formation of hematite and goethite. Reflectance spectroscopy in the visible and infrared regions are coupled with Mossbauer spectroscopy in this study to characterize the ferric material in montmorillonites containing Fe3+, as well as carbonates or sulfates, in the interlayer region. Morris R. V.* Lauer H. V. Jr. Golden D. C. Terrestrial Impact Melts as Analogues for the Hematization of Martian Surface Materials Visible and near-IR reflectivity and Mossbauer data were obtained on powders of hydrothermally altered impact melt sheets from West Clearwater Lake, Manicouagan, and Ries (Polsingen) impact structures. The data support previous interpretations that martian bright regions spectra can be interpreted by a ferric-bearing phase that has a relatively featureless absorption edge together with some well-crystalline (bulk) hematite to account for the 860-nm hematite band. The data also show that bands at wavelengths longer than 900 nm, which are characteristic of martian dark regions, occur when both hematite and pyroxene are present. It thus follows that hematization of Mars can be attributed, at least in part, to hydrothermal alterations of impact melt sheets. Impact heating could also form bulk Hm from nanophase ferric oxides. Merenyi E.* Edgett K. S. Singer R. B. Deucalionis Regio, Mars: Evidence for a Unique Mineralogic Endmember and a Crusted Surface A small equatorial region south of Sinus Meridiani, Deucalionis Regio, has been found spectrally distinct from other regions as seen in a high spectral resolution telescopic image of the meridian hemisphere of Mars. Analysis of Viking IRTM and other related data suggest that Deucalionis Regio has a crusted surface. The crust-bonding minerals may contribute to the spectral uniqueness of this region. Burns R. G.* Chemical Weathering on Mars: Rate of Oxidation of Iron Dissolved in Brines Salts believed to occur in martian regolith imply that brines occur on Mars, which may have facilitated the oxidation of dissolved Fe2+ ions after they were released during chemical weathering of basaltic ferromagnesian silicate and iron sulfide minerals. Calculations show that the rate of oxidation of Fe2+ ions at -35 degrees C in a 6M chloride-sulfate brine that might exist on Mars is about 10^6 times slower that the oxidation rate of iron in ice-cold terrestrial sea water. Moore J. M.* Bullock M. A. Stoker C. R. Mars Brine Formation Experiment The presence of water-soluble cations and anions in the martian regolith has been the subject of speculation for some time. Viking lander data provided evidence for salt-cemented crusts on the martian surface. This evidence is in the form of lander imagery of friable soil layers and planar fragments of disturbed soil, and the detection of abundant sulfur and some evidence for chlorine in the soil. Martian sulfur is very likely to exist in the form of sulfate, and chlorine in the form of chlorides, chlorates, or perchlorates. Following the Viking landings, there was considerable discussion of other salt-forming materials that "should" exist on the martian surface, including carbonates and nitrates. Very recently, Earth-based IR spectroscopic evidence has been obtained indicating the presence of carbonates as well as sulfates and other hydrates on the martian surface. Carbonates have been detected in the SNC meteorites that are commonly believed to have a martian origin. If the crusts observed at the two Viking landing sites are, in fact, cemented by salts, and these crusts are globally widespread, as IRTM-derived thermal inertia studies of the martian surface seem to suggest, then evaporite deposits, probably at least in part derived from brines, are a major component of the martian regolith. The composition of liquid brines in the subsurface, which not only may be major agents of physical weathering, but may also presently constitute a major deep subsurface liquid reservoir, is currently unconstrained by experimental work. Wenrich M. L.* Christensen P. R. A Study of Carbonates, Sulfates, and Phosphates Using Thermal Emission Spectroscopy Thermal emission spectroscopy is useful for identifying mineralogies including carbonates, sulfates, and phosphates. Each of these groups of minerals has a distinct emissivity profile that allows for general identification (e.g., carbonate). Laboratory data are being collected that suggest the potential for determining specific composition of these minerals (e.g., calcite, magnesite). Previous studies of Mars suggest that the above groups of minerals should be present. On Mars fine-grained mineralogies are likely to be intimately mixed due to aeolian activity. Mixtures of calcite with palagonite will be studied to determine the volume percent requirement for salt identification and to understand the complexities of fine-grained mixtures observed by thermal emission. Further work with mixtures will include sulfate and phosphate mineralogies. Christensen P. R.* Harrison S. T. Barbera P. Ruff S. Thermal-Infrared Emission Spectroscopy of Natural Surfaces: Application to Coated Surfaces Thermal-infrared vibrational spectroscopy has great potential for remotely determining the composition of planetary surface materials, taking advantage of fundamental molecular vibrational modes that produce spectral features in the range from 3 to >50 micrometers. Much of the current interest in the thermal infrared wavelength region stems from the deployment of multispectral scanners, such as the Thermal Infrared Multispectral Scanner (TIMS) and the Mars Observer Thermal Emission Spectrometer (TES). In order to support these missions, it is necessary to develop an appropriate library of laboratory measurements. Presley M. A.* Christensen P. R. Thermal Conductivity Measurements of Particulate Materials Under Martian Conditions The mean particle diameter of surficial units on Mars has been approximated by applying thermal inertia determinations from the Mariner 9 Infrared Radiometer and the Viking Infrared Thermal Mapper data together with thermal conductivity measurements. Several studies have used this approximation to characterize surficial units and infer their nature and possible origin. Such interpretations are possible because previous measurements of the thermal conductivity of particulate materials have shown that particle size significantly affects thermal conductivity under martian atmospheric pressures. The transfer of thermal energy due to collisions of gas molecules is the predominant mechanism of thermal conductivity in porous systems for gas pressures above about 0.01 torr. At martian atmospheric pressures the mean free path of the gas molecules becomes greater than the effective distance over which conduction takes place between the particles. Gas particles are then more likely to collide with the solid particles than they are with each other. The average heat transfer distance between particles, which is related to particle size, shape, and packing, thus determines how fast heat will flow through a particulate material. Wednesday, March 17, 1993 Moon Rocks: From the Highlands to the Maria to Antarctica 8:30 - 12:00 a.m. Room C Chair(s): O. B. James H. Takeda Taylor S. R.* Norman M. D. Esat T. M. The Mg-Suite and the Highland Crust: An Unsolved Enigma Most of the rocks returned from the highlands are polymict breccias, pulverized by the massive bombardment. However, some monomict breccias with low siderophile element contents are considered to be "pristine" rocks that represent the original igneous components making up the highlands crust. Three principal pristine constituents make up the lunar highland crust: ferroan anorthosites, the Mg-suite, and KREEP. Jolliff B. L.* Bishop K. M. Apollo 17 Materials Viewed from 2-4 mm Soil Particles: Pre-Serenitatis Highlands Components Among the highlands lithologies of 2-4-mm rock fragments in North Massif soil 76503, we have found a compositional group, low in incompatible element concentrations, that we interpret as representing the pre-Serenitatis surface. A component of these materials is an igneous-textured lithology that we believe formed in large impact melts. These are compositionally similar to, and possibly precursors of, many of the granulitic breccias that appear to be mixtures of ferroan and magnesian-suite rocks. The polymict, or old, upper- crustal breccias, along with granulitic breccias and the endogenous igneous lithologies found particularly at the North Massif stations, constitute the poorly consolidated portions of North Massif. Highland samples from the South Massif, on the other hand, are enriched in materials of the competent, impact- melt breccias formed by the Serenitatis impact. The competent melt-breccias contain clasts of most of the pre-existing surface materials, but they also contain components not found in the rocks of the poorly consolidated massif materials. Cushing J. A.* Taylor G. J. Norman M. D. Keil K. The Granulite Suite: Impact Melts and Metamorphic Breccias of the Early Lunar Crust The granulite suite consists of two major types of rocks. One is coarse-grained and poikilitic with many euhedral crystals of olivine and plagioclase. These characteristics indicate crystallization from a melt; the poikilitic granulites are impact melt breccias. The other group is finer-grained and granoblastic, with numerous triple junctions; the granoblastic granulites are metamorphic rocks. Compositional groups identified by Lindstrom and Lindstrom contain both textural types. Two-pyroxene thermometry indicates that both groups equilibrated at 1000 to 1150 degrees C. Calculations suggest that the granoblastic group, which has an average grain size of about 80 micrometers, was annealed for <60,000 years at 1000 degrees C, and for <2500 years at 1150 degrees C. Similar equilibration temperatures suggest that both groups were physically associated after impact events produced the poikilitic melts. Granulitic impactites hold important information about the pre-Nectarian bombardment history of the Moon, and the composition and thermal evolution of the early lunar crust. Norman M. D.* Alibert C. McCulloch M. T. Fragments of Ancient Lunar Crust: Ferroan Noritic Anorthosites from the Descartes Region of the Moon Noritic anorthosite clasts from breccia 67016 have bulk compositions similar to that of the upper crust of the Moon, and petrogenetic affinities with pristine ferroan anorthosites. Rb-Sr and Sm-Nd isotopic compositions of mineral separates from one of these clasts suggest very old (>= 4.4 Ga) ages, but interpretation of these data is complicated by the multistage history of the clasts, which involved magmatic crystallization, brecciation, subsolidus recrystallization, and sulfide metasomatism. These clasts record some of the earliest events on the Moon, including early crust formation, accretionary bombardment. and degassing of the lunar interior. Premo W. R.* Tatsumoto M. U-Pb Isotopic Systematics of Ferroan Anorthosite 60025 Preliminary U-Pb isotopic data from separates of ferroan anorthosite 60025 confirm its antiquity at ~4.42 Ga. Three Pb-Pb isochron ages involving different sets of mineral separates vary by only 20 m.y., but indicate derivation of the sets from isotopically distinct magma sources. If this anorthosite was a monomict cumulate product formed during the Moon's early primary differentiation stage, then residual liquids of crystallizing magmas were evolving isotopically, even at the centimeter scale, over the duration of the crystallization period. Another explanation is that this sample is simply a polymict breccia and that the Pb isotopic results are a result of subsequent mechanical mixing of mineral assemblages from various cumulate piles formed coevally at ~4.42 Ga from isotopically distinct magma sources. Snyder G. A.* Taylor L. A. Halliday A. N. Geochronology and Petrogenesis of the Western Highlands Alkali Suite: Radiogenic Isotopic Evidence from Apollo 14 Several rocks of alkalic affinity, from the western highlands of the Moon, have been analyzed for their Nd and Sr isotopic compositions. One sample yields a Sm-Nd mineral isochron of 4110 + 41 Ma. This age, in conjunction with U-Pb zircon ages on two other alkalic rocks from the Apollo 14 landing site, suggests a distinct western highlands alkalic "event" which was approximately 100 Ma in duration. Since the last dregs of the lunar magma ocean probably crystallized prior to 4.3 Ga, this alkalic "event" may have included the re- melting of evolved plutons or the remobilization of urKREEP trapped liquid from upper mantle cumulates. Shih C.-Y.* Wiesmann H. Garrison D. H. Nyquist L. E. Bogard D. D. Chronology of Lunar Granite 12033,576: Resetting of Rb-Sr and K-Ca Isochrons Lunar granite 12033,576 is a subsample of the "large" (~1 g) felsite 12033,507, which was identified from a collection of 4-10 mm particles from the 12033 soil sampled from the north rim of Head Crater in the eastern part of Oceanus Procellarum. Discordant ages of ~3.6, ~0.8, ~3.9 and ~2.2 Ga for this lunar granite were obtained, respectively, by the K-Ca, 39Ar-40Ar and U- Pb zircon methods in previous studies and by the Rb-Sr method in this study. Assuming the granite crystallized ~3.9 Ga ago (zircon age), and was shocked by meteoritic impacts at 0.8 Ga ago (39Ar-40Ar age), the intermediate apparent ages by the Rb-Sr and K-Ca methods can be interpreted as reset by diffusion of the parent and daughter nuclides. The Rb-Sr age is less resistant to resetting than the K-Ca age, but more resistant than the 39Ar-40Ar age. Yanai K. Takeda H. Lindstrom M. M.* Tatsumoto M. Torigoe N. Misawa K. Warren P. H. Kallemeyn G. W. Koeberl C. Kojima H. Takahashi K. Masuda A. Nishiizumi K. Consortium Reports on Lunar Meteorites Yamato 793169 and Asuka 881757, a New Type of Mare Basalt Consortium studies on lunar meteorites Yamato 793169 and Asuka 881757 (formerly Asuka-31) were performed to characterize these new samples from unknown locations in the lunar mare. Both meteorites are coarse- grained mare rocks having low Mg/Fe ratios (bulk mg' = 31-35) and low TiO2 (1.5-2.5% in homogenized bulk samples). They are intermediate between VLT and low-Ti mare basalts. Although these meteorites are not identical to each other, their mineral and bulk compositions, isotopic systematics, and crystallization ages are remarkably similar and distinct from those of all other mare basalts. They appear to represent a new type of low-Ti mare basalt that crystallized at about 3.9 Ga. These meteorites are inconsistent with the canonical correlation between the TiO2 contents and ages of mare basalts and suggest that our knowledge of lunar volcanism is far from complete. Takeda H.* Arai T. Saiki K. Ti-bearing Oxide Minerals in Lunar Meteorite Y793169 with the VLT Affinity Lunar meteorite Yamato 793169, previously classified as a VLT mare basalt, contains considerable amounts of Ti-bearing oxides in the mesostasis area (2 vol%). Mineralogical study of these oxides revealed that they are isolated grains of ilmenite, ulvospinel, and chromite, which are formed at the last stage of crystallization. Ti/(Ti+Al+Cr) and Cr/(Cr+Al) versus Fe/(Mg+Fe) variations of these phases are not in the same trends as in the low Ti pigeonite basalts of Apollo 12 and 15 in spite of its higher TiO2 contents. Warren P. H.* Lindstrom M. M. Consortium Study of Lunar Meteorites Yamato-793169 and Asuka-881757: Geochemical Evidence of Mutual Similarity, and Dissimilarity vs. Other Mare Basalts Compositions of bulk powders and separated minerals from two meteorites derived from the mare lava plains of the Earth's Moon, Yamato-793169, and Asuka-881757, indicate a remarkable degree of similarity to one another, and clearly favor lunar origin. However, these meteorites are unlike any previously studied lunar rock. In both cases, the bulk-rock TiO2 content is slightly greater than the level separating VLT from low-Ti mare basalt, yet the Sc content is much higher than previously observed except among high-Ti mare basalts. Conceivably, the Sc enrichment in A881757 reflects origin of this rock as a cumulate from a mare magma of "normal" Sc content, but this seems unlikely. Mineral-separate data suggest that most of the Sc is in pyroxene, and a variety of evidence weighs against the cumulus hypothesis as a major cause for the high Sc. The remarkable similarity between Y793169 and A881757 suggests the possibility that they were derived from a single source crater on the Moon. MARE BASALTIC LUNAR METEORITES: Warren P. H. and Lindstrom M. M. PRISTINE NONMARE ROCKS: Warren P. H. Torigoye N.* Misawa K. Dalrymple G. B. Tatsumoto M. U-Th-Pb, Sm-Nd, and Ar-Ar Isotopic Systematics of Lunar Meteorite Yamato 793169 U-Th-Pb, Sm-Nd, and 40Ar-39Ar isotopic studies were performed on Yamato (Y)- 793169, an unbrecciated diabasic lunar meteorite whose chemical composition is close to low Ti (LT) and very-low-Ti (VLT) mare basalts. The isotopic data indicate that the meteorite was formed earlier than 3.9 Ga from a source with low U/Pb and high Sm/Nd and was disturbed by a thermal event at 751 Ma. Due to the small sample size (104 mg), a plagioclase crystal and glass grains were handpicked for Ar analysis, leaving four fractions for the U-Th-Pb and Sm-Nd studies; a fine-grained fraction (<63 micrometers; Fine) and three medium- grained fractions (63-150 micrometers). Medium-grained fractions were divided by density: a heavy fraction (rho > 3.3) consisting mainly of pyroxene (PX1), a lighter fraction (rho < 2.8)) consisting of plagioclase (PL), and a middle density fraction (predominantly pyroxene; PX2). The fractions were washed with acetone and alcohol, and then leached in 0.01 HBr and 0.1N HBr in order to remove any terrestrial Pb contamination. Analysis of the HBr leaches revealed that this meteorite was heavily contaminated with terrestrial Pb during its residence in Antarctic ice. Nyquist L. E.* Shih C.-Y. Wiesmann H. Bansal B. M. 146,147Sm-142,143Nd Formation Interval for the Lunar Mantle and Implications for Lunar Evolution Small anomalies in the isotopic abundance of 142Nd have been measured for two A17 high-Ti basalts, ilmenite basalt 12056, olivine-pigeonite basalt 12039, feldspathic basalt 12038, and two KREEP basalts. These anomalies correlate with 147Sm/144Nd for the basalt source regions as calculated from initial 143Nd/144Nd ratios in the basalts, and are interpreted to be from decay of 146Sm (t(sub)1/2 = 103 Ma) in distinct lunar mantle reservoirs. A three-stage model for evolution of 143Nd/144Nd and 142Nd/144Nd yields reservoir 147Sm/144Nd ratios which, with the 142Nd/144Nd ratios in the basalts, form a "mantle isochron" giving a lunar mantle formation interval of 94 -20/+23 Ma (2 sigma). Calculated reservoir Sm/Nd ratios are in the range expected from some earlier models of basalt petrogenesis. The isochron value of 142Nd/144Nd at 147Sm/144Nd(sub)CHUR = 0.1967 is within error limits of the average 142Nd/144Nd measured for an L6 chondrite, an H5 chondrite, and the Orgueil carbonaceous chondrite. Evolution of 143Nd and 142Nd for high-Ti basalt 70135 was modeled precisely, starting from chondritic relative REE and Nd-isotopic abundances and using the initial (146Sm/144Sm)(sub)0 ratio inferred from a previous study of angrite LEW86010 as the initial solar system value of this parameter. We infer that the initial Sm/Nd ratio in precursor lunar materials was very nearly chondritic (within ~8%) prior to lunar differentiation. Neal C. R.* Taylor L. A. Petrogenesis of Apollo 12 Mare Basalts, Part 1: Multiple Melts and Fractional Crystallization to Explain Olivine and Ilmenite Basalt Compositions Mare basalts returned by the Apollo 12 mission have been divided into four groups on the basis of mineralogy and whole-rock chemistry: (1) Olivine Basalts; (2) Pigeonite basalts; (3) Ilmenite Basalts; and (4) Feldspathic Basalts. James and Wright (1972) and Rhodes et al. (1977) concluded that the olivine and pigeonite groups were comagmatic and that the within group variations are due to fractional crystallization of olivine and minor Cr- spinel, with pigeonite replacing olivine in the pigeonite basalts. Rhodes et al. concluded that the parental compositions for these suites were probably represented by the vitrophyres, and the olivine basalts are comprised essentially of cumulates and the pigeonites of evolved end members. However, Neal et al. (1992a,b, 1993)have demonstrated, using trace-element considerations, that the Apollo 12 olivine and pigeonite suites are not related. The ilmenite basalts were studied extensively by Dungan and Brown (1977) who noted that both cumulates and evolved fractionates were present within this group. In their modeling, Dungan and Brown used the vitrophyre compositions as parents. Neal et al. demonstrated that the feldspathic suite is probably comprised of only one member--12038. The ilmenite and olivine basalts are demonstrated to be the products of several nonmodal partial melting events of a single source followed by closed-system fractional crystallization. Jerde E. A.* Snyder G. A. Taylor L. A. On the Composition of neuKREEP: QMD Contamination at Apollo 11? The Group A basalts of Apollo 11 differ in many respects from other high-Ti basalts of the region. Chemically, they are the only high-K (>2000 ppm K) variety or high-Ti basalt and are enriched in incompatible trace elements relative to other basalts from both the Apollo 11 and Apollo 17 sites. In addition, Group A basalts are the youngest of all high-Ti basalts, with an age of 3.56 +- 0.02 Ga. The cluster of compositions is consistent with the Apollo 11 Group A basalts representing a single flow. Papanastassiou et al. have also indicated the uniqueness of these basalts, based particularly on relatively young Rb-Sr model ages (3.8-3.9 Ga). A model for the formation of the Group A basalts was presented by Jerde et al., wherein the Apollo 17 orange volcanic glass is the parent liquid. Fractionation of this composition, coupled with the assimilation of incompatible-element-rich material, results in compositions akin to those of the Apollo 11 Group A basalt population. Orange glass of similar major-element composition is present at the Apollo 11 site as well, although complete trace element analyses are not available. New modeling results using the Apollo 11 orange glass major elements are grossly similar to those obtained using the Apollo 17 orange glass. indicating ~30% fractionation Wednesday, March 17, 1993 Ordinary and Enstatite Chondrites 1:30 - 5:00 p.m. Room A Chair(s): K. Keil M. Prinz Rubin A. E.* Euhedral Metallic-Fe-Ni Grains in Extraterrestrial Samples Metallic Fe-Ni is rare in terrestrial rocks, being largely restricted to serpentinized peridotites and volcanic rocks that assimilated carbonaceous material. In contrast, metallic Fe-Ni is nearly ubiquitous among extraterrestrial samples (i.e., meteorites, lunar rocks, and interplanetary dust particles). Anhedral grains are common. For example, in eucrites and lunar basalts, most of the metallic Fe-Ni occurs interstitially between silicate grains and thus tends to have irregular morphologies. In many porphyritic chondrules, metallic Fe-Ni and troilite form rounded blebs in the mesostasis because their precursors were immiscible droplets. In metamorphosed ordinary chondrites metallic Fe-Ni and troilite form coarse anhedral grains. Some of the metallic Fe-Ni and troilite in meteorites and lunar rocks has also been mobilized and injected into fractures in adjacent silicate grains where local shock-reheating temperatures reached the Fe-FeS eutectic (988 degrees C). In interplanetary dust particles metallic Fe-Ni most commonly occurs along with sulfide as spheroids and fragments. Yang C. W.* Williams D. B. Goldstein J. I. Preliminary AEM Study of the Microstructure and Composition of Metal Particles in Ordinary Chondrites The purpose of this study is to examine the microstructure and composition of the metal particles in ordinary chondrites using analytical electron microscopy (AEM) techniques. Since the phases produced within the metal particles are very fine, the application of various AEM techniques for structural and chemical characterization is critical. However, thin specimen preparation for AEM study has proven very difficult because of the matrix silicate that is present. This is the first AEM study of the metal particles in chondrites. A type 6 chondrite, Saint Severin (LL6), was selected for examination because the metal phases have been reheated into the single-phase taenite region (>700 degrees C), and cooled slowly to lower temperatures. Acombination of electron optical instruments was employed including a field emission gun (FEG) JEOL 840F high-resolution scanning electron microscope (HRSEM), a JEOL 6300F FEG-HRSEM, a Philips 400T AEM, and a JEOL 733 electron probe microanalyzer (EPMA). Podosek F. A.* Brannon J. C. Perron C. Pellas P. Elevated Initial 87Sr/86Sr in Ordinary Chondrite Metal The metal phase of ordinary chondrites has been found to contain a complex assemblage of nonmetal phases, evidently formed when elements such as Si, Cr, and P, originally dissolved in the metal, were subsequently oxidized and exsolved. We have investigated the Rb-Sr isotopic system in samples of H- chondrite metal, finding small but nontrivial amounts of Sr, apparently concentrated toward the surface of metal grains, and characterized by pronounced excesses of 87Sr relative to what would be expected for the normal chondrite age and the observed Rb/Sr ratio. The high 87Sr/86Sr in chondritic metal can plausibly account for the characteristically elevated initial 87Sr/86Sr found in ordinary chondrite phosphates, but it remains unclear when and where the metal experienced the high Rb/Sr environment needed to account for such high 87Sr/86Sr. Burkland M. K.* Swindle T. D. Baldwin S. L. Studies of the Release of Radiogenic 129Xe from Bjurbole: Evidence Against Simple Diffusion Models The I-Xe system, based on the decay of 15.7 Ma 129I, is a potentially precise chronometer of early solar system events. However, it is not known how the system responded to thermal events during the post-formation histories of ordinary chondrites, severely limiting the usefulness of this system. An extended heating study of Bjurbole suggests that at least three components are responsible for the thermal release of radiogenic 129Xe and that the system may be less susceptible to thermal resetting than some calculations indicated. Kehm K.* Nichols R. H. Jr. Hohenberg C. M. McCoy T. J. Keil K. I-Xe Structure of Ilafegh 009 and Shallowater: Evidence for Early Formation and Rapid Cooling of Impact-derived Enstatite Meteorites Enstatite meteorites have proven to be ideal samples for past studies of the I-Xe system. This work focuses on two enstatite meteorites that were formed by impact processes. Ilafegh 009 is a clast-free impact melt rock from the EL chondrite parent body. The Shallowater aubrite probably formed when a fully molten planetesimal collided with a solid planetesimal, mixing fragments of the solid planetesimal into the enstatite mantle of the molten planetesimal. A complex three-stage cooling history resulted from this mixing and later breakup and reassembly of the parent body. The present study indicates that the I-Xe structure of these two meteorites resulted from in situ decay of live 129I and that both experienced xenon closure of the iodine host phase at approximately the same time. I-Xe cooling rates are consistent with the cooling rates derived from mineralogic and petrologic studies of these objects. The similarities in ages suggest that the region of the nebula in which enstatite parent bodies formed must have experienced an intense early bombardment. Scott E. R. D.* McCoy T. J. Keil K. Post-Metamorphic Brecciation in Type 3 Ordinary Chondrites Type 3.1-3.9 ordinary chondrites can be divided into two kinds: those in which the compositions of chondrule silicates are entirely consistent with metamorphism of type 3.0 material, and those in which the compositional heterogeneity appears to be too extreme for in situ metamorphism. We present petrologic data for three LL3 chondrites of the second kind, Ngawi, ALHA77278 (both type 3.6), and Hamlet (type 3.9), and compare these data with results for the first kind of LL3-4 chondrites. Given that chondrules form in the nebula and that metamorphic equilibration occurs in asteroids, our new data imply that Ngawi, A77278, Hamlet, and many other type 3 ordinary chondrites are postmetamorphic breccias containing materials with diverse metamorphic histories; they are not metamorphic rocks or special kinds of "primitive breccias." We infer also that metamorphism to type 3.1-3.9 levels produces very friable material that is easily remixed into breccias and lithified by mild shock. Thus, petrologic types and subtypes of chondrites indicate the mean metamorphic history of the ingredients, not the thermal history of the rock. The metamorphic history of individual type I or II, porphyritic chondrules in type 3 breccias is best derived from olivine and pyroxene analyses and the data of McCoy et al. (1991) for unbrecciated chondrites. The new chondrule classification scheme of Sears et al. (1991) and DeHart et al. (1991) appears to provide less information about the original state and metamorphic history of individual porphyritic chondrules and should not replace existing classification schemes. Crozaz G.* Hsu W. Anomalous REE Patterns in Unequilibrated Enstatite Chondrites: Evidence and Implications We present here a study of REE microdistributions in unequilibrated enstatite chondrites (EOCs). Although the whole rock REE contents are similar in both unequilibrated and equilibrated chondrites, the host minerals of these refractory elements are different. In the least equilibrated ordinary chondrites (UOCs), the REE reside mainly in glass, whereas, in their more equilibrated counterparts, the bulk of the REE is in calcium phosphate, a metamorphic mineral that formed by oxidation of phosporous originally contained in metal. In the smaller group of enstatite (E) chondrites, calcium phosphate is absent and the phase that contains the highest REE concentrations is a minor mineral, CaS (oldhamite), which contains approximately 50% of the total Ca present. In E chondrites, elements typically considered to be lithophiles (such as Ca and Mn) occur in sulfides rather than silicates. This indicates formation under extremely reducing conditions, thus in a region of the solar nebula distinct from those that supplied the more abundant ordinary and carbonaceous chondrites. Previously, we observed a variety of REE patterns in the oldhamite of UECs; they range from almost flat to some with pronounced positive Eu and Yb anomalies. Here, we searched for complementary REE patterns in other minerals from E chondrites and found them in the major mineral enstatite. Whenever Eu and Yb anomalies are present in this mineral, they are always negative. REE in E CHONDRITES: Crozaz G. and Hsu W. Weisberg M. K.* Prinz M. Fogel R. A. Shimizu N. The Formation of FeO-rich Pyroxene and Enstatite in Unequilibrated Enstatite Chondrites: A Petrologic-Trace-Element (SIMS) Study FeO-rich pyroxenes (Fe-Pyx) are a major constituent of some of the chondrules and fragments in unequilibrated enstatite chondrites (UEC). They contain structurally oriented zones of Cr-, Mn-, V-rich, FeO-poor enstatite (En) that has a red cathodoluminescence and is associated with mm-sized blebs of low-Ni, Fe-metal, and in some cases silica. This represents clear evidence for the process of reduction. The Fe-pyx is nearly always rimmed by minor element-poor En having a blue CL. More commonly, red and blue En occur as larger grains in chondrules and fragments throughout the UEC's, unassociated with Fe-pyx, constituting the major silicate phases. REE abundance patterns of the Fe-pyx are LREE-depleted and red En has a similar pattern. Blue En rims, however, have a flat REE pattern at ~1xChondritic (Ch). The Fe-pyx in UEC's originated from an earlier generation of chondrules that formed in a nebular region that is more oxidizing than that required for enstatite chondrites. Following solidification these chondrules were introduced to a more reducing nebular environment. Some of the red En crystals that are common throughout UEC's may be the products of reduction of Fe-pyx. Blue En rims grew onto the surfaces of the Fe-pyx and red En via condensation from the nebula. Zhang Y.* Benoit P. H. Sears D. W. G. Lewis CLiff 87057: A New Metal-rich E3 Chondrite with Similarities to Mt. Egerton, Shallowater and Happy Canyon The Antarctic meteorites LEW87057, LEW87220, LEW87223, LEW87234, LEW87237, and LEW87285 were described as paired E3 chondrites by Brian Mason (Antarctic Meteorite Newsletters, 12(1) and 15(1,2)). LEW87223 is texturally unusual, containing abundant chondrules and jagged troilite grains that are enclosed in metal. Our INAA data show that the siderophile element abundance in two splits of LEW87223 are higher than the EH range, while chalcophile elements (Cr excepted) are highly depleted. Mineral compositions are unlike those normally found in EH or EL chondrites or aubites, and in some respects resemble those of several anomalous enstatite meteorites such as Shallowater, Mt. Egerton, and Happy Canyon. The bulk and mineral compositions are consistent with the addition of EL chondrite metal to an EL3 chondrite with the removal of sulfides other than troilite, which is now Cr-rich. We suggest that this meteorite is an EL3 chondrite into which metal was introduced and sulfides redistributed during an event involving impact melting and brecciation. Brearley A. J.* Jones R. H. Chondrite Thermal Histories from Low-Ca Pyroxene Microstructures: Autometamorphism vs Prograde Metamorphism Revisited In order to constrain the thermal histories of chondritic meteorites we have carried out a detailed study of the microstructures of low-Ca pyroxenes produced experimentally and in types 4 and 5 ordinary chondrites. We have performed cooling experiments on synthetic MgSiO3 at cooling rates between 2 and 10,000 degrees C/hr from the protopyroxene stability field into that of orthopyroxene (OPX) and annealed the products of these experiments for a variety of annealing times. There are clear microstructural differences between samples that have been cooled and those that have been subsequently annealed. A comparison of the microstructures observed in our experimental samples with those in H4-5 ordinary chondrites shows that they cannot have experienced a single stage cooling history, as proposed for the autometamorphism model. Benoit P. H.* Sears D. W. G. The Great 8 Ma Event and the Structure of the H-Chondrite Parent Body We have recently identified two groups of H5 chondrites with distinct thermal histories, one of which (the "unusual" group) has cosmic ray exposure ages of ~8 Ma and is only found in the Antarctic meteorite collection, and the other, which is found in both Antarctic and non- Antarctic collections. We suggested that these groups are indicative of changes in the meteorite flux over the last million years. We have extended our work to H4 chondrites and find that the groups are also apparent. The most likely origin of the unusual group of H-chondrites is thermal processing during the 8 Ma event, which was the source of large numbers of meteoroid fragments. Wolf S. F. Lipschutz M. E.* Multivariate Statistical Analysis: Principles and Applications to Coorbital Streams of Meteorite Falls Multivariate statistical analysis techniques (linear discriminant analysis and logistic regression) can provide powerful discrimination tools that are generally unfamiliar to the planetary science community. Their use can be illustrated by the following example. Dodd et al. (1992) used fall parameters to identify a group of 17 H chondrites ("Cluster 1") that were part of a coorbital stream that intersected Earth's orbit in May, from 1855-1895, and can be distinguished from all other H chondrite falls. Using multivariate statistical techniques, Dodd et al. (1992) demonstrated that a totally different criterion, labile trace element contents--hence thermal histories--of 13 Cluster 1 meteorites are distinguishable from those of 45 non-Cluster 1 H chondrites. Here, we focus upon the principles of multivariate statistical techniques and illustrate their application using nonmeteoritic and meteoritic examples. Yanai K.* Kojima H. Regolith Breccia Consisting of H and LL Chondrite Mixture Antarctic meteorite Yamato-8424(Y-8424) is a regolith breccia that is homogenized mixture of H and LL chondrite components . The breccia consists mainly of a fine-grained material with mineral fragments of olivine, pyroxene and Fe-Ni metal with traces of plagioclase. Wednesday, March 17, 1993 Mars: Tectonism and Volcanism 1:30 - 5:00 p.m. Room B Chair(s): W. B. Banerdt J. Plescia Zuber M. T.* Wrinkle Ridges, Reverse Faulting, and the Depth Penetration of Lithospheric Stress in Lunae Planum, Mars Tectonic features on a planetary surface are commonly used as constraints on models to determine the state of stress at the time the features formed. Quantitative global stress models applied to understand the formation of the Tharsis province on Mars constrained by observed tectonics have calculated stresses at the surface of a thin elastic shell and have neglected the role of vertical structure in influencing the predicted pattern of surface deformation. Wrinkle ridges in the Lunae Planum region of Mars form a concentric pattern of regularly spaced features in the eastern and southeastern part of Tharsis and formed due to compressional stresses related to the response of the martian lithosphere to the Tharsis bulge. Structurally, the ridges are on the order of 1 km wide, several hundreds of meters high, and are interpreted to be the surface manifestation of reverse faulting in combination with a smaller component of folding. The ridges are located in Early Hesperian-aged plains materials that are interpreted to be flood basalts with thicknesses <1 km. As observed in the exposures of valley walls in areas such as the Kasei Valles, the surface plains unit is underlain by an unconsolidated impact-generated megaregolith that grades with depth into structurally competent lithospheric basement. The ridges have alternatively been hypothesized to reflect deformation restricted to the surface plains unit ("thin-skinned deformation") and deformation that includes the surface unit, megaregolith, and basement lithosphere ("thick-skinned deformation"). The nature of ridge formation has important implications for the interpretation of geophysical models of the Tharsis region. If the ridges formed normal to the maximum compressive stress direction at the surface, then these features can be used to constrain the state of stress determined in elastic spherical shell models. If, however, the ridges nucleated at depth and propagated to the surface, then the orientations of the ridges may have been controlled by a different stress state. Even if ridge-related faults nucleated within the surface plains unit, it is necessary to understand the extent to which vertical mechanical structure affects the near-surface state of stress. Banerdt W. B.* Horizontal Stresses Induced by Vertical Processes in Planetary Lithospheres Understanding the state of stress in the elastic lithosphere is of fundamental importance for planetary geophysics, as it is the link between the observed geologic structures on the surface and the processes that form and modify these structures. As such it can provide valuable constraints for the difficult problem of determining interior structure and processes. On the Earth, most large-scale, organized deformation can be related to lateral tectonics associated with plate dynamics; however, the tectonics on many extra-terrestrial bodies (such as the Moon, Mars, and most of the outer-planet satellites) appears to be primarily vertical in nature, and the horizontal stresses induced by vertical motions and loads are expected to dominate the deformation of their lithospheres. The largest stress contributions from vertical loading come from the flexure of the lithosphere, which induces both bending moments and membrane stresses. We are concerned here only with non- flexural changes in the state of stress induced by processes such as sedimentary and volcanic deposition, erosional denudation, and changes in the thermal gradient that induce uplift or subsidence. This analysis is important both for evaluating stresses for specific regions in which the vertical stress history can be estimated, as well as for applying the proper loading conditions to global stress models. It is also of interest for providing a reference state of stress for interpreting stress measurements in the crust of the Earth. Most of the previous work on this subject has been directed toward the latter problem. Tanaka K. L.* Chadwick D. J. Extensional History of Mars' Tharsis Region Graben structures in the outer part of the Mars' Tharsis region follow calculated stress patterns that are based on Tharsis topographic and gravity anomalies and flexural response of the lithosphere. However, Tharsis extension is not evenly distributed--most extension is concentrated in the Alba, Tempe, Valles Marineris, and Syria-Thaumasia volcanotectonic provinces. Our preliminary results suggest that the overall extension of Tharsis produced by loading is about 8 km (based on the assumption that the Sirenum province exemplifies the overall extension of Tharsis), whereas the total circumferential extension of the Tharsis region estimated thus far is about 33 km (we have not made an estimate for the Syria-Thaumasia province, and evidence in some other areas may be buried). We conclude that local tectonism, possibly caused by intrusion and passive and active rifting, produced most of the extension in the Tharsis region. Chadwick D. J.* Lucchitta B. K. Fault Geometries and Extension in the Valles Marineris, Mars The central troughs of the Valles Marineris system on Mars are generally regarded to be of tectonic origin because they are linear and bounded by faults. Their radial orientation to the Tharsis rise suggests that at least their incipient formation was influenced by the formation of the rise, but the exact role of Tharsis is not yet well understood. Collapse and erosion may have significantly modified the original tectonic troughs. Watters T. R.* Zimbelman J. R. Scott D. H. Arcuate and Circular Structures in the Tharsis Region: Evidence of Coronae on Mars Arcuate and circular structures are evident in the Tharsis region of Mars. They involve concentric graben and fracture systems and are often associated with volcanic centers. The most prominent example is Alba Patera, a low-relief volcanotectonic center. It is surrounded by a graben system comprised of the Alba and Tantalus Fossae and part of the Ceranius Fossae system that are thought to have formed during the Early Amazonian. The graben concentric to Alba Patera form an annulus that is 600 km in diameter. Many of the north- trending graben of Ceranius Fossae were clearly influenced by Alba Patera. Graben are deflected around and in some cases merge with the Alba-concentric system. Other prominent circular structures have been described on the Tempe Terra plateau. The annulus of graben and fractures forming these structures is approximately 250 km in diameter. Graben formed over a period extending from the Late Noachian through the Hesperian with the most pronounced arcuate graben forming during the Noachian. These structures are also associated with volcanic centers. One of the graben sets encircles a volcanic dome and a calderalike depression. As in the case of Alba Patera, northeast-trending graben associated with Tempe and Mareotis Fossae are deflected and often merge with the concentric graben. Another set of arcuate graben occur near the prominent scarp that marks the western boundary of the Lunae Planum plateau. These graben appear to have been part of a circular structure that had a diameter on the order of 600 km, although this can not be confirmed because of subsequent extensive faulting and erosion. The graben that comprise this system formed over a period extending from the Late Noachian through the Early Hesperian. Baloga S. M.* Spudis P. D. Reconstruction of the Dynamics of the 1800-1801 Hualalai Eruption: Implications for Planetary Lava Flows The 1800-1801 eruption of alkalic basalt from the Hualalai volcano, Hawaii provides a unique opportunity for investigating the dynamics of lava flow emplacement with eruption rates and compositions comparable to those that have been suggested for planetary eruptions. Field observations suggest new considerations must be used to reconstruct the emplacement of these lava flows. These observations are (1) The flow traversed the 15 km from the vent to the sea so rapidly that no significant crust formed and an observation of the eruption reported that the flow reached the sea from the vent in approximately 1 hour; (2) The drainage of beds of xenolith nodules indicates a highly fluid, low-viscosity lava; (3) Overspills and other morphologic evidence for a very low viscosity host fluid; (4) No significant longitudinal increase in flow thickness that might be associated with an increase in the rheological properties of the lava; and (5) the relatively large size of channels associated with the flow, up to 80 m across and several kilometers long. Bruno B. C.* Taylor G. J. Lopes-Gautier R. M. C. Quantifying the Effect of Rheology on Plan-View Shapes of Lava Flows This study aims at quantifying the effect of rheology on the plan-view shapes of lava flows. Plan-view shapes of lava flows are important because they reflect the processes governing flow emplacement and may provide insight into lava flow rheology and dynamics. In our earlier investigation we reported that plan-view shapes of tholeiitic basalts are fractal, having a characteristic shape regardless of scale. We also found we could use the fractal dimension (a parameter that quantifies flow margin convolution) to distinguish between the two major types of basalts: a'a and pahoehoe. Encouraged by these earlier results, we are currently developing a similar method for use on silicic flows, and present our preliminary work. Lopes-Gautier R.* Bruno B. G. Taylor G. J. Rowland S. Kilburn C. R. J. Martian Lavas: Three Complementary Remote Sensing Techniques to Derive Flow Properties Several remote sensing techniques have been developed to determine various prope Mouginis-Mark P.* The Influence of Oceans on Martian Volcanism Geomorphological evidence for episodic oceans on Mars has recently been identified. This idea of large bodies of water on Mars is innovative and controversial compared to the more generally accepted view of a "dry Mars," but also enables some of the more enigmatic volcanic landforms to be reinterpreted in a self-consistent model. This hypothesis can be used to develop new models for the mode of formation of several volcanic landforms in the W. Tharsis and S.E. Elysiumia regions of Mars. Gregg T. K. P.* Williams S. N. Explosive Mafic Volcanism on Earth and Mars Deposits within Amazonia Planitia, Mars, have been interpreted as ignimbrite plains on the basis of their erosional characteristics. The western flank of Hecates Tholus appears to be mantled by an airfall deposit, which was produced through magma-water interactions or exsolution of magmatic volatiles. Morphologic studies, along with numerical and analytical modeling of martian plinian columns and pyroclastic flows, suggest that shield materials of Tyrrhena and Hadriaca Paterae are composed of welded pyroclastic flows. Terrestrial pyroclastic flows, ignimbrites and airfall deposits are typically associated with silicic volcanism. Because it is unlikely that large volumes of silicic lavas have been produced on Mars, we seek terrestrial analogues of explosive, mafic volcanism. Plinian basaltic airfall deposits have been well documented at Masaya, Nicaragua, and basaltic ignimbrite and surge deposits also have been recognized there. Ambrym and Yasour, both in Vanuatu, are mafic stratovolcanoes with large central calderas, and are composed of interbedded basaltic pyroclastic deposits and lava flows. Zavaritzki. a mafic stratovolcano in the Kurile Islands, may have also produced pyroclastic deposits, although the exact nature of these deposits is unknown. Masaya, Ambrym, and Yasour are known to be located above tensional zones. Hadnaca and Tyrrhena Paterae may also be located above zones of tension resulting from the formation and evolution of Hellas Basin, and, thus, may be directly analogous to these terrestrial mafic, explosive volcanoes. Plescia J. B.* Geology of Biblis Patera, Ulysses Patera, and Jovis Tholus, Mars There are a variety of constructional volcanic features in Tharsis. These features range from Olympus Mons and the Tharsis Montes shields, to the small low shields and fissure eruptions that characterize much of the volcanic plains, to the smaller volcanic constructs in the northeast and western parts of Tharsis. Here, I describe the geology of the western group, which includes Biblis Patera, Ulysses Patera, and Jovis Tholus. Each of these volcanoes has had a unique, and complex geologic history. Robinson M. S.* Mouginis-Mark P. J. Zimbelman J. R. Wu S. S. C. Chronology, Eruption Duration, and Atmospheric Contribution of Apollinaris Patera, Mars Geologic mapping from Viking image data of the martian volcano Apollinaris Patera has allowed identification of seven major events that have shaped its current morphology. Derivation of new topographic data has allowed accurate estimates of the volume of erupted products from which estimates of an eruption duration are presented for the edifice and its corresponding atmospheric water contribution. Wednesday, March 17, 1993 Lunar Volcanic Glasses and Regolith 1:30 - 5:00 p.m. Room C Chair(s): J. Delano D. S. McKay Delano J. W.* Compositional Heterogeneity Within a Dumbbell-shaped Apollo 15 Green Glass: Evidence for Simultaneous Eruption of Different Magmas The Apollo 15 green glasses that occur most prominently in 15425, 15426, and 15427 consist of compositionally distinct groups. The six groups that have thus far been defined are thought to represent different batches of magma that were erupted as fire-fountains, probably at different times in lunar history; although isotopic analyses of representative glasses within some of these groups have not yet found any differences in age at the l-100 Ma precision. While the petrogenesis of these picritic magmas has been contentious, the discovery of a compositionally heterogeneous, dumbbell-shaped green glass raises additional questions and problems about their origin. Shearer C. K.* Papike J. J. Basaltic Magmatism on the Moon. A Perspective from Volcanic Picritic Glasses It is widely accepted that basaltic magmas are products of partial fusion of peridotite within planetary mantles. As such they provide valuable insights into the structure and processes of planetary interiors. Those compositions that approach primary melt compositions provide both a clearer vision of planetary interiors and a starting point at which to understand basaltic evolution. Within the collection of lunar samples returned by the Apollo and Luna missions are homogeneous, picritic glass beads of volcanic origin. These glass beads provide a unique perspective concerning the origin of mare basalts, the characteristics of the lunar interior and processes culminating in the in the early differentiation of the Moon. In this presentation, we report our ion microprobe derived trace element data from all picritic glasses previously identified. We place this trace element data and literature isotopic and experimental data on the picritic glasses within the framework of mare basaltic magmatism. Finnila A. B.* Hess P. C. Rutherford M. J. Assimilation in Lunar Basalts and Volcanic Glasses: Implications for a Heterogeneous Mantle Source Region Several scientists have called on assimilation of anorthositic crustal material or KREEP compositions to explain various lunar lithologies. In order to address the practicality of such processes, we have outlined some techniques for calculating how much assimilation is possible in magma chambers and dikes based on thermal energy balances and simple fluid mechanical constraints. In a previous effort we demonstrated that dissolution of plagioclase in an iron-free basalt was too slow to contaminate magmas, and that the energy cost of melting plagioclase-rich crustal matenal was prohibitive both in magma chambers and in dike conduits. In the present work we have extended this analysis to include dissolution rates in an orange glass composition and to quantitatively predict the maximum contamination possible due to assimilation of both lunar crustal material and KREEP. McKay D. S.* Wentworth S. J. Grain Surface Features of Apollo 17 Orange and Black Glass Lunar soil sample 74220 and core samples 74001/2 consist mainly of orange glass droplets, droplet fragments, and their crystallized equivalents. These samples are now generally accepted to be pyroclastic ejecta from early lunar volcanic eruptions. It has been long known that they contain surface coatings and material rich in volatile condensable phases including S, Zn, F, Cl, and many volatile metals. Meyer (1990) summarizes the voluminous published chemical data and calculates the volatile enrichment ratios for most of the surface condensates. In an attempt to more completely understand this enrichment of surface volatiles, we have searched for carbon and carbon- bearing phases on droplet surfaces. Rutherford M. J. Fogel R. A.* C-O Volatiles in Apollo 15 and Apollo 17 Picritic Glasses A15 and A17 primitive picritic glasses have been examined by FTIR for the presence of dissolved CO species to determine the role of CO gasses in driving lunar fire fountains. A15 green and yellow glasses were extensively studied and found to be free of dissolved C species down to FTIR detection limits (10- 100 ppm; species and sample specific). Preliminary data on A17 orange glasses are similarly devoid of FTlR detectable CO species. Re-analyses of the CO driving mechanism theory for mare volcanism demonstrates the need to determine the fO2 of the lunar interior, the factor that most critically determined the role of C gasses in the fire-fountaining events. Oxygen fugacities equivalent to IW-0.5 and above imply dissolved CO3= in the primitive glasses at levels above FTIR detection. fO2 below IW-0.5 implies concentrations of CO3= below FTIR detection. Recent data suggesting lunar mantle fO2 of IW-2 or less, strongly mitigates against finding FTIR measurable dissolved C03=, consistent with the findings of this study. Colson R. O.* Graphite "Solubility" and CO Vesiculation in Basalt-like Melts at One Atm The identity and source of the vapor phase that caused lunar lava fountaining and vesiculation in lunar basalts continues to be of interest because of its implications for the composition and state of the lunar interior and because of its implications for lunar resources. In light of the apparent near-absence of H2O on the Moon, it has been suggested that the vapor phase may be CO2-CO. This premise is supported by the presence of carbon on the surface of volcanic glass beads. However, although the rapid exsolution of CO2 from a melt during decompression may be consistent with fire fountaining, it fails to provide a satisfying explanation for vesiculation in mare basalt where exsolution of the gas phase would more reasonably be related to cooling/crystallization at low pressure rather than decompression from high pressure. Also, geochemical trends in lunar volcanic glasses suggest that their source has an oxygen fugacity more reducing than the iron-wustite buffer, an oxygen fugacity that is inconsistent with presence of dissolved CO2-CO at depth. I report results of experiments in which a vesicular "basalt" is produced from a melt equilibrated with graphite and pure CO gas at one atmosphere pressure. The vesiculation is apparently related to exsolution of CO or a CO species during cooling of the melt or growth of quench crystals. Additionally, particulate carbon dispersed through the quenched sample suggests that elemental carbon is either in solution in the melt prior to quenching or tends to go into suspension perhaps as colloidlike particles. These two observations may provide insight into the nature of fire fountaining and vesiculation on the Moon. Hess P. C.* The Ilmenite Liquidus and Depths of Segregation for High-Ti Picrite Glasses Lunar picrite glasses represent primitive and perhaps near-primary liquids that have suffered only minor degrees of crystallization or near-crustal modification. These glasses are multisaturated with olivine and othopyroxene at pressures from 20-25 kb. I argue that high TiO2 mare glasses were indeed equilibrated with orthopyroxene and were segregated from the lunar mantle at mean depths of 400-500 km. Longhi J.* Liquidus Equilibria of Lunar Analogs at High Pressure Melting experiments have been performed in the range of 20 to 40 kbar on partially crystallized synthetic glasses in order to test the accuracy of the polybaric fractional fusion model for picritic lunar green glasses. Results show that the model predicts the position of the olivine(ol) + orthopyroxene(opx) liquidus boundary within the uncertainty of the measurements, but that details of the calculations are subject to change because of new crystal/liquid partitioning data for olivine and pyroxene. Wagner T. P.* Grove T. L. Origins of the High-Ti Lunar Ultramafic Glasses We have performed a series of high-pressure melting experiments on a synthetic analogue of the Apollo 14 black glass. At 15 kb and 1400 degrees C, olivine is the liquidus phase. At 25 kb and 1450 degrees C, orthopyroxene and Cr-spinel crystallize. A multiple saturation point with olivine + orthopyroxene + spinel on the liquidus occurs at 20 kb and 1440 degrees C. We propose that the high- Ti ultramafic glass evolved from a low-Ti green glass generated deep in the lunar interior that assimilated shallow-level, late-stage cumulates of magma ocean differentiation. Heat for assimilation is provided by superheat of the low-Ti magma and melts high-Ca pyroxene and ilmenite cumulates at a depth of 100 km. This model is a variant of that proposed by Hubbard and Minear and rejected by Ringwood and Kesson. Norris J. A.* Keller L. P. McKay D. S. Impact Glasses from the Ultrafine Fraction of Lunar Soils The chemical compositions of microscopic glasses produced during meteoroid impacts on the lunar surface provide information regarding the various fractionation processes that accompany these events. To learn more about these fractionation processes, we studied the compositions of submicrometer glass spheres from two Apollo 17 sampling sites using electron microscopy. The majority of the analyzed glasses show evidence for varying degrees of impact- induced chemical fractionation. Among these are HASP glasses (High-Al, Si- Poor), which are believed to represent the refractory residuum left after the loss of volatile elements (e.g., Si, Fe, Na) from the precursor material. In addition to HASP-type glasses, we also observed a group of VRAP glasses (volatile-rich, Al-poor) that represent condensates of vaporized volatile constituents, and are complementary to the HASP compositions. High-Ti glasses were also found during the course of this study, and are documented here for the first time. Korotev R. L.* Morris R. V. Lauer H. V. Jr. Composition and Maturity of the 60013/14 Core The 60013/14 double drive tube (62 cm deep) is one of three regolith cores taken 35-40 m apart in a triangular array on the Cayley plains at station 10' (LM/ALSEP), Apollo 16. This trio, which includes double drive tube 60009/10 (59 cm deep) and deep drill core 60001-7 (220 cm), is the only such array of cores returned from the Moon. The top 45 cm of 60013/14 is mature, as is surface reference soil 60601 taken nearby. Maturity generally decreases with depth, with soil below 45 cm being submature. The zone of lowest maturity (34 <= Is/FeO < 50) extends from 46 to 58 cm depth, and corresponds to the distinct region of light-colored soil observed during core processing. In the other two cores, most of the compositional variation results from mixing between fine-grained, mature soil with 10-11 micrograms/g Sc and coarse- grained ferroan anorthosite consisting of >99% plagioclase with <0.5 micrograms/g Sc. This is most evident in 60009/10, which contains a high abundance of plagioclase at about 54 cm depth (minimum Sc: 3-4 micrograms/g); a similar zone occurs in 60001-7 at 17-22 cm (MPU-C), although it is not as rich in plagioclase (minimum Sc: 6-7 micrograms/g). Compositional variations are less in 60013/14 than in the other two cores (range: 7.9-10.0 micrograms/g Sc), but are generally consistent with the "plagioclase dilution" effect seen in 60009/10, i.e., most 60013/14 samples plot along the mixing line of 60009/10. However, a plagioclase component is not the cause of the lower maturity and lighter color of the unit at 46-58 cm depth in 60013/14. Many of the samples in this zone have distinctly lower Sm/Sc ratios than typical LM area soils and plot off the mixing trend defined by 60009/10. This requires a component with moderately high Sc, but low-Sm/Sc, such as feldspathic fragmental breccia (FFB) or granulitic breccia. A component of Descartes regolith, such as occurs at North Ray crater (NRC) and which is rich in FFB, could account for the composition of these soils (i.e., a 3:1 mixture of 60601 and NRC soil). It seems unlikely that NRC ejecta would occur half a meter deep at the LM station, thus this low-Sm/Sc component may result from an older, local crater that penetrated the Cayley surface layer and excavated underlying Descartes material, as did North Ray crater. There is no evidence for such a unit or component in the other two cores. Soil below the light-colored unit (58-62 cm) has "typical" Sm/Sc ratios, but the lowest absolute Sc concentrations, i.e., it is compositionally equivalent to a mixture of surface soil and plagioclase such as that in ferroan anorthosite. This is the only soil that might be related to the plagioclase-rich units in the other two cores. Except for the mature soil at the top of each core and, perhaps, the plagioclase-rich layers, there is little compositional evidence for any common unit among the three cores. Soil corresponding to the mare-glass-bearing unit (MPU-B) and regolith-breccia-bearing unit (MPU-A) of 60001-7 do not occur in 60013/14 or 60009/10. Kerridge J. F.* Kim Y. Kim J. S. Marti K. Nitrogen Isotopic Signatures in Agglutinates from Breccia 79035 Agglutinates in the size range 125-175 micrometers from regolith breccia 79035 are substantially depleted in N compared with bulk 79035. Isotopically, agglutinate N closely resembles that found previously in ilmenite separates. The minimum delta15N value found during stepwise pyrolysis of agglutinates is significantly heavier than that observed for bulk 79035. The major host phase for trapped N in 79035, and the host phase of the lightest isotopic component(s), remain unidentified. Brilliant D. R.* Franchi I. A. Pillinger C. T. Multiple Nitrogen Components in Lunar Soil Sample 12023 Nitrogen is one of the enigmatic elements in lunar soils and breccias. The large range in delta 15N values found within lunar soils was initially attributed to a secular increase in the 15N/14N ratio of 50% within the solar corona, and hence in the implanted nitrogen within the lunar regolith. However, more recent explanations have proposed a two- (or many) component mixing model of solar wind nitrogen with some hypothetical nonsolar components. Such components could include indigenous lunar nitrogen, nitrogen contained in interstellar grains in primitive meteorites, and magnetospheric nitrogen from the terrestrial atmosphere. This study reports the identification of at least 4 components, only two of which have a solar wind origin. Thursday, March 18, 1993 Carbonaceous Chondrites and Chondrules 8:30 - 12:00 a.m. Room A Chair(s): J. Gooding L. Grossman Prinz M.* Weisberg M. K. Clayton R. N. Mayeda T. K. Oxygen Isotopic Relationships Between the LEW85332 Carbonaceous Chondrite and CR Chondrites LEW85332, originally described as a unique C3 chondrite, was shown to be a C2 chondrite with important linkages to the CR clan (CR chondrites, ALH85085, Acfer 182). An important petrologic aspect of LEW85332 is that it contains anhydrous chondrules and hydrated matrix, and new oxygen isotopic data on chondrules, matrix and whole rock are consistent with the petrology. Chondrules fall on the equilibrated chondrite line (ECL), with a slope near 1, which goes through ordinary chondrite chondrules. This contrasts with the CR chondrule line that has a lower slope due to hydrated components. LEW85332 chondrules define a new carbonaceous chondrite chondrule line, parallel to the anhydrous CV chondrule line (CCC), consistent with the well-established concept of two oxygen isotopic reservoirs. Matrix and whole rock fall on the CR line. The whole rock composition indicates that the chondrite is dominated by chondrules, and that most of them contain light oxygen similar to that of anhydrous olivine and pyroxene separates in the Renazzo and Al Rais CR chondrites. Kallemeyn G. W.* The Al Rais Meteorite: A CR Chondrite or Close Relative? Although the classificational group 'CR' was first put forth by McSween (1979) more than 10 years ago, it included only the Al Rais and Renazzo meteorites. It has only been the relatively recent discovery of several CR-related chondrites in Antarctica and the Sahara that has provided the necessary research material for an extensive group description and classification (Weisberg et al., 1993); Bischoff et al., 1993). Some 22 separate specimens representing at least 6 falls are now purportedly members of the CR group. In light of all this new data, an old question can once again be raised as to whether or not Al Rais should be classified in the same distinct group as Renazzo. Brandstatter F.* Kurat G. Ivanov A. V. Palme H. Spettel B. Mineralogy Versus Bulk Composition of the Carbonaceous Chondrite Clast Kaidun II Kaidun is an unusual chondritic meteorite. Its main mass is of CR-type (Kaidun I) and contains CI-like (Kaidun II), EL, and EH clasts. As already noted, the classification of Kaidun II based on mineralogy and bulk chemical criteria does not allow an unambigious assignment to CI. Here we report on the bulk chemical composition and the mineralogy of the Kaidun II clasts. The results of our investigation show that Kaidun II has some characteristics of both CI and CM chondrites. A possible relationship to the texture and composition of micrometeorites is indicated. Steele I. M.* Fe/Mn in Olivine of Carbonaceous Chondrites Olivines in primitive meteorites show a range of Fe/Mn both within one grain and among grains, suggesting that they have recorded changing conditions during or after growth. Because olivine should be an early forming phase, Fe/Mn is used here to infer these earliest conditions. Initial Fe/Mn in cores of isolated, euhedral forsterite in both C2 and C3 meteorites ranges from 25 to 35, but differs at grain edge. Murchison (C2) forsterites show Fe/Mn approaching 1.0 at the grain edge while Ornans Fe/Mn is near 60 at grain edge. These values are lower than the matrix Fe/Mn for both meteorites and the distinct difference in zoning profile indicates different processes operating during and after grain growth. Hua X.* Buseck P. R. Olivines in the Kaba Carbonaceous Chondrite and Constraints on Their Formation Kaba is unique in containing almost pure fayalitic olivine (Fo(sub)0.1). Its coexistence with pure forsterite (up to Fo(sub)99.6) and normal (Fo(sub)92 to Fo(sub)59) and reversely (Fo(sub)0.4 to Fo(sub)4.7) zoned olivines suggests that the Kaba olivines are in thermodynamic disequilibrium and experienced a complicated history. The fayalite is sufficiently pure that it is unlikely that it could have been produced by fractional crystallization. A gas-solid reaction under oxidizing conditions (H2O/H2 ratio ~10) is probably responsible for its formation. Browning L. B.* McSween H. Y. Jr. Zolensky M. E. Determining the Relative Extent of Alteration in CM Chondrites The aqueous alteration of CM chondrites provides a record of the processes attending the earliest stages of parent body evolution. However, resolving the alteration pathways of chondritic evolution requires a means for distinguishing the relative extent of alteration that individual samples have experienced. We propose three new indices for gauging the relative degree of alteration in CM chondrites based on modal and compositional analyses of seven CM falls. The proposed alteration parameters are consistent with the basic tenets of several previous models and correlate with additional indices to produce an integrated method for determining the relative extent of alteration. The model predicts the following order of progressive alteration: Murchison (MC) < Bells (BL) < Murray (MY) < Cochabamba (CC) < Mighei (MI) < Nogoya (NG) < Cold Bokkeveld (CB). Woolum D. S.* Poelstra K. Alexander C. Ireland T. LREE Variability in CM Matrices: Another Look at Meteorite "Compaction Ages" Recent neon-21 analyses indicate that individual grains in some CM meteorites show the effects of enhanced exposures to energetic particles beyond that which can be attributed to conventional cosmic ray exposure. The exposures clearly occurred prior to the final compaction of the CM meteorites. These exposures were of such a magnitude that they require that we either have to revise our understanding of regolith dynamics and the timescales involved in regolith activity of meteorite parent bodies, or must invoke an exposure of the irradiated grains to an enhanced energetic particle environment, an environment that would most likely be produced by an early active Sun. Constraints on the compaction ages of the meteorites that exhibit the large precompaction exposures could play a critical role in resolving this issue. Gilmour I.* Pillinger C. T. Extraction and Isotopic Analysis of Medium Molecular Weight Hydrocarbons from Murchison Using Supercritical Carbon Dioxide The large variety of organic compounds present in carbonaceous chondrites poses particular problems in their analysis,not the least of which is terrestrial contamination. Conventional analytical approaches employ simple chromatographic techniques to fractionate the extractable compounds into broad classes of similar chemical structure. However, the use of organic solvents and their subsequent removal by evaporation results in the depletion or loss of semivolatile compounds as well as requires considerable preparative work to assure solvent purity. Supercritical fluids have been shown to provide a powerful alternative to conventional liquid organic solvents used for analytical extractions. Besides their strong solvating ability, supercritical fluids have several characteristics that make them useful for the quantitative recovery of organic compounds from carbonaceous chondrites. The solvent strength of a supercritcal fluid is directly related to its density so that modification of the extraction pressure and temperature enables species of different solubility to be extracted. The polarity of a supercritical fluid can also be controlled by the addition of solvent modifiers such as methanol. The low critical temperature of a supercritcal fluid such as carbon dioxide (31 degrees C) can be used to avoid the decomposition of thermally labile components. Finally, although liquid when in the supercritical state, fluids such as carbon dioxide retain the low viscosity and subsequent penetration of the sample matrix of a gas; after extraction the use of a gas makes sample recovery much simpler. Misawa K.* Fujita T. Kitamura M. Nakamura N. Refractory Precursor Components in an Allende Ferromagnesian Chondrule Chemical and petrological studies of chondrules revealed that they were formed through melting of pre-existing solid precursor materials, and that one of the refractory lithophile precursors was a high temperature condensate from the nebular gas and related to Ca, Al-rich inclusions (CAIs). Sheng et al. found relict spinel grains with isotopically fractionated Mg in plagioclase-olivine inclusions from CV chondrites and suggested that the major fractionation processes were common to CAIs and chondrules. We have determined the Mg isotopic composition of five barred olivine chondrules and one coarse-grained rim from the Allende (CV3) meteorite. A reproducibility of instrumental isotope fractionation is +-2 per amu. The precision of the ^26Mg/^24Mg data after normalization for mass fractionation can be as good of 0.5 per mil (2 sigma (sub)mean). The Mg analytical results are given in Table 1 and indicate that delta ^25Mg/^24Mg and delta^26Mg of the chondrules are normal within errors. Jones R. H.* Complex Zoning Behavior in Pyroxene in FeO-rich Chondrules in the Semarkona Ordinary Chondrite A detailed understanding of the properties of silicate minerals in chondrules is essential to the interpretation of chondrule formation conditions. This study is further work in a series of petrologic studies of chondrules in the least equilibrated LL chondrite, Semarkona (LL3.0). The objectives of this work are a) to understand chondrule formation conditions and nebular processes and b) to use the data as a basis for understanding the effects of metamorphism in more equilibrated chondrites. FeO-rich pyroxene in the chondrules described shows complex zoning behavior. Low-Ca clinopyroxene, orthopyroxene, pigeonite, and augite are all observed in various associations with one another. Coexisting olivine phenocrysts are also FeO-rich and strongly zoned. Compositional and zoning properties are similar to those observed in boninites and are interpreted as resulting from rapid cooling of individual chondrules. Krot A. N.* Rubin A. E. Chromite-rich Mafic Silicate Chondrules in Ordinary Chondrites: Formation by Impact Melting Chromium-rich chondrules constitute <0.1% of all ordinary chondrite (OC) chondrules and comprise three groups: chromian-spinel chondrules, chromian-spinel inclusions, and chromite-rich mafic silicate (CRMS) chondrules. Chromian-spinel chondrules (typically 100-300 micrometers in apparent diameter) exhibit granular, porphyritic, and unusual textures and occur mainly in H chondrites. Their morphologies are distinct from the irregularly shaped chromian-spinel inclusions of similar mineralogy. Chromian-spinel chondrules and inclusions consist of grains of chromian- spinel embedded in plagioclase (Pl) or mesostasis of Pl composition. Many also contain accessory ilmenite (Ilm), high-Ca pyroxene (Px), merrillite (Mer), and rare olivine (Ol); some exhibit concentric mineral and chemical zoning. CRMS chondrules (300-1100 micrometers in apparent diameter) are generally larger than chromian-spinel chondrules and occur in all metamorphosed OC groups. Most CRMS chondrules are nearly spherical although a few are ellipsoidal with a/b aspect ratios ranging up to 1.7. Textures include cryptocrystalline, granular, radial, barred, and porphyritic varieties; some contain apparently relict grains. The chondrules consist of chromite (Chr), Ol and Pl, along with accessory Mer, troilite (Tr), metallic Fe-Ni (Met), Px, and Ilm. The mesostasis in CRMS chondrules is nearly opaque in transmitted light; thus, they can be easily recognized in the optical microscope. Based on the similarity of mineralogy and chemistry between CRMS chondrules of different textures (opaque chromite-rich mesostasis, skeletal morphology of Ol grains, similar bulk compositions) we suggest that these chondrules form a genetically related population. Huang S.* Benoit P. H. Sears D. W. G. The Group A3 Chondrules of Krymka: Further Evidence for Major Evaporite Loss During the Formation of Chondrules Like Semarkona (type 3.0), Krymka (type 3.1) contains two distinct types of chon Lofgren G. E.* DeHart J. M. Dickinson T. L. Experimentally Reproduced Relict Enstatite in Porphyritic Chondrules of Enstatite Chondrite Composition We recently proposed a model for the origin of porphyritic pyroxene (PP) chondrules in enstatite chondrites that contain phenocrysts of enstatite with blue cathodoluminescence (CL) set in a matrix of radial, dendritic enstatite with red CL. The model is based upon studies of chondrules in ordinary chondrites that suggest that they form as mechanical aggregates of crystalline and other material that are then partially melted and cooled. When melting is essentially complete so that only submicroscopic nuclei remain, the texture that forms is porphyritic and all the phenocrysts will be newly formed with virtually undetectable relict cores. If melting is significantly less complete, however, relatively large relict crystals will remain. Such relict crystals were first described in chondrules in enstatite chondrites by Nagahara (1981) and Rambaldi (1981) and identified by their distinctly different CL and chemistry compared with the dominant crystals in the chondrule. In this case the pyroxene phenocrysts that we consider relict have distinctive levels of Mn and Cr and Al, which produce a characteristic blue CL. We have completed experiments in which previously grown blue CL En (relict) is enclosed in a matrix of red CL En with a coarsely radial texture. Thursday, March 18, 1993 Mars: Surface and Atmospheric Processes 8:30 - 12:00 a.m. Room B Chair(s): V. Gulick K. Herkenhoff Gulick V. C.* Baker V. R. Fluvial Erosion on Mars: Implications for Paleoclimatic Change Fluvial erosion on Mars has been nonuniform in both time and space. Viking orbiter images reveal a variety of different aged terrains exhibiting widely different degrees of erosion. Based on our terrestrial analogue studies, rates of fluvial erosion associated with the formation of many of the valleys on Mars is probably on the order of hundreds of meters per million years, while rates of erosion associated with the formation of the outflow channels probably ranged from tens to hundreds of meters in several weeks to months. However, estimated rates of erosion of the martian surface at the Viking Lander sites are extremely low, on the order of 1 micrometer/yr or less. At most, this would result in a meter of material removed per million years, and it is unlikely that such an erosion rate would be able to produce the degree of geomorphic work required to form the fluvial features present elsewhere on the surface. In addition, single-terrain units are not eroded uniformly by fluvial processes. Instead fluvial valleys, particularly in the cratered highlands, typically are situated in clusters surrounded by vast expanses of uneroded surfaces of the same apparent lithologic, structural, and hydrological setting. Clearly, throughout its geologic history Mars has experienced a nonuniformity in erosion rates. By estimating the amount of fluvial erosion on dissected terrains and by studying the spatial distribution of those locations that have experienced above-normal erosion rates, it should be possible to place further constraints on Mars' paleoclimatic history. Leyva I. A.* Clifford S. M. The Seismic Response of an Aquifer to the Propagation of an Impact Generated Shock Wave: A Possible Trigger of the Martian Outflow Channels? Aquifer dilation from shock waves produced by the 8.4 magnitude Alaskan earthquake of 1964 led to water and sediment ejection from the ground up to 400 km away from the earthquake's epicenter. Groundwater disturbances were observed as far away as Perry, Florida (~5500 km), where well water fluctuations with an amplitude of as much as 2.3 m were reported. The martian cratering record provides evidence that the planet has experienced numerous seismic events of a similar, and often much greater, magnitude. Given this fact, and the photogeologic evidence for abundant water in the early crust, we have investigated the response of a basalt aquifer to the propagation of compressional waves (P-waves) produced by impacts in the 33-1000-km-diameter size range. The resulting one-dimensional changes in effective stress and pore pressure were calculated--as a function of both distance and time--based on the following assumptions: (1) that all of the seismic energy radiated by an impact is transmitted as a single compressional wave, (2) that both the host rock and groundwater are compressible, and (3) that there is no net flow between the water-filled pores. Parker T. J.* Gorsline D. S. Constraints on the Rate of Discharge and Duration of the Mangala Valles Flood Interest in Mangala Valles remains high within the planetary science community. This is justified because the survey mission images provide us with nearly complete coverage of the system at high resolution. Upcoming high resolution topography from the Mars Observer Laser Altimiter will enable the application of flood discharge models with an unprecedented level of detail. Betts B. H.* Murray B. C. Thermal Studies of Martian Channels and Valleys Using Termoskan Data The Termoskan instrument on board the Phobos '88 spacecraft acquired the highest spatial resolution thermal data ever obtained for Mars. Included in the thermal images are 2 km per pixel observations of four major channel and valley systems: Shalbatana Vallis (extending within the Termoskan data from approximately 2 degrees S, 46 degrees W to 5 degrees N, 44 degrees W); Simud Vallis, Hydraotes Chaos, and the associated outflow channel connecting it with the eastern end of Vallis Marineris (all connected from 7 S, 36 W to 5 N, 37 W); Al-Qahira Vallis (20 degrees S, 199 degrees W to 14 degrees S, 195 degrees W); and Ma'adim Vallis (20 degrees S, 183 degrees W to 16 degrees S, 184 degrees W). Simultaneous broadband visible channel data were obtained for all but Ma'adim Vallis. We find that all four valley systems have higher inertias than their surroundings, consistent with previous thermal studies of martian channels. We conclude that the higher inertia is probably due to some intrinsic difference associated with the channel floors, rather than due only to dark aeolian deposits as suggested by some previous studies. Our conclusion is based largely upon the localized nature of the dark deposits in contrast with the thermal homogeneity of the channel floors. Thus, these channels and valleys show an uncommon (for Mars) connection between morphology and inertia. Lucchitta B. K.* Isbell N. K. Howington-Kraus A. Sedimentation, Volcanism, and Ancestral Lakes in the Valles Marineris: Clues from Topography Compilation of a simplified geologic/geomorphic map onto a digital terrain model of Valles Marineris has permitted quantitative evaluations of topographic parameters. The study showed that, if their interior layered deposits are lacustrine, the ancestral Valles Marineris must have consisted of isolated basins. If, on the other hand, the troughs were interconnected as they are today, the deposits are most likely of volcanic origin, and the mesas in the peripheral troughs may be table mountains. The material eroded from the trough walls was probably not sufficient to form all of the interior layered deposits, but it may have contributed significantly to their formation. De Hon R. A.* Classification of Martian Deltas A simple, broad, morphologic classification of martian deltas, based primarily on planimetric shape, includes (1) digitate deltas, (2) fan- shaped deltas, and (3) re-entrant deltas. A fourth, somewhat problematical, class includes featureless plains at the end of many valley systems. Morphologies of deltas reflect conditions prevalent on the surface at the time of their formation. The limited range of deltaic forms present suggests a restricted range of environmental influences. Most martian deltas are high-constructive deltas in which fluvial action dominates, most are small by terrestrial standards, and most indicate short-lived inflow into basins of standing water that lack significant wave or current action. Herkenhoff K. E.* Thermal Inertia and Radar Reflectivity of the Martian North Polar Erg: Low-Density Aggregates The north polar layered deposits on Mars appear to be the source of the dark material that comprises the north polar erg. The physical properties and chemical composition of the erg material therefore have important implications for the origin and evolution of the Martian layered deposits. Viking bistatic radar and infrared thermal mapping (IRTM) data indicate that the bulk density of the erg material is lower than that of the average Martian surface. These data are consistent with hypotheses involving formation of filamentary sublimation residue (FSR) particles from erosion of the layered deposits. The color and albedo of the erg and of the layered deposits, and the presence of magnetic material on Mars, suggest that the dark material is composed of low- density aggregates of magnetic dust grains, perhaps similar to FSR particles created in laboratory experiments. Zent A. P.* Roush T. L. Spectral Analysis of Chemisorbed CO2 on Mars Analog Materials The goal of this work is to estimate the mass of C02 that may have been removed to a quasistable reservoir on the martian surface by chemisorption, and to estimate the spectral effects of chemisorbed C02 in remotely sensed martian spectra. Our approach is to characterize the conditions most favorable for the formation of carbonate on common terrestrial oxide minerals, and to search for infrared spectral bands that result from chemisorption of C02 molecules onto oxide and other Mars analogue materials. Crisp D.* Bell J. F. III Near-Infrared Spectra of the Martian Surface: Reading Between the Lines Moderate-resolution near-infrared (NIR) spectra of Mars have been widely used in studies of the Martian surface because many candidate surface materials (weathered basalts, iron minerals, carbonates, bicarbonates, sulfates, hydrates, etc.) have distinctive absorption features at these wavelengths. Recent advances in NIR detector technology and instrumentation have also encouraged studies in this spectral region. The use of moderate spectral resolution (200 < lambda/delta lambda/ < 2000) has often been justified for NIR surface observations because the spectral features produced by most surface materials are relatively broad, and easily discriminated at this resolution. In spite of this, NIR spectra of Mars are usually very difficult to interpret quantitatively. One problem is that NIR surface absorption features (particularly for mineral mixtures) are often only a few percent deep, requiring observations with great signal-to-noise ratios. A more significant problem is that gases in the Martian atmosphere (CO2, H2O, and CO) contribute numerous absorption features at these wavelengths. Ground-based observers must also contend with variable absorption by several gases in the Earth's atmosphere (H2O, CO2, O3, N2O, CH4, O2). The strong CO2 bands near 1.4, 1.6, 2.0, 2.7, 4.3, and 4.8 micrometers largely preclude the analysis of surface spectral features at these wavelengths. Martian atmospheric water vapor also contributes significant absorption (several percent) near 1.33, 1.88, and 2.7 micrometers, but water vapor in the Earth's atmosphere poses a much larger problem to ground-based studies of these spectral regions. The third most important NIR absorber in the Martian atmosphere is CO. This gas absorbs most strongly in the relatively-transparent spectral windows near 4.6 and 2.3 micrometers. It also produces 1-10% absorption in the solar spectrum at these NIR wavelengths. This solar CO absorption cannot be adequately removed by dividing the Martian spectrum by that of a star, as is commonly done to calibrate ground-based spectroscopic observations, because most stars (even "solar-analog" stars, which have similar color temperatures) do not have identical amounts of CO absorption in their spectra. Moroz V. I.* Titov D. V. Gektin Yu. M. Naraeva M. K. Selivanov A. S. Altitude Profile of Aerosols on Mars from Measurements of Its Thermal Radiation on Limb Measurements of the thermal (range 7-13 micrometers) radiation of Mars with the high space resolution (~2 km) were made by the TERMOSKAN experiment of the Phobos mission. Some of results were published earlier but only the surface radiation was analyzed in detail. However, some part of these measurements was made near the limb of the planet. The atmosphere gives important input here in the planetary thermal radiation. Beyond the limb the atmosphere is the only source. The task of this work is to estimate some characteristics of the atmosphere using brightness profiles of the thermal radiation near the limb. An appropriate model of the temperature profile T(h) is necessary for such an analysis. A set of T(h) models (nominal, maximal, and minimal) was defined using various sources including MARSGRAM, Viking-1 lander data, its theoretical considerations, and boundary layers models. On the next step the possible input of the atmospheric gaseous emissions (wing of CO2 15-micrometer band) was estimated. It was found that even for the maximal T(h) this input is no more than a few percent of the measured radiation beyond the limb. Consequently the aerosols are responsible for almost all measured emission. The analysis of the observed profile showed that these aerosols have two components: (1) exponential with the scale height about 10 km and (2) some layered structure (two layers with maxima about 23 and 33 km consisted probably of ice). Murphy J. R.* Pollack J. B. Dust-Dynamic Feedbacks in the Martian Atmosphere: Surface Dust Lifting The emplacement of dust into the martian atmosphere requires a dynamical interaction between the overlying atmosphere and particles upon the surface. This is the case for both localized lifting events and planetary-scale storms. The identification of which specific components of the atmospheric circulation are responsible for dust raising at particular seasons and locations is a fundamental question in regard to understanding the genesis of lifting events and possible feedbacks between lifted dust and continuation or cessation of lifting. We have been developing numerical models with which to study a wide range of questions related to suspended dust in the martian atmosphere. Our present model is comprised of interactively coupled 3-D dynamical and aerosol transport/microphysical models. With this model we are investigating the nature of possible feedbacks between surface dust lifting and the amplification/damping of near-surface wind and thermal fields and their implications for additional lifting. These studies allow us to identify preferred locations for dust lifting (e.g., large-scale topographic slopes) and the particular circulation component(s) (e.g., overturning circulation, thermal tides, baroclinic waves) responsible for the lifting. Dollfus A.* Ebisawa S. Dust in the Martian Atmosphere: Polarimetric Sensing Telescopic optical polarimetry over planet Mars enables us to detect and localize scatterers in the martian atmosphere. Polarization discriminates between crystal clouds and dust particles. For dust grains, sizes can be derived. There is an increase in size from very small grains forming the permanent dust haze to submicrometer-sized grains for the local dust veils and the several micrometer-sized pieces found in the dust storm clouds. Mellon M. T.* Jakosky B. M. The Effects of Orbital and Climatic Variations on Martian Surface Heat Flow Large changes in the orbital elements of Mars on timescales of 10^4 to 10^6 years will cause widely varying climate, specifically surface temperatures, as a result of varying insolation. These surface temperature oscillations will produce subsurface thermal gradients that contribute to the total surface heat flux. We investigate the thermal behavior of the martian regolith on orbital timescales and show that this climatological surface heat flux is spatially variable and contributes significantly to the total surface heat flux at many locations. Bell J. F. III* Calvin W. M. Pollack J. B. Crisp D. An Observational Search for CO2 Clouds on Mars C02 ice clouds were first directly identified on Mars by the Mariner 6 and 7 infrared spectrometer limb scans. These observations provided support for early theoretical modeling efforts of C02 condensation. Mariner 9 IRIS temperature profiles of north polar hood clouds were interpreted as indicating that these clouds were composed of H20 ice at lower latitudes and C02 ice at higher latitudes. The role of C02 condensation on Mars has recently received increased attention because (a) Kasting's (1991) model results indicated that C02 cloud condensation limits the magnitude of the proposed early Mars CO2/H2O greenhouse, and (b) Pollack et al.'s (1990) GCM results indicated that the formation of C02 ice clouds is favorable at all polar latitudes during the fall and winter seasons. These latter authors have shown that C02 clouds play an important role in the polar energy balance, as the amount of C02 contained in the polar caps is constrained by a balance between latent heat release, heat advected from lower latitudes, and thermal emission to space. The polar hood clouds reduce the amount of C02 condensation on the polar caps because they reduce the net emission to space. Thursday, March 18, 1993 Lunar Geology 8:30 - 12:00 a.m. Room C Chair(s): J. Granahan P. D. Spudis Stacy N. J. S. Campbell D. B.* Earth-based Measurement of Lunar Topography Using Delayed Radar Interferometry Radar interferometry has been applied to radar mapping of the surface of Venus to resolve the ambiguity in the backscatter from the areas with the same delay-Doppler coordinates. For observations of the Moon these points are usually isolated by the small extent of the area illuminated by the radar beam so interferometric techniques can be used to determine a third dimension to the location of the radar backscatter. Recent observations of Sinus Iridum (45.1 degrees N, 31.0 degrees W) using the Arecibo Observatory S-Band radar system (12.6 cm wavelength) in April and August 1992 have yielded a pair of images viewed with a very small 0.0420 degrees difference in incidence angle. These images can be used as a delay interferometric pair and have been correlated to generate phase fringes that are related to topography. The spatial resolution of the images are 18 m in delay and 33 m in cross range (Doppler). The anticipated topographic resolution once the phase fringes have been unwrapped is approximately +-10 m. Bahar E.* Haugland M. Interpretation of Lunar and Planetary Electromagnetic Scattering Using the Full Wave Solutions Bistatic radar experiments carried out during the Apollo 14, 15, and 16 missions provide a very useful dataset with which to compare theoretical models and experimental data. Vesecky et al. report that their model for near grazing angles compares favorably with experimental data. However, for angles of incidence around 80 degrees, all the analytical models considered by Vesecky et al. predict values for the quasispecular cross sections that are about half the corresponding values taken from the Apollo 16 data. In this work, questions rasied by this discrepancy between the reported analytical and experimental results are addressed. The unified full wave solutions are shown to be in good agreement with the bistatic radar taken during Apollo 14 and 16 missions. Using the full wave approach, the quasispecular contributions to the scattered field from the large-scale surface roughness as well as the diffuse Bragg-like scattering from the small-scale surface roughness are accounted for in a unified self-consistent manner. Since the full wave computer codes for the scattering cross sections contain ground-truth data only, it is shown how it can be reliably used to predict the rough surface parameters of planets based on the measured data. Cooper B. L.* Carter J. L. Using Lunar Sounder Imagery to Distinguish Surface from Subsurface Reflectors in Lunar Highlands Areas We have developed a method using the Apollo 17 Lunar Sounder imagery data that appears capable of filtering out off-nadir surface noise from highland area profiles so that subsurface features may now be detected in highlands areas as well as mare areas. Previously this had been impossible because the rough topography in the highlands areas created noise in the profiles that could not be distinguished from subsurface echoes. The new method is an image-processing procedure involving the computerized selection of pixels that represent intermediate echo intensity values, then manually removing those pixels from the profile. Using this technique, a subsurface feature with a horizontal extent of about 150 km, at a calculated depth of approximately 3 km, has been detected beneath the crater Riccioli in the highlands near Oceanus Procellarum. This result shows that the ALSE data contain much useful information that remains to be extracted and used. Mendell W. W.* Wieczorek M. A. Thermogeologic Mapping of the Moon from Lunar Orbit The Infrared Scanning Radiometer (ISR) onboard the Apollo 17 Command-Service Module (CSM) mapped thermal emission of the lunar surface from orbit. Measured temperature values span the diurnal range of lunar temperatures (85 K to 400 K) and have an accuracy of approximately +-2 K. Surface spatial resolution at nadir is 2.2 km. This Apollo data is being revisited using data presentation software for the Macintosh computer, which was not available 20 years ago, even on mainframes. The new thermal images exhibit subtleties in the delineation of geophysical surface units that were unappreciated in the original survey of the data. Looking first at nighttime thermal emission from the ground tracks over Oceanus Procellarum to Mare Orientale, we have confirmed and expanded on earlier observations of regolith differences between mare and highlands and of a scheme for relative age-dating of larger impact craters of the Copernican age. We see an impact crater near Lenz, just north of Orientale, which exhibits an extraordinarily fresh ejecta blanket. Photography of this area is extremely poor, but we can see the feature in the Galileo data. We plan to derive geophysical surface properties of the overflown region using thermal models of regolith structures. Tompkins S.* Pieters C. M. Mustard J. F. Distribution and Geologic History of Materials Excavated by the Lunar Crater Bullialdus Bullialdus Crater is a 61-km, Eratosthenian-age impact crater located on the western edge of Mare Nubium. Previous analysis of the spatial distribution of materials in the area using nine telescopic near- infrared spectra suggested a possible three-layer structure prior to the impact event: two shallow gabbroic layers and one deeper noritic layer (from a potential depth of 5.5 km). The initial interpretation of this stratigraphy was that Bullialdus may have tapped a layered mafic pluton, such as have been invoked to explain the existence of Mg-suite rocks. High-spatial-resolution CCD images of Bullialdus were analyzed to better map the spatial distribution of the observed lithologies, and to assess the plausibility of the pluton interpretation. Kadel S. D.* Greeley R. Neukum G. Wagner R. The History of Mare Volcanism in the Orientale Basin: Mare Deposit Ages, Compositions and Morphologies The eruptive history of mare basalts in the Orientale Basin has been studied, using Lunar Orbiter IV high-resolution photographs, Zond 8 photographs, and recently acquired Galileo EM-1 multispectral images. This work represents a refined set of compositional data, incorporating the use of a linear mixing model for mare compositions, crater count data, and a comprehensive morphologic analysis of Orientale Basin mare deposits. Evidence for multiple eruptive episodes has been found, with compositions ranging from medium- to high-Ti basalt (<4 to >6 wt% TiO2). Eruptive styles included flood, rille- forming and shield-forming eruptions. Impact crater densities of mare units in the Orientale Basin enable determination of the ages of these deposits, using the method of Neukum et al. Earliest eruptions of mare basalt in the basin occurred at >=3.70 Ga and the latest eruptions occurred at about 2.3-2.5 Ga. Hence, mare volcanism occurred over a period of nearly 1.5 Ga. Hiesinger H.* Jaumann R. Neukum G. GLL Imaging Team Earth-based and Galileo SSI Multispectral Observations of Eastern Mare Serenitatis and the Apollo 17 Landing Site Both the Apollo 17 and the Mare Serenitatis regions have been observed by Galileo during its fly-by in December 1992. We used Earth-based multispectral data to define mare units that then can be compared with the results of the Galileo SSI data evaluation. Spudis P. D.* Hawke B. R. Lucey P. G. Geology and Deposits of the Serenitatis Basin GEOLOGY AND DEPOSITS OF THE SERENITATIS BASIN Paul D. Spudis1, B. Ray Hawke2, Paul G. Lucey2 1Lunar and Planetary Institute, Houston TX 77058.2Planetary Geoscience, SOEST, Univ. Hawaii, Honolulu HI 96822.The Serenitatis Basin is prominent on the nearside of the Moon, just east of Mare Imbrium. Originally thought to be one of the oldest lunar basins, reinterpretation of both geological relations and Apollo 17 isotopic data suggest instead that Serenitatis is one of the youngest basins, having formed in the Nectarian Period about 3.87 Ga ago. As part of our continuing effort to understand the geology of multiring basins on the Moon and to use basins as probes of the deep lunar crust, we here report results for theSerenitatis Basin. Our examination of Serenitatis was stimulated in part by a new effort to reexamine the geology of the Apollo 17 landing site. Hawke B. R.* Blewett D. T. Lucey P. G. Taylor G. J. Peterson C. A. Bell J. F. Robinson M. S. Bell J. III Coombs C. R. Jaumann R. Hiesinger H. Neukum G. Spudis P. D. Remote Sensing Studies of the Northeastern Portion of the Lunar Nearside During the Galileo spacecraft encounter with the Earth-Moon system in December 1992, a variety of spectral data and imagery were obtained for the eastern limb region as well as much of the lunar nearside. In order to support this encounter, we have been collecting near- infrared spectra and other remote sensing data for that portion of the northeastern nearside (NEN region) for which the highest resolution Galileo data were obtained. Analysis of spectra obtained for highlands units in the NEN region indicates that most surface units are dominated by anorthositic norite. To date, no pure anorthosites have been identified in the region. Several dark-haloed impact craters have exposed mare material from beneath highlands-rich surface units. Hence, ancient mare volcanism occurred in at least a portion of the NEN region. Endogenic dark-haloed craters in the region are the sources of localized dark mantle deposits (LDMD) of pyroclastic origin and at least two compositional groups are present. Blewett D. T.* Hawke B. R. Lucey P. G. Bell J. F. III Jaumann R. Hiesinger H. Neukum G. Spudis P. D. Spectral and Multispectral Imaging Studies of Lunar Mantled Mare Deposits Near-IR reflectance spectra (0.6-2.5 micrometers) and CCD images in the extended visible range (0.4-1.0 micrometers) obtained with Earth- based telescopes have been used to investigate the composition and origin of formations in the Schiller-Schickard region of the Moon. Of particular interest are the Schickard light plains, which represent an area of mantled mare basalt, or cryptomare. Here local preexisting mare basalts were eroded and incorporated into a highlands-rich deposit by ejecta from the Orientale Basin. Spectral observations of mature and immature highland and mare surfaces, as well as dark-halo crater materials, provide information on the mafic mineralogy of features in the area. Analyses of the "1 micrometer" absorption band and spectral mixing models indicate that selected spots in the light plains contain on the order of 50% mare basalt. CCD image cubes can be used to map the amount of basalt in the light plains and evaluate changes with radial distance from Orientale. Head J. W.* Wilson L. Mode of Emplacement of Lunar Mare Volcanic Deposits: Graben Formation Due to Near Surface Deformation Accompanying Dike Emplacement at Rima Parry V Theoretical analyses, together with the observed style of emplacement of lunar mare volcanic deposits, strongly suggest that mare volcanic eruptions are fed by dikes from source regions at the base of the crust or deeper in the lunar mantle. Some dikes intrude into the lower crust, while others penetrate to the surface and are the sources for voluminous outpourings of lava. Still others stall near the surface generating a near-surface extensional stress field. We have investigated the hypothesis that some lunar linear rilles (graben) are the near-surface manifestations of dikes intruded to shallow depths. For a specific example (Rima Parry V) we show that the geometry of the faults implies a mean dike width of about 150 m and depth to the dike top of about 500 m, values consistent with other theoretical and observational data on lunar dike geometry. Localized pyroclastic deposits along Rima Parry V are evidence for the presence of near-surface magma, and are interpreted to be the result of degassing and pyroclastic eruption subsequent to the emplacement of the dike. Koehler U.* Jaumann R. Neukum G. Age Determinations and Earth-based Multispectral Observations of Lunar Light Plains The history of Light plains still remains doubtful, but there are good arguments--mainly obtained by age determinations and supported by multispectral observations--for an endogenic (magmatic) instead of an (exclusively) impact-related origin. Helfenstein P.* Veverka J. Head J. W. Pieters C. Pratt S. Mustard J. Klaasen K. Neukum G. Hoffmann H. Jaumann R. Rebhan H. McEwen A. S. Belton M. Galileo Photometry of Apollo Landing Sites As of December 1992, the Galileo spacecraft performed its second and final flyby (EM2) of the Earth-Moon system, during which it acquired Solid State Imaging (SSI) camera images of the lunar surface suitable for photometric analysis using Hapke's photometric model. These images, together with those from the first flyby (EMl) in December 1989, provide observations of all of the Apollo landing sites over a wide range of photometric geometries and at eight broadband filter wavelengths ranging from 0.41 micrometers to 0.99 micrometers. We have completed a preliminary photometric analysis of Apollo landing sites visible in EM1 images and developed a new strategy for a more complete analysis of the combined EM1 and EM2 datasets in conjunction with telescopic observations and spectrogoniometric measurements of returned lunar samples. Thursday, March 18, 1993 Asteroid and Planetary Core Formation and Metal-Rich Meteorites 8:30 - 12:00 a.m. Room D Chair(s): G. W. Lugmair H. Newsom Keil K. Wilson L.* Explosive Volcanism and the Compositions of the Cores of Differentiated Asteroids Eleven iron meteorite groups showing correlations between Ni and siderophile trace elements that are predictable by distribution coefficients between liquid and solid metal of fractionally crystallizing metal magmas, are interpreted to be fragments of the fractionally crystallized cores of 11 differentiated asteroids. Many of these groups crystallized from S-depleted magmas, which we propose resulted from removal of the first partial melt (a Fe,Ni-FeS cotectic) by explosive pyroclastic volcanism. We show that these dense, negatively buoyant melts can be driven to asteroidal surfaces by the combination of an excess pressure in the melt and the presence of buoyant bubbles of gas, which decrease the bulk density of the melt. We also show that in typical asteroidal materials, veins will form that grow into dikes and serve as pathways for migration of melt and gas to asteroidal surfaces. Since cotectic Fe,Ni-FeS melt consists of about 85 wt% FeS and 15 wt % Fe,Ni, removal of small volumes of eutectic melts results in major loss of S but only minor loss of Fe,Ni, thus leaving sufficient Fe,Ni to form sizeable asteroidal cores. Jurewicz S. R.* Jones J. H. Experimental Segregation of Iron-Nickel Metal, Iron-Sulfide, and Olivine in a Thermal Gradient: Preliminary Results Speculation about the possible mechanisms for core formation in small asteroids raises more questions than answers. Petrologic evidence from iron meteorites, pallasites, and astronomical observations of M asteroids suggests that many small bodies were capable of core formation. Recent work by Taylor suggests that extensive silicate partial melting (preferably 50 vol% or greater) is required before metal can segregate from the surrounding silicate and form a metal core. Although some qualitative experimental data exists, little is actually known about the behavior of metals and liquid sulfides dispersed in silicate systems. Experiments were performed to: 1) evaluate the potential for metal/sulfide/silicate segregation in a thermal gradient; and 2) obtain quantitative data of the wetting parameters of metal-sulfide melts among silicate grains. Our preliminary results indicate that thermal diffusion may play an important role in metal/sulfide/silicate segregation. Zhou Y.* Steele I. M. Chemical Zoning and Diffusion of Ca, Al, Mn, and Cr in Olivine of Springwater Pallasite The pallasites, consisting mainly of Fe-Ni metal and olivine, are thought to represent the interior of a planetary body that slowly cooled from high temperature. Although the olivines are nearly homogeneous, ion microprobe studies revealed variations of Ca, Ti, Co, Cr, and Ni near grain edges. These variations were thought to represent diffusion in response to falling temperature of the parent body. Pallasite cooling rates have been estimated based on kamacite-taenite textures, but results differ by x100. In principle elemental profiles in olivine can allow estimates of cooling rate if diffusion coefficients are known; in addition, given a cooling rate, diffusion coefficients could be derived. Data are presented that show that apparent diffusion profiles can be measured for Al, Ca, Cr, and Mn that qualitatively agree with expected diffusion rates and have the potential of providing independent estimates of pallasite cooling rates. Holzheid A.* Borisov A. Palme H. Siderophile Elements in the Upper Mantle of the Earth: New Clues from Metal-Silicate Partition Coefficients New, precise data on the solubilities of Ni, Co, and Mo in silicate melts at 1400 degrees C and fO2 from IW to IW-2 are presented. The results suggest NiO, CoO as stable species in the melt. No evidence for metallic Ni or Co was found. Equilibrium was ensured by reversals with initially high Ni and Co in the glass. Mo appears to change oxidation state at IW-1, from MoO(sub)3 to MoO(sub)2. Metal-silicate partition coefficients calculated from these data and recent data on Pd indicate similar partition coefficients for Pd and Mo at the conditions of core formation. This unexpected result constrains models of core formation in the Earth. Newsom H. E.* Maehr S. Core Formation in the Moon: The Mystery of the Excess Depletion of Mo, W and P We have evaluated siderophile element depletion models for the Moon in light of our improved statistical treatment of siderophile element abundance data and new information on the physics of core formation. If core formation occurred in the Moon at the large degrees of partial melting necessary for metal segregation, according to recent estimates, then a significant inconsistency (not seen in the eucrite parent body) exists in the depletion of the incompatible siderophile elements Mo, W, and P, compared to other siderophile elements in the Moon. The siderophile data, with the exception of Mo, are most consistent with terrestrial initial siderophile abundances and segregation of a very small core (<1 wt%) in the Moon. Our improved abundance estimates and possible explanations for these discrepancies are discussed below. Haack H.* Scott E. R. D. Rubio G. S. Gutierrez D. F. Lewis C. F. Wasson J. T. Brooks R. R. Guo X. Ryan D. E. Holzbecher J. Systematic Chemical Variations in Large IIIAB Iron Meteorites: Clues to Core Crystallization Analysis of numerous individual iron meteorites has shown that fractional crystallization of iron cores results in variations in chemical concentration of the solid core that span several orders of magnitude. The magnitude and direction of the resulting spatial gradients in the core can provide clues to the physical nature of the core crystallization process. We have analyzed suites of samples from three large IIIAB irons (Cape York, 58t; Chupaderos, 24t; Morito, 10t) in order to estimate local chemical gradients. Initial results for the concentrations of Ge, Pd, Pt (Massey group), Ir, Au, As, Co, Os, and Rh (Dalhouse group), and P (Arizona group) show significant ranges among the Cape York and Chupaderos samples and marginally significant ranges among the Morito samples. Measurements of Au, Ir, Co, Ni, Cu, Ga, As, W, Re (from UCLA) and Ni and Co (Arizona group) are in progress. We find a spatial Ir gradient in Chupaderos with a magnitude similar to the one reported for Agpalilik (Cape York iron) by Esbensen et al. (1982). Walker R. J.* Morgan J. W. Horan M. F. Grossman J. N. Rhenium-Osmium Isotope Systematics of Ordinary Chondrites and Iron Meteorites Using negative thermal ionization mass spectrometry, Re and Os abundances were determined by isotope dilution and 187Os/186Os measured in 11 ordinary chondrites, and also in 1 IIB and 3 IIIB irons. In addition, 1860s/1880s and 1890s/l880s ratios were precisely determined for three unspiked ordinary chondrites as a means of constraining the intensity of any neutron irradiation these meteorites may have experienced. Esat T.* Bennett V. Re-Os Dating of IIIAB Iron Meteorites Recently Creaser et al. and Volkening and Heumann have demonstrated the efficient production of large (10E-11A) ion beams by negative thermal ionisation mass spectrometry (NTIMS) using standard laboratory solutions of Os compounds. Horan et al. have applied NTIMS to a group of 7 IIA iron meteorites and obtained a Re-Os closure age of 5496+-152 m.y. The initial 187Os/186Os ratio was 0.8007+-0.0029. In addition they analysed 3 IIIA meteorite samples that indicated an age of 4554+-180 m.y. and an Os initial of 0.812+-0.0075 that does not overlap with the initial for the IIA irons. We have been independently pursuing a similar program with the direct aim of determining possible variations in the initial 187Os/186Os ratio or Re-Os closure age of different classes of iron meteorite. We have applied NTIMS to Os extracted from the most common group of iron meteorites the IIIAB. These meteorites are believed to be of magmatic origin, formed by fractional crystallisation of molten cores of asteroidal bodies. The present results point to a significantly lower initial 187Os/186Os ratio of 0.7731+-0.0050 than previously determined. Creaser R. A.* Papanastassiou D. A. Wasserburg G. J. Rhenium-Osmium Isotope Systematics of Group IIA and Group IVA Iron Meteorites We are investigating the Re-Os isotope systematics of two groups of magmatic iron meteorites (IIA, IVA) in an attempt to establish precise "total rock" isochrons by the Re-Os system. The ^l87Re-^l870s isotope system is recognized as a method by which the ages of iron meteorites can be directly determined, and provide information on the timing of FeNi segregation and core formation in planetesimals. The Re-Os isotope system permits the direct absolute dating of the metal phase in iron meteorites. Indirect dating of iron meteorites has been achieved in the past through the Rb-Sr, K-Ar, and most recently, Sm-Nd for silicate inclusions, where present. Relative dating has been obtained directly by extensive studies of the short-lived system ^107Pd-^107Ag for the metal and sulfide phases and indirectly using ^129I-^129Xe in silicate and sulfide inclusions. The major experimental difficulties with Re-Os analyses derive from: a) the multiple oxidation states of Eisenhour D. D. Buseck P. R. Palme H.* Zipfel J. Micro-Zoning in Minerals of a Landes Silicate Inclusion There is an increasing number of meteorites with chondritic bulk composition but completely different textures than the conventional chondrite groups. Winonaites, acapulcoites, and silicate inclusions in IAB-iron meteorites have in common coarse grain size and highly equilibrated mineralogy with frequent 120 degree triple junctions, and they record a significantly lower degree of oxidation than ordinary chondrites. They all have equilibration temperatures, based on Ca-exchange among pyroxenes, of around 900 to 1100 degrees C. However, on cooling, disequilibrium features may develop: (a) Olivine in IAB inclusions has lower Fa content than equilibrium Fs content of pyroxenes requires. (b) CaO-zoning in olivine was established at temperatures of around 500 degrees C, several hundred degrees below pyroxene equilibration temperatures. Obviously, olivine responded faster to changes in fO2 (Fa in olivine) and temperature (Ca-zoning) than pyroxenes. Differences in diffusion coefficients can readily explain the observed trends. We report on much more subtle zoning features in pyroxenes. TEM observations reveal large compositional gradients in Ca, Na, Cr, Ti, and Fe within the first micrometer of cpx and opx crystals. Stewart B. W.* Papanastassiou D. A. Wasserburg G. J. Sm-Nd Systematics of Silicate Inclusions in Iron Meteorites: Results from Caddo (IAB) The timing of events leading to the formation of silicate-rich and metal-rich re planetesimals remains an important problem in the study of planetary formation a differentiation in the early solar system. The IAB irons are especially importan are considered to represent a magmatic differentiation series. Iron meteorites p particular challenge for chronological studies, due to the relative paucity of p serving as hosts for radioactive parent-daughter nuclides. Recent work using the system, following on the pioneering work by Herr et al. and Luck and Allegre, ap promising, but investigations up to now have concentrated on whole rock isochron Silicate clasts enclosed within iron meteorites can provide information about th chronology and thermal history of irons. Extensive work on Rb-Sr, K-Ar, and I-Xe reported on silicate inclusions in iron meteorites. We report the initial result Sm-Nd study of an inclusion within the Caddo IAB iron, the first Sm-Nd isotopic silicate clast embedded within an iron meteorite. Our results include measuremen standard long-lived 147Sm-143Nd (tau = 152 AE) system, as well as the shorter-li 142Nd (tau = 0.149 AE) system, which has been shown to be very useful in deciphe history of the early solar system [10-15]. The Caddo silicate clast was describe et al., who kindly provided us with a major part of the inclusion. The inclusion coarse-grained, consisting predominantly of olivine, clinopyroxene, and plagiocl lesser amounts of orthopyroxene, Fe-Ni metal, sulfide, and phosphate. The relati grain size (up to 3 mm) and 120 degree grain boundaries suggest extensive metamo high temperature. Based on a study of a thin section, there is evidence for meta along grain boundaries in some regions of the inclusion, suggesting that the Fe- was molten when the silicate clast was incorporated. Metamorphic recrystallizati have occurred during this event. Jacobsen S. B.* Harper C. L. Jr. Constraints on the Differentiation of the Earth from Coupled 146,147Sm-142,143Nd Systematics The coupled Sm-Nd systematics are a powerful (albeit analytically challenging) tool for investigating the geodynamic history of the Earth. We have previously reported evidence for a 33 ppm difference of an Isua sample relative to our terrestrial standard. Interpretation yields a formation age range for the depleted mantle (DM) source reservoir of 4.45-4.55 Ga. This is consistent with an epoch of LREE-enriched melt extraction from the mantle (proto-crust formation), soon after magma ocean freeze-up following the putative Moon forming giant impact. Harper C. L. Jr.* Jacobsen S. B. 142Nd/144Nd in Bulk Planetary Reservoirs, the Problem of Incomplete Mixing of Interstellar Components and Significance of Very High Precision 145Nd/144Nd Measurements Apart from the challenge of very high precision 142Nd/144Nd ratio measurement, accurate applications of the coupled 146,147Sm-142,143Nd systematics in planetary differentiation studies require very precise knowledge of the present-day (post-146Sm decay) 142Nd/144Nd ratios of bulk planetary objects (BP). The coupled systematics yield model ages for the time of formation of Sm/Nd-fractionated reservoirs by differentiation of Sm/Nd-unfractionated bulk planetary reservoirs. Estimates of (142Nd/144Nd)BP and (143Nd/144Nd)BP therefore provide the critical baseline relative to which these model ages are referenced. In the 147Sm-143Nd systematics, 143Nd/144Nd variations are mostly large (>200 ppm), and therefore small (viz., <20 ppm) variations in initial 143Nd/144Nd ratios generally can be ignored. However, in the case of 146Sm- 142Nd, the range of 142Nd/144Nd divergence for differentiated planetary reservoirs is much smaller (<~100 ppm). Consequently 146,147Sm-142,143Nd model ages are sensitive to small variations in bulk planetary 142Nd/144Nd (both present-day and initial). One major unanswered question is whether or not Nd shelf standards (CIT Ndb/Ames metal, La Jolla, NASA-JSC/Ames metal) have 142Nd/144Nd identical to the bulk Earth or otherwise might record some degree of radiogenic evolution in an early-fractionated reservoir. Our discussions of early Earth differentiation based on 142Nd/144Nd in Isua and Acasta samples have employed a working assumption: (142Nd/144Nd)Ndb= (142Nd/144Nd)Bulk Earth. This requires experimental justification and is apparently contradicted by chondrite 142Nd/144Nd measurements, which have been interpreted to indicate: (142Nd/144Nd)JSC/Ames metal = ((142Nd/144Nd)CHUR + 35+/-8 ppm). At present, interpretations of the early Earth and Moon hinge largely on this issue. Because Ba in bulk chondrite samples exhibits similar magnitude nuclear anomalies, attributable to incomplete mixing of interstellar components, a critical question is whether or not nuclear effects are also present in 142Nd/144Nd, both in bulk chondrites and between planetary objects. Thursday, March 18, 1993 Stars, Stardust, and Isotope Anomalies 1:30 - 5:00 p.m. Room A Chair(s): D. Papanastassiou A. M. Davis Dong Q. W. Thiemens M. H.* Development of a Molecular Beam Technique to Study Early Solar System Silicon Reactions Silicon monoxide is one of the major gas phase silicon-bearing components observed in astronomical environments. Silicon oxide serves as the major rock-forming material for terrestrial and meteoritic bodies. It is known that several gas phase reactions produce mass- independent isotopic fractionations that possess the same delta 17O/delta l8O ratio observed in Allende inclusions. The general symmetry dependence of the chemically produced mass independent isotopic fractionation process suggests that there are several plausible reactions that could occur in the early solar system that may lead to production of the observed meteoritic oxygen isotopic anomalies. An important component in exploring the role of such processes is the need to experimentally determine the isotopic fractionations for specific reactions of relevance to the early solar system. It has already been demonstrated that atomic oxygen reaction with CO, a major nebular oxygen-bearing species, produces a large (~90 permil), mass-independent isotopic fractionation. The next hurdle regarding assessing the involvement of symmetry-dependent isotopic fractionation processes in the presolar nebular is to determine isotopic fractionation factors associated with gas phase reactions of metallic oxides. In particular, a reaction such as O+ SiO --> SiO2 is a plausible nebular reaction that could produce a delta 17O~delta 18O fractionation based upon molecular symmetry considerations. While the isotopic fractionations during silicate evaporation and condensation have been determined, there are no isotopic studies of controlled, gas phase nucleation processes. In order to carefully control the reaction kinetics, a molecular beam apparatus has been constructed. This system produces a supersonic, collimated beam of SiO molecules that is reacted with a second beam of oxygen atoms. An important feature of molecular beams is that they operate at sufficiently low pressures and high temperature in the jet that avalanche nucleation and clustering processes may be avoided. Yurimoto H.* Nagasawa H. Mori Y. Inter- and Intra-Crystalline Oxygen Isotope Distribution of Fassaites in Allende CAI Calcium, aluminum-rich inclusions (CAIs) from carbonaceous chondrite are believed to be among the first solids to have formed in the early solar system and thus to preserve a record of conditions and processes that prevailed early in the solar nebula. We have measured inter- and intracrystalline oxygen isotope distributions of fassaite in Type B1 CAI of Allende meteorite. Secondary ion mass spectrometry (SIMS) using negative Au primary ion was applied to in situ oxygen isotope ratio analysis on an polished thin section of the CAI. Nittler L. R.* Walker R. M. Zinner E. Hoppe P. Lewis R. S. Identification of an Interstellar Oxide Grain from the Murchison Meteorite by Ion Imaging We report here the first use of a new ion-imaging system to locate a rare interstellar aluminum oxide grain in a Murchison acid residue. While several types of carbon-rich interstellar grains, including graphite, diamond, SiC, and TiC, have previously been found, isotopically anomalous interstellar oxide grains have proven more elusive. For example, Virag et al. found no evidence of an interstellar component in a study of 26 corundum grains from Murchison. Measurements by Amari et al. on an additional 92 hibonites and 41 corundum grains in a smaller grain size fraction of Murchison gave similar negative results. A positive result was however reported by Huss et al., who found one corundum grain, out of 23 studied from Orgueil, that had a large 26Mg excess corresponding to an (26Al/27Al)(sub)0 value of 9 x 10^-4 and was thus of possible interstellar origin. A total of 550 Orgueil oxide grains and some 760 Murchison oxide grains were studied here. One corundum grain from Murchison, 83-5, showed an 18O depletion (d18O = -244 +- 24 permil) and an 17O enrichment (d17O = 1072 +- 59 permil) indicative of an interstellar origin. Grain 83-5 is also enriched in 26Mg leading to an inferred (26Al/27Al)(sub)0 of 8.7 x 10^- 4, similar to that measured by Huss et al. in their Orgueil sample. Additional data on many more oxide grains are needed to determine whether this agreement indicates a single population of presolar corundum grains or is purely coincidental. Lewis R. S.* Srinivasan B. A Search for Noble-Gas Evidence for Presolar Oxide Grains We present early results from an ongoing search for isotopically distinctive noble gases as evidence for presolar oxide grains. With some qualifications, we do not see such evidence in spinel rich acid residue fractions from the Allende meteorite. We remain hopeful that less abundant mineral separates may yet be fruitful. Amari S.* Zinner E. Lewis R. S. Interstellar Graphite in Murchison: Continued Search for Isotopically Distinct Components We have measured C-, N-, and O-isotopic ratios of individual graphite grains from the Murchison density fractions KFB1 (2.1-2.15 g/cm3) and KFC1 (2.15-2.20 g/cm3). According to the C-isotopic composition, KFC1 consists of two populations: one has light C while the other has heavy C. In addition to these two populations, KFB1 also contains grains whose C-isotopic ratios are approximately normal. In view of these isotopic distributions it is difficult to understand why the Kr-isotopic compositions of these two separates are so different. In contrast to the lighter density fractions KE1 (1.6-2.05 g/cm3) and KFA1 (2.05-2.10 g/cm3), the separates KFB1 and KFC1 do not seem to contain many grains with significant 18O excesses. Huss G. R.* Hutcheon I. D. Wasserburg G. J. Do SiC Grains in Orgueil Differ from those in Murchison? Studies of individual presolar SiC grains have shown that most are enriched in 29Si, 30Si, and 13C, and depleted in l5N, compared to solar-system abundances, and that many have large excesses of 26Mg, most plausibly from in situ decay of 26Al. Stone et al. observed that SiC from a family of platy SiC grains define a linear array on a 3-isotope plot that does not pass through normal solar-system Si. In contrast, Si-isotope data from over 100 3-4 micrometer SiC grains from Murchison form an elongate ellipse enclosing the Stone et al. linear array but also including 'normal' solar-system Si. To investigate whether this difference in Si isotopes indicates different populations of SiC in the two meteorites and to improve the characterization of Orgueil SiC, we used the PANURGE ion microprobe to measure Si, C, N, and Mg isotopes and Al and Na concentrations in a suite of 2-5 micrometer Sic grains from a new sample of Orgueil. Yates P. D.* Micrometeorite Pre-solar Diamonds from Greenland Cryoconite? An acid-resistant residue prepared from Greenland cryoconite has been investigated to determine whether the micrometeorite component within the cryoconite contains presolar material analogous to that found in primitive chondritic meteorites. The residue has been analysed for carbon content and stable isotopic composition, by electron probe microanalysis (EPMA) for major element chemistry and then by a combination of X-ray diffraction (XRD) and transmission electron microscopy (TEM) to elucidate the structure of any constituent mineral phases. The cryoconite sample, which was collected ca.25 km inland of the ice margin at the latitude of Sondre Stromfjord on the west coast of Greenland, was processed by Dr.J.W.Arden at the University of Oxford following procedures used on bulk meteorite samples for the isolation of presolar dust components. Verchovsky A. B.* Russell S. S. Pillinger C. T. Fisenko A. V. Shukolyukov Yu. A. Are the C-delta Light Nitrogen and Noble Gases Located in the Same Carrier? Light nitrogen and the HL family noble gas components of C(sub)delta appear to be separable by high-resolution pyrolysis experiments. Thus C(sub)delta is not an homogeneous material and probably consists of debris of many stars. The question of whether the N and Xe(HL) actually reside in different carriers continues to be addressed. Clayton D. D.* El Eid M. Brown L. E. Carbon and Nitrogen in Type II Supernova Diamonds Abundant diamonds found in meteorites seem either to have condensed within supernova interiors during their expansions and coolings or to have been present around those explosions Either alternative allows implantation of Xe-HL prior to interstellar mixing. A puzzling feature is the near normalcy of the carbon isotopes, considering that the only C-rich matter, the He-burning shell, is pure 12C in that region. That last fact has caused many to associate supernova carbon with 12C carbon, so that its SUNOCONS have been anticipated as very 12C-rich. We show that this expectation is misleading because the 13C-rich regions of Type IIs have been largely overlooked in this thinking. We follow the idea that the diamonds nucleated in the 12C- rich He shell, the only C-rich site for nucleation, but then attached 13C-rich carbon during turbulent encounters with overlying 13C-rich matter. That is, the initial diamonds continued to grow during the same collisional encounters that cause the Xe-HL implantation. Instead of interacting with the small carbon mass having 13/12=0.2 in the upper He zone, however, we have calculated the remnants of the initial H-burning core, which left behind 13C-rich matter as it receded during core hydrogen burning. Howard et al. described why the velocity mixing would be essential to understanding the implantation of both the Xe-H and Xe-L components. Velocity mixing is now known to occur from the X-ray and gamma-ray light curves of supernova 1987A. Sharp C. M.* Wasserburg G. J. Molecular Equilibria and Condensation Sequences in Carbon Rich Gases Chemical equilibria in stellar atmospheres have been investigated by many authors. Lattimer, Schramm, and Grossman presented calculations in both O rich and C rich environment and predicted possible presolar s. A recent paper by Cherchneff and Barker considered a C rich composition with PAHs included in the calculations. However, the condensation sequences of C bearing species have not been investigated in detail. In a carbon rich gas surrounding an AGB star, it is often assumed that graphite (or diamond) condenses out before TiC and SiC. However, Lattimer et al. found some conditions under which TiC condenses before graphite. We have performed molecular equilibrium calculations to establish the stability fields of C(s), TiC(s), and SiC(s) and other high temperature phases under conditions of different pressures and C/O. The preserved presolar interstellar dust grains so far discovered in meteorites are graphite, diamond, SiC, TiC, and possibly Al2O3. Wasserburg G. J.* Gallino R. Busso M. Raiteri C. M. AGB Stars as a Source of Short-lived Radioactive Nuclei in the Solar Nebula The purpose of this study is to estimate the possible contribution of some short-lived nuclei to the early solar nebula from AGB sources. Low mass AGB stars appear to provide a site for synthesis of the main s process component for solar system material with an exponential distribution of neutron irradiations that varies directly as exp[-tau/tau(sub)0] (where tau is the time integrated neutron flux with a mean neutron exposure tau (sub)0) for solar abundances with tau (sub)O = 0.28 mb^-l. Previous workers have estimated the synthesis of key short-lived nuclei that might be produced in AGB stars. While these calculations exhibit the basic characteristics of nuclei production by neutron exposure, there is need for a selfconsistent calculation that follows AQB evolution and takes into account the net production from a star and dilution with the cloud medium. Thursday, March 18, 1993 Outer Solar System 1:30 - 5:00 p.m. Room B Chair(s): L. J. Horn R. Malhotra Matson D. L.* Veeder G. J. Johnson T. V. Blaney D. L. Goguen J. D. A Decade's Overview of Io's Volcanic Activity Over the past decade some aspects of Io's volcanic activity have changed greatly, while others have essentially remained constant. This contrast has emerged from our study of multiwavelength, infrared observations of Io's thermal emission. From 1983 to 1992 we observed the disk-integrated flux density of Io from the NASA Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Our spectral coverage allows us to separate out the emission components due to volcanic thermal anomalies, which are warmer than the background emission caused by solar heating. Our temporal coverage allows us to resolve individual eruptions and also to obtain the disk-integrated flux density as a function of longitude (or, equivalently, orbital phase angle). Characteristics that persisted over the decade involve (1) Loki's location and intensity of emission, (2) the leading hemisphere emission, and (3) the average heat flow. The variable aspects of Io over the decade include (1) Loki's hotter area(s) and (2) the outbursts in the leading hemisphere. Kargel J. S.* Crustal Structure and Igneous Processes in a Chondritic Io Liquid sulfur can form when metal-free C1 or C2 chondrites are heated. It may be obtained either by direct melting of native sulfur in disequilibrated C1 or C2 chondrites or by incongruent melting of pyrite and other sulfides in thermodynamically equilibrated rocks of the same composition. Hence, Lewis considered C2 chondrites to be the best meteoritic analogue for Io's bulk composition. Metal-bearing C3 and ordinary chondrites are too chemically reduced to yield liquid sulfur and are not thought to represent plausible analogues of Io's bulk composition. An important aspect of Lewis' work is that CaSO4 and MgSO4 are predicted to be important in Io. Real C1 and C2 chondrites contain averages of, respectively, 11% and 3% by mass of salts (plus water of hydration). The most abundant chondritic salts are magnesium and calcium sulfates, but other important components include sulfates of sodium, potassium, and nickel and carbonates of magnesium, calcium, and iron. It is widely accepted that chondritic salts formed by low-temperature aqueous alteration. Even if Io originally did not contain salts, it is likely that aqueous alteration would have yielded several percent sulfates and carbonates. In any event, Io probably contains sulfates and carbonates. Hogenboom D. L.* Kargel J. S. Ganasan J. P. Lee L. Phase Equilibria of the Magnesium Sulfate-Water System to 4 Kbars Magnesium sulfate is the most abundant salt in carbonaceous chondrites, and may be important in the low-temperature igneous evolution and aqueous differentiation of icy satellites and large chondritic asteroids. Accordingly, we are investigating high-pressure phase equilibria in MgS04-H20 solutions under pressures up to four kbars. An initial report was presented two years ago. This abstract summarizes our results to date including studies of solutions containing 153%,17%, and 22% MgSO4. Briefly, these results demonstrate (1) that increasing pressure causes the eutectic and peritectic compositions to shift to much lower concentrations of magnesium sulfate, and (2) the existence of a new low-density phase of magnesium sulfate hydrate. Goldsby D. L.* Kohlstedt D. L. Durham W. B. Rheology of Water and Ammonia-Water Ices Creep experiments on fine-grained water and ammonia-water ices have been performed at one atmosphere and high confining pressure in order to develop constitutive relationships necessary to model tectonic processes and interpret surface features of icy moons of the outer solar system. The present series of experiments explores the effects of temperature, strain rate, grain size, and melt fraction on creep strength. In general creep strength decreases with increasing temperature, decreasing strain rate, and increasing melt fraction. A transition from dislocation creep to diffusion creep occurs at finer grain sizes, higher temperatures, and lower strain rates. Croft S. K.* Porosity and the Ecology of Icy Satellites The case for a significant role for porosity in the structure and evolution of icy bodies in the solar system has been difficult to establish. Here are presented a relevant new dataset and a series of structure models including a mechanical compression, not thermal creep, model for porosity that accounts satisfactorily for observed densities, moments of inertia, geologic activity, and sizes of tectonic features on icy satellites. Pappalardo R.* Greeley R. Structural Evidence for Reorientation of Miranda About a Paleo-Pole Two structural arguments support the premise that Miranda has reoriented with respect to Uranus over its geologic history. Orientations of major extensional and compressional features are consistent with a major satellite reorientation similar to that previously suggested based on cratering asymmetry. Furthermore, structures within Elsinore Corona provide supporting evidence that this reorientation took place about a paleo-pole located near (-75 degrees, 80 degrees), suggesting a second minor reorientation of Miranda. Denk T.* Jaumann R. Neukum G. Disk-resolved Spectral Characteristics of Saturn's Medium-sized Satellites Voyager l and 2 images of six saturnian satellites have been investigated in the orange, green, and violet spectral range. The objective of this study is to examine and compare spectral differences of surface features in order to obtain information about compositional trends. Horn L. J.* Russell C. T. Mass of Saturn's A Ring The mass of Saturn's A ring is reestimated using the behavior of spiral density waves embedded in the ring. The Voyager photopolarimeter (PPS) observed the star delta Scorpii as it was occulted by Saturn's rings during the Voyager 2 flyby of Saturn in 1981, producing a radial profile of the rings. We examined forty spiral density waves in the Voyager PPS data of the A ring, including 10 weaker waves that have not been previously analyzed, using an autoregressive power spectral technique called Burg. The strengths of this new method for ring studies are that weaker, less extended waves are easily detected and characterized. This method is also the first one that does not require precise knowledge of the resonance location and phase of the wave in order to calculate the surface mass density. Uncertainties of up to 3 km are present in the currently available radial scales for Saturn's rings. Strom R. G.* Croft S. K. Triton's Cratering Record and Its Time of Capture Recent impact crater counts on the Voyager 2 high-resolution images of Triton have resulted in a more accurate crater size/frequency distribution down to about 3 km diameter. These counts reveal a size/frequency distribution characterized by a differential -4 slope. This is consistent with the observation that there are no craters larger than 27 km diameter on the 20% of Triton viewed at resolutions capable of detecting them. A -4 slope is deficient in large craters and at the very low crater density on Triton no craters larger than about 30 km are expected on just 20% of the satellite. Boyce J. M.* A Structural Origin for the Cantaloupe Terrain of Triton Cantaloupe terrain is unique to Triton. It is Triton's oldest terrain and includes an ~250,000-km-sq region displaying sparsely cratered, closely spaced, nearly circular dimples about 30-40 km across. This terrain is found on no other planet because only on Triton did the final major global thermal pulse (1) cause complete (or nearly complete) interior melting resulting in a cooling history where large thermal stresses shattered and contorted a thin, weak lithosphere, and (2) occur after heavy bombardment so that the surface features were preserved. The cantaloupe terrain is composed of intersecting sets of structures (folds and/or faults) that have developed as a result of global compression generated by volumetric changes associated with cooling of Triton's interior. Further, it is proposed that these structures developed after the period of heavy bombardment, and resulted from the last major global thermal epoch in Triton's unique history (either caused by tidal or radiometric heating). Initially, as the body cooled and the structures formed, their surface topography was most likely modified by thermal relaxation of the warm surface ices. Schenk P.* Jackson M. P. A. Diapirs and Cantaloupes: Layering and Overturn of Triton's Crust It has recently been proposed that cantaloupe terrain formed as a result of instability and overturn (i.e., diapirism) of Triton's crust. Morphologic evidence implicates compositional layering within Triton's crust as the driving mechanism for this overturn. We review the morphologic evidence for this origin and evaluate some of the implications. Hansen C. J.* Paige D. A. A Pluto Thermal Model The recent discovery of nitrogen on Pluto suggests that Pluto's volatile cycles may be similar to those on Neptune's moon Triton. Here, we report the first results of our efforts to apply a thermal model that we developed to study the seasonal nitrogen cycle on Triton to the case of Pluto. The model predicts volatile behavior as a function of time to calculate frost deposit depth, polar cap boundaries, temperature of the frost and substrate, and atmospheric pressure, assuming nitrogen frost deposits in solid-vapor equilibrium with nitrogen in the atmosphere. Levison H. F.* Stern S. A. Mapping the Stability Region of the 3:2 Neptune-Pluto Resonance Pluto and Charon are most likely the remnants of a large number of objects that existed in the Uranus-Neptune region at early epochs of the solar system. Numerical integrations have shown that, in general, such objects were ejected from the planetary region on timescales of ~10^7 years after Neptune and Uranus reached their current masses. It is thought that the Pluto-Charon system has survived to current times without being dynamically removed in this way because it is trapped in a set of secular and mean motion resonances with Neptune. The best-known Pluto-Neptune orbit coupling is the 3:2 mean motion resonance discovered almost 30 years ago by C. Cohen and E. Hubbard. These workers showed that the resonance angle, delta = 3 lambda(sub)P - 2 lambda (sub)N - omega(sub)P, where omega(sub)P is the longitude of perihelion of the Pluto-Charon system, and lambda(sub)N and lambda(sub)P are the mean longitude of Neptune and Pluto-Charon respectively, librates about 180 degrees with an amplitude, A(sub)delta, of 76 degrees. Thursday, March 18, 1993 Future Lunar Exploration 1:30 - 5:00 p.m. Room C Chair(s): M. B. Duke D. A. Morrison Head J. W.* Belton M. Greeley R. Pieters C. McEwen A. Neukum G. McCord T. Lunar Scout Missions: Galileo Encounter Results and Application to Scientific Problems and Exploration Requirements On the basis of U.S. and Soviet orbital and surface exploration and results from Earth-based observations, we have a first-order understanding of the major stages in the formation and evolution of the Moon. The Moon is known to have formed an initial lunar highland primary crust, and a secondary basaltic crust due to mantle partial melting. Impact craters at all scales have modified the crust and the largest basins may have excavated through the crust into the mantle. Exploration of Mercury and Mars has shown many similarities to the Moon, indicating that the Moon is a key to the understanding of the evolution of one- plate planets. The Moon may have formed from the impact of a Mars-sized object into a proto-Earth. These factors indicate that the Moon is a baseline and a cornerstone in planetary exploration in reference to such fundamental questions as planetary origin, the impact record, and the formation and evolution of primary and secondary planetary crusts. In addition, some of our most fundamental information about the Moon was collected during the Apollo human exploration expeditions and many questions and issues will only be resolved by future long visits and extended exploration traverses. Pieters C. M.* Head J. W. McCord T. B. The MinMap Team MinMap: An Imaging Spectrometer for High Resolution Compositional Mapping of the Moon MinMap has been selected by the Lunar Scout program to characterize and map the mineral composition of the Moon. The instrument will be built as a collaborative effort between Brown University, SETS Technology Inc., and Ball Aerospace Corp. MinMap is a visible to near-infrared imaging spectrometer that contains 192 spectral channels from 0.35-2.4 micrometers with signal-to-noise >200 and 256 cross-track spatial elements. The spectrometer design has a 6 degrees field of view (FOV) and utilizes grating dispersive elements and two dimensional detectors (no moving parts). An "image cube" of data is produced that contains two dimensions of spatial information and one dimesnsion of spectral information. All spectral channels and cross-track spatial elements are recorded simultaneously with spacecraft motion scanning the second spatial dimension. The high spectral resolution and continuous spectral range of MinMap are designed to measure the diagnostic absorption features of principal lunar minerals and their lithologic mixtures. Since the optical properties of lunar materials change in a regular manner upon exposure to the space environment, this spectral range is also quite sensitive to variations in exposure history (soil maturity). Nominal measurement strategy is to obtain full global data of the Moon at 180 m/pixel from a 450-km polar orbit during the first month or two of operation. A 100-km orbit is anticipated for the remaining part of a one-year mission allowing higher-resolution data (~80 m/pixel) to be obtained for targeted regions. MinMap exceeds LExSWG's measurement recommendations and will provide the highest spatial resolution compositional map of lunar rocks and soils currently planned for orbital missions. Since all spectral channels are co-registered and obtained simultaneously, "image cube" data swaths will be available for analysis almost immediately. Neukum G.* The High Resolution Stereo Camera (HRSC) for the Lunar Scout-I Mission The High Resolution Stereo Camera (HRSC) is a planetary imaging system developed by the German Aerospace Research Establishment (DLR) with the involvement of the German Space Industry under the leadership of the German Space Agency (DARA) for the Russian Mars 94 and Mars 96 missions. The same instrument, virtually unmodified, is ideal for imaging the Moon. If flown on a Lunar Scout spacecraft, the HRSC will be operated so that it will produce data suitable for generation of a global lunar geodetic net, a global stereo image dataset (both datasets produced at an orbit altitude of 200 km approximately) and high-resolution stereo imagery of areas of interest to the scientific community from an orbit altitude of 100 km (resolution is a function of orbit altitude). All data will be digital. Moss C. E.* Burt W. W. Edwards B. C. Martin R. A. Nakano G. H. Reedy R. C. Gamma-Ray Spectrometer for Lunar Scout II We review the current status of the Los Alamos program to develop a high- resolution gamma-ray spectrometer for the Lunar Scout II mission, which is the second of two Space Exploration Initiative robotic precursor missions to study the Moon. This instrument will measure gamma rays in the energy range of ~0.1- 10 MeV to determine the composition of the lunar surface. The instrument is a high-purity germanium crystal surrounded by an CsI anticoincidence shield and cooled by a split Stirling cycle cryocooler. It will provide the abundance of many elements over the entire lunar surface. Auchampaugh G.* Barraclough B. Byrd R. Drake D. Feldman W. Moss C. Reedy R. The Los Alamos Neutron Spectrometer for Lunar Scout-I Mission We review the current status of the Los Alamos program to develop a neutron spectrometer for the Lunar Scout I mission, which is the first of two such missions to obtain global compositional, gravity, topography, and image maps of the lunar surface during nominal one-year missions. The neutron spectrometer will measure fast and slow (epithermal and thermal) neutrons in the ranges of 0.5 MeV to 25 MeV and 0.01 eV to more than 1 keV, respectively. The neutron spectrometer will consist of two independent instruments, a fast- neutron one and a thermal and epithermal one. The measured neutron fluxes are very sensitive to hydrogen in the top meter of the lunar surface and provide additional information about lunar composition. Clark P. E. Evans L. G. Trombka J. I.* Remote Sensing X-Ray Fluorescence Spectrometry for Future Lunar Exploration Missions Measurement of discrete line X-ray emission from space can be used to obtain both qualitative and quantitative elemental surface composition information. Remote orbital X-ray elemental analysis is the measurement of characteristic X-rays following the interaction of solar X-rays with the surface of a given solid body. Thus, X-ray emission from a surface is strongly dependent on the incident solar spectrum as well as on the chemical composition of the surface. In this paper, the relationship between the solar flux and the major lunar element fluorescence and scattered X-ray flux is characterized. A model had been developed to calculate lunar surface X-ray fluorescence emission spectra under a variety of solar conditions, and in particular conditions that might be expected for a Lunar Scout mission proposed for 1995 or 1996. This time period should be during solar minimum. Cheng A. F.* Lunar Scout Two Spacecraft Gravity Experiment Measurement of the gravity field of the Moon has a high science priority because of its implications for the internal structure and thermal history of the Moon, and it has a high priority for future exploration activities because of the influence of lunar gravity on spacecraft navigation and orbit maintenance. The current state of knowledge in the lunar gravity field (and the uncertainty in the knowledge) is based primarily on data accumulated from the Lunar Orbiter and Apollo programs. Data are sparse and emphasize the equatorial band (+-30 degrees) on the nearside of the Moon. There are no tracking data on the farside and only the Lunar Orbiter V provides a small amount of high-inclination data. A host of gravity models developed from different combinations of tracking data have large discrepancies in their predictions of spacecraft motion and orbit lifetimes. There are also large disagreements in the Mercator projections of the gravity acceleration from each model, especially on the farside, where the contours tend to have no obvious relationship with the local topography. The science and engineering requirements for global gravity field mapping will be satisfied with continuous radiometric tracking of Lunar Scout I in a low polar orbit using the Deep Space Network, and Lunar Scout II in a high elliptical orbit. Shoemaker E. M.* Nozette S. Clementine: An Inexpensive Mission to the Moon and Geographos The Clementine Mission, a joint project of the Strategic Defense Initiative Organization (SDIO) and NASA, has been planned primarily to test and demonstrate a suite of lightweight sensors and other lightweight spacecraft components under extended exposure to the space environment. Although the primary objective of the mission is to space- qualify sensors for Department of Defense applications, it was recognized in 1990 that such a mission might also be designed to acquire scientific observations of the Moon and of Apollo asteroid (1620) Geographos. This possibility was explored jointly by SDIO and NASA, including representatives from NASA's Discovery Program Science Working Group, in early 1991. Besides the direct return of scientific information, one of the benefits envisioned from a joint venture was the development of lightweight components for possible future use in NASA's Discovery-class spacecraft. In January 1992, SDIO informed NASA of its intent to fly a "Deep Space Program Science Experiment," now popularly called Clementine; NASA then formed an advisory science working group to assist in the early development of the mission. Lucey P. G.* The Clementine Instrument Complement The recent successes of the Galileo SSI imaging experiment at the Moon and Gaspra show the utility of multispectral imaging of planetary objects. "Clementine" is the planetary community's "code name" for the SDIO (Space Defense Initiative Organization) mission to the Moon and the asteroid Geographos. This mission is designed as a long-term stressing test on sensors and space systems developed for SDIO. In the course of this test Clementine will obtain science data using a varied and powerful array of remote sensing instruments that were developed by or for Lawrence Livermore National Laboratory in Livermore, California. Clementine carries five cameras, one for navigation and four for science experiments. In addition, a laser ranger is included that will serve as a laser altimeter. The Clementine cameras cover a wider range of spatial resolutions and wavelength range than did Galileo and are almost ideally suited to mapping of mafic rock types as are present on the Moon and expected at Geographos. The technical characteristics of the Clementine cameras are listed in the tables. McEwen A. S.* Clementine: Anticipated Scientific Datasets from the Moon and Geographos The Clementine spacecraft mission is designed to test the performance of new lightweight and low-power detectors developed at the Lawrence Livermore National Laboratory (LLNL) for the Strategic Defense Initiative Office (SDIO). A secondary objective of the mission is to acquire useful scientific data, principally of the Moon and the near-Earth asteroid Geographos. The spacecraft will be in an elliptical polar orbit about the Moon for about 2 months beginning in February of 1994, and it will fly by Geographos on August 31. Clementine will carry seven detectors, each weighing less than about 1 kg: two Star Trackers, wide-angle uv/vis, wide-angle Short Wavelength IR (SWIR), Long- Wavelength IR (LWIR), and LIDAR (Laser Image Detection And Ranging) narrow- angle imaging and ranging. Additional presentations about the mission, detectors, and related science issues are in this volume. Spudis P. D.* Lucey P. G. Contributions of the Clementine Mission to Our Understanding of the Processes and History of the Moon CONTRIBUTIONS OF THE CLEMENTINE MISSION TO OUR UNDERSTANDING OF THEPROCESSES AND HISTORY OF THE MOON Paul D. Spudis1 and Paul G. Lucey2 1.Lunar and Planetary Institute, Houston TX 77058 2. University of Hawaii,Honolulu HI 96822The Clementine mission [1] will provide us with an abundance of informationabout lunar surface morphology, topography, and composition, permitting us toinfer the history of the Moon and the processes that have shaped that history. This information can be used to address fundamental questions in lunar scienceand allow us to make significant advances towards deciphering the complexstory of the Moon. The Clementine mission will also permit a first-orderglobal assessment of the resources of the Moon and provide a strategic base ofknowledge upon which future robotic and human missions to the Moon can build. Meyer C.* Opportunity for Early Science Return by the Artemis Program The purpose of the Artemis Program is to gather vital scientific and engineering data by conducting robotic exploration missions on the lunar surface both prior to and concurrent with human missions. The Artemis Program includes rapid, near-term development of a variety of small experimental and operational payloads, a low-cost capacity to deliver these payload to any location on the lunar surface, and the analysis of the data returned. The Artemis Program will provide opportunities to improve the understanding of lunar geosciences, to demonstrate the Moon's unique capacity as an astronomical platform to study the universe, to conduct scientific and technology development experiments, and to prepare for and complement human missions. Lofgren G. E.* The First Lunar Outpost: The Design Reference Mission and a New Era in Lunar Science The content of the First Lunar Outpost (FLO) Design Reference Mission has been formulated and a "strawman" science program has been established. The mission consists of two independent launches using heavy lift vehicles that land directly on the lunar surface. A habitat module and support systems are flown to the Moon first. After confirmation of a successful deployment of the habitat systems, the crewed lunar lander is launched and piloted to within easy walking distance (2 km) of the habitat. By eliminating the Apollo-style lunar orbit rendezvous, landing sites at very high latitudes can be considered. A surface rover and the science experiments will accompany the crew. The planned stay time is 45 days, 2 lunar days and one night. A payload of 3.3 metric tons will support a series of geophysics, geology, astronomy, space physics, resource utilization, and life science experiments. Sample return is 150 to 200 kg. The rover is unpressurized and can carry 4 astronauts or 2 astronauts and 500 kg of payload. The rover can also operate in robotic mode with the addition of a robotics package. The science and engineering experiment strategy is built around a representative set of place holder experiments. Chicarro A. F.* Mission to the Moon: An ESA Study on Future Exploration The increasing worldwide interest in the continuation of lunar exploration has determined ESA to carry out an investigation of the motivations to return to the Moon to establish a permanent or a semipermanent manned lunar base. This study also considers the possible role Europe could play in the future exploration and possible utilisation of the Moon. The study concentrated in this first phase mainly on scientific questions, which were divided into three main areas: (1) Science of the Moon, which includes all investigations concerning the Moon as a planetary body (geophysics, geochemistry, geology, origin of the Earth-Moon system, interaction of the Moon with its environment); (2) Science from the Moon, using the Moon as a platform, hence including observatories in the broadest sense (interferometry, very-low-frequency astronomy, astrometry, planetary astronomy, solar physics and solar wind plasma, high energy physics); and (3) Science on the Moon, which includes not only questions related to human activities in space but also the development of artificial ecosystems beyond the Earth (exobiology, radiation biology and protection, artificial ecosystems and biological support systems, human physiology and medicine). Thursday, March 18, 1993 Exploration Poster Session 7:00 - 9:00 p.m. LPI Vorder Bruegge R. W. Davies M. E. Horan D. M. Lucey P. G. Pieters C. M. McEwen A. S. Nozette S. Shoemaker E. M. Squyres S. W. Thomas P. C. The Clementine Mission Science Return at the Moon and Geographos The Clementine Mission is being built and flown by the Naval Research Laboratory under the sponsorship of the Strategic Defense Initiative Organization of the United States Department of Defense in joint- cooperation with NASA, and will explore the Moon and the near-Earth asteroid (NEA) 1620 Geographos with lightweight sensors developed by the Lawrence Livermore National Laboratory. A NASA Science Team for this mission will be selected by way of an NRA in April 1993. The instrument suite includes imaging cameras that cover a spectral range from the near-ultraviolet to the mid-infrared, a laser ranger, and, potentially, a charged particle telescope. To be launched in early 1994, Clementine will be in lunar orbit from February through May 1994, at which time it will depart the Moon for a flyby of 1620 Geographos in August 1994. This mission represents an outstanding opportunity for scientists interested in the Moon and asteroids. It is anticipated that the data returned from this mission will permit: an assessment of global lunar crustal heterogeneity at a resolution of less than 1 km; an assessment of the lithologic heterogeneity of Geographos at a scale of 100 m or better; and an assessment of surface processes on Geographos on the order of 10 m. This abstract describes the basic mission of Clementine and some of the key scientific questions that will be addressed. Additional material on the Clementine mission, its data handling and processing, and its instrument suite is presented elsewhere in this volume. Altenberg B. H. Franklin H. A. Jones C. H. Thermodynamics of Lunar Ilmenite Reduction With the prospect of returning to the Moon, the development of a lunar occupation would fulfill one of the goals of the Space Exploration Initiative (SEI) of the late 1980s. Processing lunar resources into useful products, such as liquid oxygen for fuel and life support, would be one of many aspects of an active lunar base. Ilmenite (FeTiO3) is found on the lunar surface and can be used as a feedstock to produce oxygen. Understanding the various ilmenite- reduction reactions elucidates many processing options. Defining the thermodynamic chemical behavior at equilibrium under various conditions of temperature and pressures can be helpful in specifying optimal operating conditions. Differences between a previous theoretical analysis (Hernandez and Franklin, 1992) and experimentally determined results (Gibson et al., 1988) has sparked interest in trying to understand the effect of operating pressure on the hydrogen-reduction-of-ilmenite reaction. Taylor L. A. Jerde E. A. McKay D. S. Gibson M. A. Knudsen C. W. Kanamori H. Production of O2 on the Moon: A Lab-Top Demonstration of Ilmenite Reduction with Hydrogen Estimates of the costs of transporting materials from Earth to the Moon are around $25,000 per pound. Therefore, it is imperative that we learn to utilize the resources on the Moon to partially offset these "astronomical" expenses. The production of oxygen on the Moon utilizing indigenous materials is crucial to the establishment and development of an autonomous lunar colony. Besides obvious biologic needs, this lunar liquid oxygen (LLOX) could result in tremendous cost savings on fuel for effective transportation systems, particularly with its export to low-Earth orbit. Over 20 different process concepts have been proposed and evaluated for the production of oxygen from lunar materials. Simplicity, low energy, easily attainable feedstock, and low resupply mass are the keywords for the process(es) that will ultimately be selected for the initial production of oxygen on the Moon. One of these schemes, which has received considerable study to date, is the hydrogen reduction of ilmenite. In fact, Carbotek, Inc. (Houston. TX) has patented an ilmenite, hydrogen reduction technique involving a three-stage, fluidized-bed process for the production of LLOX. We explain a lab-top demonstration unit of the basic concepts of this oxygen generation process that has been constructed by our group at the University of Tennessee. It utilizes many of the principles that must be addressed in designing an effective production plant for operation on the Moon. Gibson M. A. Knudsen C. W. Brueneman D. J. Kanamori H. Ness R. O. Sharp L. L. Brekke D. W. Allen C. C. Morris R. V. Keller L. P. McKay D. S. First Oxygen from Lunar Basalt The Carbotek/Shimizu process to produce oxygen from lunar soils has been successfully demonstrated on actual lunar samples in laboratory facilities at Carbotek with Shimizu funding and support. Apollo sample 70035 containing approximately 25% ilmenite (FeTiO3) was used in seven separate reactions with hydrogen gas varying temperature and pressure: FeTiO3 + H2 --> Fe + TiO2 + H2O. The experiments gave extremely encouraging results as all ilmenite was reduced in every experiment. The lunar ilmenite was found to be about twice as reactive as terrestrial ilmenite samples. Analytical techniques of the lunar and terrestrial ilmenite experiments performed by NASA Johnson Space Center include iron Mossbauer spectroscopy (FeMS), optical microscopy, SEM, TEM, and XRD. The Energy and Environmental Research Center at the University of North Dakota performed three SEM techniques (point count method, morphology determination, elemental mapping), XRD, and optical microscopy. Agosto W. N. Production of Electronic Grade Lunar Silicon by Disproportionation of Silicon Difluoride Waldron has proposed to extract lunar silicon by sodium reduction of sodium fluorosilicate derived from reacting sodium fluoride with lunar silicon tetrafluoride. Silicon tetrafluoride is obtained by the action of hydrofluoric acid on lunar silicates. While these reactions are well understood, the resulting lunar silicon is not likely to meet electronic specifications of 5 nines purity. Dale and Margrave have shown that silicon difluoride can be obtained by the action of silicon tetrafluoride on elemental silicon at elevated temperatures (1100-1200 C) and low pressures (1-2 torr). The resulting silicon difluoride will then spontaneously disproportionate into hyperpure silicon and silicon tetrafluoride in vacuum at approximately 400 C. On its own merits, silicon difluoride polymerizes into a tough waxy solid in the temperature range from liquid nitrogen to about 100 C. It is the silicon analogue of teflon. Silicon difluoride ignites in moist air but is stable under lunar surface conditions and may prove to be a valuable industrial material that is largely lunar derived for lunar surface applications. Taylor L. A. Chambers J. G. Patchen A. Jerde E. A. McKay D. S. Graf J. Oder R. Evaluation of Lunar Rocks and Soils for Resource Utilization: Detailed Image Analysis of Raw Materials and Beneficiated Products The rocks and soils of the Moon will be the raw materials for fuels and construction needs at a lunar base. This includes sources of materials for the generation of hydrogen, oxygen, metals, and other potential construction materials. For most of the bulk material needs, the regolith. and its <1-cm fraction, the soil will suffice. But for specific mineral resources, it may be necessary to concentrate minerals from rocks or soils, and it is not always obvious which is the more appropriate feedstock. Besides an appreciation of site geology, the mineralogy and petrography of local rocks and soils is important for consideration of the resources that can provide feedstocks of ilmenite, glass, agglutinates, anorthite, etc. In such studies, it is very time-consuming and practically impossible to correlate particle counts (the traditional method of characterizing lunar soil petrography) with accurate modal analyses and with mineral associations in multimineralic grains. But X- ray digital imaging, using X-rays characteristic of each element, makes all this possible and much more (e.g., size and shape analysis).We demonstrate an application of beneficiation image analysis in use in our lab (Oxford Instr. EDS and Cameca SX-50 EMP) to study mineral liberation from lunar rocks and soils. Elphic R. C. Funsten H. O. III Hervig R. L. Solar Wind-Induced Secondary Ions and Their Relation to Lunar Surface Composition Based on previous laboratory experiments simulating solar wind sputtering of lunar surface materials, it appears that solar wind ions sputter secondary ions in sufficient numbers to be measured from low-altitude lunar orbit. Solar wind protons are hundreds of times less efficient than those used in standard secondary ion mass spectrometry; nevertheless secondary ions of Na, Mg, Al, Si, K, Ca, Mn, Ti, and Fe were observed sputtered from sample simulants of mare and highland soils. These secondary ion fluxes depend both on concentration in the soil and on probability of ionization; yields of easily ionized elements such as K and Na are relatively much greater than those for the more electronegative elements and compounds. The principal geochemical indicator elements Na, Mg, Al, Si, K, Ca, Ti, Mn, and Fe are nevertheless detectable. However, it has not been shown that the observed secondary ions vary strictly one-to-one with their concentrations in the geochemically distinct soil simulants used here. Shelfer T. D. Wills E. L. Agresti D. G. Pimperl M. M. Shen M. H. Morris R. V. Nguyen T. Combined Backscatter Mossbauer Spectrometer/X-ray Fluorescence Analyzer (BaMS/XRF) for Extraterrestrial Surfaces We have designed and tested a prototype combined backscatter Mossbauer spectrometer and X-ray fluorescence analyzer (BaMS/XRF). A space-qualified instrument based on this design would be suitable for in situ use on planetary missions to the surfaces of the Moon (Artemis and lunar outpost), Mars (MESUR), asteroids, or other solid solar system objects. The BaMS/XRF instrument is designed to be capable of concurrent sample analyses for the mineralogy of iron-bearing phases and elemental composition without the need for sample preparation. Wilson T. L. Svoboda R. CERN-derived Analysis of Lunar Radiation Backgrounds The Moon produces radiation, which background-limits scientific experiments there. Early analyses of these backgrounds have either failed to take into consideration the effect of charm in particle physics (because they pre-dated its discovery), or have used branching ratios that are no longer strictly valid (due to new accelerator data). We are presently investigating an analytical program for deriving muon and neutrino spectra generated by the Moon, converting an existing CERN computer program known as GEANT that does the same for the Earth. In so doing, this will (1) determine an accurate prompt neutrino spectrum produced by the lunar surface; (2) determine the lunar subsurface particle flux; (3) determine the consequence of charm production physics upon the lunar background radiation environment; and (4) provide an analytical tool for the NASA astrophysics community with which to begin an assessment of the Moon as a scientific laboratory vs. its particle radiation environment. This will be done on a recurring basis with the latest experimental results of the particle data groups at Earth-based high-energy accelerators, in particular with the latest branching ratios for charmed meson decay. This will be accomplished for the first time as a full 3-Dimensional simulation. LaFave N. Wilson T. L. Lunar LIGO: A New Concept in Gravitational Wave Astronomy For three decades, physicists have been in search of an elusive phenomenon predicted by Einstein's general theory of relativity: gravitational radiation. These weak vibrations of spacetime have, thus far, eluded conclusive Earth- based detection due in part to insufficient detector sensitivity and noise isolation. The detection of gravitational waves is crucial for two reasons. It would provide further evidence for the validity of Einstein's theory of relativity, the presently accepted theory of gravitation. Furthermore, the ability to identify the location of a source of a detected gravitational wave event would yield a radical new type of astronomy based on non-electromagnetic emissions. We continue our study of a lunar-based system, which can provide an important complement to Earth-based analysis because it is completely independent of the geophysical sources of noise on Earth, while providing an Earth-Moon baseline for pin-pointing burst sources in the universe. We also propose for the first time that a simplified version of the LIGO beam detector optical system, which we will call LLIGO (Lunar LIGO), could be emplaced on the Moon as part of NASA's robotic lander program now under study (Artemis). Murphy D. L. Vondrak R. R. Effects of Levitated Dust on Astronomical Observations from the Lunar Surface It is believed that a substantial population of levitated dust is present in the terminator region of the Moon. Stray light scattered by this dust layer may contaminate astronomical observations made from the lunar surface using infrared, visible, and ultraviolet light. The evidence for dust levitation stems from Surveyor vidicon images of horizon glow, anomalous brightness in photographs of the solar corona taken by Apollo astronauts while the spacecraft was just inside the Moon's shadow, and observations by Apollo astronauts of streamers just prior to lunar orbital sunrise or just after lunar orbital sunset. It has been proposed that the differential charging of the lunar surface in the terminator region due to photoemission and the consequent strong local electric fields comprise the mechanism responsible for this levitation. Although quantitative data on the levitated lunar dust distribution are meager, it is possible to estimate column densities and sizes. In this paper we summarize the estimates of particulate sizes and number densities of previous authors, and construct a nominal terminator dust distribution, as a function of particulate radius and altitude above the lunar surface. Using the model we estimate the brightness of scattered sunshine for three wavelength bands. For the results in the visible wavelengths, we compare the estimated brightness with the known brightness of selected astronomical objects and discuss the implications for lunar-based astronomy. Landheim R. Greeley R. Des Marais D. Farmer J. D. Klein H. Mars Exobiology Landing Sites for Future Exploration The selection of landing sites for exobiology is an important issue for planning for future Mars missions. This report presents results of a recent site selection study that focused on potential landing sites described in the Mars Landing Site Catalog. In addition, we will review basic exobiology science objectives in Mars exploration, and outline the procedures used in site evaluation and prioritization. Matsushima K. Saito J. Utashima M. Koshiishi H. A Mission Concept of Phobos/Deimos Exploration The exploration of space resources will be a part of space developments in the 21st century. Among the extraterrestrial bodies in the solar system, minor bodies, especially asteroids, are very attractive targets not only because of their scientific value, but also because they are promising extraterrestrial resources for future utilization. In considering the exploration of these bodies, the minute characterization of their component materials is the most important instrumental problem. Thursday, March 18, 1993 Lunar Remote Sensing Poster Session 7:00 - 9:00 p.m. LPI Fischer E. M. Pieters C. M. Measuring and Distinguishing Compositional and Maturity Properties of Lunar Soils by Remote VIS-NIR Spectroscopy Space weathering on the lunar surface affects the spectral/optical character of an exposed lunar soil in three ways: the reflectance is reduced, absorption band depths are reduced, and a red-sloped continuum is created and increased with exposure. As a result, the spectrum of a lunar soil is dependent upon both the degree of exposure at the lunar surface and the original composition. It is critical to the remote analysis of lunar soils to differentiate between the optical effects of maturity and the effects of composition. In the laboratory, it is possible to determine and consequently distinguish the degree of exposure, or soil maturity, as measured by parameters such as Is/FeO (mature defined as Is/FeO >= 60), and the composition, as measured by various chemical and petrographical techniques. Lunar soils returned by the Apollo missions provide important ground truth for developing methods for remotely measuring the maturity and the concentration of Fe-bearing minerals in lunar soil. The ground truth spectral data analyzed here are from the John Adams lunar soil spectra collection. Soils collected from or near highland terrains are emphasized in the present discussion. The mineralogical makeup of mare soils results in behavior somewhat different from highland soils. Lucey P. G. Clark B. E. Hawke B. R. Mixing Model Analysis of Telescopic Lunar Spectra We have analyzed very-high-quality reflectance spectra of the lunar surface from the University of Hawaii lunar spectral data collection using a spectral mixing model. The spectra analyzed are those of 45 mare sites and 75 highland sites. The spectra were selected on the basis of very high signal-to-noise ratios based on error bars and point-to-point scatter, and on quality of removal of telluric water bands. The spectral mixing model used seven components, not all of which were used in each fit. Four of the components were mineral spectra of an orthopyroxene, a clinopyroxene, an olivine, and an anorthite, measured at the Brown University's RELAB. All the minerals were 45- 90-micrometer splits. Lunar soil contains other components that have the effect of reddening and darkening the soil, as well as reducing spectral contrast. In addition, lunar soil contains spectral neutral bright material (likely very-fine-grained feldspar) that serves to reduce spectral contrast and bnghten soils. Early attempts to fit many of the spectra pointed out the need for a component that has a very broad smooth absorption feature centered near 1.1 micrometers. Glass is a good candidate for this component. For the bright component we used a flat reflectance of 70% to represent fine-grained feldspar. For the "glass" component we used a telescopic spectrum of a pyroclastic glass present on the Aristarchus Plateau that is characterized by a strong smooth band centered at 1.07 micrometers. This spectrum was converted to albedo using the map of Pohn and Wildey. In addition to exhibiting the glass band, this spectrum is very red and has a low albedo. On the assumption that the dark component and the red component are agglutinates, which is reasonable but not necessarily true (Pieters, pers. comm, 1992), we sought a dark red component. To derive its properties we modeled the spectrum of an Apollo 16 soil (16xxx), and assumed the dark red component to comprise 60% of the soil, appropriate to agglutinate abundance in mature soil. We adjusted the albedo and slope of a straight line representing the dark red component until the contrast and albedo of the model spectrum matched the soil spectrum. The mixing was done after conversion to single scattering albedo using the equations of Hapke and average single particle phase functions for silicates measured by Mustard and Pieters. Larson S. M. Collins J. Singer R. B. Johnson J. R. Melendrez D. E. Lunar Phase Function Effects on Spectral Ratios Used for Resource Assessment Groundbased telescopic CCD images of 36 selected locations on the Moon were obtained in 5 "standard" bandpasses at 12 phase angles ranging from -78 degrees to +75 degrees to measure phase function effects on the ratio values we have used to quantify the abundance of TiO2 and qualitatively indicate soil maturity. Consistent with previous studies, we find that the Moon is "bluer" at small phase angles, but that the effect on the ratio values for TiO2 abundance for the phase angles of our data is on the order of the measurement uncertainties throughout the range of abundances found in the mare. The effect is more significant as seen from orbiting spacecraft over a range of selenographic latitudes. Robinson M. S. Hawke B. R. Edwards K. Lucey P. G. Clark B. E. Preliminary Results from Mariner 10: High Resolution Images of the Moon In November 1973 the Mariner 10 spacecraft acquired high-esolution images of both the Earth and the Moon as it began its voyage to Venus and then Mercury. The best images had a resolution of ~1 km and were taken from an unusual viewpoint, above the lunar North Pole. At this time the Moon was illuminated such that the eastern limb, including ~30 degrees of the farside, was visible. Two high-resolution mosaics were acquired during this period that provide excellent views of regions of the Moon poorly seen from the Earth. These include the Frigoris, Humboldtianum, Marginis, and Smythii regions. These images also covered expanses of highlands not visible from the Earth. These data were unique in that they were the only useful robotic spacecraft images of the Moon; and they remained so until December 1990 when the Galileo spacecraft made its first encounter with the Moon. We have acquired these Mariner 10 lunar images and are currently using them in conjunction with Earth-based telescopic spectra as well as Apollo and Lunar Orbiter photographic data to investigate the nature of deposits comprising the Northeast Nearside (NEN) of the Moon. These Mariner 10 frames have proved useful for photogeologic, photometric, and photoclinometric (R. Kirk) analyses; they have also been used in support of the second Galileo lunar encounter of December 1992 (see other abstracts this issue). Head J. W. Mustard J. Antonenko I. Hawke B. R. Modes of Formation of Lunar Light Plains and the Detection of Cryptomaria Deposits The early volcanic and impact histories of the Moon are closely linked, and the record preserved in surface morphology and samples is tightly convolved because of the interaction of the two processes. The deconvolution of the record is an important goal in order to assess early volcanic flux and the mode of emplacement of large crater and basin deposits. For example, lunar light plains have been variously interpreted as volcanic, impact, and volcanic covered by impact deposits (e.g., cryptomare). The development of criteria for the determination of the origin of light plains and the detection of cryptomaria is thus a key to the deconvolution of this early record. In this paper we outline the various hypotheses for the origin of, and potential modes of occurrence of light plains and cryptomaria, and develop criteria for their recognition and documentation. We use the example of the Schiller- Schickard and Balmer cryptomaria to illustrate the application of these techniques to the problem of light plains interpretation and cryptomaria documentation. Peterson C. A. Hawke B. R. Lucey P. G. Taylor G. J. Blewett D. T. Spudis P. D. Spectral Reflectance Studies of the Humorum Basin Region A portion of the mare-bounding (MB) ring of Humorum Basin is composed of pure anorthosite while other parts of the ring are composed of noritic anorthosite. An episode of mare volcanism emplaced basaltic units in the region northwest of the MB ring after the Humorum impact event. Subsequently, large impacts emplaced a veneer of highlands material atop the basalt flows. Some mare material could have been mixed with this highlands debris either by local mixing by secondary craters or by vertical mixing. Spectra for most other highlands units in the region indicate a noritic anorthosite lithology. Spectra of mare basalts in Mare Humorum and nearby mare flooded craters show relatively deep absorption bands due to the presence of abundant high-Ca pyroxene. An analysis of spectra for a small number of craters in the highlands west of the outer ring of Humorum reveals the presence of high-Ca pyroxene. This suggests the possible presence of an extensive gabbroic province. Gladstone G. R. McDonald J. S. Boyd W. T. EUVE Observations of the Moon During its all-sky survey, the Extreme Ultraviolet Explorer (EUVE) satellite observed the Moon several times at first and last quarters, and once near the Dec. 10, 1992, lunar eclipse. We present here a preliminary reduction and analysis of this data, in the form of EUV images of the Moon and derived albedos. Extreme ultraviolet observations of the Moon are of considerable interest, since it has been speculated that lunar EUV emissions are primarily due to L- and M-shell X-ray fluorescence and may provide a useful diagnostic of surface elemental abundances. Davies M. E. Colvin T. R. Belton M. J. S. Greeley R. Galileo SSI Team Galileo EM-2 Contributions to the Lunar Control Network A local control network is being developed using Galileo images that cover the region north of the Apollo area and lie between 10 and 100 degrees east longitude. This network is tied to the Apollo control network and will have a positional accuracy of approximately 500-1500 m. This region has been photographed by Earth-based telescopes and the Mariner 10 and Lunar Orbiter spacecraft, but the Galileo images are preferred for control because of their superior viewing angles, resolution, and Galileo's geometrically stable sensor. Williams D. A. Greeley R. Neukum G. Wagner R. Multispectral Studies of Selected Crater- and Basin-filling Lunar Maria from Galileo Earth-Moon Encounter 1 New visible and near-infrared multispectral data of the Moon were obtained by the Galileo spacecraft in December 1990. These data were calibrated with Earth-based spectral observations of the nearside to compare compositional information to previously uncharacterized mare basalts filling craters and basins on the western nearside and eastern farside. A Galileo-based spectral classification scheme, modified from the Earth-based scheme developed by Pieters, designates the different spectral classifications of mare basalt observed using the 0.41/0.56 micrometers reflectance ratio (titanium content), 0.56 micrometers reflectance values (albedo), and 0.76/0.99 micrometers reflectance ratio (absorption due to Fe2+ in mafic minerals and glass). In addition, age determinations from crater counts and results of a linear spectral mixing model were used to assess the volcanic histories of specific regions of interest. These interpreted histories were related to models of mare basalt petrogenesis in an attempt to better understand the evolution of lunar volcanism. McEwen A. S. Becker T. L. Robinson M. S. Klaasen K. P. Heffernan C. Sunshine J. M. Galileo SSI Team Lunar Multispectral Mosaics from Galileo's Second Earth-Moon Flyby Galileo's Solid-State Imaging (SSI) experiment acquired about 800 images of the Moon from the second Earth-Moon flyby (EM2) in December of 1992. Ten major sequences were acquired; each consists of mosaics of the entire or nearly entire visible and illuminated surface from each viewing geometry in at least six spectral filters (effective wavelengths for the Moon of 420, 564, 660, 756, 890, and 990 nm). The geometries of LUNMOS numbers 3, 4, 5, and 6 were designed to provide stereo data at the best possible resolutions. Preliminary science results are described in a series of abstracts in this volume. The purpose of this abstract is to describe the sequences, calibration, processing, and mosaicking, and to present a set of color products in a poster session. Thursday, March 18, 1993 Lunar Samples Poster Session 7:00 - 9:00 p.m. LPI Wentworth S. J. Lindstrom D. J. Martinez R. R. McKay D. S. Petrology and Geochemistry of VLT Glasses from Double Drive Tube 79001/2 As a part of our more general studies of soils from Apollo 17 double drive tube 79001/2 (station 9, Van Serg Crater), we are analyzing glasses from the 79001/2 core by a multidisciplinary approach including SEM/EDS and INAA. We recently reported preliminary results for a wide variety of 79001/2 glasses, including common and unusual compositional types of both mare and highland origin. Our efforts are currently focused on VLT (very low-Ti; TiO2 < 1 wt%) mare glasses, which are common in 79001/2 and have also been found in other Apollo 17 soils. One of our primary objectives is to determine whether any or all of the Apollo 17 VLT glasses represent pristine volcanic compositions. In addition, we wish to define the range of VLT glass compositions and possible relationships between the glasses and VLT lithic samples, for which some geochemical data have been obtained previously. Hughes S. S. Dasch E. J. Nyquist L. E. Petrologic Models of 15388, a Unique Apollo 15 Mare Basalt Mare basalt 15388, a feldspathic microgabbro from the Apennine Front, is chemically and petrographically distinct from Apollo 15 picritic, olivine- normative (ON) and quartz-normative basalts. The evolved chemistry, coarse texture, lack of olivine, and occurrence of cristobalite in 15388 argue for derivation by a late-stage magmatic process that is significantly removed from parental magma. It either crystallized from a magma evolved from the more mafic Apollo 15 basalts, or it crystallized from a currently unrepresented magma. Rb-Sr and Sm-Nd isotopic systematics yield isochron ages of 3.391 +- 0.036 and 3.42 +- 0.07 Ga, respectively, and epsilon(sub)Nd = 8.6 +- 2.4, which is relatively high for Apollo 15 mare basalts. In contrast to chemical patterns of average Apollo 15 ON basalts and Apollo 15 picritic basalt, 15388 has a strongly positive LREE slope, high Ti, shallower HREE slope, and a slightly positive Eu anomaly. These features argue against 15388 evolution by simple olivine fractionation of a parental ON or picritic basalt magma although olivine is a dominant liquidus phase in both potential parents. Benoit P. H. Sears D. W. G. Natural Thermoluminescence Profiles in Lunar Cores and Implications for Meteorites Meteorites and lunar samples have been irradiated by high energy cosmic rays, typically for millions of years. In addition to producing isotopic changes, the irradiation creates ionization that may be recorded in the form of stored thermoluminescence (TL) in certain minerals, the most important of which is feldspar. One aspect of interpreting the TL of these samples is the effect of "shielding" or depth control, which is particularly important for meteorites, since they have lost an unknown amount of mass during atmospheric entry. Here we report theoretical calculations that we compare with samples from lunar cores for which we have excellent stratigraphic control. We then discuss the implications for these results for the TL of meteorites, which have a different irradiation geometry. We find that, in general, calculated profiles are similar to those observed in lunar samples and meteorites. Additional effects, such as orbital (thermal) history and terrestrial age, must also be considered in the case of meteorites. Brinton K. L. F. Bada J. L. Arnold J. R. A Reexamination of Amino Acids in Lunar Soil Amino acids in lunar soil provide an important indicator of the level of prebiotic organic compounds on the Moon. The results provide insight into the chemistry of amino acid precursors, and furthermore, we can evaluate the survival of organics upon impact. The amino acid contents of both hydrolyzed and unhydrolyzed hot-water extracts of Apollo 17 lunar soil were determined using ophthaldialdehyde/N-acetyl cysteine (OPA/NAC) derivatization followed by HPLC analysis. Previous studies of lunar amino acids were inconclusive, as the technique used (derivatization with ninhydrin followed by HPLC analysis) was unable to discriminate between cosmogenic amino acids and terrestrial contaminants. Cosmogenic amino acids are racemic, and many of the amino acids found in carbonaceous meteorites such as Murchison, i.e. alpha-amino- ibutyric acid (aib), are extremely rare on Earth. The ninhydrin method does not distinguish amino acid enantiomers, nor does it detect alpha-alkyl amino acids such as aib, whereas the OPA/NAC technique does both. Takeda H. Miyamoto M. Mineralogy and Cooling History of Magnesian Lunar Granulite 67415 We investigated Apollo granulite 67415 by mineralogical techniques to gain better understanding of cooling histories of lunar granulites. We estimated cooling rates from chemical zoning of olivines in magnesian granulitic clasts by computer simulation of diffusion processes. The cooling rate of 10 degrees C/yr obtained is compatible with a model of the granulite formation, in which the impact deposit was cooled from high temperature or annealed, at the depth of about 25 m beneath the surface. Jolliff B. L. A Monazite-bearing Clast in Apollo 17 Melt Breccia A phosphate-rich clast in a pigeonite-plagioclase mineral assemblage occurs in Apollo 17 impact-melt breccia 76503,7025. The clast, measuring 0.9 x 0.4 mm in thin section, contains 3.3% (volume) apatite [Ca5P3O12(F,Cl)], 0.8% whitlockite [Ca16(Mg,Fe)2REE2P14O56], and trace monazite [(LREE)PO4]. Major minerals include 26% pigeonite, En53-57Fs34-35Wo8-13, and 69% plagioclase, An84-92Ab7-15Or0.6-1.1). Troilite, ilmenite, and other accessory minerals constitute <1% of the assemblage and Fe metal occurs along fractures. Also present in the melt breccia as a separate clast is a fragment of felsite (K,Ba-feldspar, "ternary" plagioclase, and a silica phase-quartz[?]). Based on the association of these clasts and their assemblages, a parent lithology of alkali-anorthositic monzogabbro is postulated. Monazite occurs in the phosphate-bearing clast as two <10-micrometer grains intergrown with whitlockite. The concentration of combined REE oxides in monazite is 63.5% and the chondrite-normalized REE pattern is strongly enriched in LREE, similar to lunar monazite in 10047,68 and to terrestrial monazite. Thorium concentration was not measured in monazite, but based on oxide analyses of ~100% (including interpolated values for REE not measured), substantial Th concentration is not indicated, similar to monazite in 10047,68. Measured monazite/whitlockite REE ratios are La: 11, Ce: 8, Sm: 3.6, Y: 0.9, Yb: 0.5. Longhi J. Vander Auwera J. The Monzonorite-Anorthosite Connection: The Petrogenesis of Terrestrial KREEP There is a suite of rocks typically associated with Proterozoic massif anorthosites that bear some interesting similarities to lunar KREEP. In many cases these rocks are plutonic and have traditionally been referred to as the jotunite-mangerite-+-charnockite-+-syenite suite. However, in the Rogaland district of southwestern Norway, where they are referred to as "monzonorites," these rocks are also present as fine-grained dikes and as the chill margin of a layered intrusion, and thus approximate magmatic liquid compositions are readily obtained by chemical analysis. Monzonorites are typically enriched in incompatible lithophile elements such as K (alkali feldspar is present), the rare earths (REE), and P. They have intermediate to low Mg, low-Ca pyroxene, and more evolved types have low Ti/Sm ratios. Much debate has developed over attempts to explain the link between monzonorites and massif anorthosites. One feature seems clear: monzonorites and associated anorthosites have different initial isotopic ratios, so a simple relation is not possible. However, there is apparently a continuum in major elements between the monzonorites and gabbros believed to represent magmas parental to the anorthosites. This continuum suggests a link via high-pressure fractionation coupled with assimilation. Although more complicated, this scenario is similar that evoked for the early Moon: following the formation of ferroan anorthosites, continued fractional crystallization of the residual liquids at the base of the crust led to the formation of KREEP. An attempt is made to establish a link between monzonorites and high-Al gabbros, which are nearly always found as ancillary intrusions associated with anorthosites, and which may record processes in lower crustal magma chambers. Premo W. R. U-Pb Isotopic Ages and Characteristics of Ancient (>4.0 Ga) Lunar Highland Rocks A review of the present (and certainly sparse) U-Pb isotopic database for ancient (>4.0 Ga) lunar crustal rocks indicate that both early-forming anorthosites and high-Mg suite rocks are formed from sources with a wide variety of 238U/204Pb (micrometer) values as early as 4.44 Ga. The U-Pb data from Apollo 16 anorthosites 67075, 60025, and 62337 indicate source-micrometer values between ~35 and ~350; whereas values for high-Mg rocks are tentatively considered to be over 500, similar to KREEP sources. Our present interpretation is that either early cumulates were formed slowly in a magma environment that allowed isotopic reequilibration or mixing over a long period of crystallization, or these cumulates formed from rapidly-evolving (both geochemically and isotopically) residual liquids. It is also possible that an early bombardment period played a role in the mixing process. Sutton S. R. Bajt A. Rivers M. L. Smith J. V. X-ray Microprobe Determination of Chromium Oxidation State in Olivine from Lunar Basalt and Kimberlitic Diamonds The synchrotron X-ray microprobe is being used to obtain oxidation-state information on planetary materials with high spatial resolution. We report initial results on chromium in olivine from various sources including laboratory experiments, lunar basalt and kimberlitic diamonds. The lunar olivine was dominated by Cr2+ whereas the diamond inclusions had Cr2+/Cr3+ ratios up to about 0.3. The simplest interpretation is that the terrestrial olivine crystallized in a more oxidizing environment than the lunar olivine. Thursday, March 18, 1993 Mars: Geologic/Geophysical Processes Poster Session 7:00 - 9:00 p.m. LPI Frey H. Reidy A.-M. Do Large Impact Basins in the Southern Hemisphere of Mars Control the Distribution of Polar Structures and Deposits? Among the outstanding problems in martian geology are the cause of the off- axis and asymmetric distribution of the southern polar layered terrain and residual ice deposits and the cause of the orientation of scarps, valleys and re-entrant canyons that occur there. A perhaps related problem region is the apparently small number of large (D > 500 km) impact basins seen in the relatively well-preserved cratered terrain of the south polar region. Previously only the 850-km-wide South Polar Basin was easily recognized. We have been mapping the south polar region in detail, searching for evidence of ancient, highly degraded impact basins that may have escaped earlier notice, for two reasons: (a) to determine whether the apparent absence of large impact basins is due to incomplete mapping and recognition or a fundamental characteristic of the martian crust related to the origin of the martian crustal dichotomy, and (b) to determine whether ancient impact basins, if they exist, exert some control on the distribution of volcanic and polar deposits in the southern hemisphere and on the topography on which these deposits lie. We previously described several promising candidates, including a large pre- Hellas basin in the Malea Planum region and an older but comparably sized basin overlapping South Polar. We concentrate on the possible influence of the candidate basins in localizing the asymmetric distribution of polar deposits and in controlling the orientation of structures found within these deposits. Kargel J. S. Geomorphic Processes in the Argyre-Dorsa Argentea Region of Mars Among many indications of possible ancient martian glaciation are sinuous eskerlike ridges in southern Argyre Planitia and the Dorsa Argentea region. But, in photogeology, other interpretations are always possible, and what appears eskerlike to one set of eyes may appear quite different to another. Interpretations of these ridges are about as numerous as observers, who collectively have suggested nine distinct hypotheses! Martian sinuous ridges have been interpreted as wrinkle ridges, lava flows, igneous dikes, clastic dikes, linear sand dunes, spits or bars, examples of inverted stream topography, or glacial crevasse fill. With Mars Observer en route to Mars the prospects for a narrowing of the debate are bright. The esker hypothesis will gain support if Mars Observer images show that the ridges contain boulders, that the ridges are layered and contain channel structures, that the ridges are modified by thermokarst, or that the ridges occur in close, logical associations with other glacial landforms such as flutes, oriented grooves, and moraines. In the meantime, the evidence presented below bolsters the esker hypothesis, challenges certain alternative ideas, and draws a tentative geomorphic connection between the sinuous ridges of Argyre and those of Dorsa Argentea. Scott D. H. Mars: New Evidence for Origin of Some Valles Marineris Layered Deposits The discovery of layered deposits in the walls of a deep trough in Lunae Planum has implications for the origin of similar-appearing deposits in some canyons of Valles Marineris. Although layering is visible in the competent, cliff-forming upper walls of the canyons, the dissimilarity in appearance between canyon walls and soft rounded hills of layered deposits on canyon floors, as well as their contrasting patterns of erosion, has been considered strong evidence that their modes of origin were different. Most workers agree that the wall rocks are volcanic flows derived from fissure vents and other volcanic sources in the region. However, several hypotheses have been advanced to account for the softer-appearing stratified floor deposits. Chief among them is the proposal that the floor deposits are waterlaid sediments that accumulated in large lakes within the canyons and include materials eroded from canyon walls, eolian deposits, and subaqueous volcanic eruptives. Dohm J. M. Scott D. H. Relation Between Ages and Elevations of Martian Channels Mapping highland and lowland channels and reassessing their time- stratigraphic positions show that many channels in the cratered highlands are younger than Noachian, that all channels in the lowland plains are Hesperian or younger, and that although martian channels formed throughout the planet's history, the rate of their formation appears to have declined with time. This detailed examination of the age relation between martian terrains and the channels that transect them was made possible by the completion of the global geologic maps of Mars and the availability of other geologic maps at scales of 1:5,000,000, 1:2,000,000, and 1:500,000. The present study focuses on the sources (heads) of the channels of the initial study to see if there is a relation between channel age and elevation. Price K. H. Geologic Mapping of Harmakhis and Reull Valles Region, Mars: Evidence for Multiple Resurfacing and Drainage Events Numerous mesas and irregular, channel-cut escarpments south of Harmakhis and Reull Valles suggesting at least two resurfacing events with subsequent periods of erosion are revealed from detailed geologic mapping of three adjacent 1:500,000-scale quadrangles (MTM -40262, -40267, -40272). Also, two distinct episodes of surface water runoff, each from different styles of water release, are evident. Geologic mapping is based on 1:500,000-scale photomosaics the Harmakhis Vallis region, and is an outgrowth of preliminary regional geologic mapping on a 1:2,000,000-scale on the east rim of Hellas Basin. Chapman M. G. Basal Scarp, Paleoglacier, and Fissure Flows of Elysium Mons, Mars Geological mapping at 1:500,000 scale of the Granicus Valles area west of Elysium Mons (MTM quadrangles 30227, 30222, and 25227) indicates that (1) the oldest deposits in the area are Upper Hesperian lavas of the Elysium shield; (2) a basal scarp formed by Early Amazonian faulting around the northwest flank of Elysium Mons triggered growth of Elysium Fossae; (3) a glacier or an ice sheet west and north of the scarp was of long duration; (4) water and lava flows from Elysium Fossae became dominant during and after glacial recession; and (5) the last volcanic- fissure-related flows resulted in linear chains of small domes on the Elysium shield. Holloway J. R. Domanik K. J. Cocheo P. A. Experimental Constraints on CO2 and H2O in the Martian Mantle and Primary Magmas We present new data on the stability of hornblende in a martian mantle composition, on CO2 solubility in iron-rich basaltic magmas, and on the solubility of H20 in an alkalic basaltic magma. These new data are combined with a summary of data from the literature to present a summary of the current state of our estimates of solubilities of H20 and CO2 in probable martian magmas and the stability of hornblende in a slightly hydrous mantle. The new results suggest that hornblende stability is not sensitive to the Mg/(Mg+Fe) ratio (mg#) of the mantle, that is, the results for terrestrial mantle compositions are similar to the more iron-rich martian composition. Likewise, CO2 solubility in iron-rich tholeiitic basaltic magmas is similar to iron-poor terrestrial compositions. The solubility of H20 has been measured in an alkalic basaltic (basanite) composition for the first time and it is significantly lower than predicted for models of water solubility in magmas. The lack of mg# dependence observed in hornblende stability and on CO2 solubility means that in many cases terrestrial results can be applied to martian compositions. This conclusion does not apply to other phenomena such as primary magma compositions and major mantle mineral mineralogy. Kuramoto K. Matsui T. Was Martian Mantle Wet? A Possible Consequence of Rapid Core Formation Degassing of H20 in the planetary interior possibly plays an important role in the evolution of the surface environment as well as geologic activity on the terrestrial planets. Mars may be such a planet that well preserves the materials and the geologic features directly related to the early evolution of H20. In this study, we investigate H20 content in the interior of proto-Mars during accretion and also during core formation. Parfitt E. A. Wilson L. Pinkerton H. Thermal and Rheological Controls on Magma Migration in Dikes: Examples from the East Rift Zone of Kilaeau Volcano, Hawaii Long-lived eruptions from basaltic volcanoes involving episodic or steady activity indicate that a delicate balance has been struck between the rate of magma cooling in the dike system feeding the vent and the rate of magma supply to the dike system from a reservoir. We describe some key factors, involving the relationships between magma temperature, magma rheology, and dike geometry, that control the nature of such eruptions. Kauhanen K. Fractal Geometry of Some Martian Lava Flow Margins: Alba Patera Fractal dimension for a few lava flow margins on the gently sloping flanks of Alba Patera were measured using the structured walk method. Fractal behaviour was observed at scales ranging from 20 to 100 pixels. The upper limit of the linear part of log(margin length) vs. log(scale) profile correlated well to the margin length. The lower limit depended on resolution and flow properties. Zimbelman J. R. Comparison of Flank Modification on Ascraeus and Arsia Montes Volcanoes, Mars Geologic mapping of the Tharsis Montes on Mars is in progress as part of the Mars Geologic Mapping Program of NASA. Mapping of the southern flanks of Ascraeus Mons at 1:500,000 scale was undertaken first, followed by detailed mapping of Arsia Mons, and mapping of Pavonis Mons will begin later this year. Results indicate that each of the Tharsis volcanos displays unique variations on the general "theme" of a martian shield volcano. Here we concentrate on the flank characteristics on Ascraeus Mons and Arsia Mons, the northernmost and southernmost of the Tharsis Montes, as illustrative of the most prominent trends. McGovern P. J. Solomon S. C. Aspects of Modelling the Tectonics of Large Volcanoes on the Terrestrial Planets Analytic solutions for the response of planetary lithospheres to volcanic loads have been used to model faulting and infer elastic plate thicknesses. Predictions of the distribution of faulting around volcanic loads, based on the application of Anderson's criteria for faulting to the results of the models, do not agree well with observations. Such models do not give the stress state in the load itself, but only suggest a state of horizontal compressive stress there. Further, these models have considered only the effect of an instantaneously emplaced load. They do not address the time evolution of stresses, nor do they consider the effect of a load that grows. A finite element approach allows us to assign elements to the load itself, and thus permits calculation of the stress state and stress history within the edifice.The effects of episodic load growth can also be treated.When these effects are included, models give much better agreement with observations. Tanaka K. L. Dohm J. M. Complex Structure of the Thaumasia Region of Mars The Thaumasia region was the first center of Tharsis tectonism, and it is the most complex and poorly understood. We are therefore compiling a geologic map of the entire Thaumasia region (lat. 15 degrees to 50 degrees to 115 degrees) at 1:5,000,000 scale. This region is mostly made up of the Thaumasia plateau, which includes the high plains of Syria, Sinai, and Solis Plana and surrounding highlands; the highlands are fractured by Thaumasia, southern Claritas, Coracis, Melas, and Nectaris Fossae. Our preliminary structural analysis of the most complexly faulted area in the region (the central part, at lat. 30 degrees to 45 degrees S., long. 80 degrees to 100 degrees) indicates that, unlike other regions of Mars, Thaumasia has undergone extensive deformation by both small- and large-scale extensional and compressional structures. These results indicate that the early (Noachian) style of tectonism commonly involved lithospheric-scale deformation, in contrast to most younger tectonism (which mainly affected the upper parts of the crust above mechanical discontinuities); this difference may be due to a weaker (and thus more readily deformable) early lithosphere in this region. Anderson R. C. Lineament Analysis and Tectonic Interpretation for the Tharsis Region, Mars The Tharsis region of Mars is critical to any study of martian tectonics. This region (65 degrees N and 65 degrees S latitude; 45 degrees W to 157.5 degrees W longitude) is characterized by an asymmetrical dome-shaped topographic high approximately 800 km across. Affecting over 25% of the surface area, this region has been the center of most of the major tectonic and volcanic activity that has taken place on the martian surface. Lineament studies are the primary tool available for studying tectonic processes on terrestrial planets such as Mars. At least three major lineament systems can be delineated in the Tharsis region; north-south- and east-west-trending lineament systems are superimposed on an older northwest-trending lineament system. Four centers of uplift have been identified based on the occurrence of radial fracture patterns. Thus, preliminary results indicate that the formation of the Tharsis Dome may not have resulted from a single uplift event, but may instead have resulted from as many as four uplift events. The northwest trending fracture may represent a pre-existing zone of weakness that contributed to the early formation of the Tharsis Dome. Mellon M. T. Jakosky B. M. Postawko S. E. Equatorial Ground Ice on Mars: Steady-State Stability Current martian equatorial surface temperatures are too warm for water ice to exist at the surface for any appreciable length of time before subliming into the atmosphere. Subsurface temperatures are generally warmer still and, despite the presence of a diffusive barrier of porous regolith material, it has been shown by Smoluchowski, Clifford and Hillel, and Fanale et al. that buried ground ice will also sublime and be lost to the atmosphere in a relatively short time. We investigate the behavior of this subliming subsurface ice and show that it is possible for ice to maintain at a steady-state depth, where sublimation and diffusive loss to the atmosphere is balanced by resupply from beneath by diffusion and recondensation of either a deeper buried ice deposits or ground water. Aguirre-Puente J. Costard F. M. Posado-Cano R. Rates of Fluvio-Thermal Erosion on Mars On Mars, most of the outflow channels start from chaotic terrains and exhibit a rather straight valley of 1500 km long and 25 km wide with few tributaries. Different interpretations of these outflow channels have been proposed. Lucchitta considers them as possible glacial valleys, and Komar proposes some comparisons with submarine rivers. Carr suggests that these valleys were produced by catastrophic release of water from confined aquifers and Baker considers them as highly turbulent catastrophic floods. In order to take into account the cold climate conditions of Mars, the presence of ground-ice and the large scale of outflows, thermal erosion was first proposed by Costard. From a quantitative point of view, three thermal mathematical models were analyzed and discussed by Aguirre-Puente et al. Cabrol N. A. Grin E. A. Dollfus A. Dawidowicz G. An Ancient Inner Lake in Ma'Adim Vallis The survey of Ma'adim Vallis and crater Gusev area by the new topographic Aeolis southeast map allows to document the hydrodynamics processes that occurred in the valley. The inner Ma'adim Vallis hypsometric and volume deposits analyzes point out that Gusev impact was subsequent to the first incision of Ma'adim Vallis in the craterized upland. The study of the deposit distributions as revealed by the topography leads to reconstruct the chronology of three major events in the Aeolis Region: Ma'adim incision, Gusev impact, and Apollinaris Patera/flow interactions. Thornhill G. D. Rothery D. A. Murray J. B. Day T. Cook A. C. Muller J.-P. Iliffe J. C. Discharge Rates in Ma'Adim Vallis, Mars A digital elevation model (DEM) of a small part of the martian channel Ma'adim Vallis has been produced using the Frankot and Chellappa shape- from-shading algorithm. Software developed by the Dept. of Photogrammetry and Surveying at University College London uses this technique to extract slope information from the grey levels of image pixels. This technique has been applied to a Viking Orbiter image of part of Ma'adim Vallis, and measurements of the channel depth and bed- slope of a channel incised into the floor of Ma'adim Vallis have been made. These results have been used to calculate order-of-magnitude estimates for discharge rates through the channel. The maximum values calculated are three orders of magnitude less than those for N. Kasei Vallis, and are similar to values cited for the Missoula floods. However, when more realistic values of the water depth are used, discharge rates comparable with those for the Mississippi River result. Erard S. Cerroni P. Coradini A. Composition of the Martian Aerosols Through Near-IR Spectroscopy Near-infrared spectroscopy is a powerful technique for studying the composition of planetary surfaces, as the main minerals exhibit absorption bands in this spectral range. It has given important information on the mineralogy and petrology of Mars in the past twenty years, although it is well known that a large fraction of light is scattered by the airborne particles before reaching the surface. The measured signal is thus the sum of two different contributions that should be studied separately: one from the surface and one from the aerosols that depends on their density, size distribution, and composition. Data from the ISM imaging spectrometer are used here to derive the aerosol spectrum. They consist in sets of spectra (from 0.76 to 3.16 micrometers) of ~3000 pixels ~25 x 25 km2 in size. The resulting spectrum exhibits both water-ice and clay mineral features superimposed on a scattering continuum. Calvin W. M. Martin T. Z. Hansen G. B. Spatial Variation in the Seasonal South Polar Cap of Mars as Observed by Mariner 7 Spatial variations in infrared spectra of the seasonal south polar cap of Mars were noted by the original Mariner 7 IRS instrument team as well as in subsequent examinations of the Mariner 7 data set. Up to now, however, there has been little effort to synthesize this information to understand variations in the south seasonal cap as a function of position or latitude. We attempt to quantify the spectra by providing estimates of CO2 grain sizes as well as upper limits on the amount of water frost contained in the polar collar. Bridges N. T. Martian Particle Size Based on Thermal Inertia Corrected for Elevation-Dependent Atmospheric Properties Thermal inertia is commonly used to derive physical properties of the martian surface. If the surface is composed of loosely consolidated grains, then the thermal conductivity derived from the inertia can theoretically be used to compute the particle size. However, one persistent difficulty associated with the interpretation of thermal inertia and the derivation of particle size from it has been the degree to which atmospheric properties affect both the radiation balance at the surface and the gas conductivity. These factors vary with atmospheric pressure so that derived thermal inertias and particle sizes are a function of elevation. Here, by utilizing currently available thermal models and laboratory information, a fine component thermal inertia map has been convolved with digital topography to produce particle size maps of the martian surface corrected for these elevation-dependent effects. Such an approach is especially applicable for the highest elevations on Mars, where atmospheric back radiation and gas conductivity are low. Nemchinov I. V. Perelomova A. A. Shuvalov V. V. Determination of Cosmic Bodies Size-Velocity Distribution by Observation of Current Impacts on Mars Collisions of cosmic bodies with terrestrial planets involve many physical processes such as deceleration and ablation during their flight through an atmosphere, the impact at a surface accompanied by cratering, melting and evaporation of surface material, generation of shock waves, etc. If body velocity is high enough, then thermal radiation is important. All these processes on Mars proceed differently from on the other planets because of the low density of its atmosphere. In particular, this leads to the fact that smaller bodies of sizes of the order of 0.1-10 m strike the planet surface without being decelerated and produce some effects that may be detected by equipment placed on board artificial satellites, by a network of stations at the surface of Mars, and even from the Earth. These observations can be used to determine size-velocity distribution of such bodies in the solar system. Flynn G. J. Organic Matter on the Early Surface of Mars: An Assessment of the Contribution by Interplanetary Dust Calculations by Anders and Chyba et al. have recently revived interest in the suggestion that organic compounds important to the development of life were delivered to the primitive surface of the Earth by comets, asteroids, or the interplanetary dust derived from these two sources. Anders has shown that the major postaccretion contribution of extraterrestrial organic matter to the surface of the Earth is from interplanetary dust. Since Mars is a much more favorable site for the gentle deceleration of interplanetary dust particles than is Earth, model calculations show that biologically important organic compounds are likely to have been delivered to the early surface of Mars by the interplanetary dust in an order-of-magnitude higher surface density than onto the early Earth. Thursday, March 18, 1993 Mars Surface Mineralogy and Spectroscopy Poster Session 7:00 - 9:00 p.m. LPI Blaney D. L. Crisp D. Using High Spectral Resolution Spectrophotometry to Study Broad Mineral Absorption Features on Mars Traditionally telescopic measurements of mineralogic absorption features have been made using relatively low to moderate (R=30-300) spectral resolution. Mineralogic absorption features tend to be broad so high resolution spectroscopy (R>10,000) does not provide significant additional compositional information. Low to moderate resolution spectroscopy allows an observer to obtain data over a wide wavelength range (hundreds to thousands of wavenumbers) compared to the several wavenumber intervals that are collected using high resolution spectrometers. However, spectrophotometry at high resolution has major advantages over lower resolution spectroscopy in situations that are applicable to studies of the Martian surface, i.e. at wavelengths where relatively weak surface absorption features and atmospheric gas absorption features both occur. Roush T. L. Orenberg J. B. Pollack J. B. Derivation of the Midinfrared (5.0-25.0 micrometers) Optical Constants of Hydrous Carbonate and Sulfate There is ample theoretical and observational evidence suggesting liquid water was once stable at the surface of Mars. Because water is essential to the evolution of life, it is important to understand the types of environments in which the liquid water was present. For example, if water were present early in Mars' history, then this raises the possibility that biological activity may have evolved, only to eventually become extinct as liquid water became scarce. Alternatively, if liquid water were stable only later in Mars' history, then it becomes problematic to envision mechanisms by which biological activity evolved and remained viable without water until more favorable conditions existed. Even without biological activity, atmospheric carbon dioxide dissolved in water can assist the chemical weathering of primary igneous minerals producing common secondary phases such as hydrates, carbonates, and sulfates. While the identification of hydrates, carbonates, and sulfates on Mars cannot provide direct evidence of biological activity, it can provide significant information regarding the presence and duration of an environment that would support the presence of liquid water at the surface. The specific mineralogy of these secondary phases can provide insight into the environments of their formation. For example, slow precipitation that occurs in large standing bodies of water, e.g., oceans or lakes, commonly results in the formation of calcite, magnesite, dolomite, siderite, and rhodochrosite. Rapid precipitation that occurs in ephemeral bodies of water, e.g., hypersaline lakes or playas, can result in the formation of all of the above phases as well as aragonite, vaterite, hydrated carbonates, alkali carbonates, bicarbonates, and other poorly ordered phases. Bishop J. L. Pieters C. M. Pratt S. F. Patterson W. The Effects of Atmospheric Pressure on Infrared Reflectance Spectra of Martian Analogs The use of terrestrial samples as analogues of Mars soils are complicated by the martian atmosphere. Spectral features due to the martian atmosphere can be removed from telescopic spectra of Mars and ISM spectra of Mars, but this does not account for any spectral differences resulting from atmospheric pressure or any interactions between the atmosphere and the surface. We are examining the effects of atmospheric pressure on reflectance spectra of powdered samples in the laboratory. Contrary to a previous experiment with granite, no significant changes in albedo or the Christiansen feature were observed from 1 bar pressure down to a pressure of 8 micrometers Hg. However, reducing the atmospheric pressure does have a pronounced affect on the hydration features, even for samples retained in a dry environment for years. Geissler P. E. Singer R. B. Komatsu G. A Mineralized Zone in Western Candor Chasma, Mars Spectral evidence from Viking and Phobos orbiting spacecraft suggests the local development of crystalline ferric oxides in a small region within Mars' equatorial Valles Marineris canyon system. This is the same area noted for its anomalous coloration in Viking Orbiter image 583A by McEwen. The unique hue of the region in Viking color data is due to a reduced green filter reflectance, relative to violet and red, in comparison to surrounding materials of similar albedo or average reflectance. For this reason the region does not appear spectrally distinctive in later Survey Mission images that were acquired without the green filter. Erard S. Cerroni P. Coradini A. Automatic Definition of Spectral Units in the Equatorial Regions of Mars ISM was the first instrument that acquired spectra of small areas (25 x 25 km2) on a planetary surface. The dataset consists mainly of images made up of about 3000 such pixels, each one corresponding to a near-infrared spectrum (0.76 to 3.15 micrometers in 128 spectral channels) with a high signal-to- noise ratio. These data were used to define spectral units and to constrain the mineralogy of surface materials in the equatorial regions of the planet. Future spaceborne imaging spectrometers (e.g., OMEGA onboard the Mars '94 Russian spacecraft) are expected to provide several hundred times as much data as ISM, so there is a strong need for fast and reliable processing methods. The present work is an attempt to define spectral units in the region of Syrtis Major-Isidis Planitia by means of G-mode analysis. The method allows the spectra to cluster according to their similarities; different levels of classification can be achieved by tuning a threshold of confidence. Pierazzo E. Singer R. B. Wavelength Dependence of Limb-Darkening of Mars from Visible and Near-IR Telescopic Spectral Imaging We are investigating the photometric properties of Mars using Earthbased telescopic spectral imaging obtained during 1988. Each spatial pixel consists of 300 spectral channels from 0.44 to 1.02 micrometers, calibrated to radiance factor (rF) through a careful procedure involving standard star observations in 1988 and 1990. Calibrated data include 3 spectral images with solar phase angle (g) of 12 degrees and 4 images with g of 4 degrees. Pixels near the sub- Earth point have a spatial footprint of about 280 x 150 km. Preliminary work using Hapke photometric function reveals a dependence of both single- scattering albedo and roughness parameter on wavelength. The former is in agreement with observed spectra of the martian surface, but the latter is unexpected. Zolotov M. Yu. Krot T. V. Moroz L. V. K, U, and Th Behavior in Martian Environmental Conditions The speculations on K, U, and Th behavior in the martian environments show that aeolian and aqueous processes leads to the preferential accumulation of K, U, and Th in fine dust material. The separation of K, U, and Th on Mars is smaller in scale to that on Earth. McEwen A. S. Soderblom L. A. Global and Regional/Seasonal Color Mosaics of Mars Four regional mosaics of Mars acquired during different seasons, along with their composite as a single global mosaic, have been completed in two colors (red and violet) at scales of 1/16 and 1/64 degrees/pixel. These mosaics were put together from a set of 51 separate mosaics, each acquired from a single Viking Orbiter spacecraft orbital revolution. Special techniques were developed and applied to suppress large variations between mosaics introduced by highly variable, optically thin, condensate hazes. The techniques utilize a combination of the spatial characteristics of the hazes (generally broad, low- frequency) along with their modulation of the regional color ratios (strongly enhancing the violet/red ratios). Photometric-function normalization was applied following the haze removal. Most of the single-orbit mosaics consist of red and violet or red, green, and violet filters, but a few mosaics with only red-filter data were included to fill gaps in global coverage at high northern latitudes. Global coverage is ~99% complete in red-filter mosaics and ~95% and ~60% complete in corresponding violet-and green-filter mosaics, respectively. All of the mosaics are geometrically tied to the 1/256 degrees per pixel Mars Digital Image Map (MDIM), which is available on Compact Disk (CD), and which will be used as the base map for Mars Observer datasets. Early in 1993, the single-orbit color mosaics will be distributed to the science community in a six-volume set of CDs. Xu P. Greeley R. Convex Set and Linear Mixing Model A major goal of optical remote sensing is to determine surface compositions of the Earth and other planetary objects. For assessment of composition, single pixels in multispectral images usually record a mixture of the signals from various materials within the corresponding surface area. In this report, we introduce a closed and bounded convex set as a mathematical model for linear mixing. This model has a clear geometric implication because the closed and bounded convex set is a natural generalization of a triangle in n-space. The end members are extreme points of the convex set. Every point in the convex closure of the end members is a linear mixture of those end members, which is exactly how linear mixing is defined. With this model, some general criteria for selecting end members could be described. This model can lead to a better understanding of linear mixing models. Landry J. C. England A. W. Far-Infrared Spectra of CO2 Clathrate Hydrate Frosts As a product of our interest in remote sensing of planetary ices, we have grown frost samples of CO2 clathrate hydrate by depositing water vapor on a cooled surface and pressurizing the resulting water frost with CO2 gas. At pressures above the dissociation pressure of the clathrate, the samples exhibit an absorption peak at 75 cm^-1. At pressures below the dissociation pressure, the peak disappears. Since the free CO2 molecule does not have rotational or vibrational absorption in this region, the absorption is attributed to a CO2 rattling mode within a clathrate cage. Hansen G. B. Martin T. Z. Modeling the Reflectance of CO2 Frost with New Optical Constants: Application to Martian South Polar Cap Spectra New measurements of the absorption coefficients of CO2 ice, in most of the spectral range 0.2 to 3.9 micrometers where absorption coefficients are below 1.5 per cm, have recently been made. Although these measurements are preliminary, they contain spectral detail not seen previously in the literature. Therefore, it is useful to combine these new data with older data from spectral regions of stronger absorption and reformulate models of the albedo or reflectance of CO2 frost. These models can then be adjusted in an attempt to match measurements of martian polar deposits, such as the set of spectra returned by the IRS instrument on Mariner 7 (1969). Schaefer M. W. Local Topography of Mars and its Relationship to Surface Weathering Processes There is a growing body of evidence in favor of the importance of aqueous sedimentary processes on Mars. It is important to understand the role that surface weathering processes have played in the development of the present morphology of the martian surface. Such an understanding is important not only for its relevance to the study of volatile sources and sinks on Mars through time, but also for its relevance to martian geologic and tectonic history. Digital topography is an important part of geologic and geomorphic studies, useful in distinguishing between different lithologies and between different types of weathering. Preliminary work is underway to develop tools for the analysis of the Mars Observer topography dataset. Held P. Teucher R. Klingelhofer G. Foh J. Jager H. Kankeleit E. A Mossbauer Spectrometer for the Mineralogical Analysis of the Mars Surface: First Temperature Dependent Tests of the Detector and Drive System Part of the scientific payload of the Mars-96 mission is a 57Fe- Mossbauer (MB) spectrometer installed on a small rover to be placed on the surface of Mars. The instrument is under development at the University of Darmstadt. This instrument, with some modifications, is also included in the scientific payload of the proposed MARSNET mission of the European Space Agency (ESA). A similar instrument is currently under development in the U.S. The reason for developing a Mossbauer spectrometer for space applications is the high abundance of the element iron, especially on the surface of Mars. The elemental composition of martian soil was determined during the Viking mission in 1976 but not its mineralogical composition. It is believed that the martian soil is composed mainly of iron-rich clay minerals, with an iron content of about 14(+-2) wt%, partly magnetic. Of extremely great interest are the oxidation state of the iron, the magnetic phases, and the mineral composition of the Mars surface. To these questions MB spectroscopy can provide important information that is not available by other methods. We report on first tests of parts of the experimental setup in the temperature range +20 degrees C to -70 degrees C, roughly corresponding to the temperature range on the surface of Mars. Questions concerning the signal/noise ratio (s/n) are also discussed. Evlanov E. N. Frolov V. A. Prilutskii O. F. Veselova G. V. Rodin A. M. Klingelhofer G. Mossbauer Spectrometer for Mineralogical Analysis of the Mars Surface: Mossbauer Source Considerations Development of space rocketry and cosmic instruments has made it possible to create interplanetary stations to be sent to solar system bodies. In the last decade of the century the planet Mars will be in the focus of planetary science problems. Russia, USA (NASA) and Europe (ESA) plan to do a new step in the study of the planetary system by spacecraft missions to Mars. One part of the programs of these missions is martian surface measurements of iron, which is a dominant element in both the martian soil and rocks (about 13% by weight). The chemistry of iron in space is strongly coupled to the chemistry of abundant elements (for example, hydrogen, carbon, oxygen) and it is this coupling of chemical cycles of abundant elements that gives us the possibility of understanding some features of the chemical evolution of matter. In this connection of extremely great importance for the understanding of the evolution of the solar system are the oxidation state of the iron and its mineral composition of the martian surface. While highly successful, the Viking landers had no instrumentation to answer these questions. Such instrumentation has to be specifically sensitive to mineralogy. For this purpose the backscattering Mossbauer spectrometer (MS-96) was proposed to be installed on a rover to be launched on board the Russian spacecraft Mars-96 mission to Mars. Due to power and mass restrictions three systems of the device MS-96 (velocity transducer, detector, and electronic components) have been extremely miniaturized in comparison to a standard system. We offer for consideration a radioactive source to find out what characteristics it should have to be suitable for purposes of the experiment. Madsen M. B. Knudsen J. M. Vistisen L. Hargraves R. B. Suggestion for Extended Viking Magnetic Properties Experiment on Future Mars Missions An array of permanent magnets, with the purpose of establishing if the magnetic particles on Mars are present as discrete or as composite particles, has been constructed. Kemurjian A. Linkin V. Friedman L. International Testing of a Mars Rover Prototype Tests on a prototype engineering model of the Russian Mars 96 Rover were conducted by an international team in and near Death Valley in the United States in late May, 1992. These tests were part of a comprehensive design and testing program initiated by the three Russian groups responsible for the rover development. The specific objectives of the May tests were (1) Evaluate rover performance over different Mars- like terrains, specifically a boulder field like that at the Viking 1 site, and sand dunes; (2) Evaluate state-of-the-art teleoperation and autonomy development for Mars rover command, control, and navigation; (3) Organize an international team to contribute expertise and capability on the rover development for the flight project. Grant J. A. Schultz P. H. Rover Mounted Ground Penetrating Radar as a Tool for Investigating the Near-Surface of Mars and Beyond In spite of the highly successful nature of recent planetary missions to the terrestrial planets and outer satellites a number of questions concerning the evolution of their surfaces remain unresolved. For example, knowledge of many characteristics of the stratigraphy and soils comprising the near-surface on Mars remains largely unknown, but is crucial in order to accurately define the history of surface processes and near-surface sedimentary record. Similar statements can be made regarding our understanding of near-surface stratigraphy and processes on other extraterrestrial planetary bodies. Ground penetrating radar (GPR) is a proven and standard instrument capable of imaging the subsurface at high resolution to tens of meters depth in a variety of terrestrial environments. Moreover, GPR is portable and easily modified for rover deployment. Data collected with a rover-mounted GPR could resolve a number of issues related to planetary surface evolution by defining shallow stratigraphic records and would provide context for interpreting results of other surface analyses (e.g., elemental or mineralogical). A discussion of existing GPR capabilities is followed first by examples of how GPR might be used to better define surface evolution on Mars and then by a brief description of possible GPR applications to the Moon and other planetary surfaces. Anderson D. L. Fourier Domain Target Transformation Analysis in the Thermal Infrared Remote sensing uses of principal component analysis (PCA) of multispectral images include band selection and optimal color selection for display of information content. PCA has also been used for quantitative determination of mineral types and abundances given endmember spectra. I report the preliminary results of the investigation of target transformation PCA (TTPCA) in the fourier domain to both identify endmember spectra in an unknown spectrum, and to then calculate the relative concentrations of these selected end members. Thursday, March 18, 1993 Comets and Asteroids Poster Session 7:00 - 9:00 p.m. LPI Thiel K. Kolzer G. Lorenz E. Kochan H. Gebhard J. Grun E. Synoptic Observations of Near Surface Processes of an Insolated Ice-Dust Body Under Space Conditions: The Case of KOSI 9 and 10 The recent experiments of the Comet Simulation Project, KOSI 9, and KOSI 10 for the first time yielded synoptic results of physically correlated phenomena near the surface of insolated porous ice dust mixtures (90% H2O, 10% olivine, 0.1% fine dispersed carbon, sample mean density 0.40 +- 0.01 g/cm^3) under space conditions. Correlations were observed, e.g., between (1) grain size of emitted dust and dust mantle formation, (2) surface avalanches and local surface temperature, and (3) lateral variations of the surface temperature and the thickness of the dust mantle at the end of the experiment. The dynamic surface processes and the observed structural parameters of the dust mantle and the emitted mineral residues are discussed in view of real comets. Ulamec S. Svedhem H. Kochan H. Measurements of the Dielectric Properties of Simulated Comet Materials as Part of the KOSI-10 Experiment The dielectric constant of the snow-mineral-mixture used for the comet simulation in the German KOSI 10 experiment was measured in the radio frequency range from 2 to 4 GHz. The traditional microwave bridge method was used, but instead of using a waveguide, which contains the sample material, small lambda/4 antennas were used as sensors. A change in the dielectric properties indicates a change in density and/or composition, respectively. The method is presented as an analytical tool for measuring such density or composition changes during alternation snow- dust materials. Clark B. E. Bell J. F. Fanale F. P. Lucey P. G. First Results of the Seven-Color Asteroid Survey The new Seven-Color infrared filter system (SCAS), designed specifically to capt mineralogical information present in asteroid spectra, is composed of seven broa allow for IR observations of objects as faint as 17th magnitude. The first test occurred in July 1992. In four nights at the IRTF on Mauna Kea, Hawaii, over 67 observed. Five of the observations were to test the new system for accuracy rela observations with the high-resolution 52-Color Infrared Survey, and with the Eig Asteroid Survey (ECAS). In three cases, the match to previous data is good. In t match to previous observations is not so good. In addition, 60 S type asteroids measured with the SCAS system. Forty of those asteroids have also been observed ECAS system. Among the new observations is infrared data of 371 Bohemia, a main that was classified "QSV" according to its UBV colors in the taxonomic system of There are no corresponding ECAS data for 371. Q-type asteroids are of special in are proposed to be the elusive parent bodies of the ordinary chondrite meteorite are Earth-crossing asteroids and have not yet been observed in the infrared (exc 371). Positive identification of a large main belt Q type would be of major impo scheme of the geological structure of the asteroid belt. Without visible wavelen the classification of 371 Bohemia remains ambiguous. We show an attempt to conjo SCAS data with ECAS data of both a typical Q-type asteroid and an average S type thus illustrate the importance of visible wavelength data to the SCAS system. In without ECAS data of 371 Bohemia we cannot use its spectral characteristics to i possible parent body of ordinary chondrite meteorites. Granahan J. C. Smith G. Bell J. F. New K Type Asteroids Several new K-type asteroids were identified during near-infrared spectral observations on July 30, 1992, at NASA's infrared telescope facility (IRTF) at Mauna Kea, Hawaii. These K asteroids are 513 Centesima, 633 Zelima, 1129 Neujmina, 1416 Renauxa, 1799 Koussevitzky, and 1883 Rauma. A K asteroid is an asteroid that possesses an S-type spectra in visible wavelengths and a C-type spectra visible in near- infrared wavelengths. These objects are usually misclassified as S asteroids on the basis of visible spectra alone. This type was first detected by the 52 infrared color asteroid survey also conducted at the IRTF. Our observations utilized a new seven color infrared asteroid filter system that allows near-infrared data to be collected from asteroids as faint as 16th V magnitude. Jarvis K. S. Vilas F. Gaffey M. J. Iron Oxide Bands in the Visible and Near-Infrared Reflectance Spectra of Primitive Asteroids High-resolution reflectance spectra of primitive asteroids (C, P, and D class and associated subclasses) have commonly revealed an absorption feature centered at 0.7 micrometers attributed to an Fe2+ - Fe3+ charge transfer transition in iron oxides and/or oxidized iron in phyllosilicates. A smaller feature identified at 0.43 micrometers has been attributed to an Fe3+ spin- forbidden transition in iron oxides. In the spectra of the two main-belt primitive asteroids 368 Haidea (D) and 877 Walkure (F), weak absorption features that were centered near the location of 0.60-0.65 micromters and 0.80-0.90 micrometers prompted a search for features at these wavelengths and an attempt to identify their origin(s). Zolensky M. E. Le L. Galindo C. Morris R. V. Lauer H. V. Jr. Vilas F. Diffuse Reflectance Spectra of Orthopyroxene, Olivine, and Plagioclase as a Function of Composition and Structure Although many similarities exist between meteorite spectra and "primitive" asteroids, there are unexplained discrepancies. These discrepancies do not appear to arise from grain size effects. Assuming that primitive meteorites did in fact originate from the "primitive" asteroids, we believe that there are two testable explanations for the observed spectral discrepancies: compositional or structural differences. We have begun to synthesize and collect reflectance and Mossbauer spectra of pertinent materials, beginning with olivine, pyroxene, and plagioclase (all found in primitive meteorites), and to assess the possible effects composition may have on spectral features. Our study focuses on the combination of composition and structural effects. Burbine T. H. Bell J. F. How Diverse is the Asteroid Belt? For approximately twenty years, many different asteroid taxonomies, which used many different observational datasets, have been developed to try to group asteroids into classes that contain members with similar spectral characteristics. However to understand the structure of the asteroid belt, the resulting classes are only useful if they are grouping together asteroids with somewhat similar mineralogies and thermal histories. Until recently, these taxonomies have focused on spectral reflectance data from 0.3 to 1.1 micrometers and visual albedo. But in the last five years, observational data sets (e.g. 0.8 to 2.5 micrometer spectra, CCD spectra, 3 micrometer spectra, radar albedos) for a small number of asteroids have been compiled that can give a better mineralogical interpretation, but whose use in asteroid taxonomy has been relatively limited. Analyses of these "supplementary" datasets show that most asteroid classes contain members with different compositions and/or thermal histories. To understand the diversity of the asteroid belt, the number of objects with these observations must be expanded and used in the next generation of taxonomies. Kring D. A. Cat Mountain: A Meteoritic Sample of an Impact-melted Chondritic Asteroid Although impact cratering and collisional disruption are the dominant geologic processes affecting asteroids, samples of impact melt breccias comprise <1% of ordinary chondritic material and none exist among enstatite and carbonaceous chondrite groups. Because the average collisional velocity among asteroids is sufficiently large to produce impact melts, this paucity of impact-melted material is generally believed to be a sampling bias, making it difficult to determine the evolutionary history of chondritic bodies and how impact processes may have affected the physical properties of asteroids (e.g., their structural integrity and reflectance spectra). To help address these and related issues, the first petrographic description of a new chondritic impact melt breccia sample, tentatively named Cat Mountain, is presented. Ryan E. V. Asteroid Collisions: Target Size Effects and Resultant Velocity Distributions To study the dynamic fragmentation of rock and to simulate asteroid collisions, we use a two-dimensional, continuum damage numerical hydrocode that models two-body impacts. This hydrocode monitors stress wave propagation and interaction within the target body, and includes a physical model (Grady and Kipp fragmentation theory) for the formation and growth of cracks in rock. With this algorithm we have successfully reproduced fragment size distributions and mean ejecta speeds from laboratory impact experiments using basalt, and weak and strong mortar as target materials. Using the hydrocode we have determined that the energy needed to fracture a body has a much stronger dependence on target size than predicted from most scaling theories. In addition, velocity distributions obtained indicate that mean ejecta speeds resulting from large-body collisions do not exceed escape velocities. Davis D. R. Farinella P. Paolicchi P. Bagatin A. C. Cellino A. Zappala E. Deviations from the Straight Line: Bumps (and Grinds) in the Collisionally Evolved Size Distribution of Asteroids Dohnanyi (1969, 1971) established that the size distribution of a collisionally relaxed population is a power law with a -3.5 slope (incremental diameter), provided that: (1) collisional parameters are independent of size and (2) there is no lower limit on the population size for shattering impacts. In the course of studying collisional effects over a wide range of asteroid sizes (centimeters up to hundreds of kilometers range), we have investigated the consequences of relaxing the second condition, while maintaining the first one. De Angelis G. A Method to Determine Asteroid Poles The determination of spin axis and shape is well known to be of fundamental importance for studies about the rotational and physical properties of asteroids. In particular, knowledge that the pole coordinate distribution is random or not could indicate the probable non-Maxwellian distribution of asteroid spin axes, while the distribution in terms of size and shape could place important constraints on the theories about the collisional history of some individual asteroids, of asteroid families, and of the asteroid population as a whole. Many kinds of methods have been developed to determine pole coordinates. One way is to use the epochs of maximum light in the observed photometric lightcurves and to calculate the number of synodic cycles between them. The synodic period changes with time in a way depending on the coordinates of the north pole, which can be calculated. This method has been expressed in many ways, but the collective term "Epoch method" (E method) is used for all the pole determination methods that, like photometric astrometry, are based on observed epochs in the lightcurves. These methods are not based on any specified model of the asteroid, so from them you can derive no shape information. On the contrary, sidereal period and sense of rotation can be derived from these methods. Thursday, March 18, 1993 Outer Solar System Poster Session 7:00 - 9:00 p.m. LPI Kuehn D. M. Barnet C. D. Beebe R. F. Vertical Structure Models of the 1990 Equatorial Disturbance of Saturn In September 1990, an atmospheric disturbance in the form of an abnormally high albedo area developed in the equatorial region of Saturn. Events of this nature are exceedingly rare for this planet as they have been detected in the equatorial region on only two other occasions in over a century. In ongoing monitoring of the atmospheres of the outer planets, CCD imaging observations of Saturn by New Mexico State University's Tortugas Mountain Station were made before, during, and after the disturbance's formation through both broad-band filters and narrow-band visible/near-IR filters centered in methane absorption bands. Also, multispectral Hubble Space Telescope observations were made within weeks of the event and later in 1991. These observations have been calibrated and scans of reflectivity at constant latitude are being modeled with a vertically inhomogeneous, multiple scattering model previously used to model Jupiter's South Equatorial Belt brightening event in 1989. In addition, the reflectivity of the disturbance as a function of the scattering angles is being obtained so as to model this feature's vertical structure in particular. Burnett D. S. Ellis S. B. Rice A. Epstein S. Source of Na for the Io Atmosphere The physics and geology of Io have been extensively studied, but there has been little discussion of the chemistry. Relatively little is known about Io chemistry, but there are constraints. Further, it will be a long time before improvements will result from direct observation, given the severe difficulties with the Galileo mission. We are interested in exploring, via laboratory simulation experiments, plausible thermochemical and photochemical processes which determine the nature and amounts of surface constituents of Io. Tufts B. R. Photogeological Analysis of Europan Tectonic Features Preliminary photogeological analyses of the Pelorus Linea and Sidon Flexus regions of Europa were conducted to explore the proposal by Schenk et al. that lateral motion of crustal blocks has occurred in a "rift zone" including possible strike-slip, tension fracturing, and geometric plate rotation about an Euler pole. These analyses revealed features interpreted as tensional structures and block rotation in a strike-slip regime consistent with the Schenk et al. hypotheses and implied the presence of at least two stages of crustal deformation consistent with a chronology developed by Lucchitta et al. Confirmation of regional scale Euler pole rotation was ambiguous, however. Up to 80 km of possible extension was identified in the rift zone; to accommodate this, "cryosubduction" is speculatively proposed here as a mechanism for recycling Europan "ice lithosphere." Croft S. K. Geomorphology of Triton's Polar Materials One of the Triton's most debated puzzles is the nature, distribution, and transport of its atmospheric volatiles. The full potential of constraints provided by detailed observations of the morphology and distribution of the polar deposits has not been realized. The objective of this study is characterization of the morphology, distribution, stratigraphy, and geologic setting of Triton's polar materials. Schenk P. Moore J. M. Geology of the Southern Hemisphere of Triton: No Polar Cap The bright southern hemisphere, comprising Uhlanga Regio, is perhaps the most poorly understood geologic province on Triton. The entire bright southern hemisphere has been described as a bright polar "cap," implying a seasonal origin, or as a permanent geologic terrain distinct from the equatorial terrains. Also, thermal models have predicted seasonal migration of frosts and ices from the presently sun-lit south latitudes to the dark northern latitudes. The distribution of frosts and geologic history of this region must be determined observationally. We reexamine the geology of this terrain with the goal of answering these questions. Schenk P. Moore J. M. 3-D Moons: The Voyager Stereo Atlas of the Outer Solar System Comprehension and analysis of geologic features on any planet is enhanced manyfold by a clear perception between albedo and topography. On many of the icy satellites significant albedo contrasts due to mixtures of dark rocky and bright icy materials can be associated with topographic features. Subtle topographic features can be masked by albedo variation and under high solar illumination albedo and topography can be difficult to separate. To this end we are compiling an atlas of stereo image pairs of the outer solar system based on Voyager imaging for the investigation of various geologic problems and for general use. Lunine J. I. Evolution of Triton's Volatile Budget Triton's volatile budget provides important links to planetary formation processes in the cold outer solar nebula. However, the budget has been modified by processes subsequent to the accretion of this body. It is of interest to assess whether certain formation environments can be ruled out for Triton on the basis of its current volatile abundances, and also to quantify some of the post-accretional processes by which the abundances have been modified. Benner L. A. M. McKinnon W. B. Orbital Simulations of Satellite Escape/Capture and the Origin of Satellites such as Triton We investigate satellite escape/capture in the context of the restricted, circular three-body problem as applied to the Sun, Neptune, and Triton. We have computed a large number of coplanar prograde and retrograde orbital simulations over a range of initial distances and velocities. The satellite starts at superior conjunction within ~2 Hill radii of Neptune and has a velocity orthogonal to the Sun-planet line. Orbits with these initial conditions can be reflected with respect to time, so an escape is simply the reverse of a capture. We numerically integrate the equations of motion to compute the satellite's position until it escapes, collides with Neptune, or after ~100 planetary years, fails to escape, when computations cease. The intial distance x and velocity v in the restricted problem uniquely define the Jacobi constant C, a conserved energylike quantity. Plots of the simulation outcomes in the prograde and retrograde C, x-phase spaces reveal distinct zones in which temporary satellites approach the planet closely enough that permanent capture can be effected by gas drag with a protoplanetary nebula or by collision with a preexisting satellite. Single and double close-flybys constitute the most common possible capture orbits. Long-term (>= 10 planetary years), multiple flyby orbits occur near the stability limits between bound and unbound orbits, and are more common among retrograde captures. Kelly W. D. Wood C. L. Tidal Interaction: A Possible Explanation for Geysers and Other Fluid Phenomena in the Neptune-Triton System Discovery of geyserlike plumes on the surface of Triton was a highlight of Voyager 2's passage through the Neptune planetary system. Remarkable as these observations were, they were not entirely without precedent. Considering the confirmed predictions for the 1979 Voyager jovian passage, it was logical to consider other solar system bodies beside Io where tidal effects could be a significant factor in surface processes. It was our intuition that the Neptune-Triton gravitational bond acting at high inclination to the Neptune equator and the fact that Neptune was a fluid body with significant oblateness would produce tidal and mechanical forces that could be transformed into thermal energy vented on Triton's surface. Prior to the Voyager flyby, others have noted that capture and evolution of Triton's orbit from extreme eccentricity to near circular state today would have resulted in significant tidal heating , but these analysts disregard current day forces. Our calculations indicate that the time varying forces between Neptune- Triton fall midway between those exerted in the Earth-Moon and Jupiter- Io systems, and considering the low level of other energy inputs, this source of internal energy should not be ignored when seeking an explanation for surface activity. In each planet-satellite case, residual or steady-state eccentricity causes time-varying stresses on internal satellite strata. In the case of Jupiter the residual eccentricity is due largely to Galilean satellite interactions, particularly Io-Europa, but in the case of Neptune-Triton, it is the effect of Triton's inclined orbit about an oblate primary. Forni O. Federico C. Coradini A. Phase Transitions and 2D Spherical Convection in a Large Icy Satellite Three ice phase transitions, based on experimental and theoretical results, have been incorporated into a 2D spherical convection model. These phase transitions are the exothermic Ice I-II, the endothermic Ice II-VI and the exothermic Ice VI-VIII phase transitions. The fluid is assumed isoviscous and the anelastic liquid formulation has been used. The object is an icy satellite whose physical characteristics resemble those of Ganymede or Callisto. Structural models, similar to those of Mueller and McKinnon, with different core sizes, i.e., with different degrees of differentiation, have been studied. The shells, i.e., the mantle of the satellite, is heated both internally and from below to account for the decaying radiogenic heating and the heat flow from the solid core. The lower boundary of the mantle is rigid and isothermal, the upper boundary is isothermal. Calculations with rigid and shear stress free upper boundary are carried out in order to assess the role of the different boundary conditions. Two different Rayleigh numbers, depending on the assumed value of the viscosity, have been used in the calculations and the thermal evolution of the satellite has been studied. Thursday, March 18, 1993 Interplanetary Dust: Laboratory Studies and Results from Spacecraft Poster Session 7:00 - 9:00 p.m. LPI Engrand C. Maurette M. Kurat G. Brandstatter F. Perreau M. A New Carbon-rich Phase ("COPS") in Antarctic Micrometeorites The contemporary flux of micrometeorites with sizes >50 micrometers reaching the Earth's surface each year (about 20,000 tons/a) is much larger than the value of about 100 tons/a reported for conventional meteorites up to masses of about 10,000 tons. Moreover, on the average, Antarctic micrometeorites contain at least as much carbon as does Orgueil, the most C-rich meteorite. Micrometeorites are thus responsible for most of the carbon accreted by the Earth. In this paper we report SEM observations of a new C-rich "dirty magnetite" phase observed as tiny inclusions in both melted and unmelted micrometeorites. This phase, which is enriched in C, O, P, S, Fe, frequently shows Ni contents in excess of 0.2%, strongly suggestive of an "extraterrestrial" origin. We also discovered this "COPS" phase in the fusion crust of Murchison. It appears likely that COPS is a product of meteoroid reprocessing during frictional heating in the Earth's atmosphere and/or its fast "weathering" in the upper atmosphere. Upon "catalyzed" hydrolysis this phase might have facilitated the functioning of micrometeorites as "microchondritic-reactors" for the synthesis of prebiotic molecules on the early Earth. Maag C. R. Tanner W. G. Stevenson T. J. Borg J. Bibring J.-P. Alexander W. M. Maag A. J. The Effect of an On-Orbit Near Encounter on the Number Flux Density of Micron Sized Particles Many materials and techniques have been developed by the authors to sample the flux of particles in Low Earth Orbit (LEO), and through regular in situ sampling of the flux in LEO, the materials and techniques have produced data that compliment the data now being amassed by the Long Duration Exposure Facility (LDEF) research activities. Several recent flight experiments have been conducted on the space shuttle as part of an ongoing program to develop an understanding of the Spatial Density as a function of size (mass) for particle sizes 1-10-6 cm and larger. During the course of one of the missions, the space shuttle corrected its altitude to evade an upper stage. The results of this near encounter provided unexpected data. Love S. G. Joswiak D. J. Brownlee D. E. Densities of 5-15 micrometer Interplanetary Dust Particles We have measured the densities of about 100 5-15-micrometer stratospheric IDPs. Great care was taken to minimize selection bias in the sample population. Masses were determined using an absolute X-ray analysis technique with a transmission electron microscope, and volumes were found using scanning electron microscope imagery. Unmelted chondritic particles have densities between 0.5 and 6.0 g/cc. Roughly half of the particles have densities below 2 g/cc, indicating appreciable porosity, but porosities greater than about 70% are rare. IDPs with densities above 3.5 g/cc usually contain large sulfide grains. We find no evidence of bimodality in the unmelted particle density distribution. Chondritic spherules (melted particles) have densities near 3.5 g/cc, consistent with previous results for deep-sea spherules. Bradley J. P. Keller L. Thomas K. L. Vander Wood T. B. Brownlee D. E. Carbon Analyses of IDPs Sectioned in Sulfur and Supported on Beryllium Films Carbon is the only major element in interplanetary dust whose abundance, distribution, and chemical state are not well understood. Information about carbon could clarify the relationship between the various classes of IDPs, conventional meteorites, and sources (e.g., comets vs. asteroids). To date, the most reliable estimates of C abundance in IDPs have been obtained by analyzing particles on thick-flat Be substrates using thin-window energy- dispersive spectroscopy in the SEM. These estimates of C abundance are valid only if C is homogeneously distributed, because detected C X-rays originate from the outer 0.1 micrometer of the particle. An alternative and potentially more accurate method of measuring C abundances is to analyze multiple thin sections (each <0.1 micrometer thick) of IDPs. These efforts, however, have been stymied because of a lack of a suitable noncarbonaceous embedding medium and the availability of C-free conductive substrates. We have embedded and thin-sectioned IDPs in glassy sulfur, and transferred the thin sections to Be support films ~25 nm thick. The sections were then analyzed in a 200-KeV analytical TEM. S sublimes rapidly under vacuum in the TEM, leaving nonembedded sections supported on Be. Apart from quantitative C (and O) analyses, S sectioning dramatically expands the range of analytical measurements that can be performed on a single IDP. Huang H.-P. Gilmour I. Pillinger C. T. Zolensky M. E. Removal of Carbonaceous Contaminants from Silica Aerogel Capture of micrometeorite material from low Earth orbit or dust grains around active comets for return to terrestrial laboratories, capable of praticing the most up-to-date technigues of chemical isotopic and mineralogical analysis, will greatly enhance our knowledge of primitive material in the solar system. The next generation of space-launched cosmic dust collectors will undoubtedly include extremely low density target materials such a silica aerogel as the decelerating and arresting medium. This material has been found to be clean from the point of view of inorganic elements and is thus acceptable for the purpose of harvesting grains to be studied by, for example PIXE, INAA, or SXRF. On the contrary, however, the process used in making aerogel leaves substantial carbon and hydrogen containing residues that would negate their suitability for collection and subsequent investigation of the very important CHON particles. Attempts to precondition aerogel by solvent extraction or heating at 500 degrees C and 750 degrees C in air for 24 hours or under a vacuum of 2 x 10(sub)-7 torr at 260 degrees C were largely ineffective except that pyrolysis did reduce volatile species. In this investigation we have examined the use of supercritical fluids for the purpose of extracting organic residues. The logic of the new approach is that beyond the supercritical point a substance has the solvating properties of a liquid but the viscosity characteristics of a gas. For example carbon dioxide becomes supercritical at a pressure of 73 atmospheres and a temperature of 31 degrees C; in consequence it can transform to a very powerful and ultraclean solvent. It can dissolve organic matter from low molecular weight up to molecules containing 90 carbon atoms. On release of pressure the fluid reverts to a gas that can easily be pumped away and removed from the substrate being extracted. Tsou P. Brownlee D. E. Albee A. L. Intact Capture of Hypervelocity Particles on Shuttle The capability for capturing micrometeoroid analogs intact through laboratory simulations in passive underdense media offers a valuable tool for cosmic dust research [Tsou 1984, Tsou 1990, Tsou 1992]. However, no amount of laboratory simulations can replace actual space experiments due to the technical inability to launch high fidelity analogs of micrometeoroids in ground laboratories and then reliably simulating high hypervelocities (greater than 7 km/s). Fluffy composite micrometeoroid analogs tend to break up and have not survived laboratory launches at beyond 4 km/s, however, a new technique of launching cluster analogs was developed for the two-stage light-gas gun at NASA Ames Vertical Gun Range [Tsou 1988]. Plasma drag or electrostatic accelerators may be able to achieve higher speeds than two-stage light-gas guns but only with very limited projectile compositions and may result in severe projectile deformation [Tsou 1990]. Consequently, a space demonstration of intact capture has been sought since 1983. This abstract reports initial results from our STS-47 flight, which carried the first Shuttle Sample Return Experiment (SRE) with silica aerogel capture media, September 1992. Flynn G. J. Sutton S. R. Bajt S. Klock W. New Low-Ni (Igneous) Particles Among the C and C? Types of Cosmic Dust Low-Ni particles with major element abundances, optical properties, and morphologies sufficiently similar to chondritic IDPs to receive JSC Cosmic Dust Catalog classifications of C or C?-types have been shown to have trace element contents and mineralogies similar to igneous material. Examination of the JSC Catalog EDX spectra by Cooke et al. has shown that 13% of the C-type and 38% of the C?-type particles are potentially low-Ni particles. We have identified two new low-Ni particles, and shown that an additional fragment from the L2002*C cluster has an igneous composition. A newly analyzed fragment of the W7066*A cluster has a chondritic composition. The W7066*A cluster is important because it has yielded a fragment of igneous composition and another fragment having high concentrations of He and Ne suggesting an extraterrestrial origin. Flynn G. J. Sutton S. R. Bajt S. Klock W. Thomas K. L. Keller L. P. Depletions of Sulfur and/or Zinc in IDPs: Are They Reliable Indicators of Atmospheric Entry Heating? The degree of heating of IDPs on Earth atmospheric entry is important in distinguishing cometary particles from main-belt asteroidal particles. Depletions in the volatile elements S and Zn have been proposed as chemical indicators of significant entry heating. The S and Zn contents of cosmic dust particles have been correlated with physical indicators of atmospheric entry heating, such as the production of magnetite and the loss of solar wind implanted He. The results indicate that the Zn content of IDPs is a useful indicator of entry heating, but the S content seems to be less useful. Blanford G. E. Using Solar Flare Track Densities to Determine the Origin of Interplanetary Dust Particles Sandford theoretically explored the use of solar flare track densities in interplanetary dust particles (IDPs) to distinguish whether they were of cometary or asteroidal origin. He determined that there were differences in the characteristic distributions of track densities that would occur from these two possible sources. Flynn examined the heating of IDPs on atmospheric entry and concluded that IDPs must be predominantly from asteroidal sources because these asteroidal particles would have sufficiently low velocities to survive atmospheric heating with little or no modification whereas cometary particles would ordinarily have velocities that are much higher. This paper looks at what we can learn by combining the approaches of Sandford and Flynn. Perreau M. Engrand C. Maurette M. Kurat G. Presper Th. C/O Atomic Ratios in Micrometer-size Crushed Grains from Antarctic Micrometeorites and Two Carbonaceous Meteorites Antarctic micrometeorites (AMMs) have similarities (but also differences) with primitive meteorites, such as unequilibrated mineral assemblages. To further assess such similarities we have measured the carbon content of micrometeorites and meteorites (Orgueil and Murchison), as determined in a random selection of micrometer-sized crushed grains, with an analytical transmission electron microscope. Such analyses yield the C/O atomic ratio, the major and minor elements contents, and the textural features of the grains on a scale of about 100 nanometers. An important proportion of micrometeorites from both the 100-400-micrometer and the 50-100-micrometer size fractions contains much more carbon than CI chondrite Orgueil. The average C-content of all micrometeorites in these two size fractions amounts to 1.8xCI and 0.8xCI, respectively (CI refers to the bulk C-content of Orgueil, of about 3.5% by weight). Carbon is usually not homogeneously distributed in the micrometeorite but is concentrated in C-rich grains. So far most of these grains are amorphous, and seem to be associated with an oxidized Fe-rich phase (possibly a variety of "dirty" magnetite). About 5% of them have the composition of "COPS," a phase additionally enriched in O, P, and S. Peng H. Xu P. First Discovery of the Organic Materials in Deep-Sea Iron Cosmic Spherule The dust impact mass analyser (PUMA) carried by the spacecraft Vega 1, Vega 2, and Giotto has provided the first direct measurements of the physical and chemical properties of cometary dust. The results indicate that most of the cometary dust particles are rich in light elements such as H, C, N, and O, suggesting the validity of models that describe the cometary dust as including organic material. Up to now, organic material has not been found in deep-sea cosmic spherules. We have determined the existence of this material in the deep-sea iron cosmic spherules collected from the North Pacific. Tanner W. G. Maag C. R. Alexander W. M. Sappenfield P. Determination of Parameters for Hypervelocity Dust Grains Encountered in Near-Earth Space The authors have been primarily interested in the determination of the population of micrometeoroids and space debris and have sought to interpret the hole size in a thin film or in a micropore foam returned from space with theoretical calculations describing the event. In order to augment the siginficance of the theoretical calculations of the impact event, an experiment designed to analyze the charge production due to hypervelocity impacts on thin films also produced data that described the penetration properties of micrometer and submicrometer-sized projectiles. The thin film penetration sites in the 500 angstrom and 1000 angstrom aluminum films were counted and a size distribution function was derived. In the case of the very smallest dust grains, there were no independent measurements of velocities like that which existed for the larger dust grains (d(sub)p <= 1 micrometer). The primary task then became to assess the relationship between the penetration hole and the particle diameter of the projectile that made the hole. The most promising means to assess the measure of the diameters of impacting grains came in the form of comparing cratering mechanics to penetration mechanics. Future experimentation will produce measurements of the cratering as opposed to the penetrating event. Particles encountered by surfaces while being flown in space will degrade that surface in a systematic manner even when the impact is with small hypervelocity particles, d(sub)p <= 10 micrometers. Though not to a degree that would precipitate a catastrophic failure of a system, the degradation of the materials comprising the interconnected system will occur. It is the degradation of the optical systems and the subsequent embrittlement of other materials that can lead to degradation if not to failure. It is to this end that research has been conducted to compare the primary consequences for experiments that will be flown to those that have been returned. Crowell L. B. Model of Optical Scatter from Microimpacts on the Hubble Telescope The investigation and modeling of optical scatter from damaged or contaminated mirrors is of interest to those who wish to estimate the working life cycle of an optical system. A space-borne telescope faces a threat to its survivability from naturally occurring micrometeoroids and man-made debris. High velocity, in the range of 4.0 to 14. km/s, impacts of small particles in the range of 1 to 100 micrometers, will produce small craters on the impacted surface. These microcraters will typically have a size range of 3 to 300 micrometers for debris impacts and 6 to 600 micrometers for micrometeoroid impacts. If the microcraters accumulate on a telescope mirror, there will be an increase in optical scatter and a loss of image resolution. Thursday, March 18, 1993 Meteorites: Thermoluminescence, Cooling Rates, and Instrumentation Poster Session 7:00 - 9:00 p.m. LPI Sears D. W. G. Benoit P. H. Lu J. Sears A. S. R. Cathodoluminescence Observations of In Situ Aqueous Destruction of Chondrules in the Murchison CM Chondrite A low-power cathodoluminescence (CL) mosaic of a 1 x 2 cm section of the Murchison CM chondrite contains chondrules of two types, one containing olivines with red CL, and thought to be the equivalent of group A chondrules of Sears et al., and one without observable CL, probably equivalent of their group B chondrules. Both types of chondrule contain dust mantles with a characteristic uniform red CL due to very-fine-grained forsterites, but the mantles of group A chondrules have complex internal structure and are much thicker than the relatively thin rims on group B chondrules. The variety of internal textures and the progressive integration of the objects with the surrounding matrix suggests to us that these features are large group A chondrules in the process of being destroyed by aqueous alteration. Prior to aqueous alteration, group A chondrules in CM chondrites were larger than group B chondrules, which is contrary to the situation in ordinary chondrites, and were apparently destroyed more rapidly than group B chondrules by aqueous alteration. Bennett M. E. McSween H. Y. Metallographic Cooling Rates of L-Group Ordinary Chondrites Shock metamorphism appears to be a ubiquitous feature in L-group ordinary chondrites. Brecciation and heterogeneous melting obscure much of the early history of this meteorite group and have caused confusion as to whether L chondrites have undergone thermal metamorphism within onion-shell or rubble-pile parent bodies. Employing the most recent shock criteria, we have examined 55 Antarctic and 24 non-Antarctic L chondrites in order to identify those that have been least affected by post-accretional shock. Six low-shock samples (those with shock grade less than S4) of petrographic types L3-L5 were selected from both populations and metallographic cooling rates were obtained following the technique of Willis and Goldstein. All non-Antarctic L6 chondrites inspected were too heavily shocked to be included in this group. However, four shocked L6 chondrites were analyzed in order to determine what effects shock may impose onmetallographic cooling rates. Molin G. M. Tribaudino M. Brizi E. Zaoyang Chondrite Cooling History from Pyroxene Fe2+-Mg Intracrystalline Ordering and Exolutions The Zaoyang ordinary chondrite fell as a single 14.15-kg mass in Hubey province (China) in October 1984 and was classified as a nonbrecciated H5 chondrite, shock facies b. Cooling rate in pyroxenes can be calculated down to about 1000 degrees C by using fine textures and at still lower temperatures (700 to 200 degrees C) by intracrystalline ordering processes. The crystal chemistry of clinopyroxene and orthopyroxene from the matrix of the H5 Zaoyang chondrite has been investigated by X-ray structure refinement and detailed microprobe analysis. By comparison with terrestrial pyroxenes, cell and polyhedral volumes in clino- and orthopyroxenes show a low crystallization pressure. Fe2+ and Mg are rather disordered in M1 and M2 sites of clino- and orthopyroxenes; the closure temperatures of the exchange reaction are 600 and 512 degrees C respectively, which is consistent with a quite fast cooling rate, estimated of the order of one degree per day. The closure temperature for the intercrystalline Ca-Mg exchange reaction for clino- and orthopyroxenes is 900 degrees C, as calculated from a matrix orthopyroxene showing clinopyroxene lamellae about 10 micrometers thick. Kinetic evaluations based on the thickness of exolved lamellae give a cooling rate of not more than a few degrees per 10^4 years. Harvey R. P. Bennett M. L. McSween H. Y. Jr. Pyroxene Equilibration Temperatures in Metamorphosed Ordinary Chondrites Ordinary chondrites are divided into petrographic types based on observed mineralogical and textural properties consistent with progressive thermal metamorphism from low grade (type 3) to high (type 7). Regardless of the exact cause of the metamorphism, higher-type chondrites should retain information concerning peak temperatures reached and for what duration. Using the two-pyroxene geothermometer of Lindsley, we have calculated the equilibrium temperatures for 26 H, L and LL type 5 and 6 ordinary chondrites, to investigate the relative peak temperatures and equilibrium-states reached by these various meteorite classes. Ganguly J. Yang H. Ghose S. Cation Ordering in Orthopyroxenes and Cooling Rates of Meteorites: Low Temperature Cooling Rates of Estherville, Bondoc and Shaw The cooling rates of meteorites provide important constraints on the size of their parent bodies and their accretionary and evolutionary histories. However, the cooling rates obtained so far from the commonly used metallographic, radiometric, and fission-track methods have been sometimes quite controversial, such as in the case of the mesosiderites and the meteorite Shaw. The metallographic method suggests an extremely slow cooling rate for the mesosiderites, <=1 degree C/Ma, which seems difficult to reconcile with the brecciated nature of the silicates in these rocks, suggesting exposure to near-surface environments. For Shaw, which is an L- group chondrite consisting of three intermingled lithologies, the metallographic method suggests a relatively rapid cooling rate, ~3 degrees C/10^3 years at 450-550 degrees C, wherever metal occurs in this meteorite, which is in contrast to the very slow cooling rate, 1 degree C/Ma at T <= 350 degrees C, deduced by the fission track method. We have undertaken a systematic study of the cooling rates of meteorites using a different approach, which involves single-crystal X-ray determination of Fe2+-Mg ordering in orthopyroxenes (OPx) in meteorites, subject to bulk compositional constraints, and numerical simulation of the evolution of the ordering state as a function of cooling rate, within the framework of the thermodynamic and kinetic principles governing cation ordering. We report the results obtained for OPx crystals from Shaw and two mesosiderites, Estherville and Bondoc. Allton J. H. Gooding J. L. Calorimetric Thermometry of Meteoritic Troilite: A Feasibility Study Two solid-state phase transitions in troilite (FeS) can be readily measured by differential scanning calorimetry (DSC) on samples of only a few milligrams. Troilite from the Mundrabilla iron meteorite displays a DSC fingerprint that is distinct from that of terrestrial troilite from Del Norte Co., California; their response to subsequent heating also differs significantly. Further work may establish whether troilite thermometry of meteorites is possible using DSC. Rivers M. L. Thorn K. S. Sutton S. R. Jones K. W. Bajt S. Wavelength Dispersive Analysis with the Synchrotron X-ray Fluorescence Microprobe A wavelength dispersive spectrometer (WDS) was tested on the synchrotron X-ray fluorescence microprobe at Brookhaven National Laboratory. Compared to WDS spectra using an electron microprobe, the synchrotron WDS spectra have much better sensitivity and, due to the absence of bremsstrahlung radiation, lower backgrounds. The WDS spectrometer was successfully used to resolve REE L fluorescence spectra from standard glasses and transition metal K fluorescence spectra from kamacite. Thursday, March 18, 1993 Undifferentiated Meteorites Poster Session 7:00 - 9:00 p.m. LPI Goswami J. N. Srinivasan G. Ulyanov A. A. Grosnaja ABCs: Magnesium Isotope Compositions Three CAIs from the Grosnaja CV3 chondrite were analysed for their magnesium isotopic compositions by the ion microprobe. The selected CAIs represent three distinct types: GR4 (compact Type A), GR7 (Type B), and GR2(Type C). Petrographic studies indicate that all the three Grosnaja inclusions were subjected to secondary alterations. The magnesium isotopic studies were carried out using a Cameca IMS-4F ion microprobe. The Mg-Al data for none of the inclusions define a well-behaved pattern even though the presence of radiogenic 26Mg was found in some of the analysed phases. The inferred initial 26Al/27Al in all such cases was less than the canonical value of 5 x 10^-5. The absence of well-behaved isotopic systematics in Grosnaja ABCs has most probably resulted fromsecondary alterations and not due to heterogeneous distribution of 26Al in the solar nebula. Keller L. P. Heterogeneous Plagioclase Compositions in the Maralinga CK4 Chondrite One of the characterisitic features of CK chondrites is the wide compositional range displayed by feldspar grains in matrix relative to the narrow range of compositions exhibited by the highly equilibrated olivines and pyroxenes. Recently, it was suggested that these heterogeneous feldspar compositions may have been strongly influenced by shock metamorphism. In this report, it is shown that the apparent range of feldspar compostions in Maralinga probably results from annealing during parent body thermal metamorphism rather than shock. Morse A. D. Newton J. Pillinger C. T. A Hydrogen Isotope Study of CO3 Type Carbonaceous Chondrites; Comparison with Type 3 Ordinary Chondrites Meteorites of the Ornans type 3 carbonaceous chondrites exhibit a range in degree of equilibration, attributed to differing amounts of thermal metamorphism. These differences have been used to split the C03 chondrites into petrologic subtypes from 3.0, least equilibrated, to 3.7, most equilibrated. This is similar to the system of assigning the type 3 ordinary chondrites into petrologic subtypes 3.0 to 3.9 based upon thermoluminescence (TL) and other properties; however, the actual range of thermal metamorphism experienced by C03 chondrites is much less than that of the type 3 ordinary chondrites. The least-equilibrated ordinary chondrites show evidence of aqueous alteration and have high D/H ratios possibly due to a deuterium-rich organic carrier. The aim of this study was to determine whether the C03 chondrites, which have experienced similar secondary conditions to the type 3 ordinary chondrites, also contain a similar deuterium-rich carrier. To date a total of five C03 meteorites, out of a set of 11 for which carbon and nitrogen isotopic data are available, have been analyzed. Ornans has not been analyzed yet because it does not appear to fit in with the metamorphic sequence exhibited by the other C03 chondrites; it also has an extremely high delta D value of +2150o/oo, unusual for such a comparatively equilibrated meteorite (type 3.4). Initial results indicate that the more-equilibrated C03s tend to have lower delta D values, analogous to the higher petrologic type ordinary chondrites. However, this is complicated by the effects of terrestrial weathering and the small dataset. Skinner W. R. Leenhouts J. M. Size Distributions and Aerodynamic Equivalence of Metal Chondrules and Silicate Chondrules in Acfer 059 The CR2 chondrite Acfer 059 is unusual in that original droplet shapes of metal chondrules are well preserved. We determined separate size distributions for metal chondrules and silicate chondrules; the two types are well sorted and have similar size distributions about their respective mean diameters of 0.74 mm and 1.44 mm. These mean values are aerodynamically equivalent for the contrasting densities, as shown by calculated terminal settling velocities in a model solar nebula. Aerodynamic equivalence and similarity of size distributions suggest that metal and silicate fractions experienced the same sorting process before they were accreted onto the parent body. These characteristics, together with depletion of iron in Acfer 059 and essentially all other chondrites relative to primitive CI compositions, strongly suggests that sorting in the solar nebula involved a radial aerodynamic component and that sorting and siderophile depletion in chondrites are closely related. Brearley A. J. Bajt S. Sutton S. R. SXRF Determination of Trace Elements in Chondrule Rims in the Unequilibrated CO3 Chondrite, ALH A77307 We have determined the concentrations of Ni, Cu, Zn, Ga, Ge, and Se in five chondrule rims in the CO3 chondrite ALH A77307 (3.0) using the synchrotron X- ray fluorescence (SXRF) microprobe at Brookhaven National Laboratory. The data show that the trace element chemistry of rims on different chondrules is remarkably similar, consistent with data obtained for the major elements by electron microprobe. These results support the idea that rims are not genetically related to individual chondrules, but all sampled the same reservoir of homogeneously mixed dust. Of the trace elements analysed Zn and Ga show depletions relative to CI chondrite values, but in comparison with bulk CO chondrites all the elements are enriched by ~1.5 to 3.5 x CO. The high concentrations of the highly volatile elements Se and Ga and moderately volatile Zn (1.5 to 2 x CO) in rims show that matrix is the major reservoir of volatile elements in ALH A77307. Ash R. D. Pillinger C. T. Carbon in Weathered Ordinary Chondrites from Roosevelt County A suite of Roosevelt County ordinary chondrites of known terrestrial age have been analyzed for carbon content and isotopic composition. Initial results indicate that significant carbon contamination is evident only in samples with a terrestrial age greater than 40ka. These samples are of weathering grade D and E and contain three times more carbon than the less weathered samples. The soil in which they were preserved has a carbon content of ~1.5%. DeHart J. M. Lofgren G. E. Cathodoluminescence Properties of Components in Enstatite Chondrites As a group, the enstatite chondrites are notable by the extremely low FeO content of most of their silicates. This property predisposes many of these materials to emit cathodoluminescence (CL). Since examination of the CL properties of meteoritic components in ordinary and carbonaceous chondrites has proven to be a useful technique, we have initiated a survey of the enstatite chondrites in order to better characterize the chemical and physical properties of their luminescing phases. Because of the diversity encountered in this study, it is first necessary to describe the number and types of materials observed to emit CL in these meteorites. Miura Y. Noma Y. Identification of New Meteorite, Mihonoseki (L), from Broken Fragments in Japan A new meteorite of Mihonoseki fallen in Shimane-ken was identified by fine broken pieces by using energy-dispersive scanning electron microprobe analysis. It shows fusion-crust (i.e., Fe-Si melt), meteoritic minerals (kamacite. taenite, troilite, amorphous plagioclase, etc.), and chondrules with clear glassy rims. Mineralogical and petrological data of several fine grains suggest that Mihonoseki is an L3/4 chondritic meteorite and is the first such identification of a meteorite to fall in Japan. This prompt identification method of meteorite fragments will be applied to the next lunar, martian, and asteroid explorations, as well as meteorite falls on the terrestrial surface. Zolensky M. E. Weisberg M. K. Barrett R. A. Prinz M. Mineralogy and Dark Clasts in Primitive vs. Differentiated Meteorites The presence of dark lithic clasts within meteorites can provide information concerning asteroidal regolith processes, the extent of interactions between asteroids, and the relationship between meteorite types, micrometeorites, and interplanetary dust particles. Accordingly, we have been seeking and characterizing dark clasts found within carbonaceous chondrites, unequilibrated ordinary chondrites, howardites, and eucrites. We find that unequilibrated chondrites in this study contain fine-grained, anhydrous unequilibrated inclusions, while the howardites often contain inclusions from geochemically processed, hydrous asteroids (type 1 and 2 carbonaceous chondrites). Eucrites and howardites contain unusual clasts, not easily classified. Friday, March 19, 1993 CAIs 8:30 - 11:00 a.m. Room A Chair(s): G. MacPherson Meeker G. P.* Quick J. E. Paque J. M. Limited Subsolidus Diffusion in Type B1 CAI; Evidence from Ti Distribution in Spinel Most models of calcium aluminum-rich inclusions (CAI) have focused on early stages of formation by equilibrium crystallization of a homogeneous liquid. Less is known about the subsolidus cooling history of CAI. Chemical and isotopic heterogeneities on a scale of tens to hundreds of micrometers suggest fairly rapid cooling with a minimum of subsolidus diffusion. However, transmission electron microscopy indicates that solid state diffusion may have been an important process at a smaller scale. If so, chemical evidence for diffusion could provide constraints on cooling times and temperatures. With this in mind, we have begun an investigation of the Ti distribution in spinels from two type-B1 CAI from Allende to determine if postcrystallization diffusion was a significant process. Deloule E.* Kennedy A. K. Hutcheon I. D. El Goresy A. Isotopic and Trace Element Characteristics of an Unusual Refractory Inclusion from Essebi The isotopic and chemical properties of Ca-Al-rich inclusions (CAIs) provide important clues to the early solar nebula environment. While the abundances of refractory major and trace elements are similar to those expected for high- temperature condensates, the variety of textural, chemical, and isotopic signatures indicate most CAIs experienced complex, multistage histories involving repeated episodes of condensation, evaporation, and metamorphism. Evidence of multiple processes is especially apparent in an unusual refractory inclusion from Essebi (URIE) described by El Goresy et al. The melilite (mel)- rich core of URIE contains polygonal framboids of spinel (sp) and hibonite (hb) or sp and fassaite (fas) and is surrounded by a rim sequence consisting of five layers. In contrast to rims on Allende CAIs, the mineralogy of the URIE rim layers becomes increasingly refractory from the core outward, ending in a layer of spinel-A1203 solid solution + Sc-rich fassaite. The chemical and mineralogical features of URIE are inconsistent with crystallization from a homogeneous melt, and El Goresy et al. proposed a multistep history involving condensation of sp + hb and aggregation into framboids, capture of framboids by a refractory silicate melt droplet, condensation of rim layers, and alteration of mel to calcite and feldspathoid. The PANURGE ion probe was used to investigate the isotopic and trace-element characteristics of URIE to develop a more complete picture of the multiple processes leading to formation and metamorphism. Greenwood R. C.* Morse A. Long J. V. P. Petrography, Mineralogy, and Mg Isotope Composition of Victa: A Vigarano CaAl4O7-bearing Type A Inclusion Thermodynamic calculations predict that Ca-dialuminate (CaAl4O7) condenses from a cooling gas of solar composition after hibonite and before melilite. Although Ca-dialuminate has now been recorded from Ca Al-rich inclusions (CAIs) in at least nine meteorites, compared to hibonite it is a relatively rare phase. As pointed out by Michel-Levy et al. the absence of Ca-dialuminate from most hibonite-bearing inclusions poses a serious problem for the condensation model of CAI formation. Here we describe an inclusion that contains abundant Ca-dialuminate partially altered to a hercynite-rich (FeAl2O4) assemblage. The evidence from VICTA indicates that compared to all other phases in type A inclusions, Ca-dialuminate is the most susceptible to secondary alteration; a feature that may explain its restricted occurrence. Unaltered Ca-dialuminate and melilite in VICTA display excess 26 Mg* indicative of in situ decay of 26Al. Casanova I.* Grossman L. Distribution of Vanadium and Melting of Opaque Assemblages in Efremovka CAIs A petrographic and chemical study of compact Type A CAIs from the Efremovka CV3 chondrite strongly suggests that the opaque assemblages (OAs) that they contain were molten at temperatures below the CAI silicate solidus, and that the V-rich magnetite presently observed in association with OAs formed by in situ oxidation of their FeNi. Lindstrom D. J.* Keller L. P. Martinez R. R. INAA of CAIs from the Maralinga CK4 Chondrite: Effects of Parent Body Thermal Metamorphism Maralinga is an anomalous CK4 carbonaceous chondrite that contains numerous Ca-, Al-rich inclusions (CAIs), unlike the other members of the CK group. These CAIs are characterized by abundant green hercynitic spinel intergrown with plagioclase and high-Ca clinopyroxene, and a total lack of melilite. We have used Instrumental Neutron Activation Analysis (INAA) to further characterize the meteorite, with special focus on the CAIs. High-sensitivity INAA was done on eight sample disks about 100-150 micrometers in diameter obtained from a normal 30-micrometer thin section with a diamond microcoring device. The CAIs are enriched by 60-70X bulk meteorite values in Zn, suggesting that the substantial exchange of Fe for Mg that made the spinel in the CAIs hercynitic also allowed efficient scavenging of Zn from the rest of the meteorite during parent body thermal metamorphism. Less mobile elements appear to have maintained their initial heterogeneity. Simon S. B.* Grossman L. Hsu A. Petrography and Origin of Refractory Inclusions from the Murray and Murchison C2 Chondrites By freeze-thaw disaggregation we have recovered a total of 47 refractory inclusions. New discoveries include a hibonite-pyroxene spherule from Murray, a CaAl4O7-bearing spherule from Murchison, and a Sc-fassaite-bearing ultrarefractory inclusion from Murchison. Beckett J. R.* Stolper E. The Stability of Hibonite and Other Aluminous Phases in Silicate Melts: Implications for the Origin of Hibonite-bearing Inclusions Phase fields in which hibonite (Hib) and silicate melt coexist with spinel (Sp), CaAl4O7 (CA2), gehlenitic melilite (Mel), anorthite (An), or corundum (Cor) in the system CaO-MgO-Al2O3-SiO2-TiO2 (CMAST) were determined and activity models developed for melilite (Mel) and Hib solid solutions. Experimentally determined partition coefficients for Ti between Hib and coexisting melt, D(sub)Ti, vary from 0.8 to 2.1 and generally decrease with increasing TiO2 content in the liquid (L). Based on Ti partitioning between Hib and melt, bulk inclusion compositions and Hib-saturated liquidus phase diagrams, the Hib in Fluffy Type A inclusions (FTAs) from Allende and at least some of the Hib from Hib-rich inclusions is relict; much of the Hib from Hib- glass spherules probably crystallized from a melt under nonequilibrium conditions. Bulk compositions for all of these Ca-Al-rich inclusions (CAIs) are consistent with an origin as Mel + Hib + Sp + perovskite (Pv) proto- inclusions in which Mel was partially altered. In some cases, the proto- inclusion was partially or completely melted with vaporization occurring over a period of time sufficient to remove any Na introduced by the alteration process but frequently insufficient to dissolve all of the original hibonite. If equilibration temperatures based on Hib-bearing CAIs reflect condensation in a cooling gas of solar composition, then Hib + Cor condensed at ~1260 degrees C (referenced to 10^-3 atm) and Hib + Sp +- Mel at ~1215 +-10 degrees C. Simple thermochemical models for the substitution of trace elements into the Ca-site of meteoritic Hib suggest that virtually all Eu is divalent in early condensate Hibs but that Eu^2+/Eu^3+ decreases by a factor of 20 or more during the course of condensation, primarily because the ratio is proportional to the partial pressure of Al, which decreases dramatically as aluminous phases condense. The relative sizes of Eu and Yb anomalies in meteoritic Hibs and CAIs may be influenced by this effect. Wang J.* Davis A. M. Hashimoto A. Clayton R. N. Diffusion-controlled Magnesium Isotopic Fractionation of a Single Crystal Forsterite Evaporated from the Solid State Though the origin of calcium- and aluminum-rich inclusions (CAIs) in carbonaceous chondrites is still a disputed issue, evaporation is no doubt one of the most important processes for the formation of CAIs in the early solar nebula. The mechanism for production of large isotopic mass fractionation effects in magnesium, silicon, oxygen, and chromium in CAIs can be better understood by examining isotopic fractionation during the evaporation of minerals. We have performed new evaporation experiments on single-crystal forsterite. We measured the magnesium isotopic distribution near the evaporating surfaces of the residues using a modified AEI IM-20 ion microprobe to obtain rastered beam depth profiles. A theoretical model was used to explain the profiles and allowed determination of the diffusion coefficient of Mg++ in forsterite at higher temperatures than previous measurements. The gas/solid isotopic fractionation factor for magnesium for evaporation from solid forsterite was also determined and found to be nearly the same as that for evaporation of liquid Mg2SiO4. Nagahara H.* Evaporation in Equilibrium, in Vacuum, and in Hydrogen Gas Evaporation experiments were conducted for SiO2 in three different conditions; in equilibrium, in vacuum, and in hydrogen gas. Evaporation rate in vacuum is about two orders of magnitude smaller than that in equilibrium, which is consistent with previous works. The rate in hydrogen gas changes depending on hydrogen pressure. The rate at 10^-7 bar of hydrogen pressure is as small as that of free evaporation, but at 10^-5 bar of hydrogen pressure it is larger than that in equilibrium. In equilibrium and in vacuum, the evaporation rate is limited by decomposition of SiO2 on the crystal surface, but it is limited by a diffusion process for evaporation in hydrogen gas. Therefore, evaporation rate of minerals in the solar nebula can be shown neither by that in equilibrium nor by that in vacuum. Paillat O.* Wasserburg G. J. Self Diffusion of Alkaline-Earth in Ca-Mg-Aluminosilicate Melts, Experimental Improvements on the Determination of the Self-Diffusion Coefficients Experimental studies of self-diffusion isotopes in silicate melts often have quite large uncertainties when comparing one study to another. We designed an experiment in order to improve the precision of the results by simultaneously studying several elements (Mg, Ca, Sr, Ba), during the same experiment thereby greatly reducing the relative experimental uncertainties. Results show that the uncertainties on the diffusion coefficients can be reduced to 10%, allowing a more reliable comparison of differences of self-diffusion coefficients of the elements. This type of experiment permits us to study precisely and simultaneously several elements with no restriction on any element. We also designed an experiment to investigate the possible effects of multicomponent diffusion during Mg self-diffusion experiments by comparing cases where the concentrations of the elements and the isotopic compositions are different. The results suggest that there are differences between the effective means of transport. This approach should allow us to investigate the importance of multicomponent diffusion in silicate melts. Friday, March 19, 1993 Heat Sources for Chondrule/CAI Melting 11:00 - 12:00 a.m. Room A Chair(s): A. E. Rubin Connolly H. C. Jr.* Hewins R. H. Lofgren G. E. Flash Melting of Chondrule Precursors in Excess of 1600C. Series 1: Type II (B1) Chondrule Composition Experiments Several questions in chondrule production remain an enigma despite years of experiments. What were the melting temperatures experienced by chondrules? What were the physical characteristics of chondrule precursors? How and why did volatile elements (i.e., Na) found within chondrules survive the formation process? We present the initial results of a series of experiments designed to investigate the above questions by using flash melting to duplicate the melting stage of chondrule formation. Hood L. L.* Horanyi M. Gas-Grain Energy Transfer in Solar Nebula Shock Waves: Implications for the Origin of Chondrules Meteoritic chondrules provide evidence for the occurrence of rapid transient heating events in the protoplanetary nebula. Astronomical evidence suggests that gas dynamic shock waves are likely to be excited in protostellar accretion disks by processes such as protosolar mass ejections, nonaxisymmetric structures in an evolving disk, and impact on the nebula surface of infalling "clumps" of circumstellar gas. Previous detailed calculations of gas-grain energy and momentum transfer have supported the possibility that such shock waves could have melted pre- existing chondrule-sized grains. The main requirement for grains to reach melting temperatures in shock waves with plausibly low Mach numbers is that grains existed in dust-rich zones (optical depth greater than 1) where radiative cooling of a given grain can be nearly balanced by radiation from surrounding grains. Localized dust-rich zones also provide a means of explaining the apparent small spatial scale of heating events. For example, the scale size of at least some optically thick dust-rich zones must have been relatively small (less than 10 km) to be consistent with petrologic evidence for accretion of hot material onto cold chondrules. The implied number density of millimeter-sized grains for these zones would be greater than 30 per cubic meter. In this paper, we make several improvements of our earlier calculations to (i) include radiation self-consistently in the shock jump conditions and (ii) include heating of grains due to radiation from the shocked gas. In addition, we estimate the importance of momentum feedback of dust concentrations onto the shocked gas, which would tend to reduce the efficiency of gas dynamic heating of grains in the center of the dust cloud. Wasson J. T.* Multiplicity of Chondrule Heating Events and the Coarsening of Chondrule Textures Our studies of compound chondrules show that most chondrules experienced at least two thermal events that produced appreciable amounts of melt. I suggest that chondrules were subjected to a hierarchy of brief heating events, the number increasing exponentially with decreasing intensity, and that some of intermediate intensity deposited enough heat to partially melt mesostasis glass and promote the growth of mafic mineral grains. This scenario can account for textures that require improbably low monotonic cooling rates in laboratory simulations, and also for the rarity of glassy chondrule textures despite the ease with which these are produced in the laboratory. Eisenhour D.* Buseck P. R. Primordial Lightning: Evidence Preserved in Chondrites An increasing body of evidence suggests that transient heating events were important in forming or modifying many constituents of chondritic meteorites. For example, chondrule compositions and textures imply high cooling rates (~5 to >2000 degrees C/hr) with only limited exposure to elevated temperatures (>1200 degrees C). Similarly, experimental studies of CAIs exhibiting igneous textures suggest cooling rates from 0.5 to 1000 degrees C/hr, and thermal pulses as short as milliseconds may be responsible for CAI rim formation. Additionally, the spectrum of organic compounds observed in chondrites suggests that transient heating may also have played an important role in their formation. The exact nature of these transient events is less clear. A variety of mechanisms have been proposed (e.g., shock melting, drag heating, volcanism, electric discharges, and magnetic reconnection). However, a consensus is lacking as to which or what combination of these mechanisms was responsible for the range of features we observe in chondrites today. Much of the difficulty lies in the inability to identify features unique to any given process. Here we discuss electric discharges ("lightning") as a transient heat source and describe its unique signature preserved in chondrites. Friday, March 19, 1993 Comets and Asteroids 8:30 - 12:00 a.m. Room B Chair(s): D. T. Britt S. Murchie Schulz R.* A'Hearn M. F. Birch P. V. Bowers C. Kempin M. Martin R. C2, CN and Dust in Comet Wilson (1987VII) Column density profiles in C2, CN, and blue and red continua (BC and RC) were constructed from two-dimensional images of the coma of Comet Wilson (1987VII). Our analysis showed that the continuum profiles decrease with the nuclear distance (rho) as 1/rho. From the C2 and CN profiles we determined the parent and the daughter lifetimes as well as the production rates in terms of the vectorial model. Fomenkova M.* Chang S. Mass and Spatial Distribution of Carbonaceous Component in Comet Halley Cometary grains containing large amounts of carbon and/or organic matter were discovered by in situ measurements of cometary dust composition during VEGA and GIOTTO flyby missions. In accordance with the classification for the data of PUMA 1 and PUMA 2 mass spectrometers on board the VEGA spacecraft, particles with a ratio of C to any rock-forming element (Mg, Si, Fe, Ca, etc.) >10, were categorized as CHON. There are 464 such particles in PUMA 1 data and 51 in PUMA 2 data. Application of cluster analysis to these grains revealed several distinct compositional classes, namely: [H,C,N,O], [H,C,N], [H,C], [H,C,O], [C,N], [C,O] [C,N,O], and [C]. Similar classes were identified among particles analyzed by PIA. Also, about a third of all particles fell into groups [H] and [O] characterized by abundances of these elements beyond chemically reasonable limits. Hartmann W. K.* The Physical Mechanism of Comet Outbursts: An Experiment During a series of impact experiments into regolithlike powders at the NASA Ames Research Center Vertical Gun Facility in 1976, I observed and filmed a unique anomalous even that may illuminate outburst mechanisms in comets. During one test, a new batch of basalt powder (half the mass in particles less than 800 micrometers in diameter) retained some air pressure while the vacuum chamber was being evacuated. As a result, the projectile impacted into gas-charged regolith. Instead of ejecting the normal, relatively negligible amount of debris, the disturbance triggered a major eruption that lasted for at least 17 seconds. The experimental results have been recently reanalyzed with reference to cometary phenomena. Grimm R. E.* McSween H. Y. Jr. Heliocentric Zoning of the Asteroid Belt by Aluminum-26 Heating The dependence of asteroid spectral class (and inferred thermal history) on heliocentric radius has been held to be the result of heating by a solar energy source, most likely electrical induction, during the formation of the planetary system. Such variations in thermal history can be more simply explained by incorporating different amounts of the radionuclide 26Al--whose decay products are observed in meteorites--into planetesimals. These differences occurred naturally as a function of the increasing amount of time required to accrete objects farther from the Sun, during which 26Al decayed from its initial concentration in the solar nebula. Both theory and isotopic evidence suggest that increases in accretion time across the asteroid belt are of order several half-lives of 26Al, which is sufficient to produce the inferred differences in thermal history. Britt D. T.* Howell E. S. Bell J. F. Lebofsky L. A. 1.2- to 3.5-micrometer Observations of Asteroid 4179 Toutatis The close Earth approach of the Apollo asteroid 4179 Toutatis during the winter of 1992-1993 has provided a unique opportunity for detailed ground- based observations of a near earth asteroid (NEA). Because of their relatively small size NEAs are usually far too faint to be observable by most ground- based instruments. This opposition by Toutatis was, however, exceptionally favorable. Toutatis approached within 0.03 AU of Earth and was as bright as 11th visual magnitude. This made the object observable in a wide variety of wavelengths including radar, thermal IR, near IR, and visual. Kelley M. S.* Gaffey M. J. An Initial Perspective of S-Asteroid Subtypes Within Asteroid Families Many main belt asteroids cluster around certain values of semi-major axis (a), inclination (i), and eccentricity (e).Hirayama was the first to notice these concentrations, which he interpreted as evidence of disruptions of larger parent bodies. He called these clusters "asteroid families." The term "families" is increasingly reserved for genetic associations to distinguish them from clusters of unknown or purely dynamical origin (e.g. the Phocaea cluster). Members of a genetic asteroid family represent fragments derived from various depths within the original parent planetesimal. Thus, family members offer the potential for direct examination of the interiors of parent bodies that have undergone metamorphism and differentiation similar to that occuring in the inaccessible interiors of terrestrial planets. The condition that genetic family members represent the fragments of a parent object provides a critical test of whether an association (cluster in proper element space) is a genetic family. Compositions (types and relative abundances of materials) of family members must permit the reconstruction of a compositionally plausible parent body. The compositions of proposed family members can be utilized to test the genetic reality of the family and to determine the type and degree of internal differentiation within the parent planetesimal. The interpretation of the S-class mineralogy provides a preliminary evaluation of family memberships. Clark B. E.* Lucey P. G. Bell J. F. Fanale F. P. Spectral Mixing Models of S-type Asteroids This paper presents the results of an attempt to determine S-type asteroid mineralogies with the use of Hapke theory spectral mixing modeling. Previous attempts to understand the spectral variations present in this single class of asteroids have concentrated on spectral parameters such as absorption band center wavelengths, band area ratios, and geometric albedos. The procedure taken here is to utilize the Hapke spectral reflectance model to calculate single-scatter albedo as a function of wavelength for a suite of candidate endmember materials. These materials are then mixed linearly in single-scatter albedo space and the mixture is converted, assuming intimate particle mixing, back to reflectance for the spectrum matching routine. A total of 39 S-type asteroids selected from the Bell et al. survey, have been matched with mixture model spectra. Sunshine J. M.* Pieters C. M. Determining the Composition of Olivine on Asteroidal Surfaces Olivine has been remotely identified as a major component on many S- and A-type asteroids based on qualitative analyses of broad absorption features near 1.0 micrometer in their telescopic reflectance spectra. Laboratory studies have shown that this diagnostic olivine absorption feature is composed of three individual absorption bands resulting from electronic transitions of Fe+2 in distorted M1 and M2 sites. Both the overall absorption feature and each of the individual absorption bands are also known to vary systematically as a function of olivine composition. Thus, it should be possible to remotely determine the composition of olivine present on the surfaces of the various olivine- rich asteroids. However, extrapolation of laboratory studies to compositional interpretation of asteroid spectra is complicated by several factors: observational noise; lower spectral resolution; compositional heterogeneities (including the presence of other phases such as pyroxenes and/or metal); and the effects of alteration processes. Hiroi T.* Pieters C. M. Zolensky M. E. Comparison of Reflectance Spectra of C Asteroids and Unique C Chondrites Y86720, Y82162, and B7904 Reflectance spectra (0.3-2.6 mm) of 11 carbonaceous (C) chondrite powders (<100 or <125 micrometers) including"unique" ones Y86720, Y82162, and B7904, have been measured and compared with those of 14 C asteroids. Among those C chondrites, only three "unique" ones had close counterparts among C asteroids (Y86720 and 1 Ceres, Y82162 and 704 Interamnia, and B7904 and 31 Euphrosyne). Mixing calculations of those C chondrites by Hapke's isotropic model were also performed to improve fits of reflectance spectra of all the 14 C asteroids. If the grain-size distributions of those asteroid surfaces are similar to those C chondrite powders, the result suggests that all tested C asteroids contain a high amount of heated C chondrite material, such as Y86720, Y82162, and B7904. Vilas F.* Hiroi T. Zolensky M. E. Comparison of Visible and Near-Infrared Reflectance Spectra of CM2 Carbonaceous Chondrites and Primitive Asteroids Spectra of primitive asteroids (here defined as C, P, and D classes and associated subclasses) have been compared to the limited number of spectra of CM2 carbonaceous chondrites. An absorption feature located at 0.7 micrometers attributed to an Fe2+ - Fe3+ charge transfer absorption in iron oxides in phyllosilicates is apparent in some of the CM2 carbonaceous chondrite spectra and many of the asteroid spectra. Sawyer has found a correlation between the area of the 0.7-micrometer feature and the mean semimajor axis of the asteroids. Spectra of a larger sample of carbonaceous chondrites, including seven CM2 chondrites, covering a spectral interval of 0.30 - 2.5 micrometers have recently been obtained using the Relab instrument at Brown University. These spectra have been compared with spectrophotometric asteroid observations in a separate abstract. We have taken those spectra of CM2 chondrites and isolated the UV, visible, and near-infrared spectral regions in order to compare them with high-quality narrowband reflectance spectra. Murchie S.* Erard S. The Spectrum of Phobos from Phobos 2 Observations at 0.3-2.6 micrometers: Comparison to Previous Data and Meteorite Analogs The surface of Phobos has been proposed to consist of carbonaceous chondrite or optically darkened ordinary chondrite ("black chondrite"). Measurements of Phobos's spectrum are key evidence for testing these hypotheses. Disk- integrated measurements were obtained by the Mariner 9 UV spectrometer, Viking Lander cameras, and groundbased observations. In 1989 disk-resolved measurements of Phobos and Mars were obtained by three instruments on Phobos 2: the KRFM spectrometer, which covered the wavelength range 0.32-0.6 micrometer; the ISM imaging spectrometer, which covered the wavelength range 0.76-3.16 micrometers; and the VSK TV cameras, whose wavelength ranges overlap those of KRFM and ISM. Here we report analysis of the Phobos 2 measurements completed since earlier results were reported. We validated calibration of the Phobos measurements using observations of Mars for reference, and compared them with pre-1989 measurements. We also combined spectra from the three detectors to produce an integrated spectrum of Phobos from 0.3-2.6 micrometers. Phobos 2 results agree well with previous measurements, contrary to some reports. The general shape of the spectrum is consistent with both proposed analogues. However, position and depth of the previously unobserved 1-micrometer absorption are more diagnostic, and indicate the composition of typical surfaces to be more consistent with black chondrite. Chapman C. R.* Neukum G. Veverka J. Belton M. Cratering on Gaspra The October 1991 Galileo flyby of Gaspra shows that its crater population is dominated by fresh craters several hundred meters in diameter and smaller. They appear to represent a production population because the spatial density is relatively low (few overlaps) and because fresh craters are very abundant; equilibrium could be attained at diameters near to or below the resolution limit of the best image. These craters are the first direct record of the population of main-belt asteroids some tens of meters in diameter. Bottke W. F. Jr.* Nolan M. C. Greenberg R. Collision Lifetimes and Impact Statistics of Near-Earth Asteroids We have examined the lifetimes of Near-Earth asteroids (NEAs) by directly computing the collision probabilities with other asteroids and with the terrestrial planets. We compare these to the dynamical lifetimes, and to collisional lifetimes assumed by other workers. We discuss the implications of the differences. Friday, March 19, 1993 Educating Young People in Earth and Planetary Sciences 8:30 - 12:00 a.m. Room C Chair(s): J. O. Annexstad J. M. Boyce Taylor G. J.* Lindstrom M. M. Sparking Young Minds with Moon Rocks and Meteorites What could be more exciting than seeing pieces of other worlds? The Apollo program left a legacy of astounding accomplishments and precious samples. Part of the thrill of those lunar missions is brought to schools by the lunar sample educational disks, which contain artifacts of six piloted trips to the Moon. JSC is preparing 100 new educational disks containing pieces of meteorites collected in Antarctica. These represent chunks of several different asteroids that were collected in one of the most remote, forbidding environments on Earth. These pieces of the Moon and asteroids represent the products of basic planetary processes (solar nebular processes, initial differentiation, volcanism, impact), and, in turn, these processes are controlled by basic physical and chemical processes (energy, energy transfer, melting, buoyancy, etc.). Thus, the lunar and meteorite sample disks have enormous educational potential. Lindstrom M. M.* NASA Partners in Space Team Spaceship Earth: A Partnership in Curriculum Writing As the Apollo astronauts left Earth to venture onto the surface of another planetary body, they saw their home planet in a new global perspective. Unmanned NASA missions have given us a closer look at all the other planets in our solar system and emphasized the uniqueness of Earth as the only place in our solar system that can sustain life as we know it. Spaceship Earth is a new science curriculum that we developed to help students and teachers explore the Earth, to see it in the global perspective, and to understand the relationships among life, the planet, and the Sun. We use astronaut photographs, especially shuttle pictures, as well as groundbased studies to help students to understand global Earth Science and integrate various aspects of physical, life, and social science. Lebofsky L. A.* Lebofsky N. R. Teaching Planetary Sciences to Elementary School Teachers: Programs that Work Planetary sciences can be used to introduce students to the natural world, which is a part of their lives. Even children in an urban environment are aware of such phenomena as day and night, shadows, and the seasons. It is a science that transcends cultures, has been prominent in the news in recent years, and can generate excitement in young minds as no other science can. Planetary sciences also provides a useful tool for understanding other sciences and mathematics, and for developing problem solving skills that are important in our technological world. However, only 15 percent of elementary school teachers feel very well qualified to teach earth/space science, while better than 80% feel well qualified to teach reading; many teachers avoid teaching science; very little time is actually spent teaching science in the elementary school: 19 minutes per day in K-3 and 38 minutes per day in 4-6. Allton J. H.* Allton C. S. Encouraging Interest in Space Exploration and Planetary Science Among Navajo Primary Students Two educational projects were initiated and organized by two individuals, implemented by 15-20 Houston, Texas, volunteers who contributed time and money, and helped by many more NASA and contractor professionals. Principal goals for both projects were (1) encouraging interest in space exploration and planetary science among students with little exposure to science and engineering, (2) relate space exploration and planetary science to Navajo culture and environment, (3) motivate students with little interest or incentive to study math and science, and (4) complete a project within one year with a limited number of volunteers on a small budget. A space/science fair was sponsored at a small Indian boarding school in Arizona, and the winners toured the Johnson Space Center. The following year winners traveled to Houston and trained at "Mars Camp," culminating in a simulated rover traverse geology sampling mission on Mars. Barlow N. G.* Encouraging Female Interest in Science and Mathematics The United States has begun a new initiative to improve the science literacy of our students. Classroom curricula are being rewritten with an increased emphasis on the understanding of concepts rather than memorization of facts and figures. Studies have shown that students learn more when they are actively involved in a project. Science fairs and science camps have been the traditional means by which students have participated in science activities beyond the classroom environment. However, studies reveal that young women and students of various ethnic backgrounds have not participated as fully in these activities as white males, either because they have not been informed about the event or more commonly because they have not been encouraged to participate by parents, teachers, counselors, and other adults. Today a number of programs are being offered specifically for these groups that have traditionally been underrepresented in scientific and engineering fields. Examples of such programs for young women are Expanding Your Horizons workshops, programs with Girl Scouts, Girls Clubs of America Operation SMART program, and mentoring programs initiated by various professional women's organizations. Postawko S. E.* Morrisey M. L. Taylor G. J. Mouginis-Mark P. Schools of the Pacific Rainfall Climate Experiment The SPaRCE program is a cooperative rainfall climate field project involving high school and college level students and teachers from various Pacific island and atoll nations. The goals of this program are both research and educational. Metzger S. M.* Do It Yourself Remote Sensing: Generating an Inexpensive High-Tech, Real Science Lake Mapping Project for the Classroom The utilization of modest equipment and software revealed bottom contours and water column conditions of a dynamic water body. Classroom discussions of field techniques and equipment capabilities followed by exercises with the data sets in cause-and-effect analysis all contributed to participatory education in the process of science. This project is presented as a case study of the value of engaging secondary and collegiate level students in planning, executing, and appraising a real world investigation that they can directly relate to. Tatsumura M. J.* Taylor G. J. Mouginis-Mark P. J. Tumuli and Tubes: Teaching Scientific Techniques Planetary and space science is the best way to teach basic chemistry, physics, and math. Einstein once said that "man is drawn to the mysterious and it is from that that we achieve true art and science." Planets and the processes that shape them are especially mysterious and fascinating to students, young and old, and because of this, planetary geology kindles interest that draws them further into the world of science. At the very least, they are engaged enough to learn how science works, a key ingredient in scientific literacy. This paper describes a project involving field measurements on Kilauea volcano, Hawaii, by a Geology 101 honors class. Annexstad J. O.* Melchior R. C. A Space Studies Curriculum for Small Colleges and Universities Space Science curricula have generally been the purview of the large and/or research-oriented universities with related departmental status. The advent of the NASA Space Grant Consortium Program has opened the field to a number of smaller, less research-oriented schools. At Bemidji State University, we have instituted a Space Studies minor at the undergraduate level that is designed to address the interdisciplinary nature of the space environment. This curriculum is targeted to supplement majors ranging from Geology to Elementary Education. Guinness E. A.* Arvidson R. E. Martin M. Dueck S. The Planetary Data System Educational CD-ROM The Planetary Data System (PDS) is producing a special educational CD-ROM that contains samples of PDS datasets and is expected to be released in 1993. The CD-ROM will provide university-level instructors with PDS-compatible materials and information that can be used to construct student problem sets using real datasets. The main purposes of the CD-ROM are to facilitate wide use of planetary data and to introduce a large community to the PDS. To meet these objectives the Educational CD-ROM will also contain software to manipulate the data, background discussions about scientific questions that can be addressed with the data, and a suite of exercises that illustrates analysis techniques. Students will also be introduced to the SPICE concept, which is a new way of maintaining geometry and instrument information. The exercises will be presented at the freshman through graduate student levels. With simplification, some of the material should also be of use at the high school level. French L. M. Planetary Science and Astronomy in the Middle School Classroom Astronomy and planetary science are of great interest to most middle and secondary school students. Many who think traditional biology, chemistry, physics, or earth science courses are dry or irrelevant are fascinated by the aesthetic and philosophical appeal of astronomy, and by the adventure of exploring other worlds. This high intrinsic interest makes planetary science and astronomy ideal media for teaching basic scientific and quantitative reasoning to students who might otherwise give up on science. During the period 1989-1992 the author integrated planetary science and astronomy into a fundamental course on physical science for eighth graders. Presented by Title Only Alexeev V. A. Peculiarities of Distributions of the Cosmic-Ray Exposure Ages of H Chondrites Falls and Finds The well-known peak in the distribution of the cosmic-ray exposure ages of H chondrites at 6-7 m.y. can be employed as a mark in the comparison of different populations of H chondrites. It is found that the age corresponding to the maximum of non-Antarctic falls is higher by 15+/-5% than for non-Antarctic finds. Antarctic H chondrites occupy an intermediate position. This effect is probably due to the process of weathering. Alexeev V. A. Dispersion of the Ratios of Cosmogenic Isotopes of Noble Gases in Chondrites of Different Cosmic-Ray Exposure Ages The dispersion of ratios of (He-3/Ne-21)c and (Ne-22/Ne-21)c depending on the cosmic-ray exposure ages of meteorites is analysed. The dispersion is increased with decrease of age. This effect may be stipulated by the presence of a more significant portion of meteorites of small preatmospheric sizes among meteorites of small radiation ages in comparison to meteorites of higher exposure age. Alexeev V. A. Ustinova G. K. Search for Effects of a Supernova Explosion 30-40 Thousand Years Ago in Chondrites The relative increases in Al-26 and Mn-53 equilibrium radioactivity of chondrites with different cosmic-ray exposure and terrestrial ages due to possible supernova explosion 30-40 thousand years ago have been calculated. Altemir D. A. Cold Press Sintering of Simulated Lunar Basalt In order to predict the conditions for which the lunar regolith may be adequately sintered, experiments were conducted in which samples of simulated lunar basalt (MLS-1), were pressed at high pressures and then heated in an electric furnace. This sintering process is called cold press sintering since the material is pressed at room temperature. Although test articles were produced that possessed compressive strengths comparable to that of terrestrial concrete, the cold press sintering process requires very high press pressures and sintering temperatures in order to achieve that strength. Additionally, the prospect of poor internal heat transfer adversely affecting the quality of sintered lunar material is a major concern. Therefore, the author concludes that cold press sintering will most likely be undesirable for the production of lunar construction materials. Altemir D. A. General Overview of an Integrated Lunar Oxygen Production/Brickmaking System On the moon, various processing systems would compete for the same resources, most notably power, raw materials, and perhaps human attention. Therefore it may be advantageous for two or more processes to be combined such that the integrated system would require fewer resources than separate systems working independently. The synergistic marriage of two such processes--lunar oxygen production and the manufacture of bricks from sintered lunar regolith--is considered. Arvidson R. E. Friedman L. D. Stadum C. J. Marslink The Marslink Project focuses on the development, dissemination, and evaluation of instructional materials about the composition, structure, and dynamics of the martian atmosphere, surface, and interior. A multiyear effort is underway so that middle and high school students can follow a full martian year of seasonal changes using data from NASA's Mars Observer spacecraft, and the Mars Observer Project has time to process and release the appropriate datasets. Monthly activity packets for students, teacher enhancement sessions, a teacher resource center, an on-call Mars data specialist, daily SPACELINK electronic bulletin board updates, and semi-annual NASA Select TV broadcasts will constitute the basis of the Marslink Project. Marslink would take advantage of: (a) the excitement of keeping abreast of activities within NASA's Mars Observer Mission from 1993 through 1996 and working with data acquired by Mars Observer and Viking spacecraft; (b) science expertise and involvement in the Mars Observer Mission at Washington University; (c) Development Team of middle and high school teachers, and research scientists who would work jointly at their home institutions and at summer writing workshops to construct activity packets; (d) dissemination capabilities associated with The Planetary Society and a number of supporting institutions; and (e) formal evaluations led by James Barufaldi, University of Texas. Monthly packets would be designed using a theme-oriented, hands-on approach for use primarily in earth science, physics, and chemistry courses. Selected material would also be of use in physical sciences and biology courses. Packets, where appropriate, would include comparisons between Mars and Earth. The Marslink Activity Book would be published by the National Science Teachers Association at the end of the project and would include background discussions, instructional materials, and project evaluations. Aubele J. C. Venus Small Volcano Classification and Description The high resolution and global coverage of the Magellan radar image dataset allows detailed study of the smallest volcanos on the planet. A modified classification scheme for volcanos less than 20 km in diameter is described. It is based on observations of all members of the 556 significant clusters or fields of small volcanos located and described by this author during data collection for the Magellan Volcanic and Magmatic Feature Catalog. This global study of approximately 10^4 volcanos provides new information for refining small volcano classification based on individual characteristics. Total number of these volcanos was estimated to be 10^5 to 10^6 planetwide based on pre- Magellan analysis of Venera 15/16, and during preparation of the global catalog, small volcanos were identified individually or in clusters in every C1-MIDR mosaic of the Magellan dataset. Basal diameter (based on 1000 measured edifices) generally ranges from 2 to 12 km with a mode of 34 km, and follows an exponential distribution similar to the size frequency distribution of seamounts as measured from GLORIA sonar images. This is a typical distribution for most size-limited natural phenomena, unlike impact craters, which follow a power law distribution and continue to infinitely increase in number with decreasing size. Using an exponential distribution calculated from measured small volcanos selected globally at random, we can calculate total number possible given a minimum size. The paucity of edifice diameters less than 2 km may be due to inability to identify very small volcanic edifices in this dataset; however, summit pits are recognizable at smaller diameters, and 2 km may represent a significant minimum diameter related to style of volcanic eruption. Guest et al. discussed four general types of small volcanic edifices on Venus: (1) small lava shields; (2) small volcanic cones; (3) small volcanic domes; and (4) scalloped margin domes ("ticks"). Steep-sided domes or "pancake domes," larger than 20 km in diameter, were included with the small volcanic domes. For the purposes of this study, only volcanic edifices less than 20 km in diameter will be discussed. This forms a convenient cutoff since most of the steep-sided domes ("pancake domes") and scalloped margin domes ("ticks") are 20 to 100 km in diameter, are much less numerous globally than are the smaller diameter volcanic edifices (2 to 3 orders of magnitude lower in total global number), and do not commonly occur in large clusters or fields of large numbers of edifices. Badjukov D. D. Petrova T. L. Vaporization by Shock Loading of Albite, Jadeite, and Pyrex Glass: Experimental Study Produced by shock experiments impact melts of albite, jadeite, and pyrex glass demonstrate a loss of both Na and Al relative to Si, which can be due to selective vaporization. It is suggested that the high volatility of Al is related to volatility of a Na-Al compound of proposed NaAlO2 composition. The similar loss of Al and Na seems to be possible during tektite-forming processes. Badjukov D. D. Koslov E. A. Zhugin Yu. N. Abakshin E. V. Shock Transformations in Quartzite We report results of studies of experimental shock metamorphism in a quartzite sample. Shock pressure increases in the experiment from the rim to the center of the bowl-shaped sample due to design of the recovery assembly. The section along an equatorial plane shows a progressive development of shock metamorphism. On the basis of our observations it is proposed that diaplectic glass can be a product of quenching of a melt. Basilevsky A. T. Weitz C. M. Regional Geology of the Vega Landing Sites: Tentative Results of Photogeologic Mapping The regional geology of the two Vega landing sites, where geochemical measurements on the venusian surface were made, was studied using Magellan C1-MIDRP imagery and MIT-processed altimetry for the large region which we will call the Vega region (22.90 degrees N to 22.90 degrees S and 154.12 degrees W to 211.12 degrees E). The results of the analysis were compiled in the form of a synoptic geologic map of about 1:10 m scale. Basu A. McBride K. Wentworth S. J. McKay D. S. Apollo 16 Core 60013/14 as a Product of Path I and Path II Regolith Evolution Processes We have made a petrographic study of 12 samples along the 62 cm profile of Apollo 16 core 60013/14, taken at Station 10' near the lunar module. The basic core description and maturity variations have been discussed earlier. This core is important because it enables a stratigraphic comparison with nearby cores 60001-7 and 60009/10. These cores form a triangle with sides 35-40m. FMR data show a maturity-match between 60009/10 and 60013/14. Unfortunately, segments of the core 60001-7 are too disturbed for any cm scale comparison. Bell J. F. III Morris R. V. Adams J. B. Changes in Hawaiian Palagonite Fe Mineralogy Associated with Thermal Alteration: Implications for Mars We have studied six Hawaiian palagonitic tephra samples (PH-l through PH-6) from a site where a Mauna Loa lava flow has partially embayed a Mauna Kea cinder cone. Tephra samples that were not affected by the lava flow (PH-5, PH- 6) consist of partially palagonitized coarse-grained glassy Hawaiitic particles whose iron mineralogy is dominated by nanophase ferric oxide (np-Ox) and olivine. Samples closest to the lava flow (PH-1 through PH-4) have been strongly altered and their iron mineralogy is dominated by np-Ox, magnetite, and hematite. This suite of samples has many spectral similarities to martian bright regions, and thus this localized thermal alteration event may be an analogue to similar processes on Mars that lead to the production of small amounts of crystalline ferric oxides within a poorly crystalline or amorphous palagonitic matrix. Bischoff A. Alkali-Granitoids as Fragments Within the Ordinary Chondrite Adzhi-Bogdo: Evidence for Highly Fractionated, Alkali-Granitic Liquids on Asteroids Adzhi-Bogdo is an ordinary chondrite regolith breccia (LL3-6) that fell October 30, 1949, in Gobi Altay, Mongolia. The rock consists of submillimeter- to centimeter-sized fragments embedded in a fine-grained clastic matrix. The breccia contains various types of clasts, some of which must be of foreign heritage. Based on chemical compositions of olivine some components have to be classified as L-type. Components of the breccia include chondrules, impact melts (some are K-rich, similar to those found in other LL-chondrites, highly recrystalized rock fragments ("granulites"), pyroxene-rich fragments with achondritic textures, and alkali-granitoidal fragments that mainly consist of K- feldspar and quartz or tridymite. Probably, this is the first report on granitoids from asteroids. It can be ruled out that these fragments represents huge rock assemblages of the parent body like granites do on Earth. Therefore, to avoid misunderstandings, these rocks will be designated as granitoids. Black S. Macdonald R. Fallick A. E. Kelly M. Stable Isotope Analyses of the Peralkaline Volcanics Gregory Rift Valley, Kenya Delta^18O analyses of the Naivasha rhyolites, basalts, Menengai trachytes, and the Yatta phonolite are presented together with D/H analyses of the Naivasha rhyolites. Delta^18O results vary from 5.7 per mil to 8.90 per mil, which is within the reported range of delta^18O analyses for continental volcanics. Closure temperatures calulated from the basalts and rhyolites show equilibration to be at magmatic temperatures. D/H values range from -40 per mil to -148 per mil indicating that the rhyolites have undergone large scale degasssing. Black S. Macdonald R. Kelly M. 230Th-238U Series Disequilibrium of the Olkaria Rhyolites, Gregory Rift Valley, Kenya: Residence Times 67% of the rhyolites analyzed show U-Th disequilibrium, ranging from 27% excess thorium to 36% excess uranium. Internal and whole rock isochrons give crystallization/formation ages between 65 ka and 9 ka; in every case these are substantially older than the eruptive dates. The residence times of the rhyolites (U-Th age minus the eruption date) have decreased almost linearly with time, from 45 ka to 7 ka, suggesting a possible increase of activity within the system related to increased basaltic input. The long residence times are mirrored by large 222Rn fluxes from the centers, which cannot be explained by larger U contents. Black S. Macdonald R. Kelly M. 230Th-238U Series Disequilibrium of the Olkaria Rhyolites Gregory Rift Valley, Kenya: Petrogenesis Positive correlations of (^238U/^230Th) vs. Th show the rhyolites to be products of partial melting. Positive correlations of U and Cl and U and F show that the U enrichment in the rhyolites is associated with the halogen contents, which may be related to the minor phenocryst phase fractionation. Instantaneous Th/U ratios exceed time-integrated Th/U ratios providing further evidence of the hydrous nature of the Olkaria rhyolite source. Black S. Macdonald R. Kelly M. 230Th-238U Series Disequilibrium of the Olkaria Basalts Gregory Rift Valley, Kenya: Petrogenesis Strong mixing trends on a (^230Th/^232Th) vs. Th diagram show that the basalts are mixed magmas that have undergone interaction with the crust. Instantaneous Th/U ratios are less than time integrated ones but these exceed the Th/U ratios in the MORB and OIB sources. This indicates that the mantle may have undergone some metasomatic fluxing; crustal contamination of the basalts will also enhance these ratios. Black S. Macdonald R. Kelly M. 230Th-238U Series Disequilibrium of the Olkaria Basalts, Gregory Rift Valley, Kenya U-Th disequilibrium analyses of the Naivasha basalts show a very small (^238U/^230Th) ratios that are lower than any previously analysed basalts. The broadly positive internal isochron trend from one sample indicates that the basalts may have source heterogeneities; this is supported by earlier work. Bogard D. D. Garrison D. H. Noble Gases in LEW88516 Shergottite: Evidence for Exposure Age Pairing with ALH77005 LEW88516 contains excess amounts of radiogenic 40Ar and 129Xe that are slightly greater than those observed in ALH77005, but in the same relative proportion as much larger excesses observed in EET79001. Cosmogenic 3He and 21Ne abundances in LEW88516 are very similar to those for ALH77005 and are consistent with a common initiation of cosmic ray exposure ~2.8 Myr ago for four of the five shergottites. Exposure of these four shergottites could have been under different shielding in a common meteoroid, or in several objects. Boss A. P. Graham J. A. Chondrule Formation by Clumpy Accretion onto the Solar Nebula Chondrule textures and compositions appear to require rapid heating of precursor grain aggregates to temperatures in the range 1500 K to 2100 K, cooling times on the order of hours, and episodic and variable intensity events in order to produce chondrule rims and chemically distinct groups. Nebula shock waves have been proposed by Hood and Horanyi as a physical mechanism that may be capable of meeting the meteoritical constraints. Motivated by astronomical observations of the close environments of young stars, we suggest that the source of the nebula shock waves may be clumpy accretion onto the solar nebula--that is, episodic impacts onto the nebula by discrete cloud clumps with masses of at least 10^22 g. If the cloud clumps are massive enough (roughly 10^26 g), the resulting shock wave may be able to propagate to the midplane and process precursor aggregates residing in a dust subdisk. Brakenridge G. R. Modern Shelf Ice, Equatorial Aeolis Quadrangle, Mars As part of a detailed study of the geological and geomorphological evolution of Aeolis Quadrangle, I have encountered evidence suggesting that near-surface ice exists at low latitudes and was formed by partial or complete freezing of an inland sea. The area of interest is centered at approximately -2 degrees, 196 degrees W. It lies within the "Younger channel and floodplain material, undivided" unit mapped at this location by the Greeley and Guest (1987) 1:15,000,000 geological map. The area also lies within one of the several broad regions of low thermal inertia discussed by Paige (1992) as potential locations of stable ground ice at low latitudes. As seen in a suite of Viking Orbiter frames obtained at a range of ~600 km (frames 725A35-A19), the plains surface at this location is very lightly cratered or uncratered, and it is thus of late Amazonian age. It exhibits an unusual pattern of intersecting, very-low-relief linear or sinuous ridges that may define ridge-interior, polygon-shaped, mainly angular but sometimes rounded ice cakes and ice floes. The ice, if it exists, is now frozen together into extensive composite floes that exhibit features analogous to terrestrial shelf ice, such as shear zones and pressure ridges. Scattered ~10-km-diameter craters form isolated, still- subaerial uplands that are more heavily cratered: these uplands are apparently older than the surrounding "ice floes" and are embayed by the ice-covered sea or the grounded ice. Extant topographic data indicate that the Amazonian plains at this location occupy a trough whose surface lies at least 1000 m below the Mars datum. A reasonable hypothesis is that quite recent surface water releases, perhaps associated with final evolution of large "outflow chasms" to the south, but possibly from other source areas, filled this trough, that ice floes formed almost immediately, and that either grounded ice or an ice-covered sea still persists. In either case, the thin (a few meters at most) high-albedo, low-thermal inertia cover of aeolian materials was instrumental in allowing ice preservation, and at least the lower portions of this dust cover may be cemented by water ice. Detailed mapping using Viking stereopairs and quantitative comparisons to terrestrial shelf ice geometries are underway. Brandt D. Durrheim R. J. Reimold W. U. Geophysical Signature of the Pretoria Saltpan Impact Structure and a Possible Satellite Crater The Pretoria Saltpan Crater is located in the southern portion of the Bushveld Igneous Complex, some 40 km north-northwest of Pretoria, South Africa. An origin by impact for this crater structure was recently confirmed. The results of the only gravity reconnaissance carried out over the crater to date failed to support an impact origin. With the aid of recent results obtained from a central drill- core, it was necessary to carry out more geophysical work that would include a gravity profile of higher resolution. A second, smaller, circular depression(about 400 m in diameter) to the southwest of the crater is suggestive of a twin crater.This site had never been investigated, and thus various geophysical surveys were conducted. Britt D. T. The Spectral Effects of Subsolidus Reduction of Olivine and Pyroxene The surfaces of atmosphereless bodies are subjected to a variety of chemical, thermal, accretionary, and shock processes related to their regolith environment. These processes are responsible for a number of alterations that occur in regoliths. Alterations include particle size commutation, implantation of solar wind gases, formation of agglutinates, spectral darkening, and, in the lunar case, the development of the very strong red continuum slope in the visible and near infrared spectra. A great deal of work has pointed to the role of agglutinates as the principal agent for darkening and reddening the lunar soil. The measures of regolith maturity are strongly linked to the accumulation of agglutinates. Recent work has suggested that the finest fractions of agglutinitic glass are major source of the spectral red slope. In particular, the red slope is most strongly associated with the agglutinitic glasses that are rich in blebs of sub-micron sized metal particles. It is thought that these metal particles, because of their size and scattering efficiently relative to the wavelength of light, are responsible for the red continuum slope. This fine fraction of metal particles is produced primarily by reduction of Fe^2+ from silicates. One mechanism for the reduction process is the reaction of solar implanted wind protons with the regolith soil during impact events. In this case the presence of hydrogen creates a reducing environment and the thermal pulse from the impact greatly speeds the reaction kinetics. To explore other reducing and thermal environments a series of experiments were done using samples in evacuated capsules buffered by Tantalum and heated to subsolidus temperatures. Brown C. D. Grimm R. E. Viscous Relaxation of the Moho Under Large Lunar Basins An isoviscous layer-isoviscous halfspace viscous relaxation model is used to compare relaxed Moho profiles of the lunar Orientale and Serenitatis Basins to the currently observed mantle uplifts of older basins. A change in the relative viscosities of the layer and halfspace does not cause any difference in the extent of relaxation with time, as long as the layer viscosity is greater than the halfspace viscosity. Therefore the Moho relaxation behaves as if the mantle is inviscid. The relaxed Orientale starting profile generally offers a poor fit to the older basins than the relaxed Serenitatis starting profile, although some of the misfit can be attributed to the large differences in local crustal thickness around many basins. The relaxed Serenitatis profile provides a good fit to the Nubium and Tranquilitatis uplifts, while the relaxed Orientale profile is best compared to Nectaris. Using the Orientale-Nectaris fit, an approximate upper bound on the mantle viscosity is calculated. This estimate, 10^21 Pa-s, agrees with the commonly assumed value. It is concluded that viscous relaxation is a probable mechanism for the subdued mantle relief under large lunar basins, and that the results demonstrate a significant lateral heterogeneity in the nearside rheology during the pre-Nectarian and Nectarian. Burba G. A. Yana Ring Structure, North-Eastern Siberia: A Possible Counterpart of Coronae on Venus An arch of the Verkhoyansky Range in northeast Siberia, Russia, is considered here as the western and southern parts of the rim of a large ring structure, referred to hereafter as Yana Ring Structure (YRS), named after the Yana River. It's general appearance is similar to coronae on Venus and could be considered as the terrestrial counterpart of coronae. Burba G. A. Bobina N. N. Shashkina V. P. Russia's Contribution to Regional Geologic Mapping of Venus: 1992 Progress Report Twenty-six sheets of geologic map in Magellan C1 format were produced by six geologists and three cartographers in Russia during 1992. Forty-five more sheets are in progress. The work is coordinated by the Vernadsky Institute. Burba G. G. Jr. Meshcherskaya V. A. Landscape and Geomorphic Survey of Zhamanshin Area, Northern Kazakhstan: Preliminary Report on 1992 Field Trip Data The geographical expedition to the Zhamanshin Crater area in June 1992 resulted in a landscape and geomorphic survey of this impact structure. Forty-two observational points in a 300-sq-km area provide new data on the geographical processes in the impact crater. The data obtained can be applied to the search for unknown impact structures anywhere through the traces of such features in landscapes. Bustin R. Gibson E. K. Jr. Wentworth S. J. Volatiles in Fourteen Interplanetary Dust Particles: A Comparison with CI and CM Chondrites In an attempt to classify the nature of volatiles within interplanetary dust particles (IDPs) we have studied 14 IDPs using a laser microprobe/mass spectrometer (LM/MS)technique. Volatile abundances and distributions found for the IDPs are compared with those measured for carbonaceous chondrites in order to determine if the IDPs are related to the parent bodies of these primitive meteorites. Chen J. H. Wasserburg G. J. Papanastassiou D. A. Th and U Abundances in Chondritic Meteorites We present new precise analyses of ^232Th/^238U in CI and CM meteorites. The relative abundance of these nuclides is important in estimates of the age of r-process elements. The cosmochronology based upon the ^232Th/^238U ratio (kappa) as was first proposed by Fowler and Hoyle depends on the precise determinations of these two different elements in meteorites and on the production ratios. Both parameters are subject to substantial errors. Recent recalculations of this chronology have used selected values from compilations but do not adequately address the errors in terms of a reliable data base. Morgan and Lovering provided extensive neutron activation analyses for ordinary chondrites, which yield an average kappa of 3.6+-0.4. Their work on carbonaceous chondrites showed a wide range in kappa from 2 to 6. More recent investigations by isotopic dilution have established: a)highly variable kappa from 2.7 to 11 in Allende Ca-A1-rich inclusions and a value of 3.6 in the Orgueil CI1 chondrite; b)a range from 2.71 to 6.63 for 7 L-type chondrites and a range from 2.7 to 4.4 for 6 L, H, and LL chondrites. The compilations of Anders and Ebihara and Anders and Grevesse are based on a single Th analysis of Orgueil. While we consider their data selection to be reasonable, we do not consider this as an adequate basis for cosmochronological calculations. Cintala M. J. Smith S. Horz F. Impact Comminution of Glasses: Implications for Lunar Regolith Evolution Glasses are important parts of every lunar regolith sample, often exceeding the mass of any other single component. Thus, a complete understanding of regolith evolution must include the behavior of particulate glasses under impact bombardment. In ongoing laboratory investigations of regolith evolution, a basalt hornfels and three glasses (obsidian, tephra, and a lead-rich, synthetic glass) each were subjected to a total of 25 impacts by stainless-steel spheres at a nominal 1.4 km s^-1. Results show that the tephra and obsidian were the weakest, while the basalt hornfels was the strongest. Compared to the four monomineralic targets used in an earlier series of experiments, the tephra and obsidian were weaker than even quartz and feldspar, the least resistant of the minerals. The hornfels was more resistant than either of the strongest minerals in the experiments, olivine and pyroxene. While the hornfels possessed little or no interstitial glass, the abundant, relatively weak plagioclase would have behaved similarly to glass. We would expect that a glassy basalt would exhibit a similar behavior to the hornfels, and are in the process of testing the hypothesis with another experimental series. Should the glassy basalt be stronger than the pure glasses, it would indicate that the sources of the glass component in the finest fractions of regoliths are dependent on the state of the glass itself (e.g., agglutinates, glass droplets, interstitial glass in basalts or impact melts, etc.) Clifford S. M. Mars: The Initial Emplacement of Ground Ice in Response to the Thermal Evolution of its Early Crust Given the geomorphic evidence for the widespread occurrence of water and ice in the early martian crust, and the difficulty involved in accounting for this distribution given the present climate, it has been suggested that the planet's early climate was originally more Earth-like, permitting the global emplacement of crustal H20 by direct precipitation as snow or rain. The resemblance of the martian valley networks to terrestrial runoff channels, and their almost exclusive occurrence in the planet's ancient (~4 b.y. old) heavily cratered terrain, is often cited as evidence of just such a period. An alternative school of thought suggests that the early climate did not differ substantially from that of today. Advocates of this view find no compelling reason to invoke a warmer, wetter period to explain the origin of the valley networks. Rather, they cite evidence that the primary mechanism of valley formation was groundwater sapping, a process that does not require that surface water exist in equilibrium with the atmosphere. However, while sapping may successfully explain the origin of the small valleys, it fails to address how the crust was initially charged with ice as the climate evolved towards its present state. Therefore, given the uncertainty regarding the environmental conditions that prevailed on early Mars, the initial emplacement of ground ice is considered here from two perspectives: (1) that the early climate started warm and wet, but gradually cooled with time, and (2) that it never differed substantially from that of today. Clifford S. M. The Role of the Geothermal Gradient in the Emplacement and Replenishment of Ground Ice on Mars Knowledge of the mechanisms by which ground ice is emplaced, removed, and potentially replenished are critical to understanding the climatic and hydrologic behavior of water on Mars, as well as the morphologic evolution of its surface. Because of the strong temperature dependence of the saturated vapor pressure of H20, the atmospheric emplacement or replenishment of ground ice is prohibited below the depth at which crustal temperatures begin to monotonically increase due to geothermal heating. In contrast, the emplacement and replenishment of ground ice from reservoirs of H20 residing deep within the crust can occur by at least three different thermally driven processes, involving all three phases of water. In this regard, Clifford (1991) has discussed how the presence of a geothermal gradient as small as 15 K km^-l can give rise to a corresponding vapor pressure gradient sufficient to drive the vertical transport of 1 km of water from a reservoir of groundwater at depth to the base of the cryosphere every 10^6-10^7 years. This abstract expands on this earlier treatment by considering the influence of thermal gradients on the transport of H20 at temperatures below the freezing point. Cloutis E. Olivine-rich Asteroids, Pallastic Olivine and Olivine-Metal Mixtures: Comparisons of Reflectance Spectra The recent acquisition of high-resolution 0.3-2.6 micrometer reflectance spectra of a number of olivine-rich asteroids permits analysis of their surface compositions to be made on the basis of new and existing laboratory spectral data for pallasitic olivines and olivine-metal mixtures. Analysis of the spectral data for the latter has revealed a number of spectral parameters that can be used to constrain olivine and metal abundances, grain sizes, and olivine compositions. Colson R. O. Jolliff B. L. Crystal-Chemistry and Partitioning of REE in Whitlockite Partitioning of REE in whitlockite is complicated by the fact that two or more charge-balancing substitutions are involved and by the fact that concentrations of REE in natural whitlockites are sufficiently high that simple partition coefficients are not expected to be constant even if mixing in the system is completely ideal . The present study combines preexisting REE partitioning data in whitlockites with new experiments in the same compositional system and at the same temperature (~1030 degrees C) to place additional constraints on the complex variations of REE partition coefficients and to test theoretical models for how REE partitioning should vary with REE concentration and other compositional variables. With this dataset, and by combining crystallographic and thermochemical constraints with an SAS simultaneous-equation best-fitting routine, it is possible to infer answers to such questions as "what is the speciation on the individual sites Ca(B), Mg, and Ca(IIA) (where the ideal structural formula is Ca(B)18Mg2Ca(IIA)2P14O56)," "how are REE charge-balanced in the crystal," and "is mixing of REE in whitlockite ideal or nonideal." This understanding is necessary in order to extrapolate derived partition coefficients to other compositional systems and provides a broadened understanding of the crystal chemistry of whitlockite. Connolly H. C. Jr. Hewins R. H. The Experimental Production of Matrix Lumps Within Chondrules: Evidence of Post-Formational Processes The processes that acted upon chondrules after their formation are as important as clues to the nature of the early solar nebula as are the exact processes that formed chondrules. Recent experiments have studied the rim forming processes and the effects the processes have on chondrules. We present information on how matrix inclusions found within chondrules may have been formed and the potential usefulness of this information. Cresswell R. G. Beukens R. P. Rucklidge J. C. Separation of Spallation and Terrestrial 14C in Chondrites Weathering products and contamination severely hamper our ability to accurately measure the C-14 spallation component in meteorites, but can give insights into a sample's terrestrial history. A procedure has been developed to measure the C-14 in these components using CO and CO2 separations from temperature extractions from 200 to 500 mg of material. The Bruderheim (L6) chondrite was chosen as a standard following the practice of previous workers, cross-checked against Peace River (L6), Abee (EH4), and Juvinas (EUC). Low- temperature fractions (<900 degrees C) give C-14 signatures consistent with a modern terrestrial C-14 source; melt fractions show elevated levels attesting to a spallogenic origin. Higher yields of CO in the melt fraction are less affected by the low levels of experimental contamination than the CO2. This fraction gave a mean CO:CO2 ratio in Bruderheim of 81.6+-7.7; the ratio of the spallation component is 79.8+-8.1. These values suggest equilibrium release of gases on the olivine-silica-pyroxene-iron buffer. This is corroborated by approximately equal release of the two components at 900 degrees C. The chondrites gave an average saturation level of 54.3+-2.9 dpm/kg; the achondrite gave 49.6+-2.0 dpm/kg. No clear correlation with oxygen content is apparent, though shielding effects have yet to be evaluated. Croft S. K. Tectonics on Triton Tectonic features on Triton have been mapped as part of a larger study of the geology of Triton. Few purely tectonic structures are found on Triton: some grabens and possibly some compressive ridges. However, most of the other structures seen (primarily cryovolcanic in origin) exhibit tectonic control. A regional tectonic network has dominant orientations of: N-S, E-W, NE-SW, and NW-SE. Most of the orientations are consistent with tidal deformations related to Triton's decreasing orbital radius. Localized quasi-concentric patterns may be due to interior processes such as mantle plumes. Crumpler L. S. Comparison of the Distribution of Large Magmatic Centers on Earth, Venus, and Mars Volcanism is widely distributed over the surfaces of Venus, Earth, and Mars. Anomalous centers of magmatic activity occur on each planet and are characterized by evidence for unusual concentrations of volcanic centers, long-lived activity, unusual rates of effusion, extreme size of volcanic complexes, compositionally unusual magmatism, and evidence for complex geological development. The purpose of this study is to compare the characteristics and distribution of these magmatic anomalies on Earth, Venus, and Mars in order to assess these characteristics as they may relate to global characteristics and evolution of the terrestrial planets. Crumpler L. S. Head J. W. Aubele J. C. Large Volcanoes on Venus: Examples of Geologic and Structural Characteristics from Different Classes Large volcanoes (>100 km) characterized by radial lava flows and similar evidence for a topographic edifice are widely distributed over the surface of Venus and are geologically diverse. Based on the global identification of more than 156 examples and preliminary geologic mapping, large volcanoes range from those characterized geologically as simple lava edifices to those bearing evidence of complexly developed volcanic and structural histories. Many large volcanoes exhibit characteristics transitional to other large magmatic center types such as coronae and novae. In this study we examine the geology and structure of several type examples of large volcanoes not addressed in previous studies, which are representative of several of the morphological classes. Crumpler L. S. Aubele J. C. Head J. W. Synthesis of Global Thematic Mapping, Venus: Geologic Correlations/Questions for the Magellan Gravity Mission An important characteristic of the Magellan mission and the resulting data is that it is both global in coverage as well as extremely high in resolution. As a result, global questions may be addressed based on the constraints of extremely detailed geological evidence. An important first step in the task of interpreting the global geological record will be a synthesis of the primary geological characteristics and their relationship to significant questions. The following is a brief synthesis based on a comparison of mapped distributions, initial interpretations, and some questions that these results raise. Crumpler L. S. Aubele J. C. Head J. W. The Magellan Volcanic and Magmatic Feature Catalog A catalog summarizing the location and characteristics of 1663 volcanic and magmatic centers identified in Magellan radar images of the surface of Venus is in final preparation to be submitted as a Geological Society of America Special Paper. The following is a brief summary preview of the contents and methods used in assembling the final data set, the organization of the catalog, and other notes of interest to potential users. Davis A. M. Brownlee D. E. Iron and Nickel Isotopic Mass Fractionation in Deep-Sea Spherules Magnetite-wustite spherules collected from deep-sea sediments are thought to have originally been Fe-Ni metal particles at the top of the atmosphere that were oxidized and melted during entry into the Earth's atmosphere. Some likely sources for the metal particles are Fe-Ni interplanetary dust particles (IDPUs) and metal or sulfide from stony IDPUs that separated after melting. Davis et al. [1991] reported that four of these spherules are enriched in the heavy isotopes of Fe, with enrichments of 8-23 permil/amu. Herzog et al. [1992] analyzed a separate collection of spherules and found enrichments in the heavy isotopes of Ni of 10-25 permil/amu. We have developed a technique for analysis of both Fe and Ni isotopes on the same ion microprobe spot and have applied this technique to a number of deep-sea spherules in order to better understand the processes leading to isotopic mass fractionation. Eight spherules show Fe and Ni isotopic mass fractionation, with Fe and Ni enriched in the heavy isotopes by 10-19 permil/amu and 4-32 permil/amu, respectively. If the mass fractionations are due to Rayleigh fractionation during evaporation, these spherules lost 76 to 94% of their original mass. Davis P. A. Tanaka K. L. Small Volcanoes in Tempe Terra, Mars: Their Detailed Morphometry and Inferred Geologic Significance The Tempe Terra province contains a variety of volcanic landforms that range in size from small vents (less than 10 km in diameter) to moderately sized volcanoes (150 km diameter). The volcanoes are aligned along the dominant northerly and northeasterly trends of the faults in this region and many of the volcanoes occur on grabens. Some workers have speculated on the nature of some of the volcanoes on the basis of their general morphology, shadow- measurement height, lateral dimensions, and geologic setting. As part of a larger study, we have obtained detailed photoclinometric profiles across five of the more conspicuous small volcanoes in the Tempe Terra region. From these data, we extracted for each volcano its flank width and edifice height and the width and depth of its summit crater. We statistically compared these dimensions for each volcano with a set of average dimensions for each type of terrestrial volcanic feature listed in Pike and Clow (1981) U. S. Geol. Surv. Open-File Rep. 81-1038. These comparisons indicate that the morphometries of the martian volcanoes 1, 2, and 3 most closely match Earth's cratered basaltic lava shields, and the morphometries of volcanoes 4 and 5 are similar to those of terrestrial basaltic tuff rings. Davis P. A. Tanaka K. L. Golombek M. P. Shallow Crustal Discontinuities and Graben and Scarp Formation in the Tharsis Region of Mars We recently obtained photoclinometric profiles across all simple grabens and erosional landforms (e.g., troughs, pits, wall-valley heads, and scarps that are bounded above and below by flat surfaces) that occur within Tempe Terra. These data, together with similar data that we obtained for Syria, Sinai, and Lunae Plana and the Alba Patera region, allow regional examination of shallow crustal discontinuities between latitude 30 degrees S and 50 degrees N and longitude 50 degrees W and 112 degrees W. The profile for each simple graben was used with an appropriate structural model to estimate the depth to the base of the faulted layer. The depths of erosional wall scarps may also indicate the depths of mechanical discontinuities such as a local lithologic or cryospheric boundary. Examination of these data indicates a surprisingly consistent set of shallow crustal discontinuities for the Tharsis region at depths of 0.4-0.6 km, 1.0-1.4 km, and 2 km; the maximum depth of the features in most study areas appears to be about 4 km. Dawson C. B. Crumpler L. S. Characteristics of Arachnoids from Magellan Data Current high-resolution Magellan data enable more detailed study of arachnoids, first identified by Barsukov et al. as features characterized by a combination of radar-bright, concentric rings and radiating lineations, named "arachnoids" on the basis of their spider and weblike appearance. Identification of arachnoids in Magellan data has been based on SAR images, in keeping with the original definition. However, there is some overlap by other workers in identification of arachnoids, coronae (predominantly bright rings), and novae (predominantly radiating lineations), as all of these features share some common characteristics. Features used in this survey were chosen based on their classification as arachnoids in Head et al.'s catalog and on SAR characteristics matching Barsukov et al.'s original definition. 259 arachnoids have been currenlty identified on Venus, all of which were considered in this study. Fifteen arachnoids from different regions, chosen for their "type" characteristics and lack of deformation by other regional processes, were studied in depth using SAR and altimetric data to map and profile these arachnoids in an attempt to better determine their geologic and altimetric characteristics and possible formation sequences. Delano J. W. Oxidation State of the Earth's Upper Mantle During the Last 3800 Million Years: Implications for the Origin of Life A popular, as well as scientifically rigorous, scenario for the origin of life on Earth involves the production of organic molecules by interaction of lightning (or other forms of energy) with a chemically reducing atmosphere in the early history of Earth . Experiments since the 1950s have convincingly demonstrated that the yield of organic molecules is high when the atmosphere contains molecular hydrogen, methane, ammonia, and water vapor. Additional work has also shown that such a highly reducing atmosphere might not, however, have been sufficiently long-lived in the presence of intense solar ultraviolet radiation for life to have formed from it. One way of maintaining such an atmosphere would be to have a continual replenishment of the reduced gases by prolonged volcanic outgassing from a reducing Earth's interior. The length of time that this replenishment might need to continue is in part constrained by the flux of asteroids onto the Earth's surface containing sufficient energy to destroy most, if not all, life that had developed up to that point in time. If a reducing atmosphere is a key ingredient for the origin of life on Earth, the time of the last environmental sterilization due to large impacts would be an important constraint. In a deep marine setting (e.g., hydrothermal vent), the last global sterilization might have occurred at 4200-4000 Ma. On the Earth's surface, the last global sterilization event might have occurred at 4000-3700 Ma. If these are meaningful constraints, how likely is it that a reducing atmosphere could have survived on the Earth until about 3800 Ma ago? Due to the importance of replenishing this atmosphere with reducing components by volcanic outgassing from the mantle, geochemical information on the history of the mantle's oxidation state would be useful for addressing this question. Geochemical and experimental data suggest that extrusive mafic volcanics derived from the upper mantle have had oxidation states near the fayalite- magnetite-quartz buffer throughout the last 3800 Ma. At magmatic temperatures, the gases released from volcanos having this oxidation state would have been, as they are today, composed dominantly of carbon dioxide and water vapor, and hence would not contain the ingredients for maintaining a reducing atmosphere. Consequently, geochemical data do not favor the survival of a reducing atmosphere until about 3800 Ma. Alternative venues and pathways for the origin of life need to be investigated (e.g., hydrothermal vents along oceanic ridges). Delano J. W. Liu Y.-G. Schmitt R. A. Soret Diffusion: A Possible Cause of Compositional Heterogeneity Within Tektites Although a terrestrial origin for tektites is well established on geochemical evidence, the physical processes involved in tektite formation have been largely ignored by geochemists and petrologists. For example, two observations that potentially bear on the physics of their origin are the following: (1) The flange of an australite is often more heterogeneous than its core; and (2) microtektites are compositionally more diverse than tektites. Dikov Yu. P. Gerasimov M. V. Yakovlev O. I. Wlotzka F. The Correlation of Alkalis and Aluminum During High-Temperature Volatilizationof Albite and Nepheline Our previous study of high-temperature evaporation of mafic targets has shown a systematic enrichment of newly formed condensates in silicon and alkalis and depletion in refractory elements (Al, Ca, Ti). This effect, which is consistent with the volatility of the elements, was also tested for several experiments with acidic targets. In one case it was the impact evaporation of granite targets using a light-gas gun facility. Analyses of the condensate collected during the impact show that it was enriched in Al and Na so that the composition of the condensate became close to nepheline. In the work of Nagahara an incongruent evaporation of alkali feldspars under stationary conditions has been also proposed. These discoveries required a more detailed study of high-temperaure evaporation of monomineralic Al-silicate targets. Dolginov A. Z. Polarity Reversals and Tilt of the Earth's Magnetic Dipole There is evidence that the terrestrial magnetic field is connected with the Earth's mantle: (1) there are magnetic anomalies that do not take part in the westward drift of the main field, but are fixed with respect to the mantle; (2) the geomagnetic pole position flips in a particular way by preferred meridional paths during a reversal; and (3) magnetic polarity reversals are correlated with the activation of geological processes. These facts may be explained if we take into account that a significant horizontal temperature gradient can exist in the top levels of the liquid core because of the different thermoconductivity of the different areas of the core-mantle boundary. This temperature inhomogeneity can penetrate in the core because fluxes along the core boundary (the thermal wind) can be strongly suppressed by a small redistribution of the chemical composition in the top of the core (see Dolginov this volume). The nonparallel gradients of the temperature, density and composition on the top of the core create a curled electric field that produces a current and a magnetic field. This seed field can be amplified by motions in the core. The resulting field does not forget the seed field distribution and in this way the field on the Earth Dolginov A. Z. Does the Thermal Wind Exist Near the Earth's Core Boundary? The temperature distribution in the Earth's core determines many important processes such as convective motion, magnetic field generation, matter exchange between the core and the mantle, the thermal flux, etc. This distribution depends on conditions on the core-mantle boundary and on the distribution of the thermal conductivity in the mantle. Seismic tomography shows that large horizontal temperature and compositional gradients exist at the core-mantle boundary. The simple assumption that these inhomogeneities are extended into the top of the core contradicts the common opinion that the horizontal temperature gradient (the thermal wind) wipes them out in a short time. However, this conclusion has been obtained without taking into account that the core volume is closed and the motion, if started, can lead to a small redistribution of composition that stops this motion. Dolginov Sh. Sh. Precession of Uranus and Neptune and Their Magnetic Fields The strength of the dipole magnetic fields of a planet Hp,can be estimated, relative to that of the Earth at the epoch of the observation, He. Dolginov Sh. Sh. On the Unique Structure of the Magnetic Fields of Uranus and Neptune The magnetic fields of Uranus and Neptune, which have comparable dipole, quadrupole, and octupole harmonics, are unique in the present-day solar system, but they resemble the geomagnetic field at the epochs of excursions and reversals known from paleomagnetic data. Dorofeyeva V. A. Makalkin A. B. Mironenko M. V. Vityazev A. V. Recondensation of Chondritic Material in the Early Solar System: Results of Thermodynamic Simulation We have performed a thermodynamic simulation of recondensation of evaporated meteoritic material. We suggested that evaporation and recondensation occurred in impact events during intercollisions of planetesimals during the early evolution of the solar system. The source material adopted for our modeling are chondrites CI Orgueil and H5 Richardton, as the examples of the opposite extremes regarding volatile content and oxidation state. We calculated equilibrium mineral compositions of the closed systems of the Orgueil's and Richardton's elemental composition at the P-T conditions characteristic for the explosion cloud formed at a planetesimal collision. These P-T conditions are 10^-4 bar and 1500 and 2000 K. Before discussing the results in more detail, we only mention the difference in the oxidation state of iron obtained for both source materials, with respect to the system of solar composition. Dorofeyeva V. A. Makalkin A. B. Vityazev A. B. On High-Temperature Formation of Iron-rich Olivine in the Early Solar System Kinetic restriction on the formation of fayalite-rich olivine in equilibrium with the cooling solar-composition gas appears to exist at lower temperature ~500 K. We offer a high-temperature formation mechanism (different from simple condensation) that operates at high hydrogen depletion (up to 300 times) relative to solar abundance. We show how the necessary depletion rate decreases with temperature (thermodynamical equilibrium is suggested). Consequences for planet formation are considered. Drake D. Analytic Expression for Epithermal Neutron Spectra Amplitudes as a Function of Water Content The epithermal portion of an equilibrium neutron spectrum in a planetary body is a sensitive function of the water content of its material. The neutrons are produced at high energies but moderate by elastic and inelastic scattering until they are captured by surrounding nuclei or escape. This paper presents an analytic expression for the epithermal amplitude as a function of water content and compares it to computer generated amplitudes. Duke M. B. Morrison D. A. Exploration Planning in the Context of Human Exploration and Development of the Moon It is widely believed that the next step beyond low Earth orbit in attaining the United States' stated goal of "expanding human presence beyond the Earth" should be to reestablish a lunar capability, building on the Apollo program, and preparing the way for eventual human missions to Mars. The Moon offers important questions in planetary and Earth science, can provide a unique platform for making astronomical observations of high resolution and sensitivity, and can be in the development path for unlocking resources of the inner solar system to support space activities and return benefits to Earth. NASA's Office of Exploration has undertaken the planning of future lunar exploration missions, with the assistance of the Solar System Exploration Division in matters dealing with the quality of scientific data and the manner in which it will be made available to the scientific community. The initial elements of the proposed program include the Lunar Scout missions, which consist of two small identical spacecraft in polar orbit around the Moon, which can accomplish most of the objectives associated with previous proposals for Lunar Polar Orbiters (Lunar Observers). These missions would be followed by "Artemis" landers, capable of emplacing up to 200-kg payloads anywhere on the Moon. In addition, the exploration program must incorporate data obtained from other missions, including the Galileo lunar flybys, the Clementine high orbital observations, and Japanese penetrator missions. In the past year, a rather detailed plan for a "First Lunar Outpost (FLO)" which would place 4 astronauts on the lunar surface for 45 days has been developed as a possible initial step of a renewed human exploration program. In the coming year, the FLO concept will be reviewed and evolve to become more highly integrated with planning for the initial human exploration of Mars, which could come perhaps five years after the reestablishment of lunar capability. Both programs could benefit from the common development of systems and subsystems, where that is sensible from a performance perspective. Eugster O. Hofmann B. Niedermann S. Thalmann Ch. On the Origin of 4He and 40Ar in Natural Gold In a first report on our investigations of noble gases in native gold we demonstrated that placer gold contains an excess of radiogenic 4He and 40Ar relative to the concentrations expected from in situ decay of U, Th, and K, respectively, during the geologic age of about 30 Ma of the samples. We also showed that the U/Th-4He age of 36 Ma of vein-type gold from the Southern Alps agrees with its K-Ar formation age derived from associated muscovite and biotite. We now studied the question of the origin of the 4He and 40Ar excesses of placer gold. We conclude that gold contains two components of noble gases, a low-temperature component from fluid inclusions or phases that release noble gases at 800 degrees C and a high-temperature component released when gold melts (1064 degrees C). In some samples, extremely high U and K concentrations or an unreasonably high formation age would be required to explain the observed 4He abundances. Thus, trapped 4He and 40Ar must be present in gold. Feldman S. C. Franklin H. A. Terrestrial Case Studies of Ilmenite Exploration and Lunar Implications The Space Exploration Initiative (SEI) includes space resource utilization as one of the four architectures to achieve U.S. goals in space. Space resource utilization will make use of lunar resources to support more long-term activities on the lunar surface. Lunar ilmenite and regolith are two of the materials that can be mined and processed for lunar oxygen production. During this investigation, several sources were reviewed to assess terrestrial exploration methods used for locating ilmenite resources. These sources included published reports on terrestrial ilmenite exploration methods, analytical methods, case histories, chemical and physical properties, and associations with other minerals. Using a terrestrial analogue and taking into account the differences between terrestrial and lunar environmental conditions, rocks, and minerals, exploration methods and analytical instruments can be recommended for a lunar orbiter and lander for the purpose of assessing lunar resources. Finn V. J. Dolginov A. Z. Baker V. R. Transmantle Flux Tectonics Venus, Earth, and Mars have surfaces that display topographic domes and depressions with quasicircular planimetric shapes, relief of 0 to several kilometers, and large spatial scales (102 to 104 km). Our morphostructural mapping reveals hierarchical arrangements of these features. They are explained by a model of long-acting mantle convection, as a particular case of convection in a stratified and random inhomogeneous medium, which develops the form of a hierarchy of different convective pattern scales, each arising from different levels in the mantle. The hypothesis of transmantle flux tectonics parsimoniously explains a diversity of seemingly unrelated terrestrial planetary phenomena, including Earth megaplumes, global resurfacing epochs on Venus, and cyclic ocean formation and global climate change for Mars. Fisenko A. V. Verchovsky A. B. Semjonova L. F. Shukolyukov Yu. A. The Fractionation of Noble Gases in Diamonds of CV3 Efremovka Chondrite It has been shown that in diamonds of Efremovka CV3 the noble gases with normal isotopic compositions are fractionated to different degrees while the correlation of isotopic anomalous components is nearly constant. Fisenko A. V. Kashkarov L. L. Semjonova L. F. Pillinger C. T. Diamond Thermoluminescence Properties of Different Chondrites It has been found that TL glows of diamonds depend on (1) the origin of diamonds and (2) the degree of chondrite metamorphism. Fisenko A. V. Ljul A. Yu. Semjonova L. F. Ignatenko K. I. The Unusual Metallic Particles in Krymka LL3.0 Chondrite The composition, structural peculiarities, and possible formation scenario of unusual metallic particles in Krymka LL3.0 chondrite are shown. Fisenko A. V. Semjonova L. F. Bolsheva L. N. Grachjova T. V. Verchovsky A. B. Shukolyuko Yu. A. On Possibility of Diamond Formations in Radiation Process The possibility of diamond formation in the radiation process was checked studying diamond contents in a carburanium sample. Diamonds were not found and this result is discussed. Fisenko A. V. Semjenova L. F. Verchovsky A. B. Russell S. S. Pillinger C. T. The Carbon Isotopic Composition of Novo Urei Diamonds We discuss the carbon isotopic composition of diamond grains isolated from the Novo Urei meteorite. A diamond separate was obtained from 2 g of whole rock using chemical treatments aimed at obtaining very pure diamond. X-ray diffraction of the residue, which represented 5000 ppm of the parent mass, indicated only the presence of the desired mineral. The diamond crystals were 1-30 micrometers in diameter, and some grains had a yellow color. The chemical treatments were followed by a size separation to give a 1-10-micrometer and a 5-30-micrometer fraction, which were named DNU-1 and DNU- respectively. Frey H. Bills B. G. Kiefer W. S. Nerem R. S. Roark J. H. Zuber M. T. Free-Air and Bouguer Gravity Anomalies and the Martian Crustal Dichotomy We compare free-air and Bouguer gravity anomalies from a 50 x 50 field, derived from re-analysis of Viking Orbiter and Mariner 9 tracking data and using a 50 x 50 expansion of the current Mars topography and the GSFC degree 50 geoid as the equipotential reference surface, with the martian crustal dichotomy. The spherical harmonic topography used in this study has zero mean elevation, and differs from the USGS maps by about 2 km. In this field the dichotomy boundary in eastern Mars lies mostly at -1 to -2 km elevation. Gerasimov M. V. Dikov Yu. P. Yakovlev O. I. Wlotzka F. Trapping of Water Vapor from an Atmosphere by Condensed Silicate Matter Formed by High-Temperature Pulse Vaporization The origin of planetary atmospheres is thought to be the result of bombardment of a growing planet by massive planetesimals. According to some models, the accumulation of released water vapor and/or carbon dioxide can result in the formation of a dense and hot primordial atmosphere. Among source and sink processes of atmospheric water vapor the formation of hydroxides was considered mainly as rehydration of dehydrated minerals (forsterite and enstatite). From our point of view, the formation of hydroxides is not limited to rehydration. Condensation of small silicate particles in a spreading vapor cloud and their interaction with a wet atmosphere can also result in the origin of hydrated phases which have no genetic connections with initial water bearing minerals. Here we present results of two experiments of a simulated interaction of condensed silicate matter which originated during vaporization of dry clinopyroxene (Na -- 1.95, Mg -- 7.13, Al -- 4.29, Si --18.00, Ca -- 6.05, Ti -- 0.31, Fe -- 2.51 atm%) in a wet helium atmosphere. Golombek M. P. Banerdt W. B. Importance of Expansion and Contraction in the Formation of Tectonic Features on the Moon The lack of globally distributed tectonic features on the lunar surface has been used to argue against significant changes in the radius of the Moon since the formation of the presently observed surface, which dates to the end of heavy bombardment about 3.9 Ga. This observation has been used previously to limit the maximum stresses to 100 MPa that could be supported by the lunar lithosphere without the formation of globally distributed tectonic features, which in turn limits the maximum radius change to +-1 km for a purely elastic lithosphere. In a previous abstract Golombek et al., limits on the elastic expansion or contraction of the Moon were reexamined with respect to realistic failure stresses necessary to produce actual lunar tectonic features. In addition, limits on the permanent (plastic) strain that could be accommodated by non-mascon grabens and wrinkle ridges were considered with more severe constraints placed on the total reasonable expansion and contraction of the Moon since 3.9 Ga. In this abstract, considerations of the distribution and mechanisms of formation of lunar tectonic features are used as an additional argument against their formation due to a planetary radius change or their accommodating much permanent plastic planetary expansion or contraction. Greeley R. Kuzmin R. Costard F. Anderson F. S. Geringer M. A. Landheim R. Wenrich M. L. Mars Analog Site Study (MASS) Many proposed missions to Mars involve landed vehicles, including the Mars 94/96 (Russia), Mars Environmental Survey (MESUR, US), and the Marsnet (ESA) missions. Most landers involve in situ measurements of rock and soil compositions, study of local geology by imaging, and establishment of seismic and meteorological networks. The selection of landing sites on Mars is a complex process that must meet engineering constraints and scientific objectives, using available and anticipated data. The goal of the MASS project is to conduct an "end-to-end" test of the site selection process using Earth analogues. Greenberg R. Nolan M. C. Bottke W. F. Jr. Kolvoord R. A. Collisional and Dynamical History of Gaspra Interpretation of the impact record on Gaspra requires understanding of the effects of collisions on a target body of Gaspra's size and shape, recognition of impact features that may have different morphologies from craters on larger planets, and models of the geological processes that erase and modify impact features. Grinspoon D. H. Evolutionary Implications of a Steady-state Water Abundance on Venus In 1987 Grinspoon proposed that the data on hydrogen abundance, isotopic composition, and escape rate were consistent with the hypothesis that water on Venus might be in steady state rather than monotonic decline since the dawn of time. This conclusion was partially based on a derived water lifetime against nonthermal escape of approximately 10^8 years. Others have questioned this conclusion. De Bergh et al. found H20 lifetimes of >10^9 years. Donahue and Hodges derived H20 lifetimes of 0.4-5 x 10^9 years. The most sophisticated analysis to date of near-IR radiation from Venus' nightside reveals a water mixing ratio of approximately 30 ppm. Recent re-analysis of Pioneer Venus Mass Spectrometer data are consistent with a water abundance of 30 ppm. Gurov E. P. Gurova H. P. Rakitskaya R. B. Yamnichenko A. Yu. The Karakul Depression in the Pamire--The First Impact Structure in Central Asia The Karakul depression was picked out as a possible impact structure by the study of space images of Tadjikistan. Its striking similarity with some complex impact craters such as Dellen and Gosses Bluff is evident. This paper discusses our investigations of the Karakul depression in 1987 and from 1989 to 1991. Gurov E. P. The Acraman Impact Structure: Estimation of the Diameter by the Ejecta Layer Thickness The important role of gigantic crater formation in the geological history of the Earth was established by the example of the K/T boundary event. The discovery of the iridium anomaly in the sedimentary rocks of Vendian in the western part of the Ukrainian shield allows us to postulate its origin in connection with a great impact of that time. The only large impact structure of that age is the Acraman Crater in southeastern Australia. Haas J. R. Haskin L. A. Luhr J. Rasskazov S. Assimilation of Solids During Ascent of Magmas from the Bartoy Field of the Baikal Region, Siberia Most investigators ascribe mare basalt magma genesis to partial melting at depths of ~130 to >400 km within the cumulate pile deposited from a lunar magma ocean. Mare basalts share with mid-ocean ridge basalts the characteristic of relative depletion in LREE and other incompatible trace elements that arises from melting within "used" mantle, from which crust- forming elements have already been separated. Some mare basalt types do not show the classical La-Nd depleted mare basalt REE distributions, however, and some types are isotopically heterogeneous. These differences have been ascribed to assimilation, mainly AFC-style, of KREEPy highland material overlying the source region. Might such assimilation occur during magma ascent through the KREEPy material? To gain information from a terrestrial setting on possible assimilation during ascent, we have studied a suite of Quaternary nepheline-hawaiites and nepheline-mugearites from the Bartoy cinder cone complex of the Baikal Rift, Siberia. The Bartoy magmas originated from >80 km deep, and erupted through thick Archean crust. We find evidence for assimilation of ~31 wt% xenocrysts of garnet, aluminous clinopyroxene, kaersutite, and olivine, all presumably from the basalt source region, but no appreciable assimilation of overlying crust, consistent with isotopic constraints. Magmatic superheat made available by rapid ascent and decompression accounts adequately for the energy of assimilation; no accompanying fractional crystallization is required or evident. Hartung J. B. Corvid Meteoroids and a Giordano Bruno Ray are Genetically Related Both Corvid meteoroids and the Giordano Bruno (GB) crater are products of recent events. On June 25, or 26, 1178, Corvid meteoroids and a portion of GB ejecta were at the same place in the Solar System and moved in the same direction (right ascension = 12 degrees and declination = +19 degrees). The ground track of this direction is the same as that of the most prominant GB ray (azimuth = 237 degrees). These "coincidences" could not have occurred by chance and, therefore, support the conclusions that the GB impact occurred on June 26, 1178, and that Corvid meteoroids are high-velocity ejecta fragments from that impact. Finally, those fragments ejected with somewhat lower velocities failed to escape from the Earth-Moon system and produced the prominant ray extending southwest from the GB crater. Head J. W. Magee Roberts K. Wilson L. Pinkerton H. Lava Flow-Field Morphological Classification and Interpretation: Examples from Venus Recent analyses suggest that thermal constraints will act to limit the maximum length of an advancing lava flow being fed at a given volume or mass effusion rate from a vent. These constraints can be characterized through the Gratz number, which has a large value at the vent and decreases down flow; under a wide range of conditions, motion apparently ceases when the Gratz number has decreased to a value close to 300. In cooling-limited flows, effusion from the vent should be steady; the flow front thickens, eventually stops due to this cooling, and the central channel does not drain. If the vent remains active, a break-out flow will form from some point on the margin of the initial flow unit. If flows on planetary surfaces can be shown to be cooling limited, eruption rates can be estimated. In this analysis, we illustrate the morphological characteristics of various flow configurations, and we describe the application of these concepts to a flow length histogram for a hypothetical flow field and then apply this to an example on Venus. Head J. W. Ivanov M. Tessera Terrain on Venus: Implications of Tessera Flooding Models and Boundary Characteristics for Global Distribution and Mode of Formation Mapping of tessera terrain using Magellan global high-resolution data has shown that it comprises about 10% of the surface area of Venus, is not randomly distributed, is extremely highly deformed relative to intervening plains, lies at a wide range of elevations, is embayed by and largely predates adjacent volcanic plains, is generally negatively correlated with broad lowlands and volcanic rises, may underlie a considerable percentage of the superposed volcanic plains, and has linear/tectonic margins for about 27% of its boundaries. In this paper we investigate further the distribution and origin of tessera through analysis of the changing nature of tessera occurrences during sequential flooding, and assessment of the nature and distribution of Type II (linear/tectonic) tessera boundaries. Head J. W. Parmentier E. M. Hess P. C. Chemical Differentiation, Thermal Evolution, and Catastrophic Overturn on Venus: Predictions and Geologic Observations Observations from Magellan show that (1) the surface of Venus is generally geologically young; (2) there is no evidence for widespread recent crustal spreading or subduction; (3) the crater population permits the hypothesis that the surface is in production, and (4) relatively few impact craters appear to be embayed by volcanic deposits suggesting that the volcanic flux has drastically decreased as a function of time. These observations have led to consideration of hypotheses suggesting that the geological history of Venus may have changed dramatically as a function of time due to general thermal evolution and/or thermal and chemical evolution of a depleted mantle layer, perhaps punctuated by catastrophic overturn of upper layers or episodic plate tectonics. We have previously examined the geological implications of some of these models; here we review the predictions associated with two periods of Venus history, (1) stationary thick lithosphere and depleted mantle layer and (2) development of regional to global instabilities, and compare these predictions to the geological characteristics of Venus revealed by Magellan. Herd R. A. Pinkerton H. Bubble Coalescence in Magmas The most important factors governing the nature of volcanic eruptions are the primary volatile contents, the ways in which volatiles exsolve, and how the resulting bubbles grow and interact. In this contribution we assess the importance of bubble coalescence. The degree of coalescence in alkali basalts has been measured using image analysis techniques and is suggested to be a process of considerable importance. Binary coalescence events occur every few minutes in basaltic melts that have a vesicularity greater than around 35%. Hess P. C. Parmentier E. M. Overturn of Magma Ocean Ilmenite Cumulate Layer: Implications for Lunar Magmatic Evolution and Formation of a Lunar Core We explore a model for the chemical evolution of the lunar interior that explains the origin and evolution of lunar magmatism and possibly the existence of a lunar core. A magma ocean formed during accretion differentiates into the anorthositic crust and chemically stratified cumulate mantle. The cumulate mantle is gravitationally unstable with dense ilmenite cumulate layers overlying olivine-orthopyroxene cumulates with Fe/Mg that decreases with depth. The dense ilmenite layer sinks to the center of the Moon forming the core. The remainder of the gravitationally unstable cumulate pile also overturns. Any remaining primitive lunar mantle rises to its level of neutral buoyancy in the cumulate pile. Perhaps melting of primitive lunar mantle due to this decompression results in early lunar Mg-rich magmatism. Because of its high concentration of incompatible-heat-producing elements, the ilmenite core heats the overlying orthopyroxene-bearing cumulates. As a conductively thickening thermal boundary layer becomes unstable, the resulting mantle plumes rise, decompress, and partially melt to generate the mare basalts. This model explains both the timing and chemical characteristics of lunar magmatism. Heymann D. Chibante L. P. F. Geochemistry and Cosmochemistry of Fullerenes III: Reaction of C60 and C70 with Ozone C60 and C70 dissolved in toluene were treated with O2 gas containing 2.6 volume% ozone and with O3-free oxygen. No reaction products were detected for 0.1 mole of O2 passed through the solution, but destruction of C60 was clearly detectable for a dose of 10^-6 moles of O3. C70 was destroyed more slowly than C60. Among the substances remaining in solution, we identified C60O, C70O, C60O2, C60O3, and C60O4. C60 crystals exposed to O3 at room temperature became less soluble in toluene in a matter of days, but oxides were apparently not formed. Horz F. Cintala M. J. Bernhard R. P. See T. H. Impact Penetration Experiments in Teflon Targets of Variable Thickness Approximately 20.4 m^2 of Teflon thermal blankets on the nonspinning Long Duration Exposure Facility (LDEF) were exposed to the orbital debris and micrometeoroid environment in low-Earth orbit (LEO) for ~5.7 years. Each blanket consisted of an outer layer (~125 micrometers thick) of FEP Teflon that was backed by a vapor-deposited metal mirror (Inconel; <1 micrometers thick). The inner surface consisted of organic binders and Chemglaze thermal protective paint (~50 m thick) resulting in a somewhat variable total blanket thickness of 180 to 200 m. There was at least one of these blankets, each exposing 1.2 m^2 of surface area, on nine of LDEF's 12 principal pointing directions, the exceptions being Rows 3, 9 and 12. As a consequence, these blankets represent a significant opportunity for micrometeoroid and debris studies, in general, and specifically they provide an opportunity to address those issues that require information about pointing direction (i.e., spatial density of impact events as a function of instrument orientation). During deintegration of the LDEF spacecraft at the Kennedy Space Center, all penetration holes >=300 micrometers in diameter were documented and were recently synthesized in terms of spatial density as a function of LDEF viewing direction by See et al. The present report describes ongoing cratering and penetration experiments in pure Teflon targets, which are intended to establish the relationships between crater or penetration-hole diameters and the associated projectile dimensions at laboratory velocities (i.e., 6 km/s). The ultimate objective of these efforts is to extract reliable mass-frequencies and associated fluxes of hypervelocity particles in LEO. Ivanova M. A. Assonov S. S. Shukolyukov Yu. A. First Investigation of Noble Gases in the Dengli H3,8 Chondrite The Dengli (H3,8) meteorite weighing 243.5 g is a find from the Karakum desert. It' s a complex microbreccia containing unusual clasts that are more typical for regolithic breccias than for H-chondrites. Based on Xe, Kr, and Ar contents and their isotopic compositions, the Dengli doesn't differ significantly from other H chondrites. Its exposure age is 7.6 m.y. That's near to the common possible data of exposure age (6.2+0.2 m.y.) of 350 H chondrites. Dengli's K/Ar age (4.01 Ga) coincides with the ages of many other H chondrites. Thus the Dengli isn't regolithic breccia and it probably formed during accretion of its parent body. Ivanova M. A. Kononkova N. N. Petaev M. I. Silica-bearing Objects in the Dengli H3.8 and Gorlovka H3-4 Chondrites Silica-bearing objects are enigmatic components of the olivine-normative ordinary chondrites. Several papers has been devoted to the study of these objects in various chondrite types. While a relatively large body of information has been collected, the origin of these objects is still controversial. Here we report new data on silica-bearing objects in the unequilibrated H chondrites Dengli and Gorlovka. The crystallization history of these objects could be explained on the basis of the phase diagram of the Q-Ol-Pl (Al2O3) system, but the origin of the silica-rich liquids remains unclear. Ivliev A. I. Kashkarov L. L. Badjukov D. D. Induced Thermoluminescence Study of Experimentally Shock-Loaded Oligoclase Artificially induced thermoluminescence (TL) in the oligoclase samples, which were shock-loaded up to 27 GPa, was measured. Essential increase of the TL sensitivity in relation to the total gamma ray irradiation dose was observed only in the sample at 27 GPa pressure. This result can be explained by the initiation of the additional radiation damage in the so highly shocked oligoclase crystal lattice. Ivliev A. I. Kashkarov L. L. Korotkova Yu. Yu. Shock-Thermal History of Kapoeta Howardite Matter on Data of Thermoluminescence Analysis of Individual Mineral Grains Artificially induced thermoluminescence (TL) was measured in OPx grains from Kapoeta howardite. TL glow curves for individual OPx grains are significantly distinguished from each other both by their shape and TL sensitivity: Five groups of OPx grains were characterized. The observed variety of glow curves of OPx grains could be caused by their distinct shock-thermal events at the regolith stage of meteorite parent body formation. Jakes P. Wolfbauer M. P. Transition Metals in Superheat Melts A series of experiments with silicate melts doped with transition element oxides was carried out at atmospheric pressures of inert gas at temperatures exceeding liquidus. As predicted from the shape of fO2 buffer curves in T-fO2 diagrams the reducing conditions for a particular oxide-metal pair can be achieved through the T increase if the released oxygen is continuously removed. Experimental studies suggest that transition metals such as Cr or V behave as siderophile elements at temperatures exceeding liquidus temperatures if the system is not buffered by the presence of other oxide of more siderophile element. For example the presence of FeO prevents the reduction of Cr2O3. The sequence of decreasing siderophility of transition elements at superheat conditions (Mo, Ni, Fe, Cr) matches the decreasing degree of depletion of siderophile elements in mantle rocks as compared to chondrites. Jerde E. A. Taylor L. A. Searching for neuKREEP: An EMP Study of Apollo 11 Group A Basalts The Apollo 11 and 17 landing sites are characterized by the presence of high- Ti basalts (TiO2 > 6%). The Group A basalts of Apollo 11 have elevated K compositions (>2000 ppm;) and are enriched in incompatible trace elements relative to the other types of high-Ti basalt found in the region. These unique basalts also are the youngest of all high-Ti basalts, with an age of 3.56 +- 0.02 Ga. Recent modeling of the Apollo 11 Group A basalts by Jerde et al. has demonstrated that this unique variety of high-Ti basalt may have formed through fractionation of a liquid with the composition of the Apollo 11 orange glass, coupled with assimilation of evolved material (dubbed neuKREEP, and having similarities to lunar quartz monzodiorite). Assimilation of this material would impart its REE signature on the liquid, resulting in the elevated REE abundances observed. Minerals such as whitlockite, which contain a large portion of the REE budget, can be expected to reflect the REE characteristics of the assimilant. To this end, an examination of the whitlockite present in the Apollo 11 Group A basalts was undertaken to search for evidence of the neuKREEP material assimilated. Jolliff B. L. Korotev R. L. Haskin L. A. An Iridium-rich Iron Micrometeorite with Silicate Inclusions from the Moon We have found a 0.1-mg iron micrometeorite containing meteoritic silicate inclusions in an agglutinate from 2-2.5 cm deep in regolith core 60014. The metal is 93% iron, 6.5% nickel, 0.5% cobalt, ~150 ppm iridium, and <2 ppm gold. Although the Ir concentration is higher than that reported previously for any iron meteorite group, it lies on the extrapolation to low Ni and high Ir concentrations of several meteorite groups on Ni,Ir plots (groups IIC,D,E, and IIIAB,E,F). Tiny, subrounded silicate inclusions comprise low-Ca pyroxene (En83), olivine (Fo80), and albitic and potassic feldspars, as mixtures of minerals or glasses. Minor phases include oldhamite (CaS) and, tentatively, hercynite (FeAl2O4). The inclusions have pyroxene FeO/MnO of ~25 and olivine FeO/MnO of 40-60. In comparison with known iron meteorites, the inclusions are most similar to those in type IIE, e.g., Weekeroo Station, Colomera, and Kodaikanal. As far as we know, this is the first observation of an iron meteorite with silicate inclusions from a lunar sample. No metal fragments with meteoritic, nonmetallic inclusions were reported in several previous, exhaustive studies of soil particles. Jones A. C. The Cooling Rates of Pahoehoe Flows: The Importance of Lava Porosity Many theoretical models have been put forward to account for the cooling history of a lava flow; however, only limited detailed field data exist to validate these models. To model accurately the cooling of lava flows, data are required, not only on the heat loss mechanisms, but also on the surface skin development and the causes of differing cooling rates. This paper argues that the causes of such variations in the cooling rates are attributed, primarily, to the vesicle content and degassing history of the lobe. Jones J. H. Walker D. Partitioning of Nb, Mo, Ba, Ce, Pb, Th and U Between Immiscible Carbonate and Silicate Liquids: Evaluating the Effects of P2O5, F, and Carbonate Composition Previously we have reported carbonate liq./silicate liq. partition coefficients (D) for a standard suite of trace elements (Nb, Mo, Ba, Ce, Pb, Th, and U), and Ra and Pa as well. In brief, we have found that immiscible liquid partitioning is a strong function of temperature. As the critical temperature of the carbonate-silicate solvus is approached, all partition coefficients approach unity. Additionally, for the overwhelming majority of the partitioning elements, lnD is a linear function of "ionic field strength," z/r, where z is the charge of the partitioned cation and r is its ionic radius. Kano N. Yamakoshi K. Matsuzaki H. Nogami K. Chemical and Isotopic Compositions in Acid Residues from Various Meteorites To study the prehistory of the solar system for possible direct evidences for processes of nucleosynthesis in the presolar stage and detection of extinct radioactive nuclide, we are planning to carry out systematic isotopic investigations on Ru, Mg and so on in primordial samples. In this paper, we present the results of chemical compositions of acid residues obtained from three types of meteorites [Canyon Diablo (IA), Allende (CV3), and Nuevo Mercurio (H5)] and the preliminary results of Ru isotopic compositions. Kargel J. S. The Rheology and Composition of Cryovolcanic Flows on Icy Satellites The rheologic properties of terrestrial lavas have been related to morphologic features of their flows, such as levees, banked surfaces, multilobate structures, and compressional folds. These features also have been used to determine rheologies and constrain the compositions of extraterrestrial flows. However, with rare exceptions, such features are not resolvable in Voyager images of the satellites of outer planets. Often only flow length and edge thickness of cryovolcanic flows can be measured reasonably accurately from Voyager images. The semiempirical lava-flow model presented below is a renewed effort to extract useful information from such measurements. Kashkarov L. L. Korotkova N. N. Skripnik A. Ya. Complex Radiation-Thermal History of Kaidun Meteorite on Data of Track Study of Silicate Minerals The results of track study of 80 individual silicate mineral crystals (ol,px,plag) picked out from Kaidun meteorite are presented. A wide range of observed track density value distributions indicate the complex irradiation history of Kaidun minerals. In one anortite crystal having two track groups with different parameters the preaccretion irradiation traces were observed in all probability. Kashkarov L. L. Kalinina G. V. Different Radiation and Metamorphic History of the Kainsaz CO 3.2 Chondrules Track and thermoluminescence parameters in chondrules from the Kainsaz CO 3.2 chondrite have been studied. Obtained results indicate their individual shock-thermal history on the early preaccretion stage of the meteorite parent body formation. Kennedy A. E. Lofgren G. E. Wasserburg G. J. An Experimental Study of Trace Element Partitioning Between Perovskite, Hibonite and Melt: Equilibrium Values The presence of perovskite (CATiO3) and hibonite (Ca Al(sub)12O(sub)19) within different regions of Calcium-, Aluminum-rich Inclusions (CAI) and the trace element concentrations of these minerals in each circumstance, constrain models of precursor formation, nebular condensation, the thermal history of inclusions with relict perovskite and hibonite and the formation of the Wark- Lovering rim. At present mineral/melt partition coefficient (D) data for hibonite are limited to a few elements in simple experimental systems, or to those derived from hibonite-glass pairs in hibonite/glass microspherules. Similarly, there is only limited data on perovskite D that are applicable to meteorite compositions. Apart from the importance of partitioning studies to meteorite research, D values also are invaluable in the development of thermodynamic models, especially when data is available for a large number of elements that have different ionic charge and radii. In addition, study of the effect of rapid cooling on partitioning is crucial to our understanding of meteorite inclusions. To expand our knowledge of mineral/melt D for perovskite and hibonite, we have instituted a study similar to that of Kennedy et al. where D values are obtained in both equilibrium and dynamic cooling experiments. As an initial phase of this study we have measured mineral/melt D for major elements (Ca, Mg, Al, Ti, and Si), 15 rare earth elements (La-Lu) and 8 other elements (Ba, Sr, U, Th, Nb, Zr, Hf, and Ge) in perovskite and hibonite grown under equilibrium conditions, in bulk compositions that are respectively similar to Compact Type A (CTA) CAI and to a hibonite/glass microspherule. Experimental mixes were doped with REE at 20-50x chondritic (ch) abundances, Ba at 50 ppm, Sr, Hf, Nb, and Zr at 100 ppm and, U and Th at 200 ppm. Trace element abundances were measured with the PANURGE ion microprobe using the techniques and standards of Kennedy and Hutcheon. Major element compositions were obtained by electron microprobe analysis. Kilburn C. R. J. Lava Crusts and Flow Dynamics Lava flows can be considered as hot viscous cores within thinner,solidified crusts. Interaction between crust and core determines a flow's morphological and dynamical evolution. When the lava core dominates, flow advance approaches a steady state. When crusts are the limiting factor, advance is more irregular. These two conditions can be distinguished by a timescale ratio comparing rates of core deformation and crustal formation. Aa and budding pahoehoe lavas are used as examples of core- and crustal-dominated flows respectively. A simple model describes the transition between pahoehoe and aa in terms of lava discharge rate, underlying slope, and either the thickness or velocity of the flow front. The model shows that aa morphologies are characterized by higher discharge rates and frontal velocities and yields good quantitative agreement with empirical relations distinguishing pahoehoe and aa emplacement on Hawaii. Kirk R. L. Morgan H. F. Russell J. F. The Cartography of Venus with Magellan Data Maps of Venus based on Magellan data are being compiled at 1:50,000,000, 1:5,000,000 and 1:1,500,000 scales. Topographic contour lines based on radar altimetry data are overprinted on the image maps, along with feature nomenclature. Map controls are based on existing knowledge of the spacecraft orbit; photogrammetric triangulation, a traditional basis for geodetic control for bodies where framing cameras were used, is not feasible with the radar images of Venus. Preliminary synthetic aperture radar (SAR) image maps have some data gaps and cosmetic inconsistencies, which will be corrected on final compilations. Eventual revision of geodetic controls and of the adopted Venusian spin-axis location will result in geometric adjustments, particularly on large-scale maps. Kirk R. L. Edwards K. B. Morgan H. F. Soderblom L. A. Stoewe T. L. Global Magellan Image Map of Venus at Full Resolution During its first 243-day mapping cycle, the Magellan spacecraft succeeded in imaging 84% of the surface of Venus at resolutions on the order of 100 meters; subsequent cycles have increased the total coverage to over 97% and provided redundant coverage of much of the planet with differing viewing geometries. Unfortunately, this full-resolution global dataset is in the form of thousands of individual orbit tracks (F-BIDRs) whose length-to-width ratio of nearly 1000: 1 makes them minimally useful unless mosaicked. The Magellan project has produced full-resolution mosaics (F-MIDRs) only for selected regions on the planet, whereas a global set of mosaics has been made only at threefold degraded resolution (C1-MIDRs). Furthermore, although the F-MIDRs, which are approximately equidimensional, are much better suited for scientific interpretation than the F-BIDRs, they are still an unwieldy dataset: over 1500 quadrangles, each showing a region only about 600 km on a side, would be required to cover the entire planet. The USGS has therefore undertaken to produce and distribute a global, full-resolution set of mosaics of the Magellan image data in a format that will be efficient for both hardcopy and digital use. Korochantsev A. V. Nikolaeva O. V. Terrestrial Bitumen Analogue of Orgueil Organic Material Demonstrates High Sensitivity to Usual HF-HCl Treatment The relationship between the chemical composition and the interlayer spacing (d002) of organic materials (OMs) is known for various terrestrial OMs. We improved this general trend by correlation with the corresponding trend of natural solid bitumens (asphaltite-kerite-anthraxolite) up to graphite. Using the improved trend we identified bitumen analogues of carbonaceous chondrite OMs residued after HF-HCl treatment. Our laboratory experiment revealed that these analogues and hence structure and chemical composition of carbonaceous chondrite OMs are very sensitive to the HF-HCl treatment. So, usual extraction of OM from carbonaceous chondrites may change significantly structural and chemical composition of extracted OM. Korotev R. L. Bishop K. M. Composition of Apollo 17 Core 76001 Core 76001 is a single drive tube containing a column of regolith taken at the base of the North Massif, station 6, Apollo 17. The core material is believed to have accumulated through slow downslope mass wasting from the massif. As a consequence, the core soil is mature throughout its length. Results of INAA for samples taken every half centimeter along the length of the core indicate that there is only minor systematic compositional variation with depth. Concentrations of elements primarily associated with mare basalt (Sc, Fe) and noritic impact melt breccia (Sm) decrease slightly with depth, particularly between 20 cm and the bottom of the core at ~32 cm depth. This is consistent with petrographic studies that indicate a greater proportion of basalt and melt breccia in the top part of the core. However, Sm/Sc and La/Sm ratios are remarkably constant with depth, indicating no variation in the ratio of mare material to Sm-rich highlands material with depth. Other than these subtle changes, there is no compositional evidence for the two stratigraphic units (0-20 cm and 20-32 cm) defined on the basis of modal petrography, although all samples with anomalously high Ni concentrations (Fe-Ni metal nuggets) occur above 20 cm depth. Krol E. Lang B. Comparative Magnetic and Thermoanalytical Study of Two Enstatite Chondrites: Adhi Kot and Atlanta With allowance for the discussion of classification of enstatite chondrites and their relation to aubrites we submit magnetic and thermoanalytical data obtained by us for consideration as additional arguments. Our study covered the Adhi Kot(EH4) and Atlanta(EL6) meteorites belonging to two distinct groups of enstatite chondrites. Applying AF demagnetization we measured the intensity of natural remanent magnetization (NRM) and determined the mean magnetic susceptibility of the samples. We also obtained the differential thermal (DTA) and thermogravimetric curves for the meteorites under study. Langenhorst F. Deutsch A. Orientation of Planar Deformation Features (PDFs) in Quartz Differently oriented single-crystal quartz was shocked experimentally at pressures of 20 to 32 GPa and preshock temperatures up to 630 degrees C. Based on this systematic investigation we can demonstrate that the orientation of planar deformation features in quartz is not only dependent on shock pressure but also on preshock temperature and shock direction. Moreover, the orientation of PDFs is strongly influenced by the set-up in recovery experiments. Lavrukhina A. K. The Astrophysical Interpretation of Isotope Anomalies in Graphite and SiC Grains of Chondrites The C, N, and Mg isotopic compositions in graphite and SiC grains of carbonaceous chondrites can be explained by nuclear processes in massive O,B second-generation stars past a stage of WR star with intensive stellar wind, where grain condensation had taken place. Lavrukhina A. K. Lavrentjeva Z. A. Ljul A. Yu. Ignatenko K. I. On Origin of the Olivine Inclusions from the Kainsaz CO Carbonaceous Chondrite Olivine inclusions and chondrules of Kainsaz have been formed in a unique process of dust matter melting. Lavrukhina A. K. Ustinova G. K. On Possible 53Mn Heterogeneity in the Early Solar System We show the effects of the influence of shock wave propagation on the energy spectrum of accelerated particles that lead to different production rates of radionuclides, in particular Mn-53, on small scales in the early solar system. Lenardic A. Kaula W. M. Models of Thermal/Chemical Boundary Layer Convection: Potential Application to Venus The upper boundary layer of Venus is comprised of at least two distinct chemical components, mantle and crust. Fluid dynamical models of convection within Venus' mantle have been primarily of the thermal boundary layer type. Models assessing the ability of convection mantle flows to deform the crust have been undertaken but models exploring the effects of a variable-thickness crust on mantle convection have been largely lacking. A venusian crust of variable thickness could couple back into, and alter, the mantle flow patterns that helped create it, leading to deformation mechanisms not predicted by purely thermal-boundary-layer convection models. We explore this possibility through a finite-element model of thermal/chemical-boundary-layer convection. Model results suggest that a crust of variable thickness can serve as a mantle flow driver by perturbing lateral temperature gradients in the upper mantle. Resulting mantle flow is driven by the combination of free convection and nonuniform crustal distribution. This combination can lead to a flow instability manifest in the occurrence of episodic mantle lithosphere subduction initiated at the periphery of a crustal plateau. The ability of a light, near-surface, chemical layer to potentially alter mantle flow patterns suggests that mantle convection and the creation and/or deformation of such a chemical layer may be highly nonseperable problems on timescales of 10^8 years. Lenardic A. Kaula W. M. On the Relationship Between Tectonic Plates and Thermal Mantle Plume Morphology Models incorporating platelike behavior, i.e., near-uniform surface velocity and deformation concentrated at plate boundaries, into a convective system, heated by a mix of internal and basal heating and allowing for temperature dependent viscosity, have been constructed and compared to similar models not possessing platelike behavior. The simplified numerical models are used to explore how platelike behavior in a convective system can affect the lower boundary layer from which thermal plumes form. A principal conclusion is that platelike behavior can significantly increase the temperature drop across the lower thermal boundary layer. This temperature drop affects the morphology of plumes by determining the viscosity drop across the boundary layer. Model results suggest that plumes on planets possessing platelike behavior, e.g., the Earth, may differ in morphologic type from plumes on planets not possessing platelike behavior, e.g., Venus and Mars. Liu Y.-G. Schmitt R. A. Chondritic Ratios of Fe/Cr/Ir in Kerguelen Plateau (Hole 783C) K/T Carbonate-rich Sediments Support Asteroid-Cometary Impact at K/T Time In the study of marine carbonate sediments from Holes 577 and 577B, Shatsky Plateau (Rise), a net extraterrestrial Fe/Ir = C1 chondritic ratio at the K/T boundary was reported. Applying a similar procedure to Hole 738C (Kerguelen Plateau) data, we obtain Fe/Cr/lr ratios similar to C1 or C2 chondritic ratios. Liu Y.-G. Schmitt R. A. Geochemical Evidences for Two Chondritic-like Cometary or Asteroid Impact Before and at the K/T Boundary Mass extinctions caused by multiple impacts of cometary showers within a short time period have been proposed. Detailed calculation of temporal profile shows that cometary showers last 3 Ma, with the bulk of the comets arriving within 1 Ma. A number of geological and palaeontological evidence supports multiple impacts and their connection with mass extinctions. Observations include clustered crater ages, stratigraphic horizons of impact ejecta closely spaced in time, and evidence for stepwise mass extinctions spanning intervals of 1-3 Ma. For the K/T boundary, three candidates, Popigai, Manson, and Yucatan, have been proposed as impact craters. Two distinct strata at the K/T boundary in western North America have been interpreted as evidence for two sequential impacts. If multiple impacts occurred within a time span of 1 Ma, then multiple Ir enrichments should be observed. Liu Y.-G. Reinhardt J. W. Schmitt R. A. Earth's Partial Pressure of CO2 Over the Past 100-500 Ma. Evidence from Ce Anomalies in Mostly Shallow Seas (<200 m) as Recorded in Carbonate Sediments, II In a previous abstract in this volume we reported the direct relationship of Ce anomalies, Ce^A*, recorded in 0.2-119 Ma CaCO3 sediments to the Ce anomalies, Ce^A, in the parental Pacific deep sea water and their relationship to atmospheric P(CO2) relative to the present P(CO2). Because carbonate samples from ocean basins, obtained from D.S.D.P. and O.D.P., are restricted to <130 Ma, we have analyzed via INM continental CaCO3 samples that were deposited in ancient oceans and shallow sea platforms <200 m over central USA, central Europe, China, and Saudi Arabia/Oman. Shallow sea conditions over continents as well as platforms may have been subjected to more localized variations because of their complex chemical regimes as well as being at higher temperatures relative to the <200 m Pacific Ocean's mixed layer. This has been observed in the spread of trace elements in 250-Ma China carbonates and in carbonates from the 150-Ma oil-rich Arab Formation. Lovell A. J. Schloerb F. P. McGill G. E. Resolving Topographic Detail on Venus by Modeling Complex Magellan Altimetry Echoes Magellan's altimeter is providing some of the finest-resolution topography of Venus achieved to date. Nevertheless, efforts continue to improve the topographic resolution whenever possible. One effort to this end is stereoscopic imaging, which provides topography at scales similar to that of the SAR. However, this technique requires two SAR images of the same site to be obtained and limits the utility of this method. We present another method to resolve topographic features at scales smaller than that of an altimeter footprint, which is more globally applicable than the stereoscopic approach. Makalkin A. B. Dorofeyeva V. A. Vityazev A. V. Possible Sources of H2 to H2O Enrichment at Evaporation of Parent Chondritic Material One of the results obtained from thermodynamic simulation of recondensation of the source chondritic material is that at 1500-1800 K it's possible to form iron-rich olivine by reaction between enstatite, metallic iron, and water vapor in the case of [H2O]/H2] ~0.1. This could be reached if the gas depletion in hydrogen is 200-300 times relative to solar abundance. To get this range of depletion one needs some source material more rich in hydrogen than the carbonaceous CI material, which is the richest in volatiles among chondrites. In the case of recondensation at impact heating and evaporation of colliding planetesimals composed of CI material, we obtain insufficiently high values of the [H2]/[H2O] ratio. In the present paper we consider some possible source materials and physical conditions necessary to reach gas composition with [H2]/[H2O] ~10 at high temperature. Martinez I. Agrinier P. Guyot F. Ildefonse Ph. Javoy M. Scharer U. Hornemann A. CO2 Production by Impact in Carbonates?: An ATEM and Stable Isotope (C, O) Study Carbonates may have been a common target for large impacts on the Earth and possible related CO2 outgassing would have important consequences for the composition of the atmosphere. To estimate volatile release during such impacts, isotopic ratios (13C/12C and 18O/16O) were determined on highly shocked carbonate samples in combination with SEM and analytical transmission electron microscopy (ATEM) investigations. The study was performed on both naturally and experimentally shocked rocks, i.e., 50-60 GPa shocked limestone-dolomite fragments from the Haughton impact crater (Canada), and carbonates shocked in shock recovery experiments. For the experiments, unshocked carbonates consisting of a mixture of dolomite and calcite from the Haughton area were used. Naturally shocked samples were collected in the polymict breccia near the center of the Haughton crater (Anomaly Hill). Marzari F. Vanzani V. Weidenschilling S. J. Dust Grain Resonant Capture: A Statistical Study A statistical approach, based on a large number of simultaneous numerical integrations, is adopted to study the capture in external mean motion resonances with the Earth of micrometer-sized dust grains perturbed by solar radiation and wind forces. We explore the dependence of the resonant capture phenomenon on the initial eccentricity e(sub)0 and perihelion argument omega(sub)0 of the dust particle orbit. The intensity of both the resonant and dissipative (Poynting-Robertson and wind drag) perturbations strongly depends on the eccentricity of the particle while the perihelion argument determines, for low inclination, the mutual geometrical configuration of the particle's orbit with respect to the Earth's orbit. We present results for three j:j+1 commensurabilities (2:3, 4:5, and 6:7) and also for particle sizes s = 15, 30 micrometers. This study extends our previous work on the long- term orbital evolution of single dust particles trapped into resonances with the Earth. Matsuzaki H. Yamakoshi K. Size Distribution of Interplanetary Iron and Stony Particles Related with Deep-Sea Spherules To study the origin and evolution of interplanetary dust, it is very important to investigate size distribution. Here changes of the size distribution of meteoroid particles due to ablative effects during atmospheric entry were investigated by numerical computer simulation. Using the results, the preatmospheric size distribution of interplanetary dust particles could be estimated from that of ablated spherules taken from deep-sea sediments. We are now analyzing deep-sea spherules from some aspects and examining if we could get any information about the interplanetary dust. McKay D. S. Carter J. L. Boles W. W. Allen C. C. Allton J. H. JSC-1: A New Lunar Regolith Simulant Simulants of lunar rocks and soils with appropriate properties, although difficult to produce in some cases, will be essential to meeting the system requirements for lunar exploration. In order to address this need a new lunar regolith simulant, JSC-1, has been developed. JSC-1 is a glass-rich basaltic ash that approximates the bulk chemical composition and mineralogy of some lunar soils. It has been ground to produce a grain size distribution approximating that of lunar regolith samples. The simulant is available in large quantities (>2000 lb; 907 kg). JSC-1 was produced specifically for large- and medium-scale engineering studies in support of future human activities on the Moon. Such studies include material handling, construction, excavation, and transportation. The simulant is also appropriate for research on dust control and spacesuit durability. JSC-1 can be used as a chemical or mineralogical analogue to some lunar soils for resource studies such as oxygen or metal production, sintering, and radiation shielding. McKay G. Ogawa T. Miyamoto M. Takeda H. More on the Cooling History of Angrite LEW 86010 Antarctic angrite LEW 86010 has many chemical and mineralogical characteristics that suggest it is closely related to Angra dos Reis (ADOR). However, these meteorites have had very different thermal histories. Olivines and pyroxenes in ADOR are nearly homogeneous, suggesting very slow cooling or extensive subsolidus equilibration. In contrast, LEW 86010 pyroxenes are extensively zoned in both major and trace elements, while olivines are nearly homogeneous, suggesting that cooling of this sample was slow enough to homogenize olivines, but too fast to homogenize pyroxenes. Mikouchi T. Takeda H. Mori H. Miyamoto M. McKay G. Exsolved Kirschsteinite in Angrite LEW86010 Olivine Mineralogy of kirschsteinite exsolution in olivine from Antarctic meteorite LEW86010 has been studied by single crystal X-ray diffraction technique. The LEW86010 olivine crystals have exsolution lamellae of kirschsteinite about 15 micrometers wide. Determination of crystallographic orientation of exsolved kirschsteinite in an olivine grain has been made. Weak reflections of exsolved kirschsteinite share common crystallographic orientation with the host olivine. The cell dimensions of the exsolved phase (a 4.87+/-0.05A b 11.14+/-0.10A c 6.36+/-0.05A) and intensities were well in accord with those of kirschsteinite previously reported. The section oriented perpendicular to the a axis shows exsolution lamellae in two directions parallel to (031) and (0-31) . The lamellae are up to 10 micrometers wide and the spacings between them were usually 50-100 micrometers. Miyamoto M. Takeda H. The Thickness of Eucritic Crust in the HED Parent Body Because cumulate eucrites are generally thought to be located at the lower part in a eucritic crust on a diogenitic mantle in the HED parent body, the burial depth of cumulate eucrites gives information on the thickness of the eucritic crust. We estimated the burial depth and cooling rate of cumulate eucrites, Serra de Mage and Moore County on the basis of the width of augite lamellae and compositional gradients of Ca in pyroxenes by numerically solving the diffusion equation. We obtained the burial depth of the eucrites of 7-8 km and cooling rate of 0.00016- 0.0002 degrees C/yr. Moroz L. V. Fisenko A. V. Semjonova L. F. Pieters C. M. Optical Effects of Regolith Processes on S Asteroids as Simulated by Laser Impulse Alteration of Ordinary Chondrite The spectral properties of some powdered chondrites and minerals altered by laser impulse are studied in order to estimate possible optical effects of regolith processes (micrometeoritic bombardment). Gradual reduction of overall reflectance and spectral contrast, the increase of continuum slope, the increase of spectrally derived olivine/pyroxene ratio and Fs content of orthopyroxene with increasing alteration degree show that regolith processes could affect optical properties of surface material more heavily than has been previously appreciated. Murali A. V. Jordan J. L. Helium-3 Inventory of Lunar Samples: A Potential Future Energy Resource for Mankind? Solar wind is the principal source for the volatile elements (H, C, N, and noble gases) in the lunar samples, which are enriched in the finest fraction of the Moon's comminuted regolith. Some of these volatiles (H, N, He, and C) are regarded as potential lunar resources that can support the inhabitants of a lunar base and provide fuels for transportation. Various studies indicated that these volatiles may be extracted by heating the lunar soils to approximately 700 degrees C. At this temperature near-quantitative release of hydrogen and helium, and approximately 20-30% of the release of nitrogen and carbon and their compounds occurs. Murchie S. Mustard J. Bright Soil Units on Mars Determined from ISM Imaging Spectrometer Data The lithology of bright martian soil provides evidence for chemical and physical processes that have modified the planet's surface. Data from the ISM imaging spectrometer, which observed much of the equatorial region at a spatial resolution of ~22 km, cover the NIR wavelength range critical to ascertaining the presence and abundance of Fe-containing phases, hydroxylated silicates, and H2O in the bright soil. ISM data previously have revealed spatial variations in depth of the 3.0-micrometer H2O absorption suggesting differences in water content, a weak absorption at 2.2 micrometers indicative of metal-OH in phyllosilicate, and variations in the 1-micrometer Fe absorption indicative of differences in Fe mineralogy. This abstract summarizes first results of a systematic investigation of spectral heterogeneity in bright soils observed by ISM. At least seven "units" with distinctive properties were discriminated. Comparison of their spatial distributions with Viking data shows that they generally correspond with previously recognized morphologic, color, and thermal features. These correspondences and the units' spectral attributes provide evidence for lithologic differences between the soils in different geologic settings. Murer Ch. Baur H. Signer P. Wieler R. Solar Noble Gases Revealed by Closed System Stepped Etching of a Metal Separate from Fayetteville Solar He, Ne, and Ar in a Fe-Ni separate from the chondrite Fayetteville are analysed by closed system stepped oxidation. We report here data of the first 15 steps comprising 55% of the total solar gases. 4He/36Ar and 20Ne/36Ar are quite constant at values about 20% below those of present day solar wind (SWC). In this, Fe-Ni differs from lunar ilmenites where 4He/36Ar and 20Ne/36Ar in the first steps are several times below SWC. Thus, metal retains SW-noble gases even better than ilmenite, almost without element fractionation. Nevertheless, the isotopic composition of SW-He, -Ne, and -Ar in the first steps of the metal sample are identical to those found in a recently irradiated lunar ilmenite, indicating that ilmenites and chondritic metal both contain isotopically unfractionated SW noble gases. A preliminary analysis of a smaller Fayetteville metal separate shows Ne from solar energetic particles (SEP-Ne) with 20Ne/22Ne < 11.5. Murty S. V. S. Nitrogen and Light Noble Gases in Parsa Enstatite Chondrite Nitrogen and light noble gases have been analysed in three bulk samples and an aubritic nodule in Parsa, an EH3 chondrite. While the three bulk samples show varying amounts of tapped He, Ne of solar composition, the nodule is totally devoid of trapped He, Ne. The N contents in bulk samples vary from 119 ppm to 197 ppm with delta 15N (per mil) ranging from -22.6 to -31.2, while for the nodule N=140 ppm with delta 15N (per mil) = -13.4. The spread in both N and delta 15N in Parsa is due to heterogeneous distribution of N-bearing minerals with differing delta 15N signatures. The higher N contents of the nodule as well as its delta 15N signatures, as compared to normal aubrites, is suggestive that it may not be genuine aubrite. Na C. Y. Barker E. S. Stern S. A. Esposito L. W. SO2 on Venus: IUE, HST, and Ground-based Measurements, and the Active Volcanism Connection UV observations by Pioneer Venus from 1978 to 1986 detected a large decline of SO2 above the clouds of Venus. The decline of SO2 has been confirmed by the International Ultraviolet Explorer (IUE) observations made in 1979 and 1987. Pioneer Venus also observed a decline in sulfuric acid aerosols from the polar regions of Venus. Further still, both Pioneer Venus and Galileo have detected radio signals that have been attributed to lightning activities in the atmosphere of Venus. Based on these observations, Esposito proposed that Venus may be volcanically active, and the large decline of SO2 above the clouds can be interpreted as the recovery of the atmosphere following the injection of SO2 from a volcanic eruption. If this hypothesis is correct then remote sensing of the Venus atmosphere may provide important clues to the current state of Venus geology. Nakamura N. Morikawa N. REE and Other Trace Lithophiles in MAC88177, LEW88380 and LEW88763 In order to investigate the petrogenesis of primitive achondrites, we have carried out precise isotope dilution analyses of REE, Sr, Ba, Rb, K, Li, Ca, Mg, and Fe for three new meteorites; (Lodranites) MAC88177, LEW88280, (Brachinite) LEW88763, together with analyses for Acapulco. Present results are compared with those of other primitive achondrites previously reported by us, N. Torigoye et al., and others. Nazarov M. A. Brandstatter F. Kurat G. Igneous Rock from Severnyi Kolchim (H3) Chondrite: Nebular Origin The discovery of lithic fragments with compositions and textures similar to igneous differentiates in unequilibrated ordinary chondrites (UOCs) and carbonaceous chondrites (CCs) has been interpreted as suggestive that planetary bodies existed before chondrites were formed. As a consequence, chondrites (except, perhaps CI chondrites) cannot be considered primitive assemblages of unprocessed nebular matter. Here we report on our study of an igneous clast from the Severnyi Kolchim (H3) chondrite. The results of the study are incompatible with an igneous origin of the clast but are in favor of a nebular origin--similar to that of chondrules. Nazarov M. A. Badjukov D. D. Barsukova L. D. Kolesov G. M. Naidin D. P. The Koshak Section: Evidence for Element Fractionation and an Oxidation Event at the K/T Boundary The Koshak site (44 47'N; 51 40'E) is a new K/T section located about 125 km EEN of the Fort Shevchenko city, Mangyshlak, Kazakhstan. In this paper we report results of geochemical and mineralogical studies of this section, which indicate a deep element fractionation and an oxidation event at the K/T boundary. Nazarov M. A. Ariskin A. A. The Erevan Howardite: Petrology of Glassy Clasts and Mineral Chemistry The Erevan howardite is a polymict regolith breccia containing xenoliths of carbonaceous chondrites. In this work we studied glassy clasts, which could be considered as primary quenched melts, and mineral chemistry of the breccia. The study reveals that the Erevan howardite consists of common rocks of the HED suite. However, unique glassy clasts, which are present some eucritic melts, were identified. The mineral chemistry and the simulation of crystallization of the melts suggest that the compositions of the melts reflect those of some primary lithologies of EPB. Neal C. R. Taylor L. A. Petrogenesis of Apollo 12 Mare Basalts, Part 2: An Open System Model to Explain the Pigeonite Basalt Compositions Original petrogenetic models suggested that the pigeonite basalts were the evolved equivalents of the olivine basalts. Rhodes et al. (1977)concluded that the olivine and pigeonite basalts were co-magmatic, but Neal et al. (1992a,b, 1993) have demonstrated that these two basaltic groups are distinct and unrelated. The pigeonite suite is comprised of porphyritic basalts with a fine-grained groundmass and range continuously to coarse-grained microgabbros with ophitic to graphic textures. Although it was generally recognized that the pigeonite basalts were derived from the olivine basalts by olivine + minor Cr-spinel fractionation, the compositional gap between these groups is difficult to reconcile with such a model. Indeed, Baldridge et al. (1979) concluded that these two basaltic groups could not have been co-magmatic. In this paper, we suggest an open-system AFC model for pigeonite basalt petrogenesis. The assimilant is lunar anorthositic crust and the r value used is 0.6. While the choice of assimilant composition is difficult to constrain, the modeling demonstrates the feasibility of this model. Nemchinov I. V. Alexandrov P. E. Artemiev V. I. Bergelson V. I. Rybakov V. A. On Magnetodynamic Effects Initiated by a High-Speed Impact of a Large Cosmic Body Upon the Earth's Surface The impact of a large cosmic body with typical size R ~ 1 km (mass M ~ 4-10 Gt for a stony or icy body) moving with velocity V ~ 50-70 km/s (kinetic energy of the order of 10^21 J or 10^6 Mt of TNT) on the Earths surface leads to a full vaporization of a body and a significant part of substance of the upper layers of the Earth and even to the ionization of this vapor cloud. As a result, a hypersonic jet of air and erosion plasma is formed. The kinetic energy E(sub)j of the jet can reach 10-20% of the kinetic energy of a cosmic body or even more. The magnitude of E(sub)j is far above the mass of atmosphere in the jet expansion cone. Thus the jet will propagate practically interstitially with the constant mean velocity U ~ 10-20 km/s and even higher. The interaction of this plasma jet with the Earths magnetic field causes magnetodynamic effects similar to those that are produced by cosmic nuclear explosions, but on a larger scale. Nemchinov I. V. Popova M. P. Shubadeeva L. P. Shuvalov V. V. Svetsov V. V. Effects of Hydrodynamics and Thermal Radiation in the Atmosphere after Comet Impacts Radiation phenomena in the atmosphere after impacts of cosmic bodies have special features in comparison with surface nuclear explosions. First, initial concentration of energy after the impact is lower and, second, a wake after the passage of the meteoroid through the atmosphere has a dramatic effect on atmospheric flow and radiation transfer. Consequently, scaling laws cannot be employed for prediction of the flow in the atmosphere and the light flux on the Earths surface. Nikishin A. M. Burba G. A. Geologic Mapping of Northern Atla Regio on Venus: Preliminary Data The Northern part of Atla Regio within the frame of Cl format Magellan photomap 15N197 was mapped geologicaly at scale 1:8,000,000. Nikishin A. M. Bobina N. N. Borozdin V. K. Burba G. A. Beta Regio Rift System on Venus: Geologic Interpretation of Magellan Images Magellan SAR images and altimetric data were used to produce a new geologic map of the northern part of Beta Regio within the frames of C1- 30N279 map sheet It was part of our contribution into Cl-format geologic mapping efforts The original map is at 1:8,000,000 scale. Nikishin A. M. Borozdin V. K. Bobina N. N. Burba G. A. Beta Regio -- Phoebe Regio on Venus: Geologic Mapping with the Magellan Data The geologic maps of C1-15N283 and C1-OON283 sheets were produced (preliminary versions) with Magellan SAR images. This work was undertaken as a part of Russia's contribution into C1 geologic mapping efforts. The scale of the original maps is 1:8,000,000 and the maps are reproduced here at a reduced size. Nikolaeva O. V. Largest Impact Features on Venus: Non-Preserved or Non-Recognizable? Conventional explanation of a lack of impact craters with diameters >300 km on Venus [Schaber et al.,1992] is that they formed during the intense bombardment era and had lunar-like morphology, but they are not preserved now because of rapid viscous relaxation of their topograpy or/and high endogenous reworking of surface. Other explanation invokes failure to recognize these larger craters because of their non-lunar-like morphology from the moment of formation [Nikolaeva et al.,1986; Hamilton,1992], since larger gravity of Venus relatively to the Moon results in that largest craters on Venus may form within the mass of shock melted material while comparably sized lunar craters would be still almost "dry" [Melosh, 1989; Cintala and Grieve, 1991; Grieve and Cintala, 1992; O'Keefe and Ahrens, 1992]. To test this hypothesis, morphologies and rim-crest diameters of the largest peak-ring and Orientale- type basins and all the larger impact features on Moon, Mercury, Mars, and Venus were compiled and compared to rim-crest diameters of model craters with different melt volume/transient-cavity volume ratios from [Cintala and Grieve, 1991]. Results show that the final diameters of model craters formed at depth of melting about twice of transient cavity depth correspond to changeover from a planet-similar morphology of all the smaller basins on any terrestrial planet to a planet-specific morphology of all the larger basins on Moon(?), Mercury, and Mars. On Venus, these largest impact features are not found and instead, a Venus-specific morphology of the largest concentric coronae appears in this size range. The coronae were suggested to form over sites of mantle upwelling and modified by subsequent volcanism and gravitational relaxation [e.g., Squyres,1992]. The results here suggest that mantle upwelling - the first and necessary step of the corona formation models - may be induced by impact event (as a result of transient cavity collapse) and operated under cover of hot, slowly cooled impact melt in the areas of thinned crust and/or thermally active regions. Norman M. D. Griffin W. L. Ryan C. G. Volatility in the Lunar Crust: Trace Element Analyses of Lunar Minerals by PIXE Proton Microprobe In situ determination of mineral compositions using microbeam techniques can characterize magma compositions through mineral-melt partitioning, and investigate fine-grained or rare phases that cannot be extracted for analysis. Abundances of Fe, Mn, Sr, Ga, Zr, Y, Nb, Zn, Cu, Ni, Se, and Sb were determined for various mineral phases in a small number of lunar highlands rocks using the PIXE proton microprobe. Sr/Ga ratios of plagioclase and Mn/Zn ratios of mafic silicates show that the ferroan anorthosites and Mg-suite cumulates are depleted in volatile lithophile elements to about the same degree compared to chondrites and the Earth. This links the entire lunar crust to common processes or source compositions. In contrast, secondary sulfides in Descartes breccia clasts are enriched in chalcophile elements such as Cu, Zn, Ni, Se, and Sb, and represent a potential resource in the lunar highlands. Oehler A. Dummel A. Goniospectrometric Properties of a White Standard Reflection spectroscopy is one of the fundamental tools in planetary remote sensing. Most of the laboratory work is done in terms of bidirectional reflectance R in the wavelength range of reflected sunlight (250 nm-2500 nm). Bidirectional reflectance R is defined as the radiance of a surface relative to the radiance of an ideal Lambertian reflector, identically illuminated (radiance coefficient in Hapke, 1981). For quantitative laboratory work the absolute scale of R has to be known. In addition, for many purposes it is necessary to know the dependence of R from phase angle for a large range of geometries. In many cases this can only be achieved if the goniospectrometric properties of the white standard that is used are known with high precision. For a long time Halon has been used as a white standard by most laboratories. Because Halon is no longer available we have now analysed the goniospectrometric properties of Spectralon(TM) SRS-99 with the DLR- goniospectrometer. Spectralon is manufactured by Labsphere Inc, North Sutton, NH 03260, USA. Oglesby J. P. Lindsay W. L. Sadeh W. Z. Characterization of Minnesota Lunar Simulant for Plant Growth Processing of lunar regolith into a plant growth medium is crucial in the development of a regenerative life support system for a lunar base. Plants, which are the core of such a system, produce food and oxygen for humans and, at the same time, consume carbon dioxide. Because of the scarcity of lunar regolith, simulants must be used to infer its properties and to develop procedures for weathering and chemical analyses. The Minnesota Lunar Simulant (MLS) has been identified to date as the best available simulant for lunar regolith. Results of dissolution studies reveal that appropriately fertilized MLS can be a suitable medium for plant growth. The techniques used in conducting these studies can be extended to investigate the suitability of actual lunar regolith as a plant growth medium. Olsen E. J. Hutcheon I. Tambo Quemado: Extraordinary Concentrations of REE and Refractory Trace Elements Caused by Artificial Heating The finders of the IIIB iron Tambo Quemado (TAMQ) had artificially heated the 130-kg. mass to 1000 degrees C for about 1 hour. In the interior, schreibersite melted. The iron also contained typical phosphate inclusions (graftonite). One such inclusion was sampled and studied by SEM, electron microprobe, and ion microprobe to determine the effect of this heating "experiment" on the mineralogy and chemistry. Graftonite apparently melted and a small amount of oxygen entered the system. Below the 940 degree C eutectic of the Fe-O-P system, graftonite recrystallized with phosphoran wustite. In addition, silica was added to the inclusion forming a new silicophosphate phase with a Si:P ratio of 1:2. Past ion microprobe work has shown that graftonite and other phosphate phases in IIIAB irons are always very depleted in lithophile trace elements, which supports their origin by oxidation of siderophile phases, metal with dissolved phosphorus or schreibersite. These three phases in TAMQ, graftonite, silicophosphate, and phosphoran wustite, contain REE at or significantly above C1 normalized values and refractory lithophile elements that vary below and above C1 normalized values. We conclude that temperature gradients in the heated mass permitted diffusion of very low traces of lithophile trace elements from one inclusion to concentrate in another. This suggests that short-term reheating can easily affect trace element distributions in these irons and that trace element, especially REE patterns may not be pristine in some instances. Pinkerton H. Norton G. A Comparison of Calculated and Measured Rheological Properties of Crystallising Lavas in the Field and Laboratory Models of most magmatic processes, including realistic models of planetary lava flows require accurate data on the rheological properties of magma. Previous studies suggest that field and laboratory rheological properties of Hawaiian lavas can be calculated from their physicochemical properties using a non-Newtonian rheology model. The present study uses new measurements of the rheological properties of crystallizing lavas to show that this is also true for lavas from Mount Etna. Rheological measurements on quenched Etna basalts were made in a specially designed furnace using a Haake Rotovisco viscometer attached to a spindle that has been designed to eliminate slippage at the melt- spindle interface. Using this spindle, we have made measurements at lower temperatures than other workers in this field. From these measurements, Mount Etna lavas are Newtonian at temperatures above 1120 degrees C and they are thixotropic pseudoplastic fluids with a yield strength at lower temperatures. The close agreement between calculated and measured rheology over the temperature range 1084-1125 degrees C supports the use of the non-Newtonian rheology model in future modeling of planetary lava flows. Plescia J. B. Eruption History of the Tharsis Shield Volcanoes, Mars The Tharsis Montes volcanoes and Olympus Mons are giant shield volcanoes. Although estimates of their average surface age have been made using crater counts, the length of time required to build the shields has not been considered. Crater counts for the volcanoes indicate the constructs are young; average ages are Amazonian to Hesperian. In relative terms; Arsia Mons is the oldest, Pavonis Mons intermediate, and Ascreaus Mons the youngest of the Tharsis Montes shields; Olympus Mons is the youngest of the group. Depending upon the calibration, absolute ages range from 730 Ma to 3100 Ma for Arsia Mons and 25 Ma to 100 Ma for Olympus Mons. These absolute chronologies are highly model dependent, and indicate only the time surficial volcanism ceased, not the time over which the volcano was built. Pohn H. A. Schaber G. G. Crater Destruction on the Venusian Highlands by Tectonic Processes It is apparent that few, if any, craters as old or highly modified as Imbrian craters on the surface of the Moon are present on the venusian highlands, or indeed anywhere on the planet's surface. Degraded craters such as those seen on the Moon, Mercury, or Mars are conspicuously absent. Furthermore, virtually all the impact craters on the venusian surface show modification only by extensional tectonics, whereas the venusian highlands show modification by compression, strike-slip movement, and finally by extension. Presumably at an earlier time, the surface of Venus resembled the surfaces of the other inner planets. The relatively recent resurfacing event that produced the plains units may be a mechanism for covering the older craters in the plains, but these vast outpourings of lava cannnot be invoked as a mechanism for covering the craters on the venusian highlands. If the plains units had covered the highlands, the entire venusian surface would appear to be as smooth as the plains units that are embayed by and, therefore, postdate the highlands. The last major tectonic events to affect the highlands such as those seen on Ovda Regio appear to be thrust faulting with consequent folding, followed by at least three episodes of strike-slip faulting and finally extensional faulting. To test whether such tectonic movements could have destroyed highlands craters, a preliminary experiment was conducted by using a Lunar Orbiter mosaic (Fig. 1, Lunar Orbiter IV-169- H1, Hevelius region) of the lunar uplands as an analogue for the ancient venusian highlands. Price M. Suppe J. Studying Venus Using a GIS Database A Geographic Information System (GIS) can significantly enhance geological studies on Venus because it facilitates concurrent analysis of many sources of data, as demonstrated by our work on topographic and deformation characteristics of tesserae. We are creating a database of structures referenced to real-world coordinates to encourage the archival of venusian studies in digital format and to foster quantitative analysis of many combinations of data. Contributions to this database from all aspects of venusian science are welcome. Provaliv A. A. Ivanov B. A. Impact Disturbance of the Venus Atmosphere Experimental simulations of the atmosphere-surface interaction during high-velocity impact are presented. At Venus an atmospheric vortex, generated with impact, may interacts with a local wind. Some surface features, observed on Magellan images, may be related to the simulated effect. Rietmeijer F. J. M. Micrometeorite Dynamic Pyrometamorphism: Nonstoichiometric Clinoenstatite (CLEN) Polymorphs of enstatite are common phases in many meteorites. They contain clues on their formation and the thermal evolution of their host rock, which includes shock metamorphism. Rare, micrometer-sized, CLEN whiskers and thin platelets in chondritic porous micrometeorites were interpreted as solar nebula condensates that remained unaffected during atmospheric entry flash-heating. This CLEN formed by (1) protoenstatite (PEN) inversion whereby the surface energy of the micrometer-sized PEN crystals aided the OREN-CLEN transformation or (2) by metastable growth. Ca-poor, Mg,Fe-pyroxene with unequilibrated, intraparticle, Mg/(Mg+Fe) distributions occurs in most chondritic micrometeorites. These distributions are a parent body signature that survived dynamic pyrometamorphism because the duration of the thermal spike during atmospheric entry is too short, but this conclusion does not consider the ultrafine grain size of micrometeorites. The maximum temperature and duration of the heating event will depend on the kinetic energy and entry angle of the incoming micrometeorite. But lacking detailed petrological data for an individual particle, its thermal profile during atmospheric entry cannot be deduced from its mass alone as a function of entry angle. In order to constrain dynamic pyrometamorphism in unmelted micrometeorites I have determined the petrological composition and silicate mineralogy in nonchondritic micrometeorites L2005T13, L2005E40, and L2006A28. Rosenblatt P. Pinet P. Thouvenot E. Comparative Hypsometric Analysis of Both Earth and Venus Topographic Distributions Previous studies have compared the global topographic distribution of both planets by means of differential hypsometric curves. For the purpose of comparison, the terrestrial oceanic load was removed and a reference base level was required. It was chosen on the basis of geometric considerations and reflected the geometric shape of the mean dynamical equilibrium figure of the planetary surface in both cases. This reference level corresponds to the well- known sea level for the Earth; for Venus, given its slow rate of rotation, a sphere of radius close to the mean, median, and modal values of the planetary radii distribution was considered and the radius value of 6051 km arbitrarily taken. These studies were based on the low-resolution (l00 x l00 km^2) coverage of Venus obtained by the Pioneer Venus altimeter and on the 1 degree x 1 degree terrestrial topography. But, apart from revealing the distinct contrast existing between the Earth's bimodal and the strong Venus' unimodal topographic distribution, the choice of such a reference level is inadequate and even misleading for the comparative geophysical understanding of the planetary relief distribution. Russell J. F. Schaber G. G. Named Venusian Craters Schaber et al. compiled a database of 841 craters on Venus, based on Magellan coverage of 89% of the planet's surface. That database has been expanded to 912 craters ranging in diameter from 1.5 to 280 km, derived from coverage of approximately 98% of Venus' surface. About 150 of the larger craters were previously identified by Pioneer Venus and Soviet Venera projects and subsequently formally named by the International Astronomical Union (IAU). Sazonova L. Feldman V. Korotaena N. Preliminary Estimation of Tagamite Cooling Conditions (Puchezh-Katunki Astrobleme, Russia) The velocities and initial temperatures of crystallization from super- heated shocked melts have been estimated as a result of the investigation of the structures and mineral association in impact melt rocks of Puchezh-Katunki astrobleme. The dependence of the these parameters vs. the body thickness of impact melt rocks has been found. Schaber G. G. Chadwick D. J. Venus' Impact-Crater Database: Update to ~98% of the Planet's Surface A total of 912 impact craters between 1.5 and 280 km in diameter have now been identified on about 98% of Venus' surface. This total includes an additional 71 craters (mostly in the southern hemisphere) not in the crater inventory given in Schaber et al. for 89% of the planet. (Extrapolated to 100% of the surface, the inventory should consist of 930 craters >1.5 km in diameter.) The total crater inventory for 98% of the planet is only slightly larger (912 versus 882) than that extrapolated from the inventory of 135 craters when only 15% of the planet had been mapped by Magellan. The remarkably consistent density of about 2.02 impact craters per 10^6 km^2 emphasizes the completely random nature of the crater population's distribution. About 400 craters recognized from Magellan mapping have been assigned names. Schmidt G. Zhou L. Wasson J. T. Iridium in Sediments Containing Large Abundances of Australasian Microtektites from DSDP Hole 758B in the Eastern Indian Ocean and from DSDP Hole 769A in the Sulu Sea Excess Ir found in sediments at the Cretaceous/Tertiary (K/T) boundary and in other (eg., Pliocene) sediments from deep sea drilling cores is widely interpreted as evidence of major impact events. The Australasian tektites originated in an impact event ~0.77 Ma ago; microtektites have been found in deep-sea sediment cores from throughout the Indian Ocean, the Philippine Sea, and western Pacific Ocean, but Ir has not been previously reported in these horizons. The deep-sea record of tektites is of particular interest, because in contrast to most continental occurrences, the stratigraphy preserves the original depositional position. Recently several cores having exceptionally high contents of Australasian microtektites have been investigated, Glass and Wu found shocked quartz associated with the microtektites. We used neutron activation to determine concentrations of Ir and other elements in two cores bearing microtektites, one from Deep Sea Drilling Project (DSDP) hole 758B in the Eastern Indian Ocean and one from DSDP hole 769A in the Sulu Sea (near Mindanao, Philippines). The sedimentation age for the microtektite layers in core 758B lies between 0.73-0.78 Ma and agrees well with the mean laser-fusion 40Ar/39Ar age of Australasian tektites of 0.77+-0.02 Ma by Izett et al. We are able to resolve a small positive Ir enhancement in 758B. Core 769A shows too much scatter to allow resolution of an Ir peak. Schultz R. A. Strength and Deformation Properties of Basaltic Lava Flows on Planetary Surfaces Basaltic rocks are thought to constitute a volumetrically significant rock type on the Moon, Mercury, Mars, and Venus, in addition to the Earth. Spacecraft images of surfaces with known or suspected basaltic composition on these bodies, particularly on Venus, indicate that these rocks have been deformed in the brittle regime to form faults and perhaps dilatant cracks, in addition to folding and more distributed types of deformation [1]. Semenova A. S. Nazarov M. A. Kononkova N. N. Lunar Basalt Meteorite EET 87521: Petrology of the Clast Population The Elephant Moraine meteorite EET 87521 was classified as a lunar mare basalt breccia, which is composed mainly of VLT basalt clasts. Here we report about our petrological study of lithic clasts and monomineralic fragments in the thin sections EET 87521,54 and EET 87521,47,1 prepared from the meteorite. The results of the study show that EET 87521 consists mainly of Al-rich ferrobasalt clasts and olivine pyroxenite clasts. The bulk composition of the meteorite can be well modelled by mixing these lithic components that appear to be differentiates of the Luna 24 basalt melt. KREEP and Mg-rich gabbro components are minor constituents of EET 87521. Senftle F. E. Thorpe A. N. May L. Barkatt A. Adel-Hadadi M. A. Marbury G. S. Izett G. Sigurdsson H. Maurasse F. J.-M. R. Magnetic Properties and Mossbauer Analyses of Glass from the K-T Boundary, Beloc, Haiti The experimental magnetic susceptibility, the temperature-independent component of the magnetic susceptibility, the magnetization, and the Curie constant have been measured for a number of specimens of glass from the K-T boundary found at Beloc, Haiti, and the results are compared with those of similar measurements of tektites. Because the Fe3+/Fe2+ ratio is needed to calculate the magnetic parameters, Mossbauer spectroscopic measurements were also made. The data were consistent with the classification of the Beloc glasses as tektites. Shearer C. K. Papike J. J. Origin of the Apollo 15 Very Low Ti Green Glass. A Perspective from the Compositional Diversity in the Very Low Ti Glasses The very low Ti green glasses from the Apollo 15 site have intrigued scientists for over 20 years. Their primitive composition has been used to understand magmatic processes and the structure of the Moon. The compositional variability observed in the Apollo 15 glass population has long been a point of debate. Initial studies did not recognize the compositional diversity in the glasses. Stolper et al. documented the major element variability and concluded it could not be produced by magmatic processes and therefore concluded that these glasses must be of impact origin. Subsequent studies confirmed a volcanic origin for the glass population and attempted to elucidate magmatic processes to account for its compositional variability. Models that have been proposed for these glasses include (1) the crystallization of single or multiple phases (olivine, pyroxene, Fe metal, immiscible sulfide), (2) the incompatible behavior of Ni and Co during multiple phase crystallization at extremely low fO2, and (3) magma or source mixing. All of these models have problems. Type (1) models appear not to be consistent with recent trace element studies on the glasses, model (2) is dependent on the debatable incompatible behavior of Ni and Co, and in models of type (3) the origin and nature of mixing models are somewhat unconstrained. This study compares the Apollo 15 green glasses with the very low Ti picritic glasses from other landing sites. Snyder G. A. Taylor L. A. Halliday A. N. Chronology and Genesis of High-Ti Mare Volcanism: Melting of Cogenetic Depleted and Enriched Reservoirs An understanding of the chronology and petrogenesis of volcanic rocks is paramount to unraveling the complexities of the composition and structure of the mantle of the Moon. High-Ti mare basalts represent the best-sampled suite of lunar volcanic rocks and probably represent melts of the uppermost mantle of the Moon. Compiled Nd and Sr isotopic data for high-Ti basalts, combined with weighted average ages determined by various techniques, yield a complex melting history for the high-Ti mantle source. Melting occurred during three distinct episodes (3.84 Ga, 3.75-3.69 Ga, and 3.56 Ga) separated by hiatuses of 100 Ma or more. The first episode included the melting of a relatively trapped liquid (KREEPy)-rich source beneath the A-11 landing site. Later melting of this source produced magmas that were relatively trapped liquid- free at the A-11 site. Synchronous melting at the A-17 landing site produced magmas with variable proportions of this evolved trapped liquid component. The final phase of volcanism occurred at the A-11 site and involved the assimilation of an evolved "neuKREEP" component. Continued melting of a similar source, which was nearly exhausted of its ilmenite component, beneath the A-12 landing site may have lead to production of magmas parental to the A- 12 ilmenite basalts. Snyder G. A. Taylor L. A. Halliday A. N. Magnesian Anorthosites from the Western Highlands of the Moon: Isotope Geochemistry and Petrogenesis Breccias from the Apollo 14 landing site have provided a wealth of information on the genesis of the lunar highlands. Various pristine rock types have been discovered in re1ative abundance, including rare ferroan anorthosites, and alkali-suite and magnesian-suite rocks. Mineral-chemical and radiogenic isotopic data are reported here for a new1y discovered Mg-suite anorthosite from Apollo 14, sample 14303,347. Meyer et al. reported U-Pb zircon analyses of Mg-suite highlands rocks from the western limb of the Moon. We have compiled these ages and generated a weighted average age of 4211 + 5 Ma, some 200 Ma younger than ferroan anorthosites. Utilizing this age for Mg anorthosite 14303,347, our data result in an initial epsilonNd value of -1.0 and initial 87Sr/86Sr of 0.69915. Based on trace-element, isotopic, and mineral-chemical data, the western highlands Mg suite is interpreted to be crustal precipitates of a picritic magma, which assimilated KREEPy trapped liquid from upper-mantle cumulates during its transport to the crust. Stooke P. J. Linear Structures on the Small Inner Satellites of Saturn Shape modelling methods developed for nonspherical worlds were applied to four satellites of Saturn, Prometheus, Pandora, Janus, and Epimetheus, resulting in the first detailed shaded relief maps of their surfaces. Ridges and valleys are described with their implications for satellite history and asteroid 951 Gaspra. They probably result from fracturing during break-up of parent bodies and/or later large impacts. Prometheus and perhaps Gaspra may be coated with debris from parent body fragmentation as well as more recent regolith. Finn V. J. Baker V. R. Dolginov A. Z. Morphotectonics of Venus Venus topography can be mapped morphostructurally to reveal nested hierarchical patterns of quasicircular upland/lowland complexes. These patterns are interpreted as surficial effects of hierarchically structured, long-acting mantle convection. Beta Regio, Alpha Regio, and Artemis illustrate this process of dynamical interaction between the deforming lithosphere and the convecting mantle. Surkov Y. A. Head J. W. Kremnev R. Nock K. T. Discovery Venera Team Discovery Venera Surface-Atmosphere Geochemistry Experiments Mission Concept The phenomenal increase in our understanding of Venus provided by the Magellan Mission has raised a series of focused, fundamental scientific questions about the geochemistry of the surface of Venus, the nature of the lower atmosphere, and the relationship of the lower atmosphere and surface. First, surface geochemical measurements from the Venera/Vega spacecraft showed that widely spaced regions of the venusian plains are made of basalts; thus basalts are significant and may be the only component of the venusian crust. But we lack information on the composition of several key elements of Venus geology: (1) Tessera terrain (which may be outcrops of continental-like nonbasaltic crustal material) and steep-sided domes/festoons are promising candidates for nonbasaltic geochemically evolved material. The composition of the lower part of the venusian crust is unknown: however, ejecta from large venusian craters provides us with the possibility of sampling this material on the surface. (2) Bulk chemistry (structure and dynamics) of the venusian atmosphere is known. The altitude profiles of water vapor content and minor admixtures relevant to redox conditions in the lower atmosphere(<20 km altitude) remain uncertain. Lack of that knowledge means that we do not understand the fine chemical structure of the main mass of the venusian atmosphere. (3) Thermodynamic models predict that igneous materials on the surface of Venus should react with gases of the venusian atmosphere. but because the water vapor content and redox conditions in the lower atmosphere are not well known, we do not understand the nature of venusian weathering: oxidation, sulfatization, carbonatization, hydration. The answers to these questions are critical to the understanding of Venus, the most Earth-like of the terrestrial planets. Takatori K. Tomeoka K. Tsukimura K. Takeda H. Hydrothermal Alteration Experiments of Olivine with Varying Fe Contents: An Attempt to Simulate Aqueous Alteration of the Carbonaceous Chondrites Hydrothermal alteration experiments of olivine with several Fe/Mg ratios were carried out under acidic and neutral conditions, and TEM observations were made on the run products. Well-developed tubular crystals of serpentine (chrysotile) were synthesized from Fo100 under both acidic and neutral conditions, and from Fo92 and Fo80 under acidic conditions. Abundance and size of chrysotile apparently depend on the Fe contents of olivine, i.e., with increasing Fe contents, less and smaller chrysotile was formed. Acidity of the solution plays an important role for the formation of chrysotile. Platy and fibrous crystals of phyllosilicate, probably serpentine, were obtained from Fo50 and Fo20 treated under acidic conditions, which are most similar to the phyllosilicates in the CI and CM chondrites. Framboidal aggregates of Fe-rich grains (presumably Fe-hydroxide) were formed from Fa100 and Fo20, but no phyllosilicate was formed from Fa100. Tanaka K. L. Schultz R. A. Large, Ancient, Compressional Structures on Mars Our search for large compressional structures similar to the Coprates rise reveals that the rise is part of a group of about three dozen structures (here named the "Thaumasia fold belt") that encircles the southern part of the Tharsis region. Similar structures are scattered over Mars. All appear to be Noachian or Early Hesperian in age, suggesting that lithospheric-scale buckling was a common form of early martian compressional deformation. Teterev A. V. Nemchinov I. V. The Sandbag Model of the Dispersion of the Cosmic Body in the Atmosphere The strength of extraterrestrial bodies depends on their structure, composition, dimensions, and history, but usually is low enough that fragmentation due to aerodynamic stresses begins sufficiently high above the surface of the Earth. Teterev A. V. Misychenko N. I. Rudak L. V. Romanov G. S. Smetannikov A. S. Nemchinov I. V. Atmospheric Breakup of a Small Comet in the Earth's Atmosphere Aerodynamic stresses can lead to the deformation and even to the destruction of the meteoroids during their flight through the atmosphere. The pressure at the blunt nose of the cosmic body moving at very high speed through the dense layers of the atmosphere may be much larger than the tensile or the compressive strength of the body, so the use of hydrodynamics theory is validated. The estimates show that the transverse velocity of the substance of the body U is of the order of [rho(sub)a/rho(sub)o]^1/2V, where V is the velocity of the body, rho(sub)o is its density, and rho(sub)a is the density of the atmosphere. Therriault A. M. Reid A. M. Reimold W. U. Original Size of the Vredefort Structure, South Africa The Vredefort structure is located approximately 120 km southwest of Johannesburg, South Africa, and is deeply eroded. Controversies remain on the origin of this structure with the most popular hypotheses being (1) by impact cratering about 2.0 Ga, (2) as a cryptoexplosion structure about 2.0 Ga, and (3) by purely tectonic processes starting at about 3.0 Ga and ending with the Vredefort event at 2.0 Ga. In view of recent work in which the granophyre dikes are interpreted as the erosional remnants of a more extensive impact melt sheet, injected downward into the underlying country rock, the impact origin hypothesis for Vredefort is here adopted. Tompkins S. Mustard J. F. Pieters C. M. Forsyth D. W. Objective Determination of Image End-Members in Spectral Mixture Analysis Spectral mixture analysis has been shown to be a powerful, multifaceted tool for analysis of multi- and hyperspectral data. The essence of the first phase of the approach is to determine a set of image end members that best account for the spectral variance in an image cube within a constrained, linear least squares mixing model. The selection of the image end members is usually achieved using a priori knowledge and successive trial-and-error solutions to refine the total number and physical location of the end members. However, in many situations a more objective method of determining these essential components is desired. We approach the problem of image end member determination objectively by using the inherent variance of the data. Unlike purely statistical methods such as factor analysis, this approach derives solutions that conform to a physically realistic model. Underwood J. R. Jr. Casacchia R. Woronow A. Teeling M. J. Geology of Galileo Regio Quadrangle, Ganymede Galileo Regio quadrangle, including most of Galileo Regio, contains two basic geologic units: older dark, furrowed, and heavily cratered material and younger light, grooved, relatively less-cratered material. Dark material dominates, the light material occurring to the W and SW in about 5% of the quadrangle. Dominant structural features, of uncertain origin, are the northwest-trending arcuate furrows and associated orthogonal and oblique furrows in the dark material; grooves in the light material are not as well ordered nor as intensely developed as elsewhere on Ganymede. Fifteen palimpsests, located partly or totally in the quadrangle and representing the period when the crust became sufficiently strong to record impact events, are subdivided into three relative-age classes, as are the numerous impact craters, some of which are dome-, moat-, and rampart-like. Galileo Regio is believed to have formed when, because of cooling, differentiation, and expansion, the thin, weak, icy lithosphere broke into blocks and light material filled the space between, or overspread those blocks that subsided, or both. Grooves formed in most of the light material and both light and dark material were cratered by impact, the most recent of which created bright-ray craters, the youngest features visible in the quadrangle. Ustinova G. K. Lavrukhina A. K. On Modelling Nuclear Reactions in Meteorites An original method of experimental modelling of the depth distribution of radionuclides in spherical thick targets irradiated by protons in 4- pi geometry on JINR (Dubna) synchrocyclotron is described. Some results are presented. Veeder G. J. Tedesco E. F. IMPS Albedo and Diameter for Asteroid 243 Ida 243 Ida is the second asteroid target of the Galileo mission. The Infrared Astronomical Satellite (IRAS) detected Ida several times during its 1983 sky survey. The IRAS Minor Planet Survey (IMPS) yields a total of 13 usable observations during 6 sightings of Ida. These data result in a geometric visual albedo of 0.24 and a mean diameter of 28 km for Ida. The IMPS catalog updates and extends the IRAS Asteroid and Comet Survey through asteroid number 4679. File versions of IMPS final products will be available from the National Space Science Data Center (NSSDC). The input for IMPS processing includes updated visual absolute magnitudes and orbital elements for each asteroid. H and G are 9.94 and 0.15 for Ida. IMPS also includes a correction for low flux densities (less than ~1 Jansky). In the case of Ida, 3 observations at 12 micrometers, 6 at 25 micrometers, and 4 at 60 micrometers were considered acceptable for analysis. Most of these do have flux densities less than 1 Jansky with a value of approximately 5 for their estimated SNR. The 25 micrometer observations as plotted in the figure are consistent with the variation expected for the cross section of Ida with rotation. Ida is a main belt asteroid with an S taxonomic classification. The spectra of S asteroids tend to be dominated by pyroxene with visual albedos from 0.1 to 0.3. The IMPS average albedo of 0.24 (+- 0.07) for 243 Ida is in the upper range observed for S asteroids. Vityazev A. V. Dorofeyeva V. A. Makalkin A. B. Recondensation in the Early Solar System: Physical Conditions and Source Material Many physical processes in the solar nebula led to fractionation of the gaseous phase of the preplanetary material. We list some of these processes with the corresponding type of fractionation in terms of Rock/Ice/Gas. Recondensation from fractionated gas phase yields different compositions of condensates. We consider in some detail one type of fractionation, impact heating with evaporation and subsequent recondensation of material during intercollisions of planetesimals. For thermodynamical simulation of recondensation in such a process one needs (a) P-T parameters and (b) some initial source material to be heated, evaporated, and recondensed, so these two problems are the main subject of the present abstract. In another paper we discuss the results of the thermodynamical simulation. Wright I. P. Douglas C. Pillinger C. T. Further Carbon Isotope Measurements of LEW 88516 Douglas et al. have previously analysed the carbon content and isotopic composition of a crushed sample (subsample 13) of the shergottite LEW 88516. The powder, which was prepared from a relatively large portion of the meteorite in order to obtain a representative sample, was distributed among the scientific community. However, it is probable that the preparation procedure was not optimised for the purposes of carbon measurements. Indeed, it was found that LEW 88516,13 contained over 1200 ppm carbon, a concentration that is greater than that present in any other SNC meteorite. That a close relative, ALH A77005, contains only 141 ppm carbon seems to implicate the preparation procedure as being responsible for the apparently high carbon content of LEW 88516. However, it may also be the nature of the fine powder that has resulted in contamination. The observation of a high carbon content in LEW 88516,13 highlights the extreme difficulty of trying to obtain representative samples of whole meteorites for this kind of investigation. Presented are some further measurements of LEW 88516 that should serve to clarify some of the issues raised by the previous investigation. Wu S. Robertson P. B. Grieve R. A. F. Shock Attenuation at the Slate Islands Revisited The Slate Islands, off the north shore of L. Superior, represent the emergent portion of the central uplift of a complex crater with an estimated diameter of 30 km. Previous systematic studies of the shock metamorphism at the Slate Islands have concentrated on the orientation of shatter cones, complemented by paleomagnetic data, and observations of the distribution of planar deformation features in quartz to locate the "shock center" and to estimate the rate of shock pressure decay from the center. The impact origin of the Slate Islands, however, has not been accepted by Sage, who attributes the shock metamorphic effects to the intrusion of diatreme breccias. This argument, in turn, has been used by others to dispute the established relationship between shock metamorphism and impact and to argue that the existence of planar deformation features in quartz from K/T boundary deposits does not require the occurrence of a hypervelocity impact event. Because of Sage's continuing assertion that the planar deformation features at Slate Islands are associated with breccia dikes and have, therefore, an endogenic origin, we have extended the earlier analysis to a more spatially extensive suite of samples than observed previously. Yamakoshi K. Ohashi H. Noma M. Sakurai H. Nakashima K. Nogami K. Omori R. Geometrical Analysis of the Microcraters Found on LDEF Samples In this work, diameters(D) and depths(T) of microcraters found on LDEF samples were measured and their origins were deduced by the (D/T) ratios, which distinguish projectile materials. From the results, one iron and several stony projectiles could be recognized. Yoder C. F. The Connection Between Venus' Free Obliquity and Its CMB Oblateness The most striking feature of Venus rotational state is its slow retrograde rotation, which is apparently maintained by a balance between solid tidal friction and thermal tidal torques. Solid tides tend to drive the spin toward sychronous rotation while thermal tides drive it away. A balance is achieved at a specific rate because of the inverse frequency dependence of the thermal tide to the semi-diurnal heating. Atmospheric models have been constructed to estimate the thermal tidal torque based on ground heating. The solid friction dissipation factor Q~50 can be deduced assuming rotation has achieved steady state. The most perplexing feature of Venus orientation is its non-zero free obliquity epsilon~1.5 degrees relative to its orbit. Although solid tides and perhaps atmospheric tides tend to increase the free obliquity on a time l/Kt=4QCa^3omega/(3k(sub)2n^2M(sub)sunR^3)~lxl0^8yr, viscous friction(CMF) at a mantle boundary(CMB) resulting from the differential angular orientation of the core and mantle spin axes should have damped the free obliquity on a time scale as short as 10^6 yr. 0ne means of achieving a balance similar to that controlling rotation is to introduce a comparatively large CMB ellipticity ec=(C(sub)c-A(sub)c)/C(sub)c=(a(sub)c-c(sub)c)/a(sub)c to reduce delta epsilon s potential frequency dependence of the tides as on the quadratic dependence of CMF on delta epsilon if the layer is turbulent. Yoneda S. Sylvester P. J. Simon S. B. Grossman L. Hsu A. Trace Element Compositions of Spinel-rich Refractory Inclusions from the Murchison Meteorite Three spinel-rich, perovskite-bearing, pyroxene-rimmed, hibonite-free inclusions from the Murchison C2 chondrite have volatility-controlled, modified group II REE patterns. Two have high Eu/Yb ratios, two have small positive Ce anomalies and one has high refractory siderophiles for a group II inclusion. Two additional spinel-rich inclusions have subchondritic Ce/La ratios and negative Eu and Yb anomalies, the Eu anomalies being larger than the Yb ones. One of these inclusions is enriched in La relative to all HREE and the other is enriched in HREE relative to all LREE except La, possibly suggesting the presence of an ultrarefractory component in the latter. One hibonite-rich inclusion has a group III REE pattern with a negative Ce anomaly. A calcium dialuminate-bearing inclusion known to have crystallized from a melt at 2100-1800K has a modified group II REE pattern, rather than a pattern that would be predicted from the evaporation expected from such a droplet, e.g., a group III or ultrarefractory pattern. Volatile element contents of these inclusions are among the lowest found in any refractory inclusions. Zipfel J. Palme H. Chemical Composition of New Acapulcoites and Lodranites The bulk compositions of two Antarctic lodranites, MAC 88177 and FRO 90011, and two acapulcoites, ALH 81261 and Monument Draw, were determined with instrumental neutron activation analysis. Acapulcoites have essentially chondritic major and trace element abundances but achondritic texture. They consist of entirely recrystallized, fine-grained mineral assemblages. Chondrules are extremely rare; one relict radiating pyroxene chondrule was described in Monument Draw (MD). The coarse-grained lodranites also have achondritic textures, but are different compositionally with depletions in Al, Na, and incompatible elements, probably a result of separation of a partial, feldspar-rich melt. MAC 88177 is significantly more depleted in incompatible elements than FRO 90011, suggesting a higher degree of partial melting for the MAC lodranite. The chemical data support a genetic relationship between lodranites and acapulcoites inferred earlier from oxygen isotopes, petrology, and mineral composition. Zuber M. T. Bills B. G. Frey H. V. Kiefer W. S. Nerem R. S. Roark J. H. Possible Flexural Signatures Around Olympus and Ascraeus Montes, Mars The effective elastic thickness (h) of the lithosphere provides a measure of the thermal and mechanical state of a planet's shallow interior. An estimate of h in the vicinity of a feature that constitutes a load on a planetary surface can be determined from the flexural response of the lithosphere to the load. This approach has been applied to Mars by calculating radial stresses associated with lithospheric flexure associated with surface loads and comparing the results to the positions of circumferential graben surrounding the major martian shield volcanos and mascon basins. However, many prominent surface loads on Mars, most notably the Olympus Mons volcano, do not exhibit flexural graben. In these instances application of the above method can provide only a lower limit of effective elastic thickness. An alternative method of determining h is to calculate the vertical displacements associated with the flexural loading and to compare the amplitude and shape of the flexural profile to observed topography. This method has not been applied to flexural problems on Mars because of the poor resolution of martian topographic data. However, previous analyses have shown that the lithosphere around major volcanic shields should exhibit vertical deflections of order 1 km over horizontal baselines of order 100 km. We were thus motivated to search for the presence of flexural troughs in the existing Mars topography data.