Monday, March 18, 1996 OUTER PLANET SATELLITES 8:30 a.m. Room A Chair(s): D. Domingue A. G. Davies Matson D. L. Johnson T. V.* Blaney D. L. Veeder G. J. Galileo's Upcoming View of Io--A Set of Predictions Many studies of Io have been done since the Voyager flybys. We have extrapolated from these to predict how Io may have changed during the 16 years since the Voyager flybys. We expect that Io's global heat flow will be about 1014 W, or about 2.5 W m-2. There will be numerous thermal anomalies and they will be consistent with basaltic-type volcanism. Many small, hot sources (e.g., temperatures greater than 600 K and diameters of a few tens of kilometers) will appear and quickly fade. Up to ten percent of Io's surface may be affected by new lava flows since Voyager. SO2 gas will be concentrated locally in plumes and other vents. During daytime, large frost covered regions will be cooler than expected possibly due to solid-state greenhousing and will effectively trap mospheric SO2. McEwen A. S.* Isbell N. R. Edwards K. E. Pearl J. C. Temperatures on Io: Implications to Geophysics, Volcanology, and Volatile Transport Voyager 1 IRIS spectra of Io from 4- to 55 micrometers wavelength have been further analyzed. We report two advances here: improved pointing corrections for the IRIS observations and improved background thermal models. The pointing corrections allow detailed mapping of thermal anomalies relative to surface features, thus enabling improved geologic interpretations. The identification of low-temperature (T) thermal anomalies due to cooling flows and pyroclastics, and to determination of Io's heat flow. The background Ts also provide important constraints on models of Io's atmosphere and volatile transfers. A new model for Io's surface that has emerged from this and other studies includes 3 basic thermophysical units: (1) bright, SO2-rich regolith with a high thermal inertia over ~80% of the surface; (2) darker, SO2-poor regolith with a low thermal inertia over ~18% of Io; and (3) high-T hot spots over ~2% of the surface. We present new evidence that unit 2 contains ubiquitous low-T geothermal anomalies from cooling volcanic deposits. Davies A. G.* Fire Fountains on Io A rift zone fire fountain model and a silicate lava flow model are used to explain the Io thermal IR outburst of 9th January, 1990. This is the best observed large thermal outburst from Io. The dataset is multispectral with respect to time, and is therefore unique. These data cannot be fitted by some scenarios, such as an active lava flow or cooling body of magma, or an overturning lava lake. The 9th January 1990 event can be modelled as a two-component silicate eruption, consisting of a rift eruption producing a line of large fire fountains, feeding a large clastogenic lava flow. Hogenboom D. L.* Kargel J. S. Reiter M. L. Pressure-Volume-Temperature and Phase Relationships of Sulfur Dioxide We have initiated a project to investigate solid-liquid phase equilibria and to make pressure-volume-temperature measurements of SO2 under the range of conditions in which SO2 is likely to exist in Io and Europa. Our goals are (1) to study this system at pressures from 0.1 MPa (1 bar) to 400 MPa, (2) to investigate phase equili-bria in certain binary chemical systems with SO2, and (3) to apply these data to problems of magmatic processes on Io and Europa and to these satellites' early volatile evolution. In our first data run pure sulfur dioxide melted at a temperature of 206.5 K at 78.7 MPa (787 bars). (The melting point at 0.1 MPa is 197.6 K.) No subsolidus transitions were observed between 206 K and 125.6 K at pressures of 65-80 MPa. Thermal modeling suggests that SO2 should melt in Io at depths of tens to hundreds of meters (~1-10 MPa pressure) and in Europa at depths of tens of kilometers (several hundred to a thousand megapascals). On Io, liquid SO2 should tend to erupt explosively, whereas on Europa SO2 should tend to sink and form sulfuric acid solution in a subcrustal brine ocean. Stern L. A. Kirby S. H.* Durham W. B. Polycrystalline Methane Clathrate: A New Method of In Situ Synthesis, and Its Phase Stability and Rheology at High Pressures Methane is an important molecule in the outer solar system and is expected to form the icy clathrate structure (CH4-6H2O) in appreciable amounts on the solid planets and moons of the outer solar system where both water and methane are abundant. Methane clathrate also occurs commonly in offshore methane-bearing sediments, and is the largest untapped hydrocarbon source on Earth. Prior work on this compound has been hampered by the difficulty in synthesizing cohesive polycrystalline aggregates suitable for physical property measurement. We have successfully made such aggregates, deformed them at low temperatures and elevated pressures and examined their deformation microstructures as guides to the deformation processes that control their rheology. We found that the growth and deformation properties of this compound to be very peculiar and interesting. Kargel J. S.* Consolmagno G. J. Magnetic Fields and the Detectability of Brine Oceans in Jupiter's Icy Satellites Salt water's electrical conductivity prompted us to investigate whether electromagnetic induction might occur in a salty ocean in Europa or Ganymede. An ocean could be detected in three ways. (1) Oceanic convection could induce electromagnetic fields or even an internal dynamo. (2) If brine reaches a moon's surface in electrical contact with ambient plasma, then a unipolar current, an induced magnetic field, and Alfven waves would be set up by Europa's motion through Jupiter's magnetic field. (3) Even if the ocean is electrically insulated from the plasma, Europa's motion through Jupiter's inclined, offset magnetic field causes the strength of the magnetic field sensed by Europa to oscillate by a factor of two every orbit; this would produce oscillatory electric and magnetic fields. Europa and Ganymede might have highly nondipolar, tilted, but very strong magnetic fields. _ Dombard A. J.* McKinnon W. B. Formation of Grooved Terrain on Ganymede: Extensional Instability Mediated by Cold, Diffusional CREEP The formation of grooved terrain is an outstanding problem in planetary geology, as it is intimately related to the major event in Ganymede's geological history: the replacement of more than half of the satellite's ancient, dark terrain by bright terrain. The morphology of the bright terrain is dominated by sets of subparallel, linear-to-sinuous ridges and troughs -- the grooves. The predominant model for groove formation is a necking-type extensional instability. The most complete analysis to date is due to Herrick and Stevenson, who applied the Basin-and-Range formation model of Fletcher and Hallet (as first proposed by Fink and Fletcher). In this model the instability arises when a strong plastic or brittle layer and underlying viscous substrate undergo extension. Herrick and Stevenson found that on Ganymede it is very difficult to generate a sufficiently strong instability to account for the spacing of the grooves. They modeled the ductile lower layer as dominated by dislocation-mediated creep with power-law stress exponents of ~= 4-5, consistent with available experimental measurements. Recent experiments by Goldsby and Kohlstedt and Durham et al. indicate that in some geological situations the creep of ice can be dominated by grain-size sensitive, diffusional mechanisms with power-law exponents of ~= 1-2. In addition, Herrick and Stevenson used present-day surface temperatures. Here, we argue that because of the dimmer younger Sun and the relatively high albedo of bright terrain at the time of groove formation, the relevant surface temperatures were significantly lower. Both incorporation of a more Newtonian ductile region and lower temperatures serve to better decouple the brittle surface layer from the substrate, leading to stronger instabilities. As a consequence, we find that it is possible to generate sufficiently strong instabilities at the proper topographic wavelengths to form the grooved terrain on Ganymede. Pappalardo R.* Head J. W. The Galileo Imaging Team Grooved Terrain on Ganymede: Science Issues to be Addressed Through Galileo Imaging Grooved terrain on Ganymede consists of sets of subparallel ridges and troughs, typically organized in structural cells within broader swaths of bright terrain. Morphological evidence generally indicates an extensional-tectonic origin for the ridges and troughs, likely as normal fault blocks. However, the detailed emplacement mechanism, stratigraphy, deformation style, and evolution of grooved terrain remain uncertain. Here we examine some of the outstanding questions regarding the origin and evolution of grooved terrain and discuss how planned Galileo observations will address these issues. Schenk P.* Origins of Palimpsests and Impact Basins on Ganymede Large impact structures on icy satellites differ radically from their terrestrial planet counterparts, and in so doing reveal much about icy satellite surfaces and cratering processes. A particularly persistent puzzle has been the nature and origin of palimpsests on Ganymede and Callisto. Palimpsests, circular low albedo scars with very low relief, are prime targets for observation by Galileo. Internal structure or composition may be important in controlling crater morphology. In order to develop an up to date paradigm for palimpsest formation we have made a systematic study of large impact structures on these bodies, the first since 1982, incorporating stereo observations and extensive measurements of basin ejecta and other basin structures. We find that the outer palimpsest boundary corresponds to the inner (pedestal) ejecta facies and basin morphology changed radically with time on Ganymede, perhaps due to decreasing thermal gradients. Moore J. M.* Mellon M. T. Zent A. P. Mass Wasting and Ground Collapse in Terrains of Volatile-Rich Deposits as a Solar System-Wide Geological Process: The Pre-Galileo View The polar terrains of Mars are covered in many places with irregular pits and retreating scarps, as are some of the surfaces of the outer-planet satellites (e.g., Io, Europa, and Triton). These features are diagnostic of exogenic degradation due to the loss of a volatile rock-forming matrix or cement. It has gone generally unrecognized that the same (or very similar) geologic process responsible for the martian polar terrains also operates on some of the outer-planet satellites. The development of many of the scarps on Triton, and the depressions they surround, appears to involve scarp recession, as the planimetric traces of these scarps are inconsistent with formation either by faulting or as flow fronts. Scarp recession can occur on the Earth when a structural or stratigraphic inhomogeneity near the scarp base is mechanically weakened. On Triton, where conditions are unfavorable for some of the processes which cause scarp retreat such as erosion by abrasion from wind, rainfall, or channelized running fluid, mechanical weakening of material exposed in the face of a scarp probably involves the loss of a cementing or matrix-forming material by sublimation. Black G. J.* Campbell D. B. Ostro S. J. The Icy Galilean Satellites: 70 cm Wavelength Radar Properties The radar properties of the icy Galilean satellites at cm wavelengths are dramatically different from those of most inner solar system objects. For the terrestrial planets, the Moon, and most smaller objects, specific radar cross sections are typically on the order of 0.1 while circular polarization ratios, defined to be the ratio of the echo power received in the same circular sense (SC) as transmitted to that received in the opposite sense (OC), range from ~0.1 to 0.4. At wavelengths of 3.5cm and 13cm, total radar albedos for Europa, Ganymede, and Callisto, are ~2.5, ~1.5, and ~0.7 respectively, and circular polarization ratios lie in the range ~1.2-1.5. In addition, reflections from terrestrial planets include specular reflection from near the sub-radar point. No such specular return is seen in the echoes from the icy Galilean satellites; the scattering closely follows a diffuse cos^n theta law at all incident angles, with n ~1.6. These measurements show no significant variation with wavelength from 3.5cm to 13cm. Here we report on the reduction of measurements made at a much longer wavelength, 70cm, in 1988 with the Arecibo telescope. Observations were made on 7 days, one day on Europa and three each on Ganymede and Callisto, with ~1 hour of integration time on each day. Single-day signal-to-noise ratios were very small and all three days were averaged to obtain significant detections of Ganymede and Callisto. Even so, Callisto was not detected in the OC channel. Europa was not detected in either circular polarization on the single day in which it was observed. Good detections of Ganymede were obtained in both channels. Roush T. L.* Near-IR (0.8-2.5 micrometers) Optical Constants of Water Ice at 100K Water ice is an abundant constituent on surfaces throughout the outer solar system. Warren provides a compilation of the optical constants of water ice (T ~ 269 K) from the UV to the thermal-IA. Bertie et al present the optical constants of water ice (T ~ 173 K) that include part of the near-IA (lambda = 1.25-2.5 micrometers). While both of these data are useful in modeling the interaction of incident solar energy with icy bodies, these temperatures are too warm for application in the outer solar system where temperatures are generally below 150 K. Ockman grew thick crystals of water ice at temperatures of ~266 K and then measured the transmission of these samples at temperatures near 100 K. These data can be used to estimate the optical constants of hexagonal water ice at temperatures appropriate for the outer solar system. Stockstill K. R.* Larson S. M. Vilas F. Gaffey M. J. Clues to the Composition of the Iapetus Dark Material: A 0.67-Micrometer Absorption Feature The composition and origin of the dark material on the leading hemisphere of Iapetus remains a mystery. Evidence for phyllosilicates exists in the form of a 3.0-m water of hydration absorption also found in the photometry of many C- class asteroids. The steep spectral slope, however, suggests that organics are present in the dark Iapetus material. Reflectance spectra covering the wavelength interval of 0.38 to 0.88 m were obtained of the trailing (bright) hemisphere of Iapetus on 9/22/93 (~3.9 days from W elongation) and of the leading (dark) hemisphere on 10/25/93 (~1.6 days from E elongation) using the University of Arizona 1.54-m telescope with a narrowband CCD spectrograph having a dispersion of 11 Angstroms/pixel. The checkerboard linear mixing model defined and used by Bell et al (1985) was applied to these spectra to remove the spectral effects of the bright polar caps on Iapetus from the spectrum of the dark material. A steeply-sloped spectrum consistent with the photometry previously derived with an absorption feature centered at 0.67 micrometers was produced. Monday, March 18, 1996 ORDINARY CHONDRITES 8:30 a.m. Room B Chair(s): G. W. Lugmair C. M. O'D. Alexander Lugmair G. W.* Shukolyukov A. MacIsaac Ch. Radial Heterogeneity of 53-Mn in the Early Solar System and the Place of Origin of Ordinary Chondrites The earlier results of our studies of 53Mn-53Cr isotope systematics (T(sub)1/2 of 53Mn = 3.7 My) in angrites (LEW 86010, Angra dos Reis) and in some eucrites (Chervony Kut [CK], Juvinas [JI3V1) have suggested that the resolution of small age differences between these differentiated objects is possible if the assumption is true that 53Mn was homogeneously distributed at least in that region of the solar nebula where the meteorite parent bodies formed. The fact that the present day and the initial 53Cr/52Cr ratios in both meteorite classes are significantly higher than the terrestrial and lunar values led us to test the 53Mn-53Cr systematics in several ordinary chondrites, the putative building blocks of planetesimals. We found that these chondrites possess uniform and higher than terrestrial bulk 53Cr/52Cr ratios (0.5 epsilon; 1 epsilon = l x 10^-4) but more than a factor of two lower than the bulk eucrites. Thus, it became clear that the original assumption of 53Mn homogeneity on a large scale may not be valid and that the low abundance on earth of the 53Mn decay product may be the consequence of a solar system wide phenomenon rather than, say, simply depletion at 1 AU due to volatility. The idea of a radially heterogeneous 53Mn distribution in the nebula was suggested and tests of this hypothesis were undertaken. Srinivasan G.* Russell S. S. MacPherson G. J. Huss G. R. Wasserburg G. J. New Evidence for 26Al in CAI and Chondrules from Type 3 Ordinary Chondrites We have known since 1976 that 26A1 (tl/2 = 7.2 x 105 yrs) was alive in the early solar system, at a level of (26Al/27Al)o z 5 x 10-5 in calcium-aluminum inclusions (CAI). However, several outstanding questions remain. Little evidence for 26A1 has been found in other chondritic material, and none has been found in differentiated meteorites. These results might imply that 26A1 was heterogeneously distributed in the nebula or by mineralogic site in nebular dust, or they might reflect differences in time of formation. There are strict limitations on finding evidence of 26A1 in normal chondrules with bulk Al/Mg ~ 0.1, since even quenched, perfectly preserved, late-stage glasses would have low Al/Mg. Primary plagioclase crystals provide the only possibility, but these only crystallize rarely in melts within the compositional range of normal chondrules. Also, metamorphism can erase the evidence in high-AI/Mg phases. To address these issues, we have conducted a search for chondrules and CAI with high-Al/Mg phases suitable for ion-probe measurement in type 3 ordinary chondrites. Previous work has revealed evidence for 26Al in a plagioclase bearing, olivine-pyroxene class from Semarkona (LL3.0; (26Al/27Al)o = 7.7+/-2.1 x 10-6)), a plagioclase-rich object from Bovedy (L3.7?; 2.5+/-1.2 x 10-7), in separated plagioclase from St. Marguerite (H4; 2.0+/-0.6 x 10-7), an isolated hibonite grain from Dhajala (H3.8; 8.4+0.5 x 10-6), and in Al2O3 and hibonite grains ((26Al/27Al)o = 2-5 x 10-5; [GRH, unpublished]) from acid residues of Semarkona, Bishunpur (LL3.1), and Krymka (LL3.1). We have identified and measured Al-Mg isotope systematics in two CAI and seven chondrules from ordinary chondrites of low metamorphic grade and have found clear evidence for 26A1 in both CAI and in two chondrules. Brazzle R. H.* Hohenberg C. M. An Update on the I-Xe Chronometer The I-Xe system, long regarded as a sensitive chronometer for early meteorite evolution, has yet to achieve a clear meaningful interpretation. We have been studying the I-Xe system on single mineral phases (phosphates from ordinary chondrites with high-precision Pb-Pb ages) in an attempt to eliminate possible non-chronometric complications, and achieve the simplest interpretation possible. Thus far, we have found two phosphate samples (H6 Guarena and H6 Kernouve) whose I-Xe ages are consistent with their Pb-Pb ages, and one sample (L6 Bruderheim) that appears inconsistent. These I-Xe ages are listed in Table 1 as a range of Dt referenced to Acapulco phosphate. Bruderheim is shocked, which may have reset the I-Xe chronometer, and Guarena and Kernouve ages have large uncertainties due to small sample size. Thus, at this time, we can report no convincing evidence of the chronological validity of the I-Xe system. However, an analysis of six additional phosphate separates (three of which have been Pb-Pb dated by Gopel et al.) is in progress, and updated results will be reported at this meeting. Alexander C. M. O'D.* OC Chondrule, Rim and Matrix Compositions: A Model The peak temperatures chondrules experienced, based on their liquidus temperatures, range from 1500 K to 2000 K. At these temperatures and nebular pressures most elements are volatile, but generally the alkali metals and S have been the only major elements considered as such. However, correlated variations in Mg and Al abundances, if they are not due to precursor compositions, require the loss of up to 50% of the SiO2 from some chondrules. The more volatile FeO would have been lost to an even greater extent. Here it is shown that if the material lost from chondrules recondenses onto the fine grained material which survived the chondrule forming process, chondrule rim and matrix compositions can be explained. Scott E. R. D.* Krot A. N. Browning L. B. On the Origin and Evolution of Minerals in Chondrite Matrix Material We have developed the outline of a model to account for the mineralogical diversity of matrix, rims and chondrules in all chondrite groups. We infer that FeO-poor, fine-grained, amorphous silicates and metallic Fe,Ni, which condensed in the nebula above 800 K were major precursor materials for FeO-poor chondrules (type I). At lower ambient nebular temperatures, amorphous silicate materials became enriched in FeO providing precursor materials for FeO-rich chondrules (type II). The higher proportion of FeO-rich chondrules in O chondrites relative to C chondrites can be attributed to the formation of more chondrules at lower ambient temperatures in the O chondrite source region. Recycling of some chondrules accounts for FeO-rich relict grains in FeO-poor chondrules. Amorphous material may have been hydrated below 400 K in the nebula. Chondrite matrix materials mostly formed by nebular and asteroidal processing of amorphous nebular materials that accreted to chondrules. Some matrix grains were reheated, probably during chondrule formation to make larger, well ordered phases which were relatively unaffected by subsequent nebular and asteroidal processing. Ebihara M.* Shinotsuka K. Kong P. Redistribution of Trace Elements During Metamorphism of Ordinary Chondrites In ordinary chondrites, contents of volatile elements, e.g., carbon and noble gases, are known to be variable; their contents decrease with increasing of petrologic types. Indeed, the bulk contents of these volatiles were proposed as parameters usable for the subclassification of unequilibrated ordinary chondrites (UOCs). In contrast, the bulk contents of refractory elements are highly uniform in chondritic meteorites, regardless of their chemical and petrological groups. In this study, we traced the distribution trends of refractory trace elements in UOCs and EOCs, and attempted to characterize the metamorphism of ordinary chondrites based on the distribution of trace elements in constituent minerals in chondrites. Mo and W were focused as refractory siderophiles, and REEs, Th and U were focused as refractory lithophiles. Nakamuta Y.* Motomura Y. Metamorphic Temperature Estimation for Equilibrated Ordinary Chondrites by a Plagioclase Thermometer Individual plagioclase grains of about 50 micrometers in size from H6, L6 and LL6 chondrites were X-rayed by a Gandolfi camera after the analysis of chemical compositions by an electron probe microanalyzer. The X-ray powder pattern recorded on a film was scanned by a microdensitometer and taken into a personal computer for precise examinations. The distance between 1-31 and 131 reflections in the x-ray powder pattern which is very sensitive to the structural state and correlates to the equilibration temperature of plagioclase was determined precisely by applying a profile-fitting technique. The distance between 1-31 and 131 reflections was corrected for the influence of Or content by using the correction diagram. The maximum temperatures for H6, L6 and LL6 chondrites obtained by using the relation diagram between [2 theta(131)-2 theta(1-31)] and equilibration temperature are 755, 840, and 830 degrees C, respectively. Yamaguchi A.* Scott E. R. D. Keil K. Petrology of Unique Impact Melt Rock, Ramsdorf (L Chondrite) We have studied a unique impact melt rock, the Ramsdorf L chondrite. We have discovered that Ramsdorf contains not only clast-poor impact melt but also a chondritic portion (>60 g) with prominent chondritic texture. Detailed mineralogical studies suggest that most (~90 vol%) of the chondritic portion was melted by shock. Ramsdorf was produced by melting of a type 3-4 chondrite and crystallization in situ. The associated shock pressure could have been ~75-90 GPa, with a post shock temperature of ~1400 degrees-1600 degrees C. The shock stage of this rock may be the highest among ordinary chondrites and other rocks, namely shock stage S6+. Rubin A. E.* LL6 Galim (a) and EH Galim (b): Redox Reactions in a Heavily Shocked Polymict Breccia Seven pebbles of centimeter size were recovered from a meteorite shower observed near the town of Galim in Cameroon in November, 1952. The main lithologies of the pebbles were described by Christophe Michel-Levy and Bourot-Denise. At least three (and probably six) of the pebbles constitute Galim (a), a heavily shocked (shock stage S6) LL6 chondrite; one pebble constitutes Galim (b), an impact-melted EH chondrite containing fragmental chondrules and euhedral enstatite grains. Because both the LL6 and EH stones are unweathered, they are almost certainly from the same fall, indicating that Galim is a polymict breccia. Galim (a) contains reversely zoned chromite grains with appreciably lower FeO/(FeO+MgO) ratios than those in typical LL5-6 chondrites. Galim (b) lacks graphite, Si-bearing kamacite, Ti-bearing troilite and other sulfides (e.g., niningerite, oldhamite) characteristic of EH chondrites. It appears that Galim (b) was an EH clast that was oxidized during shock melting while in contact with its LL host; Galim (a) was reduced, during either the same shock event or subsequent thermal metamorphism. Sharp T. G.* Chen M. El Goresy A. Microstructures of High-Pressure Minerals in the Sixiangkou L6 Chondrite: Constraints on the Duration of Shock Events in Chondrites High-pressure minerals in the Sixiangkou (L6) chondrite contain micro-structures that can be used to constrain the time scale of shock metamorphism. Crystallization of majorite-pyrope garnet and magnesiowustite from a silicate melt at high pressure and temperature required micrometer-scale diffusion at high pressure. Large polycrystalline ringwoodites and majorites, formed by solid-state transformations, show evidence of annealing and dislocation recovery. These diffusion-controlled processes are inconsistent with the micro-second time scales generally accepted for shock metamorphism of chondrites. Chen M. Sharp T. G. El Goresy A.* Wopenka B. Xie X. The High-Pressure Assemblage Majorite-Pyrope Solid Solution + Magnesiowustite: A New Constraint on the High Pressure and Temperature History of Shock Melt Veins in Chondrites Two coexisting distinct high pressure assemblages were discovered in shock melt veins of the Sixiangkou (L6) chondrite: (1) majorite-pyrope(sub)ss + magnesiowustite and (2) ringwoodite + low-Ca majorite. The majorite-pyrope(sub)ss + magnesiowustite evidently crystallized from a dense melt of bulk Sixiangkou composition that was produced by shock-fusion under high pressures and temperatures, whereas ringwoodite and low-Ca majorite in the second assemblage were formed by solid state transformation of olivine and low-Ca pyroxene originally present in the meteorite. The two high pressure assemblages indicate a duration over a time on the order of seconds under a high pressure (20-24 GPa) and high temperature (2050-2300 degrees C) regime. Romstedt J.* Schultz L. Metzler K. Nuclear Tracks and Noble Gases in Ten Unequilibrated H-Chondrites from the Sahara We present nuclear track densities as well as concentrations and isotopic composition of He and Ne of ten unequilibrated H-chondrites. All meteorites are recent finds from the Algerian Sahara and have low shock stages (S1-S2). According to the presence of trapped solar gases and solar-cosmic-rays- irradiated olivines two of the investigated chondrites are regolith breccias, two other meteorites show loss of cosmogenic ^3He accompanied by shortening of nuclear tracks. For most of the meteorites the shielding depth and preatmospheric radii are estimated. Monday, March 18, 1996 MARTIAN ATMOSPHERIC AND FLUVIAL PROCESSES 8:30 a.m. Room C Chair(s): J. F. Bell T. Parker Jakosky B. M.* Loss of Martian Volatiles to Space and Evolution of Climate: A Consistency Check There has been a lot of discussion recently regarding the nature of the early martian climate as inferred from surhce geomorphological and from geochemical arguments. I get the sense that a lot of people believe that these results are not consistent and that there is no consensus as to what early Mars was like. I would like to try to summarize what we do know and what we do not know from each approach, and what the real constraints are that are imposed by each technique. Bell J. F. III* Switala A. E. Crisp D. WFPC2 Science Team HST WFPC2 Observations of Mars Surface Mineralogy and Atmospheric Condensates We used the Hubble Space Telescope (HST) Wide Field/Planetary Camera-2 (WFPC2) to take images of Mars on UT 23-26 February and 11 April 1995, just after opposition. Mars was imaged using the Planetary Camera and discrete filters at ultraviolet (160, 255, 336 nm) and near infrared (1042 nm) wavelengths to place improved constraints on the airborne dust and ice optical depths and the ozone column abundances. These images are also being used to produce high-spatial resolution maps of the surface CO2 and H2O frost deposits and other surface albedo features. We also imaged Mars using the WFPC2's Linear Ramp Filter (LRF) in order to spatially map variations in the surface and dust ferric and ferrous iron mineralogy. The LRF is a continuously variable narrowband filter that allows 1.25% spectral resolution images to be obtained at nearly any wavelength between 370 and 980 nm. An HST command interpretation error prevented us from obtaining the exact LRF wavelengths that we requested; however, LRF images were obtained at 565, 619, 640, 724, 850, 870, 889, and 918 nm. Zent A. P.* New Estimates of the Adsorption of H2O on Martian Surface Materials The model of H2O adsorption on basalt that has been used in the literature for years is based on a set of measurements made at pressures and temperatures somewhat higher than appropriate to Mars. Empirical representations of this data have been developed, and used extensively to estimate the total abundance of H2O adsorbed in the regolith, the diffusion scale of H2O over a variety of depths, the behavior of H2O in the diurnal boundary layer, and the extent and distribution of ground ice. We recently reported on the measurement of H2O adsorption onto Mauna Kea palagonite, and found adsorbate densities very different than predicted by the current isotherms. Extrapolation of an unrealistic adsorption isotherm accounts for much of the apparent discrepancy: the empirical adsorption isotherm heretofore used to estimate H2 O adsorption on basalt at Mars-like conditions implicitly assumes adsorbents are homogeneous. Isotherms appropriate to heterogeneous adsorbents are presented, along with new adsorption data at Mars-like conditions. Adsorption onto the regolith, diffusion, and ice nucleation are discussed in light of the new results. Craddock R. A.* Tanaka K. L. Estimates of the Range in Flow Velocities Associated with the Circum-Chryse Outflow Channels To know what we're getting out of the Mars Pathfinder "grab bag" landing site it is imperative that the detailed geology and hydraulic history of the circum-Chryse outflow channel complex be understood ahead of time. Crude estimates of the maximum channel flow velocities can be made simply by knowing the depth and slopes of the outflow channels themselves. Although these characteristics have been derived in part by stereophotogrammetry, they are subject to a considerable amount of error. Fortunately some Earth-based radar data exist which are both reasonably accurate and provide the spatial coverage necessary for determining the slopes of some of the channels. Using these data, the bed shear stress of a flow, or the retarding stress at the base of a flow, Tb, can be estimated from the depth-slope formula ~b = pghS where p is the density of the fluid, g is gravitational acceleration, h is the flow (or channel) depth, and S is the slope of the channel. This is equal to the bottom stress created by a flow, tau, where tau = pCf-u2 and Cf is a dimensionless drag coefficient and u is the mean flow velocity. Thus, the mean flow velocity for a channel can be calculated from -u = (ghS/Cf) 1/2. The dimensionless drag coefficient can be adjustment for gravity by the expression Cf = g(n21hll3) where n is the Manning roughness coefficient (units of s/ml/2), which has been derived empirically from terrestrial observations. Application of an appropriate Manning roughness coefficient, n, to Martian outflow channels is uncertain, so a range of reasonable values (0.015 to 0.035) is used. Estimates of the mean flow velocities were calculated from this method, however, at best these represent maximum values. Large-scale geologic mapping indicates that most channels were subjected to multiple episodes of flooding, which suggests that the channels may not have been completely full of water at any one time (i.e., bankfull discharge). This method is also not directly applicable to Simud and Tiu Valles because the Earth-based radar data indicate a positive downslope gradient, which mav be due to modification (e.g.slack-water deposition or tectonism) post-dating channel formation. Komatsu G.* Baker V. R. Cataclysmic Flood Spillways: Comparison of Mars and Earth Examples A remarkable paleohydrological pattern is developed in the great system of Pleistocene spillways connecting the Black, Caspian, and Aral Sea basins. Expansion of the paleolakes occupying these basins in the Pleistocene might be attributed to progressive changes in water-balance parameters, including temperature, precipitation, runoff, and evaporation. This explanation is emphasized by Chepalyga (1984). However, evidence for cataclysmic flood erosion in the spillways indicates that very rapid, massive influxes of water played a major role in their origin. Clifford S. M.* Hydraulic and Thermal Constraints on the Development of the Martian Valley Networks The resemblance of the Martian valley networks to terrestrial runoff channels, and their almost exclusive occurrence in the planet's heavily cratered highlands, suggested to many early investigators that the networks were the product of rainfall--relics of a significantly warmer and wetter climate that existed early in the planet's history. However, in response to mounting geologic and theoretical arguments against the existence of a warm early Mars, efforts to explain the genesis of the networks have refocused on potential contributing endogenetic conditions and mechanisms. This abstract examines the hydraulic and thermal constraints that would have been imposed on the development of the networks by the existence of a colder climate. Costard F.* Aguirre-Puente J. Greeley R. Guillemet G. Makhloufi N. Martian Fluvial-Thermal Erosion: Laboratory Simulation Examination of the Martian conditions leads to the idea that analogies may be considered between the Martian and Terrestrial periglacial climates. Particularly, the formation processes for Martian outflow channels and Siberian valleys seem to be similar: the presence of ground-ice and wide rivers, both on Mars and in Siberia, suggests the occurence of thermal erosion during the flooding event. Thermal erosion is the result of ground thawing produced by the heat transfer from the water flow to the frozen ground. The dynamics of these flows under cold climate conditions induce the propagation of a thawing line in the ground and a rapid bank recession, with removal of sediments equal to 19 to 24 m/year. In order to estimate the thermal erosion efficiency in Mars, we have utilized a one-dimensional model. To test the ablation model, a hydraulic channel has been built. The purpose of the experimental study is to mesure the propagation of the thawing line in a simulated Martian ground ice and the thermal erosion rate. Parker T. J. Highlights from 1:500K Geologic Mapping of Central and Southern Argyre Planitia The floors of the Argyre and Hellas basins contain etched layered material that is probably thick sedimentary deposits from channels that once flowed into them. Argyre is unique, however, because Uzboi Vallis flowed out of the basin, implying overflow of a lake within the basin. This makes it the largest impact basin on Mars with channels both draining into it and flowing out from it. Geologic mapping of the central and southern Argyre Basin has revealed a long and complex history of water and wind-related processes acting to shape the region over martian geologic time. The following paragraphs summarize my inferences about the geologic history of Argyre. The more important conclusions are: (I) There is no evidence to suggest that Argyre Planitia is younger than any other large impact basin on Mars; (II) the channels radial to the basin are subsequent streams that developed after the elevated rim (or multiple rings) of Argyre had been breached by valley networks draining both inward and outward from an ancient drainage divide at the basin's rim; (III) Uzboi Vallis is late Noachian in age, and probably drained a lake within Argyre northward through Ladon Valles, Margaritifer Valles, through a predecessor to Ares Valles, and into Chryse Planitia; (IV) The layered material in the basin interior is lacustrine and ranges in age from late Noachian through early Amazonian; (V) the sinuous ridges developed with the accumulating interior sediments and emanate from Surius Vallis, and are likely aqueous sedimentary structures, either lacustrine barriers or glacial eskers; (VI) the debris aprons in Charitum Montes are very young (late Amazonian), but not modern, as they are lightly cratered. Mosangini C. Ori G. G.* Sedimentary Evolution of the Hydroates Chaos, Mars The Hydroates Chaos is a rather peculiar example of such a type of features. It is located between the Chrysie Planita and the eastern tip of the Valles Marineris. This area is extremely rich of chaotic terrains, which are the source for the largest set of outflow channels on Mars. The Hydroates Chaos (HC) is the origin of two outflow channels from its northern rim, but another channel flows into the Chaos from the south. Thus, the HC acted as a real "lake" receiving water from southern chaotic areas and pouring water to the north in the Chrysie Planitia. This peculiarity has generated a complex geological history that is fossilised in several features. Maxwell T. A.* Terrestrial Landscape Response to Climatic Cycling and Martian Paradigms Over the past 20 years of martian geologic investigations, paradigms for the origin of dendritic highland channels have altered between surface runoff and subsurface release with consequent headward erosion via sapping. Even the highest resolution Viking images have been unable to provide a definitive suite of geomorphic relations, primarily because of limited coverage, but also because of varied relations between drainage channels and their superposed materials. In light of the continuing difficulties for a unique solution to highland channel formation, and as a result of new field investigations of terrestrial paleochannels in the northeastern Sahara, it seems opportune to re-evaluate some of the basic principles by which martian channels are interpreted. Some of the terrestrial observations were made in the late 1970's by McCauley, Breed, and El-Baz, but these concentrated on the Viking lander observations and analogous terrestrial aeolian features. The past 20 years have seen a great increase in knowledge of the timing of climatic change in the northeast Sahara, as well as new field investigations of areas not visited before by scientists familiar with the problems of martian geomorphology. Following a brief summary of the late Tertiary history of the now hyperarid desert of northern Sudan and southern Egypt, I list four commonly stated (or tacitly assumed) guiding principles for martian channel interpretation and discuss the merits of applying these guides to channels on both Earth and Mars. Slade M. A.* Jurgens R. F. Haldemann A. F. C. Radar Imaging and Topography of Ares/Maja Valles, Isidis, and Tritonis Lacus Pathfinder Landing Sites The 1995 Mars Opposition provided coverage of sub-Earth latitudes between 16 degrees N. and 22 degrees N. - a range of latitudes not covered by 3.5-cm radar during any previous Mars' closest approach. Fortuitously the Mars Pathfinder potential landing sites are constrained (by solar power, aerobraking, and communications engineering considerations) to be in Chryse Planitia, Isidis Planitia, and Amazonis Planitia, in areas which were sampled at high resolution during this Opposition. The preliminary prime landing site at the mouth of Ares Vallis in southeastern Chryse Planitia in particular was available at closest approach when the radar returns were strongest. The 100 x 200 km Ares Vallis landing ellipse covers an area that been rejected as a Viking Lander site in 1976, in part due to Goldstone CW radar echoes with low signal-to-noise (SNR). The Goldstone radar system at 3.5-cm is now 12 dB more sensitive than the 13-cm system in use at the time of the Viking landing site assessment, and thus definitive results on the potential problems of this site have been obtained. We present updated and integrated evaluations for landing hazards and radar imaging at the Ares Vallis site based on two types of radar observations: radar delay-Doppler profiles (ranging for short) and Continuous Wave (CW) radar spectra. Radar observations also cover Maja Vallis, Marte Vallis, Cerberus, and Isidis. Robinson C. A.* Neukum G. Hoffmann H. Marchenko A. Basilevsky A. T. Ori G. G. A Suggested Geological Development for Ares Vallis, Mars Ares Vallis emerges close to the equator at 18 degrees W and is the easternmost channel of a complex affecting the southern rim of the Chryse Basin. At its mouth are rich arrays of sculptured landforms and remnants of the plateau in which the channel was incised. Our crater count and sedimentological studies of the channel indicate that until 3.5Ga ago it may have formed by glacially-related processes, with some fluvial activity, and that at more northerly locations, localized volcanic resurfacing followed until 1.6 Ga ago. Wenrich M. L.* Christensen P. R. A Formational Model for the Martian Polygonal Terrains On Mars, polygonal terrains are present within the northern lowlands in both the Acidalia and Utopia Planitae. The polygons vary in appearance but generally are 5 to 25 km in diameter and are defined by boundary fractures. Neither desiccation of sediment, fracturing of cooling lava, frost-wedging, nor deep-seated, uniform horizontal tension is a solely satisfactory process for generating the polygons. Here we propose a mechanism for the origin of the polygonal terrains that is based on density-driven free convection of pore fluid in a 1.31 to 5.25 km-thick layer of wet sediment overlying a frozen subsurface and subsequent desiccation with fracturing occurring preferentially above subsurface topographic highs. Monday, March 18, 1996 SPECIAL PLENARY SESSION: MASURSKY LECTURE SERIES 1:30 p.m. Room C Chair(s): D. C. Black Ingersoll A. P.* Probing Questions about Jupiter Monday, March 18, 1996 LUNAR BASINS: THEORY, OBSERVATIONS, AND EXPERIMENTS THIS SESSION IS DEDICATED TO THE MEMORY OF KEITH RUNCORN 2:30 p.m. Room A Chair(s): G. Ryder G. J. Taylor Takata T.* Formation of Large Impact Basins on the Moon: Numerical Simulations of Impact Cratering on Spherical Targets Numerical simulations of impact cratering on spherical bodies are presented in order to investigate the impact parameters for the formation of South Pole-Aitken (SP-Aitken) basins, whose diameter is larger than the radius of the moon. Results show that some 50 % of the impact energy to form the same size of SP-Aitken basin on the plane target is required to form it on the moon. The excavated depth is approximately estimated as 100 km. The result indicates that possible basin materials of SP-Aitken are (1) mixture of the upper mantle, lower crust, and the exotic ejecta, or (2) undifferentiated primitive mantle materials. Moreover, the impact of a body of about 300km-radius with the velocity of <= 2 km/s can not only form SP-Aitken basin, but supply the excess mass of the crust on the far-side as ejecta. Schultz P. H.* Nature of the Orientale and Crisium Impacts The Crisium and Orientale impact basins contain positive gravity anomalies quite different in size relative to their massif-ring diameters. Earlier studies suggested that contrasting basin mascons could reflect the evolving lunar thermal history, but recent Clementine data indicate other causes also must contribute to their gravity signatures. Impact angles provide a reasonable alternative explanation for these differences. Such a perspective underscores the effect of different scaling relations controlling the penetration, excavation, and modification stages of crater formation that become increasingly evident at large scales. Further, the proposed trajectories affect depths of excavation, asymmetric extents of crustal failure, and sequence of ejecta emplacement. Warren P. H.* Global Inventory of Lunar Impact Melt as a Function of Parent Crater Size The size-frequency distribution of lunar impact melt has been estimated by extrapolation from the crater size-frequency distribution estimates of Strom and Neukum (1988). Results indicate that the overwhelming proportion of all lunar impact melt originated during formation of a relative few large basins (most notably South Pole Aitken). This proportion is so high that among the near-surface population of lunar rocks (available as samples) impact "melt rocks" formed as basin-splash ejecta are probably more abundant than melt rocks formed as portions of large, homogeneous central sheets of impact melt. Haskin L. A.* McKinnon W. B. Benner L. A. M. Could Imbrium Ejecta be the Source of the High-Th Material in the Van de Graaff Region of the Moon? Could most of the Th-rich material in lunar highland samples be Imbrium ejecta? The farside highlands gamma-ray data show a high Th concentration only near the Imbrium antipode, near Van de Graaff and M. Ingenii. This Th-rich area lies at the edge of the South-Pole-Aitken Basin, far from the KREEPy Procellarum region, and supports a widespread distribution of KREEPy material, which may derive from the residual liquid of the magma-ocean. Here, we investigate an alternative explanation, that the Th at Van de Graaff may be contained within Imbrium ejecta. We previously speculated (orally, LPSC XXVI) that the Imbrium bolide impacted into a Th-rich geochemical province that is a primary feature of lunar differentiation. The province consists of subregions of high Th concentration, an apparent patchwork of KREEPy lava flows, mare basalt flows, and Th-rich highland material with an average surface concentration as high as ~8 ppm. Spudis P. D.* Hawke B. R. Lucey P. G. Taylor G. J. Stockstill K. R. Composition of the Ejecta Deposits of Selected Lunar Basins from Clementine Elemental Maps The composition of basin ejecta can be studied to probe the lunar crust and to understand better the process of large-body impact. Our method uses remote-sensing data to measure the composition of ejecta for many basins of differing sizes, ages, locations around the Moon. We have used orbital maps of chemical concentration made by Apollo as well as Earth-based spectra for selected spots within the ejecta. Until now, no basin has had complete coverage of its entire ejecta blanket, as the Apollo groundtracks typically covered a small fraction of the deposit and the telescopic data were 1-2 km diameter spots. Data from the Clementine mission provide global coverage at uniform viewing conditions, resolution, and geometry in a variety of spectral bands in the visible and near-infrared. Recently, Lucey et al. have reported on a new technique to extract iron and titanium concentrations for the lunar surface from Clementine global images. We have used these new maps to provide our first estimate of the mean composition of the ejecta from a variety of near and far side basins. We have compared these estimates to our previous results for parts of the ejecta blanket and are re-examining our previous estimate for bulk crustal composition based on these analyses. Results support the idea that the lunar crust is lower in Fe (i.e., richer in Al) than previously estimated, supporting the concept of a global magma ocean on the early Moon. Lucey P. G.* Taylor G. J. Hawke B. R. Spudis P. D. Iron and Titanium Concentrations in South Pole-Aitken Basin: Implications for Lunar Mantle Composition and Basin Formation Recently, we reported on a technique to determine FeO content of soils from multispectral images of the Moon and applied this technique to global, low-resolution Clementine images to create a global iron map of the Moon. In this abstract, we utilize FeO and a new technique that allows us to map the concentration of titanium globally across the Moon. Using these data we find that the composition of the interior of the huge South Pole-Aitken (SPA) basin is unlike any major lunar rock type or model lunar mantle composition, although its ejecta (the highlands terrain immediately surrounding the basin rim) is typical of the lunar highlands (FeO of about 3-4 wt.%, TiO2 ~ 0.3 wt.%). The elevated FeO content of the basin floor may result from exposure of rocks from the mantle of the Moon. The TiO2 abundances suggest that the floor of SPA could be a mixture of approximately equal parts of typical LKFM (lower crustal composition) and low-Ti mantle rocks with 10-16 wt.% FeO. This FeO content is in the range of likely mantle compositions. This mixture would have a Th concentration of about half the level in LKFM, or about 2-4 ppm, as observed in the northern floor materials of SPA basin. Ryder G.* Relationships Among Impact Melt Samples From the Taurus-Littrow Massifs, Apollo 17 Landing Site Impact melt rocks dominate the samples collected from the lower slopes of the massifs bordering the Taurus-Littrow Valley during the the Apollo 17 landing mission. A large number of them, most of which have poikilitic groundmasses, form a reasonably tight chemical cluster and they have been virtually unanimously interpreted as melt created in the Serenitatis event itself. A large number of samples from the South Massif have aphanitic textures and are more clast-rich; they form a more chemically disperse and distinct group. Several authors have suggested that these samples were also produced during the Serenitatis impact. However, the mineral clast populations and the chemical trends within these groups differ and there is a hiatus between them. Further, multiple analyses of individual aphanitic rocks show that there are quite distinct compositions among them. Thus the only evidence to suggest that they are from the same event is that they are from the same landing site, which I suggest is barely an argument. Most likely, the aphanites represent a number of smaller impacts around the landing site after the Serenitatis event. Other impact melts distinct from each other and from both the poikilitic and aphanitic groups attest to the history of impacting in the region over a fairly restricted time range. Dalrymple G. B.* Ryder G. 40Ar/39Ar Laser Step Heating Ages of Some Apollo 17 Melt Rocks and the Age of the Serenitatis Impact We are attempting to refine the chronology of lunar basin formation in order to test the several hypotheses for the early bombardment history of the Moon by large impactors, which range from gradually declining early bombardment to late cataclysmic bombardment only. Our approach is to use a laser system to obtain high-resolution 40Ar/39Ar ages on carefully characterized samples of impact melt rocks. The use of the laser system allows us to obtain high resolution age spectra on very small (sub-milligram) samples, thus avoiding or minimizing the effects of older clasts. Using these methods, we have determined an age of 3,893 +/- 9Ma for the Serenitatis impact, which is significantly older than our maximum age for the Imbrium impact of 3,867 Ma measured using the same methods. James O. B.* Siderophile Elements in Lunar Impact Melts Define Nature of the Impactors This paper reports results in a continuing study of published siderophile-element data for lunar rocks. The current emphasis is study of melt rocks thought to have been generated by the impacts that formed the largest lunar craters and the basins, to try to determine compositions of the impactors. This paper extends last year's study of lunar melt rocks by correcting the data for indigenous Ni, analyzing a much larger data set of lunar samples and meteorites, and evaluating effects of volatility of Au and Ge. The results support the conclusions of last year's study--the Apollo 16 impactors were IAB irons (or asteroids containing IAB iron), the Apollo 14 and 15 impactors were probably similar, and the Apollo 17 impactor was an EH chondrite. Monday, March 18, 1996 STARDUST 2:30 p.m. Room B Chair(s): P. Hoppe B. S. Meyer Verchovsky A. B.* Pillinger C. T. The Carriers of Noble Gases in Presolar Diamonds--A Case of Alice in Wonderland? The carrier of the P3 noble gas component in nanometre diamonds from primitive meteorites is still enigmatic. On the one hand it was found to be closely associated with the HL carrier: for P3 rich samples Xe-P3/Xe-HL ratio is remarkably constant. As might be expected, these two components are not separable during laboratory chemical treatment, although they are easily resolved by stepped heating. The fact that P3 noble gases are released at low temperature (300-800 degrees C) during pyrolysis led to the suggestion that they are located near diamond surface. Moreover the association of P3 noble gases released with chemically active gases in the same temperature range can be interpreted to indicate that P3 gases are evolved as a result of chemical reactions destroying diamond surface. At the present time it is not absolutely clear what is happening at the diamond surface during stepped pyrolysis. With no added oxygen, we observe CO + CO2 in the low temperature products in amounts corresponding to 10-15% of the total carbon: we therefore have argued that surface absorbed oxygen is responsible for the reactions. Huss and Lewis supposed that a reaction with hydrogen, also present on the diamond surface, might take place to induce restructuring of the outermost layers during pyrolysis. In our experience, destruction of diamond surface rather than modification is a real fact but how it relates to the release of P3 gases is not quite clear. Incidentally, the release of chemically active gases is observed irrespective of whether or not the P3 component is present in diamond. The same is true for the presence of surface bonded hydrogen and oxygen. Thus, if the mechanism of P3 gas loss under natural conditions is the same as that in the laboratory experiments, then later on the diamond surface acquires a new layer of oxygen and/or hydrogen. In other words the extraneous elements could act as agent(s) for release of P3 noble gases and need not represent a common constituent(s) of the carrier. Hoppe P.* Kocher T. A. Strebel R. Eberhardt P. Amari S. Lewis R. S. Origin of Circumstellar SiC Grains With Low 12C/13C Ratios: A Multiple Star Scenario In a search for the rare SiC grains of types A and B (12C/13C <= 10 we have screened approximately 2000 SiC grains from Murchison separate KJE (average size 1.14 micrometers by ion imaging and subsequently analyzed candidate grains together with approximately 200 SiC grains from Murchison separate KJC (average size 0.67 micrometers) by conventional analysis technique with the University of Bern ion microprobe. Together with our data previously obtained for separate KJE we now have C-, N-, MgAl-, and Si-isotopic data of 73 A and B grains (not all elements on all grains. The C- and N-isotopic data can be reproduced remarkably well by a simple model mixing CNO- and He-burning products. The previously postulated origin of the A and B grains in J-type carbon stars and CH stars is supported by our new data. For one grain with 14N/15N = 7, however, a nova origin is a realistic possibility. Stephan T.* Jessberger E. K. TOF-SIMS Analysis of Interstellar SiC Grains Ten interstellar SiC grains from the Murchison CM2 meteorite that were previously investigated at Washington University have been analyzed using Time-Of-Flight Secondary-Ion-Mass-Spectrometry (TOF-SIMS). During the last years we have demonstrated the potential of this technique for the analysis of small samples like interplanetary dust particles or micrometeorites. Simultaneous detection of all secondary ions (positive or negative) at high transmission of the spectrometer (20-80%) and sufficient mass resolution (m/ m <= 6000 at 50% peakheight) for separation of most molecular interferences from elemental peaks at high lateral resolution (beam diameter: ~0.2 microns) allow to obtain extensive information on individual SiC grains without completely destroying the sample. During a typical TOF-SIMS analysis only a few monolayers are consumed whereas conventional SIMS techniques, mainly with double focussing mass spectrometers (DF-SIMS), are more destructive and small samples like typical SiC grains are easily sputtered away. Therefore, TOF-SIMS seems to be ideally suited for the analysis of these grains with a typical size of a few microns. Nevertheless, TOF-SIMS suffers from some of the same problems like all SIMS techniques, e.g., with quantification. Most attempts to measure isotopic ratios yielded unsatisfactory results so far because in many cases the required precision (typically in the order of permill) could not be achieved due to low count rates and therefore high statistical errors and/or due to non-sufficient mass resolution for a complete separation of interferences from hydrates (e.g., 12C1H from 13C). Mendybaev R. A.* Beckett J. R. Grossman L. Stolper E. Kinetics and Mechanisms of Volatilization of SiC and SiO2 in the Solar Nebula We determined the evaporation rate of SiC and SiO2 in H2-CO2 and noble gas-H2-CO2 gas mixtures. The evaporation rate of SiO2 in H2-CO2 with log fO2 = IW - 2.8 is close to that for SiC, but the rate increases significantly as log fO2 is decreased from IW - 3 to IW - 6. In contrast, rates for SiC are independent of fO2 of the gas. Dilution by He has a negligible effect on the evaporation rate of SiC and SiO2, but reaction rates in Ne- and Ar-bearing gas mixtures are slower. These effects are interpreted in terms of the dependence of diffusion rates of gaseous species on the composition of the gas mixtures used in the experiments. Retardation of the overall evaporation rate of SiC due to diffusion of gaseous reactants to the surface of SiC or of product gaseous species from it has only a small effect on the very short lifetimes of interstellar SiC grains exposed to the solar nebula. _ Huss G. R.* Wasserburg G. J. Sources of SiC Grains with Low 12C/13C and High 15N/14N and 26Al/27Al The isotopic characteristics of most presolar SiC grains (mainstream grains) from primitive chondrites are consistent with an origin in low-mass AGB stars. Small groups of grains require a different origin, such as the X grains which appear to have come from supernovae. Grains with 12C/l3C ratios below ~20, which make up 5-10% of the presolar SiC in chondrites, are not currently understood. Standard models of partial H burning at the base of the stratified stellar envelope during the Main Sequence followed by First, Second, and Third Dredge-up cannot produce 12C/l3C ratios below ~20 because: 1) homogenization of the envelope dilutes material processed by H burning (l2c/l3c z 3.5) with unprocessed envelope material (12c/l3c ~ 89), and 2) 12c is mixed into the envelope from the He shell during Third Dredge-up, the process which produces a carbon star. These grains also tend to have higher (26AI/27Al)o and 15N/I4N ratios than mainstream SiC grains. Low 12C/l3C ratios can be produced by hot bottom burning at the base of the convective envelope during the AGB phase, which occurs in stars more massive than ~4 M solar mass, but if too much nuclear processing occurs, most of the C is converted to 14N and the star cannot become a carbon star. Models of slower deep circulation below the standard convective envelope, which transports material into the H-burning zone in 1-2 M solar mass stars, appear capable of producing low 12C/l3C and 18O/~fiO ratios while still permitting the stars to become carbon stars. But, since 26AI is efficiently produced only at temperatures above ~30x106 degrees, higher 26AI is not predicted by a deep circulation model in 1-2 M solar mass stars. The base of the convective envelope may achieve such temperatures in more massive stars during hot bottom burning, allowing extra production of 26Al, but no AGB model appears capable of explaining high 15N/I4N ratios, particularly when the ratios exceed the solar value. Nittler L. R.* Oxygen and Aluminum Isotopic Ratios in Presolar Al2O3 from the Tieschitz Meteorite As part of our ongoing efforts to enumerate the compositions and stellar sources of presolar oxide grains in meteorites, we used ion-imaging to locate forty new circumstellar Al2O3 grains in a residue of the Tieschitz (H3.6) meteorite. We located an additional 8 presolar grains by direct high-mass-resolution measurements in the ion microprobe, including several that would have been missed by isotopic ratio ion-imaging. Most of the grains have O-isotopic compositions and inferred initial 26 Al/27 Al ratios that lie within previously observed ranges, and are reasonably well explained by models of nucleosynthesis in red giant stars. One grain has an 16 O/17 O ratio of 71, much lower than any previously observed, and also lower than current theoretical predictions for stellar nucleosynthesis. Although the total number of presolar oxide grains identified in meteorites is approaching one hundred (out of some 5 x 10^4 measured grains), none have the extreme excesses in 16 O needed to explain the O-isotopic compositions of CAIs. Kehm K.* Amari S. Hohenberg C. M. Lewis R. S. 22Ne-E(L) Measured in Individual KFC1 Graphite Grains from the Murchison Meteorite We have analyzed 46 individual graphite grains from Murchison KFC1 (2.15-2.20 g/cm^3) first by ion microprobe to determine C and N isotopic compositions and then by laser gas-extraction to determine He and Ne compositions. This work is part of our ongoing investigation of noble gas compositions in individual SiC and graphite grains from the Murchison meteorite. Each of the measured grains was inferred to be circumstellar (CS), based on isotopic anomalies in at least one element. Three of the 46 grains contained 22Ne-E(L) in measurable quantities. Clemett S. J.* Messenger S. Chillier X. D. F. Gao X. Walker R. M. Zare R. N. Indigenous Polycyclic Aromatic Hydrocarbon Molecules in Circumstellar Graphite Grains Exotic carbon isotope ratios observed in some graphite grains extracted from primitive meteorites point to a circumstellar origin. We have used microprobe two-step laser desorption/laser ionization mass spectrometry (micrometer L^2 MS) to analyze individual circumstellar graphite grains for organic matter in the form of polycyclic aromatic hydrocarbons (PAHs). To date, we have studied 124 graphite grains, extracted from the Murchison CM2 (105 grains), Tieschitz H3 (8 grains), Indarch E4 (6 grains), and Acfer 094 (5 grains) meteorites. Approximately 70% of the grains showed appreciable concentrations of PAHs. Molecule specific isotopic analyses show that most of the PAHs are isotopically normal and could have been added to the grains at various stages in their evolution. However, there is a significant number of molecules that are isotopically similar to the parent grains and are hence indigenous to the circumstellar grains themselves. Bernatowicz T. J.* Cowsik R. Amari S. Lewis R. S. Constraints on Stellar Grain Formation from Circumstellar Graphite in the Murchison Meteorite We have previously described the internal structure and composition of micrometer-sized circumstellar graphite spherules from the Murchison meteorite (separate KFC1; 2.15-2.20 g/cm3 ) that have an onion-like external morphology. Our TEM studies show that the KFC1 spherules (modal size ~1.5 micron) generally consist of nanocrystalline carbon mantled by well-crystallized graphite. The spherules often contain internal crystals of Ti, Zr and Mo refractory carbides (5-200 nm; modal size ~10 nm) that formed prior to (and sometimes served as seeds for) carbon condensation. We have since numerically modeled the growth of graphite spherules and TiC crystals in stellar outflows, to put constraints on the physical conditions responsible for their formation. A related study making use of our observations has been presented by Sharp and Wasserburg. Amari S.* Zinner E. Lewis R. S. Ca and Ti Isotopic Compositions of Size-Separated SiC Fractions from the Murchison Meteorite Titanium isotopic patterns of presolar SiC grains from Murchison change with grain size; delta ^50Ti/delta ^46Ti and delta ^50Ti/delta ^49Ti ratios decrease with increasing grain size. This trend can be interpreted by assuming that SiC grains with different sizes come from AGB stars with different metallicities; coarser SiC grains formed in AGB stars of lower metallicity. Fisenko A. V.* Verchovsky A. B. Semejenova L. F. Pillinger C. T. Isotopic Composition of Carbon in Colloidal and Non-Colloidal Diamonds from Efremovka The isotopic composition of carbon has been measured in colloidal and non-colloidal fractions of presolar diamonds from Efremovka CV3 to investigate the diamond heterogeneity. Differences in combustion temperature (higher for the non-colloidal material) and isotopic composition (5 per mil heavier for the same fraction) have been observed. Monday, March 18, 1996 SPECIAL SESSION: GALILEO MISSION TO JUPITER RESULTS FROM ENCOUNTER 2:30 p.m. Room C Chair(s): T. V. Johnson R. E. Young Johnson T. V.* Galileo Orbiter, A New Moon for Jupiter Young R. E.* Galileo Probe Mission Overview Orton G.* Galileo Probe Entry Conditions from Remote Sensing Seiff A.* Kirk D. B. Knight C. D. Young R. E. Schubert G. Mihalov J. Blanchard R. C. The Structure of Jupiter's Atmosphere as Measured by the Galileo Probe Owen T. C.* Niemann H. B. Hunten M. Atreya S. K. Way S. Chemical Composition Measurements of the Atmosphere of Jupiter with the Galileo Probe Mass Spectrometer Hunten D. M.* Von Zahn U. The Helium Abundance on Jupiter as Measured from the Galileo Probe Ragent B.* Colburn D. Avrin P. Orton G. Rages K. Yee S. Blamont J. The Clouds of Jupiter: Early Results of the Galileo Probe Mission Nephelometer Experiment Atkinson D. H.* Measurement of a Zonal Wind Profile on Jupiter by Doppler Tracking of the Galileo Entry Probe: Preliminary Results Sromovsky L. A.* Collard A. D. Fry P. M. Revercomb H. E. Best F. A. Orton B. Tomasko M. G. Lemmon M. Freedman R. S. Hayden J. Preliminary Results of the Galileo Net Flux Radiometer Experiment Lanzerotti L. J.* Rinnert K. Gehmel G. Gliem F. O. Krider E. P. Uman M. A. Measurements of Radio Frequency Waves in Jupiter's Atmosphere Fischer H.* Pelke E. Wibberebz G. Lanzerotti L. J. Mihalov J. D. Energetic Particle Measurements in the Innermost Part of the Jovian Magnetosphere Head J.* Galileo Imaging Team Gallilean Satellite Encounters: The Next Steps Monday, March 18, 1996 REFLECTANCE THEORY/SPACE WEATHERING EXAMPLES FROM THE MOON 2:30 p.m. Room D Chair(s): B. Hapke T. V. V. King Shepard M. K. Campbell B. A. Surface Roughness, Optical Shadowing, and Radar Backscatter The topography of natural surfaces at scales of a few meters or less is commonly referred to as roughness. These variations in height and slope, their magnitude, and the changes in structure as a function of scale length are of fundamental importance to interpretation of geologic emplacement regimes and subsequent modification. For most planetary studies and many terrestrial situations, no in situ observations of the ground are available, and remote sensing data are used to infer the nature of the terrain. For optical, infrared, and microwave measurements, surface roughness and its scale-dependence have a large impact on the brightness, polarization, angular scattering properties, and wavelength-dependence of reflected energy. The link between surface roughness and specific remote sensing properties for many types of observations, however, remains elusive. We focus here on the nature of roughness and its scale-dependence for terrestrial rocky surfaces, and the effect of such changes on optical shadowing and radar backscatter. Domingue D.* Cheng A. Bidirectional Surface Reflection Modeled Using Microscopic Roughness Photometry and spectroscopy are fundamental to remote sensing of planetary surfaces. A photometric model based upon radiative transfer theory has come into wide use for analyzing measured spectral reflectances to infer scattering properties of particulate surfaces, compositions of surface materials, and textural parameters. This photometric model included effects of microscopic extinction shadow hiding and multiple scattering. The model disagreed significantly with observation only in that it predicted a sharp brightening near the poles and illuminated limb of low albedo bodies. To remove this so-called "limb brightening" effect from the model, Hapke introduced a macroscopic roughness correction, involving the surface tilt parameter theta that accounted for macroscopic slopes and shadowing on the surface of the body, without attempting to model the surface structure in detail. Hapke B.* DiMucci D. Nelson R. Smythe W. The Nature of the Opposition Effect in Frost, Vegetation and Soils Two different mechanisms can cause an opposition effect: shadow-hiding and coherent backscatter. Because each mechanism samples different properties, it is important to the interpretation of earth and planetary remote sensing observations to understand which dominates in a given medium. This question can be answered by measuring the circular polarization ratio, the ratio of the intensity of circularly polarized light reflected with the same helicity as incident to that reflected with the opposite helicity. If the opposition surge is caused primarily by shadow-hiding, the circular polarization ratio decreases with decreasing phase angle, while if it is caused primarily by coherent backscatter, the circulation polarization ratio increases. We have measured the circulation polarization ratio as a function of phase angle for lunar and terrestrial soils, vegetation and H2O frost. Typical samples of our results are shown in the figure, which plots reflectance, r, and the circulation polarization ratio against phase angle. The reflectances of all samples possess opposition effects 5-10 wide. The circulation polarization ratio increases with decreasing phase angle for the frost and dry terrestrial soil, showing that the opposition effects in these media are dominated by coherent backscatter. However, the circulation polarization ratio decreases for broad-leafed vegetation and moist terrestrial soil, showing that shadow-hiding is the important mechanism for these media. Nelson R. M.* Hapke B. W. Smythe W. D. Horn L. J. Can Coherent Backscattering Principles Form the Basis of a New Remote Sensing Technique for Measuring Particle Size in a Planetary Regolith? Coherent backscattering has been suggested as a contributor to the opposition effect observed in planetary regoliths and in particulate materials studied in the laboratory (Hapke, 1990 and Muinonen, 1990). Hapke discusses specific behavior for the reflected component of polarized monochromatic light incident on a particulate surface. We performed goniometer measurements of the angular scattering properties of particulate materials under illumination by monochromatic light. We found behavior that is consistent with the hypothesis that coherent backscattering is the principal contributor to the opposition effect (Hapke et al., 1993, Nelson et al., 1995). We have observed this in chemically homogeneous powders as well as natural substances including lunar samples. King T. V. V. Calvin W. M. Clark R. N. Influence of Particle Grain Size Distribution on Theoretical Reflectance Calculations A combined laboratory and theoretical investigation is being conducted to determine the most accurate means of representing the particle size distribution of a mineral mixture for use in theoretical reflectance calculations. Radiative transfer theory is being used to model the laboratory reflectance spectra of mineral separates and mixtures which mimic carbonaceous and ordinary chondrites (the theoretical mixtures will be generated using the known chemistry, mineralogy, optical constants and grain size distributions which have been previously determined). Comparison of theoretical spectra computed using a single grain size (for example 52 micrometers) and the computed spectra using a cumulative power-law distribution to represent the same average grain size in mineral separates and mixtures show differences in the strength of absorption features which could affect estimates of minerals abundance. Hartman B. N. Domingue D. Verbiscer A. Scattering of Light by Individual Particles and the Implications for Models of Planetary Surfaces Hapke's theory has been widely used to describe surface physical characteristics by modeling planetary surface reflectance variations with viewing geometry. Incorporated into Hapke's model is a function describing the scattering behavior of individual particles. The scattering of light by individual particles within a planetary regolith is a key aspect of any model that attempts to describe the photometric properties of a planetary surface. Early studies modeled regolith particles using Mie theory, which satisfactorily describes both transmitted (forward scattered) and reflected (back scattered) components of light from perfect spheres. However, the Mie theory is inadequate for describing the scattering behavior of particles with irregular shapes or internal scatterers. The double Henyey-Greenstein function can mathematically describe both components of scattered light by a range of particle shapes. There are two forms of the double Henyey-Greenstein function: the first has two independent parameters (excluding the phase angle) and assumes the width of the back and forward scattered components are equal, and another function with three independent parameters which allows the widths of the back and forward scattered components of the particle scattering function to vary. The objective of our study was to examine the difference in the quality of fit between the two double Henyey-Greenstein functions in describing single particle scattering behavior as used in Hapke's model. We specifically evaluated the difference in quality of fit between the two models and quantified the significance of that difference using rms values. The key to obtaining a unique solution to Hapke's model is to avoid adding unnecessary parameters to the model; this would constrain the types of data sets to which the model can be applied. Thus the goal is to minimize the number of parameters while still physically describing the surface being modeled. The approach we took was two-fold. The first part of our project was to examine the laboratory data of McGuire and Hapke and apply both double Henyey-Greenstein models and examine the quality of fit between the models and the data. The purpose of this part of the project was to determine if there is a significant difference in the quality of fit between the two double Henyey-Greenstein functions. The second part of our project involved applying Hapke's model to the terrestrial snow data to compare these snow particles with McGuire and Hapke's artificial particles for structural correlations. We then applied Hapke's model to disk-integrated phase curve measurements of the Galilean satellites and compared our single particle scattering function results with the artificial particle and snow particle results to attempt to describe the particle structures of the material comprising the regoliths of these satellites. Herzog S. G.* Mustard J. F. Reflectance Spectra of Five-Component Mineral Mixtures: Implications for Mixture Modeling The surfaces of the terrestrial planets consist largely of particulate materials, or soils, whose compositions and particle size distributions are products of geologic processes. The reflectance spectrum of a particulate surface represents a systematic combination of the mineralogy and particle size distribution of the material. Both composition and grain size information are useful in geologic interpretation of a surface, but are convolved in a single reflectance spectrum. We used spectral mixture analysis to separate these parameters by modeling the reflectance of five-component laboratory soils which vary in grain size. We find that successful prediction of endmember compositions requires some knowledge of grain size distribution, and that the fine particles dominate the mixed spectrum in excess of volume fractions and intimate mixture model predictions. Johnson J. R. Lucey P. G. Horton K. A. Williams T. Winter E. M. Stocker A. D. An Explanation for the Observed Spectral Contrast Reduction Between Field and Laboratory Infrared Measurements of Soils Comparison of emission spectra (7-14 m) of pristine soils in the field with bidirectional reflectance spectra of soils obtained in the laboratory shows that laboratory spectra tend to have less contrast than field spectra. We investigated this phenomenon by measuring emission spectra of both pristine (in situ) and sampled soils (prepared as if for transport to the laboratory). The sampled soils had much less spectral contrast than the pristine soils in the reststrahlen region near 9 m. We hypothesize that this effect is due to a difference in grainsize distribution of the optically active layer (i.e., fine particle coatings). This concept was proposed by Salisbury et al. to explain their observations that soils washed free of small particles adhering to larger grains exhibited greater spectral contrast than unwashed soils. Unrecognized, this phenomenon could influence interpretations of remote sensing data since it is a common practice to use spectra of materials obtained in the laboratory to interpret spectra obtained remotely. Basu A.* McKay D. S. Comparison of Agglutinates from Apollo Sites Agglutinates are the principal constructional, i.e., authigenic products of space weathering on the moon and possibly on other atmosphere-free planetary bodies. Production of agglutinates is accompanied by production of new phases such as glass, fine-grained Fe0, possibly Fe-Ni metal, and some chemical fractionation including partial loss of volatile components. Thus, the concentration of agglutinates affects remotely sensed properties of planetary surfaces. Agglutinates also serve as recorders of lunar soil evolution and presence or absence of critical clast-types may provide some stratigraphic datum. Wentworth S. J.* Morris R. V. Shelfer T. D. Nguyen T. Q. Keller L. P. McKay D. S. Patina on Apollo 17 Breccia 76015: SEM Petrography and Mossbauer Spectrometry A number of rocks in the lunar sample collection have surface patinas which formed as a result of exposure to the space weathering environment on the surface of the moon. It is important to understand patinas and their relationships to the rocks on which they formed because it is likely that patinas on surface- exposed rocks on the moon and other planetary bodies would affect data obtained by either remote sensing or unmanned landers. We have undertaken a multidisciplinary study of patinas on selected Apollo samples using a wide range of techniques including scanning electron microscopy (SEM), backscattered electron imaging (BEI), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), backscatter Mossbauer spectrometry (BaMS), spectral reflectance spectroscopy, ferromagnetic resonance (FMR) analysis, and wavelength dispersive spectrometry (WDS). SEM/EDX results for patina on high-Ti mare basalt 75075 were reported by McKay et al in LPSC XXVI. Our current focus is on the patina on crystalline matrix breccia 76015. Our initial results show that the 76015 patina is very complex with characteristics that vary widely over a short distance. The 76015 patina is a good example of classic patina with typical lunar surface exposure features such as microcraters and glass pancakes, and is defined here as accretionary microcratered pancake-bearing (AMP) patina. Keller L. P.* Wentworth S. J. McKay D. S. An Electron Microscope and Microspectrophotometry Study of Patina on Apollo 17 Breccia 76015 The surfaces of lunar materials that are exposed to the space environment are modified by a number of processes which alter their chemical and physical characteristics. These surface modifications on lunar rocks result in the formation of patina. The presence of patina obscures the underlying mineralogy and can strongly affect the optical properties of the substrate. Patinas are important in the further exploration of the Moon and other airless bodies in the Solar System because of their effects on remotely-sensed data (both spectral and chemical). We present here the results of a detailed study of a small region of patina on 76015 as part of a consortium study on the nature and effects of space weathering on the lunar surface (see companion abstract Wentworth S. J. et al.). Allen C. C.* Morris R. V. McKay D. S. Experimental Approaches to Space Weathering The process of space weathering in lunar soil (maturation) includes the combined effects of micrometeorite impacts and solar wind interactions. Impacts melt small volumes of soil containing solar wind hydrogen and carbon. The melt quenches rapidly to agglutinitic glass in a strongly reducing environment. Glass formation under these conditions causes reduction of Fe2+ in the glass to nanophase (~4-33 nm) iron metal (np-Fe0). Space weathering also produces distinct changes in the reflectance spectra of lunar soils. With increasing maturity overall soil albedo is reduced, spectral contrast is diminished, and the continuum slope is increased. We are attempting to duplicate the effects of space weathering in the laboratory using hydrogen reduction at subsolidus temperatures. Alterations induced in the mineralogical and optical properties of 17 lunar soils resemble changes caused by natural space weathering. Tuesday, March 19, 1996 THE LUNAR HIGHLANDS: MACRO TO MICRO 8:30 a.m. Room A Chair(s): S. C. Solomon M. J. Rutherford Solomon S. C.* Simons M. The Isostatic State of the Lunar Highlands from Spatio-Spectral Localization of Global Gravity, Topography, and Surface Chemistry The nature of isostatic support of topography in the lunar highlands provides important clues to the processes of crustal formation and early crustal evolution. The chemistry of a crust formed by crystallization in a lunar magma ocean could be either heterogeneous or nearly homogeneous in density, depending on the horizontal length scales for convection in the cooling magma ocean and the mixing processes (e.g., impact mixing, remelting, serial magmatism) during crustal cooling. In a crust that is compositionally homogeneous on lateral scales of tens of kilometers and greater, elevation differences would be supported by an Airy compensation mechanism (i.e., variations in crustal thickness), whereas lateral variations in crustal composition and therefore crustal density at such length scales would contribute to isostatic support of elevation differences through a Pratt mechanism. Even if the earliest lunar crust varied only radially in density, lateral variations might now be present if a layered crust has been differentially excavated by large impacts or if the earliest crust was variously intruded by later mantle-derived magmas. The total volume of the lunar crust, a quantity of importance for constraining models of largescale differentiation and bulk composition, is sensitive to the assumed mechanism of highland isostasy. Zuber M. T.* Smith D. E. Neumann G. A. Lemoine F. G. What Factors Control the Long Wavelength Shape of the Moon? To first order the present shape of the Moon is a sphere with maximum positive and negative deviations of about 8 km, both occurring on the far side (160 degrees, 75 degrees S; 240 degrees E, -10 degrees N) in the areas of Korolev and S.P.-Aitken. These departures from a sphere are the combined result of various processes in the Moon's early history. The two largest global-scale features are the center of mass/center of figure (COM/COF) offset, for which various possible causes have been offered, and the polar flattening, probably due to an earlier rotation rate, both of which are of the order of 2 km. In addition there are smaller wavelength deviations, due primarily to impact basins. Wieczorek M. A.* Phillips R. J. The Structure and Compensation of the Lunar Highland Crust Using newly obtained gravity and topography data as well as a near global surface iron concentration map obtained from the Clementine mission, the structure and compensation of the lunar crust have been investigated. Geoid to topography ratios (GTRs) are tested against single layer Pratt and Airy compensation models, as well as dual-layered Airy models. For the single layer Airy model it is found that compensation occurs intracrustally at a depth of approximately 40 km. This is consistent with the existence of a 20 km seismic discontinuity at the Apollo 12 and 14 sites (which lie 1.7 km below the mean planetary radius.) For the two layer model it is necessary for the crust to be vertically stratified with an upper crustal thickness lying between 0 and 26 km. Lateral variations in crustal density are found to be more than an order of magnitude less than expected for a Pratt compensation model that fits the GTR data. We are currently attempting to determine the relative amounts of compensation that would occur at the upper-lower crustal interface and Moho due to the emplacement of a surface load. Tompkins S.* Pieters C. M. Classification of Central Peak Compositions on the Moon The history of the Moon's primary crust is broadly understood within the framework of the magma ocean theory. On a regional scale, complex histories are expected, which may include multiple episodes of basin-forming impacts and volcanism, as well as smaller local events. Between the global view and the focused local one, however, our knowledge of crustal evolution remains largely theoretical and model-dependent. Since central peaks of craters 50-200 km in diameter are believed to represent lithologic units excavated from 5-20+ km depth, variations in crustal composition to these depths can be analyzed through a survey of impact crater central peaks. The global nature and high spatial resolution of the Clementine multispectral data is well suited to this task. Korotev R. L.* The Composition of the Prebasin Crust in the Central Highlands of the Moon The Apollo 16 regolith consists of a large amount of material derived from the prebasin crust, i.e., (1) plutonic ferroan anorthosite and brecciated derivatives (>90% plagioclase), (2) a variety of noritic anor-thosites (plutonic, feldspathic fragmental breccias [FFBs], granulitic breccias [GrBs], feldspathic impact-melt breccias), and (3) a minor amount of gabbronorites of highland affinity. However, the site is sufficiently close to nearside mare basins that the regolith also contains a substantial fraction of basin ejecta as well as some mare-derived materials (MDMs) delivered to the site by volcanism and impacts since filling of the basins with mare basalt. These syn- and postbasin products include (4) mafic impact-melt breccias [MIMBs, i.e., "LKFM" and "VHA"], (5) MDMS, i.e., glasses and some crystalline mare basalt, and (6) meteoritic material (largely from micrometeorites) accumulated in the regolith since basin for-ma-tion ~3.9 Ga ago. The MIMBs, which are rich in incompatible trace elements, were formed during the time of basin formation by impacts large enough to penetrate the outer feldspathic crust and melt mafic underlying material, although not all of the several known varieties at the Apollo 16 site may actually have been formed by impacts that produced basins. The Central Highlands, as sampled by the Apollo 16 mission, differs from highlands regions distant from mare basins in its high abundance of mafic syn- and postbasin material. For example, some feldspathic lunar meteorites (ALHA81005, Yamato-86032, MAC 88104/5, QUE93069) contain virtually no MDMSor MIMBs. Peterson C. A.* Hawke B. R. Lucey P. G. Taylor G. J. Blewett D. T. Spudis P. D. Anorthosite on the Lunar Nearside and Farside The distribution of anorthosite (rock composed of at least 90% plagioclase feldspar) on the lunar surface can provide important information regarding the geologic history of the Moon. Evidence suggests that the early Moon was covered by a magma ocean which differentiated as it crystallized, forming a plagioclase flotation crust and a cumulate pile of denser mafic minerals. Subsequent bombardment of the lunar surface has disrupted the original flotation crust, and most of the remnants have been obscured by more mafic deposits, but the distribution of the outcrops of pure anorthosite that have been identified holds important implications for the evolution of the lunar crust. Pieters C. M.* He G. Tompkins S. Anorthosite and Olivine Layers of the Farside Upper Crust at Tsiolkovsky Clementine multispectral image mosaics for the farside crater Tsiolkovsky have been calibrated and processed (116 five-color frame sets). Calibrated 5-color spectra at Tsiolkovsky are used to identify several distinct lithologic compositions known from lunar samples: anorthosite, olivine-rich troctolite, noritic anorthosite, and low-titanium basalt. Using Tsiolkovsky multispectral images as a guide to context and lithologic diversity, the compositional information implied by the 5-color spectra provide areas insight into the structure of the farside upper crust. The dominant rock type of the walls of Tsiolkovsky and the surrounding megaregolith (as excavated by small craters) is anorthosite. Tsiolkovsky central peaks, however, exhibit distinct layers of anorthosite and a lithology dominated by olivine. Either Tsiolkovsky, and other craters that exhibit similar compositional complexity, have fortuitously impacted into localized pockets or layers of lithologic diversity, or such compositional diversity is common in the highland crust between 5-3.0 km. Jolliff B. L.* Hsu W. Geochemical Effects of Recrystallization and Exsolution of Plagioclase of Ferroan Anorthosite The minor- and trace-element composition of plagioclase of ferroan anorthosite is of special significance because it is used to infer the composition and the geochemical and petrologic evolution of the magma from which it crystallized. It is therefore important to understand not only the distribution of trace elements between plagioclase and melts of relevant temperature, pressure, and composition, but also whether any post-crystallization mechanisms have modified the primary plagioclase compositions. Two potential modification mechanisms are recrystallization and exsolution, driven by either late-magmatic, subsolidus, or impact processes. Snyder G. A.* Crozaz G. Taylor L. A. Probing the Crust of the Earth's Moon: Trace-Elements in Minerals from Post-Magma Ocean Highlands Rocks The evolution of the lunar highlands crust involved two distinct stages: (1) the formation of anorthositic to leuconoritic crust as flotation cumulates (FANs) from an incipient lunar magma ocean ~4.4-4.5 Ga ago; and (2) the modification of this early crust from 4.4 to 3.9 Ga through the crystallization of mafic and ultramafic plutonic rocks (and occasional, minor felsic rocks). The petrogenesis of these later mafic and ultramafic plutonic rocks has been the subject of intense debate. We present ion probe trace-element data on both individual pyroxene and plagioclase grains from these post-magma ocean highlands rocks. Parental liquids were calculated from these analyses and are similar in composition to differentiating KREEP basalts. Papike J. J.* Fowler G. W. Layne G. D. Shearer C. K. Ion Microprobe Investigation of Plagioclase and Orthopyroxene from Lunar Mg-Suite Norites: Implications for Calculating Parental Melt REE Concentrations and for Assessing Postcrystallization REE Redistribution The lunar Mg suite, which includes dunites, troctolites, and norites, makes up to 20-30% of the Moon's crust down to a depth of ~60 km. The remainder is largely anorthosite. This report focuses on norites (which consist mostly of orthopyroxene and plagioclase) because we have found that both phases are effective recorders of their parental melt compositions. Our previous studies reported trace element data for orthopyroxene while this study reports trace element data for plagioclase. We estimated melt REE concentrations from both phases and found that, although there is some evidence of postcrystallization REE distribution, both phases still preserve an igneous signature. The estimated melt REE concentrations are similar to high-K KREEP especially for melts parental to A-15 norites. This study provides strong support for mineral trace element inversion estimates for parental melts. Even though these lunar crustal lithologies experienced high temperature annealing, the cores of plagioclase and pyroxene continue to carry a record of their igneous crystallization history. Rutherford M. J.* Tonks B. Holmberg B. Experimental Study of KREEP Basalt Evolution: The Origin of QMD and Granite at the Base of the Lunar Crust A number of recent studies have been directed at determining the petrogenetic relationship of KREEP-rich lunar lithologies, such as lunar granites and quartz monzodiorite (QMD), to one another and to some original "ur-"KREEP. It has been proposed that KREEP or urKREEP is the residuum formed by extreme fractionation of the lunar magma ocean, but pristine KREEP basalts such as 15386 have an mg* which is apparently much too high for such a residual melt. Emphasizing this point, Warren has suggested that the more evolved QMD may be used to represent urKREEP. However, the petrogenesis of QMD is not well known; previous work indicated that fractionation of a KREEP basalt composition at 1 atm produces a QMD residual melt composition just before becoming immiscible, but it is not known if this same process occurred at depth. It is generally accepted that immiscibility was involved in the genesis of lunar granites in the near-surface environment; it has not been established whether immiscibility is stable at the base of the lunar crust (3 kbar). Two recent discoveries bear on the general problem: 1) the ages of lunar granites have been shown to extend from 4.4 to 3.9 Ga, and 2) modeling indicates that crystal cumulates of the lunar magma ocean would have overturned on a short time scale tending to mitigate against the strong decrease in the mg* of the late stage cumulates [8]. These developments give additional potential importance to magmas such as the A15 KREEP basalt, 15386. Accordingly, experiments have been done at 3 kbar in order to determine what melt compositions are produced by fractionation A15 KREEP at the base of the lunar crust in contrast to those produced at the surface . Are QMD-like compositions produced by fractional crystallization at the base of the crust, and can immiscibility occur at this depth? Tuesday, March 19, 1996 CAIs AND CARBONACEOUS CHONDRITES 8:30 a.m. Room B Chair(s): A. M. Davis L. A. Leshin Simon S. B.* Davis A. M. Richter F. M. Grossman L. Experimental Investigation of the Effect of Cooling Rate on Melilite/Liquid Distribution Coefficients for Sr, Ba, and Ti in Type B Refractory Inclusion Melts It is well established that Type B1 refractory inclusions were once at least partially molten. These inclusions are thought to represent closed magmatic systems, but attempts to model the trace element contents in melilite in these CAIs have not met with much success. Observed abundances of most trace elements tend to be significantly higher than those predicted using equilibrium melilite/liquid distribution coefficients. Boundary layers have been proposed as an explanation, but in the case of Sr, its D of ~0.8 is too high to give rise to boundary layers sufficiently enriched in Sr to account for the observed enrichments. We have investigated the possibility that Ds increase with increasing cooling rate. We found that Ds for Ti, Sr and Ba are virtually invariant with akermanite and with cooling rate, and the models still do not fit the observations, especially for Ba, which is off by a factor of ~20. The results indicate that a process occurred during the crystallization of melilite that we have yet to understand. _ Davis A. M.* Richter F. M. Simon S. B. Grossman L. The Effect of Cooling Rate on Melilite/Liquid Partition Coefficients for Y and REE in Type B CAI Melts Type B CAIs are widely believed to have crystallized from melts. Beckett et al. measured melilite/liquid partition coefficients for La, Ce and Tm for a Type B CAI bulk composition cooled at 2 degrees C/hr and found that they were proportional to the gehlenite to akermanite ratio of crystallizing melilite. Davis et al. reported REE concentrations in melilite from three Allende Type B CAIs. They modeled melilite crystallization in these CAIs using REE partition coefficients and found two important discrepancies: (1) melilite in CAIs is consistently higher in REE than the amount predicted; and (2) REE patterns in CAI melilite become more steeply enriched in LREE than HREE with increasing akermanite content, whereas significant fractionation of REE from one another is not predicted. Davis et al. proposed that these differences were caused by the formation of trace element-rich boundary layers in melt adjacent to growing melilite crystals and that the fractionation of REE from one another was due to differing diffusion rates of REE in the melt, which caused slower-diffusing LREE to become more enriched in the boundary layer. In order to test for kinetic effects, we have measured melilite/liquid partition coefficients for Y and 9 REE (reported here) and Ti, Sr and Ba in synthetic Type B CAIs grown at a variety of cooling rates. We find that REE partition coefficients for melilite decrease, rather than increase, with increasing cooling rate. Cooling rates in excess of 2 degrees/hr cannot explain REE distributions in Type B CAI melilite. Paque J. M.* High-Temperature Differential Thermal Analysis: Tool for Studying Nucleation Characteristics of Melts Many Ca-Al-rich inclusions (CAIs) are believed to have formed from melts, probably from the melting of pre-existing solids. Experimental petrologists attempt to duplicate these conditions in the laboratory and determine the set of experimental parameters that best correspond to the observed textures and chemistry. Since textures are highly dependent on nucleation characteristics of the melt, it is of interest to determine the temperature of crystallization of phases under varying conditions. Typically, this has been done by quenching individual samples at intervals during the cooling sequence and examining the experimental charges to determine the phases present. Under dynamic conditions many phases, such as melilite and anorthite, do not crystallize reproducibly due to the stoicastic nature of crystallization. Differential Thermal Analysis (DTA) has been routinely used at temperatures below the range of crystallization of CAIs, and generally under isothermal condition, to determine melting points, phase transitions and solid state reactions. I am extending this method to higher temperatures and dynamic crystallization conditions. The use of High Temperature Differential Thermal Analysis (HT-DTA) provides a means of determining the exact temperature of crystallization of major phases, and examining the nucleation characteristics of a melt under varying conditions without the need for numerous quenched samples. HT-DTA is used here to study the effect of time at maximum temperature on the crystallization of melilite from a bulk composition representing a Type A/B1 CAI. Schirmeyer S.* Bischoff A. Stephan T. Jessberger E. K. Lithium-bearing Phases in Ca,Al-rich Inclusions from CM-Chondrites: Indication of Nebular Alteration Processes Imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS) is an excellent technique for investigating the micro-distribution of elements within different components of chondrites. We used this technique to study the elemental distribution of Lithium within the CM chondrites Cold Bokkeveld, Yamato 791198, and Banten. Lithium can be detected by TOF-SIMS with high sensitivity. Lithium enrichments were exclusively found within CAIs and mainly occur in the Fe-rich phyllosilicates that are located between the outer Wark-Lovering rim and the core of the Ca,Al-rich inclusions. Since Li-enrichments have not been found within the Fe-rich phyllosilicates of the matrix, it is suggested that those within CAIs were formed by alteration in a different environment. We suggest a nebular origin for alteration of Ca,Al-rich inclusions. Weisberg M. K.* Prinz M. Boesenberg J. S. Kozhushko G. Clayton R. N. Mayeda T. K. Ebihara M. E. A Petrologic and Oxygen Isotopic Study of Six Allende Dark Inclusions: Evaluation of Nebular vs. Asteroidal Models for Their Origin Dark lithic inclusions (DIs) are important components in chondrites because they are records of complex events that clearly predate incorporation into their host chondrites. In CV and CR chondrites most DIs show a bulk chemical and oxygen isotopic association with their host chondrites. Allende DIs have recently sparked considerable debate as to whether their unusual characteristics represent primary aggregates of materials overprinted by metasomatic exchange in the nebula or are secondary materials which were affected by aqueous alteration and subsequent dehydration by heating after accretion. To address the broad question as to their origin and relationship to their host chondrite we carried out a petrologic and oxygen isotopic study of six newly found DIs from the Allende CV3 chondrite. We conclude that some of the characteristics of Allende DIs are difficult to explain by a process of heating/dehydration of pre-existing phyllosilicates, but are also difficult to explain entirely by a process of condensation. The issues are complex and require experimental study to help resolve some of the questions. Brearley A. J.* Prinz M. Dark Inclusions in the Allende Meteorite: New Insights from Transmission Electron Microscopy We have studied 2 dark inclusions in the Allende meteorite using transmission electron microscopy. One inclusions consists exclusively of subrounded aggregates of FeO-rich olivine and the second consists of chondrules which have been partially replaced by FeO-rich olivine set in a fine-grained matrix. The microstructures of olivine and diopside in both DIs are complex and both contain abundant pores and submicron inclusions of pentlandite, ilmenite and spinel. These phases are typically rimmed by a thin mantle of poorly graphitized carbon (PGC). The presence of PGC provides strong evidence that the olivine and diopside formed under similar conditions, after formation of the complex hydrocarbon precursors of the PGC at low temperatures. Many of the microstructures appear to be best reconciled with formation in a parent body environment as a result of aqueous alteration followed by metamorphism. Krot A. N.* Scott E. R. D. Zolensky M. E. Origin of Fayalitic Olivine Rims and Plate-ike Matrix Olivines in the CV3 Chondrite Allende We suggested earlier that nearly all of the matrix and the margins of chondrules in Allende were converted to phyllosilicates which were subsequently dehydrated. To test this model, we have studied two moderately altered (Type B,) Allende dark inclusions and Allende host by optical microscopy, scanning electron microscopy and electron microprobe analysis. We infer from our new data that hydrothermal alteration of dark inclusions after accretion caused various degrees of replacement of the original matrix material, chondrule rims, chondrules and isolated forsterite grains by phyllosilicates, formation of secondary sulfide rims around replaced chondrules and isolated forsterite grains, and redistribution of Ca, Fe, Mg, Si, Na, and Mn. Subsequent dehydration of phyllosilicates (saponite and serpentine) produced fayalitic olivine rims around forsterite grains, plate-like olivines and anhedral nephline grains which typically enclose plate-like fayalitic olivines in matrix, chondrule rims and replaced portions of chondrules and forsterite grains. Veins consisting of fayalitic olivine, hedenbergite and andradite were formed at the same time. The compositional, textural and mineralogical similarities between fayalitic olivine rims and matrix olivine in dark inclusions and Allende host and the presence of fayalitic veins in Allende confirm our earlier suggestion that Allende experienced hydrothermal alteration and subsequent dehydration. Zolensky M. E.* Krot A. N. Mineralogical and Compositional Study of an Allende Dark Inclusion Using optical microscopy, scanning electron microscopy (SEM), electron microprobe analysis (EPMA) and transmitted electron microscopy (TEM), we studied several chondrules and isolated forsterite grains in a Type A/B Allende dark inclusion 5a2. All chondrules and isolated forsterite grains in this dark inclusion show various degrees of replacement by fayalitic olivine which occurs in two textural types: compact rims around forsterite grains and plate-like euhedral and subhedral olivine grains surrounded by finer-grained olivine, chromite, hercynite, Fe-Ni sulfides, nepheline and sodalite. We found that plate-like fayalitic olivines in the Allende dark inclusion contain abundant lozenge-shaped voids and planar dislocations which are characteristic for olivine grains produced by dehydration of phyllosilicates. Based on these observations we conclude that Allende dark inclusions experienced various degrees of aqueous alteration and subsequent thermal metamorphism which produced fayalitic olivine rims and plate-like fayalitic olivines and their interstitial mineral assemblage. Hanowski N. P.* Brearley A. J. Chondrule Alteration in the CM Carbonaceous Chondrite, LEW 90500 We have carried out a detailed analysis of 15 chondrules in the CM chondrite LEW 90500, and a characteristic alteration behavior for each of four principal chondrule types has been established. Backscattered electron images and electron microprobe data clearly indicate a strong control on the degree and composition of the hydrous alteration products by the abundance and composition of the primary phases in the chondrule. Alteration initially affected metals and sulfides as well as mesostasis glass. Subsequently pyroxene and Fe-rich olivine were altered followed by the incipient alteration of Mg-rich olivine. Consequently, chondrule types with a high abundance of phases which consistently are most susceptible to aqueous alteration show the highest degree of alteration. The uniformity in alteration behavior for each chondrule type suggests that alteration took place in situ on a parent body. Exchange reactions with a relatively Mg-rich fluid overprinted the primary Fe-rich alteration products of the mesostasis. Browning L. B.* Bourcier W. L. Tochilinite: A Sensitive Indicator of Alteration Conditions on the CM Asteroidal Parent Body Each CM chondrite experienced a different degree of aqueous alteration. As a group, then, these meteorites preserve tangible evidence of asteroidal reactions that were interrupted at many different stages of completion. Geochemical modeling of CM reaction progress should elucidate the nature of the accreted CM materials and the specific types of asteroidal processes and conditions that subsequently influenced them. However, most of the minerals in CM chondrites are stable under a wide range of environmental conditions, which hinders efforts to capitalize on the diverse degree of CM alteration. Petrologic evidence suggests that Fe-rich tochilinite, the widespread mineralic component of CM chondrites previously referred to as "poorly characterized phase (PCP)", may be the most sensitive indicator of the conditions of CM alteration. This possibility has not previously been explored because thermodynamic data for tochilinite are lacking. We have estimated the thermodynamic properties of tochilinite from mixing equations and then calculated its stability limits with associated non-silicate phases as a function of PS2, PO2, and PCO2. The resultant phase relations : a) are consistent with mineral association in CM chondrites, b) indicate that the CM fluids were S-depleted and extremely reducing, c) imply the possibility of H2 gas seeps on the CM parent body, and d) suggest that the alteration of CM materials occurred at significant asteroidal depths. Hutcheon I. D.* Phinney D. L. Radiogenic 53Cr* in Orgueil Carbonates: Chronology of Aqueous Activity on the CI Parent Body The petrographic and mineralogical features characteristic of CI carbonaceous chondrites, including carbonate- and sulfate-filled veins, magnetite morphologies and the ubiquitous presence of carbonates provide strong evidence for the action of aqueous fluids (water) on the CI parent planetesimal. Although commonly described as "early", the time scale of aqueous alteration is not well constrained. Dolomitic (Ca-Mg) carbonates in Orgueil are characterized by very low 87Sr/86Sr ratios, consistent with the lowest values reported for Allende, and must have been deposited within ~50 Ma of Allende refractory inclusions, while 53Cr* excesses in some, but not all, carbonates in two CI chondrites suggest an even earlier onset of aqueous activity. We present here a new study of the Mn-Cr isotope system in Mn-rich carbonates (breunnerite) in Orgueil, providing unambiguous evidence of large excesses of radiogenic 53Cr* from the in situ decay of short-lived 53Mn (t1/2 ~ 3.7 Ma). The inferred initial abundance of 53Mn, 53Mn/55Mn ~ 2 x 10-6, implies early formation of these Orgueil carbonates, within ~16 Ma of Allende Ca-, Al-rich inclusions. Leshin L. A.* Rubin A. E. McKeegan K. D. Oxygen Isotopic Compositions of Olivine and Pyroxene from CI Chondrites The oxygen isotopic compositions of carbonaceous chondrites are complicated since individual components of these meteorites have distinct isotopic compositions that have been affected, to varying degrees, by both nebular and parent body processes. We present the first measurements of oxygen isotopic compositions of rare olivine and pyroxene in the CI chondrites. By combining these data with oxygen isotopic analyses of secondary minerals in these rocks previously reported by Clayton and co-workers, we hope to place quantitative constraints on the physical processes that occurred during formation of these meteorites. The oxygen isotopic composition of separated olivine and pyroxene grains from Orgueil and Ivuna were measured using SIMS. The delta ^(18)O values of the olivines range from ~+3 to +13 and the delta^(17)O values range from ~+1 to +7. The delta^(18)O values of the pyroxenes range from ~+4 to +10 and the delta^(17)O values range from ~+2 to +5. Observations that will be explained in the interpretation of the data are: (1) CI pyroxene and olivine grains have essentially identical oxygen isotopic compositions which are distinct from those measured in other carbonaceous chondrites. (2) All data points except two are indistinguishable from terrestrial values at the one sigma level, and the two exceptions fall only slightly below the terrestrial fractionation line. We consider the data taken as a whole to be consistent with terrestrial values, as well as the slightly positive delta^(17)O values reported previously for matrix and magnetite in CI chondrites. (3) Due to the small range in d values observed relative to the uncertainties, it is not possible to distinguish whether these data fall on a line of approximately slope 1 or slope 1/2. Tuesday, March 19, 1996 MARS: VOLCANIC AND TECTONIC PROCESSES 8:30 a.m. Room C Chair(s): J. H. Fink D. E. Smith Mouginis-Mark P. J.* The Long Lava Flows of Elysium Planitia, Mars The existence of several unusually long (>100 km) lava flows to the north of Elysium Mons, Mars, has been previously reported. Further mapping of the same flow field is reported here, since it is now seen to include a total of 59 individual lava flows that range in length from 12 to 246 km. These flows are located predominately to the north and west of Elysium Mons, in the region between 26 degrees - 34 degrees N, 207 degrees - 224 degrees W. The flows are of interest for several reasons: a) they can be used to infer the distribution of dikes within the perimeter of Elysium Mons; b) some flows are very long compared to Quaternary terrestrial lava flows and so provide morphologic clues to the emplacement process of these unusual terrestrial examples; and c) they may place real-life constraints on recent numerical models that predict the formation of long lava flows via tube-fed eruptions. Arvidson R. E.* Harmon J. Campbell B. Guinness E. A. Geologic Analysis of Arecibo Radar Images of Mars Five equatorial to mid-latitude regions of Mars were covered by the Arecibo S-band (12.6 cm) system and reduced to specific cross section images that show backscatter in same-sense and oppositely polarized circular polarization. The purpose of this paper is to summarize analyses of surface properties inferred from the data, Viking Orbiter images, and comparisons to data acquired over lava flows in Hawaii and the Lunar Crater Volcanic Field, Nevada with the AIRSAR system. Detailed descriptions of the work will be presented in subsequent papers by Harmon et al., in preparation. A prime result of the analyses is that two very young units display very high same- sense backscatter, with values comparable to rough, young a'a flows on Earth. They are: (a) the plains member of the Olympus Mons Formation that girdles Olympus Mons in N, E, and S directions from the shield; and (b) the young channeled and/or plains volcanism terrain located in Elysium and Amazonis Planitiae (Achu; Cerberus Plains). We suggest that these surfaces developed after weathering was unable to destroy roughness associated with flows and channels; i.e., volcanism, channeling, and the efficacy of weathering were all related. Lescinsky D. T.* Fink J. H. Lava and Ice Interaction on Mars: Application of Terrestrial Observations and Laboratory Simulations Observations of terrestrial examples of lava and ice interaction have been combined with laboratory simulations to provide a framework for investigating lava and ice interaction on Mars. Identification of large- and small-scale characteristic features will indicate whether large quantities of meltwater were trapped or whether lava flow confinement occurred. These features should also enable the distinction between interaction with glacial ice or permafrost. Relationships between lava flow morphology and eruption parameters determined by laboratory simulation can be applied to martian deposits to estimate lava composition and effusion rate, and consequently constrain mantle processes. Fink J. H.* Griffiths R. W. Modeling Lava Domes as Temperature-dependent Bingham Materials The venusian "pancakes" brought silicic lava domes to the attention of many planetary geoscientists. Lava domes have also been compared with certain constructs on the Moon, Mars and icy satellites. The principal unresolved questions about domes are how their overall shape (morphology) and surface structure (texture) relate to their composition and mode of emplacement. Because few terrestrial domes have been carefully monitored while active, various attempts have been made to model their growth in the laboratory and in theory. Here we present the results of 21 new analog experiments using suspensions of polyethylene glycol (PEG) wax and kaolin (clay) powder injected into tanks of cold water. The resulting extrusions exhibit a progression of morphologic and textural features common to natural domes, with transitions depending on the relative rates of eruption and cooling. The identification of similar features on extra-terrestrial flows may expand ourability to infer their composition and cooling history. Crown D. A.* Peitersen M. N. Downflow Morphologic Variations in Hawaiian Lava Flows: Implications for Modeling Planetary Lava Flow Emplacement Models for the emplacement of lava flows on planetary surfaces utilize morphologic properties of flows to constrain eruption conditions, cooling history, and/or the physical properties of lavas. A variety of thermal and rheologic models have been used to assess the types of volcanic processes that have occurred on other planets. Most models of flow behavior use average values of flow width and/or flow thickness based on a limited number of measurements. The precision of these measurements, in particular of flow thickness, is highly variable. Changes in flow dimensions along the length of a lava flow represent the combined influences of a series of complex phenomena, including variations due temperature-dependent rheologic properties, variations due to time-variable supply conditions, and changes in the nature and geometry of the underlying surface, as well as the development of a brittle crustal layer. Variations in downflow dimensions thus provide important information regarding eruption and flow emplacement history, but are rarely used in quantitative analyses of flow dynamics. In order to interpret and utilize downflow morphologic variations of planetary lava flows, data from well documented terrestrial flows are being analyzed. Michaels G.* Greeley R. Lava, Debris, and Pyroclastic Flow Deposits: Analysis and Identification Using Curvature Spectra Evidence of rhyolite, andesite, basalt, debris or pyroclastic flow deposits on planets such as Mars and Venus can give important clues to the nature of volcanic processes, crustal evolution and volatile abundance and history. The positive identification of lahars or debris flows on Mars is evidence for the presence of water. Despite this importance, it is often difficult to recognize flow deposit type with confidence from remote observation. Williams D. A.* Lesher C. M. Summary of Field Evidence for Thermal erosion by Channelized Archean and Proterozoic Komatiite Lava Flows Thermal erosion is inferred to have occurred during the emplacement of several types of terrestrial and lunar lava flows, to have possibly generated some of the martian channels and venusian canali, to have formed embayments in some Archean and Proterozoic greenstone belts , and to have generated most magmatic sulfide deposits. Hot, low-viscosity komatiite lavas may be the best analogs for some planetary lavas, and studies of the morphologies and geometries of the embayments in Archean and Proterozoic greenstone belts therefore provide the best means to calibrate mathematical models of thermal erosion. The study of thermal erosion by Archean and Proterozoic komatiites is complicated by the large scale of the flows, superimposed heterogeneous structural disruption and pervasive metamorphism, and sporadic exposure, which make determinations of vent locations and flow geometries virtually impossible. Nevertheless, igneous textures are often preserved and studies of surface outcrops, underground exposures, and drill cores can provide much data on the morphologies and compositions of komatiite lava flows and associated thermal erosion features. Mustard J. F.* Sunshine J. M. Mafic Mineralogy of Martian Volcanics: Variations in Space and Time The high spatial resolution, spectral coverage, and high signal to noise of the ISM data have permitted the determination of a mineralogic basis for the spectral properties of weakly altered regions of Mars. Through the use of the Modified Gaussian Model (MGM), it was demonstrated that weakly altered mafic materials are dominated by two-pyroxene basalts, analogous to the basaltic SNC meteorites. In addition, differences in spectral properties between Eos Chasma and Nili Patera were best modelled as a change in relative modal abundance of low (LCP) and high (HCP) calcium pyroxene, with Nili Patera enriched in high-calcium pyroxene relative to Eos Chasma. There is a great deal of diversity within other spectra of weakly altered crust identified thus far, and some of this diversity is due to mixing with dust and alteration products, and some to mineralogic variations. We have examined the mineralogic basis for this diversity to determine how regionally extensive these mineralogic differences are, the degree of homogeneity of surface compositions, and if other compositional units can be identified. For all areas examined, the weakly altered materials on Mars are best modelled as two-pyroxene basalts enriched in HCP. Syrtis Major, a low relief, Early Hesperian, shield volcano, has a homogeneous composition with the highest relative abundance of HCP. Ophir Planum, a region of ridged, volcanic(?) plains, is also enriched in HCP, but has ~12% less than Syrtis, while the floor of Valles Marineris is the ~20%-25% less HCP than Syrtis. These differences can be interpreted to indicate a change in source mantle composition from relatively fertile (Syrtis Major, Ophir Planum) to depleted (Valles Marineris), which would be consistent with the relative ages and environments of formation. Smith D. E.* Zuber M. T. The Shape of Mars and the Topographic Signature of the Hemispheric Dichotomy Analysis of occultation data from the Mariner 9 and Viking Orbiter spacecraft has yielded the first accurate model for the shape of Mars. The hemispheric dichotomy, which was previously considered to be a relatively sharp 1-3 km change in height from the topographically high southern hemisphere to the low northern hemisphere, is not a fundamental feature of the shape of the planet. The dichotomy is a consequence of tha bout 3 km offset between the center of mass and center of figure of Mars, and the boundary along most of its length consists of broad, gradual surface slopes over distances of thousands of kilometers. This result is supported by analysis of high spatial resolution Earth-based radar topographic profiles. Any successful model for the origin of the dichotomy must explain a planet with an ellipsoidal shape and a long wavelength, gradual topographic transition between the northern and southern hemispheres. Turcotte D. L.* Martian Tectonics: Why the Global Dichotomy and the Tharsis Uplift Two of the major tectonic features on Mars are the global crustal dichotomy and the Tharsis uplift. One explanation for the dichotomy is that hemispheric subduction occurred during the early evolution of Mars. The lithosphere in the northern hemisphere was subducted one or more times removing the primordial crust. As the planet cooled and the global lithosphere stabilized, heat was lost by continuous volcanism in one region, the Tharsis uplift. This volcanic construct can be best explained by pressure release melting in an ascending limb of a global mantle convection system. Frey H. V.* Bills B. G. Lyons S. N. Roark J. H. Regional Crustal Thickness Variations on Mars We generated models of crustal thickness for Mars using both Mars50c and GMM-1 based on the assumption that gravity anomalies are due only to variations in surface and crust/mantle topography and crust and mantle densities are laterally constant, for a range of crust and mantle densities, and assumed average crustal thickness. Here we discuss regional variations in crustal thickness for one such model, with average thickness of 65 km and crust/mantle density contrast 0.5 (crust 3.0, mantle 3.5) gm/cc. Crustal thickness ranges from 140 km below Olympus Mons to less than 10 km below the Hellas and Isidis basins. Crust below Argyre is five times thicker than below Hellas, while that in Elysium is 85 km thick. Most (but not all) heavily cratered terrain is thicker than the crustal average, while most lowlying plains are only 25-45 km thick. Across the crustal dichotomy boundary zone the crustal thickness changes by 25 to 35 km over less than 500 km in some (but not all) places. Tanaka K. L.* Dohm J. M. Watters T. R. Possible Coronae Structures in the Tharsis Region of Mars Coronae, as defined on Venus, constitute broad (several tens to hundreds of kilometers across) circular to elliptical topographic features with associated volcanism and radial and concentric tectonic structures. On Venus, coronae vary widely in character and size; they appear to be structures formed by rising mantle diapirs that produce uplift and relaxation of the lithosphere and commonly volcanism. Three similar structures have been recently proposed for Mars. Here, based on a preliminary reconnaissance, we propose that as many as 15 coronae occur in the Tharsis region of Mars. Most of these have been previously interpreted as the sites of deep-seated intrusives. Alternatively, some may be controlled or formed by impact, collapse, or other geologic processes. Schultz R. A.* Extension and Strain Across Valles Marineris Extension and strain are calculated for six traverses across the Martian Valles Marineris troughs. Extension accommodated by the large trough normal faults varies systematically along the length of the trough system, increasing from 5-10 km near the eastern and westem termini to a maximum value of 20-30 km near the center, assuming 60 degree fault dip angles and +1 km in trough relief. The spatial gradient in extension exceeds reasonable uncertainties in both topography and fault dip, demonstrating an inhomogeneous deformation field along strike, similar to other well documented examples of normal faults and grabens on Earth. Strain also varies with position, typically between 5% and 20%. Smaller values of strain reported for the Valles Marineris region, <2%, that include large expanses of unextended terrain represent spatial averages that may underestimate the magnitude of strain at the troughs. Tuesday, March 19, 1996 MARE BASALTS: GENERATION, EMPLACEMENT, COMPOSITION, AND DISTRIBUTION 1:30 p.m. Room A Chair(s): J. W. Delano R. C. Friedman Taylor G. J.* Lucey P. G. Hawke B. R. Spudis P. D. Composition of the Lunar Maria We have developed analytical techniques to accurately determine the FeO and TiO2 contents from reflectance spectra, and applied these to a global data set (35-km spatial resolution) obtained by the Clementine mission. The data show that lunar mare basalts vary continuously in composition from very low-Ti to high-Ti basalts. Basalts with TiO2 contents in the range 2-4 wt.% are most abundant, though basalts with lower TiO2 are significant. Abundance decreases with increasing TiO2: only 5% of the mare surfaces contain >7.5 wt.% TiO2. This is consistent with the relative abundances of very-low to high-Ti cumulates produced in a magma ocean. Some maria have notably lower FeO (by at least 5 wt.%) than typical; these include Nectaris, Frigoris, and Orientale. These maria may be composed of basalts richer in Al2O3 than aluminous mare basalts in the sample collection. Longhi J.* Investigation of the Origin of Hi-Ti Basalts by Polybaric Fractional Fusion New experiments at 25 and 30 kb help to define the ilmenite saturation surface in liquids relevant to melting of the mare basalt source region. These data have been incorporated into new calculations of polybaric fractional fusion. The calculations show that it is possible to generate composite liquids, similar in composition to the very-high-Ti (VHT) red and black volcanic glasses, by the same process of polybaric fractional fusion proposed for the very-low-Ti green glasses by [1]. The only important difference is the composition of the VHT source, which was the result of adding 10 % of a 1:1 mixture of ilmenite and clinopyroxene to the green glass source composition of Longhi (1992) Lunar Planet. Sci. XXII, 343-353. However, calculated densities of the instantaneous melts are distinctively higher than those of the depleting source. As the melt sinks through the rising source, it freezes and becomes denser still. In order to generate the VHT magmas, the frozen VHT primary melts must remain within the source. To do this, the frozen melt must remain in discrete, isolated lenses rather than accumulate into a single mass. Alley K. M.* Parmentier E. M. Thermal Convection in an Initially Compositionally Stratified Fluid Heated from Below: Application to a Model for the Evolution of the Moon Stable compositional stratification resulting from the formation and early differentiation of a planet may play a significant role in its thermal evolution. Plumes generated by unstable thermal boundary layers in a fluid of sufficiently strong stable compositional stratification rise to their height of neutral buoyancy, developing a thermally and compositionally mixed layer which thickens with time. One model of mare basalt formation supposes that during initial differentiation, dense illmenite and radioactive element enriched cumulates resulting from the final stages of magma ocean solidification sink to the center of the Moon. This provides a flux of heat into the base of the newly stably stratified lunar mantle, leading to the development of a thickening mixed layer. Melting at the top of such a mixed layer may provide a source for mare basalts, producing melt at appropriate depths and time interval after solidification of the magma ocean. The purpose of this study is to use numerical solutions of the equations governing buoyant viscous flow to further investigate this simple theoretical model of lunar evolution by better quantifying the rate of development of a mixed layer. Delano J. W.* Apollo 14 Green Volcanic Glasses with a Compositional Memory of Melting Processes in the Lunar Mantle The picritic volcanic glasses are generally believed to be samples of primary magmas from the lunar mantle. Although the twenty-four groups define a wide compositional range (e.g., factor of 70 in Ti), these magmas have experimentally determined, multiple-saturation points within the limited pressure-range of 15-25 kilobars. Assuming that these picritic magmas were generated by an isobaric, batch melting process, the source regions of these primary magmas were located at depths of 300-500 km. Although the isobaric, batch melting process was a popular concept in terrestrial MORB petrology until the mid-1980s, dynamic melting processes are now considered to be more realistic for MORB petrogenesis. These concepts have recently been applied to the lunar picritic compositions. Since the mineralogy, phase relations, and bulk composition are known with much less certainty for the lunar mantle than for the terrestrial upper mantle, dynamic melting models for the primary lunar magmas are comparatively underconstrained and non-unique. The challenge becomes even greater if these superheated picritic magmas also assimilated complex, late-stage cumulates during their ascent through the uppermost mantle and lower crust. The fact that the highest-Ti picritic melts become negatively buoyant at >15-20 kilobars provides new constraints on the depths and melt-extraction processes. Welten K. C.* Lindstrom D. J. Martinez R. R. Trace Elements in Vitrophyric Glass and Lithic Clasts from 78526, A VLT-Basaltic Impact Melt Since the discovery of very low-Ti (VLT) basalts as tiny clasts and glasses in Apollo 17 drill cores, VLT basalts have been recognized as an important lunar rock type. Lunar sample 78526 (8.77 g), our largest specimen of VLT basaltic composition, is an impact melt composed of vitrophyric green glass with minor lithic and mineral inclusions. Major and trace element abundances of bulk 78526 are intermediate between those of Apollo 17 VLT basalt fragments, but the lithic clasts are less well-defined because no trace element data are available. This work utilizes a micro-coring technique to extract microgram-sized samples from a thin-section of 78526, and micro-INAA techniques to obtain trace element abundances. The results show that the two major clasts are distinct from bulk 78526, but are similar to several VLT basalt clasts found in a highland breccia, 72235. Gillis J. J.* Spudis P. D. The Composition and Geologic Setting of Lunar Far Side Maria The dichotomy in the distribution of maria between the Earth facing side and the far side of the Moon has evoked many questions concerning the emplacement mechanisms, compositional variation, and source regions of lunar basalts. The near side maria have been well analyzed using a variety of Earth based, and lunar surface and satellite information. The Clementine mission has provided the first global mineralogical and chemical maps for the Moon. These data will allow us to gain knowledge of many lunar far side basalt deposits for the first time. Although the far side maria represents only about 1% of the surface of the Moon, these deposits provide insight into the crustal evolution, thermal history, and the interior of the Moon. Information on the composition, age, and volume of mare deposits will further our understanding of these questions. We are now studying several data sets, including multi-spectral images, crater statistics, and altimetry, to reconstruct and understand lunar volcanic evolution. Ultraviolet/visible (UVVIS) image data provide information of the composition of mare deposits. Ratios 415/750 provide information of titanium content and the 950/750 absorptions is correlated with the concentration of Fe2+ and mafic minerals. Crater statistics provide relative age determination, which can be used to estimate absolute ages. Knowing the range in ages of all the mare units on the far side will allow us to determine the duration of magmatism on the far side. Digital elevation maps from the Clementine altimetry allow us to estimate the deviation of mare units relative to the mean lunar radius to test if a systematic correlation in height of lava eruption surfaces and age might reflect a corresponding increase in depth of the zones of magma generation with time. Preliminary calibrations and multi-spectral mosaics have been generated for mare deposits within Antoniadi, Campbell, Compton and for Mare Moscoviense. Friedman R. C.* Blewett D. T. Taylor G. J. Lucey P. G. FeO and TiO2 Variations in Mare Imbrium We are investigating regional variations in mare basalts to determine whether there are changes in the magma sources over time or evolution of lava during the course of emplacement. Mare Imbrium, with its multiple high-volume Eratosthenian flows, is a prime location for examining magma compositions in three spatial and temporal contexts: along lengthy flows, between major eruptive events (represented by at least three phases of lava flows), and between younger (Eratosthenian) and older (Late Imbrium) mare filling lavas. We have used Clementine and Galileo spectral reflectance data, calibrated to determine FeO and TiO2 contents to high accuracy. These data show significant differences between older and younger Imbrium mare, which suggest distinctly different geochemical provinces: one like Apollo 15 basalts and one like yellow glass 15010,3189 and richer in TiO2 and Th. The flows from three eruptive events, attributed to the same source but spread over 0.5 Gy, show minor variations in composition, suggesting modest evolution through time of the magma source due to the repeated high volume melting events. Two long (400-600 km) Eratosthenian flows show no consistent variation downstream, indicating little fractional crystallization took place during flow emplacement. von Frese R. R. B.* Tan L. Potts L. V. Merry C. J. Bossler J. D. Lunar Crustal Modeling of Mare Orientale from Clementine Satellite Observations Clementine satellite gravity and altimetry observations were analyzed in lunar spherical coordinates for density variations that may be related to the crustal features and evolution of Mare Orientale. The analysis considered 2-degree gridded free-air gravity anomaly and LIDAR topographic estimates for a 68-degree by 68-degree region of the moon centered roughly on Mare Orientale. At this scale of coverage, the gravity anomalies appear to be dominated almost completely by crustal density variations related to surface topography, crustal isostasy, mantle topography, and meteorite impact. The topography of the study region includes over 11 km of relief, the gravity effect of which we modeled in lunar spherical coordinates by Gauss-Legendre quadrature integration assuming a topographic density of 2.8 g/cc. We observed substantial positive and negative correlations between the free-air and topographic gravity anomalies that seriously limit the utility of simple Bouguer gravity anomalies for lunar subsurface studies. Using the wavenumber correlation spectrum between the free-air and topographic gravity anomalies, we designed correlation filters to extract the the correlative anomalies in the two data sets. Hiesinger H.* Jaumann R. Neukum G. Head J. W. III Mare Australe: New Results from Lunar Orbiter and Clementine UV/VIS Imagery Mare Australe is an old lunar impact basin which is visible in its entire shape only in spacecraft images but not from Earth. We used Lunar Orbiter IV images as well as Clementine UV/VIS images acquired in 1994 in order to study basalt ponds of the Mare Australe basin. We performed crater counts, measured the size of the basalt ponds and the nearest neighbor distance and compared our results for Mare Australe to previously published results for the Mare Orientale region and the South Pole-Aitken basin. Yingst R. A.* Head J. W. Characteristics of Mare Deposits on the Eastern Limb of the Moon: Implications for Magma Transport Mechanisms Lunar volcanic history has been examined in light of geomorphological and stratigraphic constraints placed upon the surface features. Compositional and petrological analyses have provided models for the conditions of mare parent magma generation . The connection between lunar magma source regions and volcanic surface features remains unclear, however, both conceptually and quantitatively with respect to our understanding of transport mechanisms. It has been suggested that mare emplacement was controlled by propagation of dikes driven by the overpressurization of diapir-like source regions stalled below the cooling lunar highland crust. Recent analyses of the characteristics of lava ponds in the South Pole/Aitken and Orientale/Mendel-Rydberg basins based on Clementine, Lunar Orbiter and Zond data have provided evidence that supports this theory. In this contribution we report on an analysis of the areas, volumes, modes of occurrence and crustal thicknesses for mare deposits in the Marginis and Smythii basins, and investigate implications for magma transport mechanisms. Wilson L.* Head J. W. Lunar Linear Rilles as Surface Manifestations of Dikes: Theoretical Considerations Linear and arcuate flat-floored rilles and v-shaped fractures are common tectonic features on the planets, are interpreted as extensional in nature, and can have a variety of modes of origin, including flexure, broad elastic expansion in the substrate, hydrofracturing, sliding of near-surface rocks along a detachment, and dike intrusion. In order to establish criteria for these different modes of origin on the Moon, and to utilize these in a better understanding of the range of tectonic forces that have influenced the lunar crust and lithosphere, we develop a theoretical treatment for the penetration of magma-filled cracks (dikes) to the vicinity of the lunar surface, and outline the predicted range of tectonic and associated volcanic features and processes. We then examine the range of rilles and associated features observed on the Moon and analyze them in the context of these predictions. Weitz C. M.* Head J. W. III McKay D. S. Eruption and Emplacement of Lunar Pyroclastic Glasses as Inferred from the 74001/2 Section We have examined the 26 thin sections made along the length of the 68-cm deep 74001/2 core. The core is composed of ~99% submillimeter orange volcanic glasses and their crystallized equivalents (black beads). The Taurus-Littrow dark mantle deposit on the southeastern edge of Mare Serenitatis represents a mixture of these volcanic beads with other lunar soils. The source(s) location and eruption conditions that produced the volcanic beads at Taurus-Littrow and other regional dark mantle deposits is still under debate. In this study, we have examined thin sections and determined the cooling histories for the volcanic beads based upon their textures. In addition, several digital elemental images were made of four of the thin sections taken at different depths within the core. Our goal is to use the petrology of the beads to determine the eruption conditions in the fire fountain and the most likely location and number of vent sources associated with the Taurus-Littrow deposit. Eventually, we intend to apply our results from the Taurus-Littrow study to other regional dark mantle deposits on the Moon using Clementine multispectral data. Neumann G. A.* Zuber M. T. Coherence Analysis of Lunar Mare Basins Clementine GLTM-2 altimetry and GLGM-2 gravity from Clementine and historical tracking provide free-air and Bouguer gravity maps at the lunar surface for wavelengths>200 km. A preliminary study found that lunar topography is not compensated by a single mechanism and maintains significant rigidity over 3 b.y. time scales. Interpretation of these data via flexural isostatic models constrains the flexural rigidity of the lithosphere lie in the range 4x10^23 to 10^25 N m for most nearside basins. These correspond to effective elastic thicknesses of 40 to 120 km. While the coherence method gives relatively robust estimates of lithospheric thickness, interpreting the Bouguer anomaly (BA) in this fashion requires some caution. The uncertainty in the gravity data increases dramatically at short wavelengths, and makes estimation of the relative proportion of subsurface to surface loadings difficult. In addition, most short-wavelength topography results from impact processes that violate the assumption that such loadings are statistically uncorrelated. Nevertheless, coherence analysis can provide a lower bound on lithospheric strength. Tuesday, March 19, 1996 COMETS AND ASTEROIDS 1:30 p.m. Room B Chair(s): B. E. Clark A. W. Harris Weissman P. R.* Levison H. F. The Size Distribution of Cometary Nuclei The solar system's inventory of comets is stored in two dynamically long-livedreservoirs, the Oort cloud at semimajor axes of ~3-100 x 10^3 AU, and theKuiperbelt at distances of 30 to ~100 AU, beyond the orbit of Neptune.Although the Oort cloud is not yet observationally accessible, the Kuiper belt is, and a total of 32 objects have been discovered in trans-Neptunian orbits as of the end of 1995 (Weissman and Levison 1996).In addition, ~30 cometary-sized objects have been detected in the Kuiper belt using the Hubble Space Telescope (Cochran et al. 1995).These discoveries, as well as other means of estimating the population of the Kuiper belt at various size ranges, make it possible to construct a size distribution for icy bodies in this region of the solar system.Since the Kuiper belt is the likely source of the short-period comets (Duncan et al. 1988) and because the long-period comets likely formed in the adjacent Uranus-Neptune zone, this size distribution may be applicable to cometary sized bodies in the Oort cloud as well as the Kuiper belt. Malhotra R.* The Fate of Neptune Planetesimals Recent observational confirmation of the existence of small bodies near and beyond the orbit of Neptune is a very exciting development in solar system dynamics. The population of this Kuiper Belt for the region between 30 AU and 50 AU heliocentric distance is presently estimated at approximately 35,000 objects larger than 100 km and upwards of 10**9 cometary size objects (less than about 10 km in size). It appears likely that the orbital distribution of these objects was largely determined by dynamical processes in the early history of the solar system that preferentially swept most Kuiper Belt objects into the narrow stable libration zones at Neptune's mean motion resonances, particularly into the 3:2 and the 2:1 resonances located at semimajor axes of 39.5 AU and 47.8 AU, respectively. Here I show that a small but notable fraction of objects in initially near-circular, low-inclination orbits in the vicinity of Neptune would have been captured at the L4 and L5 Lagrange points and survived in that dynamical state the entire orbital evolution of Neptune during the early history of the Solar system. Harris A. W.* The Rotation Rates of Very Small Asteroids: Evidence for 'Rubble Pile' Structure I have recently updated my data base of asteroid rotation rates. The new file includes reliable rotation periods of 688 asteroids. Most of the trends reported earlier remain apparent in the expanded data set. In this paper I concentrate on the rotation statistics of the smallest asteroids, D < 10 km. The main conclusion is that there are no asteroids rotating fast enough that they are in a state of tension, that is, that they would fly apart if they had no tensile strength. In earlier studies, it was not clear whether this is because small asteroids indeed have no tensile strength, or only because such fast rotation is statistically improbable enough that one would not expect to find an end-member of the distribution spinning faster than that limit. A histogram of the spin distribution of the smallest asteroids of the present expanded data set indicates that the distribution at the fast end of rotation frequency is in fact truncated, thus implying that even these small asteroids have no tensile strength, that is, they are not monolithic, and may be "rubble piles." Rivkin A. S.* Bottke W. F. Hypovelocity Impacts in the Asteroid Belt The mean impact speed in the asteroid belt is roughly 5 km/s. Coincidentally, that is also roughly the speed of sound in iron-nickel meteorites and in basalt. The speed of sound in ordinary chondrites is about 3.5 km/s. Because of this, other things being equal, a higher percentage of impacts into chondritic parent bodies will generate shock waves than impacts into metallic or basaltic bodies. Without a shock wave, much less material may be vaporized and lost to space. Similarly, impacts involving metallic or basaltic impactors may be less likely to disrupt and vaporize the impactor, perhaps resulting in preferential retention of these materials over chondritic material on the asteroidal surface. To investigate the frequency of "hypovelocity" (slower than the speed of sound in a material) impacts with members of the C, S and M classes, we have calculated probability distributions of impact velocities between the 682 asteroids with D > 50 km (a representative sample of asteroids in the main belt) and 21 asteroids of various classes using the technique of Bottke et al. (1994). We find that peak impact speeds can vary by 2-3 km/s from asteroid to asteroid, and because these speeds are close to the speed of sound in iron-nickel, basalt and ordinary chondrite, these relatively small changes can give rise to large differences in percentage of hypovelocity impacts from asteroid to asteroid. We also find that for a given target, C, S and M-class impactors all have very similar impact speed distributions. Because of these factors, it seems clear that discussions of impact-generated "space weathering" should take not just asteroid size into account, but also compositions and orbits. Clark B. E.* New News and the Competing Views of Asteroid Belt Geology We have collected a lot of new data in the seven years since the big blue books came out (Asteroids II, 1989, and Meteorites and the Early Solar System, 1988). Here we review how all this new data has changed our perceptions of the geology of the asteroid belt. The only way to a complete understanding of asteroid belt geology is through knowledge of the nature of the mysterious and elusive heating mechanism that differentiated the asteroids. If we can figure out what it was, then we will know something fundamental about the forces at work during the formation time of the Solar System. Some of the principal observations we can make to put constraints on the possibilities come from observations of main asteroid belt geology. The most pressing problem in the geology of the asteroid belt is in the inner belt. S-type asteroids are key to the inner main belt, but their meteorite linkage is ambiguous. Why do we care? Because--if the S-types are chondritic, then almost the entire main belt is primitive material, representing very early solar system time, but telling us almost nothing about the mysterious heating mechanism. If the S-types are differentiated, then the heating mechanism ceased functioning in the middle of the main belt at the 'dividing line' between primitive and differentiated material. This dividing line is the silicate rock melting temperature position in space of the mysterious heating event. The Bell Big Picture model of asteroid belt geology synthesized all the information available and established the position of this dividing line back in 1988. But there is now new data to consider. Giese B.* Oberst J. Kirk R. L. Zeitler W. High Resolution Digital Terrain Models of Asteroid Ida: A Comparison Between Photogrammetry and the Shape-from-Shading Method The asteroid Ida was imaged by the SSI camera during the second Galileo asteroid flyby in August 1993. We derived high resolution digital terrain models in selected regions of Ida based upon a rigorous photogrammetric analysis and compared these results with terrain models derived from a two-dimensional photoclinometric ("shape-from-shading") analysis. Our comparison shows that there are striking discrepancies between the two models depending on spatial scales of surface features. This suggests that surface morphology studies of the asteroid should not rely on just one of these models. Ghosh A.* McSween H. Y. Jr. An Extended Thermal History (100 Ma Long) for Asteroid 4 Vesta Based on Radionuclide and Collisional Heating Thermal modeling has so far been done on small asteroids with radii of 100 km or less. In this study, we attempt to model a larger body, asteroid 4 Vesta, which has a radius of 275 km. Previous models have only attempted to explain thermal metamorphism and aqueous alteration, not igneous activity and core formation. In tracing the thermal evolution of a body such as Vesta, it becomes important to account for the latent heat of fusion and the increased specific heat capacity of melts, as well as to consider the possible effects of collisional heating and the redistribution of radioactive nuclides (26Al and 60Fe) during differentiation. Wasson J. T.* Chapman C. R. Grogan K. Dermott S. F. Possible Formation of the Vesta-Family Asteroids and the Main IRAS Dust Band by an Oblique Impact on Vesta Spectroscopic studies by Binzel and Xu showed that a large fraction of the asteroids linked to Vesta on the basis of similar orbits share Vesta's unique, sharply defined basaltic reflection spectrum. These asteroids, which we will call vestoids, have diameters 10 km. Galileo studies of Gaspra and Ida show that spectral properties of FeO-rich surfaces degrade quickly; it appears that an ordinary-chondrite surface may resemble a typical S-asteroid within about 100 Ma, and the degradation of FeO-rich basaltic surfaces would be comparable. It follows that the nearly unweathered surfaces of Vesta and the vestoids may be <10 Ma old, in which case the vestoids were probably formed in the same collisional event that resurfaced Vesta. The semimajor axes of the vestoid orbits show a remarkably wide range from smaller than that of Vesta at 2.36 AU to 2.47 AU, near the 3:1 period resonance with Jupiter that appears to be responsible for bringing most stony meteorites from the Asteroid Belt to Earth. Britt D. T.* The Parent Asteroids of Ordinary Chondrite Meteorites Ordinary chondrites account for 80% of meteorite falls, but twenty years of searching have found only a very few direct spectral matches between OC's and asteroids. Those asteroids that do match are small, rare, and all but one are in the planet crossing asteroid population. The one OC match in the main belt, 3628 Boznemcova, is very small (7 km) and the only OC found out of a thousand asteroids examined by the Binzel et al. search. Why can't we find a large reservoir of ordinary chondrite parent bodies? The three commonly cited theories on OC parent bodies each have their flaws. Bell's approach, that OC's have been smashed by collisional evolution into fragments smaller than 10 km diameter has not been supported by the results of the extensive Binzel search. Chapman has suggested that the S-asteroids may be OC's that have "space weathered". S-asteroids are abundant and have "approximately" OC-like albedo and spectral features. But the work of Gaffey et al. has shown that the S class is really a grab-bag of radically different mineralogies and only the S(IV) subclass has OC-like mineralogies. In addition, extensive spectral and petrologic work with regolith OC's and space weathering experiments with OC material have shown that OC's do not "space weather" to match the spectral features of S-asteroids. But S(IV)/OC link also has several problems: (1) S asteroids have weaker absorption bands than OC's; (2) S asteroids have a much stronger spectral "red" slope from 0.6 to 1.6 microns; (3) S asteroids have a characteristic "leveling off" of the spectral slope longwards of 1.6 microns. Flynn G. J.* Are the S-Type Asteroids the Parent Bodies of Ordinary Chondrite Meteorites?: Evidence from the Interplanetary Dust Recovered from the Earth's Stratosphere The observation of dust bands associated with the Koronis family of S-type asteroids demonstrates that this family produces abundant interplanetary dust. Because of the low inclinations and eccentricities of the orbits of Koronis family members, dust from the Koronis family is enhanced in near-Earth collections. Kortenkamp et al. estimate that the Koronis family and the C-type Themis family together contribute 55% of the interplanetary dust particles (IDPs) incident on the Earth. Comparison of the chemical, physical, and mineralogical properties of the IDPs collected in the stratosphere with those properties of ordinary chondrite meteorites show few similarities, suggesting the S-type Koronis asteroids produce IDPs distinctly different in composition and structure from ordinary chondrites and that these S-asteroids are not parent bodies of ordinary chondrite meteorites. Gaffey M. J.* Asteroid 6 Hebe: Spectral Evaluation of the Prime Large Mainbelt Ordinary Chondrite Parent Body Candidate with Implications from Space Weathering of Gaspra and the Ida-Dactyl System No large mainbelt asteroid has yet been confirmed as an ordinary chondrite parent body. A number of plausible candidates have been identified based on dynamical considerations and/or spectral evidence. The fifth largest S-asteroid, 6 Hebe, is the best current candidate for one of the long sought large mainbelt ordinary chondrite parent bodies. Its proximity to the chaotic zones associated with both the 3:1 proper motion and the nu(sub)6 (or g=g(sub)6) secular resonance "escape hatches" should make Hebe a major mainbelt contributor to the terrestrial meteorite flux. Hebe also exhibits a surface assemblage (subtype S(IV), the only subgroup among the S-asteroids that might include ordinary chondrite parent bodies) which allows (or more precisely, does not exclude) an ordinary chondrite affinity. An analysis of the rotational variations of spectra obtained in June 1979 and February 1989 provide a sophisticated test of whether or not Hebe is a viable ordinary chondrite parent body. If the rotational spectral variations of Hebe are consistent with an undifferentiated silicate assemblage, there still remains the mismatch in the overall spectral slope and band intensities. It has been suggested that the Gaspra and Ida-Dactyl encounters establish that space weathering causes this discrepancy. However, there are several inconsistencies in this simple picture which suggest that while the presence of a space weathering process on Ida and Gaspra appears well established, there still remains considerable uncertainty concerning its nature and ability to spectrally modify ordinary chondrites to match the slope and band depths of S-asteroids. Hiroi T.* Pieters C. M. Zolensky M. E. Prinz M. Reflectance Spectra (UV-3 micrometers) of Heated Ivuna (CI) Meteorite and Newly Identified Thermally Metamorphosed CM Chondrites Reflectance spectra of heated Ivuna CI meteorite and some CM2 meteorite powders have been measured for the first time. Heated Ivuna samples show the same correlation between the UV and 3-micrometer absorption strengths as the CI/CM/CR meteorites and heated Murchison samples previously studied. In addition to the previously known five unusual CI/CM meteorites, thermal metamorphism has been detected for Y-82098 and EET87522 from their rounded 3-micron band. These heated CI/CMs fall into a consistent range on the UV-3 micron absorption plot. Tuesday, March 19, 1996 MARS DEEP INTERIOR 1:30 p.m. Room C Chair(s): B. G. Bills C. M. Bertka Wanke H.* Dreibus G. The Chemical Composition of the Martian Interior With the well justified assumption that the refractory elements are present on Mars in C1 abundances observed element correlations in SNC-meteorites can be used to calculate the abundance of moderately volatile and volatile elements in the Martian mantle. The FeO/MnO ratio of SNC-meteorites yields the concentration of oxidized iron in the Martian mantle. If we further assume C1 abundance ratios for Mg, Si, and Fe (total), reliable estimates of the chemical composition of mantle and core of Mars are possible. In light of the two component model of planet formation, the high depletion of chalcophile elements in the Martian mantle indicates that on Mars the volatile element depleted reduced component A and the volatiles containing oxidized component B have been equilibrated with each other during the formation of this planet. In other words, an almost homogeneous accretion is inferred for Mars contrary to the Earth for which an inhomogeneous accretion is the generally preferred model to account for the high abundances of siderophile elements in its mantle. Bertka C. M.* Fei Y. A Profile of Martian Mantle Mineralogy and Density up to Core-Mantle Boundary Pressures IWe have experimentally determined the mineralogy of a Martian mantle composition along a model marstherm up to core-mantle boundary pressures. Experiments were performed in a multi-anvil apparatus with a starting composition similar to the Dreibus and Wanke estimate of Martian mantle composition. The marstherm chosen for study was derived from the assumption of a liquid core. The modal abundance of the minerals present in the experimental run products was estimated with mass balance calculations. A density profile of the Martian mantle was calculated from the modal abundance and the equations of state of the minerals stable at high pressure and temperature. Combined with the constraint of the mass of the planet, and an estimate of the moment of inertia factor for Mars, the mantle density profile can be used to calculate the depth to the core-mantle boundary for a given core composition. Holloway J. R.* Partial Melting of the Martian Mantle: Effect on Redox State and Magmatic Carbon The strong partitioning of ferric iron relative to ferrous iron into the melt during partial melting of spinel or garnet peridotite mantle results in transport of oxygen to the surface. The effect is enhanced in a graphite-bearing mantle. Carbon in the mantle is transported to the surface as dissolved carbonate regardless of its presence as graphite or carbonate minerals. The oxidation state of SNC's is shown to be consistent with a graphitebearing mantle. Sotin C.* Labrosse S. Mocquet A. Nusselt-Rayleigh Relationship for a Fluid Heated from Below and from Within: Application to the Cooling Rate of the Martian Core Three-dimensional numerical experiments have been carried out to investigate the role of internal heating on the convection pattern and heat transfer through a fluid layer heated from below. A simple relationship between the mean temperature, the Rayleigh number, and the internal heating rate has been inverted. A thermal boundary layer analysis provides a simple relationship between the heat flux at the surface, the mean temperature, and the Rayleigh number. This relationship is verified by the numerical runs. This same analysis is applied in 3D spherical coordinates and allows us to predict the amount of heat that can be extracted from a liquid core. Recent experimental data on the melting curves of Fe and FeS in the pressure range of interest for Mars are being used to construct models of the cooling rate of the martian core. Fei Y.* Bertka C. M. The State of the Martian Core Compositional models, based on element correlations observed in SNC meteorites and cosmochemical constraints, suggest that the Martian core is enriched in FeS. Whether the Martian core is liquid or solid is dependent on its thermal state and the melting behavior of the core materials. Accurate determination of melting temperatures in the Fe-FeS system at high pressure is required to assess the state of a sulfur-bearing iron-core. Melting in the system Fe-FeS at high pressure has been studied up to 10 GPa, far below the core pressures of Mars. The results above 5 GPa are still controversial. We have carried out high-pressure melting experiments on Fe-FeS mixtures in piston-cylinder and multi-anvil apparatus. In this paper, we present our melting data in the Fe-FeS system up to core pressures of Mars. We also report a new iron-sulfur compound (Fe3S2), synthesized above 14 GPa, which affects the melting behavior in the simple binary system. Finally, we discuss the state of the Martian core in light of our new experimental results. Bills B. G.* Geophysical Constraints on the Deep Interior of Mars: Present Status and Future Prospects An improved determination of the spin pole precession rate of Mars will contribute greatly to our understanding of the internal structure of the planet. The moment of inertia of Mars is an extremely important global geophysical parameter, whose value is only known with an accuracy of 10%. The polar moment is determined as the ratio of two parameters: the gravitational oblateness, which is known to an accuracy of 0.01%, and the spin axis precession rate, which is known with an accuracy of 10%. Thus, a better determination of the spin precession rate will directly improve knowledge of the moment of inertia, with consequent tightening of constraints on internal density structure and composition. I will review present knowledge of the precession rate, compositional implications of various possible values of the moment of inertia, and near term prospects for improving knowledge of the deep interior of Mars from geodetic observations. Tuesday, March 19, 1996 PLANETARY INTERIOR PROCESSES 3:00 p.m. Room C Chair(s): Y. Fei J. Li Righter K.* Drake M. J. Evidence for a Deep Terrestrial Magma Ocean: Prediction of Siderophile Element Metal/Silicate Partition Coefficients at P, T, fO2 and Metallic and Silicate Melt Composition Expressions of the form, lnD(M) = a lnfO2 + b/T + cP/T + d(ln(1-XS)) + e(nbo/t) + f, have been derived to predict D(M) (where D is a metal/silicate partition coefficient and M = Ni, Co, Mo, W and P) as a function of P, T, fO2, and silicate and metallic melt composition (nbo/t and XS, respectively). Earth's upper mantle abundances of Ni, Co, Mo, W and P can be reconciled with simple metal-silicate equilibrium at P-T conditions near the upper/lower mantle interface (27 GPa, 2200 K, delta IW = -0.15, X(sub)S = 0.15 and nbo/t = 2.7), in a magma ocean. Li J.* Agee C. B. Pressure Effect on Partitioning of Ni, Co, S: Implications for Mantle-Core Formation Isothermal experiments up to 20 GPa show a dramatic influence of pressure on partitioning of Ni and S between molten Fe-alloy and silicate melt. Both Ni and Co become less siderophile with pressure. Pressure affects Ni much more than Co such that their decreasing partition coefficients (D alloy melt/silicate melt) reach values of 29 and 26 respectively at an extrapolated pressure of ~28 GPa. The observed abundances of Ni and Co and their near chondritic ratio can be explained by alloy-silicate chemical equilibrium at high pressure during core extraction in a magma ocean. The partitioning behavior of sulfur is the opposite of Ni and Co--it becomes more siderophile with pressure. Sulfur's enhanced affinity for Fe-alloy with depth indicates that it is likely the dominant light element in the Earth's core. Holzheid A.* Palme H. Accretion and Early History of the Earth: Constraints Imposed by Ni and Co Abundances in the Earth's Mantle The distribution of siderophile elements between the Earth's metal core and its silicate mantle provides information on the core forming process and thus on the early history of the Earth. Here we examine the behavior of Ni and Co during core formation. The Ni/Co-ratio is useful because both elements have similar cosmochemical volatility and occur in the same oxidation state (+2) in silicate melts. A major difference is their affinity to metallic iron leading to ten times higher metal-silicate partition coefficients for Ni compared to Co at around 1400 degrees C and a pressure of one atmosphere. Recent determinations of metal/liquid-silicate partition coefficients at high pressures show that separation of metal from liquid silicates within the pressure range of the upper and probably also lower mantle cannot produce the observed chondritic Ni/Co-ratio in the Earth's mantle. This favours inhomogeneous accretion models. In addition, late Earth forming planetesimals must have been undifferentiated, differntiated objects have low Ni, Co mantles. Newsom H. E.* W/Hf Fractionation in Chondrites and the Earth: Constraints on Timing of Core Formation The recent measurements of the Hf and W isotope systematics in terrestrial and meteorite material has led Lee and Halliday to conclude that terrestrial core formation occurred at least 60 Myr after the formation of the iron meteorites. This calculation depends on assumptions regarding the Hf/W ratio for the bulk silicate Earth, and for the ratio in the chondritic material from which the iron meteorites, and the Earth formed. A new study of the abundance and depletion of W in the Earth, relative to refractory lithophile elements, such as Hf, provides constraints on the terrestrial Hf/W ratio. In the chondrites, the ratio of W to refractory lithophile elements is also variable, because W did not fully participate in the metal-silicate fractionation which occurred in the solar nebula. Therefore, the uncertainties in the Hf/W ratios in the chondrites and in the Earth must be considered to determine the constraints on the timing of core formation in the Earth. In the case of the heterogeneous accretion theory, W is accreted to the Earth's primitive mantle in an additive process, which has important implications for the significance of the Hf-W isotope systematics. Gaetani G. A.* Grove T. L. The Effect of Variable fO2/fS2 Conditions on Wetting Angles in Olivine/Sulfide Melt Aggregates: Mobility of Sulfide Melt in the Earth's Upper Mantle Core formation processes are controlled by the mobility of Fe-Ni-S-O liquids in the Earth's upper mantle. Existing experimental data are inconclusive as to whether conditions exist under which olivine/sulfide melt dihedral angles are low enough that sulfide will be mobile in mantle peridotite at small melt fractions. Here we report experiements that investigate the effect of variable fO2/fS2 conditions on olivine/sulfide melt wetting characteristics. Our experimental results demonstrate that the oxygen content of the sulfide melt represents a major control on olivine/sulfide melt interfacial energy. As the O content of the sulfide melt increases the interfacial energy decreases, leading to lower dihedral angles. At fO2/fS2 conditions relevant for the present-day upper mantle the dihedral angle is low enough (53 degrees) that sulfide melt will form an interconnected network at all melt fractions. Therefore, sulfide melt will be able to segregate efficiently from an olivine-rich matrix via porous flow. At fO2/fS2 conditions reducing enough to stablize metal, the olivine/sulfide melt dihedral angle is high (86 degrees). At these conditions a significant amount (~6 vol%) of sulfide will be trapped in isolated pockets, limiting the efficiency of percolation as a transport mechanism. Minarik W. G. Ryerson F. J. Metal-Sulfide Melt Non-Interconnectivity in Silicates, Even at High Pressure, High Temperature, and High Melt Fractions We have investigated the textural microstructure of iron-nickel-sulfur melts in contact with olivine, pyroxene, and the modified-spinel polymorph of olivine. The experiments were conducted at 1500 degrees C and pressures ranging from 1 to 17 GPa. For compositions more metal-rich than the monosulfide, including the eutectic composition, the metal sulfide melt has a dihedral angle greater than 60 degrees and does not form an interconnected grain-edge fluid. Increasing pressure does not measurably alter the dihedral angles. Textural evolution results in coarsening of the sulfide melt pockets, resulting in large pockets surrounded by many silicate grains and separated from one another by melt-free grain edges. Chemical communication between these large pockets is limited to lattice and grain-boundary diffusion. Due to the large interfacial energy between sulfide melt and silicates, sulfide melts are unable to separate from solid silicate via grain-boundary percolation and remain stranded in isolated melt pockets. Sulfide melt in excess of the critical melt fraction (5-25%) will develop a transient interconnectivity as sulfide collects into larger melt pockets and interconnectivity is pinched off. Efficient separation of core-forming sulfide melts from silicate requires either melting of the silicate matrix or a very large fraction of metal-sulfide melt (perhaps as large as 40%). Jones J. H.* Walker D. Partitioning of Xe Among Phases Relevant to the Upper Mantles of the Earth and Mars Previous studies have suggested that, compared to the other noble gases, Xe may be quite compatible in solid silicate phases during partial melting in the Earth's upper mantle. Conceivably, if Xe is compatible in some major silicate phase, such as olivine, the observed Xe depletion of the Earth's atmosphere could be simply explained by retention of Xe in the mantle. Consequently, we have attempted to partition Xe between olivine, pyroxene, spinel, graphite, and silicate liquid at igneous temperatures and pressures. We find that Xe is highly incompatible in olivine, pyroxene, and spinel, but that it is slightly compatible in graphite. We suggest that the carbon-rich phases observed in mantle xenoliths are the most likely hosts for Xe in the mantle. Alternatively, graphite in crustal rocks such as shales or other pelites could also serve as a host for Xe. Whether graphite in crustal materials could also explain the depletion of Xe in the martian atmosphere is problematical. Tuesday, March 19, 1996 METAL-RICH METEORITES 1:30 p.m. Room D Chair(s): R. J. Walker J. Zipfel Chabot N. L.* Drake M. J. Magmatic Iron Meteorite Petrogenesis: Partitioning of Ag and Pd Between Solid and Liquid Metal Stimulated by new analytical data for the parent-daughter isotope system 107Pd/107Ag which appear to be inconsistent with the few laboratory determinations of partitioning behavior, we have carried out an experimental study of the partitioning of Ag and Pd between solid Fe-metal and S-bearing metallic liquid. Silver is incompatible in solid metal and partition coefficients decrease with increasing S-content in the metallic liquid. Palladium changes from modestly incompatible to modestly compatible in solid metal with increasing S-content in the metallic liquid. These new data are used to investigate Ag and Pd systematics in magmatic iron meteorites. Chen J. H.* Wasserburg G. J. Live 107Pd in Some Group II and III Irons and the Time-Scales of Fe-Ni Segregations in the Early Solar System In a recent report, we presented evidence for excesses (*) of ^107Ag* due to ^107Pd decay (tau bar = 6.5 Ma) in a wide variety of iron meteorites, some pallasites and mesosiderites. They provide unambiguous evidence of the in situ decay of ^107Pd in planetary differentiates. A key problem of early solar system chronologies has been the ability to interrelate different short-lived and long-lived chronometers. Evidence for the presence of both live ^107Pd and ^53Mn (tau bar = 5.3 Ma) in the early solar system has been found in several meteorites. However, there appear to be major discrepancies between the ^107Pd and ^53Mn chronometers. New Re-Os data on iron meteorites seem to support the small time differences for formation of iron meteorites as inferred by the Pd-Ag system. In this study, we selected samples on which Re-Os data were available. These include Coahuila (IIA) metal and Tres Castillos (IIIA) metal and sulfide. Smoliar M. I.* Walker R. J. Morgan J. W. Re-Os Isotope Systematics in the Cape York Meteorite Shower Here we report results of Re-Os isotope analyses of a suite of Cape York individuals (Agpalilik, Savik I, Thule, and Woman) along with other IIIAB members close in chemical composition to Cape York (Casas Grandes, Loreto, and Trenton). The IIIAB parent core had complicated crystallization history which cannot be described solely by fractional crystallization (e.g., scatter in Ni-Ir data along theoretical crystallization trends is much wider than analytical uncertainties). Meteorites from the Cape York shower demonstrate anomalous behavior for many trace elements, probably reflecting other major process. The high iridium part of the group IIIA irons (presumably representing the central part of the parent IIIAB core) was precisely dated by Re-Os method in this lab. Consequently, the aim of the present study is to deceipher the chronology of the Cape York irons relative to the IIIA isochron and to obtain precise Re-Os composition data in order to place additional constraints on conceivable models on the history of the IIIAB parental core. Papanastassiou D. A.* Wasserburg G. J. Shen J. J. Re-Os Diffusion into Massive Schreibersite and Possible Internal Isochrons for Iron Meteorites For age determinations it is essential to obtain a reasonable range of parent-daughter physical-chemical fractionation between whole rock samples for the determination of whole rock isochrons, and between constituent phases for the possible determination of internal isochrons. For the application of Re-Os to iron meteorite evolution, the stages of evolution which can, in principle, result in Re-Os fractionation include: (a) condensation of PGE and of FeNi in the solar nebula; (b) subsequent oxidation of part of the Fe by reaction with S; (c) segregation of massive FeS during melting on parent planetesimals; (d) melting and fractional crystallization of the metal phase; (e) element redistribution during subsolidus phase transformations, including the precipitation and exsolution of schreibersite. Shen et al. have reported on analytical techniques which yield replicate analyses of Re-Os which are in good agreement, within ñ2.5 per mil. For whole samples of iron meteorites the results show a well defined correlation line on a ^187Re-^187Os evolution diagram for iron meteorites from groups IA, IIA, IIIA, IVA, and IVB, taken together. This correlation line yields a slope of 0.07848 ñ 0.00018 (2 sigma) and initial ^187Os/^188Os = 0.09563 0.00011 (2 sigma). The corresponding age is 4.61 ñ 0.01 AE using lambda (^187Re = 1.64 x 10^-11 a^-1). Re-Os data on sulphides from two iron meteorites (Group IA) show that the sulphides are extremely depleted in Re and Os. These workers concluded that sulphide formation and segregation, in the presence of FeNi, does not affect the Re-Os system (except as a diluent) and that sulphide is not useful for the possible determination of internal isochrons. Yin Q.* Jagoutz E. Palme H. Wanke H. NUR--A Possible Proxy for CHUR Reference for the Re-Os System Derived from 187Os/188Os Ratio of the Allende CAI The knowledge of the evolution of 187 Os/188 Os and 187 Re/188 Os ratios in various reservoirs, such as bulk solar system (BSS), bulk earth (BE) and the bulk silicate earth (BSE) is important for the development of the Re-Os systematics as a geochemical tool. The Re/Os ratios in carbonaceous chondrites are unfortunately not constant and cannot be used to uniquely define the average solar system 187 Os/188 Os evolution similar to the chondritic 143 Nd/144 Nd evolution (CHUR). The Re/Os ratios in Ca, Al-rich inclusions (CAIs) of CV3 carbonaceous chondrites appear to be unfractionated both in macro- and micro-scale. Considering that the CAIs are the oldest materials found in the solar system, and Re and Os are among the first elements to condense from a gas of solar composition, the 187 Os/188 Os and 187 Re/188 Os ratios of CAIs in CV meteorites may be more representative for BSS evolution than individual meteorites. The Os isotopic composition of type B CAI from the Allende meteorite (A44) is shown to be very close to previous estimates for the chondritic reservoir by other methods. Since the noble metal budget in CAIs is controlled by tiny refractory metal Nuggets, we use the acronym NUR (a German equivalent for ONLY) for the Uniform Reservoir for the Re-Os system to characterize the average solar system evolution of 187 Os/188 Os. The proposed values for NUR are (187 Os/188 Os)NUR=0.12736 +/- 0.00016 (corresponding to 187 Os/186 Os=1.0622 +/- 0.0013) and (187 Re/188 Os)NUR =0.3952 +/- 0.0055 respectively. Zipfel J.* Mathew K. J. Marti K. Nitrogen Isotopic Composition of Metal and Graphite Separates from the El Taco (IAB) Iron Meteorite Nitrogen isotopic compositions of iron meteorites were studied by several authors to address the question of the origin of iron meteorites and their genetic relationships. It was concluded that parent body processes have only a slight effect on the primary signatures. All these results are only based on the N composition of the matrix metal. No systematic study has been performed to determine effects of parent body processes on the N isotopes in the presence of silicate inclusions. Nitrogen signatures, reflecting isotopic disequilibrium, were previously observed in Acapulco. We report first results of a detailed study of the N isotopic composition in silicate and metal phases of the IAB iron El Taco. Metal and graphite separates were analyzed by stepwise pyrolysis followed by several combustion steps using a static mass spectrometer. The new data reveal a large scale disequilibrium among the investigated phases. Benedix G. K.* Love S. G. Scott E. R. D. Keil K. Taylor G. J. McCoy T. J. Catastrophic Impact During Differentiation of the IAB-Winonaite Parent Asteroid IAB iron meteorites contain angular, unmelted, chondritic rock fragments resembling winonaites; they may be derived from a single body. Two models have been proposed to account for the silicate inclusions and the compositional range of the metal. Kracher proposed that fractional crystallization of a S-saturated core could produce the observed siderophile element trends. Wasson and coworkers suggested that these meteorites could have formed in individual impact melt pools in the megaregolith on a common parent body. Kracher's model does not adequately explain mixing of molten metal with unmelted silicates. The impact model explains the brecciation and mixing, but cannot account satisfactorily for lithologies formed by partial melting. We are exploring break-up and reassembly of a hot, partially differentiated body as an alternative model that combines the best features of the other models. Prior to the impact, sulfur-rich molten metal began to drain to the center of the body. Silicates experienced some melting as well, producing basaltic melts and olivine-rich residues. Before core formation was complete, we suggest that an impact disrupted the body, mixing silicates into molten metal (represented by IAB irons with silicate inclusions) and olivine-rich residues into unmelted silicate rock (winonaites). Preliminary results of computer modelling indicate that core material can be scrambled and mixed with silicate material from the overlying mantle of the body in a break-up and reassembly scenario. Bashir N.* Beckett J. R. Hutcheon I. D. Stolper E. M. Carbon in the Metal of Iron Meteorites Magmatic iron meteorites (e.g., group IIIAB) are believed to represent the slowly cooled cores of asteroidal-sized bodies. Concentration gradients of Ni in taenite (gamma-alloy) adjacent to kamacite (alpha-alloy) have been studied extensively following the early work of Wood and Goldstein and Ogilvie, and can be used to constrain the rate at which the meteorite cooled through -700 degrees-400 degrees C. Studies on non-anomalous irons have also shown zoning profiles in C, Co, Cu, Zn, Ga, and Ge. Zoning patterns of elements other than Ni can provide independent constraints on the thermal histories of iron meteorites at lower temperatures, provided as in the case of C, N and P, that the elements diffuse significantly faster than Ni. We report here ion microprobe measurements of the distribution Choi B.-G.* Wasson J. T. Pallasites--Chemical Compositions and Relationship to IIIAB Irons Because of their similarities in chemical (siderophile elements in metal) and oxygen isotope compositions, it has been suggested that the IIIAB-irons and main group (MG) pallasites formed the core and core-mantle interface in a single asteroid. Mantle olivine appears to have been crushed and invaded by a IIIAB-like magma. We review the petrological and chemical characteristics of pallasites and IIIAB irons and report preliminary result of our instrumental neutron activation analysis (INAA) data for pallasites and related meteorites. As inferred by Scott, metal compositions of MG pallasites can be largely modeled as the residual liquid present during crystallization of IIIAB irons. However, several volatile elements are higher than expected in the IIIAB liquid, suggesting transport through a coexisting gas phase. Our INAA data of metal from Brahin, Vermilion, and Yamato 8451, which have similar oxygen isotope compositions, shows them to be mutually unrelated. Haack H.* Rasmussen K. L. The Origin of Group IIF Iron Meteorites-Clues from Metallographic Cooling Rates With only five members, group IIF is one of the smallest groups of iron meteorites. Their high Ni concentrations give them an ataxitic texture which inhibits determination of their metallographic cooling rates using traditional techniques. Cooling rates based on alpha-bandwidths show a wide range suggesting that these irons did not cool in a common core. We find that the previously determined alpha-bandwidth cooling rates are in error due to the presence of massive schreibersite blebs in the alpha-spindles on which the cooling rates are based. We have therefore redetermined the metallographic cooling rates of the five group IIF iron meteorites based on alpha-bandwidths of alpha-lamellae devoid of schreibersite. We have determined metallographic cooling rates based on three sets of observations: (1) Ni profiles in the taenite rims surrounding alpha-lamellae, (2) Ni profiles in the outer edge of alpha-lamellae, and (3) bandwidths of alpha-lamellae. We find cooling rates of approximately 10 K/My for all five members of group IIF. Low overall Ni concentrations observed in the alpha-lamellae may, however, indicate a lower cooling rate at low temperatures. The metallographic cooling rates suggest that the IIF iron meteorites cooled slowly in a central core. Yugami K.* Takeda H. Kojima H. Miyamoto M. Distribution of Opaque Minerals in Primitive Achondrites We studied distribution of opaque phases (FeNi metal and troilite) in three "winonaites", Yamato (Y) 74025, Y75305 and Y8005 and compared them with a lodranite-like acapulcoite Elephant Moraine (EET) 84302. The thin sections of Y74025, Y75305 and Y8005 include very large opaque grains. The difference between the distribution patterns of opaque minerals in these four meteorites may reflect the different degrees of aggregation of opaque minerals. The heterogeneous distribution of materials in each thin section and between thin sections of each meteorite indicates that the heterogeneity exists in rather small scale in the parent bodies of primitive achondrites. It supports the idea that the local heterogeneity of materials in the parent body can explain the depletion of low-temperature melting materials from lodranites. _ Bogard D. D.* Garrison D. H. 39Ar-40Ar Ages of Acapulcoites and Early Parent Body Metamorphism Acapulcoite Chronology: We determined precise 39Ar-40Ar ages for five acapulcoites, ALHA81261, ALHA81187, EET84302, Monument Draw, and Acapulco (Fig. 1). These ages are 4.511 +/-0.007, 4.507 +/-0.024, 4.512 +/-0.017, 4.517 +/-0.006, and 4.510 +/-0.011 Ga, respectively. Each release, with only modest Ar loss at lower meteorite shows a constant age over a large fraction of its 39Ar temperatures. All five meteorites are consistent with a common 39Ar-40Ar age of 4.51 +/-0.01 Ga. When the age uncertainty (<0.5%) of the NL-25 hornblende age monitor is considered, the absolute K-Ar age of these five acapulcoites is 4.51 +/-0.02 Ga. Marvin U. B.* Petaev M. I. Kempton R. Preliminary Observations on Drusy Vugs in the Albion Iron Meteorite A 22.28-kg mass of iron, found in Albion, Washington, in the winter of 1966-1967 remained unknown until 1991 when the finder brought it to Carleton B. Moore at Arizona State University. Moore identified it as a meteorite and sent a sample to John A. Wasson at UCLA who classified it as a IVA fine octahedrite. This iron is unique in having vugs, ranging in length from 4 to 9 mm, scattered throughout the otherwise orderly Widmanstatten structure. Drusy spheroids cover the walls and partially fill some of the vugs. We have analyzed the constituents of one vug (Vug-A) and the metals surrounding it. The drusy spheroids consist mainly of small, irregular grains of kamacite containing 2-3.5 wt.% Ni and several rounded masses of tetrataenite (55.6 wt.% Ni). Both metals are enmeshed in troilite which appears to have invaded and corroded them. Daubreelite (FeCr2S4) in euhedral grains is scattered through the spheroids and elsewhere in the iron. It formed earlier than the troilite which contains no chromium and shows no sign of reaction where the two are in contact. A phosphide mineral in euhedral to subhedral grains, 3-15 um across, also occurs in the spheroids. Our analyses of sixteen grains yielded a consistent composition of [Ni 0.54 (Fe 0.46)3.7 P], which we are examining as a new species. The spheroids are rimmed with kamacite and show clear evidence of having grown in stages during the passage of a fluid or vapor phase. We regard the vugs as primary features inherited from bubbles in the cooling melt that remained open during formation of the Widmanstatten structures. Previous investigators have attributed masses of troilite-kamacite intergrowths to shock-melting of troilite nodules. However, we find it difficult to envision shock processes opening cavities, lining them with drusy minerals, and giving rise to a new phosphide without damaging the metal in which we find no evidence of shock effects. We are investigating further our hypothesis that the unique vugs in the Albion iron are of primary origin. Tuesday, March 19, 1996 EDUCATION SPECIAL SESSION DISPLAYS PLANETARY SCIENCE EDUCATIONAL ACTIVITIES AND TECHNOLOGY 6:30 - 9:30 p.m. LPI Dasch J. Ward E. The National Space Grant College and Fellowship Program: The First Five Years 1989-1994 NASA accepted the Congressional mandate to manage the National Space Grant College and Fellowship Program (NSGC&FP) and announced the original competition for Space Grant awards in 1989. Currently, 52 independently governed Space Grant consortia administer programs in three areas of university and NASA concern: research, education, and public service. NASA funds have remained stable at $15 million since 1991, but the consortia have leveraged matching funds, including in-kind contributions, to approximately double the Space Grant awards. The number of affiliated Space Grant institutions has grown from 86 affiliates in 1990 to over 550 affiliates in 1996. Members include many of the finest colleges and universities in the nation, internationally competitive business and industrial partners, small businesses, state and local government agencies, private colleges, community colleges, medical colleges, and other nonprofit organizations. Though the program has been fully operational in all 52 consortia for less than five years, the benefits to the taxpayer have been significant. Space Grant faculty and students obtained funding for over 300 research proposals. Since 1990, the NSGC&FP has served close to 6500 citizens by providing tuition assistance. Among these 6500 were significant numbers of students from underrepresented groups in science and engineering. Space Grant consortia leveraged $16.5 million for precollege activities and administered over 1300 precollege projects that promote NASA-related science education. Space Grant precollege programs, serving both teachers and students, provide good examples of higher education faculty working well with local school systems. The consortia administered over 600 public service programs and leveraged approximately $4.5 million. Space Grant funds provided science and technology lectures, demonstrations, science exhibits, space-related periodicals, audio and video productions of NASA-related subjects, and have helped to positively affect science literacy and NASA awareness in over one million people. Additional information about the NSGC&FP can be obtained through the Education Programs link on NASA's World Wide Web site. The NASA web address is: http//www.nasa.gov Gruener J. E. French G. D. Connell K. M. Education and Outreach Components of the Lunar Prospector Discovery Mission The Lunar Prospector mission was selected as a NASA Discovery Mission in February of 1995. The objective of the mission is to map the Moon's surface composition, magnetic fields, gravity fields and gas release events to improve the understanding of the origin, evolution, current state, and available resources of the Moon. The mission will be accomplished using a small (285 kg wet), relatively simple, spin-stabilized spacecraft in a polar orbit 100 km above the lunar surface. The spacecraft and science instruments are currently being fabricated, with launch scheduled for October 1997. Mission duration is planned to be a minimum of one year. Associated with the mission is a privately financed effort, MoonLinkTM, to involve schools and the general public in activities prior to, during, and after the actual mission. Students and interested individuals or groups in all parts of the world will get to actively interact, via the Internet, with mission controllers and scientists conducting spacecraft operations and data acquisition. Throughout the entire mission, participants will also have the opportunity to investigate uniquely one 150 km by 150 km geographic area on the lunar surface. This will help provide "ownership" in the knowledge gained about the Moon. Betts B. H. Kenealy R. Nash D. B. Successful Components of Planetary Science Field Trips Over 12,000 students in the 3rd to 8th grades have come to the San Juan Institute (SJI) over the last four years to attend SJI's field trip programs, including Journey Through the Solar System, a two hour introduction to the Solar System. We have evolved the program based upon teacher, student, and educator feedback, and have found several successful components. Some of these are relatively obvious, while others may not be. Here we wish to pass those lessons along to others who may try similar programs. The components we have identified as successful include: having a two-pronged philosophy of what is to be accomplished which consists of (1) trying to excite students about science, and helping them get over science anxiety by seeing science as fun, and (2) passing along basic planetary science and physical science information in the process; utilizing table top demonstrations; providing the students with workbooks to use during the presentation; providing hands-on activities including meteorites as ways to "touch space" (SJI has also recently put on display a local meteorite); having a drawing contest; utilizing visual aids including pictures and video; having lectures with coherent themes; having a question and answer session; giving students a tour through a working science lab facility; obtaining formal evaluations and informal feedback from teachers and students, and revising the program in response; and providing students with current planetary and space events and sky information updates. Grady M. M. Hutchison R. The Meteorite Teaching Package Programme of the Natural History Museum The Natural History Museum has prepared four packages of meteorites, complete with teaching notes, available for loan at no cost to schools and colleges in the UK. The packages are aimed at different student ages (junior, senior and college) and contain a combination of hand specimens, mounts and thin sections. The loan programme is administered by the Particle Physics and Astronomy Research Council (PPARC). Saunders R. S. Limaye S. S. Krauss R. Scientific Analysis and Display of Planetary Data with McIDAS-eXplorer The Planetary Data System (PDS) has now published extensive Mars Galileo missions as well as for the outer planets and their satellites from the Voyager missions. The compilation of radar observations of the Venus surface from the Magellan mission data has resulted in over 70 gigabytes of radar reflectivity (MIDRs and FMAPs) and radiometry images (GxDR's) and altimetry profiles (ARCDRs) covering over 98% of the surface of Venus. To facilitate access, analysis and display of these data, a software environment, McIDAS-eXplorer is now available. This planetary image access, display and analysis environment is based on a mature system that is used for weather operations, research and education at many sites around the world. The eXplorer extensions now allow analysis of most solar system targets for which spacecraft and ground based telescopic data are available in a recognizable format such as the PDS or FITS. Navigation, registration and calibration of the planetary data are an integral part of the environment. Designed to run on most UNIX workstations supporting X-windows, the environment is user extensible allowing addition of user developed applications and includes both a Graphical User Interface and a command line interface, multi-frame display and animation capability and tools for most image analysis applications such as digital enhancements, filters, cartographic projections, graphical overlays, and color composites and image classification. Lindstrom M. M. Allen J. S. Allen C. C. Eskridge G. Tobola K. Stocco K. Exploring the Solar System Teacher Workshop: Lessons Learned and Teacher Plans In June 1995 JSC planetary scientists conducted a three week teacher workshop on "Exploring the Solar System (Ex SS)" as part of the NASA Science Teacher Enhancement Program (STEP). The scientists were assisted by the JSC education branch and two lead teachers who had previously helped develop the meteorite educational activities. Additional support was provided by the Lunar and Planetary Institute, Space Center Houston, and the University of Houston, Clear Lake. Funding for the workshop which included teacher stipends and tuition for 3 hours college credit at UHCL came primarily from the STEP at NASA headquarters education office, with limited support for exploration activities from code SX. Bender K. C. Greeley R. Pappalardo R. Activities in Planetary Geology In 1982, NASA released an educational publication entitled _Activities in Planetary Geology for the Physical and Earth Sciences_ (EP-179). Prepared before Voyager data for the outer solar system were widely available, and before the Magellan and Galileo missions were even launched, this document is substantially out of date and out of print. At the request of NASA, this document has been revised, updated and expanded. The new manual is divided into 5 sections: a) geologic processes, b) impact cratering, c) planetary atmospheres, d) planetary surfaces, and e) geologic mapping. Each section includes an introduction to the topic and three or more activities. Individual activities range in grade from 4 to college, suggestions for grade appropriate modifications are included to allow for wider application. Answer keys and suggestions for conducting the activities are included for the instructor's use. NASA expects to publish this new manual by mid-1996. Tuesday, March 19, 1996 LUNAR MARIA POSTERS 6:30 - 9:30 p.m. LPI Wagner R. J. Head III J. W. Wolf U. Neukum G. Age Relations of Geologic Units in the Gruithuisen Region of the Moon Based on Crater Size-Frequency Measurements Data Base. All measurements of crater size-frequency distributions were carried out on Lunar Orbiter high resolution images from orbits LOV 183, 184 and 185 and on image LOV 182 M with medium resolution. The images were photographically processed and used as transparencies in a size of 25 by 25 cm. Photogeologic units on these images were identified using albedo and morphology as primary criteria. Chevrel S. D. Pinet P. C. Head J. W. Gruithuisen Domes Region: A Candidate for a Large Extended Lunar Nonmare Volcanism Unit While the lunar mare volcanism appears to be relatively well documented, little is known about the emplacement of lunar nonmare volcanic geologic units before and during the period of mare volcanism. As it is for the mare volcanism, the existence of nonmare volcanism has equally major implications for the thermal history and crustal evolution of the Moon. Small lunar areas known as red spots may represent good candidates for such nonmare volcanic geologic units predating or contemporaneous of the period of mare volcanism. These anomalous areas are primarily characterized by a relatively high albedo and a strong absorption in the UV relative to the visible. They are located in a variety of geological settings, exhibit a wide range of morphologies (domes, smooth plains units, rugged highland patches), and show some spectral variations interpreted as differences in composition. Many red spots are embayed by younger mare deposits, suggesting that they have originally covered a greater proportion of the lunar surface. Hiesinger H. Oberst J. Jaumann R. Neukum G. Head J. W. III Observations of Mare Humboldtianum: Topography, Ages and Multispectral Characteristics Mare Humboldtianum, a 650 km wide multi-ring impact basin on the northeastern part of the nearside of the Moon (59 degrees N, 82 degrees E) is hardly visible from Earth. This Nectarian-aged basin has been observed by the Galileo spacecraft in 1992 and the Clementine spacecraft in 1994. We combined multispectral imaging data with stereo data in order to study the basin structure and the basalt extrusions in this lunar area. Additionally we performed crater counts to date the surface age of the lava deposits. Wahlisch M. Zhang W. Roatsch T. Oberst J. Cook A. C. Hiesinger H. Jaumann R. Digital Terrain Models and Rectified Color Ratio Mosaics in the Mare Orientale Region Derived from Clementine Image Data We used Clementine UVVIS images to derive a Digital Terrain Model (DTM) and a rectified color ratio image mosaic along a narrow strip crosscutting the Northern Mare Orientale basin. Combining topography with precisely registered color filter images allow us to study the structure, mineralogy and chemistry of this multi-ringed impact basin in unprecedented detail. Bussey D. B. J. Spudis P. D. A Compositional Study of Lunar Sinuous Rilles The Clementine mission to the moon has provided, for the first time, a global multi-spectral data set of the lunar surface, mapping the moon at eleven different wavelengths at a resolution of approximately 100 m pixel^(-1). By making ratioed images of different wavelengths, it is possible to infer compositional information. We are making color-composite images of all the major lunar rilles to study the compositional relations associated with rilles to try and better understand their formation. So far approximately twelve rilles have been studied, including Rimae Hadley, Sharp, Herigonius and Prinz. Preliminary findings indicate that it is possible to obtain information on sub-rille composition using the Clementine data. Robinson M. S. Shoemaker E. M. Hawke B. R. Spectral Heterogeneity of Lunar Local Dark Mantle Deposits We are currently investigating the spectral properties of lunar dark mantle deposits (DMD) utilizing Clementine UVVIS and HIRES image data (25-150 m/pixel). Local dark mantle deposits (LDMD) are identified by their very low albedo, smooth texture, and small size relative to the regional DMDs. DMDs are mostly composed of glass spheres and LDMD are thought to be mixtures of olivine, pyroxene, host rock and glasses formed during maar-type eruptions. We find that maar deposits in Alphonsus crater, Atlas crater, and the Schrdinger basin exhibit interdeposit spectral variations. Some of the maars in the Alphonsus region show intradeposit spectral variations and may be the result of maturity differences, admixture of surrounding material, crystallinity variation and/or true mineralogical differences. Li L. Mustard J. F. He G. Mixing Across a Simple Mare-Highland Contact in the Grimaldi Basin: New Insights from Clementine UV/VIS Data The investigation of the composition of mare-highland boundaries carried out by Mustard et al. using multispectral images from the Galileo Solid State Imaging (SSI) instrument reveals the existence of three distinct mixing systematics across the mare-highland contacts in the region of southwestern Procellarum. The three basic types are narrow, moderate, and complex mixing gradients, and each implies a different set of fundamental processes that have contributed to the observed gradients. However, the 4 km resolution of the Galileo SSI data is too low to critically evaluate the exact properties of these boundaries, particularly in areas with rapidly changing abundances. The higher spatial resolution of Clementine UV/VIS data ( ~200 m/pixel, 5 filters between 0.415- 1.0 ammicrometers) allows the contact of mare-highland to be addressed in more detail. We have begun a series of studies to characterize and model mixing across mare-highland boundaries using these data, beginning with simple boundaries (sharp geologic contact, simple superposition of mare on highland). In this study, the contact between the Grimaldi mare and the highland on the southern edge is investigated through the spectral mixture analysis of Clementine UV/VIS data. Our preliminary analyses reveals the boundary consists of three mixing zones: moderate, steep, and moderate. The moderate zones on the mare and highland sides of the contact are approximately 30km wide, while the steep zone is ~6-8 km wide. We are currently examining other such simple boundaries to determine if the physical dimensions and properties are consistent across the moon, and thus a chracteristic properties of simple boundaries. ~ T~ ~ FT ~. I '1 . ~ . 171 ~.~_ ~ Antonenko I. Head J. W. Cintala M. Horz F. Experimental Studies of Lunar Dark Halo Craters: Implications for Cryptomare Thickness Measurements The use of DHCs in studies of cryptomare has necessitated a refinement in our understanding of their formation. To this end, a series of impact experiments into layered targets of sand was undertaken in order to determine the depth of mare penetration required to form a dark halo (d(sub)m). Preliminary analysis of experimental results indicates that dm may equal 3-10% of the crater depth. Caution must be used in applying this value, since experimental conditions do not exactly simulate the spectral characteristics of the Moon. Williams K. K. Zuber M. T. Re-evaluation of Mare Thicknesses Bases on Lunar Crater Depth-Diameter Relationships With the wealth of data from the Clementine mission, it is now possible to investigate the compensation states and sub-surface structure of the lunar basins. Unfortunately, considerations of the relationship between the lunar gravity and topography for mare basins are hindered by the lack of understanding of mare thicknesses. By re-evaluating the depth-diameter relationship for large lunar basins using the Clementine topography data set, it has been found that the depths of the lunar basins measured do not exceed 4.9 km. Subtracting the depth from the rim-crest to the mare surface from the maximum depth found for non-mare basins, mare thicknesses have been calculated for the lunar mare basins. These new thicknesses can now be used in investigations of the sub-surface structure of lunar basins and their compensation states. Budney C. J. Lucey P. G. Basalt Thickness in Mare Humorum: New Method and Results Basalt thicknesses in mare basins have been determined using assumptions about the pre-mare topography of partly buried craters. Differences in those assumptions have led to a factor of two difference in mare thickness estimates. Further, knowledge of thickness is restricted to areas in which buried craters are present. We have shown that craters in the mare sometimes excavate highland material from below the mare cover. Using such craters, and assumptions about their depth of excavation, we can obtain independent estimates of basalt thickness. This method allows testing of pre-mare crater topography models employed in the buried crater method, adds significantly more data points for development of isopach maps, and allows determining of thickness of stratigraphic layers other than mare basalts such as melt sheets where compositional contrast is present. Staid M. I. Pieters C. M. Craters as Indicators of Compositional Stratigraphy in Mare Tranquillitatis and Serenitatis Spectrally distinct mare deposits within the Tranquillitatis and Serenitatis basins exhibit a complex sequence of basalt emplacement representing a detailed record of lunar volcanism on the eastern limb. High spatial resolution ultraviolet-visible (UVVIS) Clementine imagery has been used to investigate the spectral properties of mare craters whose deposits suggest the excavation of compositionally distinct subsurface units. Previous interpretations of such spectrally anomalous crater materials have been hampered by the lack of diagnostic information to distinguish immature basalt regoliths. Clementine data and coordinated sample analyses are combined to investigate the stratigraphic implications of such crater materials. Results provide insight into the sequence of basalt emplacement within the two adjacent basins as well as initial investigations into the spectral properties of freshly exposed mare materials. Snyder G. A. Taylor L. A. Apollo 14 High-Al Basalts May Not Represent Earliest Volcanism on the Moon: The Enigma of Disturbed Sm-Nd and Undisturbed Rb-Sr Isotopic Systems Conventional wisdom is that high-Al mare basalts from the Apollo 14 landing site are the oldest known basalts from the Moon (as old as 4.3 Ga). As such, they represent the earliest melting of the lunar deep interior after solidification of the Moon-wide magma ocean. The samples analyzed previously have yielded Rb-Sr ages from 3.9 to 4.3 Ga and would indicate that they were coeval with KREEP basalt volcanism. Although the Rb-Sr systematics of these rocks yield well-defined ages, the Sm-Nd systematics of all but one of the samples indicate disturbance. These observations are in direct contrast to other lunar and terrestrial rocks which often show disturbance of the Rb-Sr systems, but yield well-defined Sm-Nd ages. It is our contention that the Rb-Sr ages may not be true crystallization ages, but may be indicating complete re-setting by an impact or other heating event. This heating event also could have involved mixing with highlands rocks. The disturbed Sm-Nd isotopic systems in these rocks are the indicators of this heating event which only partially reset the Sm-Nd "ages." Therefore, high-Al basalts may not represent the earliest volcanism on the Moon. Beard B. L. Taylor L. A. Scherer E. E. Johnson C. M. Snyder G. A. The Source Mineralogy of High- and Low-Ti Basalts Based on Their Hf Isotopic Composition The Lu-Hf isotopic system is ideally suited for determining the mineralogy of source regions because of the large mineral dependent Lu-Hf fractionations that occur during partial melting and crystallization. However, because of the analytical challenges of Hf isotopic analysis the Lu-Hf isotope system has not been widely applied to studies of petrogenesis. We have developed a new technique for Hf isotopic analysis and Lu and Hf concentration determination by isotope dilution mass-spectrometry that is ideally suited for Lu-Hf isotopic analysis of high-Ti basaltic rocks. Using this technique we have analyzed 5 high-Ti mare basalts; 2 Apollo 11 samples with high epsilon (sub)Nd values and 3 Apollo 17 samples with low epsilon (sub)Nd values. These new data complement the existing Hf isotopic analyses of high-Ti basalts, which consist of 4 Apollo 11 samples with low epsilon (sub)Nd values and 3 Apollo 17 samples with high epsilon (sub)Nd values. This combined data set clearly demonstrates that the Hf isotopic composition of high-Ti basalts is relatively constant over a range of epsilon (sub)Nd values. The Hf - Nd isotopic array of high-Ti basalts is in marked contrast to the Hf - Nd isotopic data of low-Ti basalts which defines a strong positive correlation with epsilon (sub)Hf = 4epislon (sub)Nd. The differences for the Hf and Nd isotopic arrays of low- and high-Ti basalts implies that the low- and high-Ti basalt source regions have been distinct over nearly the entire history of the Moon. Moreover, these source regions must have formed with distinctly different mineralogies. Shearer C. K. Papike J. J. Major and Trace Element Modeling of a Polybaric Melting Origin for Lunar (Apollo 15) Very Low Ti Picritic Magmas The high Mg# of lunar picritic glasses relative to mare basalts, as well as other attributes (high Ni, low Al, contrasting incompatible element abundances, higher multiple saturation pressures), have been interpreted as indicating that these glasses represent extremely primitive lunar magmas that approach primary magma compositions. Although these glasses have been utilized in numerous lunar studies, we still do not truly understand many aspects of these volcanic glass. A case in point is the A15 very low-Ti picritic glasses which have been used extensively in modeling many aspects of the Moon. These glasses show a range of contrasting chemical characteristics. Weitz C. M. Rutherford M. J. Head J. W. III Oxidation States During Ascent and Eruption of the 74001/2 Orange Glass Magma While 99% of the 74001/2 lunar core section is composed of orange glasses and their crystallized equivalents (black beads), some olivine phenocrysts and metallic blebs have also been identified. Searching through the 26 thin sections that represent the 68-cm deep core, we have identified 29 metallic blebs larger than 4 micrometers in size: 14 are enclosed in or trapped between olivine phenocrysts, 14 are enclosed in volcanic beads, and one metallic fragment was found by itself. Most metal blebs are spherical; the largest is 44 micrometers in diameter but most of the blebs in the volcanic beads are <10 micrometers. Microprobe analyses show that the metallic blebs can be divided into three major classes: (1) Fe-Ni-Co spheres enclosed inside or trapped between olivine crystals; (2) Fe-Ni-Co metal blebs enclosed inside volcanic beads; and (3) smaller (<5 micrometers) and very irregular shaped Fe metallic blebs in the crystallized black beads. Each of these three types of metal blebs represents a different oxidation and/or temperature history in the conduit or during eruption in the fire fountain. In this study, we have used the compositions of the blebs to determine the oxidation states when the blebs initially formed and how the conditions changed during ascent and eruption of the magma. Our goal is to apply these results in order to model the ascent and eruption of the volcanic beads that compose the regional dark mantle deposits. Togashi S. Distribution Coefficients for La, Sm, Sc and Co Between Plagioclase and Low La-Magma The concentrations of Na, La, Ce, Sm, Eu, Fe, Co and Sc in a few tens of mg samples of plagioclases and their host lavas were determined with INAA for low-La and low-alkaline tholeiites from Japan. The data suggest that Henry's law holds in the case of plagioclase down to 0.1 ppm level concentrations. The variations of the distribution coefficients (D's) for La, Ce, Sm, Co and Sc are small compared with the varieties of the host magmas, in which concentrations vary by 2 orders of magnitude. The D of Sc is minimum and is available as a criteria of the maximum contamination of the magmatic component with plagioclase. Plagioclase records quantitatively the composition of the host magma not only for incompatible elements (LREEs), but also for compatible elements (Co) during magmatic processes. These records reveal the evolution of host magmas and their origin, and are applicable to the lunar anorthosites. Tuesday, March 19, 1996 LUNAR REGOLITH POSTERS 6:30 - 9:30 p.m. LPI Hawke B. R. Blewett D. T. Bell J. F. III Lucey P. G. Campbell B. A. Robinson M. S. Remote Sensing Studies of Lunar Crater Rays The nature and origin of lunar crater rays has long been the source of major controversy. Some lunar scientists have proposed that rays are dominated by primary crater ejecta, while others have emphasized the role of secondary craters in producing rays. Pieters et al. (1985) presented the results of a remote sensing study of a portion of the ray system north of Copernicus. They provided evidence that the present brightness of the Copernicus rays in this sector is due largely to the presence of a component of highland ejecta intimately mixed with local mare basalt and that an increasing component of local material is observed in the rays at progressively greater radial distances from the parent crater. These results have been questioned and the origin of lunar rays is still uncertain [e.g., Schultz and Gault (1985)]. In an effort to better understand the processes responsible for the formation of lunar rays, we have utilized a variety of remote sensing data to study selected rays associated with Olbers A, the Messier crater complex, and Tycho. The data include near-IR reflectance spectra (0.6-2.5 um) and 3.8- and 70-cm radar maps. Dikov Yu. P. Ivanov I. V. Wlotzka F. Galimov E. M. Chemical Zoning of the Surface Layers in Luna-16 Soil Sample 1635: High Enrichment of Carbon The fine fraction (<74 micrometers) of a Luna-16 soil sample was examined by X-ray photo-electron spectroscopy (XPS) in connection with Ar-etching. We found that the surface layers show three chemically distinct zones. The middle zone, about 2500-3000 angstroms below the surface, is the most interesting. It contains up to 60% carbon, and in addition noticeable amounts of Zn (up to 4%). The remaining elements of this zone show a silicate composition. It is probable that this chemical zoning in the fine regolith particles is connected with a condensation process, and that the impact of a carbon-rich projectile was responsible for the carbon-enriched layer. The projectile could be a carbonaceous chondrite or, more likely, a comet. Shervais J. W. Shroff G. Thompson C. B. A Petrologic Survey of Six Rock Fragments from the Apollo 14 Bulk Sample: 14430, 14436, 14440, 14443, 14444, 14451 Recent petrologic studies of pristine nonmare samples from the Apollo 14 site have demonstrated the unique character of the western highlands crust. Many of the lithologies which occur here are not found at other highland sites or represent unique variations of more common lithologies. Rocks of the Apollo 14 site are geochemically distinct from similar lithologies found at eastern highland sites (Apollo 15, Apollo 16, Apollo 17, and the Luna sites) -- a fact which adds further complications to current petrogenetic models for the lunar crust. Nonetheless, an understanding of how the Apollo 14 highlands province formed and why it differs from highland crust in the east is crucial to our overall understanding of primordial lunar differentiation and petrogenesis. We report here new petrologic data on six previously unstudied rock fragments from the Apollo 14 bulk sample, which was taken from a location close to the lunar module during the first EVA. These fragments are coarser than 1 cm and were separated from the bulk sample with a 1 cm mesh seive. No work has been done on these fragments since they were described by Carlson and Walton. Basu A. Dorais M. McKay D. S. Metal Globules in Agglutinatic Glass Our preliminary analyses show that Ni/Co ratios in micrometer sized metal globules in agglutinatic glass of soil 61181 are mostly above 20, which suggests meteoritic derivation. The excess metal in lunar soils relative to lunar rocks, therefore, is likely to be meteoritic in part and possibly resides in agglutinatic glass. Sisterson J. M. Schneider IV R. J. Beverding A. Gans C. S. Kim K. Englert P. A. J. Castaneda C. Vincent J. Reedy R. C. Short-Lived Cosmogenic Radionuclide Production in Lunar Rocks; Improved Estimates for the Solar Proton Flux in Recent Solar Cycles Radionuclides are produced in lunar rocks by cosmic rays bombarding the Moon. Solar cosmic rays (SCR) can only penetrate the top 1-2 cm of the Moon's surface, while galactic cosmic rays (GCR) can penetrate several meters. The cosmogenic radionuclide archive can be analyzed to give information about the solar proton fluxes incident on the Moon over the past 2-10 million years. It is important to understand any variation in the Sun's activity over an extended time period so that the radiation hazards that might be met in space missions can be correctly assessed. Reedy, in 1977, analyzed the lunar rocks returned by the Apollo missions for the short-lived radionuclide production and compared the results with contemporary measurements of the solar proton flux. Now that there are (1) many more direct measurements of the solar proton flux available, (2) additional depth profiles measured in lunar rocks, and (3) considerable progress in measuring the reaction cross sections needed as input to the theoretical models, new estimates for solar proton fluxes over solar cycles 19 and 20 have been made. Heymann D. Fullerenes Were Not Found in Lunar Samples 10084 and 79261 Radicati di Brozolo et al. found the fullerenes C(sub)60 and C(sub)70 in and around a micrometeorite impact pit on the skin of the LDEF (Long Duration Exposure Facility) spacecraft and concluded that the fullerenes had formed from carbon of the micrometeorite. Becker et al. concluded that C(sub)60 and C(sub)70 in carbon-bearing breccias from the large Sudbury impact structure had come mostly from carbon of the impacting bolide. These observations suggested that fullerenes could have formed by analogous impacts on the Moon. One sample from the Apollo 11 site and one from the Apollo 17 site were extracted with toluene. The extracts were analyzed by High Performance Liquid Chromatography (HPLC) for the presence of C(sub)60 and C(sub)70 fullerenes. No fullerenes were detected. Tuesday, March 19, 1996 THE MOON: REMOTE SENSING AND MAPPING TECHNIQUES POSTERS 6:30 - 9:30 p.m. LPI Pieters C. M. Visible to Mid-Infrared Properties of Lunar Soils and Soil Separates In order to provide a comparative baseline for a variety of remote compositional analyses, reflectance spectra from the visible through the mid-infrared have been acquired for representative mature veils from Apollo 11, 12, 14,and 16. Similar spectra were acquired for size separates for Apollo 11 soil 10084. The finest fraction dominates both the visible and the midIR. Apollo 16 soils exhibit the shortest Christiansen Frequency (CF), a trend consistent with their feldspathic character. Finer soil size fractions, however, are observed to have longer CF, perhaps due to the effective strength of the Reststrahlen bands (RB). Altogether, features in the near-infrared are currently the most readily interpreted in terms of mineral composition and space weathering effects. Malaret E. Lucey P. G. Medium Resolution Global Lunar Mosaics of FeO, TiO2 and Is/FeO We have produced global mosaics of FeO, TiO2, and Is/FeO derived from Clementine data for distribution on CDROM. The three images are each of dimension 10000x5000 pixels in simple cylindrical projection covering the entire lunar surface imaged by Clementine (nearly 100%) at a sampling and resolution of 0.036 degree (about 1km at the equator). Giguere T. A. Blewett D. T. Lucey P. G. Taylor G. J. Hawke B. R. Adding Dimensions to the Lunar Geologic Map Using GIS We are applying Geographic Information Systems (GIS) technology to assemble a variety of lunar data types into one integrated database. The resulting combination of data will allow us to examine lunar geologic issues from a completely new perspective. To show the utility of GIS, we investigated the light plains deposits found on 4-7% of the nearside. The nature and origin of light plains have been a long-standing problem in lunar geology. Ideas for their origin include: true volcanic light plains, plains containing much impact melt, and emplacement of ejecta from basin-forming impacts. We made a spatial comparison between FeO and TiO2 derived from multispectral data acquired during the Galileo lunar flyby and digitized areas of light plains in their currently accepted locations. The chemical database provides an additional criteria for classifying the light plains regions. We have found that there are large variations in the FeO and TiO2 contents of the light plains. These units vary from relatively low FeO (6-8 wt.%) and low TiO2 (0.1 wt.%) to relatively high FeO (11-14 wt.%) and TiO2 (0.7-1.5 wt.%). Coombs C. R. Using GIS (Geographic Information System) Technology to Assess the Resource Potential of Lunar Pyroclastic Deposits Analyses of the lunar pyroclastic deposits can help address two major science theme strategies put forth by LExSWG: to better understand the formation of the Earth-Moon system, and the thermal and magmatic evolution of the Moon (LExSWG, 1992). To better visualize the interrelationships and assess the resource potential of the lunar pyroclastic sites, I have combined data collected from a variety of sources to generate a series of computer-based geographic information systems (GIS) for the major lunar pyroclastic sites; Lunar Pyroclastic GIS. An example of one data package is discussed here for the Taurus Littrow/Apollo 17 region of Mare Serenitatis. He G. Pieters C. M. Mustard J. F. The Use of Pattern Theory in Spectral Analysis of Surface Units Although evaluating the complexity of physical processes that shape planetary surfaces is often beyond the capability of any individual approach, remote sensing techniques are used quite commonly and successfully for the compositional analysis and mapping of remote surface units, and determining their stratigraphic relations. Image processing tools play a key role in the successful analysis and interpretation. With the strong physical basis for diagnostic spectral properties of rocks and minerals as background, multispectral information for small areas or separate groups of pixels can often be interpreted directly. However the geologic context and study of all spectral variability pixel by pixel for a whole region is quite impractical and requires tools that can both manipulate large image-cube arrays of data while applying analysis criteria that can be accurately interpreted. Spectral mixing analysis is one such approach, but it lacks the ability to distinguish the subtle but important details of spectra variability related to composition. Specific information about composition and physical processes active on the surface are generally deduced from the results. Edwards K. E. Colvin T. R. Becker T. L. Cook D. Davies M. E. Duxbury T. C. Eliason E. M. Lee E. M. McEwen A. S. Morgan H. Robinson M. S. Sorensen T. Global Digital Mapping of the Moon The Clementine spacecraft imaged more than 99% of the Moon's surface at resolutions of 80-250 m/pixel; we are in the process of assembling a global digital base map. A new geodetic network has been constructed from ~43,000 images and ~265,000 match points. The average relative positional error is about 80 m, less than 1 pixel. The absolute positional accuracy should be ~250 m/pixel on average, which will facilitate future lunar exploration. Previous control networks achieved accuracies of ~1-2 km/pixel on the near side and ~10-20 km/pixel on the far side. The final base map will consist of 750-nm normalized albedo. The processed dataset will be distributed on a 7-volume set of CD-ROMs and made available via the Internet. The imaging node of the Planetary Data System (PDS) has developed a program called MapMaker which facilitates the construction of digital maps with user-specified latitude-longitude limits and scale. Gaddis L. McEwen A. S. Becker T. Analyses of Galileo SSI Data from EM2: Scattered Light Effects Scattered light proved to be a small but important component of SSI data from the EM1 encounter of Galileo. In previous analyses, we characterized the effects of the removal of scattered light from the EM1 SSI 6-band multispectral data from Lunmap 14 (L14), a whole-disk imaging sequence of the Moon centered near Mare Orientale. An important result of our recalibration effort was an increase in 1-micron band depths of western limb basalts; mare ponds and deposits were shown to have mafic mineral contents comparable to many basalts of the nearside. Here we examine the effects of removal of scattered light from SSI data from EM2 ~whole-disk data from the Lunmos 9 (L9) imaging sequence. We show that scattered light is also an important component for compositional interpretation of the EM2 data, which were acquired after removal of the lens cover, and that its behavior is a complex function of surface albedo. Naydyonov A. N. Shkuratov Yu. G. Stankevich D. G. Lunar Optical Types as Revealed by Galileo The albedo-color diagram using albedo A (0.76 micrometers) and color index C (0.76 / 0.41 micrometers) images from Galileo data is presented. With the help of this diagram a map of optical types of lunar surface was prepared. McEwen A. S. A Precise Lunar Photometric Function The Clementine multispectral dataset will enable compositional mapping of the entire lunar surface at a resolution of ~100-200 m, but a highly accurate photometric normalization is needed to achieve challenging scientific objectives such as mapping petrographic or elemental compositions. The goal of this work is to normalize the Clementine data to an accuracy of 1% for the UVVIS images (0.415, 0.75, 0.9, 0.95, and 1.0 micrometers) and 2% for NIR images (1.1, 1.25, 1.5, 2.0, 2.6, and 2.78 micrometers), consistent with radiometric calibration goals. The data will be normalized to R30, the reflectance expected at an incidence angle (i) and phase angle (alpha) of 30 degrees and emission angle (e) of 0 degree, matching the photometric geometry of lunar samples measured at the reflectance laboratory (RELAB) at Brown University The focus here is on the precision of the normalization, not the putative physical significance of the photometric function parameters. The 2% precision achieved is significantly better than the ~10% precision of a previous normalization. Bondarenko N. Shkuratov Yu. G. Kreslavsky M. Correlation Between Radar and Optical Images of Lunar Surface Correlation diagrams of radar reflectance (70 cm) vs. albedo (0.65 micrometers) and color-index (0.65 mm/0.42 micrometers) and radar-optical unit maps are presented. The trend: the redder color, the higher radar reflectance was found for moderate color-index areas. Rank correlation maps were generated. High correlation seems to correspond to areas surrounding some impact craters. Graham F. G. A Search Strategy for Aluminum in the Lunar Atmosphere It's reasonable to infer the presence of aluminum in the extremely tenuous lunar atmosphere if the lunar atmosphere is produced from meteoric volatization and puttering processes which can also involve the aluminum-rich lunar highlands. The presence of aluminum vapor in the lunar atmosphere can be sought as an emission line in the visible spectrum at 396.1 nm. Normally, this line would be masked by the bright reflected solar continuum, but fortunately, it is located on the shortwave side of the Fraunhofer H line of Calcium II centered at 396.8 nm, which has an equivalent width of 1.4 nm. Hence, the Aluminum I emission line should appear as a bright "spike" in the shortwave wall of the H line profile. Tuesday, March 19, 1996 LUNAR METEORITES POSTERS 6:30 - 9:30 p.m. LPI Kita N. T. Systematics of Alkali and Pb Abundances in Meteoritic and Lunar Samples The alkali depletion is not a unique characteristic of the moon, but is common to eucrites, angrites, and the earth. Because the moon and the earth are depleted in more volatile Pb in a similar degree to both chondrites and achondrites, it is hard to assume that alkali depletion was caused by vaporization loss during the giant impact event. Alkali and volatile depletion might have originated from their source material which accreted to the planets. Symes S. J. Benoit P. H. Sears D. W. G. Thermal Histories of Lunar Meteorites and Apollo Samples: A Thermoluminescence Overview We have measured the induced thermoluminescence (TL) properties of seven lunar meteorites (three highland, four mare) in order to investigate their thermal and metamorphic histories. Most meteorites have TL sensitivities appropriate for their respective provenances, although shock has lowered the TL in two of the mare meteorites. The TL data indicate that all of our highland and mare meteorites contain a significant fraction of disordered feldspar, in contrast to Apollo highland soils, and more like Apollo highland impact melt rocks. The data suggest derivation of the lunar meteorites from a variety of depths in a melt sheet. Kring D. A. Hill D. H. Boynton W. V. A Glass-Rich View of QUE94281, A New Meteoritic Sample from a Mare Region of the Moon QUE94281 is a new lunar meteorite breccia with abundant vesicular glass that seems to both coat and be injected through the breccia. Petrologic and geochemical analyses of a bulk sample (,22) and thin-section (,5) indicate the breccia is a mixture of glass spherules, agglutinate fragments, pyroxene fragments, olivine fragments, anorthositic clasts, and feldspathic impact melt breccia clasts. The compositions of these components and the composition of the bulk breccia indicate it has a mixture of mare and highland material, but that it is dominantly associated with a mare region of the Moon containing low-Ti and/or very-low-Ti basalts. Jolliff B. L. Rockow K. M. Korotev R. L. QUE94281: Shallow Plutonic VLT Components and Highlands Components QUE94281, one of the newest lunar meteorites discovered in Antarctica, is a glassy- matrix, clast-rich regolith breccia containing a clast assemblage dominated by mineral debris from a shallow plutonic environment with compositional similarity to VLT (very-low-Ti) basalt. Abundant coarse pyroxene clasts are finely exsolved and have compositions similar to pyroxenes of known lunar VLT basalts and other lunar meteorites of basaltic composition. Pyroxene compositions follow a slow-cooling trend leading to the assemblage fayalite-hedenbergite-quartz. Coarse plagioclase clasts have An contents of ~68-95, similar to the other "basaltic" meteorites. As a proxy for the bulk composition of the meteorite, the composition of the fusion crust is very similar to the bulk composition of Yamato 793274 and reflects the dominant contribution of the VLT-composition mineral clasts. However, the clast assemblage also contains numerous subrounded, glassy and finely crystalline melt-breccias of highlands origin. Lithic clasts that have fine-grained basaltic texture are rare. Tuesday, March 19, 1996 PLANETARY INTERIOR PROCESSES POSTERS 6:30 - 9:30 p.m. LPI Walker R. J. Meisel T. Morgan J. W. Osmium Isotope Constraints on the Origin of Highly Siderophile Elements in the Earth's Primitive Upper Mantle Using negative thermal ionization mass spectrometry (NTIMS), Re and Os abundances were determined by isotope dilution, and 187Os/188Os ratios were measured in a variety of terrestrial upper mantle xenoliths, and also ordinary, enstatite and carbonceous chondrites. Our objective is to precisely constrain the Os isotopic composition of the Earth's primitive upper mantle (PUM) and compare it with the Re-Os systematics of the different chondrite classes. Ultimately, these results should provide us with a better understanding of the relative abundances of highly siderophile elements added to the Earth by late accretion, and how their abundances were modified by subsequent processes. Toda Y. Abe Y. Effect of Planetesimal Impacts on Mantle Differentiation During the period of heavy bombardment, mantle differentiation would be induced by two processes. One is the extraction of melt formed at the ascending region of mantle convection due to adiabatic decompression (endogenic process). The other is the differentiation induced by impact melting (exogenic process). We estimate the volume production rate of melt by each process to clarify quantitative importance of the exogenic processes on the early Earth. Delaney J. S. Bajt S. Dyar M. D. Sutton S. R. McKay G. Roeder P. Comparison of Quantitative Synchrotron MicroXANES (SmX) Fe^III/(Fe^II + Fe^III) Results for Amphibole and Silicate Glass with Independent Measurements In situ measurements of the oxidation states of elements such as Fe, Cr, V, Mn, Ti, Ce, Eu, Re, U can be made using synchrotron microXANES (SmX) techniques. Iron oxidation state measurements are of fundamental importance in most terrestrial systems and are useful in meteoritic and planetary materials. Oxidation state measurements of other transition metals have great implications for extraterrestrial samples that equilibrated at oxygen fugacities lower than typical terrestrial systems. The development of microbeam techniques using synchrotron x-ray sources, permits oxidation state measurements in microvolumes of sample comparable to the compositional microprobe analysis volumes. Comparison of Fe oxidation state ratios by SmX with independent data for amphibole samples and other minerals shows a good correlation between the microanalytical and bulk techniques. The spatial resolution of SmX permits zoning of oxidation states to be documented and also allows inclusions in minerals to be avoided. Analyses of Fe oxidation states can be made quantitatively in microvolumes comparable with those analyzed for composition by other microanalytical techniques. The consistence of the SmX results and conventional techniques implies that the simple calibration scheme used for SmX can be used for quantitative analyses of a broad range of terrestrial and extraterrestrial materials. Lauer H. V. Jr. Jones J. H. Lindstrom D. J. Experimental Partitioning Studies on Sc in Olivine and Pyroxene We have investigated the partitioning of Sc into olivine and pyroxene at one bar pressure in a controlled atmosphere gas-mixing furnace system using compositional systems appropriate for understanding of meteoritic (eucrite) and terrestrial basalts. Our results are in general agreement with previously published work of others and have been extended to include much higher concentrations of Sc in both olivine and pyroxene. Schwandt C. S. McKay G. A. Sector Zoning of Minor and Trace Elements in Synthetic Pyroxenes Enstatite (En80 Fs19 Wo1) crystals grown from hypersthene normative volatile-free basaltic melts under iron-wstite fugacity conditions contain dramatic sector zoning of minor (Al, Cr, Ti) elements and trace elements (Yb, other REE under investigation). Enstatite crystals are synthesized at constant furnace temperature a couples of degrees below the liquidus of 1210 degrees C, for a duration of 1 to 2 days, resulting in crystals with dimensions of 50 x 100 x 500-1000 m. The mineral/melt partition coefficients determined for each of the sectors can vary from one another by more than a factor of three. Sector development appears to be kinetically and crystallographically controlled. The largest concentration differences occur between the fastest and slowest growing sectors. Growth of these pyroxenes is fastest parallel to the c-axis, therefore in the case of Al, the {001} and {011} sectors are the most depleted and the prism sectors are most enriched, with the {010} sectors containing the most Al. To accurately determine a single equilibrium partition coefficient, one could algebraically average the partition coefficients on a sector volume percentage basis. However, sector differences between the prism sectors are minor and the prism sectors comprise the largest volume, such that use of partition coefficients determined from the prism sectors is perhaps appropriate for most modeling. Capobianco C. J. Hervig R. L. Solubility of Ru and Pd in Silicate Melts: The Effect of Melt Composition Solubilities of Ru and Pd in silicate melts having compositions in the Na2O-MgO-CaO-Al2O3-TiO2-SiO2 +/- FeO system were measured. Large changes in Ru solubility were correlated with the melt's ratio of non-bridging oxygens to tetrahedral cations, nbo/t. Ru wt% = (190)exp[-22000/T + 3.7 + 1.7(nbo/t)]. Palladium solubility showed negligible variation over the same compositional range. Thus, fractionation of Ru from Pd during magmatic differentiation could occur based simply on the difference in solubility as a function of melt basicity. We also found that substitution of FeO for MgO increases the solubility of Pd and Ru, but only slightly. Kadik A. A. Shilobreeva S. N. Minaev V. M. Kovalenko V. I. The Carbon Measurements in the Upper Mantle Minerals by the 12C(d,n)13N Nuclear Reaction The knowledge of carbon solubility in minerals of deep origin by participating in point defect equilibria could be critical to understanding the carbon contents in the mantle rocks and magmas. The carbon measurements in the mantle olivine (Ol), orthopyroxene (OPx) and garnet (Gar) minerals by 12C(d,n)13N reaction show that carbon is dissolved in this crystals in content of 5-100 ppm. The average the carbon content in the orthopyroxenes is higher than in olivines and garnets. The partial melting of carbon-bearing minerals in the absence of free carbon and fluid phase was shown to lead to formation of 300-400 ppm C or 0.1-0.15 wt% CO2 in melt This carbon content in melt is consistent with CO2 content observed in basaltic magmas. Freund F. Gupta A. Kumar D. Organic Matter from H2O and CO2 Dissolved in Minerals Complex organic compounds must have been available on the early Earth. However, where they came from or how they formed has remained a subject of intense discussion. Our work looks at the solid Earth as a possible source. We have succeeded in extracting organic molecules from inside MgO and olivine in quantities sufficient to obtain not only IR, 1H- and 13C-NMR spectra but also to purify at least on of the compounds and to grow single crystals for x-ray structure determination. The MgO crystals used in these experiments had crystallized at 2860 degrees C from a melt equilibrated with CO/CO2/H2O at 1 bar in a carbon arc furnace. The olivine crystals originated from the high pressure environment of the upper mantle. Thermodynamics requires that, as a melt saturates with reactive gases such as CO, CO2 or H2O, the crystals growing from such a melt dissolve some of these gases. During cooling, the dissolved gases undergo a redox conversion by which solute CO2 reduces to C, and solute H2O reduces to H2, counterbalanced by an aliquot of O2- oxidizing to O_. As the solubility decreases with decreasing temperature, C and H2 segregate, concentrating along dislocation lines, subgrain boundaries etc. Grasset O. Sotin C. The Liquidus of H2O-NH3 up to 1.5 GPa: Implications for the Presence of a Liquid Shell in Titan's Interior Thermal evolution, internal structure and atmospheric composition of Titan depend strongly on the physical and chemical properties of the NH3-H2O-CH4 system. Sapphire anvil cell experiments have been conducted in the pressure range [100 MPa - 1.5 GPa] and in the temperature range [130 - 300 K] to investigate the ammonia (15%) - water system. A liquidus of the system is proposed and its implications for the internal structure of the satellite are discussed. Grasset O. Parmentier E. M. Numerical Experiments of Convective Heat Transfer in a Fluid with Strongly Temperature Dependent Viscosity: Implications for Thermal Planetary Evolution Predictions of planetary thermal evolution depend on understanding the rate of heat transfer in a convecting fluid with a strongly temperature dependent viscosity. At high Rayleigh number, the heat flux for a constant viscosity fluid cooled from above is proportional tom 1/3 DT 4/3 where m is the viscosity and DT is the temperature difference across the thermal boundary layer. This relationship has frequently been applied to planetary thermal evolution by calculating the viscosity at the temperature of the well-mixed interior. However, according to scaling analysis, a conductive lid regime appears above a viscosity variation of 104-105. This regime has been observed both in laboratory experiments and in numerical models. In this regime, laboratory experiments indicate that the appropriate DT is the temperature difference across only the convecting part of the thermal boundary layer with Tm the temperature in the well mixed layer. Numerical studies for a fluid heated from within with large viscosity variations are presented. For a steady state volumetrically heated fluid cooled from above, as in the transient cooling laboratory experiments, the structure consists of a nearly isotherm well-mixed layer, an unstable boundary layer, and a stagnant conductive lid. New estimates of DTv are proposed and implications for planetary evolution are discussed. Mocquet A. Vacher P. Sotin C. Iron Content of the Martian Mantle: The Potential of Seismological Investigations Nowadays, geophysical observations on the inner structure of planet Mars are very sparse. Their interpretations are based on observations of its outer surface, of its very weak magnetic field, of its gravity field, and on geochemical arguments derived from the analysis of SNC meteorites. The composition of the martian mantle should mainly differ from the Earth's one by an enrichment in iron. In the absence of additional data, this interpretation must only be regarded as a working hypothesis, and all studies dealing with the inner structure of Mars insist on the non-uniqueness of the theoretical models. This paper describes how seismological data could provide constraints on the iron content of the mantle. Tuesday, March 19, 1996 MARS SURFACE CHARACTER AND MINERALOGY POSTERS 6:30 - 9:30 p.m. LPI O'Connell B. W. Geissler P. E. Martian Evaporite Deposits? A Comparison of Viking 3-Point Spectra Multispectral images can be used to distinguish surface materials of different composition apart from albedo differences. Here we report on a study comparing the color properties of three potential evaporite deposits on Mars. Blaney D. L. Sulfate Formation on Mars by Volcanic Aerosols: A New Look Sulfur was measured at both Viking Lander sites in abundances of 5-9 wt % SO3. Because the sulfur was more concentrated in clumps which disintegrated and the general oxidized nature of the Martian soil, these measurements led to the assumption that a sulfate duricrust existed. Two types of models for ulfate formation have been proposed. One is a formation by upwardly migrating ground water. The other is the formation of sulfates by the precipitation of volcanic aerosols. Most investigators have tended to favor the ground water origin of sulfates on Mars. However, evidence assemble since Viking may point to a volcanic aerosol origin. Bishop J. L. Dummel A. The Influence of Fine-Grained Hematite Powder on the Spectral Properties of Mars Soil Analogs; VIS-NIR Bi-Directional Reflectance Spectroscopy of Mixtures Visible to near-infrared bi-directional reflectance spectra have been measured for several mixtures of fine-grained hematite powder and chemically-treated montmorillonites. The purpose of this study is to measure the influence of a highly absorbing, fine-grained material on the spectral properties, especially NIR brightness and 2.2 m band depth, of silicate-bearing Mars analogs. Chemically- treated montmorillonites, containing ~10-20 wt.% nanophase ferric minerals and a NIR spectral brightness of ~85%, are used here as the Mars soil analog materials. The hematite powder was found to significantly darken the NIR albedo of the silicate and greatly decrease the strength of the 2.2 m band. The environmental conditions at the time of sample measurement were also observed to influence the near-infrared spectral properties of these Mars analog materials. Presley M. A. Christensen P. R. The Effect of Bimodal and Polymodal Mixtures of Particle Sizes on the Thermal Conductivity of Particulate Materials Under Martian Atmospheric Pressures Overview: New measurements of the effect of bimodal and polymodal mixtures on the thermal conductivity of particulate materials under martian atmospheric pressures are presented and discussed. The thermal conductivity of a particulate material containing a mixture of different particle sizes is the same as the thermal conductivity that the material would have if it were composed entirely of the largest particle sizes contained within that material. Arnold G. E. Bishop J. L. Junghans K. Schade U. Relationships Between the Optical Constants and Spectroscopic Features of Particulate Quartz. Implications for Remote Sensing of Planetary Surfaces Infrared reflectance spectra of particulate mineral samples depend on particle size and porosity. Analysis of remotely sensed infrared spectroscopic data requires the study of particle size effects on the spectral properties. Therefore, reflectance spectra of quartz size separates and the optical constants n and k have been measured. The relationships between spectral features, the optical constants and the Mie single scattering efficiencies are discussed. Variation of spectral features as a function of the particle size in relation to the optical constants allows us to identify at least 4 different cases of scattering and absorption behavior: (1) n=1, k is small, (2) k is large, n undergoes rapid change in the range of anomalous dispersion, (3) n < 1, k is low and (4) k is low and n >1. Martin P. Pinet P. Bacon R. Rousset A. Bellagh F. Martian Surface Mineralogy from 0.8-1.05 Micrometer Tiger Spectro-Imagery Measurements: Evidence for Compositional Heterogeneities in Terra Sirenum/Tharsis Montes Formation New 0.84-1.02 microns reflectance spectra of Mars are derived from TIGER spectro-imaging measurements obtained at Mauna Kea CFH observatory during the 1990 opposition, from a systematic survey of the Tharsis volcanic region at 200-250 km spatial resolution. The spectral interval is spanned by 240 wavelength channels, with a sampling of about 8 Angstroms. Besides a complementarity with the ISM imaging spectrometer mapping at a larger spatial resolution, the continuous wavelength coverage of the Tharsis province is of particular interest as it permits to study the spectroscopic variability through significative albedo markings. In particular, the dichotomy separating the southern cratered highlands of Terra Cimmeria/Terra Sirenum from the younger volcanic terrains of north Tharsis is investigated. Primary goals of this work are to identify the physico-chemical origins of the observed spectral variations, to map the surface units based on their spectroscopic properties, and to draw inferences in terms of compositional heterogeneities of bright and dark materials. Detailed information regarding the ferric and ferrous mineralogy of undocumented or poorly documented soils is presented, on the basis of a reliable set of about 1500 highly-resolved spectra (R~600) and with the application of mixture modeling and spectral variable analyses to the dataset. Bell J. F. III Wolff M. J. James P. B. Clancy R. T. Lee S. W. Martin L. J. Calibration and Mineralogic Analysis of HST Images of Mars Obtained During 1994-1995 Between August 1994 and August 1995 we obtained images of Mars using the Hubble Space Telescope (HST) Wide Field/Planetary Camera-2 (WFPC2), with P.B. James as the PI of our General Observer proposal team. Most of the images were obtained in 5 filters (F255W, F336W, F410M, F502N, F673N) approximately once per month during the times when Mars was greater than 50 degrees from the Sun. More frequent coverage was obtained in these five filters around the time of opposition in February 1995, and four additional wavelengths (LRF7400, LRF8600, F953N, F1042M) were added for the imaging performed in July and August 1995. The primary goal of this HST observing campaign is synoptic monitoring of Mars atmospheric and surface phenomena. Initial results from these observations have recently been reported. The primary focus of the work reported here is on the continuing analysis and interpretation of the images with emphasis on determining and mapping Mars surface mineralogy. Israel E. J. Arvidson R. E. Wang A. Jolliff B. L. New Approaches to the In-Situ Study of Martian Surface Mineralogy With the upcoming Surveyor missions to Mars, including the Sojourner rover scheduled to be launched in December, 1996, and plans for future lander and rover missions, there will be many opportunities for in-situ study of martian rocks. There is evidence from Earth-based reflectance spectra for the presence of amorphous gel-like coatings, rich in Fe3+, on martian surface materials. Additional evidence for the presence of coatings is seen in Viking Lander images, which show specular reflections due to forward scattering from boulders. Given the presence of rock coatings, the question is, will it be necessary to break rocks open or drill into them in order to analyze underlying materials, or will it be possible to use spectroscopic techniques to see beneath the coatings? To test the idea that spectroscopy can be used to infer the mineralogy of coatings and underlying materials, we are analyzing a varnished basalt from the arid environment of Lunar Crater Volcanic Field, Nevada with a laser Raman spectrometer. Golombek M. Rapp D. Size-Frequency Distributions of Rocks on Mars Predicting the size-frequency distribution of rocks at different locations on Mars is difficult owing to the limited data set (ground truth from only two sites at the surface) and yet is critical for determining potential hazards for future Mars landers. In this abstract, we review rock frequency data at the two Viking landing sites and a variety of sites on the Earth with special reference to larger rocks that could be hazardous to a lander, describe the data in terms of simple mathematical expressions, and provide a means of extrapolating the data to any location on Mars from relationships between the rock frequency curves and remote sensing data. Simpson R. A. Baron J. E. Tyler G. L. Inference of Block Populations from Radar Backscatter: Application to Pathfinder Landing Site Characterization Historically, roughness estimates for planetary surfaces on scales of centimeters to meters have been derived from dispersion of quasi-specular radar echoes.Echo broadening in time and/or frequency can be related to distribution of tilts of surface facets properly oriented to give mirror-like reflection toward the radar receiver.It has long been recognized that this model is incomplete; in most cases the effects from discrete scattering objects (e.g., rocks) will be missed, while in the worst circumstances, such as on aa basalt flows where the concept of a "facet" has little meaning, the model is hopelessly inadequate.Mie theory allows calculation of scattering from individual spherical particles in a homogeneous dielectric medium; approximations allow extension to ellipsoids.But the general problem of scattering from irregular objects near the top of a dielectric half-space (regolith) has remained intractable.Starting from first principles, however, we have succeeded in developing finite-difference time-domain (FD-TD) codes which accurately represent the scattering process in 2- and 3-dimensions and have been able to relate the numerical results to simple populations of blocks which might be found on surfaces such as near the Viking Lander sites and at the proposed Mars Pathfinder site. Crumpler L. S. The Significance of Rock Abundances at the Viking Lander Sites: Implications for Mars Pathfinder and Surveyor Landers Several missions are planned to Mars in the coming years that will place a variety of instrumented landers on the surface [Mars Pathfinder, Mars Surveyor, Mars 96]. In preparation for selection of landing sites, engineering considerations, and science goals, it is desirable to estimate the physical characteristics of the surfaces. Rock abundances on the surface are an important consideration for both science issues and site hazard estimation. Almost any potential hazard may be accommodated in engineering designs, but at the cost of size, weight, and complexity of the lander. The ability to do science at the lander sites is governed not only by the ability to function in the environment, but also by the ability to interpret the significance of the observed surface. There is currently very little knowledge, and no viable models of the geologic significance of rock populations, although recent efforts to understand the Mars Pathfinder site have spurred renewed efforts to address this issue. In this study I review the characteristics of the rock abundances as observed at the Viking Lander sites, discuss some models for the physical distribution and orizin of rock populations, and address the possible significance for proposed landers and rovers. Guinness E. A. Arvidson R. E. Shepard M. K. Specular Scattering from Rock Surfaces at the Viking Lander Sites Over the next decade one focus of Mars exploration will be a series of landers and rovers, starting with Mars Pathfinder. An important component of the landed exploration of Mars will be to determine the chemistry and mineralogy of rocks. Important objectives for future measurements of rocks on Mars will be determination of coating thickness and properties and whether sensors will measure rock coatings or the underlying rock. In addition, the coatings themselves are of interest as to their origin and the relation of coatings to rock weathering on Mars. Coated rock surfaces are present at the Viking Lander sites and are found to have a very high reflectance in the forward scattering direction (i.e., specular reflection), similar to varnished rock surfaces found in arid terrestrial settings. The presence of these coatings may indicate that rock breakdown is largely by macroscopic spalling, providing surfaces that are relatively long-lived and capable of developing coatings through on-going weathering processes. Ming D. W. Golden D. C. Gooding J. L. Morris R. V. Thompson D. R. Bell J. F. III Mineralogical and Thermal Properties of Jarositic Tephra on Mauna Kea, Hawaii: Implications for the Sulfur Mineralogy on Mars Jarosite, a ferric hydroxysulfate, occurs in some tephra samples on Mauna Kea volcano in Hawaii. The mineralogy and thermal properties of two jarositic tephra samples were characterized by X-ray diffraction (XRD) analysis and differential scanning calorimetry (DSC). XRD analysis of the <5 mm fraction of the tephra indicated nearly monomineralic jarosite in both samples. DSC signatures indicated two strong endoenthalpic transitions for jarosite at near 670 and 950 K, which represent dehydroxylation and ferric sulfate decomposition of jarosite, respectively. If jarosite exists on the surface of Mars, it should be easy to identify with DSC interfaced to an evolved-gas analyzer during a robotic mission (e.g., Mars Surveyor Program. Beiersdorfer R. E. Ming D. W. Intensive and Bulk Compositional Controls on Hydrothermally Altered Mafic Rocks--Implications on the Study of Martian Hydrothermal Metamorphism The occurrence of hydrothermally altered rocks on Mars has important implications with regard to exobiology; the planetary CO2 budget and petrology. It is highly probable that hydrothermally altered martian rocks will contain mineral assemblages similar to those found in metamorphosed mafic rocks on Earth and could be assigned to the zeolite-, prehnite-pumpellyite-, prehnite-actinolite-, pumpellyite-actinolite-, or greenschist-facies. The mineralogy of metamorphosed mafic rocks is very sensitive to changes in their intensive conditions, such as pressure (P) and temperature (T). Unfortunately, most terrestrial metamorphosed mafic rocks contain high-variance assemblages (phase-rule degrees of freedom > 3) that can not be used individually to estimate P or T. When Martian samples are returned to Earth during the early part of the next century, it is highly probable that they will contain high-variance assemblages. Fruitful interpretation of these samples will require that they be placed in petrological context with their terrestrial counterparts. Fortunately, the terrestrial high-variance assemblages typically exhibit a systematic correspondence between effective bulk composition and mineral assemblage, and can be used to interpret the P, T and fluid composition during metamorphism. _ Morris R. V. Golden D. C. Ming D. W. Thompson D. R. Reflectivity Spectra of Sulfate Alteration Products of Volcanic Tephra from the Summit of Mauna Kea Volcano: Implication for Martian Sulfate Mineralogy We report XRD and reflectivity data for samples of tephra from the summit cones of Mauna Kea Volcano that have been oxidatively altered to the hydoxysulfate minerals alunite and jarosite, presumably by hot, sulfur-bearing magmatic gas and/or fluids percolating up through the cinder cone. This process could also occur on Mars, where the heat source could be impact processes in addition to volcanic processes. It is possible that the ~900 nm band observed in certain martian spectral data results from jarosites and is evidence for these processes. Bockstein I. M. Gektin Yu. M. Removal of Brightness Artifacts from the Panoramas of Mars Surface Received from Phobos-2 Spacecraft The method is described that was used to remove brightness artifacts -- long vertical strips due to Phobos shadowing -- from two panoramas of Mars surface obtained from Phobos-2 space station. The main idea of this method is spacial filtering of shadow regions of the panoramas with the help of a sliding parallelogrammic window. Linear correction of lines inside the shadow region is realized to make its border invisible. The results of filtering are presented. Starukhina L. V. Shkuratov Yu. G. Computer Simulation of Phobos Spectrum: Implication to Surface Composition Composite Phobos spectrum proposed by the authors is simulated. Calculation show that (1) modification of the carbonaceous chondrite spectra by formation of fine grains of reduced iron in the upper surface layers of particles (similar to those in lunar regolith) do not match the Phobos spectral curve; (2) a good fit for it is obtained for submicron mixtures of clay minerals and dark red organic compounds, so Phobos spectral curve can be explained by higher H content in the organic components of Phobos surface material as compared to carbonaceous chondrites. Tuesday, March 19, 1996 MARTIAN GEODYNAMICS POSTERS 6:30 - 9:30 p.m. LPI Zuber M. T. Smith D. E. Haberle R. M. The Effect of the Atmosphere of Mars on the Planet's Rotation and Gravity Field Movements in the atmosphere of Mars cause changes in the distribution of mass and in the angular momentum that affect the planet's rotation and gravity field. As atmospheric material moves seasonally toward the summer pole the angular momentum of the atmosphere decreases and the rotation rate of the solid part of the planet increases to conserve the planet's total angular momentum. Similarly, as atmospheric material moves away from the pole the body of the planet slows down. The coupling between the atmosphere and the surface is through wind stress acting on the planet's topography. As the atmospheric mass is redistributed during the Mars year the gravity field of the planet is changed and a small seasonal variation in the gravity field is produced. Imbalance in the distribution of atmospheric material with respect to the rotation pole introduces a torque on the rotation axis that excites polar motion. Smith D. E. Zuber M. T. The Accuracy of the Long Wavelength Shape of Mars We have recently completed a re-analysis of the Mariner 9 and Viking 1 and 2 occultation data and produced a new model for the long wavelength topography of Mars. To derive this model we re-computed the orbits of the spacecrt using a new gravity field model for Mars, employed the latest planetary ephemerides and planetary dynamics models, re-estimated the atmospheric refraction corrections, and referenced the topography to an improved geoid. We believe the overall accuracy of the topographic model to be approximately 500 meters. Lyons S. N. Frey H. V. Bills B. G. Roark J. H. Mars Crustal Thickness Models from Mars50c and GMM-1 Planetary crustal thickness models can be produced from the assumption that gravity anomalies are due only to variations in surface and crust/mantle topography and crust and mantle densities are laterally constant. Assuming plausible values for crust (2.7 to 3.1) and mantle (3.0 to 3.5 gm/cc) density and mean crustal thickness (50 to 150 km), we generated such models for Mars using both Mars50c and GMM-1. Except for Tharsis these are nearly identical and show more spatial detail than earlier gravity models. We show how crustal thickness based on Mars50c depend on average crustal thickness and crust/mantle density contrast. A reasonable model for Mars has average thickness about 65 km if the density contrast is 0.5 (crust 3.0, mantle 3.5) gm/cc. This model satisfies the "constraint" on crustal thickness from the possible detection of a marsquake by Viking Lander II. Tuesday, March 19, 1996 MARS: TECTONIC PROCESSES POSTERS 6:30 - 9:30 p.m. LPI Fori A. N. Schultz R. A. Analysis of Fault Lengths Across Valles Marineris, Mars Summary. As part of a larger project to determine the history of stress and strain across Valles Marineris, Mars, graben lengths located within the Valley are measured using a two-dimensional window-sampling method to investigate depth of faulting and accuracy of measurement. The resulting degree of uncertainty in measuring lengths (+19 km - 80% accuracy) is independent of the resolution at which the faults are measured, so data sets and resultant statistical analysis from different scales or map areas can be compared. The cumulative length frequency plots show that the geometry of Valley faults display no evidence of a frictional stability transition at depth in the lithosphere if mechanical interaction between individual faults (an unphysical situation) is not considered. If strongly interacting faults are linked and the composite lengths used to re-create the cumulative lengths plots, a significant change in slope is apparent suggesting the existence of a transition at about 35-65 km below the surface (assuming faults are dipping from 50ø to 70ø). This suggests the thermal gradient to the associated 300-400øC isotherm is 53C/km to 12øC/km. Treiman A. H. Spiker K. Fault-Continuation Ridges in the Valles Marineris, Mars: Evidence for Groundwater Circulation The walls of the Valles Marineris (VM) are marked in many places by high-standing ridges that continue the traces of graben bounding faults on the plains adjacent to the VM. These fault-continuation ridges (FCR) are most prominent at Melas Labes and Candor Labes, where they form the boundaries between Ophir, Melas, and Candor Chasmae. FCR are as long as 100 km, and extend from the plains surfaces downward about 3-6 km in elevation. Available data suggests that FCR are fault zones cemented and hardened by groundwater deposits. Mest S. C. Crown D. A. Relationships Between Wrinkle Ridges and Craters in Hesperia Planum: Constraints on the Timing of Ridge Formation Wrinkle ridges in Hesperia Planum are similar in morphology to those in other regions of Mars such as Lunae Planum, the Coprates region, and Chryse Planitia and to lunar mare ridges. Martian wrinkle ridges have been classified into four types: high- and low-relief ridges, ridge rings, and rectilinear ridges. High- and low-relief ridges and ridge rings are believed to be compressional tectonic features formed in volcanic plains, whereas rectilinear ridges are thought to result from differential erosion or are the surface expressions of dikes. Watters T. R. Robinson M. S. Photoclinometric Studies of Lobate Scarps on Mars Lobate scarps are landforms on Mars that occur almost exclusively in highland material. These scarps are morphologically similar to highland scarps observed on the Moon and Mercury. They are lobate, arcuate or linear and often segmented, in plan view, and appear to be generally one-sided. Based on their morphology and the fact that many clearly deform crater floors, they are interpreted to be compressional tectonic features resulting from thrust faulting. Lobate scarps on Mars, like those on Mercury, appear to occur on at least two different scales, described here as moderate- and large-scale. Although lobate scarps are important tectonic features, reflecting significant compressional deformation of some of the oldest land masses on Mars, their morphology and dimensions have never been characterized in detail. Monoscopic photoclinometry is used in this study to extract elevation data across lobate scarps from Viking Orbiter images. From these data the amount of the crustal shortening across lobate scarps is estimated. Preliminary results indicate that large-scale scarps like Amenthes Rupes reflect at least several kilometers of crustal shortening and may be the largest compressional tectonic features on Mars. Dohm J. M. Tanaka K. L. Thaumasia Region of Mars: Chronology and Regional Distribution of Fault and Ridge Systems and Other Significant Structures We have completed a series of five paleotectonic maps that shows fault and ridge systems in the Thaumasia region as they prevailed during successive geologic periods. These structures have been classified chronologically by noting their age relative to rock units, which were dated by crater counts and by crosscutting relations. Five geologic stages were chosen based on major geologic periods defined by an unpublished 1:5,000,000-scale geologic map of the Thaumasia region. These stages are defined by sets of geologic units, and, based on the unit age ranges, we conservatively show slight overlaps between the stages. Here, we discuss the history of each major tectonic feature and structure set in the Thaumasia region. Jons H.-P. A Huge Fossil Linear Zone of Weakness in the Area of the Ancient Martian Uplands? The spatial distribution of linear escarpments and the numerous upland volcanoes which are aligned along those features indicate the existence of a linear zone of weakness which sets in south of Apollinaris Pateras, includes the Sisyphi Montes, and can be detected as far north as approximately -60 degrees latitutde. Tuesday, March 19, 1996 MARTIAN VOLCANISM POSTERS 6:30 - 9:30 p.m. LPI Peitersen M. N. Crown D. A. Downflow Width Behavior of Martian and Terrestrial Lava Flows Lava flow morphology is typically the primary type of data available for constraining planetary volcanic processes. Flow emplacement (and hence morphology) is controlled by topography, rheology, and thermal history, but the exact relationship between these factors and the resulting flow shape is not well understood. Accurate interpretation of flow dynamics is dependent upon well-constrained flow geometry. Previous modeling studies have commonly assumed relatively constant flow widths and/or cross-sectional areas, use a single value for width based upon a limited number of measurements, or do not consider width at all. Recent analysis of individual flow lobes in the Puu Oo flow field on the east rift zone of Kilauea Volcano show that flow widths vary by up to an order of magnitude. Significant width variations in Martian flows on Tyrhenna Patera, Alba Patera, and Elysium Mons are also observed. Complex flow-forms may be the result of spatially-interacting, temporally-discrete episodes in addition to contemporaneously emplaced flows. Variations in width with downflow distance from the vent may provide critical clues to flow emplacement processes and volcanic stratigraphy; furthermore, flow width is one of the few characteristics that can easily be measured from planetary mission data. The relative widths and lengths of lava flows are presumably a function of the relationship between the lateral spreading rate and flow front advance. Width/length "aspect ratios" are used to quantify this; however, they are highly dependent upon flow history interpretations. Although topography is thought to be a primary control, recent studies have demonstrated a marked insensitivity of flow morphology to topographic variations at 20 to 40 foot scales. Sakimoto S. Baloga S. Crisp J. Three-Dimensional Modelling of Leveed Lava Flows Eruption rates and lava flow emplacement times are important factors in understanding planetary volcanism. For planetary lava flows featuring well-developed central channels and lateral embanking levees, lava flow rates and emplacement times have been estimated from measured morphological parameters, such as levee and channel widths, using the isothermal Bingham flow model of Hulme. The isothermal assumption and the arbitrary fitting of a static two-dimensional model to a three-dimensional problem have been criticized, but no definitive replacement has emerged in the literature to date. Through the use of numerical modeling, we are investigating the validity of the theoretical assumptions and approach used by Hulme for leveed lava flows that have fully developed central channels. The goal of this study is to identify the conditions for which the Hulme approach provides a reasonable approximation and to isolate the parameter regimes that require a more refined theoretical treatment of flow emplacement. Lopes-Gautier R. Bruno B. Taylor G. J. Smythe W. Kilburn C. Analysis of Martian Lava Flow Properties Using Three Complementary Models Studies of lava flows on Mars must still rely largely on imaging data obtained by the Viking mission. Therefore, flow morphology remains the prime data from which effusion and magma characteristics must be inferred. This study uses three complementary lava flow models to infer emplacement characteristics of several of martian flows. We have previously used these models to study some of Alba Patera's younger flows. Here we extend the scope of the work by analysing flows from the Elysium Planitia and further examining the sources of uncertainties in the models, including the effect of flow margin erosion. We will discuss how the models can be used together as a powerful analysis tool for extra-terrestrial lavas in general. Anderson S. W. Crown D. A. Stofan E. R. Plaut J. J. Processes Affecting the Formation and Degradation of Silicic Lava Flow Surface Boulders Boulders are a ubiquitous but often neglected feature present on silicic lava flows and domes. As part of an analysis of the emplacement history of silicic extrusions, we determined boulder size distributions on young lava flows and studied the development of boulders on the active Mount St. Helens and Mount Unzen lava domes. We find measurable variations in average boulder size across a single extrusion and between different extrusions that are related to changes in emplacement conditions during eruption and flow emplacement. Challis D. Greeley R. Thermal Erosion by Lava: A Comparison of Theoretical and Experimental Modeling Lavas are hypothesized to carve channels by thermal erosion. This study models thermal erosion using Polyethylene Glycol (PEG) as an analog for lavas. PEG has been used in a variety of studies to model lava flows. Two previously formulated melt rate equations are compared to the measured erosion rates. Neither equation was found to match the experimental results equation 1 was 2 orders of magnitude too low and equation 2 was 80-90% too high. This is probably because the theoretical equations do not account for decreased heat transfer at the base of the flow. Miyamoto H. Sasaki S. Reproducing Actual Morphology of Planetary Lava Flows Assuming that lava flows behave as non-isothermal laminar Bingham fluids, we developed a numerical code of lava flows. We take the self gravity effects and cooling mechanisms into account. The calculation method is a kind of cellular automata using a reduced random space method, which can eliminate the mesh shape dependence. We can calculate large scale lava flows precisely without numerical instability and reproduce morphology of actual lava flows. Fagents S. A. Emplacement Mechanisms of Volcanic Materials on Mars The physical environmental conditions on Mars suggest that basaltic magmas containing even small amounts of typical volatile species should commonly have erupted explosively. Theoretical considerations indicate that enhanced volatile exsolution and expansion, together with the increased atmospheric trajectories of pyroclasts expected under martian conditions, should have promoted distribution of fine material over great distances. This implies that widespread pyroclastic deposits should be common in Mars' geologic record, but the identification of numerous lava flows and effusively constructed volcanic edifices indicates otherwise. Lava flows may have formed effusively on Mars if the magma reaching the surface of the planet was essentially devoid of volatiles; either as a result of depletion in the magma source region, or as a consequence of gas-loss during storage or transport. However, consideration of the details of the mechanisms of ascent and eruption of basaltic magmas under martian planetary conditions reveals that it may be possible for lava flows to form as a by-product of explosive volcanism in highly overpressured explosive eruptions. Jurgens R. F. Slade M. A. Haldemann A. F. C. Radar Imaging and Topography of Tharsis, Olympus Mons, and Albor Tholis The 1995 Mars Opposition provided coverage of a range of latitudes not covered by 3.5-cm radar during any previous Mars' closest approach: sub-Earth latitudes between 16 degrees N. and 22 degrees N. Thus this opposition presented the opportunity to examine the Tharsis/Olympus region and other highland areas at new latitudes for ground-based Goldstone radar. However, the analysis of delay-Doppler radar data in mountainous terrain requires a more complicated fitting process than for the lowland planitiae that are mostly flat planes. Two special difficulties must be dealt with: (1.) the rapid change in elevation makes it difficult to position the rather narrow range window on the specific surface, (2.) the large slope reflects most of the signal power to directions other than back to the radar. Zimbelman J. Johnston A. Lovett C. Jenson D. Geologic Map of the Ascraeus Mons Volcano, Mars Ascraeus Mons is the northernmost of the three aligned shield volcanoes that comprise the Tharsis Montes on Mars. Geologic mapping of the Tharsis Montes has been supported through the Mars Geologic Mapping program of NASA; here we report on the results obtained for Ascraeus Mons. Ascraeus Mons was initially mapped at 1:500,000 scale on four adjacent Mars Transverse Mercator (MTM) sheets, which covered the entire volcano and the surrounding materials. The four individual maps were then digitized and subsequently combined using ARC/Info commercial software. The final composite map is presented at a scale of 1:1,000,000, making it consistent with the already published composite maps of Arsia and Pavonis Montes. Mapping of Ascraeus Mons has revealed a more complex history for this volcano than is shown on previous published maps. The eastern flank is superposed by lavas originating from embayments cut into the southern flank; the contact between the shield flows and the superposed lavas is ~2 km higher here than the comparable contact on the western flank. The western flank is intensely modified by linear coalescing collapse features, some of which were sources for late-stage lavas. This result indicates that effusive activity continued well beyond the shield construction stage, leading to degradation of shield materials on the lower western flank. N-S-oriented graben west of the volcano formed within a topographic low that affected lava flow orientations in the region, but which does not show up on published topographic maps. A lobe-shaped deposit occurs west of the volcano, similar to features west of Arsia and Pavonis Montes, but here they are much smaller than those to the south. The lobe deposits include both effusive and modified materials, all of which is deformed by curious arcuate graben. Clearly this volcano had an intriguing and protracted history, now revealed through detailed geologic mapping efforts. Lucchitta B. K. Young Volcanism or Extensive Mass-Wasting Collapse Inside West Candor Chasma, Mars Anomalous color as well as unusual erosional and deposition features have been noted previously in west Candor Chasma. In this report I tentatively attribute strange stratigraphic, morphologic, and topographic relations to either relatively young volcanism or wholesale mass-wasting collapse of interior deposits. Tuesday, March 19, 1996 MARS GEOLOGIC PROCESSES POSTERS 6:30 - 9:30 p.m. LPI Herkenhoff K. E. Nowicki S. A. Plaut J. J. The North Polar Layered Terrain on Mars: A Very Young Surface THE NORTH POLAR LAYERED TERRAIN ON MARS: A VERY YOUNG SURFACE; K. E. Herkenhoff1, S. A. Nowicki2, and J. J. Plaut1, 1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109-8099, 2Augustana College, Rock Island, IL 61201-2296. The Martian polar layered deposits are probably the best source of information about the recent climate history of Mars, but their origin and the mechanisms of accumulation are still a mystery. The polar layers are sedimentary deposits that most planetary scientists believe are composed of water ice and varying amounts of wind-blown dust, but their composition is poorly constrained. Interpretation of the observed polar stratigraphy in terms of global climate changes is complicated by the significant difference in surface ages between the north and south polar layered terrains inferred from crater statistics. While no craters have been found in the north polar layered terrain, the surface of the south polar layered deposits appears to have been stable for many of the orbital/axial cycles that are thought to have induced global climate changes on Mars. Further studies of the polar layered deposits should ultimately lead to a better understanding of the climate history of Mars. Mellon M. T. Limits on the CO2 Content of the Martian Polar Deposits It has recently been suggested that large quantities of CO2 could be sequestered within the martian polar deposits as CO2 ice or CO2 clathrate hydrate, stabilized against sublimation by a non-porous overburden of water ice. I investigate the phase stability of CO2-bearing polar ices and place limits on the quantity of CO2 that can be sequestered. The magnitude of this limitation follows from CO2-bearing ices having thermal conductivities on the order of 5 to 6 times smaller than that of water ice, producing a higher geothermal gradient. By calculating the effective thermal conductivity of a mixture of ices and the resulting temperature-pressure profile of the polar deposits and comparing these profiles with the phase diagrams of water ice, CO2 ice, and CO2 clathrate hydrate, I set limits on the quantity of CO2 in the polar deposits. Zimbelman J. Crown D. Jenson D. Initial Investigation of the Enigmatic Massive Deposits in Amazonis Planitia, Mars Massive deposits in the Amazonis Planitia region of Mars, distributed within 12 degrees S to 18 degrees N latitude and 125 degrees to 220 degrees W longitude, have been mapped regionally as the Medusae Fossae Formation (MFF). They have been subdivided locally into from three to seven separate units. Several hypotheses have been published concerning the possible formation mechanism for these unique deposits. Recently four Mars Transverse Mercator (MTM) sheets at 1:500,000 scale were approved by NASA for systematic geologic mapping, with the goal of obtaining constraints on the hypotheses of formation of these materials. Base map materials were only recently received, but here we report on an initial investigation of the excellent Viking images (orbits 462 to 473 of the Survey Mission, with 30-33 m/pixel resolution) available for large portions of the Medusae Fossae materials. Digital processing and mosaicing of selected images reveals remarkable evidence of intense aeolian erosion; the Medusae Fossae materials apparently are extremely friable and easily removed by the wind. Several exhumed surfaces, exposed once MFF materials have been partially to completely stripped away, show apparently pristine morphology of lava flow fronts and impact craters. The preservation of these delicate morphologies after both burial by and exhumation from beneath the MFF place some useful constraints on the physical properties of these materials. Photoclinometry of several partially exhumed impact craters and erosional scarps of MFF indicate minimum thicknesses of 200-300 m for these materials SE of the Gordii Dorsum feature. Efforts are underway to compare the MFF to the Los Frailes ignimbrite in Bolivia as a test of the ignimbrite hypothesis of origin for the Martian materials. Parker T. J. Identifying Shore Morphology on Mars Coastal morphology is probably the most diagnostic evidence of former lakes that we can expect to find on Mars, at least until unequivocal surface compositional information becomes available in the next few years. The following points should be noted when evaluating a potential shoreline on Mars: (I) Water is an environmentally abundant volatile at Mars' position in the solar system, and liquid water has probably been stable at its surface at various times in the past. (II) Free standing liquid water is confined by gravity to an equipotential upper surface that intersects topography at an essentially constant elevation over large regions. Other liquid volatiles would be similarly confined, but gases would not, so eolian erosional boundaries are not topographically confined and are often indistinct or gradational. (III) Winds blowing across open water generate ripples and waves (or transport rafted ice in cold environments) that transfer their energy to the land, resulting in erosion, sediment transport and deposition that is focused within a few meters of the water level. (IV) Longshore transport moves sediment eroded from headlands into embayments, producing beaches that transit from erosional to depositional, with wave-cut cliffs grading along shore into constructional barriers and spits. De Hon R. A. Martian Highland Troughs: Regional Controls of Runoff and Sedimentation Large and regionally important, troughs in the highland terrain of Mars provide natural pathways that channel fluids on the surface and are major loci of sediment accumulation. Nelson D. M. Greeley R. Geologic Sequence of Formation and Evidence for Sheetwash Erosion at the Ares Vallis Region, Mars The mapping of the Ares Vallis region, Mars, has brought about the identification of 12 new geologic units, seven of which are used to refine the sequence of events which shaped the formation of this outflow channel and its surroundings. The region mapped, at 1:2M scale, spans the terrain from 30 degrees N to 15 degrees S, and 10 degrees to 45 degrees W. Also investigated was geomorphologic evidence for fluvial sheetwash over the Npl2 (Noachian Plateau, subdued member) geologic units within the Ares Vallis region. Three sites outside of this region, selected based on respective distance from presumed fluvial activity, were used to test whether other Npl2 units were smoothed in a similar manner. _ Tanaka K. L. Geologic History of Chryse Basin, Mars: New Crater Counts and Mapping Results A complex sequence of erosional, depositional, and structural events that make up the geologic history of Chryse basin has been deciphered by photogeologic methods, including geologic/geomorphic mapping at 1:5,000,000 scale (partly revised from) and new, more thorough crater counts. Apparently, four distinct episodes of outflow-channel formation and associated basin sedimentation occurred from Late Noachian to Middle Amazonian time, interspersed by local emplacement of volcanic flows, structural deformation, and mass wasting. Lancaster M. G. Guest J. E. The Distribution of Subsurface Water at Hadriaca and Tyrrhena Paterae and Surrounding Areas on Mars from Impact Crater Morphology It is well established that the surface of Mars exhibits abundant evidence for the presence of either liquid or frozen water during the course of Martian history. The origin, location, extent and transport of this water is of critical importance in the understanding of Martian geology and climate. In particular, the fluid appearance of rampart crater ejecta has been cited as evidence for subsurface ice at the time of impact. Ejecta morphology has proven to be a useful tool for studying the distribution of subsurface ice on Mars. It is possible that in some regions the concentration and distribution of subsurface ice has been affected by volcanic processes, either in the melting and/or mobilisation of existing subsurface water, and/or in the injection of juvenile water into the martian crust. The presence of water may also have affected the style of volcanic eruptions on Mars, increasing the volatile content of rising magmas and generating explosive activity. We are currently investigating the abundance and role of water in the evolution of the volcanoes Hadriaca and Tyrrhena Patera and surrounding highlands northeast of the Hellas Basin. The morphology of these volcanoes has been attributed to explosive volcanism, and to the presence of substantial amounts of water in the regolith at the time of their eruption. The location of Hadriaca Patera in a region containing channelled plains, debris flows, and pitted plains, together with the style of erosion of the volcano flanks suggests presence of volatile-rich surface materials or fluvial or periglacial activity. This work is a continuation of research undertaken by Cave in the Elysium Mons Region, where ice was found to be enriched at depth in the Elysium Lavas. We are performing a similar analysis for the volcanics of Hadriaca and Tyrrhena Paterae. A database containing information on the location, size, morphology, ejecta characteristics and degradation state of several hundred impact craters displaying ejecta in the region of Mars between the equator and 40 degrees S, and from 225 degrees to 275 degrees W is therefore being compiled. Craddock R. A. Chuang F. The Mechanism for Forming Martian Rampart Craters: Clues from Crater Morphometry Many large craters on the martian surface have ejecta blankets that terminate with a pronounced lobate ridge. These rampart craters represent features unique to Mars, and their morphology suggests that their ejecta was emplaced as a highly mobile fluid instead of ballistically. High velocity experiments into mud have duplicated many of the common morphologic characteristics of rampart craters supporting this hypothesis. However, additional experiments performed at a variety of atmospheric pressures have also been able to duplicate aspects of rampart crater morphology These results suggest that rampart craters form as the result of a low pressure atmosphere. Obviously the presence of ice or water would have an effect on the target properties (i.e., material strength) of the martian substrate, which should also influence the overall crater morphometry, thus allowing us to test these competing hypotheses. In this study we used the photoclinometric algorithm developed by Davis and Soderblom and written for the Planetary Image Cartography System to determined the shape of over 300 fresh lunar-like or rampart impact craters at a variety of diameters (3 to 80 km). Because it is probable that Ethology would also effect the strength of the target material, we confined our study to craters formed in the dissected unit of the Highland Plateau (Npld) as defined by the 1:15M-scale geologic mappers and interpretable to the scale of our images. In addition we confined our study to +30 degrees to omit variations in crater morphology seen at higher latitudes. The images used were radiometrically-calibrated (red and minus-blue filter), moderate resolution (~200 m/pixel) obtained by the Viking orbiters. Our initial results show that there is a slight difference in morphometric relations between fresh lunar-like and rampart craters, supporting the hypothesis that the unique morphology of rampart crater ejecta results from the incorporation of volatile materials contained in the target substrate. Jons H.-P. Progressive Chaotic Terrains in Volcanic Areas on Mars: Up to What Dimensions Can Such Features Grow? The spatial distribution of the largest martian volcanoes and concentrically arranged escarpments which embay those volcanoes indicate that progressive chaotic terrains and the depressions which they effect (i.e. the meltplains) can grow up to a diameter of more than thousand kilometers. Tuesday, March 19, 1996 MARS: EOLIAN PROCESSES POSTERS 6:30 - 9:30 p.m. LPI White B. R. Greeley R. Minimum Aeolian Particle Threshold on Mars Dust storms on Mars have long been known to exist. Theoretical predictions and previous wind-tunnel tests conducted at equivalent Martian pressure predict higher threshold wind speeds than were observed by the Viking Landers. We have carried out new preliminary wind-tunnel tests that suggest 10 micron-sized silt particles' threshold is strongly a function of aerodynamic "conditioning" of the surface and the state of the atmospheric stability (i.e., unstable or stable). The threshold wind speeds at a height of one meter ranged from a low of 34 m/s (U(sub)*T = 1.5 m/s) for an unconditioned surface and unstable atmosphere to a high of 85 m/s (U(sub)*T = 3.8 m/s) for a "conditioned" surface and neutral atmosphere. The conditioning refers to the level of compaction the surface has obtained. An "unconditioned" surface represents newly deposited particles that have aerodynamically settled onto an existing surface. A conditioned surface is one that has undergone several cycles of wind speed above and below threshold condition without having additional particles settling on the surface. Marshall J. R. Borucki J. Sagan C. Behavior of Windblown Sand on Mars: Results From Single-Particle Experiments Experiments are investigating the behavior of individual sand grains in the high-energy martian aeolian regime. Energy partitioning during impact of a saltating grain determines grain longevity, but it also influences the way in which the bed becomes mobilized by reptation. When single grains of sand are fired into loose beds, the bed can absorb up to 90% of the impact energy by momentum transfer to other grains; it has been discovered that the impacting grains cause circular craters even at low impact angles. Hundreds of grains can be splashed by a single high-velocity (100 m/s) impact causing more bed disturbance through reptation than previously thought. The research is supported by NASA's PG & G Program. Anderson F. S. Lo E. Greeley R. Global Sand Transport and Distribution on Mars The presence of dunes on Mars shows that saltating sand plays an important role in the evolution of the planet's surface. Although previous work focused on dust transport, numerous studies demonstrate that sand is substantially easier totransport by wind. Indirect evidence suggests that saltation may be currently active; however, definitive evidence is lacking. An improved iterative sand transport algorithm was developed based on an earlier model and use of an improved Mars General Circulation Model. The geometry of sand erosion and deposition are dependent on many factors, including the stress threshold required to move sand, sand sources, and potential sand trapping. The new sand transport model takes these factors into account and can be used to predict areas of sand erosion or accumulation. Results are then compared to observed surface features, such as dune fields, sites of dust storm locations, and properties of the surface derived from Viking IRTM data. Runs indicate: 1) net sand transport is from the northern hemisphere to the southern hemisphere, 2) northern plains erosion is prevalent, and 3) sand accumulates in the north polar region. Wennmacher A. Neubauer F. M. Patzold M. Schmitt J. Schulte K. A Search for Dust Devils on Mars A thorough search through the Viking orbiter images (PDS CD-ROM archive) of Mars for dust devils was conducted in support for the High Resolution Stereo Camera-Surface Atmosphere Interaction (HRSC-SAI) experiment on the Russian Mars-96 spacecraft. So far we focussed on the martian areas Arcadia Planitia and the two Viking lander sites where the occurence of dust devils has already been reported (by imaging or from meteorological data, respectively). Additional events were found which were not yet reported elsewhere. The prime parameters (e. g. height, diameter etc.) confirm the theoretically predicted trends. Metzger S. M. Lancaster N. Terrestrial Dust Devils; Implications for the Surface of Mars Wind action is one of the most active geomorphic processes presently at work on Mars. Dust devils appear to play a considerable role in the redistribution of fine particulates and have been cited as a likely trigger for the great dust storms that sweep that planet. On-going terrestrial analog studies conducted in southem Nevada have examined the surface conditions and aerodynamic properties which permit the formation of dust devils. This leads to certain expectations of the ground conditions to be found where Martian dust devils have been imaged (and other areas that act as dust sources, by implication of the freeconvection model for Dianet-encirclina dust storms. Tuesday, March 19, 1996 OUTER PLANET SATELLITES POSTERS 6:30 - 9:30 p.m. LPI Metzger S. M. Geomorphic Tests of the Geyser and Dust Devil Models for Triton's Plumes The Voyager Neptune Encounter in 1989 revealed plumes rising over Triton's southern hemisphere with diameters that range between tens of meters and 1.5 km to an altitude of 8 km whereupon their plumes are blown at least 100 km to the west. The plumes, in conjunction with dark surface streaks, have been explained as either nitrogen geysers developed by a solid-state greenhouse mechanism in the polar nitrogen ice or as dust devil thermal vortices forming over and entraining material from darker surface patches. This examination explored both models from a geomorphic perspective to identify and test relevant nrcdictions of each mechanism that should be visible in available imagery. Garry J. R. C. An Analysis of the Mapping Sonar on the 'Huygens' Probe to Titan As part of the Cassini mission to the Saturnian system a probe will be deployed in 2004 onto the surface of Titan. This probe, named Huygens, carries a set of acoustic devices to investigate the moon's atmosphere and surface. One part of the sensor package is an active sonar which is expected to yield topographic information about Titan's surface. A computer code has been written to model the sonar's operation over synthetic boundaries with known statistical properties. By examining the dependence of these simulated echoes on the surface properties an estimate may be made of the precision with which the landscape of Titan can be known. Gomez D. Prieto O. Rodriguez M. A. Strike Slip Tectonics on Enceladus One of the most surprising characteristics of Enceladus is its intense tectonic activity on its surface. This tectonic activity can be seen as faults and ridges that deform the ancient craterized terrains and generate the actual topography. The main structure that defines the visible tectonics of the satellite is Samarkanda Sulci, which it has been interpreted as a extensional rift but we have reinterpreted as a strike slip zone. The slip has a sinistral movement and a deduced NNW-SSE direction of compression. The changes in the orientation of the fault plane generate directional duplex, some trans-tensional and others trans-pressional. Tufts B. R. A San Andreas-sized Strike-Slip Fault on Europa Truncated linear features abutting opposite sides of an unnamed 800km gray band in Europa's southern hemisphere can be aligned by a left-lateral displacement of 42km. This is nearly identical to the displacement required to "close" a rhomboidal feature at the north end of the band. The offset was determined using orthographic reprojections of Voyager image 1372J2-001. While this gray band has been previously noted in the literature this is the first reported measurement of displacement on it. Based on this reconstruction, the gray band can be interpreted to be a dextral strike-slip fault in which the rhomboidal portion occupies a releasing bend. Extending from at least 80 degrees S, 269 degrees W to 60 degrees S, 192 degrees W, the fault is comparable in length to the San Andreas fault between San Francisco and Mexico. If scaled to the size of the Earth it would be 3250 km long, two and one-half times the total extent of the San Andreas. This strike-slip fault has implications for Europan crustal constitution and geologic history. Its existence is another demonstration that the Europan crust has been deformed by lateral motion, possibly facilitated by a subsurface decoupling layer. Pappalardo R. Coon M. D. A Sea Ice Analog for the Surface of Europa The surface of Europa is crossed by bands and ridges of a variety of specific morphologies. Reconstruction of gray and wedge-shaped bands indicates that they formed through opening and separation of lithospheric blocks atop a liquid or ductile substratum, perhaps broadly analogous to processes that operate in terrestrial sea ice. Furthermore, theoretical models of thermal balance suggest that a liquid water ocean may underlie the icy surface of Europa. We have begun investigation of whether the landforms of Europa can be explained through analogy to terrestrial sea ice processes. We propose a sequence in which cyclical tensional and compressional stresses (predicted from nonsynchronous rotation) open and close lithospheric scale ice plates to produce dark bands, triple bands, and ridges. Granahan J. C. Polk K. S. Fanale F. P. McCord T. B. Spectral/Image Data Cubes of Europa, Ganymede, and Callisto A set of spectral image cubes of Europa, Ganymede, and Callisto have been constructed from Voyager mission images in an effort to prepare for the Jovian system portion of the Galileo spacecraft mission. Unfortunately, due to the fact that this spacecraft is presently restricted by an optimal 40 bits per second downlink and a finite onboard tape data storage many general observations of these moons will be restricted. Galileo imaging experiments like the Solid State Imager (SSI) and the Near Infrared Mapping Spectrometer (NIMS) will be restricted to making specific targeted observations due to their data intensive usage of spacecraft downlink and tape storage. These spectral image cubes are designed to provide complementary spectral and spatial coverage of Europa, Ganymede, and Callisto utilizing up to date control network measurements and photometric correction algorithms. Head J. W. Pappalardo R. T. Solid State Imaging Team Galileo Ganymede: Nature of Dark Terrain and Processes of Formation and Evolution Analysis of Voyager images has led to a broad understanding of several aspects of the geology of Ganymede but there is considerable uncertainty about the nature, formation, and evolution of the dark terrain, which comprises about 50% of the surface of the planet. Here we examine the major issues, focus on specific questions that need to be resolved, and highlight areas under study and targeted by the Galileo Solid State Imaging System to help resolve these issues. Greeley R. Bender K. Sullivan R. Pappalardo R. Homan K. Galileo SSI Observation Plans for Callisto and Europa On December 7, 1995, the Galileo spacecraft successfully entered orbit around Jupiter, beginning its two-year mission to explore the planet's atmosphere, family of satellites, ring system, and magnetospheric environment. Here we present an update to previously published work of the solid-sate imaging (SSI) plans for observing Callisto and Europa during Galileo's two-year mission. Due to a problem with the on-board tape-recorder during Jupiter approach, a unique 700 m/pixel mosaic of the south pole of Europa was canceled, along with high resolution imaging of Io. As a consequence of the tape recorder anomaly and revised rules for its use, approximately 16% less tape capacity will be available for storing SSI images and other data throughout the mission. Smythe W. Lopes-Gautier R. Davies A. Carlson R. Soderblom L. Galileo NIMS Team Galileo's Near Infrared Mapping Spectrometer (NIMS) Science at Io: Objectives, Plans, and Predictions One of the primary objectives of the Galileo mission is to investigate the nature of Io's dynamic volcanism. The Near Infrared Mapping Spectrometer (NIMS) is a remote sensing instrument aboard Galileo which combines imaging and spectral capabilities and will be used to map the composition and temperature of Io's surface and plumes. NIMS has the unique capability to image and obtain spectral data simultaneously and is the first instrument of its kind to be flown in a planetary mission. The spectral range of NIMS is from 0.7 to 5.2 microns, which spans two regions: surface reflected light and emitted thermal radiation. NIMS will be able to investigate both the surface mineralogy and the temperature of Io's surface features. Howell R. R. Moustakas C. Analytical Models of Lava Flows at Ulgen, Amaterasu, and Loki Patera Analytical models of the thermal emission from silicate lava flows on Io are being extended to study the Voyager IRIS spectrum of Ulgen Patera, Amaterasu Patera, and also ground-based measurements of Loki during some of its brightenings. Previous work had found that the models provide a very good fit to the thermal emission of Loki, and provide insight into the different characteristics of the activity at Pele. The new model fits show that the activity at Ulgen is similar in character to Loki, but at much reduced intensity. Furthermore a pair of Ulgen spectra with slightly different fields-of-view allows us to eliminate assumptions concerning the background emission from the passive surface, and allow a more rigorous test of the model. Betts B. H. Nash D. B. Implications of Laboratory Infrared Reflectance Spectra for Finding H2S on Io Does H2S exist on Io's surface? We believe this is an open question with significant implications for Io's volcanic and surface processes. Current evidence for its presence depends most upon infrared bands detected from Earth that overlap not only with H2S fundamentals, but also with bands seen in SO2 ice. Thus, to better prepare for Galileo observations of IO, and as part of a broader look at H2S infrared spectra and Io, we present H2S spectra in the range 2.3-5.2 micrometers and compare them with the known Io surface constituent, SO2. In particular, we combined our lab H2S spectra and SO2 spectra with the approximate Galileo-NIMS spectral bandpasses and channel separations to create simulated NIMS spectra of H2S and SO2 frost surfaces. The 2.7 micrometers H2S band (vl + v2 and/or V3 + v2) is the most significant and unique band for distinguishing H2S from SO2 in this spectral range, but the shape and size of the extremely strong 3.9 micrometers band (vl and V3 stretching fundamentals) will be useful in determining the thickness of H2S deposits. Thin (0.1 to 1 mm) H2S frosts create deep bands relative to much thicker SO2, so on IO, NIMS should be able to distinguish very thin deposits of H2S from thicker or intermingled SO2 deposits. Tuesday, March 19, 1996 ATMOSPHERIC PHENOMENA POSTERS 6:30 - 9:30 p.m. LPI Kasting J. F. Whittet D. C. B. Sheldon W. R. The Effect of Ultraviolet Radiation on Planetary Habitability Climatic constraints on planetary habitability, specifically the requirement of liquid H2O oceans, provide a definition of the habitable zone around main sequence stars with spectral types in the early-F to mid-K range. However, it has not been demonstrated that planets orbiting such stars would have habitable surfaces when biologically-damaging energetic radiation is also considered. The large amounts of ultraviolet (UV) radiation emitted by early-type stars have been suggested to pose a problem for the evolution of life in their vicinity. It has also been suggested that a significant problem is posed by late-type stars which emit proportionally less radiation at the short wavelengths (lambda < 200 nm) required to photolyze O(sub)2, an essential step in ozone (O(sub)3) formation. The presence of O(sub)3 in a planetary atmosphere is the only shield from UV radiation in the wavelength range 220-320 nm which is capable of inflicting serious damage to organisms on Earth, and presumably elsewhere in the Universe; other components of planetary atmospheres absorb negligibly in this UV range. Here we present detailed calculations of the UV spectrum at the surface of a planet with an Earth-like atmosphere and orbiting three types of main sequence stars, F, G and K within the habitable zone, based on climactic constraints. We conclude that neither of the concerns regarding UV radiation expressed previously should be necessarily fatal to the evolution of advanced life: Earth-like planets orbiting F and K stars are shown to receive less harmful UV radiation at their surfaces than is the case for the Earth. Williams D. M. Kasting J. F. Habitable Planets with High Obliquities The obliquities of the terrestrial planets have been shown to vary chaotically and by large amounts in times less than 10 Myr, thus inviting the possibility for Earth to occasionally reach high obliquity where it might experience climatic conditions unfavorable for life. Although Earth escapes this fate by having its rotation axis stabilized by the Moon, many extrasolar Earth-like planets without large satellites should be subjected to periods of high obliquity. The number of worlds supporting life outside the Solar System, then, may be far fewer than has been suggested if high obliquities render moon-less Earths uninhabitable. Climates at high obliquity are particularly harsh on middle and high latitude continents that warm and cool rapidly in response to large insolation swings. These areas exhibit a wide range of temperatures over a seasonal cycle, with extremes reaching well above or below 273 Kelvin, making them seasonally unsuitable for water-dependent life. Marty B. Zimmermann L. Humbert F. Nitrogen Isotopic Composition of the Silicate Earth and its Bearing on Earth-Atmosphere Evolution The isotopic composition of terrestrial nitrogen in the silicate Earth is not well known, which precludes quantitative treatment of the atmospheric nitrogen evolution. In order to document it, we have performed a static mass spectrometry analysis of rare gas and nitrogen contents and isotopic ratios in mantle-derived samples. Gases were extracted by stepwise crushing of oceanic basalt glasses, a procedure which minimises the contribution of nitrogen added to the samples after lava eruption. All data can be explained by mixing between a surface component (40Ar/36Ar ratios close to the atmospheric value and delta 15N values close to 0 per mil, the atmospheric composition by definition, or slightly positive) and a mantle component with 40Ar/36Ar ratios up to 42,000 and delta 15N = -4.5 +/- 0.5 per mil. The mantle component is also characterised by a N2/40Ar +/- ratio (where 40Ar has been produced during Earth's history by the decay of 40K) comparable to the atmospheric N2/40Ar ratio, but by a N2/36Ar ratio (where 36Ar is a primordial isotope of argon) two orders of magnitude higher than the corresponding air ratio. These observations are used to constrain the early state of the terrestrial atmosphere : nitrogen in the primitive atmosphere was probably enriched in 15N, with a lower limit for its delta 15N value of +20 per mil. Ponganis K. V. Graf Th. Marti K. Isotopic Fractionations by Low-Energy Ion Implantation: A Possible Solar System Process The isotopic signatures of the noble gases found in planetary atmospheres compared to those in the solar wind have been considered diagnostic of early and ongoing solar system processes. Model explanations for the evolution of terrestrial planets' atmospheres have been numerous, but largely ignored until recently has been the process of low-energy ion implantation. Although low-energy ion implantation was suggested as a factor influencing the evolution of solar system inventories, little work has been carried out regarding the magnitude and relevance of the process, and none was done for Ne or Ar. We confirmed observations by Bernatowicz and Hagee that Kr and Xe implanted in W are isotopically fractionated by 1% to 2% per amu. We further demonstrated that argon is fractionated > 3% per amu; here we report large fractionations for Ne and Ar. The identification of suitable conditions for changing isotopic and elemental signatures of nobles gases is a first step in a search for natural environments where low-energy implantation may have occurred. Tuesday, March 19, 1996 REFLECTANCE THEORY/SPACE WEATHERING: EXAMPLES FROM THE MOON POSTERS 6:30 - 9:30 p.m. LPI Withey P. A. Nuth J. A. Coagulation of Very Small Iron Grains With 'Web-Like' Structures We report the results of a further series of experiments on 20-50 nm-sized iron particles condensed from the gas phase in a condensation flow apparatus. These iron particles coagulate efficiently to form very long chains producing a "web-like" structure at the macroscopic level. It is discovered that these pure-iron chains and webs form even in the presence of weak ambient magnetic fields (<0.2 gauss). Remanance acquisition and alternating field demagnetization studies reveal that large magnetic fields are required to significantly alter the magnetization of these iron webs indicating that they are very magnetically hard. Iron particles in protostellar nebulae may have coagulated similarly and could have aided in the collection of surrounding nonmagnetic material to form even larger grains. We suspect that these particles should be efficient absorbers of microwave radiation and may be seen in the long-wavelength spectra of protostellar nebulae. Pieters C. M. Plagioclase and Maskelynite Diagnostic Features Reflectance spectra of several lunar and terrestrial crystalline plagioclase samples and a maskelynite separate from a lunar meteorite have been acquired. All the plagioclase samples exhibit the well-known crystal field (CF) absorption near 1.3 micrometers. Maskelynite, a diaplectic glass formed from highly shocked plagioclase, exhibits two features similar to those of homogeneous glass from octahedral and tetrahedral absorptions near 1.0 and 1.9 micrometers, respectively. MGM analyses were performed to evaluate these absorption features quantitatively. As expected, the plagioclase crystal field absorptions near 1.3 are well fit by a single absorption band with MGM. Additional features frequently appear in the visible and near infrared for plagioclase samples, but are currently unassigned. The two maskelynite features, however, are not well fit with only two bands as would be expected for glass. A third band is required with properties similar to the plagioclase band, suggesting the presence of a minor remnant plagioclase structure. Shelfer T. D. Morris R. V. Nguyen T. Q. Agresti D. G. Wills E. L. Prototype Backscatter Mossbauer Spectrometer (BaMS); Instrumentation Summary and Conclusions We have completed work on development of a miniature research-grade backscatter Mossbauer spectrometer (BaMS). The BaMS instrument, built completely from scratch, provides (non-destructively and with no sample preparation) high-quality Mossbauer spectra from which the distribution of Fe among both its oxidation states and its mineralogies can be calculated. A flight-qualified version of this design would provide future missions to the surfaces of the Moon, Mars, asteroids, and other solid-surface solar-system objects, with a highly-selective mineralogical tool. Kochan H. Feibig W. Kretschmer M. Willnecker R. Gerasimov M. V. Speth B. Kohler U. Weckesser S. The New Planetary Simulation Facility of DLR--Construction, Operation, Application A promising way to study physical phenomena on planets, moons, and comets is the experimental simulation in a laboratory on Earth. In the former, meanwhile disassembled DLR Space Simulator, experiments only could be performed under vacuum conditions. The new simulation chamber, 1.5 m in diameter and 1.8 m high, is constructed as a Dewar vessel with two walls and an isolating vacuum between and can be operated with different atmospheres at variable pressures. In addition the liquid nitrogen (LN2) operated cooling system will not only support experiments at around 77K. After the cooling-down phase with full power, the cooling system is operated in an intermittent (pulsed) mode, opening experimental conditions over a wide temperature regime. This allows e.g. Mars-simulation experiments with a CO2- atmosphere, as well as the realization of cometary vacuum conditions. The chamber will be built in the DLR, Institute for Space Simulation, Koln, in the second half of January 1996 Tuesday, March 19, 1996 COMETS AND ASTEROIDS POSTERS 6:30 - 9:30 p.m. LPI Zeitler W. Oberst J. Giese B. A Close Look at the Locations of Ejecta Blocks on the Surface of the Asteroid Ida We determined high-resolution Digital Terrain Models (DTMs) of the asteroid Ida using photogrammetric analysis of Galileo SSI stereo images to study the locations of ejecta blocks in detail. This beginning study of topography and surface slopes in the neighborhood of the blocks may reveal the origin of these objects. Banerdt W. B. Pike T. Martin R. Lognonne P. A Seismic Investigation of the Interior of a Comet Seismology can be a powerful method for investigating the inner structure and dynamic processes of a comet. Whereas remote sensing and in situ experiments (with the exception of electromagnetic sounding) can only sample the outermost skin of a comet, seismic waves can potentially probe the entire volume of the body in detail. We have proposed an experiment, which has been provisionally selected, to detect and analyze the seismic activity of a the comet P/Wirtanen using the RoLand Surface Science Package of the Rosetta Mission. This novel extension of seismology to comets will provide a unique opportunity to study the dynamic activity and probe the deep internal structure of a member of this fascinating and poorly known class of objects. Bruckner J. Masarik J. Planetary Gamma-Ray Spectroscopy of a Comet Surface Planetary gamma-ray spectroscopy is a very capable tool to explore the surface of a comet nucleus. The gamma rays emitted from the surface carry qualitative and quantitative information on many elements present in the surface. We studied the gamma-ray emission of comets with different chemical compositions and surface geometries. Monte Carlo calculations were performed to simulate the cosmic-ray interactions with the comet surface and the subsequent emission of gamma rays. Our calculations show that the evaluation of gamma-ray spectra provide fairly accurate information on concentrations of all major elements. The planned gamma-ray measurements that will be carried out during the ESA comet mission Rosetta, will contribute to the detailed investigation of the surface of a comet and in conclusion to its history and origin. Wednesday, March 20, 1996 REMOTE SENSING OF THE MOON AND MERCURY: LABORATORY MEASUREMENTS AND CLEMENTINE, GALILEO, MARINER AND KUIPER OBSERVATIONS 8:30 a.m. Room A Chair(s): P. G. Lucey S. Tompkins Lucey P. G. Blewett D. T.* Johnson J. L. Taylor G. J. Hawke B. R. Lunar Titanium Content from UV-VIS Measurements Recently a technique for deriving the Fe content of the lunar surface from spectral measurements in the extended visible wavelength range was developed and applied to images obtained by Clementine cameras [Lucey et al. (1995) Science 268, 1150.]. In concert with that effort, we have been re-examining the effects of Ti content on the spectral properties of a group of lunar soil samples. Insight gained from the study of the soil spectra has suggested a method for calibrating lunar imaging data sets to TiO2. Unlike the previous "Charette relationship", the new method for determination of Ti is insensitive to Fe, and thus is applicable to much lower Ti contents. This allows Ti-mapping to be extended to the lunar highlands and low-Ti maria. TiO2 images have been prepared from Galileo and Clementine data. Clark P. E.* McFadden L. A. The Lunar Crust as Case Study: A Comparison of Iron Data Derived From Geochemical and Mineralogical Remote Sensing Techniques The abundance and distribution of iron on planetary surfaces has been of great interest to planetary scientists, because such data provide important constraints for models of planetary and solar system evolution. A variety of global iron abundance maps have been published for the Moon, the only planet to date for which compositional data derived from both chemical and mineralogical remote sensing instruments are available. Such maps, derived from different instruments and methodologies, have shown some striking differences. This problem becomes particularly important to resolve now in preparation for the encounter with asteroid 433 Eros by the NEAR mission, which will include both X-ray and gamma-ray spectrometers, from which iron abundances are derived directly, as well as visible and near infrared imaging spectrometers, from which iron-bearing mineral abundances are derived and iron abundances can be inferred. Tompkins S.* Pieters C. M. Ryder G. Spectral Variations Among Crystalline and Glassy Apollo 17 Impact Melt Rocks Remote geochemical mapping of lunar impact melt associated with complex craters is a key to better understanding of impact melt formation and the impact cratering process. Ground-based NIR spectra and Clementine multispectral images comprise a dataset with high-resolution spectral and spatial information, respectively. To interpret these data with respect to impact melt, two suites of lunar samples are being measured in NASA's Reflectance Experiment Laboratory (RELAB) at Brown University. The samples include seven Apollo 17 crystalline impact melts as well as synthesized glass equivalents, and 11 naturally occurring impact glasses from several landing sites. Acquisition and analysis of the laboratory spectra is the first step towards quantitatively modeling remote spectra to learn more about the composition and source of the melt found at complex craters. Furthermore, from the laboratory spectra, basic compositional parameters may be determined for application to Clementine data. Initial measurements of the Apollo 17 melt rocks have been completed and are presented below. Nozette S.* Lichtenberg C. L. Spudis P. Bonner R. Ort W. Malaret E. Robinson M. Shoemaker E. Clementine Bi-Static Radar Experiment: Preliminary Results The Clementine mission included a series of ad hoc bistatic radar experiments to search for ice in the lunar polar regions. Water ice and/or other cold-trapped volatiles are expected to exhibit backscatter enhancement and a high ratio of same sense (transmitted) to opposite sense polarization in the reflected radar signal, as the bi-static angle Beta approaches 0. This is due to volume scattering possibly combined with an associated Coherent Backscatter Opposition Effect (CBOE). Bi-static radar observations were made on seven Clementine orbits, four at the south pole (orbits 234,235,236,237) and three at the north pole (orbits 299,300,301). The magnitude and circular polarization ratio of the radar echo as a function of Beta was measured for selected lunar target areas. Pinet P. C.* Martin P. Costard F. Chevrel S. Daydou Y. Johnson P. E. Aristarchus Plateau: Clementine Spectro-Imaging and Geological Inferences A detailed remote sensing survey of the Aristarchus Plateau has been made in the UV-visible- near infrared domain by means of earth-based telescopic and Clementine CCD spectro-imaging techniques and the distribution of the main types of materials is proposed on the basis of their spectral characteristics. Following a preliminary investigation using a principal component (PCA) analysis, a spectral mixture analysis, based on a multiple mixing modeling technique, is presented here. It gives a comprehensive description of the distribution and amount of surface mixing of the units present at Aristarchus Plateau and in its immediate vicinity. Terazono J.* Takizawa Y. Haruyama J. Ohtake H. Kumita K. Matano Y. Kanbara H. Sanga T. Kouda R. Surface Geology Analysis of the Moon Using Data Obtained by Clementine We studied the lunar surface geology and mineralogy with using image maps which we made from the Clementine image data released by NSSDC (National Space Science Data Center). We could discriminate geological differences of the area that covers the Apollo 17 sampling sites. Our investigation using the Clementine image data will contribute to future lunar missions. Oberst J.* Wahlisch M. Zhang W. Roatsch T. Cook A. C. Jaumann R. New Data on Lunar Topography Derived from Galileo and Clementine Stereo Images We analyzed images, obtained by the Galileo and Clementine spacecraft in 1992 and 1994 using state-of-the-art photogrammetric techniques to derive Digital Terrain Models (DTMs) for selected lunar regions. These regions include: the near-side northern hemisphere, parts of Mare Orientale, and the Alpine Valley. The new topographic data allow us to study the morphology of the Moon, in particular large craters and the multi-ring impact basins in unprecedented detail. Kiefer W. S.* Dodge M. C. Uncompensated Mare Basalts as a Model for Lunar Mascons Lunar mascons are regions of positive gravity anomalies over topographic basins. First discovered by Doppler tracking of Lunar Orbiter 5, mascons are among the most prominent features in the lunar gravity field. Mascons are found at many of the near-side, circular mare basins, including Crisium, Humorum, Imbrium, Nectaris, Orientale, and Serenitatis. Acquisition of new, high resolution gravity and topography data sets by the Clementine spacecraft allows a new look at the mass distributions that produce these gravity anomalies. We show that a model with uncompensated mare basalt fill and Airy compensated basement (sub- basalt fill) topography can explain the observed mascon gravity anomalies. Moore J. M.* McEwen A. S. The Abundance of Large, Copernican-Age Craters on the Moon Recent studies have brought into question the size-frequency distribution of Copernican-age lunar craters. In particular, several large near-side craters that have been mapped as Copernican in the compilation of Wilhelms are probably older, on the basis of the size-frequency distribution of superposed craters. The craters were mapped as Copernican at least in part because of the presence of bright rays. However, the rays are only apparent over dark mare terrains, and are probably due to highlands materials excavated and deposited over the maria rather than being due to immature soils. The geologically-mapped population of large Copernican craters has also been suspect due to the apparent deficiency of these craters on the far side when compared with the near side. The far side contains very little maria, and provides a more uniform canvas to record bright-rayed craters. With the recent receipt of high-resolution and high-sun multispectral images of the far side by Clementine, it is now possible to re-evaluate the size-frequency distribution of far-side rayed craters, which are of likely Copernican age. We find a systematically lower density of far-side Copernican craters than the previous whole-Moon counts, especially at the larger sizes. We find that several large craters were previously misidentified as having bright rays due to the superposition of smaller rayed craters. These results have implications for the age of Venus' surface and the frequency and severity of impact-induced extinctions and climate changes on the Earth. Cook A. C.* Hiesinger H. Preliminary Analysis of Clementine Imagery of the Alpine Valley The lunar Alpine Valley (2W-8E, 47N-51N) is a 180 km long, 6-14 km wide, lava flooded breach in the highlands between Mare Imbrium and Mare Frigoris. Along most of its length lies a sinuous rille. We utilize Clementine stereo images to derive a new digital elevation model (DEM) of the valley, and also perform a comprehensive analysis of Clementine multispectral imagery. We present some of our preliminary findings in this abstract. Margot J. L.* Campbell D. B. Campbell B. A. Butler B. J. Lunar Dielectric Constants from Radio Thermal Emission Measurements Thermal emission from the Moon at 21 cm was measured in all four Stokes parameters with the Very Large Array (VLA). The polarization properties of the emission are determined primarily by the dielectric constant of the regolith material, the fraction of the emergent radiation that is diffused, and the surface roughness on scales larger than the wavelength. Estimates of the dielectric constant were obtained at ~90 km resolution and the smooth sphere values range from ~2 to ~4. Results are illustrated for the Crisium area where the mare regions have dielectric constants ~2.7 and highland regions have values of ~2.4. A map of the dielectric constant shows that the various regions correlate well with 70 cm radar data. Using a facet model, it is shown that the observed variations around Crisium cannot be accounted for by roughness at very large scales. Previous estimates of lunar rms surface slope are used to provide absolute measurements of the dielectric constant. For negligible diffusion of the emission by wavelength-scale structure at the surface, the values of the dielectric constant corrected for roughness are ~2.7 for Mare Crisium and ~2.5 for the surrounding highlands. These variations could be explained by near-surface density changes (mare density ~1.52 gcm^-3, highlands density ~1.41 gcm^-3), where the dichotomy is consistent with heavier basaltic materials dominating the mare regolith. Sprague A. L.* Witteborn F. C. Kozlowski R. W. H. Wooden D. H. Mid-Infrared Spectroscopy of Mercury from the Kuiper Airborne Observatory We present mid-infrared (5 - 10 \mic) spectroscopic measurements of the planet Mercury obtained from the Kuiper Airborne Observatory (KAO) using the High Efficiency Infrared Faint Object Grating Spectrograph (HIFOGS). Spectra show features characteristic of plagioclase feldspar that was previously observed near 120\deg \ mercurian longitude. The spectra also show spectral features that could be interpreted indicative of the presence of pyrrhotite (\pyrr). An analysis that fully accounts for the effects of large field of view (FOV), thermal gradients, rough surface and absolute calibration is still underway. Robinson M. S.* Lucey P. G. Calibrated Color and Albedo Maps of Mercury In order to determine the albedo and color of the mercurian surface, we are completing calibrated mosaics of Mariner 10 image data. A set of clear filter mosaics is being compiled in such a way as to maximize the signal-to-noise-ratio of the data and to allow for a quantitative measure of the precision of the data on a pixel-by-pixel basis. Three major imaging sequences of Mercury were acquired by Mariner 10: incoming first encounter (centered at 20S, 2E), outgoing first encounter (centered at 20N, 175E), and southern hemisphere second encounter (centered at 40S, 100E). For each sequence we are making separate mosaics for each camera (A and B) in order to have independent measurements. For each mosaic, regions of overlap from frame-to-frame are being averaged and the attendant standard deviations are being calculated. Due to the highly redundant nature of the data, each pixel in each mosaic will be an average calculated from 1-10 images. Each mosaic will have a corresponding standard deviation and n (number of measurements) map. A final mosaic will be created by averaging the six independent mosaics. This procedure lessens the effects of random noise and calibration residuals. From these data an albedo map will be produced using an improved photometric function for the Moon. A similar procedure is being followed for the lower resolution color sequences (ultraviolet, blue, orange, ultraviolet polarized). These data will be calibrated to absolute units through comparison of Mariner 10 images acquired of the Moon and Jupiter. Spectral interpretation of these new color and albedo maps will be presented with an emphasis on comparison with the Moon. Wednesday, March 20, 1996 SHERGOTTITES: CRYSTALLIZATION, WEATHERING, AND EJECTION 8:30 a.m. Room B Chair(s): T. J. McCoy M. M. Grady Harvey R. P.* McCoy T. J. Leshin L. A. Shergottite QUE 94201: Texture, Mineral Compositions, and Comparison with Other Basaltic Shergottites The basaltic shergottites (Shergotty, Zagami and EET A79001) exhibit evidence for complex, multi-stage petrogenesis involving a range of pressures and cooling histories, as well as complex physical processes occurring in near-surface magma bodies. The recently described 12.0 gram shergottite QUE 94201 provides an opportunity to gain further insights into this petrogenesis. The description of QUE 94201 as a coarse-grained mixture of subequal pyroxene (pigeonite, Fs(sub)21-69) and maskelynite suggests similarities to the dark, mottled lithology of Zagami described by McCoy et al, 1995. Our examination of QUE 94201 (PTS 4, 6 and 38) confirms the similarity to the dark, mottled lithology of Zagami. Both are coarse-grained, FeO-rich lithologies with pockets of late-stage phases (e.g., phosphates, Fe,Ti-oxides, fayalite-SiO2 intergrowths; essentially identical to the DN pockets of McCoy et al, 1993). However, QUE 94201 crystallized from a melt more fractionated than Zagami. McSween H. Y. Jr.* Eisenhour D. D. QUE94201, A Noncumulate Shergottite? Basaltic shergottites were initially described as cumulates, based on strong preferred orientation of pyroxene grains and non-cotectic compositions. More recent studies have interpreted these rocks as flows that contained some entrained pyroxene crystals; however, the homogeneous magnesian pyroxene cores are assumed to have grown at depth from a larger body of magma, and so in this sense were "cumulus." Although the proportion of cumulus cores has been a subject of controversy, the basic model for their formation has not been questioned (except by who described irregularly zoned pigeonite cores in Zagami). The newly recovered Antarctic shergottite QUE94201 appears not to contain any cumulus crystals. Wadhwa M.* Crozaz G. QUE94201: A New and Different Shergottite The newly found Antarctic shergottite, QUE94201, provides the opportunity to study yet another putative sample from Mars. According to, it appears to be petrographically similar to the Zagami dark mottled lithology (DML) described by. However, observed in this meteorite growth features not previously recognized in pyroxenes of other shergottites. Unlike the homogeneous blocky cores commonly found in shergottite pyroxenes, they described "pyroxene cores consisting of nuclei of magnesian pigeonite, mantled by magnesian augite which, in turn, are rimmed by ferroan pigeonite, strongly zoned to pyroxferroite". In this study, we report ion microprobe measurements of REEs and other selected trace and minor elements in whitlockite (the major REE carrier in shergottites), maskelynite, and pyroxene. Relying on the extensive electron probe data set of, we analyzed 22 spots representing the range of pyroxene compositions. Mikouchi T.* Miyamoto M. McKay G. A. Mineralogy and Petrology of New Antarctic Shergottite QUE94201: A Coarse-Grained Basalt With Unusual Pyroxene Zoning QUE94201 is a new Antarctic shergottite. Although its mineralogy is similar to previously known basaltic shergottites, QUE94201 (QUE) has distinct differences from them. QUE has larger grain sizes and contain abundant maskelynite, whitlockite and a wide impact melt vein enriched in P. The coarse-grained texture of pyroxene and maskelynite is similar to lunar meteorite Asuka881757. The largest difference of QUE from typical shergottites is pyroxene zoning trends which are very similar to pyroxenes of Apollo lunar mare basalts such as 12021. However, the low FeO/MnO ratio of pyroxene and existence of Na-rich maskelynite distinguish QUE from lunar basalts. Cooling rate calculations of pigeonite rim zoning constrain the cooling rate to be faster than 5 degrees C/yr (Burial depth: <30 m). This indicates the last stage of crystallization would occur in a shallow lava flow. McKay G.* Yang S.-R. Wagstaff J. Complex Zoned Pyroxenes in Shergottite QUE 94201: Evidence for a Two-Stage Crystallization History QUE 94201 (Q) is a new Antarctic shergottite that is petrographically similar to other basaltic shergottites. As with them, clinopyroxene was the first mineral to crystallize, and thus likely contains clues to the composition of the parent melt and details of the crystallization history. We have used the elemental mapping capability of our new Cameca SX100 electron microprobe to study details of the pyroxene zoning patterns in this sample, and have inferred from them that Q underwent a dramatic change in growth regime during pyroxene crystallization, possibly through migration from a magma chamber to a shallow intrusion or lava flow, as suggested for Zagami by McCoy et al. Kring D. A.* Gleason J. D. Hill D. H. Jull A. J. T. Boynton W. V. QUE94201, a New Martian Meteorite that May Represent a Bulk Melt Rather than a Cumulate Fraction QUE94201 is a new mafic achondrite dominated by pyroxene and plagioclase. Petrologic and geochemical analyses of a bulk chip (,16) and thin-section (,7) indicate the sample is a basaltic gabbro that is related to previously described basaltic and lherzolitic shergottites of suspected martian origin. However, unlike the cumulate fractions represented by other shergottites, QUE94201 is a plutonic rock that fractionally crystallized in what appears (petrographically) to be a closed system. QUE94201 contains more Fe-rich pyroxene and more bulk Al, Ti, and P than other shergottites, including lithology B of EETA79001, and thus appears to be more evolved. However, QUE94201 is also more LREE-depleted than other shergottites which suggests it has not assimilated as much of the LREE-enriched mantle component thought to have affected the compositions of other shergottites. Mittlefehldt D. W.* Lindstrom M. M. Martian Meteorites QUE 94201, An Unusual Basalt, and Governador Valadares, A Typical Clinopyroxenite: Geochemistry We have begun a geochemical study of the martian meteorites Governador Valadares and QUE 94201. As expected based on petrology, Governador Valadares is a typical martian clinopyroxenite, very similar in composition to Lafayette and Nakhla. Quite unexpectedly, the new martian meteorite QUE 94201 is unusual in composition. In general geochemical characteristics, it is most similar to EETA79001 lithology B. However, QUE 94201 exhibits more extreme geochemical fractionations. QUE 94201 is extremely LREE depleted, with La/Sm ~0.1 times CI, but shows little fractionation in the MREE to HREE (Sm/Yb ~0.7 times CI). QUE 94201 is enriched in the high field strength elements (HFSE), and has higher contents of P, Ti and Hf than any other martian meteorite yet found. Paradoxically, QUE 94201 is highly depleted in two other HFSE, Ta and Th. The extreme geochemical variations exhibited by martian basalts (EETA79001 lithologies A and B, QUE 94201, Shergotty and Zagami) are unusual by terrestrial standards, and suggest unusual petrogenetic processes on Mars. McCoy T. J.* Lofgren G. E. The Crystallization of the Zagami Shergottite: A 1 Atm. Experimental Study The SNC meteorites have been the subject of intense interest owing to their likely Martian origin. While numerous petrologic studies have been conducted, experimental studies of their crystallization are sparse. Petrographic studies and partial-melting experiments were conducted using Zagami and Shergotty. The partial-melting experiments produced pyroxenes of compositions comparable to the Mg-rich cores of natural shergottite pyroxenes ~1140degrees C, crystallizing ~45% pyroxene, suggesting that shergottites were cumulates. Recent petrologic studies of Zagami measured Mg-rich core abundances of 20 +/- 5%, far below the inferred 45%. These studies inferred widely different crystallization histories. Atwo-stage magmatic history with Mg-rich pyroxene crystallization in a deep-seated (1-2 kb) magma chamber followed by eruption of a thick, phenocryst-bearing lava flow and cooling at ~0.1-0.01 degrees C/hr was favored by McCoy and Brearley, while a single-stage history cooling at 5-20 degrees C/hr and undercooling was supported by Treiman and Sutton. The difference in Mg-rich pyroxene abundances, the possibility of a complex pressure-cooling history, and the lack of dynamic crystallization experiments to constrain the cooling history of Zagami all suggested that a new experimental study was needed. Our equilibrium crystallization experiments are similar, producing ~38% core-composition pyroxene at 1150 degrees C. Differences in pyroxene abundances are not due to experimental error (e.g., fO2), but reflect differences in pressure and/or composition between the experiments and natural samples. Our dynamic crystallization experiments do not produce Zagami-like textures at cooling rates => 5 degrees C/hr. Textures are dependent on both nuclei density and cooling rate. Slower cooling rates (<= 2 degrees C/hr) and relatively few nuclei produce textures more Zagami-like, but pyroxene zoning and mesostases texture are unlike Zagami. Zagami probably formed during slower cooling (< 0.5 degree C/hr), and perhaps with a multi-stage history. Wentworth S. J. Gooding J. L.* Water-Based Alteration of the Martian Meteorite, QUE94201, By Sulfate-Dominated Solutions The martian meteorite, QUE94201, contains substantial deposits of sulfate minerals at the scales of tens of micrometers and smaller. On the fusion crust, vugs contain mixtures of separate Ca- and Fe-sulfate phases as well as aluminosilicate clays and secondary silica. In the interior, Ca- and Fe-sulfates occur together along with gel-textured Fe-sulfate/phosphate; carbonates are rare or absent. The salt textures suggest origin from low-temperature, oxidizing solutions, perhaps transitional to colloidal suspensions, that were strongly dominated by sulfate with relatively little carbonate. Distinction of Antarctic from martian alteration effects requires further analysis of textural relationships among fusion crust, mineralized vugs and veins. Eugster O.* Weigel A. Polnau E. Two Different Ejection Events for Basaltic Shergottites QUE94201, Zagami and Shergotty (2.6 Ma Ago) and Lherzolitic Shergottites LEW88516 and ALH77005(3.5 Ma Ago) TWO DIFFERENT EJECTION EVENTS FOR BASALTIC SHERGOTTITES QUE94201, ZAGAMI AND SHERGOTTY (2.6 MA AGO) AND LHERZOLITHIC SHERGOTTITES LEW88521 AND ALH77005 (3.5 MA AGO); O. Eugster, A. Weigel, and E. Polnau, Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland. QUE94201 (hereafter QUE) with a total mass of 12.0 g was recognized to be a shergottite. In the framework of a consortium study we obtained interior chip 23 that was taken 3 mm from the fusion crust. The aim of our work is the .characterization of it's noble gas inventory and the dating of the ejection event on Mars. From cosmic-ray produced 3He, 21Ne, and 38Ar we infer thaZagami and obtain an average exposure age of 2.4 Ma. We conclude that QUE was ejected from Mars simultaneously with the other basaltic shergottites Shergotty and Zagami. We postulate thatthe lherzolithic shergottites ALH77005 (ALH) and LEW88516 (LEW) were ejected about 0.9 Ma earlier. In this work we also present data on LEW and Zagami. Nishiizumi K.* Caffee M. W. Exposure History of Shergottite Queen Alexandra Range 94201 Cosmogenic nuclide studies of SNC meteorites have contributed significantly to our understanding of these objects. From cosmic-ray-produced nuclides exposure histories, ejection conditions from the hypothesized martian parent body, and genetic relationships between SNC meteorites are determined. In addition to those nuclides produced by galactic cosmic rays are those produced by solar cosmic rays (SCR). Radionuclides produced by SCRs reside in the uppermost few centimeters of material and their presence in meteorites indicates the degree to which a meteorite has ablated. Previous work shows ablation is less than 1-2 cm in at least three shergottites, ALHA77005, Shergotty, and EETA79001. This suggests that the atmospheric entry velocity and/or entry angle of these shergottites is much lower than the velocity and/or entry angle of ordinary chondrites. We report here the results of cosmogenic radionuclides in the recently recovered Antarctic shergottite Queen Alexandra Range 94201 (QUE94201). We measured 10Be (half-life = 1.5 My), 26Al (0.705 My), and 36Cl (0.301 My) concentrations by AMS. Gladman B.* Burns J. A. Towards a Self-Consistent Model of Lunar and Martian Meteorite Delivery The lunar and martian meteorites present several puzzles: (1) the equal numbers of each group, (2) the much larger average mass of the martian meteorites, (3) the inferred shallow pre-launch depths of the lunar meteorites vs. the deep ones of the martian meteorites, (4) the prevalence of geologically young rocks amongst the SNCs (even though such terrain is relatively rare on Mars), and (5) the 4-pi age spectrum of the martian objects terminates at about 15 Myr. We have undertaken detailed numerical studies of the orbital history of meteoroids liberated from these bodies. By comparing these results with the age spectrum obtained from cosmic ray exposure studies of the meteorites, we develop a self-consistent model that can explain the above features, although not uniquely since surface properties of the two targets appear to play a major role. Wednesday, March 20, 1996 TERRESTRIAL IMPACT STRCUTURES, TEKTITES AND SPHERULES 8:30 a.m. Room C Chair(s): J. F. McHone B. P. Glass Brandt D.* Reimold W. U. Wendorf L. Koeberl C. Grant J. Geophysical Signatures of the Roter Kamm Impact Crater, Namibia New gravity and magnetic data have been collected in- and outside of the Roter Kamm impact crater. As in previous studies, the traverses did not extend sufficiently beyond the rims, and, hence, the regional values were not adequetely constrained. The gravity results of this study were similar to those of Fudali: a negative, near-symmetrical anomaly was obtained over the crater center, which conforms to that expected for a sediment-filled, simple bowl-shaped crater. The magnetic results of this study, however, are different from those reported earlier, most probably due to the longer profiles of this study. A slight positive magnetic anomaly was obtained over the crater interior, and, as noted by Fudali and Reimold et al., magnetic noise was observed in the vicinity of the rim. We also report on a first finding of suevitic breccia at Roter Kamm. Grant J. A.* Koeberl C. Reimold W. U. Schultz P. H. Brandt D. Franzsen A. J. The Degradation History of the Roter Kamm Impact Crater, Namibia The Roter Kamm impact crater formed 3.7 Ma in the southern Namib desert of Namibia (27 degrees 46'S; 16 degrees 18'E). The crater is surrounded and partially buried by an active eolian sand sheet and dune field, thereby hampering access and assessment of preservation state. As a result, the preservation and extent of any pristine ejecta deposits were poorly known. In an attempt to constrain the degradation state of the crater, a ground penetrating radar (GPR) was used in tandem with other geological investigations to define the surface character and shallow stratigraphy in and around the crater. Results indicate that much of the continuous ejecta deposits north, east, and south of the rim are completely eroded: the western exterior of the crater was not investigated. At least one local occurrence of only slightly modified ejecta does persist north of the rim, however, and additional ejecta may remain beneath alluvium flanking the outer rim. Dressler B. O.* Sharpton V. L. Schnieders B. Scott J. Formation of Impact Breccias at the Slate Islands Structure, Northern Lake Superior, Ontario, Canada Various types of breccias, shatter cones, impact melts and microscopic shock metamorphic features are common on the Slate Islands (48 degrees 40' N; 87 degrees 00' W) and provide convincing evidence that the structure was formed as the result of comet or asteroid impact. Target rocks consist of three main groups of Archean and Proterozoic supracrustal and intrusive rocks, about 2.7 Ga and 1.8 Ga and 1.1 Ga old respectively. The structure is about 32 km in diameter. Its age is as yet not very well constrained. A large portion of the island group appears to represent the partially eroded central uplift of a complex impact crater. Here we relate specific breccia types in the target rocks to the various phases of the impact process, namely compression (Type A pseudotachylites), central uplift and excavation (clastic matrix breccias), and crater modification (monomict, parautochthonous breccias). Type B pseudotachylites may have formed during the excavation and late crater modification stages. Bunte Breccia and suevite deposits occur on the flanks of the central uplift or on the inner flanks of the annular trough. Sharpton V. L.* Dressler B. O. The Slate Islands Impact Structure: Structural Interpretation and Age Constraints The ~7-km-wide Slate Islands group, located in northern Lake Superior ~10 km south of Terrace Bay, Ontario, represents the heavily eroded central portion of a complex impact crater. Bathymetric data suggest an original crater diameter of ~32 km. Macroscopic and microscopic shock metamorphic features are common expressions within the Archean-to-Proterozoic igneous, metamorphic, and sedimentary target complex. Field studies conducted over the last two years reveal that the islands preserve not only the uplifted basement of the central structure, as previously recognized, but also allogenic breccia deposits of the original peripheral trough. Allogenic breccias are particularly prevalent along the eastern and northern island exposures. Heterogeneous melt bodies are located within heavily brecciated units of the central uplift. The intensely deformed structural trough and rim zone are evident on the N-S GLIMPCE reflection seismic line traversing the crater ~7 km to the west of the crater center. Currently available age constraints are weak but the apparent lack of post-Jacobsville clasts in the preserved breccia units suggests a Late Proterozoic-Early Paleozoic (800-500 Ma) age. Isotopic analyses of melt rocks, essential for refining the production age of this impact crater are currently underway. Gurov E. P.* The Group of Macha Craters in Western Yakutia The group of Macha craters is placed in the marginal part of Aldan Anteclise in Macha river basin, the left tributary of Lena river. Coordinates of the craters: 60 degrees 06 minutes N, 117 degrees 35 minutes E. The Macha craters were discovered by aerovisual observations of Aldan Shield and Aldan Anteclise during the impact craters search in this region. Ivanov B. A.* Kocharyan G. G. Kostuchenko V. N. Kirjakov A. F. Pevzner L. A. Puchezh-Katunki Impact Crater: Preliminary Data on Recovered Core Block Structure Vorotilov Deep Borehole (maximum depth is 5374 m) has been drilled at the central uplift at the Puchezh-Katunki impact crater. The estimated rim crater diameter is about 40 km. Core recovery is about 70% of penetration with coring. The recovery core investigation allows to estimate the blocky structure of hard rocks inside the central uplift. Up to depth of 3 km the block size along the borehole is about 100 m. Such blocks survived with a minor or medium damage the passage of a shock wave with an amplitude 20 to 40 GPa, the downward displacement at 6 to 10 km during the transient crater growth and the consequent upward displacement to the final position inside the central uplift. The proper model of large scale crater formation should take into account the discrete character of rock deformations. With measured block size, the deformation seems to be highly localized at inter-block boundaries. McHone J. F.* Greeley R. Blumberg D. SIR-C/X-SAR Radar Studies; Impact and Aeolian Features, Borkou Region Northern Chad Spaceborne Radar Laboratory (SRL) multiwavelength and multipolarization images of the Borkou region in northern Chad reveal a landscape dominated by aeolian and impact processes. Steady winds in long-established patterns have cut into flat sandstones and shales to form a series of linear erosional grooves which serve as conduits for mobile sands. Dunes migrate many tens of kilometers along the grooves and, in one area, pass through the remnants of the Aorounga impact structure. Active dunes which appear distinctive on radar data with a critical incidence angle near 32 degrees (the angle of repose for medium sands) and look directions perpendicular to the direction of sand movement provide important clues to the local Saharan sand budget. Ocampo A. C.* Pope K. O. Shuttle Imaging Radar (SIR-C) Images Reveal Multiple Impact Craters at Aorounga, Northern Chad The recently discovered Aorounga impact crater in northern Chad was imaged by SIR-C during several passes of the space shuttle Endeavor in 1994. Our analyses of the radar images reveal that Aorounga is a multiple impact site with three or possibly four craters. Each crater is approximately 11-12 km in diameter and their centers are spaced 14-20 km apart in a NNE trending arc. The Aorounga craters are perhaps the largest group of >10 km diameter craters known on Earth and were apparently formed by the breakup of a large comet or asteroid prior to impact. Simonson B. M.* Davies D. Wallace M. Reeves S. Hassler S. PGEs and Quartz Grains in a Resedimented Late Archean Impact Horizon in the Hamersley Group of Western Australia The early Precambrian Hamersley Group of Western Australia contains two thick packages of carbonate-rich strata, the Carawine Dolomite and the Wittenoom Formation, that occupy mutually exclusive areas within the Hamersley Basin. Within each of these formations is a single horizon which contains sand- to fine gravel-size particles believed to be distal ejecta from a large bolide impact. In the Carawine Dolomite, the ejecta are restricted to a coarse-grained dolomitic debris flow deposit up to 25 m thick. In the Wittenoom Formation, the ejecta are restricted to a turbidite which is <=1.3 m thick and consists largely of sand-size carbonate and argillite intraclasts. Together, these two horizons constitute a single, unique layer that appears to have been deposited rapidly over an area >= 50,000 km2 by a single high-energy event around 2.5 Ga. Deposition is inferred to have taken place in a series of distinct stages as follows: (1.) ballistic dispersal of mostly sand-size particles from the impact site to the seafloor in the Hamersley Basin, (2.) reworking of the newly deposited ejecta in the Hamersley Basin into large symmetrical ripples by impact-generated tsunami waves, and (3.) subsequent erosion and resedimentation of most of the ejecta by one to three large sedimentary gravity flows that moved south and west down the paleoslope of the Hamersley Basin. New data will be presented concerning the two main types of ejecta found in this layer: microkrystites and quartz grains. Specifically, microkrystite-rich samples are enriched in Ir and Ru by an order of magnitude or more relative to the surrounding strata, but other siderophile elements (Pd, Pt, Au, Cr, Co, and Ni) display neither anomalously high concentrations nor chondritic interelement ratios. As for the quartz grains, their petrographic characteristics clearly indicate they are not volcanic in origin, but they do not appear to have planar deformation features like those reported from numerous other impact ejecta horizons. Glass B. P.* McHugh C. M. G. More Upper Eocene (North American?) Tektites and Impact Ejecta Off New Jersey Previous studies have shown that an 8-cm-thick layer of impact ejecta (including tektites, shocked quartz, coesite, and stishovite) occurs at Deep Sea Drilling Project (DSDP) Site 612 on the Continental Slope off New Jersey. During Leg 150 of the Ocean Drilling Project (ODP) two additional sites, drilled within 9 km of Site 612, were found to contain late Eocene impact ejecta. Site 904A, 4.9 km north (up slope) of Site 612, was found to contain a layer of tektite glass and shocked quartz. In addition to tektite glass and shocked quartz at Site 904A, we report here the discovery of coesite and clinopyroxene-bearing spherules. Site 903C was drilled 7.5 km to the northwest of Site 904. A 7-cm-thick laminated layer was identified in core 56R, section 6, from Site 903C, which was described as a "... silty to very fine sandy interval ... that may contain microtektites". The sandy interval in core 56R, section 6, appears to be equivalent in age to the upper Eocene tektite layer found at Sites 612 and 904A. We report here the discovery of impact ejecta (but no tektite glass) at Site 903C. Albin E. F.* Wampler J. M. New Potassium-Argon Ages for Georgiaites and the Upper Eocene Dry Branch Formation (Twiggs Clay Member): Inferences About Tektite Stratigraphic Occurrence The tektites of east-central Georgia, called "georgiaites," are a subset of the larger North American tektite strewn field. Previous studies have reported the potassium-argon age of Georgia tektites at 32.0 Ma, 33.7 Ma, and 34.0 Ma, respectively. Glass et al., using the 40Ar-39Ar method, obtained a 34.5 Ma age for a single Muong Nong type georgiaite. Corrected fission track ages given by Storzer et al. have a range between 1.0 and 35.8 Ma with the lower dates accounted for by post-depositional thermal alteration. However, isotopic dating of North American microtektites from Barbados suggests that the strewn field was produced at 35.4 (+/- 0.6) Ma. In addition to the lack of consistently reliable age data for Georgia tektites, their stratigraphic occurrence has yet to be satisfactorily resolved. Although the North American strewn field was deposited in the late Eocene, georgiaites have been found only on formations ranging in age from Oligocene to Pleistocene so are thought to have been transported from their original site of deposition. In this investigation we present new potassium-argon ages for the Georgia tektites and an upper Eocene formation in an effort to resolve tektite stratigraphic location in coastal main deposits of east-central Georgia. Delano J. W.* Hanson B. Liquid Immiscibility: Cause of Compositional Heterogeneity in Tektites Compositional heterogeneities within individual tektites on scales >100 microns have been described and attributed to incomplete mixing of diverse lithologies in the target during impact melting. During the impact event, these melts were heated (3000 degrees C; far above their liquidus temperatures (about 1350degrees C; and then cooled rapidly to form glass. Some tektites were transported above the Earth's atmosphere and underwent a second heating episode when they re-entered the atmosphere at hypersonic velocity. This second heating episode generated flanges. If, as implicitly assumed by tektite investigators, mixing/dissolution processes dominated during these heating events, why are compositional heterogeneities better developed in the flanges, which are products of two intense heating events, than in the core of the same tektite that underwent only one heating event? Since chemical diffusion during the second heating event should have decreased the size and magnitude of the compositional heterogeneities, an unmixing/exsolution process seems to be more compatible with the observations. The current study has found that trends of element distribution exhibited by schliere within splash-form tektites are nearly identical to those exhibited by millimeter-scale layers in Muong Nong-type tektites. Consequently, it is the view of this abstract that: (a) schlieren and layers were produced by a single, chemical exsolution process; and (b) the differing length-scales of the compositional heterogeneities reflect different temperature-time histories of the samples. Prasad M. S.* Sudhakar M. Collisions in the Ejecta Plume of the Australasian Impact Event Major impacts on earth produce glassy ejecta distributed in widespread strewn fields, a classic example of which are tektites and microtektites. Although several models have been proposed on the process of impacts and ejecta emplacement very little is actually known on the immediate post impact scenario in terms of ground truth. We present here the phenomenon of microimpacts on microtektites which, we suggest here, have taken place within the ejecta plume of the impact which generated the Australasian tektite strewn field. Microimpacts, although have been found on virtually every sq.cm of the lunar soil, they however, formed due to cosmic collisions on the moon. Since tektites and microtektites are the ejecta of major impacts on earth, the microimpacts presented here are the first such occurrence not only on earth but also have not been reported in the ejecta of any other known major impact. This study has importance in understanding cratering mechanics of major impacts on terrestrial planets. Wednesday, March 20, 1996 FROM SMALL TO SMALLER 8:30 a.m. Room D Chair(s): G. J. Flynn L. P. Keller Liou J. C.* Zook H. A. Cometary Sources of Low-Entry-Heating CPA IDPs In the all sky survey by the Infrared Astronomical Satellite in 1983, eight comets were discovered to have dust trails. Among those eight comets, the most prominent trail belonged to Tempel 2, suggesting that this comet recently has been a most prolific producer of interplanetary dust. Numerical simulations that we have carried out of the orbital evolution of 4 and 9 micron diameter dust particles from Tempel 2 shows that radiation pressure causes a significant fraction of dust particles in this size range to be directly injected into a 1:2 interior mean motion resonance with Jupiter. There, they remain trapped for thousands of years. When they escape the resonance, their orbital eccentricities are usually quite small. These, and dust grains from other similar Tempel-type comets, approach the Earth with the low velocities typical of asteroidal dust grains. A sub-group of chondritic porous aggregate (CPA) interplanetary dust particles (IDP's) that were collected from stratosphere has been shown to have undergone little heating upon entry into the Earth's atmosphere. It is possible that this sub-group of grains derive from Tempel-type comets, and not from asteroids. Genge M. J.* Grady M. M. Hutchison R. Atmospheric Alteration in Fine-Grained Antarctic Micrometeorites (AMMs) in the size range 50-100mm were examined by scanning electron microscopy and electron microprobe techniques to differentiate primary 'pre-atmospheric' characteristics from those originating from alteration in the atmosphere. Results indicate that the majority of particles (61) are completely or partially melted during atmospheric deceleration attaining peak temperatures in excess of ~1600 degrees C. A significant number of particles (18) containing relict phyllosilicates have evidently been heated to temperatures less than 600-900 degrees C. The matrix compositions of the examined AMMs broadly support previous theoretical predictions that all particles >50mm have suffered some degree of partial vaporisation or melting during aerobraking, however, they also imply that predicted peak temperatures are severely overestimated. It is infered from particle textures that cosmic dust particles >50mm can support both high thermal gradients (~1000 degrees C) and may develop hydrodynamical bow-shocks: phenomena which are specifically excluded from theoretical treatments. Matrix compositions also testify to volatile enrichments of the elements K, Cl, F, and P presumably as the result of reactions with atmospheric species. A reaction mechanism is suggested for the observed coupled enrichment of K and Cl. Flynn G. J.* Bajt S. Sutton S. R. Evidence for Weakly Bound Bromine in Large Interplanetary Dust Particles Collected from the Stratosphere Bromine concentrations exceeding 100 times the CI meteorite value have been measured in interplanetary dust particles collected from the Earth's stratosphere. Because of the high halogen content and the long duration of particle residence in the stratosphere, Br contamination is possible. Six large interplanetary dust particles were analyzed using the Synchrotron X-ray Fluorescence Microprobe at the National Synchrotron Light Source. Each particle was exposed to the x-ray beam for a continuous 4 to 16 hour period. An approximately exponential decay of the Br content with time, over a 2-hour period, was observed. Since the particles are estimated to be heated to <100 degrees C during these analyses, this easily mobilized Br is likely to be a contaminant. In all cases the Br concentration stabilized at a value which exceeded the CI meteorite value. No loss of S, Cl, K, Cu, Zn, or Ge was observed, indicating that these elements are more strongly bound. Bradley J. P.* Dukes C. Baragiola R. McFadden L. Johnson R. E. Brownlee D. E. Radiation Processing and the Origins of Interplanetary Dust The anhydrous chondritic (CP) class of interplanetary dust particles (IDPs) are considered to be pristine (unprocessed) objects because of their fragile, porous microstructures and lack of evidence of significant parent body alteration. During their recent 10^4-10^5 year lifetimes in solar orbit, IDPs accumulate high densities of implanted solar flare tracks as well as solar wind irradiated amorphous rims along their outer surfaces. Some grains in anhydrous IDPs were also exposed to an earlier irradiation episode(s). These grains are of major significance because their exposure predates accretion of IDPs and, in the case of cometary IDPs, the exposure predates comet accretion. Since comets are primitive small bodies, it is conceivable that the grains are surviving solar nebula or presolar interstellar grains. The most well-characterized grains are GEMS (glass with embedded metal and sulfides). Several researchers have noted that properties of GEMS strongly resemble those of "amorphous silicates" which are ubiquitous throughout the interstellar medium. This is the more significant because the half dozen or so known properties of interstellar silicates, as determined from astronomical observations, are "exotic" and mutually exclusive of one another. If GEMS are indeed interstellar "amorphous silicates", then one of the main raw ingredients of the Solar System has been found. Joswiak D. J. Brownlee D. E.* Bradley J. P. Schlutter D. J. Pepin R. O. Systematic Analyses of Major Element Distributions in GEMS from High Speed IDPs As we reported last year, all of the highest speed stratospheric IDPs that we have identified by the stepped He release method of Nier and Schlutter, contain GEMS as a major component. GEMS are submicron rounded units composed of glass with embedded metal and sulfide grains. Internal composition gradients in GEMS, consistent with extensive pre-accretional irradiation, have been reported by Bradley. Many of the properties of GEMS are consistent with astronomically observable effects caused by interstellar silicate grains. We report here detailed elemental analyses of the internal compositions of GEMS in a subset of IDPs selected on the basis of their high speed entry into the atmosphere, an indicator of likely cometary origin. Rietmeijer F. J. M.* The Butterflies of Principal Components: A Case of Ultrafine-Grained Polyphase Units Dusts in the accretion regions of chondritic interplanetary dust particles [IDPs] consisted of three principal components: carbonaceous units [CUs], carbon-bearing chondritic units [GUs] and carbon-free silicate units [PUs]. Among others, differences among chondritic IDP morphologies and variable bulk C/Si ratios reflect variable mixtures of principal components. The spherical shapes of the initially amorphous principal components remain visible in many chondritic porous IDPs but fusion was documented for CUs, GUs and PUs. The PUs occur as coarse- and ultrafine-grained units that include so called GEMS. Spherical principal components preserved in an IDP as recognisable textural units have unique proporties with important implications for their petrological evolution from pre-accretion processing to protoplanet alteration and dynamic pyrometamorphism. Throughout their lifetime the units behaved as closed-systems without chemical exchange with other units. This behaviour is reflected in their mineralogies while the bulk compositions of principal components define the environments wherein they were formed. Messenger S.* Walker R. M. Clemett S. J. Zare R. N. Deuterium Enrichments in Cluster IDPs Substantial enrichments in D/H, of up to +2,700 per mil, have been previously observed in roughly 1/3 of chondritic IDPs. Often D-rich IDPs are marked by significant isotopic variations between different subfragments. The most extreme example of this isotopic heterogeneity is the IDP Butterfly, in which delta D varies between -200 and +2700 per mil. Ion imaging of one fragment from Butterfly found the D highly concentrated relative to the H, with an inferred dD of at least +9,000 per mil. Recent measurements of an unusual 'cluster' IDP (Dragonfly) dramatically surpass these results, with delta D ranging up to ~ +50,000 per mil. _ Keller L. P.* Thomas K. L. McKay D. S. Carbon Petrography and the Chemical State of Carbon and Nitrogen in IDPs Carbon is a major component of interplanetary dust. The carbonaceous material in IDPs has a strong effect on the physical and optical properties of IDPs, and is the likely carrier of isotopic anomalies in hydrogen and nitrogen. Despite their abundance and importance, little is known about the nature and distribution of light elements in primitive IDPs. We report here on our TEM and electron energy-loss spectroscopy studies of carbon and nitrogen in a suite of anhydrous IDPs. Bajt S.* Chapman H. N. Flynn G. J. Keller L. P. Carbon Characterization in Interplanetary Dust Particles with a Scanning Transmission X-ray Microscope Carbon XANES (X-ray Absorption Near Edge Spectroscopy) spectra of two carbon-rich interplanetary dust particles (IDPs) and a selected set of carbon standards were measured with a Scanning Transmission X-ray Microscope (STXM) to identify the carbon bonding in IDPs. The XANES spectra of 3 regions of one IDP (L2008 F4) show a notable similarity to the C-XANES spectrum of C(sub)60 (known also as buckminsterfullerene, or buckyballs). Efforts are underway to confirm the presence of C(sub)60 in this IDP using other analytical techniques and to exclude the possibility of contamination. The peak positions in the C-XANES spectrum of the second IDP, L2008 H9, suggest that carbon is present mainly as graphite or poorly-graphitized carbon. Strait M. M.* Thomas K. L. McKay D. S. Porosity of Interplanetary Dust Particles We report here new information from our studies on the porosity of interplanetary dust particles. We have resolved some of the problems we encountered earlier and we report new results for four hydrated IDPs and two meteorites. Determination of the porosity of IDPs is important in the dynamics of collisional and orbital evolution of small-sized particles. We are using an image analysis method to make these determinations from digitized photographs of thin-sectioned particles. Earlier determinations of porosity were derived from measures of density and suggested that particles had appreciable porosity, but that porosities were probably less than 70%. Engrand C.* Deloule E. Hoppe P. Kurat G. Maurette M. Robert F. Water Contents of Micrometeorites from Antarctica Micrometeorites have high hydrogen contents regardless of their thermal alteration during atmospheric entry (1.4-20 wt.% H2O equivalent). Also, two cosmic spherules (melted micrometeoroids), retained hydrogen in astonishingly large amounts (1.1 and 2.86 wt.% H2O). COPS, essentially a P-rich, ferro-ferrioxide - hydroxide, contains between 2.9 and 3.8 wt.% H2O. The isotopic composition of the hydrogen in all objects is highly variable and has a range similar to that of hydrogen from carbonaceous chondrites (delta D -366 per mil to 501 per mil). Rietmeijer F. J. M.* An Analytical Electron Microscope (AEM) Study of Hydrous Alteration in a Smoke of Modal Forsterite Composition Many studies have simulated vapor phase condensation in astrophysical environments such as the solar nebula. They generally investigated simple (Mg,Fe)-SiO vapors that yielded fine-grained amorphous and/or crystalline smokes with pyroxene and olivine compositions. A typical study matched infrared [IR] spectra of these smokes with those of astrophysical dusts but AEM analyses of smokes at the level of their constituents were rarely attempted. These analyses of the condensation event, including autometamorphism, and thermal alteration and hydration are necessary to constrain the onset of mineralogical activity in the solar nebula, in particular the nature of protophyllosilicates. Infrared spectroscopy, X-ray diffraction, scanning electron microscopy and AEM are the most common techniques for smoke characterization. The fact that IR spectroscopy is sensitive to different properties than AEM, for example, is not always appreciated, e.g. AEM analyses of hydrated Mg-SiO smokes with distinct IR features for layer silicates showed small amounts of protophyllosilicates restricted to domains with the appropriate MgO/SiO2 ratio. I report the ongoing effort of AEM characterization of experimentally hydrated Mg-SiO smokes with a resolution that allows comparisons with AEM studies of interplanetary dust particles [IDPs] and undifferentiated meteorite matrices. Taylor S.* Lever J. Harvey R. A New Source of Micrometeorites: The South Pole Water Well In 1995 we built, tested and deployed a collector to suction particulates from the bottom of the South Pole drinking water well (SPWW) in the hope of finding large numbers of micrometeorites. The SPWW, because it melts huge amounts of firn and ice, provides an efficient way of concentrating micrometeorites, which occur ubiquitously but in low concentrations in terrestrial environments. We made 5 separate collections, traversing an area of about 30 m^2 and collecting approximately 200 g of material. Microscopic examination of the 250-425 micrometer size fraction from 2 of the 5 collections suggests that 1 of every 1000 particles in this size fraction is a melted micrometeorite. There are also translucent and transparent spherules, similar to those described by Maurette et al., which are thought to be extraterrestrial and particles which appear to be unmelted micrometeorites. Dating of the ice brackets the depositional age of any micrometeorites collected between 1000-1500 AD. Wednesday, March 20, 1996 SPECIAL PLENARY SESSION: NASA SPACE SCIENCE PROGRAMS 1:30 p.m. Room C Chair(s): D. Blanchard Huntress W. T.* Space Science Program Brinton H. C.* Planetary Research and Analysis Program Johnson T. V.* Galileo Mission Cheng A. F.* Near-Earth Asteroid Rendevous Wednesday, March 20, 1996 MARS: MINERAL SPECTROSCOPY AND SNC MINERALOGY 2:30 p.m. Room A Chair(s): J. L. Bishop A. H. Treiman Dreibus G.* Jagoutz E. Spettel B. Wanke H. Phosphate-Mobilization on Mars? Implication from Leach Experiments on SNC's Leach experiments on Shergotty, Zagami, and ALHA77005 showed that their phosphates dissolve readily with diluted acids at room temperature. Even an only slightly acidified brine-solution (MgSO4, pH = 3) dissolved more than 50% of the REEs and U whereas K remained insoluble. These experiments indicate the possibility of alteration and mobilization of phosphates in the Martian environment with the consequence of an enrichment of U relative to K in the soil compared to igneous rocks. Gleason J. D.* Kring D. A. Boynton W. V. The Role of Garnet in Martian Mantle Evolution: Further Evidence from Shergottite Rare Earth Patterns REE fractionation and isotopic decoupling effects in SNC meteorites have been attributed in the past to the presence of garnet in their mantle source regions. Quantifying the garnet effect is now possible using the parent melt REE compositions determined by for the shergottites, a group of SNC meteorites characterized by complex rare earth element (REE) patterns and 180 Ma isotopic ages. Below, we develop a multi-stage REE evolution model for a shergottite source which underwent fractional fusion at earlier stages with garnet present. Similar processes may account for (1) the decoupling of the Sm-Nd isotopic system from the Rb-Sr and U-Th-Pb isotopic systems in SNC meteorites, and (2) non-chondritic abundance ratios for certain refractory lithophile elements (e.g., high Th/La, U/La, and low Al/Ti) in SNC's. If this model is generally correct, it then requires a planet large enough to have crystallized substantial garnet in its mantle source regions, consistent with a martian origin for the SNC's. Shearer C. K.* Spilde M. N. Papike J. J. Layne G. D. Hydrothermal Systems on Mars. Insights from Sulfur Isotopic Systematics in Alteration Assemblages in Martian Meteorite Allan Hills 84001 Crustal processes and reactions during hydrothermal and biogenic activity result in extreme degrees of sulfur isotopic fractionation on Earth. For example, delta ^(34)S in terrestrial sulfides ranges from -70 per mil to +70 per mil on Earth. In contrast, delta ^(34)S values for sulfides from other planetary bodies that have been sampled (Moon, asteroids) show a very limited mass fractionation. The standard deviation in the bulk isotopic composition of sulfur in meteorites of all types is less than 0.1 per mil. However, the isotopic composition of sulfides in meteorites shows slightly more variability. Troilite in Orgueil, a carbonaceous chondrite, has a delta ^(34)S of 2.6 per mil. Kaplan and Hulston showed that sulfides in enstatite chondrites have delta ^(34)S of between +1.6 to +2.5 per mil. The delta ^(34)S in troilite from ordinary chondrites ranges from -2.7 per mil to +2.5 per mil. The slight fractionation of delta ^(34)S into these sulfides has been attributed to nebular heterogeneity, low temperature (100 degrees C) reactions between water and elemental sulfur, and oxidation of FeS in an aqueous environment. Lunar materials exhibit a much broader variation in bulk delta ^(34)S than has been observed in meteorites. Whereas bulk lunar rocks show variability on the order of +0.37 to +0.68, lunar soils have delta ^(34)S as high as +9.76 per mil. These high values in the bulk lunar soils have been attributed to preferential volatilization of ^(32)S during sputtering caused by micrometeorite bombardment. Until now, S fractionation processes on the larger terrestrial planets such as Mercury, Venus, and Mars has been only speculative. With the discovery of a possible martian meteorite with an imprint of a martian hydrothermal system, we can gain insights into S fractionation on another planet. Treiman A. H.* Norman M. Mittlefehldt D. Crisp J. 'Nakhlites' on Earth: Chemistry of Clinopyroxenites from Theo's Flow, Ontario, Canada The nakhlites (basaltic rocks rich in augite pyroxene) have been considered a uniquely martian phenomenon, testifying to fundamental differences between Mars and the Earth. However, very similar augite-rich basalt flows are present in Archaean terranes on Earth. New chemical data confirm the similarity and extend it to trace elements. Like the nakhlites, the parent magmas of one such flow (Theo's) is LREE-enriched with minimal Eu anomaly, and depleted in the most incompatible elements. Clearly, nakhlite-forming petrogenesis on Mars had a terrestrial equivalent during the Archaean. Studies of these Earth basalts will clarify the nakhlites' origin, and the nakhlites will in turn clarify petrogenetic processes in the Archaean (and subsequent) Earth. Murchie S.* Merenyi E. Singer R. Kirkland L. Visible-NIR Spectroscopic Evidence for the Composition of Low-Albedo Altered Soils on Mars Spectroscopic studies of altered Martian soils at visible and at NIR wavelengths have generally supported the canonical model of the surface layer as consisting mostly of 2 components, bright red hematite-containing dust and dark gray pyroxene-containing sand. However several of the studies have also provided tantalizing evidence for distinct 1 micrometer Fe absorptions in discrete areas, particularly dark red soils which are hypothesized to consist of duricrust. These distinct absorptions have been proposed to originate from one or more non-hematitic ferric phases. We have tested this hypothesis by merging high spatial resolution visible- and NIR-wavelength data to synthesize composite 0.44-3.14 1lm spectra for regions of western Arabia and Margaritifer Terra. The extended wavelength coverage allows more complete assessment of ferric, ferrous, and H2O absorptions in both wavelength ranges. The composite data show that, compared to nearby bright red soil in Arabia, dark red soil in Oxia has a lower albedo, a more negative continuum slope, and a stronger 3 micrometer H2O absorption . However Fe absorptions are closely similar in position and depth. These results suggest that at least some dark red soils may differ from "normal" dust and mafic sand more in texture than in Fe mineralogy, although there appears to be enrichment in a water-containing phase and/or a dark, spectrally neutral phase. In contrast, there is clear evidence for enrichment of a low-albedo ferric mineral in dark gray soils composing Sinus Meridiani. These have visible- and NIR-wavelength absorptions consistent with crystalline hematite with relatively little pyroxene, plus a very weak 3 micrometer H2O absorption. These properties suggest a Ethology richer in crystalline hematite and less hydrated than both dust and mafic-rich sand. Erard S.* Calvin W. Composite Spectra of Mars, 0.4-3.7 Micrometers Spectroscopic measurements of Mars are key observations to identify both ferric/ferrous minerals (from the 0.8-1.2 micron range) and alteration minerals (from the 2.2-5 micron range) present on the surface. The first space borne instrument to span this complete spectral range will be OMEGA/Mars 96, scheduled for July, 1997. However, IRS/Mariner in 1969 and ISM/Phobos-2 in 1989 performed observations of quality that partially cover the spectral range. This first comparison between the two data sets is intended to assess ISM calibration's accuracy at longer wavelengths, to check the permanency of shallow spectral features, and to look for possible differences in areas observed by the two instruments. Bishop J. L.* Visible and Infrared Reflectance Spectroscopy of Physical and Chemical Ferric-Smectite Mixtures as Mars Soil Analogs These experiments indicate that chemically-formed and physically-formed mixtures of fine-grained ferric oxides and smectites produce different kinds of materials and that the method of incorporating ferric species into a smectite mineral has an important influence on the spectral properties. A few wt.% ferric oxide can be present .in the smectite structure without giving visible ferric features, whereas as few wt.% ferric oxide as hematite contributes significantly to the visible and NIR spectral properties of the sample. The NIR spectral brightness and NIR band strengths of the chemically-treated montmorillonites containing ferrihydrite and ferric sulfate species are relatively similar to those of natural montmorillonite. In contrast, the NIR spectral brightness and NIR band depths are both significantly reduced when small amounts of fine-grained hematite are mixed with these smectites. Physical mixtures .Christiansen feature towards longer wavelengths and decreased the intensity of the montmorillonite features near 20 m. Chemical and physical mixtures of smectites with ferrihydrite and fine-grained hematite are good models for the bright region material on Mars because of the chemical and mineralogical compositions of these samples, as well with ferrihydrite and hematite produced shifts in the montmorillonite as their visible and infrared region spectral properties. Arnold G. E.* Bishop J. L. Schade U. Ellipsometry, Reflectance and Emittance Spectrocopy of Quartz Particle Size Separates, Palagonitic Soils and Montmorillonite. Implications for Remote Sensing of Planetary Surfaces Infrared reflectance and relative emittance of Mars analogs have been measured, resulting in emittance spectra that show inversely similar spectral features to the reflectance spectra. Absolute emittance can be derived when a wavelength range exists at which emittance equals one. This takes place at the short wavelength flank of strong absorption bands, where the refractive index of the material is close to the refractive index of air. The emittance spectra of the fine-grained palagonitic soil and montmorillonite exhibit dominant features at 6 micrometers due to the H-O-H bending mode and near 8 micrometers as the real part of the refractive index approaches a minimum. In the reststrahlen region, where the optical constants undergo rapid change, weak bands are observed for these samples. Golden D. C.* Morris R. V. Ming D. W. Bell III J. F. Yang S. V. Thompson D. R. Occurrence of a Titanium-bearing Jarosite from Hawaii Jarosite has been discovered recently as an alteration product of tephra from Mauna Kea volcano in Hawaii. These samples share common features with the remotely sensed martian spectra in the region 200 nm to 2200 nm region. A titanium bearing jarosite from Mauna Kea is examined in this paper with reference to its reflectance spectrum, chemical composition, and also its possible use as a Mars soil analog. Hamilton V. E.* Christensen P. R. Determining the Composition of Mars: Vibrational Spectroscopy of the Zagami Meteorite Zagami is a fine- to medium-grained basaltic shergottite primarily composed of pyroxene and maskelynite. If this meteorite represents a piece of the martian surface, as proposed by previous workers, then it can provide information about some of the most recent igneous processes on Mars. The Mars Global Surveyor spacecraft will carry the Thermal Emission Spectrometer for spectral mapping of the martian surface. We apply this technique by comparing the infrared spectrum of Zagami to the spectra of major rock-forming minerals. We find that the positions of absorption features in pyroxene spectra bear the most similarity to absorptions in the Zagami spectrum. Additionally, we are able to identify spectral characteristics of pyroxenes known to be present in Zagami, and we can eliminate at least one pyroxene that is not present in Zagami on the basis of spectral dissimilarities. _ Wednesday, March 20, 1996 FROM STARS TO SOLAR NEBULA 2:30 p.m. Room B Chair(s): T. J. Bernatowicz G. J. Wasserburg Meyer B. S.* The L.-S. El Eid M. F. On the Nucleosynthesis of 48Ca and 50Ti Endemic isotopic anomalies of 50 Ti exist in all carbonaceous (CI, CM, CO, and CV) meteorites. These anomalies occur in all phases of the meteorites and apparently call for small, refractory Ti-containing precursor grains. Within the refractory mineralized inclusions in these meteorites, 48 Ca anomalies accompany the 50 Ti anomalies. These anomalies are roughly correlated in the sense that excess 48 Ca associates with excess 50 Ti and deficient 48 Ca associates with deficient 50 Ti, but not is a strictly proportional way. These anomalies hold important clues, and deciphering these clues will provide important information about the formation of solid bodies in the solar system. Wasserburg G. J.* Busso M. Gallino R. The Abundances of Actinides and Short-Lived Non-Actinides in the ISM We present a simple phenomenological assessment of the abundance of various short-lived nuclei relative to ^232Th. We are not interested here in pursuing self-consistent long-term nucleosynthetic chronologies, but rather, in establishing a means of estimating the abundances of short-lived nuclei in the ISM and relating them to production of the actinides. It will be shown that a variety of distinctive SN and other sources must be responsible for the inventory of short-lived nuclei in the galaxy and the placental ISM from which the solar system formed. Foster P. N.* Boss A. P. Injection of Radioactive Nuclides from the Stellar Outflow that Triggered the Collapse of the Presolar Nebula We examine the injection of short-lived radioactive nuclides into the collapsing presolar nebula. The source of the nuclides is assumed to be the same star that instigated the collapse of the molecular cloud core. Using hydrodynamic models of triggered collapse, we employ two numerical techniques (tracer particles and a color concentration equation similar to the continuity equation) to follow the trajectories of particles carried along by the stellar outflow. The techniques yield the same overall results--roughly 20% of the particles incident on the molecular cloud core are captured by the collapsing protostar in the standard model. This percentage varies with the momentum of the incident material. The injected particles enter the collapsed cloud core in a fairly steady stream, over a time period of approximately 500,000 years. Chaussidon M.* Robert F. Boron and Lithium Isotope Variations in Chondrules: The Signature of Presolar Nucleosynthesis We report here the preliminary results of tests concerning two major implications of the boron isotope variations that were recently found in meteoritic chondrules (Chaussidon and Robert 1995, Nature 374, 337-339). (1) Freshly nucleosynthetized boron with variable 11B/10B ratios must have been preserved as solid grains in the solar nebula and implanted or embedded in the newly formed chondrules. Therefore, chondrules cooling rates do not allow a complete isotopic homogenisation which can be tested experimentally. (2) Another element, lithium, is also formed by the spallogenic reactions occurring in the presolar cloud with a spallogenic isotopic ratio (7Li/6Li) between 2 and 5 according to different models, which is lower than the bulk solar system ratio of ~12.5. Therefore, a negative correlation between the isotopic compositions of Li and B should be present in chondrules. Petaev M. I.* Wood J. A. Condensation in the Solar Nebula: Effects of Partial Isolation of Condensates from the Residual Gases High-temperature condensates are preserved in chondrites because they became isolated from the nebular environment during subsequent cooling, condensation, and reaction. We have calculated condensation sequences for models that assume a steady withdrawal of condensed matter from the reactive system as it cooled. Ruzmaikina T. V.* Formation of Chondrule Precursors in the Presolar Cloud In this paper I consider the growth of dust aggregates in the presolar cloud, taking into account their compressibility and possible compaction under the effect of higher velocity collisions. It is assumed that the aggregates could grow initially as fractals of low density and reach the masses of chondrule precursors before they begin to disrupt each other. When the kinetic energy of impacts approaches the strength of the material but is still less than it, collisions might cause compaction of the aggregates instead of destruction. The more compact aggregates experience a smaller number of collisions, and tend to avoid both further increase of mass and catastrophic destruction. This scenario can provide a population of dust aggregates with a relatively narrow size range and could be a good candidate for the chondrule precursors. Wood J. A.* Processing of Chondritic and Planetary Materials in Shocks Associated with Spiral Density Waves in the Solar Nebula An energetic nebular setting or environment has long been sought in which the material now in the planets and the chondritic meteorites was thermally processed. This paper proposes that the setting can be found behind shock fronts that accompanied the motion of spiral density waves through the solar nebula during the infall stage of nebular evolution. The substance of the nebula passed through these shock fronts many times, and each time there were opportunities for heating, melting, vaporization, chemical fractionation, and concentration of condensed matter between turbulent vortices. Only a small fraction of the potential planetary matter in the system, by responding to these opportunities, joined planetesimals and avoided being drawn into the sun. Thiemens M. H.* Xu X. Colman J. Trogler W. C. A New Isotope Effect and Its Possible Role in the Production of Excess Meteoritic 33S and Carbonaceous Material (Q?) There are several observations of sulfur isotopic anomalies in meteoritic material. This includes excess ^33S in Allende acid residues, oldhamite from Norton County and bulk ureilites. The source of these observed isotopic anomalies is at present, unresolved. Recently, excess ^33S has been observed in an organic extract from Murchison (methane sulfonic acid). This molecule also possesses deuterium excesses ranging from 660 to 2,730 per mil. Since sulfur, like oxygen, may be subject to symmetry dependent reaction chemistry, investigations exploring isotopic fractionations in astronomically relevant molecules may be of utility. Esat T. M.* Volatile Depletion in the Inner Solar System: Rayleigh Distillation and Potassium Isotopes The rocky planets, the Moon and some meteorites are depleted in elements that are volatile at temperatures below about 1200 K. Large variations in condensation or vaporisation temperatures can be invoked to account for the segregation of volatile from refractory elements. Starting with an initial hot nebula, incomplete condensation and removal of uncondensed volatiles is one alternative; partial vaporisation and removal of volatiles by heating of cold nebular material is another possibility. Incomplete mass transport across spatial temperature gradients can cause mass dependent variations in isotopes, in particular, of light mass elements. If high temperature vaporisation occurs across a liquid-vacuum interface (Rayleigh distillation), mass flow is unidirectional, resulting in complete removal of vaporised material. In a gaseous envelope mass transport could occur bidirectionally through vaporisation as well as collisional capture such that, in the limiting case, the system will approach equilibrium with no net isotope fractionation. Pepin R. O.* Relationships Among Isotopic Compositions of Solar-System Xenon and Krypton A remarkably large number of isotopically different trapped Xe and Kr compositions have been measured in meteorite carrier phases or inferred to exist in the early solar system. For Xe these include a group here collectively termed "Q-type" Xe: Xe-Q, Xe-P1, OC (ordinary chondrite)-Xe, AVCC (average carbonaceous chondrite)-Xe, and ureilite Xe (e.g., Kenna and Novo Urei), all of which have been put forward at one time or another as candidates for "primordial" solar-system Xe. Presolar meteoritic diamonds contain a second and different suite of isotopically distinct compositions: Xe-P3, Xe-HL, and Xe-P6e ("exotic"). Another, Xe-S, is prominent in interstellar silicon carbide. Each of these is accompanied in its carrier by a corresponding Kr composition. In addition an originally inferred but now detected composition called U-Xe is a promising primordial parent for nonradiogenic terrestrial Xe; both U-Xe and its associated Kr, Kr-1, appear to be present in solar wind (SW) Xe and Kr. Such an extensive and isotopically diverse set of compositions might well be expected from nucleosynthetic theory. The possibility explored here, however, is that some of them are not truly independent, but may instead reflect fractionation or mixing of a smaller subset of primary components. Wednesday, March 20, 1996 IMPACT MATERIALS AND EFFECTS 2:30 p.m. Room C Chair(s): J. A. Grant C. Koeberl Smith T. R.* Hodge P. W. Microscopic Meteoritic Material at the Dalgaranga, Odessa, and Veevers Meteorite Craters While there has been a considerable amount of study into understanding the impact craters, cratering mechanisms, the meteorites and their parent bodies, relatively little consideration has been given to the small meteoritic particles that are created as a result of the meteorite's passage through the atmosphere and subsequent impact. We present an analysis of soil samples from three young terrestrial impact craters, the Odessa craters in Texas and the Dalgaranga and Veevers impact craters in Western Australia. The Dalgaranga crater is of special interest since it is the only crater known to be formed by the impact of a mesosiderite. At the Odessa and Dalgaranga craters small meteoritic fragments are very abundant and their total calculated mass is much greater than the mass of recovered meteorites. Kargel J. S. Coffin P.* Kraft M. Lewis J. S. Moore C. Roddy D. Shoemaker E. M. Wittke J. H. Systematic Collection and Analysis of Meteoritic Materials from Meteor Crater, Arizona We have started a systematic collection and analysis of meteoritic materials from Meteor Crater, Arizona. Since our earlier report (Kargel, J.S., Kraft, M.D., Roddy, D.J., Wittke, J.H., and Lewis, J.S., 1995, Eos, v. 76, p. F337), we have found 47 small fragments of the Canyon Diablo iron meteorite. We also have collected impactite lapilli; oxidized meteorite fragments; and materials we call amalgamated meteoritic/lithic fragments (AMLs), which consist of target rocks fused with and impregnated by oxidized meteoritic iron. The composition of the impactite lapilli is consistent with admixture of about 3 parts Kaibab Formation (siliceous dolomitic limestone) and 1 part oxidized meteorite. In addition, the lapilli contain microscopic spherules of Ni-rich metal (up to 90% Ni), which can only be explained by partial oxidation of Canyon Diablo metal. Our interpretation of the lapilli is that the impact event melted and devolatilized rocks of the Kaibab Formation (siliceous limestone and dolomite), which mixed with molten meteoritic metal. If impact heated metal droplets or vapor condensates attained about 3500 K, then CO2 released from the Kaibab Formation may have thermally decomposed to CO and O2 and caused partial oxidation of the metal. Xue S. Yang Y.-L. Herzog G. F.* Hall G. S. Mass Fractionation of Germanium Isotopes in Canyon Diablo Spheroids Canyon Diablo spheroids are enriched in Ge compared to bulk Canyon Diablo. Within the spheroids, Ge is concentrated in the metallic cores and depleted in the oxide shells. The isotopic abundances are normal in the cores but are mass fractionated by ~+4 per mil/AMU in the oxide shells. We infer that most of the Ge in iron-depleted and nickel-enriched melt remains with the metal while a small part of the Ge oxidizes. A portion of that oxidized Ge evaporates, perhaps helped along by the energy liberated during the oxidation of iron, and becomes enriched in heavy Ge isotopes. Cygan R. T.* Boslough M. B. Myers S. A. Assink R. A. NMR Relaxation Behavior of Moderately Shocked Sandstone from Meteor Crater, Arizona Nuclear magnetic resonance (NMR) spectroscopy provides a sensitive and reliable tool for the identification and characterization of shocked minerals resulting from natural impact as well as from shock experiments. We previously characterized the relaxation behavior of experimentally-shocked quartz by determining the T1 relaxation times of crystalline and amorphous phases, showing that time dependence of the magnetization for each phase follows a power-law function before reaching a saturation plateau. We have now applied this method to characterize the complex mixture of silica phases and composition of moderately shocked Coconino Sandstone from Meteor Crater, Arizona. Our new results indicate that, as for the previous work, each phase exhibits power-law behavior followed by saturation. Moreover, the natural samples contain three additional phases, and relaxation analysis can be used to determine the relative abundances of quartz, coesite, stishovite, and a dense hydrated amorphous silica. Boslough M. B.* Cygan R. T. Crawford D. A. Kirkpatrick R. J. NMR Spectroscopy of Shocked Quartz from Experimental Impact Craters To quantify the extent of shock modification associated with the impact of projectiles into quartz targets, we used the NASA Ames Vertical Gun Range (AVGR) to generate impact craters and applied nuclear magnetic resonance (NMR) spectroscopy to recovered samples. Two cratering experiments were performed by using the two-stage gun to launch copper projectiles into target materials comprised of unconsolidated quartz powder and a block of Coconino Sandstone. The velocities of the 0.32 cm-diameter spherical impactors were 4.61 km/sec for the unconsolidated quartz and 4.97 km/sec for the sandstone block. The resulting impact craters have diameters of approximately 22 cm and 9 cm, respectively, for the quartz sand and solid sandstone targets. 29Si NMR spectroscopy of recovered target material from each impact experiment provides a useful method for comparing samples and degree of shock loading. The 29Si MAS (magic angle spinning) NMR spectra from samples collected across most of the crater in the quartz powder target exhibit relatively little variation in the quartz resonance. However, the shocked quartz collected from the central part of the crater exhibits extreme broadening of the NMR peak, indicating significant shock metamorphism. The samples from the impact crater formed in the sandstone block are characterized by broadened NMR peaks; the most significant broadening occurs for the material from the center of the crater and from the crater edge. Shelkov D.* Milledge H. J. Verchovsky A. B. Kaminsky F. Pillinger C. T. Preliminary C, N, He and Ar Study of Shock Produced Diamonds from the Popigai Crater and Ebeliakh Placer, Siberia Herein we present the first results of C and N isotopic measurements along with N, Ar and He content study of shock originated diamonds extracted from the impactites of Popigai crater and Ebeliakh placer deposits. Delta ^l3C values suggest that two different populations may be involved. Koeberl C.* Reimold W. U. Petrological and Geochemical Studies of Samples from the Nicor Chestnut 18-4 Drill Core, Ames Impact Structure, Oklahoma The near-circular 15-km-diameter Ames structure is located at 36 degrees 15' N and 98 degrees 12' W in southeastern Major County (NW Oklahoma). The structure, which is set in Cambro-Ordovician Arbuckle dolomite, consists of two concentric rims, an outer rim, which is about 1.5 to 3 km wide, and an inner "rim". The rocks of the outer rim consist mainly of fractured and brecciated Arbuckle dolomite. The inner "ring" (about 5 km in diameter) seems to be the eroded remnant of a central structural uplift, with rocks comprising brecciated Precambrian granite and Arbuckle dolomite. The depression is covered by Middle Ordovician Oil Creek shale. The structure is penetrated by a number of oil- and gas-producing wells in the crater rim and the central uplift. The production from these wells indicate that Ames represents one of the largest - if not the largest - single oil fields in Oklahoma. Currently the structural disturbance is buried beneath almost 3000 m of sedimentary rock. The origin of the structure has been intensely debated since the discovery of the structural anomaly, but geophysical and geological, as well as petrological and geochemical data provide very good evidence that it was formed by impact, and not by volcanism or even more esoteric processes. In the present study, we analyzed 17 samples, including impact melt breccia, from the Nicor Chestnut 18-4 core. These samples represent the largest and best examples of impact melt breccias and melt rock obtained so far from the Ames structure. One important result of the petrographic analyses is the observation that not all carbonate rocks postdate the impact, but some were clearly present among the target rocks. The chemical composition of the impact melt breccias is similar to that of other melt rocks from the Dorothy 1-19 core, as well as to the target granite, with variable carbonate admixtures. Some impact melt rocks are enriched in siderophile elements, indicating a possible meteoritic component. Holker Th.* Deutsch A. The Boltysh Impact Structure, Ukraine: Geochemistry of the Melt Sheet The contribution is devoted to test to which degree major, minor and trace elements, including REE, as well as Sr-Nd isotopic compositions are homogenised within coherent impact melt sheets. For this case study, drill core samples from the Boltysh impact structure, Ukraine, were analysed using XRF, ICP-MS, TIMS, microprobe, and SEM techniques. Despite textural variations of the impact melt rocks, the geochemistry of the impact melt rocks is extremely uniform with the exception of potassium, and initial Sr isotope ratios. Wednesday, March 20, 1996 NEWEST LUNAR METEORITES 2:30 p.m. Room D Chair(s): M. M. Lindstrom O. Eugster Lindstrom M. M.* Mittlefehldt D. W. Morris R. V. Martinez R. R. QUE94281, A Glassy Basalt-rich Lunar Meteorite Similar to Y-793274 QUE94281, the newest basaltic lunar meteorite, is a complex regolith breccia with abundant glass. This glass occurs in a variety of forms and compositions; fusion crust, splash glass coating, glass veins and glass beads. In both mineralogy and composition the breccia is intermediate between highland and mare rocks, but closer to mare samples. It is very similar to lunar meteorite Y-792374 and may be derived from the same site on the Moon. Arai T.* Warren P. H. Kallemeyn G. W. Lunar Meteorite QUE94281: A Possible Pair of Y793274 and/or EET87521 QUE94281 is a spherule-poor regolith breccia composed mainly of mineral fragments of pyroxene and plagioclase. Its overall texture is roughly intermediate between those of Y793274 and EET87521. About 2/3 of the pyroxene fragments larger than 100 mm contain pyroxene exsolution lamellae of order 1 mm in width, a remarkable similarity with Y793274 and EET87521. QUE94281 large pyroxenes also show a similar range of Fe-Mg-Ca variations to those of Y793274. Almost all the large pyroxene fragments distribute along a single crystallization trend on a Fe# (Fe/[Fe+Mg]) vs. Ti# (Ti/[Ti+Cr]) plot. The Ti# intercept at Fe# = 0.5 for this pyroxene trend suggests that the source mare basalt was a VLT variety with bulk-rock TiO2 ~ 0.8 wt%, another strong similarity with Y793274 and EET87521. QUE94281 is possibly a pair of Y793274 and/or EET87521. Nishiizumi K.* Caffee M. W. Exposure Histories of Lunar Meteorites Queen Alexandra Range 94281 and 94269 Most lunar meteorites have complex cosmic ray exposure histories having been exposed both at some depth on the moon (2 pi irradiation) before their ejection and as small bodies in space (4 pi irradiation) during transport from the moon to the earth. Finally, many lunar meteorites found in Antarctica have long terrestrial exposure ages, as is the case for many other meteorites found in Antarctica. A unique determination, in the best of cases, of the duration of each of these distinct epochs requires the measurement of four or more cosmogenic nuclides. We report here cosmogenic 10Be (half-life = 1.5 My), 26AI (0.705 My), and 36CI (0.301 My) results for the new lunar meteorites Queen Alexandra Range 94281 (QUE94281) and Queen Alexandra Range 94269 (QUE94269). From these radionuclide measurements alone we cannot completely unravel the complex history of these objects however we can set meaningful limits on their irradiation histories. Eugster O.* Polnau E. Lunar Meteorite QUE94269 - Pairing with QUE93069 Confirmed. Lunar Meteorite QUE94281 - Similarity with Y-793274 Lunar meteorite QUE94269 (3.2 g), an anorthositic breccia, was recognized to be paired with QUE93069 (21.4 g). We obtained a 0.023 mg interior chip of QUE94269 for the characterization of the solar wind- and cosmic ray-exposure history and comparison with QUE93069. The concentrations of the trapped and cosmic ray produced noble gases and their isotopic ratios are very similar in QUE94269 and QUE93069 (36Artr: 3.45 and 3.66x10-4 cm3STP/g, 21Nec: 36.6 and 40.8x10-8 cm3STP/g). They represent the most mature known lunar meteorites with respect to solar- and cosmic ray-particle irradiations. Lunar meteorite QUE94281 (23.4 g) is a basaltic breccia. It's chemical composition is similar to that of lunar meteorite Y-793274. In fact, the concentrations of trapped 36Ar in these two lunar meteorites are similar but QUE94281 shows a higher loss of He and Ne than Y-793274 and it's lunar regolith residence time (T38 = 370 Ma) appears to have been only half of that of Y-793274. Benoit P. H.* Symes S. J. K. Sears D. W. G. The Irradiation and Recent Thermal History of QUE 94281 and Other Lunar Meteorites: Evidence for Short Transit Times The recent history of lunar meteorites can be divided into three stages, including time spent on or near the lunar surface, ejection into space and transit to Earth, and time spent on Earth. Most lunar meteorites have short terrestrial ages and cosmic ray exposure (CRE) ages compared to most basaltic meteorites. It should, however, be noted that these "ages" carry large uncertainties. The lunar meteorites have significantly lower natural thermoluminescence (TL) levels than basalticmeteorites. Cosmogenic radionuclide abundances suggest this is not the result of lunar meteorites having large terrestrial ages, and we and no differences in short-term ("anomalous fading") characteristics between lunar meteorites and basaltic meteorites. We therefore interpret our TL data in terms of short cosmic ray exposure after ejection from the Moon. EET 87521, MAC 88104/5, QUE 94281, and Y 793169 have apparent space CRE durations of <10,000 years. ALH A81005, Asuka 881757,and Y 82192 have apparent CRE exposure durations in excess of 10,000 years, but these data may represent pre-ejection irradiation. Wednesday, March 20, 1996 SOLAR PROTONS AND RARE GASES 3:45 p.m. Room D Chair(s): R. C. Reedy A. J. T. Jull Jull A. J. T.* Cloudt S. Evidence for Recent Gardening or Disturbance in Lunar Core 76001 from Solar-Cosmic-Ray Records of 14C Due to their long exposure times, lunar soils and rocks provide a continuous record of galactic (GCR) and solar-cosmic-ray (SCR) intensities. Lunar cores are very useful in studies of the production of nuclides by GCR particles in very large objects. Lunar samples also contain especially good records of SCR effects and variations of SCR fluxes in the past. We report on new 14C measurements obtained as a function of depth in the Apollo 17 drive tube core, 76001. This core was described as undisturbed by NASA documentation. The core was collected at a break in the slope of North Massif, on an 11o slope. Results are shown in the figure and compared to our earlier experimental results on the apparently undisturbed Apollo 15 cores 15001-15006 and 15008. The results show a disturbance of the core data in the top few g/cm^2 of this core. Sisterson J. M.* Schneider IV R. J. Jull A. J. T. Donahue D. J. Cloudt S. Kim K. Beverding A. Englert P. A. J. Castaneda C. Vincent J. Reedy R. C. Revised Solar Cosmic Ray Fluxes Estimated Using Measured Depth Profiles of 14C in Lunar Rocks; the Importance of Good 14C Cross Section Determinations Long-lived radionuclides produced in lunar rocks by solar cosmic rays (SCR) preserve an historical record of the Sun's activity. SCR flux estimates over different time intervals can be made by using radionuclides with different half-lives. Knowing the solar proton flux in the past combined with modern direct measurements made with satellites results in better estimates of hazards that might be met in space missions. The SCR flux estimated over the time period characterized by the ^14C half life (5730 years) does not agree with those calculated using other radionuclides, and one possible explanation might be that the cross sections used in the calculation were incorrect. We present new values for the cross sections ^16 O(p,3p)^14C, Mg(p,x)^14C, and Fe(p,x)^14C at selected energies to add to those that we have reported before. Lunar rocks are usually 41-46% (by weight) oxygen, so it is very important to have good values for the cross section for the reaction ^16 O(p,3p)^14C, particularly near threshold. We present new values for this cross section and revised estimates for the SCR flux over the past 10,000 years. Garrison D. H.* Bogard D. D. Rao M. N. Reedy R. C. Determination of ~2 Ma Average Flux and Rigidity for Energetic Solar Protons from Lunar Rock 61016 Earlier NASA spacecraft measured real-time characteristics of energetic solar protons and found the flux and some other parameters to vary widely. The only way to determine some long-term, average characteristics of solar protons is by measurement of their nuclear interaction products as a function of depth within the very few lunar rocks that have oriented geometries and simple exposure histories. Most previous studies utilized radionuclides such as 26Al, 53Mn, 14C. etc. We developed techniques to measure SCR (i.e., solar-proton-produced) 21Ne, 22Ne, and 38Ar, and in lunar rocks 68815 and 61016 we reported average values for the flux and rigidity (i.e., particle energy distribution) for solar protons of >10 MeV over the past 2 Ma. However, our initial work on rock 61016 included a limited number of samples and a less sophisticated method of deriving solar proton characteristics. Recently we measured SCR Ne and Ar in additional depth samples of 61016, using procedures previously reported. Duplicate data sets between three measurement series were taken for four depths, and these values were averaged to normalize all of the data. Here we report more precise results for proton flux and rigidity determined from all our analyses of 61016. Wieler R.* Kehm K. Meshik A. P. Hohenberg C. M. Solar Noble Gases in Single Lunar Regolith Grains: Xe-Kr-Ar Fractionation in the Solar Corpuscular Radiation Confirmed Elements with a first ionization potential (FIP) below 10eV are enriched in the solar corona and solar energetic particles relative to photospheric abundances by a factor of ~4. We concluded earlier that Xe (FIP = 12.1 eV) is also enriched relative to Kr (14.0 eV) and Ar (15.8 eV) and that the mean Kr/Xe fractionation is only half as high today as it was 1-3 Ga ago. This conclusion was based on the inference--provided by noble gas data obtained with closed-system etching--that the lunar regolith conserves the relative abundances of Ar, Kr, and Xe unaltered. Here we provide further evidence for this with analyses of all five noble gases in single grains of two lunar soil samples. Becker R. H.* Schlutter D. J. Rider P. E. Pepin R. O. Reevaluation of the Solar Wind 36Ar/38Ar Ratio The isotopic composition of solar wind (SW) argon is an important parameter in the modeling of the evolution of the terrestrial planet atmospheres. Anders and Grevesse assumed a 36Ar/38Ar ratio for SW of 5.31, essentially equal to that of air. Considerable evidence has since developed which indicates that this ratio is too low. Benkert et al. have reported their best estimate for the recent SW as 5.48 +/- 0.05, determined from measurements of lunar soil 71501. Based on Ar data obtained from surface oxidation of a metal separate from the Weston meteorite and from an uncontrolled etch of lunar sample 67701, reported by our group previously, we consider even this value to be too low. Since values of 5.75 to 5.85 were reported by Black for initial low temperature (<150 degrees C) pyrolysis steps on samples of Kapoeta, which, even allowing for diffusive fractionation, imply a fairly high SW 36Ar/38Ar ratio (in the range of ~5.6 to ~5.7), we decided to analyze Kapoeta for its light solar wind gases using the acid-etching techniques developed in our laboratory based on the CSSE procedure of Benkert et al. Thursday, March 21, 1996 RESURFACING AND TECTONIC HISTORY OF VENUS 8:30 a.m. Room A Chair(s): C. D. Brown P. J. McGovern Lenardic A.* Kaula W. M. Venus: Its Great Transition The Magellan imagery shows that Venus has a crater abundance equivalent to a surface age of 300-500 Ma and a crater distribution close to random. Hence the tectonics of Venus must be quiescent compared to Earth's in the last few 100 Ma. The main debate is whether its decline is closer to monotonic or episodic, with enhanced tectonism and volcanism yet to come. The former hypothesis implies most radioactive heat sources have been differentiated upward; the latter, that they have remained at depth. The low level of activity in the last few 100 million years inferred from imagery favors the monotonic hypothesis; the low abundance of radiogenic argon favors the episodic. A problem for both hypotheses is the rapid decline of thermal and tectonic activity some 300 to 500 Ma. The nature of the convective instabilities that caused the decline, and their propagation, are unclear. Namiki N.* Solomon S. C. Potassium Budget and Fractional Degree of Mantle Melting on Venus from Venera and Vega Gamma-ray Measurements The abundance of potassium in the terrestrial planets is important for understanding interior heat production and thermal evolution as well as planetary outgassing and magmatism. Recent analyses of the impact crater population on Venus from Magellan radar images led to the hypothesis that the planet experienced nearly global volcanic resurfacing approximately 500 My ago and that volcanic activity after this event continued at a lesser, but appreciable, rate. The distinctive magmatic histories of Earth and Venus, despite their similarities in mass, solar distance, and presumably bulk composition, have been variously postulated to be the result of different styles of mantle convection on the two planets under the assumption that the heat production per mass, and in particular the K concentration, in Venus and Earth are approximately equal. Parmentier E. M.* Head J. W. Hess P. C. Lithospheric Buoyancy: Implications for Global Resurfacing on Venus Impact craters on Venus are consistent with a surface age of 300-500 Myr and their distribution on the surface is indistinguishable from a random one. Despite the relatively young age and volcanic origin of the surface, remarkably few craters are obviously flooded by volcanic deposits. The simplest interpretation is relatively rapid global resurfacing about 300-500 Myr ago. Mechanisms that could cause catastrophic and perhaps episodic global scale mantle overturn with consequent decompression melting and volcanic resurfacing are thus important to understanding the evolution of Venus. The age at which the lithosphere contains zero net buoyancy is a function of mantle potential temperature since this controls the crustal thickness. The calculated maximum age of zero net buoyancy is about 300 Myr for a mantle potential temperature of about 1500 degrees C corresponding to a crustal thickness of about 40 km. It must be regarded as a concidence that this age concides almost exactly with lower bounds for the estimated age of the Venusian surface. The calculated age depends at least weakly on parameters such as the melting model, which is not precisely known for Venus. This simple anaylsis does show that the buoyancy of the crust and depleted mantle layer are sufficient to keep the lithosphere buoyant for time scales at least comparable to the present age the Venusian surface. Relative to the much younger age of zero net buoyancy of the terrestrial oceanic lithosphere, this is accomplished by having a thicker crust. Gravitaitonal instability of dense granulite or eclogite may limit the crustal thickness to about 40 km. Head J. W. III* Basilevsky A. T. Wilson L. Hess P. C. Evolution of Volcanic Styles on Venus: Change, But Not Noachian? The Venus cratering record has been interpreted in terms of a geologically recent (~300-500 Ma ago), possibly catastrophic global tectonic and volcanic resurfacing event followed by diminished levels of activity; we examine the stratigraphic and volcanic record and its petrogenetic implications in order to test this model and understand its consequences. We find that there are definite planet-wide changes in volcanic and tectonic style and flux with time and that volcanic changes can be interpreted in terms of variations in the depth and percent partial melting. Although peak flux decreased with time, the average flux was apparently never much more than values typical of recent global volcanism on Earth. These observations suggest that early tessera-related tectonism was the major factor in the eradication of pre-existing impact craters and that post-tessera volcanism was areally widespread but volumetrically relatively low. Basilevsky A. T.* Head J. W. Regional and Global Stratigraphy of Venus: A Preliminary Assessment and Implications for the Geologic History of Venus Analysis of the Magellan imagery in numerous representative sample areas of Venus led to establishing the stratigraphic sequence of geologic formations of the planet starting with heavily deformed tessera-forming materials, followed by the suite of plains-forming volcanics, moderately deformed by widespread deformation, which, in turn, are locally overlapped by younger volcanics mostly associated with rifts. This sequence and the resulting scenario of the geologic history of Venus, put constraints on the character and rate of processes in the planet interior. The global high-resolution Magellan coverage of Venus provided a basis for establishing the characteristics of the major geologic units and structures of this planet, for assessing their stratigraphic relations, and for interpreting the geologic history of the planet. Such an endeavor will require many years of intensive mapping and analysis of the data before a detailed picture emerges. Here we report on an initial analysis of widely distributed areas on the planet and a preliminary assessment of regional stratigraphic units into the model of global stratigraphy of Venus. Further detailed studies, especially made through the NASA-sponsored Venus Geologic Mapping Program, are necessary to test the suggested stratigraphy model. Hansen V. L.* Willis J. J. Structural Analysis and Geodynamic Implications of Tessera Terrain, Venus Understanding processes of tessera formation is fundamental to Venus tectonic and geodynamic models. We examined tessera terrain in Ishtar Terra, crustal plateaus, and as inliers within the plains using high-resolution Magellan SAR imagery. We describe several major types of tesseraeeach found in specific geologic or geomorphic regions. Fold and S-C tessera terrain are found only in Ishtar Terra; lava flow and basin-and-dome terrains reside within the interior of crustal plateaus, whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus; and star terrain lies within central Phoebe. Inliers are divisible into fracture-dominated and graben-dominated tesserae, which may represent ancient flooded coronae-chasmata and crustal plateaus, respectively. Thus tesserae might form in several tectonic environments, including as a result of (1) subsurface flow in Ishtar Terra, (2) as sequences of surface-layer extension and contraction in crustal plateaus, (3) as highly-extended, previously-deformed crustal plateaus which have deflated or sunken, and become flooded and thus preserved as large plains inliers, and (4) as densely-fractured surface layersfractured as a result of corona and chasma formationwhich have since sunken and become flooded, and thus preserved as isolated, scattered, highly-fractured inliers. If these models of formation are correct, tesserae would not form a global onion skin; they would not represent a globally synchronous unit; they would not record a single period of deformation; and it would not infer a single mechanism for tesserae formation. Ivanov M. A.* Head J. W. Tessera Terrain in Ovda Regio, Venus: Preliminary Results of a Geologic Mapping Traverse Tessera terrain covers about 35 x 10^6 km^2 (~8%) of the surface of Venus; everywhere it has been examined it appears to represent the oldest stratigraphic unit. There have been only a few attempts to decipher the details of tessera internal structure. We have completed a geologic traverse across a large tessera occurrence in the central portion of Ovda Regio (at about 82 degrees E) in an attempt to understand 1) the units making up the surface of tessera and the nature of the tessera precursor terrain, 2) the sequence of events in Ovda tessera evolution, and 3) the timing of this process. We analyzed the morphology of the tessera structural facies to assess the nature of the tessera precursor terrain and the sequence of events leading to tessera formation and we used intratessera plains as a stratigraphic marker in defining stages in the formation and evolution of Ovda Regio. Johnson C. L.* Solomon S. C. Variations in Lithospheric Properties on Venus from Coronae and Chasmata Topographic flexural signatures and geoid (or gravity)/topography admittance studies provide constraints on lithospheric properties (such as thickness and strength) on Venus, but are unable to distinguish between spatial and temporal variations in such properties. Relative variations in surface age of sufficiently large areas can be distinguished however, on the basis of impact crater densities. In particular, impact crater densities for large volcanoes, the chasmata regions, and coronae with extensive associated volcanism indicate that these three classes of features are young relative to the mean surface age of the planet. We postulate that differences between coronae associated with chasmata, those on swells (excluding Beta and Atla), those in the plains, and those in corona chains reflect a combination of spatial and temporal variations in lithospheric properties. These variations are investigated using information derived from (a) studies of lithospheric thickness at coronae and chasmata, (b) variations in corona morphology with tectonic regime, (c) impact crater densities, and (d) relaxation of impact crater topography. Brown C. D.* Grimm R. E. The Thermal Evolution of Venus as Recorded by Surface Tectonics The Venus impact crater record suggests that the planet experienced a resurfacing event of virtually global proportions roughly 400 million years ago. The catastrophic resurfacing hypothesis holds that minimal volcanic and tectonic activity has occurred in the interim; a parallel argument is that the lithosphere has cooled and thickened in the post-catastrophic period. We evaluate this postulate by examining correlations between the relative ages of tectonic features and their inferred thermal gradients. Using an instability growth model, we find a high geotherm is necessary to explain the deformation wavelengths of tessera, among the oldest terrains on Venus. Flexure and gravity modeling of coronae and large volcanoes, structures of intermediate age, indicate lower geotherms at the time of formation. Artemis Chasma--a zone of lithospheric underthrusting--is believed to be relatively young; it displays coherent, plate-like tectonics and an extraordinarily low temperature gradient. Together these observations favor the monotonic cooling and thickening of the venusian lithosphere in the past few hundred million years. McGovern P. J.* Solomon S. C. Implications of Stress Modeling for Volcanic Structure and Magmatic Flux on Venus We describe a new structural model for large volcanoes on Venus emphasizing the importance of material that fills the flexural depression beneath the edifice. The moat-filling material is integral to the structure of volcanoes on Venus, in contrast to the fragmented and structurally weak character of material that fills flexural moats around volcanoes on Earth. Neglect of this moat-filling material will cause underestimation of volcanic flux by an order of magnitude. Models of resurfacing and thermal evolution of Venus must be able to accommodate a significant magmatic flux during the period of large volcano construction that was comparable to the current terrestrial intraplate eruption rate. Kreslavsky M.* Venus Cratering Record: Constraints on Resurfacing History Two facts about impact crater population on Venus, namely (1) observed rather homogeneous spatial distribution of craters, and (2) low number of lava embayed craters, constrain resurfacing history of Venus. It had been shown that the facts are consistent with the catastrophic resurfacing model, implying that almost all the surface was flooded by lava during rather short period. It had also been shown that the equilibrium resurfacing model implying constant rate of resurfacing is not consistent with the facts. Presented are advanced models of crater population evolution, which give quantitative constraint on duration of surface formation. It was found that the facts are consistent with a resurfacing history with decreasing resurfacing rate, in which RMS variation of surface age is not higher than 40% of average surface age. Collins G. C.* Basilevsky A. T. Head J. W. Ivanov M. A. Impact Crater Embayment on Venus and the Termination of Global Resurfacing The sequence of plains emplacement on Venus seems to follow a consistent pattern over large areas of the planet. The plains units can be separated into materials responsible for the global resurfacing event of Schaber et al. and materials which post-date this event. We studied embayed craters and determined the stratigraphic position of the embaying material to find if all embayed craters had been embayed since the resurfacing event, or if some were embayed by the event itself. Gilmore M. S.* Ivanov M. I. Head J. W. III Basilevsky A. T. Deformation of Craters on Tessera Terrain, Venus The Venus cratering record has been shown to have a distribution that is indistinguishable from one that is spatially random, leading to the interpretation that the planet underwent a catastrophic resurfacing event 300 (+300/-110) Ma ago. Although >80% of the surface is composed of volcanic plains, this average age applies to all terrains, including the highly deformed tessera terrain which has been shown in the vast majority of cases to be stratigraphically older than the plains. Using high resolution Magellan data, we have compiled an independent list of tessera craters on Venus, extending to smaller crater sizes the work done by Ivanov and Basilevsky. We then examined each of the deformed craters to determine whether the deformation is attributable to processes of tessera formation or processes that postdated tessera formation. These analyses have resulted in a list of craters that suffered tessera deformation prior to the emplacement of the global plains. The number density of these craters may then be used to constrain the duration over which tessera deformation occurred. Thursday, March 21, 1996 METEORITES: MARTIAN AND OTHERS 8:30 a.m. Room B Chair(s): D. A. Kring E. Jagoutz Thomas K. L.* Romanek C. McKay D. S. Keller L. P. Gibson E. K. Microanalysis of Unique Fine-grained Minerals in the Martian Meteorite ALH84001 ALH84001 is an igneous martian meteorite, consisting primarily of orthopyroxene (opx) and accessory phases including chromite, maskelynite, and carbonate. Trace amounts of augite, apatite, olivine, and pyrite have also been observed. In this work, we use transmission electron microscopy (TEM) to describe the chemistry and mineralogy of ultra fine-grained (nanometer-sized) regions in ALH84001. These regions include optically dark inclusions in the opx and Fe-rich grains rimming carbonate spheroids. These Fe-rich rims have precluded prior characterization because of their ultra fine grain size. Grady M. M.* Wright I. P. Pillinger C. T. Nitrogen and Argon in ALH 84001 Revisited: Unravelling a Martian Atmospheric Component We have measured nitrogen and argon released simultaneously upon combustion of two samples of ALH 84001 (A84). Both nitrogen and argon appeared to be heterogeneously distributed in the rock: one sample liberated very little gas above blank levels, whereas the second sample, selected because it contained abundant material from the "crush zones", was rich in both species. Using the ^40Ar/^14N ratio and delta ^15N of the gas liberated above 700 degrees C from this second sample, an attempt has been made to calculate the relative quantities of adsorbed terrestrial gases and trapped martian atmospheric species. Following from this, excess ^40Ar attributed to radiogenic production from potassium decay can be used to determine a K-Ar age of the sample. We calculate that ~17.5% of the total ^40Ar is indigenous to the sample. Assuming that the trapped component would have a ^40Ar/^14N ratio equivalent to that in the present-day martian atmosphere (~0.33), then there is a small excess of ^40Ar (amounting to about 1.5% of the total ^40Ar). Taking a reasonable estimate of the potassium abundance (100-200 ppm) implies that A84 has a K-Ar age of ~0.76-1.28 Gyr, which is much younger than ages determined in previous studies and using other methods. Jagoutz E.* Nd Isotopic Systematics of Chassigny Neodymium isotopes of the Chassigny SNC meteorite are reported. An age of 1.362 ñ 0.062 Ga is inferred. The 142Nd/144Nd has an anomaly of 0,71 ñ 0,26 epsilon which implies that the reservoir from which Chassigny was formed was depleted in LREE as early as 4,5 Ga. The positive initial 143Nd/144Nd of 15.2 ñ 0,5 epsilon also indicates an origin from a depleted resorvoir. These data strongly support the model of Jagoutz (1991). Shih C.-Y.* Nyquist L. E. Wiesmann H. Sm-Nd Systematics of Nakhlite Governador Valadares Sm-Nd systematics of two whole rock and three mineral separate samples from nakhlite Governador Valadares define a ^147Sm-^143Nd age of 1.36+/-0.03 Ga. This age is in excellent agreement with the Rb-Sr and ^39Ar-^40Ar ages obtained previously for this meteorite. The petrographic evidence suggests that this rock is a pyroxene cumulate crystallized in a subsurface sill. Thus, the isotopic age probably represents the age of such a magmatic event. The initial e^143Nd value determined for the rock at 1.36 Ga is +17+1, indicating the parent magma of the rock came from a LREE-depleted source. Our Sm-Nd age and e^143Nd data, and the previously published e^142Nd datum for the rock are consistent with the early formation of its source about 4.56 Ga ago, and late melting of the source and formation of the rock about 1.3 Ga ago. The good agreement of isotopic ages and textures among Governador Valadares, Nakhla and Lafayette strongly suggests that all nakhlites probably have undergone similar two-stage evolution. Romanek C. S.* Treiman A. H. Jones J. H. Gibson E. K. Socki R. A. Oxygen Isotopic Evidence for Aqueous Activity on Mars: Delta 180 of Lafayette Iddingsite This study was prompted by the observation that oxygen in Lafayette is significantly enriched in 18 O over than that in Nakhla, delta 18 O = 5.3 per mil vs. 4.7 per mil (all relative to SMOW) even though both meteorites have essentially the same delta 18 O for their olivines (~4.0 per mil) and augites (~4.8 per mil). A possible source of this 18 O-enriched oxygen in Lafayette is preterrestrial aqueous alteration material, which is much more abundant in Lafayette than in Nakhla. This alteration material, "iddingsite," in both meteorites consists principally of smectite clay and ferrihydrite replacing magmatic olivine and siliceous glass. To test this hypothesis, separates of augite, olivine, and "iddingsite" were extracted from Lafayette and analyzed for oxygen isotopes by laser fluorination. _ Xue S.* Yu Y. Hewins R. H. Hall G. S. Herzog G. F. Zinc Losses from and Zinc Isotopic Abundances in Residues Formed by Heating of Zn, ZnO, ZnS, and Zn-Doped Silicate Glass To find out whether 1) losses of Zn might occur and 2) leave an isotopic imprint under conditions relevant to the formation of chondrules, we carried out a set of controlled heating experiments. Silicate mixtures doped with zinc as metal, as oxide, or as sulfide lose appreciable quantities of that element --from 25% to more than 90%. When introduced as metal or as oxide, but not as sulfide, the Zn also undergoes mass-dependent fractionations up to 0.5%/AMU. Nakamura T.* Metzler K. Nagao K. Takaoka N. Noble-Gas Distribution in the Murchison, Murray, Nogoya and Yamato-791198 CM Chondrites Laser-spot analysis was carried out on the Murchison, Murray, Nogoya, and Yamato (Y)-791198 CM chondrites in order to see microdistribution of noble gases. Primordial noble gases are rich in rims around chondrules in the four meteorites. This suggests that large fractions of the primordial noble-gas carrier phases in the CM chondrites might have accreted on chondrules prior to formation of the meteorite parent bodies. Solar-type noble gases in Murchison, Murray, and Nogoya reside preferentially in the clastic areas which were probably formed by cataclastic effects due to regolith gardening. Cosmogenic noble-gas concentration in the clastic areas of Murchison is enriched by factors of three than that in the non-clastic area, suggesting that materials consisting of the clastic areas have been exposed to cosmic ray on the surface of the meteorite parent body, or the clastic areas have high abundance of preirradiated grains. Scherer P.* Loeken T. Schultz L. Changes in the Noble Gas Pattern of Hot Desert Meteorites Caused by Weathering - Correlation with Terrestrial Age and Weathering Grade We present new noble gas data on 11 ordinary H-chondrites from the Acfer region and one from Roosevelt County that confirm earlier results and provide additional information about the coherence of atmospheric noble gas contamination with the degree of weathering and terrestrial age. To study the compositional variations in heavily weathered meteorites in more detail, noble gases in four of these chondrites are determined from interior and exterior sections of the same specimen. Palme H.* Weckwerth G. Wolf D. The Composition of a New R-Chondrite and the Classification of Chondritic Meteorites A dramatic increase in the number of recovered meteorites during the last ten years led to the discovery of new meteorite classes. The Rumuruti meteorites (R-chondrites) presently comprise 10 meteorites, with only one fall, Rumuruti. Meteorites of this new group are characterized by high olivine content with predominantly FeO-rich olivine (ca. Fa 39) and the almost complete absence of metallic iron. Yet the bulk composition of these oxidized objects is similar to ordinary chondrites. It is therefore not clear, how these meteorites fit into conventional classification schemes for chondritic meteorites. Here we report XRF and INAA data on a new R-chondrite, Dar al Gani 013, from the Libyan desert. In addition, we have performed XRF-analyses on two other R-chondrites, Rumuruti and Acfer 217. The results confirm that R-chondrites are a unique group of chondritic meteorites that cannot be derived from ordinary chondrites by oxidation. Thursday, March 21, 1996 SATURN RINGS 10:30 a.m. Room B Chair(s): G. Consolmagno Horn L. J.* Rappaport N. J. Non-Linear Dispersion of Spiral Density Waves in Saturn's Rings We have analyzed the dispersion behavior of a number of spiral density waves in Saturn's A ring using the non-linear dispersion relation. We demonstrate that the dispersion behavior of the Mimas 8:5 and Prometheus 7:6 density waves is linear. On the other hand, the Mimas 5:3 density wave in Saturn's A ring disperses non-linearly over the extent of the wave and displays an unexpected difference in surface mass density between the Voyager radio science (RSS) and Voyager photopolarimeter (PPS) occultation data sets. The RSS surface mass densities were larger than the PPS values. This inconsistency may be indicative of different processes affecting the large (RSS) and small (PPS) particle populations. The dispersion behavior of these waves provides an important tool for probing the physical characteristics of the rings and estimating the ring age (which is currently thought to be much younger than the age of the solar system). Bauer J.* Lissauer J. J. Simon M. IRTF Observations of Saturn-Earth Ring Plane Crossing: Faint Outer Ring Near-IR Profiles On May 22nd and August 10th, 1995 (UT), the Earth passed through the plane of Saturn's main rings. In order to study Saturn's ring system and inner moons, we obtained several thousand images at 1.1, 1.7, and 2.2 micrometers on the nights of May 21st, 22nd, and 25th and August 6th 7th, and 8th at the NASA IRTF using the NSFCam, a 256 x 256 pixel InSb imaging detector. On all these nights but the 21st, the dark side of the rings was visible. We obtained several deeper exposures of the region beyond the East and West ring ansae of the main rings, and successfully observed Saturn's faint E and G rings. Hubbard W. B. Consolmagno G. J.* Boyle R. P. Hill R. The Shadow of Titan on Mimas and Saturn's Rings We observed the Saturn system on November 7-9, 1995, at the Vatican Advanced Technology Telescope (VATT) on Mt. Graham, Arizona, using the portable CCD (PCCD) fast occultation imaging system of the University of Arizona. Along with imaging seven of Saturn's moons, we recorded a rare mutual event on the evening of November 9 (4:25 UT November 10) where sunlight at the Saturnian moon Mimas was eclipsed by Titan. The shadows of Rhea and Titan were also seen moving across the rings of Saturn itself. Analyzing the change in brightness in this shadow during these events may provide information about the structure of Titan's atmosphere, and the precise timing of the shadow-Mimas event will give improved precision to the ephemeris of Mimas' orbit. Thursday, March 21, 1996 IMPACT STORY--MECHANICS, ATMOSPHERES, AND WORLD DESTRUCTION 10:30 a.m. Room C Chair(s): W. B. McKinnon V. L. Sharpton O'Keefe J. D.* Ahrens T. J. Planetary Strength, Central Peak Oscillation, and Formation of Complex Craters Previous theories/models explain the formation of complex craters. These include: 1) dynamic theories that assume rapid collapse of the transient crater, oscillation of the central peak, and the propagation and freezing of wave structures, to 2) quasi-static theories that assume material failure and slumping of the transient cavity walls, or viscous relaxation of the transient cavity. We numerically modeled the initial shock wave driven flow fields to the late-stage strength-and gravity-driven motions leading to iso-static equilibrium. Two (bounding) strength models can describe effects of deep regoliths to highly consolidated surfaces and give varying final crater depth, diameter and other features. Transition from simple to complex craters occurs for planetary strengths consistent with conventional strength models (e.g. ~10 kbs, at depth). In complex craters, the diameter of the inner ring corresponds to the diameter of the transient cavity. The calculated depth/diameter ratios for craters varies from 1/6 to 1/60, depending upon model strength and scaling regime. Turtle E. P.* Melosh H. J. Finite-Element Modeling of the Vredefort Impact Structure with Implications for the Collapse and Modification Stage of Large Crater Formation While the morphology of large impact craters has been studied in some detail, the processes which generate structures such as central peaks, peak rings, and multiple external rings are still poorly understood. We are using finite-element modeling to investigate the collapse and relaxation phase of impact crater formation with the specific intention of determining the mechanisms necessary to reproduce the subsurface structure observed at Vredefort in South Africa. We have incorporated the rheologic parameters for a simplified stratigraphy into a mesh which has been modified to reflect the displacements that occur during the crater excavation phase. The modeling of the subsequent relaxation of this transient crater has generated up-turning of layers at stratigraphic boundaries similar to that observed at Vredefort. In addition, the stresses produced are consistent with the collapse of the transient crater wall in the formation of a complex crater. Newman W. I.* Symbalisty E. M. D. Jones E. M. Ahrens T. J. Impact Events and the Erosion of Planetary Atmospheres: Some Surprising Results from Theory and Simulation We investigate the evolution of the atmospheric blast waves produced by a K-T sized meteoritic impact on a terrestrial planet. Extending the mathematical analysis due to Kompaneets (1960) for a localized explosion in an isothermal atmosphere, we explore the response of an adiabatic atmosphere as well as model atmospheres for Earth and Venus to a 5 times 10^30 erg event. A principal outcome is that the blast waves will not spread laterally by more than pi times the scale height, approximately 30 km. for the Earth, the result of the upward diversion of the expanding blast, into the direction of least resistance. This effect was first noted in the simulations due to Jones and Kodis (1982). Further extension of the analytic methods provides exact particle trajectories for the ejecta and the prediction that virtually no material would escape from the planet. Our calculations show a factor ten less atmospheric loss than the Vickery and Melosh model (1990) and are confirmed by Los Alamos' CAVEAT finite difference code. Boslough M. B.* Crawford D. A. Interacting Atmospheric Plumes from Bolide Swarms We have used the Sandia shock physics code, CTH, to simulate the interaction of atmospheric impact plumes generated by an array of simultaneous impact events on Earth. This work was stimulated by advances in the understanding of atmospheric impact processes since the impact of comet Shoemaker-Levy 9 (SL9), and by our desire to apply what we have learned to terrestrial processes. We performed 2-D axisymmetric calculations with pseudoperiodic boundary conditions to determine the effects of the interaction of plumes resulting from a cluster of impacts, compared to a single, isolated event. We simulated a plume from the impact of a 20 km/s, 34 meter-diameter asteroid (kinetic energy of 3 megatons) at vertical incidence, and compared its evolution and collapse to interacting plumes from arrays of impacts with near-neighbor separation distances of 40, 100, and 200 km. As expected, the closer-packed arrays lead to denser plumes that reach higher altitudes and generate higher temperatures within denser air upon collapse. These results can be applied to models for generation of layered tektites by radiative heating from an impact-heated atmosphere. Simulations of interacting plumes from bolide swarms can also be used to consider dispersed rubble-pile models for SL9. Takata T.* Comet Shoemaker-Levy 9 Impact on Jupiter: Plume, Ejecta, and Scaling of Fragment Size Three-dimensional numerical simulations of long-term plume evolution induced by the impact of comet Shoemaker-Levy 9 (SL9) are presented. Results show that at 6 minutes after the impact, the ejecta starts to fall back on the Jovian atmosphere and 'splash back' is observed. However, the initiating time of splash back largely depends on initial conditions of setting the heat source. Ejecta pattern observed by Hubble Space Telescope is also reproduced by a simple particle model, in which the fall back ejecta produces the crescent shape feature. Analytical scaling relation between the impact energy of fragments of SL9 and maximum plume heights is also presented. This scaling relation agrees with results of three-dimensional numerical analysis of impact of fragments of >= 0.5 km-size. Crawford D. A.* Boslough M. B. Using HST Plume Height Data to Place Lower Bounds on Comet Shoemaker-Levy 9 Fragment Mass and Penetration Depth Models of Comet Shoemaker-Levy 9 fragments entering the Jovian atmosphere and subsequent plume evolution are strongly constrained by the seemingly contradictory observations that plumes observed by the Hubble Space Telescope (HST) all had approximately the same maximum altitude yet the dark ejecta they left behind varied considerably in albedo and lateral extent. We modeled plume evolution for fragments with diameters of 125-2000 m using the CTH shock-physics code. Initial conditions were provided by an analytical model that has been calibrated against numerical entry simulations. The plume simulations show that maximum altitude of a specific isodensity contour is an increasing function of fragment size and mass. However, if the tops of the plumes observed by HST are derived from material originating from a specific level of the atmosphere (an isocomposition contour), then maximum plume heights are independent of fragment size provided the fragments penetrated at least 30 km below this level. For example, if the tops of the plumes originated from the visible cloud tops, then fragment masses greater than 4x10^12 g (equal to 200 m diameter fully dense water ice) are required to explain the observations. If the visible plumes originated from the NH4SH layer, then masses greater than 3x10^13 g (400 m water ice) are required. Schmidt R. M.* Housen K. R. Hassig P. J. Effect of Boundary Conditions on Calculations for Venusian Entry Shock The question of atmospheric effects in crater formation was addressed by Schmidt and Hassig (1995) and Schmidt et al.(1995) using numerical simulation. Presentation of these earlier results are somewhat obscured by scaling models that relate spacial and temporial variables to crater size and time of formation. The results shown here are unscaled and shown in prototype dimensions. The original 10-km impactor calculation performed by Hassig (1991) used a numerical artifact to create a "transparent" target allowing the impactor to pass through the surface. Hence, the calculated aerodynamic effects were due only to the entry shock wave. There was concern that vaporization might provide an additional contribution to the flow field. Likewise, a compliant cratering target would cause some vaporization while at the same time absorbing a lot of the kinetic energy in the form of coupling to the ground leading to crater formation. To evaluate the effects of these various boundary conditions, two additional calculations were run. One had a rigid surface, causing the impactor to arrest, vaporize and rebound upon contact. The second used a coarsely-zoned quartz-like material to allow crater formation to take place. In the early time, close in, there were significant differences. However at the range of expected final crater radius, the profiles all more or less coalesce. Sugita S.* Schultz P. H. Impact Vapor Generation Inferred from Run-out Flows on Venus Oblique impact craters on Venus are commonly associated with long run-out flows extending downrange. Parametric hydrocode calculations are used to assess the variables controlling the downrange off-set of the source region of such flows. We find that the impact-angle dependence of the downrange offset of the source region of the run-out flows measured from the point of impact can be readily explained by energy and momentum coupling efficiencies in the downrange impact vapor/melt. Barnouin-Jha O. S.* Schultz P. H. Atmospheric Origin of Flow Lobes Around Craters on Mars and Venus Distal flow lobes around craters on planets with atmospheres may result from the formation of waves in a ring vortex created by flow separation at the top edge of an advancing impact ejecta curtain. The number of waves in a smoke ring is controlled by two factors: 1.) the ratio of the radius of the ring vortex (Rv) to its core radius (a); and, 2.) the intensity of the flow in the ring vortex core. At laboratory scales, the number of ejecta flows from ejecta-driven ring vortices varies with these factors in the same fashion as waves of a smoke ring. Theoretical and laboratory studies indicate that the number of flow lobes created during an impact varies with the crater radius (R) to the three-fourths power (i.e. R3/4). Lobe counts on Mars and Venus confirm this correlation for those craters whose radii do not exceed the scale height of the atmosphere and where the flow in the ring vortex does not exceed the ambient speed of sound. The formation of flow lobes by ring vortices created by the motion of the ejecta curtain, are consistent, therefore, with observation at planetary scales. Variations in the number of lobes on Mars could indicate variations in target lithologies, presence of volatiles or changes in atmospheric conditions. Further work will establish which of these contributing factors most affects the number of distal lobes created during an impact. Ivanov B. A.* Pogoretsky A. V. Bingham Parameters for Fluidized Ejecta Spreading on Mars and Martian Volatiles We estimate martian ejecta flow parameters, comparing the horizontal ejecta motion after ballistic deposition with the motion of terrestrial flows of the known nature (rock avalanches, lahars, debris and pyroclastic flows). The ejecta flow rheology is approximated with the Bingham law. The ejecta spreading is measured as the ratio of the outer ejecta blanket radius, R(sub)m, to the transient cavity radius, R(sub)t. For typical values of R(sub)m/R(sub)t from 3 to 6, estimated Bingham parameters of the ejecta correspond occupy the parameter's field just between dry rock avalanches and water-saturated debris flows on Earth. While the model will be more advanced, such a comparison seems to allow a quantitative measure of volatile content and their physical nature from Viking imagery. Love S. G.* Ahrens T. J. Catastrophic Impacts in the Gravity Regime The collisional evolution of the asteroids is often modelled by analogy with small-scale, strength dominated laboratory impact experiments. Recent calculations Holsapple, however, suggest that gravity dominates over strength in determining impact behavior for silicate objects larger than ~6 km diameter. Thus, results from strength dominated impacts may not apply to most numbered asteroids or to most meteorite parent bodies. To investigate catastrophic impacts (i.e., those that erode ~50% of the target's mass) on gravity dominated objects, we wrote a three-dimensional Smoothed Particle Hydrodynamics (SPH) computer code that includes a rigorous treatment of gravity. We modelled the impacts of variously sized silicate projectiles onto silicate targets 10 to 1000 km in diameter at impact speeds of 3, 5, or 7 km/s and angles of 15, 45, or 75 degrees. The impact energy is increasingly partitioned into projectile and target kinetic energy at higher impact speeds and larger projectile:target size ratios. Particle velocity distributions are complex in shape and evolve continuously throughout each simulation. The amount of material permanently ejected from the target is roughly proportional to the projectile mass when the speed, angle, and target size are held constant. The catastrophic threshold (Q*) for 50% target mass removal occurs at projectile kinetic energy per unit target mass (specific energy) equal to 8 x 10^3, 3 x 10^4, 8 x 10^4, 3 x 10^5, and 1.5 x 10^6 J/kg for target diameters of 10, 31.6, 100, 316, and 1000 km, respectively. Projectile material was not retained on targets smaller than 1000 km diameter. The collisions modeled here may launch ejecta into closed satellite orbits. Material strongly heated in our simulations usually escapes; the retained heat produces volume averaged impact heating of <10 degrees C per event for asteroids 1000 km), sinuous, relatively narrow channels (~3 km) with almost constant width along their lengths; and "pancake domes", circular, steep-walled features >25 km in diameter. Similarly, Venusian lava flows commonly approach 1000 km in length-much longer than typical terrestrial lava flows. The uniqueness of the venusian lava forms has prompted workers to propose that exotic or evolved lava compositions were responsible for their formation. For example, carbonatite or sulfur flows have been suggested as the formational agents for Venusian canal), and others have suggested that the pancake domes are composed of rhyolitic lavas or extraordinarily frothy basalts even though it is difficult to explain how such large volumes of exotic or evolved lava could have been generated on Venus, especially in the absence of Earth-like plate tectonics. However, it is not only possible, but likely, that all of these venusian lava morphologies can be explained through the interaction between large volumes of basaltic lava and the dense, thick, hot venusian atmosphere. Rogers P. G.* Zuber M. T. Campbell B. A. Crossflow Topographic Effects on the Emplacement of Leveed Lava Flows: Implications for Venusian Lava Flows Through integration of theoretical modeling and field studies of lava flows, we can attempt to understand the flow dynamics and emplacement mechanisms that give rise to their final morphologic features. By understanding the development of such features on terrestrial flows, we hope to extrapolate analytical models to studies of lava flows on other planets. However, the final dimensions and detailed morphology of a lava flow depend upon the complex interaction between effusion rate, rheologic parameters, and underlying topography. This work examines the dependence of the commonly used Bingham flow model on topographic slopes parallel and perpendicular to the flow direction, and assesses the impact of slope uncertainties on interpretation of remote sensing data for Venus. Bridges N. T.* McGill G. E. Formation and Modification Ages of Some Steep-sided Domes on Venus Large, steep-sided domes are among the most enigmatic features on Venus. Their origin is controversial, and numerous models considering both emplacement mechanisms and compositions have been proposed. An understanding of their stratigraphic positions relative to other geologic units and structures is critical. Age relationships between domes and surrounding plains materials and structures are commonly complex. However, we find that most of the steep-sided domes examined in this study formed before or during structural modification of the surrounding plains. Bulmer M. H.* An Examination of the Transport and Emplacement of Long Run-Out Debris Masses Associated with Modified Domes on Venus Evidence for slope failures exists on Mars, the Moon, Earth and Venus, but the processes involved in the transport and emplacement of large mass movements (> 10^6 m^3) remain poorly understood. The majority of models proposed to explain the long travel distances and low coefficients of friction of large mass movements are based on the concept that debris were fluidized either by internally or externally derived processes. The mass movements G1 and G2 with the longest runouts on Venus are of particular interest since they are large and appear to have been highly mobile, tending to travel further for a given vertical drop than mass movements on Earth, Mars and the Moon. This mobility may be explained if a fluidizing agent or lubricant enhanced the locomotion of the debris, but in the absence of water on Venus another medium must be postulated. Likely influences and controls on the transport and emplacement of debris masses on Venus can be divided into those resulting from the geological and geomorphological conditions and those that result form environmental conditions. Having examined the geological and geomorphological influences on the transport and emplacement of debris masses on Venus it appears that neither the nature of the rock mass, the site of the slope failures, nor its traveled path can adequately account for the tendency for mass movements on Venus to travel further for a given vertical drop than mass movements on the other terrestrial planets. The effects of atmospheric conditions on Venus seem likely to be the important factor in explaining the apparent mobility of large rock masses over the surface of the planet. Thursday, March 21, 1996 CHONDRULES IN ORDINARY CHONDRITES 1:30 p.m. Room B Chair(s): H. Nagahara R. H. Hewins Karner J. M. Wasilewski P. J. Rietmeijer F. J. M. Nuth J. A. Preliminary Electron Microbeam Analyses of Lightning Induced Evaporation and Gas Phase Mixing Transient high-energy events such as reconnecting magnetic fields, chemical energy in amorphous solids, frictional heat (infall model) and lightning are invoked for the formation of chondrules in the solar nebula. It remains an open question whether any single one of these events, or a combination of them, is responsible for the variety of chondrule properties in meteorites. We used the Langmuir Laboratory's Triggered Lightning Research Facility on South Baldy Mountain near Socorro (New Mexico) to conduct an exploratory experiment on lightning induced alteration of refractory materials in conjunction with an ongoing experiment to study the formation of lodestone in natural deposits. Blankets of glass wool (~1 cm thick) were placed in a cylindrical container as separations between three samples of natural magnetite. The samples were placed perpendicular to its axis and were about 1 cm thick. The lightning strike passed through the container from top to bottom thereby passing through the three samples and two intervening blankets of glass wool. The passage of the lightning strike through the container left a hole in each blanket with a diameter of about 1 cm. The hole's wall in the blanket between the samples of pure magnetite sample and magnetite with some amounts of (Mg,Fe,Ca)-silicates and calcite shows an orange-brown discoloration. A spray pattern with similar discoloration occurred on this blanket surface surrounding the hole. Several millimeter-sized black spheres and (rare) black fragments were located around the hole where the lightning strike exited the blanket. Weidenschilling S. J.* Production of Chondrules by Lightning in the Solar Nebula It is widely believed that chondrules were formed in the solar nebula by transient heating events that melted precursor aggregates of dust grains. The nature of the heating events is unclear, but one persistent suggestion is some sort of electrostatic discharge or "lightning". This idea is attractive because the formation of planetesimals involved settling of particles into a dense layer in the central plane of the nebula. This layer provided an environment with high solids/gas ratio, high opacity, and potentially high concentration of precursor aggregates. Moreover, this dense dusty layer was turbulent due to shear relative to the nebular gas. Such a turbulent dusty environment offers the prospect of charge separation and generation of electrical discharges of some sort. However, arguments for and against lightning remain inconclusive because of limited understanding of the generation of terrestrial lightning and difficulties of scaling such processes to very different conditions in the solar nebula. In the present work I report on efforts to place other constraints on the chondrule-forming process in this context from models of particle settling and coagulation during the formation of planetesimals. Assuming "lightning" can occur, are other conditions met? Can shear-generated turbulence provided enough energy? Does particle coagulation produce sufficient precursor aggregates of the appropriate size? Are chondrules concentrated or sorted aerodynamically, and accreted efficiently into planetesimals? Nagahara H.* Evaporation Induced Isothermal Crystallization of Silicate Melt In order to investigate and role of evaporation and crystallization kinetics for silicate melt, isothermal vacuum experiments were carried out in the system MgO-SiO2. Due to successive evaporation, melt crystallized olivine at a fixed temperature. The evaporation rates and bulk chemical composition of residues varied with time, and reached a steady state. The pressure-composition phase diagram for the system at a fixed temperature well explains the experimental results. The results suggest a possibility of isothermal formation of chondrules (and some CAIs) at low pressures where evaporation takes place continuously. Hewins R. H.* Zanda B. Bourot-Denise M. Evaporative Loss and Degree of Melting in Semarkona Type I Chondrules Bulk compositions have been determined by broad beam techniques for Semarkona type I (FeO-poor) chondrules. The finest grained (least melted) approach CI in composition, and abundances of moderately volatile elements (K, Na, Fe, Ni, P, S) decrease as grain size (degree of melting) increases. This is unequivocal evidence of evaporative loss during chondrule formation. Zanda B.* Bourot-Denise M. Hewins R. H. Chondrule Precursors: The Nature of the S- and Ni-bearing Phases(s) Abstract Lauretta and Fegley showed that troilite made by S condensation on kamacite should include a significant amount of Ni, but they reported being unsuccessful in their search for Ni-bearing troilite in chondrites. We show the various opaque associations (OAs) found in veneers around chondrules in primitive and less primitive chondrites to be the result of parent body decomposition under different conditions of an initial Ni-bearing monosulfide solid solution (Ni-MSS). OAs in the less melted (finest grained) Semarkona chondrules are similar to the ones in veneers and differ from those in coarser grained chondrules. As fine-grained chondrules must have kept a better memory of their precursor minerals, this indicates that the dominant opaque phase in chondrule precursors was Ni-MSS, the logical end member of S condensation on kamacite in the solar nebula. Connolly H. C. Jr.* Hewins R. H. Compound Chondrules and Igneous Rims: Multiple Heating of Chondrules Understanding the formation of independent compound chondrules and igneous rims (IR) could provided insight into the frequency and intensity of melting events within the chondrule-forming region(s) of the nebula. Experimental reproduction of independent compound chondrules showed that for the experimental conditions used, independent compound chondrules could only be formed from the collision of molten or semi-molten droplets. We report here on several additional experiments with porous dust coatings on charges that attempt to reproduce independent compound chondrules and actually form the igneous/melt rims as reported. Lofgren G. E.* Le L. Thermal Recycling of Chondrules: I. Experimental Study of Remelting on Texture Recycling of chondrule material is clearly a significant part of the chondrule forming process. Recycled material includes whole chondrules, but more often fragments of chondrules comminuted down to individual crystals. These recurring relicts attest to the vigor of the forming process and its duration. Experimental study of the remelting kinetics of recycled material can provide further constraints on the nature and duration of the chondrule forming process. We have begun an experimental study of the thermal recycling history of chondrules, chondrule fragments, and minerals primarily by conducting experiments that duplicate aspects of their kinetically controlled heating and cooling (crystallization) histories. This study builds on a decade of experimentation directed towards developing a general model for chondrule crystallization. The current study is an enlargement of the dynamic crystallization chondrule model to include recycling of chondrule material during chondrule formation. Jones R. H.* Layne G. D. Trace Element Partitioning in Porphyritic, Pyroxene-rich Chondrules in Semarkona The behavior of trace elements during chondrule crystallization has not been investigated extensively. The most detailed study is that of Alexander, who determined trace element data for olivine, pyroxene and glass for chondrules from three ordinary chondrites, Semarkona, Bishunpur and Chainpur. We have measured trace element concentrations (REE, Sr, Y and Zr) of pyroxenes and mesostases in four chondrules from Semarkona, with a view to understanding the behavior of these elements during chondrule crystallization. REE elements are incompatible in all pyroxenes, but are more strongly partitioned into Ca-rich pyroxenes than low-Ca pyroxenes. Pyroxenes have negative Eu anomalies, consistent with igneous crystallization, and mesostases have complementary positive Eu anomalies, consistent with closed system fractional crystallization of individual chondrules. Partition coefficients of many of the minor and trace elements measured are higher than equilibrium values in comparable systems. This is attributed to the rapid cooling rate of the chondrules. One of the chondrules studied has notably high REE partition coefficients for augite, close to 1. This may be attributable to the high Al2O3 content of this augite which enhances charge-balanced substitutions for M3+ cations such as REE. Hanon P.* Chaussidon M. Robert F. High C and H Contents of Chondrules Carbon and hydrogen concentrations (reported hereafter as in ppm C and [H] in ppm H2O) of 33 chondrules of all petrological types and sizes, and belonging to some of the least altered and metamorphosed chondrites were determined with the CRPG Nancy ion-microprobe. Special care was taken in order to efficiently get rid of the terrestrial contamination. Before analysis, each sampling area (0~25 micrometers and ~50 micrometers for the smaller chondrules) was sputtered by the O^2- primary beam (20nA) for 5 minutes. Precise chemical concentrations for H and C were obtained for a -60V offset applied to the sample, along with an energy filtering of +/- 10V. Mass resolution (M/Delta M) of 1800 is sufficient to discriminate the 24Mg++ signal from the 12C+. Mid-ocean ridge basalts were used for calibration of C and H. Major element concentrations in phases were obtained by electron probe analysis. Then, in each chondrule, phase proportions were visually estimated allowing the calculation of a bulk concentration for the major elements. [C] and [H] were obtained by two methods : 1) using internal chondrule correlations between individual ion-probe spots for carbon (or H) and major elements contents or 2) by averaging all ion-probe [C] and [H] determinations. _ Steele I. M.* Extraterrestrial Forsterite: Review of Present Knowledge While forsterites are in nearly all primitive extraterrestrial samples, there is no obvious conclusion for origin and relation to other components. Some questions include: 1) how its heavy oxygen can be reconciled with a chondrule origin; 2) high minor element content in light of known distribution coefficients; 3) reason for two main chemical variations associated with specific sample types; 4) an origin by condensation or from melt or more complex process? For each question there are several or possibly no answers. Suggestions are given for additional studies. Grossman J. N.* The Redistribution of Sodium in Semarkona Chondrules by Secondary Processes Sodium is the most easily measured volatile element in chondrules, accessible to nearly all commonly used micro- and bulk-analytical techniques. Experiments show that Na is easily lost from chondrule melts under certain conditions, making it one of the key elements for understanding chondrule thermal histories, precursor materials, and the environment in which chondrules formed. The apparent paradox that chondrules contain abundant Na, whereas theory and experiment seem to require that it should have been evaporated during the melting event have led to speculation that, (a) the experiments are not applicable, (b) chondrules formed in peculiar environments, or (c) the distribution of Na and other volatiles, particularly sulfur, in chondrules may be secondary. The latter suggestion has been particularly fueled by the recent discovery of a chondrule with zoned mesostasis in Semarkona, which is consistent with late entry of volatiles. I report three lines of evidence that support speculation (c). First, a significant fraction of Semarkona chondrules appear enriched in Na within mesostasis near the chondrule surface. Second, fine-grained chondrules almost always show evidence for significant secondary loss of Na from their surfaces. And third, all types of olivine-bearing chondrules contain glass inclusions in the olivine in which the inclusions are strongly depleted in volatiles compared to the intergranular mesostasis. I attribute these effects to subsolidus alteration of chondrules, probably by more than one process; candidates include very light metamorphism and aqueous alteration, changing the outer portions of chondrules, and an earlier, more pervasive metasomatism of chondrule glass. The setting for each process remains uncertain. Krestina N.* Jagoutz E. Kurat G. Sm-Nd System in Single Chondrules from Tieschitz (H3) We investigated the Sm-Nd system in single chondrules from the Tieschitz (H3) meteorite. An isochran obtained for some chondrules yielded an age of 2.04 Ga, and a chondritic initial ratio. This isochron probably dated the timing of chondrule alteration. Some of the chondrules fall on 4.55 Ga reference isochron. The fractionation of the REE for one of them was caused by a process in the solar nebula at the time of chondrule formaton. Deloule E. Robert F.* Origin of Water in Meteorites: Ion-Probe Determinations of D/H Ratios in Chondrules Water contents and H isotopic compositions have been determined with the CRPG-Nancy ion microprobe in twelve chondrules (type I and type II after McSween's classification) from several deuterium-rich LL3 meteorites. The water concentration ranges from 500 to 17,000 ppm, with most of the data lying between 1000 and 5000 ppm and the dDSMOW values between -510 per mil and +1930 per mil (i.e. D/H ratios between 76 and 457 x 10^-6). Several spots were analysed in each chondrule to test their internal homogeneity. In an individual chondrule, the distribution of both water concentration and its isotopic composition could be either quite homogeneous, either extremely heterogeneous, but no systematic gradients were found. These ion microprobe measurements demonstrate that hydrogen bearing components (water and organics) were present in chondrule precursors. Thursday, March 21, 1996 K/T IMPACT AND IMPACT VAPORIZATION 1:30 p.m. Room C Chair(s): M. S. Bell J. A. Tyburczy Kinsland G. L.* Hurtado M. Ceron A. Pope K. Ocampo A. Smythe W. Bedard P. Analysis of Gravity and Topographic/Bathymetric Data Over the Chicxulub Impact Structure: A Look at Details The Chicxulub impact feature, has been characterized as being relatively circular with a diameter of 180 Km, 240 Km, or 300 Km. To arrive at these characterizations the authors have relied for the most part upon gravity data, sparse well control, a few seismic lines and a ring of cenotes (sinkholes). We are better defining the feature by analysis of the details of the gravity and topographic/bathymetric data. Bell M. S.* Sharpton V. L. Small Scale Heterogeneities in K/T Boundary Tektites From Mimbral Relict tektites from El Mimbral, Mexico are from K/T boundary deposits considered to be proximal ejecta related to the Chicxulub impact crater on the Yucatan peninsula. Our dark-brown Mimbral clasts are from the spherule bed (layer 1 of Bohor & Betterton) just above the latest Maastrichtian Mendez formation and are postulated to have been deposited initially in a shallow water environment. Relict tektites examined in this study are similar in bulk composition to those in previous reports of Mimbral tektites. Previous studies describe compositions determined to be homogenous on the 10-100 micron scale. However, spot analyses of Mimbral glass reveal small scale heterogeneities within individual fragments which were not optically apparent. Elemental mapping was utilized to illuminate heterogeneities in the range of 5 to 100 microns and to more clearly characterize their distribution. Compositional variations determined by mapping have been quantified by spot analysis and are described here. Pope K. O.* Ocampo A. C. Fischer A. G. Morrison J. Sharp Z. Carbonate Condensates in the Chicxulub Ejecta Deposits from Belize It has long been proposed that large amounts of CO2 released to the atmosphere by impact vaporization of carbonates could trigger greenhouse warming. Recent studies of the Cretaceous/Tertiary Chicxulub impact indicate that large amounts of both carbonate and sulfate were vaporized, although sulfates had a much more dramatic effect on climate in part due to the relatively small ambient sulfate reservoir of the Earth's atmosphere compared to the huge ambient reservoir of CO2. One process that could mitigate the climatic effects of these volatiles is the back reaction of impact generated oxides (CaO and MgO) with CO2 and SO2-SO3 in the vapor plume. Analyses of Chicxulub ejecta deposits from Belize confirm that the proximal ejecta in this locality are dominated by carbonate lithologies and sulfates are extremely rare. Much of the carbonate is in the form of dolomite and calcite spheroids and euhedral dolomite silt matrix. These spheroids and fine-grained matrix may have formed through condensation in the vapor plume, thus sequestering large amounts of impact generated CO2. Kyte F. T.* A Piece of the KT Bolide? A heavily altered lithic fragment of probable meteoritic origin has been recovered from sediments of the Cretaceous/Tertiary (KT) boundary from Deep Sea Drilling Project (DSDP) Site 576. DSDP Site 576 is located in the western North Pacific (32 degrees 21.4'N, 164 degrees 16.5'E) and sediments recovered at this site record a nearly complete sequence of pelagic clay sedimentation from Late Cretaceous to Recent. At 65 Ma this site was located in the central region of the ancestral Pacific basin approximately 9,000 km due west of the Chicxulub impact structure in the Yucat_n. The KT boundary has now been identified in two cores at this site recovered from Holes 576 and 576B. The KT boundary is characterized by anomalous Ir concentrations, as high as 13 ng/g, coincident with a trace mineral assemblage dominated by shocked quartz and Ni-rich magnesioferrite spinel. At Site 576, anomalous Ir (>1 ng/g) occurs across nearly 100 cm of section, but a sharp peak with the trace minerals is more localized. In Hole 576, FWHM for the Ir anomaly is ~10 cm. The breadth of the Ir anomaly has been attributed to bioturbation and possibly chemical diffusion within these homogenous pelagic clays. Schuraytz B. C.* Lindstrom D. J. Marin L. E. Martinez R. R. Mittlefehldt D. W. Sharpton V. L. Wentworth S. J. Iridium Metal in Melt Rock from the Chicxulub Impact Basin We identified a ~4-micrometer-long, ~1 x 10^-10 g "nugget" of Ir metal in a melt rock sample from the Chicxulub impact basin. This particle, which consists of an aggregate of subhedral Ir metal grains surrounded by a thin coating of silicate, was isolated in the course of our efforts to better understand the partitioning of projectile material between crater deposits and ejecta in large meteorite impact events. In addition to confirming earlier reports of Ir in melt rock and melt breccia within the K-T boundary source crater, the results of our search for physically identifiable carriers of meteoritic components provide additional evidence for extreme fractionation of siderophile elements and their concentration in sparsely disseminated trace phases. Pierazzo E.* Melosh H. J. Kring D. A. Numerical Simulation of the Chicxulub Impact Event Our understanding of the effects of impacts on the development and evolution of life is still incomplete and approximate, and very few experimental data are available to help us out. One of the best case-studies for this problem is the Chicxulub impact event. The Chicxulub crater, located in the Yucatn Peninsula, Mexico, has been associated with one of the most dramatic extinction events characterizing the evolution of life on the Earth, the extinction that marks the Cretaceous-Tertiary boundary. Unfortunately, the complex target geology at the Chicxulub site has proven to be a difficult obstacle for modeling studies and up to today, few simulations have been carried out. A good equation of state for the thermodynamic properties of the target materials (in particular carbonates, e.g., calcite, and evaporites, e.g., anhydrite) is still needed. This study reports the progress of our modeling of the target geology in the Chicxulub event. The main improvement over previous work consists in the development of an ANEOS equation of state for calcite and granite, which are necessary for an accurate simulation of the Chicxulub event. The preliminary results of our simulation indicates a CO2 production of 1-2 1015 kg, lower than in previous studies, and a production of water vapor of about 6 1012 kg, most of which will end up in the stratosphere. Tracer particles are used in the simulation to follow the trajectory and the thermodynamic conditions of the ejecta plume. Baines K. H.* Pope K. O. Ocampo A. C. Ivanov B. A. Long-term Environmental Effects of the Chicxulub Impact The impact of a large bolide into the northern Yucatan peninsula 65 million years ago resulted in unusually prolonged global environmental effects which caused the mass extinction that marks the K/T boundary. As previously discussed, the impact explosively released into the stratosphere about 100 billion tons of sulfur dioxide excavated from the the unusually sulfate-rich target rock, thereby producing a long-lasting reservoir of aerosol-generating material that severely affected the world climate. Here, we update our previous analysis of the evolution of the impact-generated stratospheric sulfur dioxide reservoir to explicitly account for (1) the large amount of water injected into the stratosphere from the Chicxulub site and (2) the diffusion of stratospheric gases back into the troposphere. We have also performed a preliminary analysis of the role of excavated salts in modifying the stratospheric abundance of ozone-destroying chlorine. Salient new results are: (1) water abundance did not limit the formation rate of stratospheric sulfuric acid hazes, (2) these reflective hazes - responsible for severe global cooling - lasted between ten and eighteen years (a marked decrease from the one-century upper limit, and (3) the stratospheric chlorine abundance increased by 1-2 orders of magnitude over the ambient concentration. Lyons J. R.* Ahrens T. J. Chicxulub Impact-Induced Vaporization: S and C Species and Their Affect on Global Climate Thick deposits of anhydrite at the Chicxulub impact site suggests that large quantities of sulfur oxides were released during the impact event. Oxidation of sulfur oxides emplaced in the stratosphere would produce long-lived sulfuric acid aerosols capable of substantially cooling Earth's surface. Recent radiative transfer modeling shows that a stratospheric H2SO4 loading of 5 Gt of sulfur is capable of reducing solar transmission below the photosynthesis limit of the biosphere, provided i) the sulfuric acid aerosols contain sufficient impurities, and ii) the reduction in solar transmission lasts for a sufficiently long time. Such calculations raise several questions: 1) How much impact-liberated S is transported to the stratosphere?; 2) Is S primarily in the form of SO2 or SO3?; 3) Are the required impurity levels plausible?. Each of these questions is addressed below. In general, the answers support the sulfuric acid aerosol scenario of Pope et al. Gerasimov M. V.* Dikov Yu. P. Yakovlev O. I. Wlotzka F. Reduction of Carbon During High-Temperature Vaporization in the Calcium-Carbonate-Silicate System Last year we reported the formation of a wide range of calcium silicates during vaporization of CaCO3-CaSO4/CaSO4 x 2H2O-SiO2 mixtures by a laser pulse, with application to the Chicxulub impact. It was shown that complex redox processes occur in the vapor cloud under dry conditions yielding part of the Si in metallic form and half the carbonas graphite. Here we investigated experimentally the Ca, Si and C high temperature chemistry using calcium carbonate-silicate targets. We used natural spurrite (Ca2SiO4 x CaCO3)and a compressed mixture of powders of CaCO3 and SiO2 with the same atomic Ca/Si ratio as in spurrite. The Ca-Si chemistry was found to be the same as in the previous research and independent of the initial state of Ca and Si in the target. Reduction of Si was found to be higher than in Gerasimov et al., amounting up to 9% of total Si. Different forms ofreduced C were identified: C as graphite, silicon carbide, and complex carbon agglomerates of fullerene type. Tyburczy J. A.* Ahrens T. J. Xu X. Epstein S. Impact-Induced Devolatilization and D/H Isotopic Fractionation of Serpentine-Metal Mixtures Impact-induced devolatilization is a process fundamental to accretion. Impact velocities of infalling planetesimals increase in proportion to planetary radius. At some point in a planet's accretion history, minimum infall velocities will be sufficient to cause incipient devolatilization of incident planetesimals, thereby creating a planet's earliest proto-atmosphere. Reaction of shock released water with iron or iron sulfides will influence both primordial atmospheric development and core formation. Previous studies have examined impact-induced devolatilization in serpentine, carbonates, sulfates, and Murchison. Our earlier studies have established that for serpentine and Murchison, impact-induced devolatilization caused D:H fractionation between released gas and residual solid, with H being preferentially partitioned into the released gas. In this study, we have shocked serpentine-iron and serpentine-pyrrhotite mixtures to examine the influence of a metal phase on the impact-induced devolatilization and D/H isotopic fractionation of water driven from serpentine by shock. _ Yakovlev O. I.* Dikov Yu. P. Gerasimov M. V. Bychkov A. M. Wlotzka F. High-Temperature Vaporization in the Albite-Orthoclase System In the present work we have investigated the relative volatilities of K, Na, Al and Si in the albite-orthoclase system under high-temperature pulse heating simulating an impact vaporization process. We have prepared seven samples with various proportions of albite to orthoclase and performed high-temperature vaporization using a powerful laser pulse. Condensed products were collected and analyzed. The main results of the experiments are: (1) the volatility of alkalis is smaller than the volatility of Si; (2) K is less volatile than Na and its volatility decreases with increasing orthoclase content of the target; and (3) volatilization proceeds with formation of a nepheline type molecular cluster (Na:Al:Si = 1:1:1) giving a condensate composed mainly of nepheline and SiO2 components. Rowan L. R.* Horz F. Zolensky M. Selective Oxidation/Reduction and Impact Melting in Experimental Metal-Silicate Craters We have produced thin veneers of crystalline to frothy projectile residues (where the projectiles varied from dunite, diopside, orthoclase or basalt cylinders) within small cm-sized craters formed in metal targets (where the targets varied from 1100 Al, Cu, SS304 or Mo). The morphology and mixing of the silicate residue with metal spherules is similar to that described in many natural impact melts including lunar samples, terrestrial impact crater melts and tektites, and shock features in some meteorites. Textural and chemical analysis suggests that local regions of these residues experienced high temperature gradients, fast rates of nucleation and crystal growth and minimal, but selective oxidation/reduction. Such inferences should help decipher the heterogeneous evolution of impact melts in terrestrial and extraterrestrial samples. Schultz P. H.* Adams M. A. Perry J. W. Goguen J. D. Sugita S. Impact Flash Spectroscopy Hypervelocity impacts into volatile-rich targets generate a self-luminous cloud that expands and decelerates in the presence of an atmosphere following theoretical expectations. Spectra of such clouds reveal prominent emission lines resulting from vaporized target components and reactions with the impactor. Because the degree of vaporization and cloud luminosity is found to vary with impact velocity, angle, and impactor/target combinations, experiments were designed using the NASA Ames Vertical Gun Range to assess the evolving spectral content. Our results establish that a wealth of compositional information can be derived from emission spectra at relatively modest impact velocities (5-6 km/s) for a wide range of carbonate and silicate targets. In contrast with much earlier studies that viewed the phenomena as a nearly instantaneous flash, our results document a complex vapor cloud that evolves in both time and space. Consequently, spectroscopy provides a powerful new tool not only for probing the impact process but also for exploiting this information to determine planetary surface compositions that might be difficult to measure or interpret with conventional remote-sensing techniques. Thursday, March 21, 1996 EDUCATION SPECIAL SESSION DISPLAYS: PLANETARY SCIENCE EDUCATIONAL ACTIVITIES AND TECHNOLOGY 6:30 - 9:30 p.m. LPI See Education Special Session Displays for Tuesday, March 19, 1996. Thursday, March 21, 1996 ORIGINS: FROM STELLAR DEATH TO LUNAR BIRTH POSTERS 6:30 - 9:30 p.m. LPI Boss A. P. Forming a Jupiter-like Companion for 51 Pegasi The recent discovery of a likely Jupiter-mass planetary companion to the star 51 Pegasi was startling because the 4.23 day period of the companion implies an orbital separation of only 0.05 AU from 51 Pegasi. If the companion is a gas giant planet formed by the mechanism believed to be responsible for the formation of Jupiter, then the companion to 51 Pegasi could not have formed at its present distance from 51 Pegasi. Radiative hydrodynamical models of protoplanetary disks show that any Jupiter-like planet must have formed at a distance from 51 Pegasi similar to that of Jupiter from our Sun, and then been dragged much closer to 51 Pegasi. Gravitational interactions with a long-lived protoplanetary disk provide an attractive means for accomplishing this planetary migration. Such a scenario implies that any Earth-like planets that also formed around 51 Pegasi disappeared into the central star long ago. Hahn J. M. Ward W. R. Resonance Trapping due to Nebula Disk Torques A protoplanet embedded in the solar nebula launches spiral density waves from its Lindblad resonances in the gas disk, and its gravitational attraction for these disturbances results in a mutual torque exerted between the protoplanet and the disk. Consequently the orbit of a sufficiently massive protoplanet may decay on a timescale shorter than the nebula lifetime, and this mechanism is most significant during the formation of the cores of the giant planets. Due to their increased mobility, migrating protoplanets may have been able to accrete large swaths of the disk and/or encounter other protoplanets. Thus disk torques may have played an important role in determining the formation history and orbit spacings of the giant planets. An interesting phenomenon also associated with orbit decay is resm nance trapping, whereby a large body is able to halt further orbit decay of smaller bodies at commensurability resonances. Examples of this effect include the trapping of planetesimals experiencing aerodynamic gas drag and dust suffering Poynting-Robertson drag. Below we address the cosmogonic implications of resonance trapping of planetary embryos experiencing orbit decay due to nebula disk torques. The following employs an approach similar to Malhotra's (1993) discussion of the gas drag trapping problem. Smith B. Nazzario R. Hyde T. W. Griffith D. Particle/Planetesimal Orbital Behavior near Proto-Jupiter It has been suggested that the accumulation time for Mars is dependent upon both the presence as well as development of proto-Jupiter. It has also been suggested that the formation of both the inner and outer planets are directly limited by the developmental time scale of proto-Jupiter. This paper examines the affect a developing proto-Jupiter would have on the orbital trajectories of particles and planetesimals ranging in size from 0.1 m to 100 m. Weidenschilling S. J. Davis D. R. Modeling Accretion in the Region of Terrestrial Planets A realistic model of accretion of planets from a swarm of planetesimals must allow for spatial variations within the swarm. Formation of a large protoplanetary embryo affects the distributions of size and orbital elements of smaller planetesimals in its vicinity, and makes them differ from the average properties of the swarm. This variation is inconsistent with the assumption of uniformity implicit in particle-in-a-box simulations. To avoid this problem, we developed a spatially resolved model that treats a hybrid population of planetesimals in a series of heliocentric zones which interact with each other and with discrete large bodies in individual orbits. We have now incorporated a gravitational stirring algorithm with separate evolution of inclinations and eccentricities, and applied this model to a swarm spanning the region of terrestrial planets, 0.5-1.5 AU. Pechernikova G. V. Vityazev A. V. Statistical Model of Earth-Moon Coaccretion and Macroimpacts The model of Earth-Moon coaccretion is developed in the framework of standard scenario of Origin of the Solar System. We take into account the evolution of population of large bodies which disappear in mutual catastrophic collisions (macroimpacts) during the planetary accretion Our model takes properly into account the additional feeding of presatellite swarm by the material ejected from outer shell of growing planet due to macroimpacts of falling bodies. This model includes five chemical reservoirs, that are circumsolar and circumplanetary disks and primitive core, mantle and permanently originating and destroying crust of the Earth. KenKnight C. E. Olivine as an Indicator of Convection in the Solar Nebula Convection at a late stage in the solar nebula formed pairs of loops through the midplane. Each pair extended radially about one atmosphere scaling height and was coherent (toroidal) in its motion for a large angle in azimuth around the Sun. Sediments were concentrated in ringlets in the midplane between vertical gas flows. Buoyancy waves propagated vertically and heated the upper atmospheres of the nebula. The convection lofted a mixture of growing crystals and smoke into the hot zone. Where aggregates of smoke on the crystals melted, the opacity of the atmospheres decreased markedly because the liquid densified the aggregate. Thus forming a liquid phase provided a "thermostat" for the con- vection. Isolated olivine crystals carried by the convection grew by this process, as revealed by an onion-like zoning of incompat- ible elements in the olivine crystals. Because the surface melts on olivine contained impurity crystals, transfers of volatiles controlled some fractionations in the nebula which have seemed puzzling. Chondrules were made from precursor solids when the convection adjusted itself episodically at shock wave speeds toward Keplerian rotation. Lee J. T. Manuel O. K.* On the Isotopic Composition of Primordial Xenon in Meteoritic Troilite and the Origin of the Chemical Elements Continuing our efforts to decipher isotopic and chemical heterogeneities in the early solar nebula, we measured xenon in troilite (FeS) from the Odessa, Canyon Diablo, and Toluca meteorites. When the FeS melts, the base isotopic composition of xenon released is terrestrial, Xe-T, plus excess radiogenic 129Xe and small amounts of excess 124Xe, 126Xe, 128Xe and 131Xe from spallation and (n,gamma) reactions. This confirms earlier observations of Xe-T in FeS separates of chondrites and iron meteorites. Xe-T is dominant in both the martian and the terrestrial atmospheres and solar xenon is much closer to mass fractionated Xe-T than to mass fractionated AVCC Xe. We conclude that Xe-T was prevalent in the inner, Fe- and S-rich region of the solar nebula where meteoritic FeS, the terrestrial planets and the Sun formed. This is consistent with the hypothesis that the solar system formed directly from heterogeneous debris of a single supernova but not with the suggestion that the base isotopic composition of solar xenon is Xe-U plus excess 134Xe and 136Xe from Xe-X. Differences between the isotopic compositions of the He and Ne that accompany Xe-X in meteorites and those of He and Ne in the solar wind suggest that intrasolar diffusion enriches lighter nuclei at the solar surface. This same process makes Xe-T in the Sun appear as Xe-U + Xe-X in the solar wind. Thursday, March 21, 1996 IMPACT STORY--MECHANICS, ATMOSPHERES, AND WORLD DESTRUCTION POSTERS 6:30 - 9:30 p.m. LPI Svetsov V. V. Simulations of the Comet Fall in the Jovian Atmosphere The fall and disintegration of the comet Shoemaker-Levy 9 in the Jovian atmosphere were numerically simulated by Ahrens et al., Takata et al., Boslough et al., Crawford et al., Zahnle et al., MacLow et al., Svetsov, and Yabe et al. The comet fragments were treated as strengthless impactors having a shape of a sphere or a cylinder. A ratio of a cylinder length to its diameter was varied from 0.33 to 3 by Crawford et al. Nemtchinov I. V. Artem'eva N. A. Ivanov B. A.* Kosarev I. B. Shuvalov V. V. Svetsov V. V. Neukum G. Hahn G. Wagner G. Luminosity of the Bolides Created by SL-9 Comet Fragments in the Jovian Atmosphere Shoemaker-Levy 9 Comet fragments intruding into the Jovian atmosphere form a wake, i.e., a hot, luminous, dissociated and ionized column. This wake expands and becomes a rarefied channel. In the dense atmosphere cracks grow in number and size and the fragment deforms as a liquid. Under aerodynamic load the meteoroid is additionally fragmented due to growth of instabilities. A cloud of small fragments decelerates, disintegrates and is vaporized. Debris and atmospheric gas escape through the rarefied wake formed at the "bolide" stage of the impact and stretched along the trajectory. A fireball, or a plume, rises to high altitudes due to nonuniformity of atmospheric density. Later, the ejected gas falls back onto the atmosphere causing atmospheric oscillations and heating. Popova O. P. Nemtchinov I. V. Estimates of Fireball's Mass Based on Luminosity Curves Investigation of the PN and EN bolides gives data on the apparent strength of the meteoroids, and premits to estimate mass influx of meteoroids with various size onto the Earth. Nemtchin