Monday, March 13, 1995 BASALTIC METEORITES: REAL AND VIRTUAL 8:30 a.m. Room A Chair(s): J. S. Delaney M. Sharma Delaney J. S.* 4 Vesta: A Thick-Skinned Parent for Basaltic Achondrites The regolith lithologies on the surface of the asteroid 4 Vesta probably sample most of the crust and possibly parts of the Vestan mantle as a result of extended impact processing. If the diogenites, eucrites and howardites are indeed samples of Vesta, and if as seems likely they represent several ejection events from Vesta, the suite of meteorites should provide representative sampling of the lithologies present at, or near, that asteroid's surface. The combination of the small size of Vesta (radius~265 km) with the reasonable cratering rates for the past 4.5 Ga implies that the regolith of the asteroid should be a globally mixed sample of the asteroid crust. The lack of young magmatic ages among achondrites further implies the crust of their parent body should be a well mixed breccia relatively unmodified by later magmatism. The lithologic composition of the howardites and polymict eucrites indicates that the eucrites and diogenites are the dominant lithologies in the crust of their parent body with volumetrically minor contributions from other lithologies such as cumulate eucrites and pigeonite rich rocks. Olivine bearing lithologies, especially magnesian olivine, are rare. Since the meteorites believed to sample Vesta have so few olivine bearing lithologies, the crust of Vesta, if derived by magmatic activity in a chondrite like precursor, must have thoroughly differentiated from an olivine dominated mantle that is essentially unsampled by the meteorites. The depth to that olivine rich mantle is presently unclear but must be sufficient that most impacts on Vesta did not excavate the mantle. To estimate the thickness of the crust we consider the structure of a Vestan planetoid derived from petrological and cosmochemical models of diogenite and eucrite genesis. Palme H.* Chakraborty S. Trace Element Fractionation in Differentiated Planetesimals by Non-Equilibrium Partial Melting A comparatively large number of meteorite parent bodies has undergone partial or complete melting. Eucrites, for example, are either partial melts or residues of fractional crystallization of a chondritic parent body. Lodranites and ureilites are solid refractory residues of partial melting processes. As melting must have occurred shortly after accretion, the decay of 26Al has been suggested as appropriate heat source. It is shown here that partial melting in a sufficiently large body heated by 26Al will lead to unusual trace element fractionations due to incomplete equilibration between melt and residual solid. Although major elements are at all times in equilibrium between melt and solid residue trace elements are not. The effect of non-equilibrium melting is most dramatically seen in the behaviour of Eu. The first phase to melt in a chondritic parent body is plagioclase, the major carrier of Eu (more than 80 %). This leads to a strong positive Eu-anomaly in the early-formed melt. During and after partial melting Eu and other trace elements will continually reequlibrate with the solid residue. A major fraction of Eu will re-partition into the minerals of the solid residue attempting to reduce the Eu-anomaly of the melt. If the melt separates immediately after formation, there is not enough time to establish trace element equilibrium between the partial melt and the residue. In a chondritic planetesimal, heated by 26Al decay, the formation of a 10 % partial melt will take several thousand years. However, because of the slow diffusion of REE in pyroxene, equilibration of the melt with the residue requires much longer times (at least 50 000 years assuming 1 mm grain size in the rsidue). Negative Eu-anomalies and other trace element characteristics in lodranites and ureilites are readily explained by non-equilibrium melting. If the eucrite parent body was heated by decay of 26Al trace equilibrium between eucritic melts and residual solid is unlikely, at least at small degrees of partial melting. The effects of non-equilibrium melting critically depend on the diffusivities of trace elements in residual minerals, the grain size of residual minerals, the phases that contain the trace elements and the time scale for formation and separation of the melt. Jones J. H.* Mittlefehldt D. W. A Petrogenetic Classification for the Eucrite Clan--Partial Melts, Fractionated Liquids, Cumulates, and Oddities The origin of the eucrites (basaltic achondrites from the asteroid belt) has been a matter of much discussion. However, the recent studies of Jurewicz et al., Jones et al., and Warren have served to renew interest and provide a better focus for this debate. The original work by Mason focused on relating eucrites and diogenites (cumulate pyroxenites) and suggested that most eucrites were fractionated liquids, having crystallized significant pyroxene. Later experimental work by Stolper suggested that at least some eucrites were primary liquids and that more fractionated eucrites could be related to these primitive magmas by fractionation of pigeonite and plagioclase. Jurewicz et al. and Jones et al. provided corroboration for Stolper's hypothesis by showing that partial melts of a chondrite were nearly indistinguishable in composition from eucrites such as Sioux County, which Stolper had designated as primitive. More recently, however, WalTen and Kallemeyn have challenged this interpretation. In these authors' view, Sioux County and similar eucrites are likely to be partial cumulates and, therefore, any similarity to experimental glasses is viewed as coincidental. Pun A.* Papike J. J. Yamato Unequilibrated Eucrites Y-74450, Y-793548, Y-82210, Y-74159, and Y-75011: Pyroxene Trace Element Systematics We have evaluated the trace element concentrations in the pyroxenes of several Yamato unequilibrated eucrites. All of the Yamato eucrites are polymict eucrites from the collections of the National Institute of Polar Research (NIPR). A total of 5 thin sections were examined for trace elements, Y-74450, 74-6; Y-82210, 61-4; Y-793548, 51-2; Y-74159, 62-1; and Y-75011,96. Y-793548, 51-2 is very weathered and clasts show obvious Fe-oxide staining. It consists of numerous fine-grained basaltic textured clasts and a few coarse grained eucritic clasts set in a brecciated matrix of pyroxene and plagioclase fragments. Y-82210, 61-4 contains large basaltic clasts with subophitic texture. Y-75011,96 consists solely of a large coarse grained eucrite clast with ophitic texture. Y-74159,62-1 is dominated by a large eucrite clast with varoilitic texture. Y-74450,74-6 contains several coarse grained eucrite fragments as well as some clasts with radiating plagioclase and pyroxene grains. It has been suggested that Y-75011 is paired with Y-74159 and possibly to Y-74450 as well. All three of these meteorites contain pyroxenes and calcic plagioclases common to various eucrites, and that some pyroxenes exhibit extensive zoning similar that found in the Pasamonte eucrite. We analyzed selected pyroxenes from moderately coarse grained unequilibrated eucrite clasts in all of the thin sections. Schwandt C. S.* Experimental Self-Diffusion Data for Orthopyroxene and its Significance for Meteorite Cooling Rates Orthopyroxene, which can contain compositional zoning of Mg and Fe, is a constituent of many meteorites. Mesosiderite meteorites are breccias consisting of roughly equal proportions of interspersed silicate clasts and Fe-Ni metal. Some clasts are magnesium-rich orthopyroxene (En[sub]83Fs[sub]17) surrounded by more iron-rich orthopyroxene (En[sub]63Fs[sub]37) overgrowths. The compositions grade transitionally from the clasts into the overgrowths. This information has been used to estimate the high temperature cooling rates of the silicate clast portions of mesosiderites. However, accurate estimation of cooling rates is dependent on the accuracy of the parameters used to calculate them. A critical parameter is the cation diffusion coefficient. Cation diffusion data for orthopyroxene has been lacking, although Ganguly and Tazzoli theoretically estimated the Mg-Fe interdiffusion coefficients from Mg-Fe order-disorder rate data for orthopyroxene. Ganguly et al. used their estimates to show that the cooling rate for the orthopyroxene clast portions of mesosiderites was three to four orders of magnitude slower than estimated by Delaney et al., who considered the diffusion coefficients for orthopyroxene to be similar to the diffusion coefficients for olivine. Recently, Schwandt et al. experimentally determined magnesium self-diffusion coefficients in orthoenstatite (En[sub]90Fs[sub]10) for diffusion parallel to each of the crystallographic directions. These experimentally determined diffusion coefficients are slightly smaller than the theoretical values of Ganguly and Tazzoli, such that the cooling rate of the orthopyroxene clasts in mesosiderites is about an order of magnitude smaller than estimated by Ganguly et al. and four to five orders of magnitude slower than estimated by Delaney et al. Yamaguchi A.* Taylor G. J. Takeda H. Keil K. Global Thermal Metamorphism on the Eucrite Parent Body We made a petrologic study of 14 eucrites. The eucrites have not only igneous textures but also metamorphic textures formed by post-crystallization processing, including brecciation, recrystallization, and melting. Abundant augite grains and abnormally thick augite lamellae in the recrystallized portions of some eucrites could be secondary products formed during thermal metamorphism. Final equilibrated temperatures, estimated by two-pyroxene geothermometer, range from ~720 to 880 degrees C, suggesting a prolonged period of metamorphism. This thermal event could have been global in scale and might have taken place after the initiall differentiation and production of a basaltic crust on the parent body. Garrison D. H.* Bogard D. D. 39Ar-40Ar Age of Ibitira: Dating the Time of Chemical Equilibration of Eucritic Pyroxenes? We determined a 39Ar-40Ar age of 4.49 Ga for the Ibitira eucrite, which may be the time of HED metamorphism that also produced chemical equilibration in eucritic pyroxenes. Shukolyukov A.* Begemann F. Pu - Xe Dating of Eucrites Pu-Xe dating method based on the measurements of Pu fission and spallation Xe is discussed. The new data for the eucrites Caldera, Jonzac, Juvinas, Padvarninkai, Pomozdino, Vetluga and for the howardite Petersburg are presented. Wadhwa M.* Lugmair G. W. Sm-Nd Systematics of the Eucrite Chervony Kut High precision ages of primitive igneous meteorites are of considerable interest since they provide insight into time scales involved in the accretion and differentiation of the parent bodies for these meteorites. Moreover, if these absolute ages can be well correlated with high resolution relative ages determined from extinct radionuclides, the time intervals between (1) the most recent nucleosynthetic event that contributed to the early solar system raw material and formation of the first solid bodies and (2) the formation of the parent bodies of different primitive meteorite groups could be better constrained. In previous work, it has been shown that the eucrite Chervony Kut (CK) has preserved evidence for the presence of live 60Fe (half life ~1.5 Ma) and 53Mn (half life ~3.7 Ma). Further, based on a comparison of the 53Mn/55Mn ratios in CK and the angrites (for which precise Pb-Pb ages are also available [3]), it was indicated that CK crystallized ~5.5 Ma before the angrites and that its absolute age may be as old as 4.5633 +- 0.0010 Ga, which is significantly older than previously determined absolute ages for other eucrites. If this is indeed true, then a comparison with the Pb-Pb ages of Allende CAIs yields the surprisingly short time interval of less than 6 Ma for the accretion and complete differentiation of the eucrite parent body. It is clear from the above discussion that it would be important to compare the chronology indicated for CK by the 53Mn relative chronometer to that determined from other radiometric methods and, in particular, to obtain a high precision absolute age for this meteorite using long-lived radioisotopes. To this end, we have made Sm-Nd isotopic and concentration measurements on pyroxene (Px) and plagioclase (Pl) mineral separates as well as on CK bulk rock (Tr) in order to study the 147Sm-144Nd and 146Sm-142Nd systematics of this eucrite. We recognize that in the current state of technology the Sm-Nd chronometer cannot resolve the apparent 5.5 Ma formation time difference between CK and the angrites indicated by the 53Mn chronometer, but it is still important to ascertain that these two chronometers are consistent. _ Sharma M.* Wasserburg G. J. Papanastassiou D. A. Quick J. E. Sharkov E. V. Laz'ko E. E. Extreme Sm-Nd Fractionation Due to Melting in the Oceanic Upper Mantle: Evidence From Polar Urals Ophiolites The observation that tectonized peridotites (dunites and harzburgites) underlie layered gabbros in ophiolite complexes has led to a paradigm about the creation of oceanic crust at spreading centers: ophiolitic harzburgites are residues of partial melting while gabbros and basalts are magma products. This hypothesis is supported by lherzolite melting experiments. However, a direct link between melt and residue has not been firmly established, i.e., it is not clear whether the tectonized harzburgite in a given ophiolite complex actually generated the overlying gabbros. Sm-Nd isotopic studies on the Trinity Ophiolite Complex show that these peridotites are unrelated to the overlying gabbros, the forrner being derived from a more radiogenic source than the latter. This observation is supported by the Pb-isotopic studies of Xigaze Ophiolites, Tibet. So far, no systematic work has been undertaken to establish the relative chronology of tectonites and layered sequences in ophiolite complexes. Low concentrations of trace elements in harzburgite/ dunite and the lack of fresh ultramafic samples have made such studies difficult. We developed a low blank technique to extract REE from 1-5 g samples of harzburgites and dunites. This procedure employs REE pre-concentration that reduces the effective sample size by a factor of 10. The samples are then,processed by conventional methods to separate Sm and Nd. Freshest samples of ophiolitic harzburgites with <1% serpentinization, were obtained from the Voykar Massif, Polar Urals, Russia. For this study we have concentrated on the samples from Polar Urals. Additionally, we have analyzed two harzburgites-one each from Vourinos and Oman. Lugmair G. W.* MacIsaac Ch. Radial Heterogeneity of 53Mn in the Early Solar System? Our recent studies of 53Mn -0 53Cr systematics (T1/2 of 53Mn = 3.7 My) in angrites (LEW 86010 Angra dos Reis) and eucrites (Chervony Kut [CK], Juvinas [JUV]) have shown that the resolution of small age differences between these differentiated objects is possible and that the eucrites are ~5.5 My older than the angrites (4.5578 Ga). The main assumption underlying this chronology is that 53Mn was homogeneously distributed at least in that region of the solar nebula There the meteorite parent bodies formed. We have further found that the present day and the initial 53Cr/52Cr ratios in both meteorite classes are significantly higher than the terrestrial value. This difference in the 53Cr/52Cr ratios was originally thought to be due to Mn/Cr fractionation in the parent planetesimals of these meteorites at a time when most of the 53Mn was still extant, possibly during the core formation process. However, a more quantitative reevaluation of the plausibility for this to occur has suggested that the major portion of this difference in the 53Cr/52Cr ratios may be difficult to obtain during planetary differentiation and that its cause may not necessarily be found by probing only the evolution of the meteorite parent bodies Mikouchi T.* Miyamoto M. McKay G. A. Porphyritic Olivine in Angrite LEW87051: Cooling Rate and Evidence for Complex Crystallization History from Relict Cores and Zoning Profiles Angrites are among the oldest basalts in the solar system and their unique mineral chemistry has offered great interests for their controversial origins. LEW86010 and LEW87051 are individual samples discovered in the Antarctic, and classified as angrites with chemical affinities to the first known angrite, Angra dos Reis. Major constituting minerals of LEW angrites are Ca-rich olivine, fassaitic clinopyroxene, and anorthite. Although fassaites in LEW86010 are zoned, olivines are homogeneous except for the exsolution lamellae of kirschsteinite. On the other hand, both fassaite and olivine in LEW87051 show chemical zonings. These evidence suggest that LEW87051 cooled more rapidly than LEW86010. Cooling rate calculation employing Fe/Mg zoning profile of LEW87051 olivine indicates that at the slowest 1000 degrees C/year cooling rate is necessary to preserve the chemical zoning of Fe/Mg, which is much faster than the previously estimated cooling rate of LEW86010 and in line with the textural evidence of both meteorites. Fe/Mg zoning profile of LEW87051 olivine also indicates the fractional crystallization under closed system. Several olivines in LEW87051 have been pointed out to have relict cores. Microprobe analysis of these olivines reveals that more Mg-rich olivine has larger core area. This fact supports that these are relict grains which survived the melting stage. The outer part of them would simultaneously crystallize with other porphyritic olivines without the relict core, because both show similar zoning profiles and their compositions are nearly identical. Love S. G.* Keil K. Mercurian Meteorites: Properties and Probabilities With the recent realization that some meteorites may come from Mars and the Moon, it is worthwhile to consider whether meteorites from Mercury could exist in our collections and, if so, whether they could be recognized. We know very little about Mercury, a state of affairs which both increases the scientific value of mercurian meteorites and aggravates the problem of identifying them. As the innermost planet, Mercury contains unique information about limits on the formation of planets. Here we review evidence supporting the possibility of rocks being delivered from Mercury to Earth, and suggest criteria which could help identify a mercurian meteorite. Monday, March 13, 1995 MARS EXPLORATION: PATHFINDER AND BEYOND 8:30 a.m. Room B Chair(s): J. D. Farmer Squyres S. W.* Carr M. H. Scientific Objectives for the Exploration of Mars Mars is one of the most important targets for solar system exploration, in large part because of its many Earth-like qualities. A number of scientific advisory bodies, including COMPLEX, the Mars Science Working Group, and the International Mars Exploration Working Group have identified a set of scientific objectives for the exploration of Mars. This abstract summarizes those objectives, and draws heavily from the work of many individuals who have served on those groups. Slade M. A.* Jurgens R. F. Mars Pathfinder Landing Site: Implications of Goldstone Radar CW and Ranging Observations The prime landing site for the Mars Pathfinder Lander/Rover has been chosen to be in Chryse Planitia in smooth terrain that seems likely to have the potential for sampling a variety of material carried by outflow from the Ares Valles and, to a lesser extent, from any Tiu Valles outflow. This site, however, was rejected as a Viking Lander site due to Goldstone CW radar echoes with low signal-to-noise. The 1995 Goldstone radar system is about 12 dB more sensitive than the radar system existing at the time of the Viking Lander decisions. Four passes over the Pathfinder prime landing site have been scheduled for January-February 1995. In addition, observations just south of the Viking Lander 1 site have been obtained in December 1994. However, repairs to the Goldstone 70-m azimuth bearing are being done during the first three weeks in January 1995. Thus we cannot say with certainty that these tracks over the "Ares" landing site will actually be performed. If the observations are successful, both continuous wave (CW) radar and radar topographic profiles will be obtained. The CW observations will yield information on the RMS slopes in the landing site area, and possibly some compositional information derived from the measured radar reflectivity. The same information (in more detail) can be obtained from the topographic profiles. Crowe D. G.* Smith P. H. Chabot N. Reynolds R. Tanner R. Doose L. Britt D. T. Singer R. Palmer J. Shinohara C. DeVries K. Friedman T. Calibration of the Imager for Mars Pathfinder (IMP) The Imager for Mars Pathfinder (IMP) is a stereo multi-spectral CCD camera scheduled to land on Mars July 4, 1997. More information on the IMP is available on the World Wide Web at: tp://seds.lpl.arizona.edu/~dcrowe/imp.html. In order to produce the best data possible to support mineralogical, morphological, and atmospheric studies, the instrument must be both characterized and calibrated. The approach is to first characterize the instrument performance, then create a calibration data base which will allow optimal ground-based data reduction for scientific utility. We will also create on-board calibration which can provide the best possible compression of non-science data for use in support tasks such as rover navigation. On-board calibration and data compression will also provide a back-up mode of science data collection in the event that the high gain antenna is not available, forcing a very low data rate low gain antenna mission. In many cases, the raw instrument performance can be greatly improved by the use of tailored algorithms. One example is the use of a two-dimensional fit to the flat fielding variation as a function of wavelength to allow on-board calibration of field responsivity in each of the 24 spectral filters (17 different wavelengths as detailed in the abstract by Britt, et. al. in this volume) using a single flat field data table. Another example is the use of a centroiding algorithm to allow angular position and distance measurement limited by the signal-to-noise ratio, rather than limited by geometrical optics. The anticipated 0.06 pixel angular resolution requires careful calibration of optical train distortions due to many causes, including objective lens distortion and scale, and non-uniformities of detector response within a pixel. The ultimate goal is to attain the highest level of precision achievable with the Flight Model hardware in radiometric, spectroradiometric, and geometric data collection. Smith P. H.* Britt D. T. Doose L. R. Singer R. B. Tomasko M. G. Gliem F. Greeley R. Sullivan R. Keller H. U. Knudsen J. M. Soderblom L. A. Update on the Imager for Mars Pathfinder (IMP) The Imager for Mars Pathfinder (IMP), the primary imaging instrument for the Mars Pathfinder Lander, has been under development at the University of Arizona and Martin Marietta Technologies since December 1993. The basic configuration of the camera, the experimental design, and a comparison between IMP and the Viking Lander Camera was presented at the 25th LPSC [Smith et al., LPSC XXV, 1293-4]. Since that time a number of significant enhancements have been incorporated into the IMP design, and the IMP Engineering Model has been delivered and extensively tested. In addition, the designs of IMP sub-experiments (Wind Socks and Magnetic Properties Arrays) have been finalized in their flight configurations. Farmer J. D.* Des Marais D. J. Greeley R. Exopaleontology at the Pathfinder Landing Site The Mars Pathfinder Mission is a Discovery Class mission that will place a small lander and rover on the surface of Mars in July of 1997. It is primarily a technology demonstration to test the feasability of a direct entry-delivery system, but carries a nominal scientific payload that includes rover-lander and instrumentation for limited mineralogical analysis. The nominal landing site was selected by the Pathfinder Team under the leadership of Dr. Matthew Golombek (JPL) based input from 60 participants at a Landing Site Workshop held last Spring at the Lunar Planetary Institute in Houston. The mission constraints for the landing site were 0-30 degrees N latitude, and below the 0.0 elevation datum. Over 20 landing sites were proposed and a nominal site was selected on southern Chryse Planitia near the terminae of the Ares and Tui based on the opportunity to sample a potentially large number lithologies in a small area (the rover will have a range of a few tens of meters from the lander). The purpose here is to review the general geological context of the landing site and the rationale for Exobiology's recommendation of the Ares site given at the workshop last spring. Because Ares and Tui Valles are sourced within terranes that may have originated by thermokarst processes, hydrothermal processes could have operated there for some time. Hydrothermal systems are presently regarded as important sites for a fossil record on Mars. Models for the formation of the outflow channels suggest that thermal spring sinters and associated aqueous mineral deposits, high priority targets for Mars Exopaleontology, could have formed in association with thermokarst processes and subsequently been delivered to the landing site in large quantities during the periodic cataclysmic outflows that created the channels. DesMarais D. J.* Carr M. H. Clark B. C. DeVincenzi D. L. Farmer J. D. Hayes J. M. Holland H. Kerridge J. F. Klein H. P. McDonald G. D. McKay C. P. Meyer M. A. Nealson K. H. Shock E. L. Ward D. M. Mars Exobiology: The Principles Behind the Plan for Exploration The search for evidence of life on Mars is a highly interdisciplinary enterprise which extends beyond the traditional life sciences. Mars conceivably had a pervasive ancient biosphere which may have persisted even to the present, but only in subsurface environments. Understanding the history of Mars' global environment, including its inventory of volatile elements, is a crucial part of the search strategy. Those deposits (minerals, sediments, etc.) which could have and retained a record of earlier biological activity must be identified and examined. Monday, March 13, 1995 LUNAR EXPLORATION STRATEGIES AND RESOURCES 10:15 a.m. Room B Chair(s): D. Morrison Taylor G.* The Moon: Unsolved Problems and How to Solve Them Many difficult and important questions remain to be answered about the origin and evolution of the Moon. These involved the formation of theEarth-Moon system, the thermal and magmatic history of the Moon, its bombardment history, formation of regolith, the history of the Sun as recorded in the regolith, and the nature of the tenuous lunar atmosphere. Furthermore, the Moon is an ideal natural laboratory for studying important processes, such as volcanism and impact, that affect all the planets. Significant advances in our understanding of the Moon and the processes that shaped it can be made from a combination of polar orbiters; sophisticated, teleoperated rovers; geophysical networks; sample return missions; and field work by humans. Wilcox D. J.* Merry C. J. Site Selection for the Interlune-1 Mission We investigated how current remote sensing and geographic information system (GIS) methods can be applied to map a promising area of the moon for lunar volatiles. Data from multispectral telescopic CCD (charged coupled device) images, an early geology map, and an elevation contour map were combined and analyzed using a GIS to produce a set of multispectral and terrain-based signatures for ten geologic units in Mare Tranquillitatis. The accuracy of this new map was assessed by comparison with an older geologic map and also by overlaying it on Orbiter orthophotos for a manual interpretation. Through this approach, we found some in-accuracies in the data sets, however, most geology formations were correctly classified and delineated. The database will be augmented with Galileo and Clementine data to improve the definition of potential sites for future lunar exploration. _ Korotev R. L.* Haskin L. A. Jolliff B. L. A Simulated Geochemical Rover Mission to the Taurus Littrow Valley of the Moon We test the effectiveness of using a simple chemical analyzer (alpha backscatter, alpha-proton, X-ray spectrometer) on a remotely operated rover to analyze soils and provide geologically useful information about the Moon during a simulated mission to a hypothetical site resembling the Apollo 17 landing site (Taurus Littrow valley). Stacy N. J. S. Campbell D. B.* A Search for Ice at the Lunar Poles Discovery of probable water ice deposits on permanently shadowed crater floors near the poles of Mercury suggested the possibility of similar deposits near the lunar poles. The interpretation that ice is responsible for the anomalous radar echoes discovered with the Goldstone and Arecibo radar systems, rests primarily on the unusual radar backscatter properties of many icy surfaces in the solar system, high backscatter cross section and greater than unity circular polarization ratio. While ice, with a dielectric constant of 3.15, does not have an intrinsically high reflectivity, it has low loss and so may serve as a weakly absorbing material which could support volume scattering mechanisms such as the coherent backscatter opposition effect. Allen C. C.* Morris R. V. McKay D. S. Oxygen Production from Lunar Soils We have conducted reduction experiments on 15 lunar soils to characterize oxygen release. The soils span the range of compositions, maturities, and pyroclastic glass contents in the Apollo collection. All were reduced in flowing hydrogen for 3 hrs at 1050 degrees C. Oxygen yield is strongly correlated to initial iron oxide abundance, averaging 77% of total FeO. Yield is relatively insensitive to mineralogy, with partial to total reduction of Fe^2+ in ilmenite, olivine, pyroxene, and glass and partial reduction of Ti^4+ in ilmenite. No correlation to sample maturity was identified. These data provide "ground truth" for oxygen resource assessment, both from orbit and on the lunar surface. Monday, March 13, 1995 ORIGINS OF PLANETARY SYSTEMS 8:30 a.m. Room C Chair(s): H. Newsom R. Malhotra Wetherill G. W.* The Formation and Habitability of Extra-Solar Planetary Systems A model that successfully predicts a number of the general features of the terrestrial planet and asteroidal regions of our Solar System has been extended to include variations in stellar mass and pre-planetary disk surface densities. The biological habitability of the resulting planetary configurations has also been investigated using the habitability criteria of Kasting et al. A principal result is that for stellar masses between 0.5 and 1.5 solar mass, the resulting planetary configurations are insensitive to differences in stellar mass. For initial surface density distributions similar to those of our Solar System, large terrestrial planets tend to concentrate near 1 AU, regardless of stellar mass. This region is habitable only for ~ 1 solar mass stars. A smaller, but significant, number of planets occur at habitable distances for all the stellar masses studied, however. Cameron A. G. W.* Time Scales in the Early Solar Nebula At the last LPSC Hutcheon presented new data showing that in aluminum minerals in chondrules the 26Al/27Al ratio had been much less than is usually found in CAI's. This was interpreted to mean that the chondrules had formed several million years after the CAI's. This led me to undertake an extensive analysis of conditions in the early solar nebula [Cameron A. G. W.], which showed that it was logical for this time delay to exist in the chondrules associated with ordinary chondrites (but not carbonaceous ones). Boss A. P.* Thermal Profiles in Protoplanetary Disks Disk temperatures control the degree of condensation of iron, silicates, and ices, and hence determine the surface density and type of solids available for initiating planetesimal accumulation. Thermal profiles for a variety of possible protoplanetary disks have been calculated using a two dimensional, radiative hydrodynamics code. Variations include changes in the disk mass, stellar mass, initial adiabat, radial density profile, energy source, and dust grain opacity. Most models assume heating due to compression associated with ongoing accretion of gas from the parent molecular cloud; disk models with heating from an alpha viscosity have also been calculated, and the results compare fairly well with previous viscous accretion disk profiles. The compressionally-heated models show that the most important parameter controlling disk thermal profiles is the disk mass, implying a temporal sequence of disk temperatures declining along with the disk mass. Foster P. N.* Boss A. P. Triggering Collapse of the Presolar Cloud with Shock Waves We examine the collision of a shock wave with a molecular cloud as a possible scenario for the formation of the solar system. We partition the initial parameter space into cases where (a) the cloud is completely disrupted, (b) the cloud undergoes collapse and forms a single object, and (c) the cloud remains relatively unperturbed. Preliminary results suggest destruction occurs for shocks with higher momentum than collapse configurations, and collapse occurs for higher post-shock temperatures than unperturbed scenarios. Chambers J. E.* Wetherill G. W. Boss A. P. Dynamical Stability of Systems of Planetary Embryos Recent work on the early stages of the formation of the inner planetary system predicts the creation, via runaway growth, of a large number of lunar-to-Mercury-sized embryos constrained to move on nearly circular coplanar orbits by dynamical friction with residual small bodies. It is not well understood how this system evolves into one in which close encounters between embryos can occur allowing them to coalesce into final planets. Gladman has shown that systems of two embryos generally avoid close encounters indefinitely if their initial orbital separation delta exceeds a critical value. We extend this work to systems of up to 20 embryos. In general, these systems are unstable with respect to close encounters. Thus multi-embryo systems are much less stable with respect to close encounters than the two-embryo case. Clayton D. D.* Jin L. Cosmic-Ray Extinct Radioactivity in Molecular Clouds We propose that 26Al, 41Ca, 53Mn, and probably 107Pd extinct radioactivities in the early solar system were the result of a special cosmic ray irradiation that happens in regions of star formation. Our analysis is normalized to the detection of 4.43 and 6.13 MeV gamma ray nuclear lines from Orion clouds by Compton Gamma Ray Observatory. The 26Al in the early solar system arose from separate source than the 26Al that pervades the interstellar medium today (stellar nucleosynthesis). We present three distinct physical pictures of the cosmic rays and of their associated reasons for 26Al. Each picture relies on a physical model based on dense cores within the molecular clouds. These magnetized cores are thick to cosmic rays below 100 MeV, and are the sites where the new solar systems are forming. Thus the cosmic rays interact and/or stop in those cores are produce 26Al only there in the high abundance found in meteorites. The three pictures are: (1) High flux of low-energy cosmic-ray O, Na, Mg and Si nuclei stopping in the cores and partially transmuting to 26Al; (2) Stopping of low-energy cosmic rays carrying 26Al/27Al = 0.1; (3) Stopping of particles preferentially accelerated from local ejecta (supernovae and Wolf Rayet stars) carrying 26Al/27Al = 0.01. Magnetic field lines converge into the clouds and converge further into the cores, focusing cosmic rays into the cores. This endows cores with high 26Al, but not the entire molecular cloud. It eliminates the awkward need to mix plasma ejecta from stars into cloud cores; the cosmic rays simply shoot in one by one, and do not emerge. Lauretta D. S.* Kremser D. T. Fegley B. Jr. Nickel Fractionation During Troilite Formation in the Solar Nebula The first reaction to incorporate sulfur into solid material in a cooling solar-composition gas is predicted to be troilite formation via the net gas-solid reaction H(sub)2S(g) + Fe(metal) = FeS(solid) + H(sub)2(g). However, the reacting metal will not be pure Fe but instead will most likely be of solar composition (~95 atom % Fe, 5 atom % Ni) and have a kamacite structure. Assuming ideal solution behavior in the metal, the pressure independent temperature at which troilite first becomes stable is 716.5K [D. S. Lauretta and B. Fegley Jr.]. Hydrogen sulfide gas was predicted by J. F. Kerridge to preferentially react with Fe resulting in continuous enrichment of Ni in the metal, eventually transforming the outer rim of the metal to a taenite structure. The predicted final product of sulfurization is a metal core with Ni content increasing radially outward from the center, having a transition from kamacite to taenite within it and a surrounding troilite rim containing very little Ni. In order to test this prediction sulfurization experiments were performed on pieces of the Sikhote Alin and Gibeon iron meteorites. The results show that initially the metal at the reaction interface becomes enriched in Ni relative to the starting composition, but Ni subsequently enters the sulfide in order to maintain chemical equilibrium with the Ni-rich alloy. Stepinski T. F.* Valagenas P. Global Evolution of Solids in Viscous Protoplanetary Disks In order to understand the large-scale architecture of the Solar System we have to study models of the solar nebula that not only are able to follow the evolution of the gas, but can keep track of the solid component as well. This work is the first step in studying the global evolution of solids in the solar nebula. We have calculated radial and transverse velocities of the solid particles relative to the accreting gas. We also have calculated a global evolution of single-size, noncoagulating particles. The case of unisize particles illustrates the differences in time evolution between the gas and the solids. It provides a valuable point of reference for the more realistic calculations. Dominik C.* Tielens A. G. G. M. Mechanical Properties of Dust Aggregates Coagulation of submicron sized dust grains is known to have been the first step towards planet formation in the early solar nebula. Nevertheless, the physics of this process is still poorly understood. On the experimental side, there are some new experiments under way which certainly will give very important insight. On the theoretical side, it is important to simultaneously develop models for the physics of coagulation which will allow us to predict the outcomes of experiments and which can thereby be tested. We have studied the frictional processes that will occur when tangential forces are applied to dust grains in contact. Numerical simulations of collisions between dust aggregates which include these processes show that small aggregates of 0.1 micrometer sized grains will survive collisions below 100cm/s almost undeformed. Velocities of approximately 1000cm/s compress the involved aggregates. At higher impact energies, both aggregates are destroyed. The critical velocities as well as the amount of compression that can be reached are strongly material dependent. Newsom H. E.* Metal-Silicate Fractionation in the Solar Nebula The fractionation of metal relative to other components in the solar nebula and during planetary formation was critical to establishing the bulk composition of meteorite and planetary materials. The data for chondrite groups indicate at least two well characterized fractionations, one affecting the "common" siderophile elements (e.g. Co, Ni, Fe), and the other affecting the refractory siderophile elements. The primary fractionation involved fractionation of Fe, Ni, and Co bearing metal, relative to silicates containing a small residual amount of Fe. The published data on new chondrite groups are consistent with this hypothesis. The secondary fractionation involved the fractionation of a phase (fremdlinge) bearing the refractory siderophile elements (e.g. Ir, Os, Re), resulting in variable refractory/common siderophile element ratios (e.g. Ir/Ni) in chondrites. The Fe abundance of most of the chondrite groups can be explained by removal of metal from a precursor, but the discovery of the metal-rich CH chondrites, and Bencubbinites indicate metal addition in some cases. Several lines of evidence suggest that most of the fractionation occurred prior to the chondrule formation event, although aerodynamic sorting of metal particles and silicate chondrules have been discovered in some chondrites. Evidence of aerodynamic sorting prior to the chondrule formation event was probably destroyed in the chondrule formation process, and the possibility of magnetic sorting events has not been investigated. Even more uncertain is the the timing of the metal-silicate fractionation relative to the earlier CAI (Calcium-Aluminum-rich Inclusion) formation process. The refractory siderophile elements had the most potential for fractionation during this process. Understanding the nebular fractionations will allow us to better constrain the initial abundances of siderophile elements in planets, and to asess the importance of other processes, such as giant impacts, for explaining the large range of metal contents in the terrestrial planets and satellites. Ganguly J.* Bose K. Kinetics of Formation of Hydrous Phyllosilicates in the Solar Nebula Calculation of equilibrium condensation sequence suggests formation of hydrous phyllosilicates in the solar nebula at T < 400 K.Lewis suggested that hydrous phyllosilicates that had formed from the equilibrium condensation of the solar nebula provided the original source of water in the region between Mars and Jupiter, including the asteroidal parent bodies of the meteorites. In a series of papers, Fegley and Prinn explored the kinetics of formation of hydrous phyllosilicates, and concluded that the time scale of formation of these minerals in the solar nebula is >10^17 s, which is >10^4 times the life time of the solar nebula. They have, thus, concluded that the hydrous minerals present in the planetary and asteroidal bodies had not formed by nebular condensation, but instead represent secondary alteration products of the anhydrous grains. The conclusion of Fegley and Prinn has inspired models of large scale aqueous alterations in the asteroidal parent bodies of the carbonaceous chondrites to account for the formation of the hydrous phyllosilicates that are commonly found in these meteorites, and are also inferred to be present in the parent bodies from their reflectance spectra. In this work, we present a critical evaluation of the works of Prinn and Fegley, and show that the available data on phyllosilicate reaction kinetics do not preclude the formation of these minerals within the life time of the solar nebula. The kinetics of formation of hydrous minerals in the solar nebula is also of great importance with regard to the question of water in the earth's mantle. If these had formed during nebular condensation, then they would have become a part of the mineralogy of the earth's mantle through planetary accretion process, which have important ramifications in the understanding of many terrestrial processes. Nagahara H.* Ozawa K. The Role of Hydrogen and Carbon in Evaporation of Forsterite in the Solar Nebula Evaporation of silicate dusts in the solar nebula is largely dependent on the gas composition, which may have resulted in chemical and isotopic fractionation. Previous experiments have shown that evaporation rate of silicates is largely enhanced in the presence of H2 gas. The role of hydrogen and carbon in evaporation of forsterite was thermochemically investigated. The results show that carbon enhances evaporation more effectively, that is, vapor pressure of the system and evaporation rate increase more largely in the presence of carbon compared to the same amount of hydrogen. In the solar nebula, evaporation rate of silicate dusts would have accelerated if organic carbon was concentrated which evaporated at lower temperatures at the time of nebular heating. Mendybaev R. A.* Beckett J. R. Grossman L. Stolper E. Cooper R. F. Bradley J. P. Evaporation Rate of Silicon Carbide in Reducing Gases Experiments were conducted to determine the rate of oxidative evaporation of CVD beta-SiC in H2-CO2 and CO-CO2 at log f (sub)O2 = IW-2.8 and IW-6.0 and temperature from 1200 to 1500 degrees C. The rate of weight loss, (4.7 +/- 0.5)10-8 mg/min/mm^2 at 1200 degrees C and (1.5 +/ 0.1)10^-5 at 1400 degrees C, is independent of log f(sub)O2, P(sub)CO, P(sub)H2O and P(sub)CO2 over the ranges investigated, and the measured E(sub)a = 563 +/- 9 kJ/mole, indicating the self-buffering mechanism, SiC(sub)(s) + 2SiO(sub)2(s) = 3SiO(sub)(g) + CO(sub)(g). If the oxidation mechanism is the same under solar nebular conditions, then the lifetime of a 10 micrometer grain in the nebula is 174 days at 1200 degrees C and 17 hr at 1400 degrees C. _ Monday, March 13, 1995 MERCURY 8:30 a.m. Room D Chair(s): M. Robinson Vilas F.* Future Exploration of Mercury Historically, scientists interested in the study of the planet Mercury always declare that this is a difficult planet to study due to its close proximity to the Sun. They're right. This same proximity has led scientists to assume that Mercury, being created in its present location, represented an extreme endmember planet in the Solar System. As a result of recent ground-based observations and theoretical models, they may be wrong: Mercury may have formed at a different location, been subjected to a bombardment that stripped the planet of its exterior, and moved to its present location. (And Jupiter thought it had problems...) The Mercury we know today is a planet where the atmospheric constituents, surface mineralogy, solar wind, and magnetic field all interact and affect one another. Thus, experiments that shed light on the attributes of one of these things stand an excellent chance of enlightening Hermeophiles about the properties or dynamics of another. For example, measuring the abundance and distribution of K, Na, and Ca and folding these data into sputtering and volatilization models may constrain the composition and petrologic type of possible feldspar units. Grard R.* Solar Photon Interaction with the Surface of the Planet Mercury Solar Photons interact directly with the surface of Mercury due to the absence of an atmosphere. Photoelectrons are emitted from the sunlit hemisphere with a mean kinetic energy of about 1.4 eV. The current density is maximum at perihelion and lies in the range 100-200 microangstrums m^-2 at the subsolar point. Photoemission increases the conductivity of the surface; it supports limited horizontal electron flows, otherwise forbidden by the highly resistive regolith and provides an electrical coupling between the planet and its magnetospheric environment, in the absence of a significant ionosphere. Killen R. M.* Morgan T. H. Benkhoff J. OH in the Mercurian Exosphere as a Function of Polar Ice Cap Evidence for polar ice caps on Mercury has been presented from radar data. It was shown that regions at both poles which are highly reflective in the radar could be due to water ices, and that such ices could be stable against evaporative loss at Mercury only in permanently shadowed polar craters. We show that a polar cap on Mercury could not be due to cometary or meteoritic impact, and would likely be eroded by vaporization by micro-meteoritic flux if it were in place for the lifetime of the solar system. However, a polar cap could be degassed from the planet's interior. If the observed polar cap is composed of water ice, a test of the amount of ice would be a measurement of OH above the pole. We calculate the OH emission tangent to the pole as a function of surface temperature and time since emplacement of the cap within 10 meters of the surface. Monday, March 13, 1995 REMOTE SENSING 9:15 a.m. Room D Chair(s): J. M. Sunshine D. L. Mitchell Campbell B. A.* Hawke B. R. Thompson T. W. Radar Studies of the Lunar Regolith Recent acquisition of Clementine multi-spectral data for the Moon have rekindled long-standing questions on the composition, vertical structure, and mechanical properties of the lunar regolith. Long-wavelength imaging radar data can be used to probe 10 m or more below the surface to address these issues, and we report here on continued analysis of the 70-cm measurements of Thompson. These data have been "calibrated" to values of backscatter coefficient by reference to previous full-disk lunar studies, which permits a direct comparison between lunar targets and relevant terrestrial AIRSAR observations at the same wavelength. We have focused on comparisons between the 70-cm radar echoes and maps of Fe and Ti abundance derived from earth-based telescopic data and Apollo orbital geochemistry experiments, and on analysis of the likely scattering mechanisms within the regolith. This work shows that: (1) there is no systematic correlation between radar backscatter from the lunar maria and multi-spectral estimates of TiO2 abundance; (2) there is likewise no systematic correlation between orbital geochemistry results and backscatter; (3) echoes from a buried substrate (i.e., an interface between "soil" and rock at some depth) do not play a major role in the observed radar variation across the maria, and any such horizon must be a very poor backscatterer. These results indicate that the radar data may be sensitive to components in the soil (perhaps free iron in agglutinates) not well represented by the spectral ratio techniques, or that the radar echoes are a better TiO2 mapping tool where the titanium content is below ~5%. _ Betts B. H.* Nash D. B. Infrared Reflectance Spectra of Selected Lunar Soils and Mineral Concentrates We have taken mid-infrared (2.2-25 micrometers, 4 cm-1 resolution) FTIR reflectance spectra of soil samples from all six Apollo sites, and of mineral separates from an A-11, lunar basalt sample. Spectra of lunar soils show a general similarity between Apollo sites, but also show several significant differences between some sites, particularly in the transition spectral region (~4.5-6.5 micrometers) and in the shape and position of the Christiansen Frequency (reflectance minimum near 8 micrometers). A-16 soils have spectra that are very similar to spectra of breccia from the same site, and significantly different from most soils at other sites (including those shown in). Presumably this is due to soils in that highland region forming largely from anorthite-rich breccias. Spectra of mineral separates from A-11 basalt sample 10058 show that the spectral properties of the major mineral components (plagioclase, pyroxene, and ilmenite) can be clearly distinguished in those separates. We also find that particle size sorts of the original sample tend to isolate certain minerals and thus the spectra of larger particles look very different than those of smaller particles due to the difference in average composition. This work is part of a comprehensive study underway to catalog and understand the mid-infrared spectral properties of lunar materials for comparison with current and future infrared emission spectroscopy of the lunar surface. Calvin W. M.* King T. V. V. Spectral Characteristics of Fe-Phyllosilicates: Comparison to Murchison and Murray We are modelling the reflectance spectra of CV and CM chondrite meteorites using a new approach that is based upon the known chemistry, mineralogy, and grain size distributions as determined by TEM and other analytical techniques. The first step in this approach is to spectrally characterize the individual mineral phases that have been identified in CV and CM chondrites. Anhyrous silicates such as olivines and pyroxenes, commonly found in CV chondrites, have been previously well characterized spectrally. The CM chondrites show significant alteration of primary minerals, resulting in compositions dominated by aqueous alteration products such as phyllosilicates, sulfates, oxides, hydroxides and carbonates. The phyllosilicates top the list of abundant phases, and in the CM chondrites in particular, Fe-rich serpentines are the most common phases. King and Clark [1989] have characterized the Mg-serpentines and chlorites, noting certain spectral similarities between chlorites and CI1 and CM2 chondrites. We present the results of an examination of the reflectance spectra of Fe-serpentines and two varieties of chamosite (chlorite group). We find these minerals can provide a reasonable spectral match to features seen in certain CM chondrites and by extension, the dark asteroids. Wang A.* Haskin L. A. Jolliff B. L. Raman Spectroscopy as a Method for In-situ Lunar Mineralogical Remote Sensing Visible and near-infrared spectroscopy (VIS-NIR) is used extensively for remote sensing of lunar mineral composition from orbit and from Earth, and mid-infrared spectroscopy (MIR) has been proposed for use from orbit. Both methods are under consideration for on-surface analysis. We suggest that Raman spectroscopy would be a superior rover-based tool for mineral identification. Sharp, non-overlapping Raman bands provide unambiguous mineral identification and, for olivine, they provide information on Mg/(Mg+Fe), an important petrologic parameter. Recent developments in lasers, energy an.,lyzers, and detectors make possible sensitive, rapid analysis with physically robust instruments of small volume, light weight, and low power. Although discussed here for lunar silicates, Raman spectroscopy is also sensitive to oxyanions such as phosphates and carbonates, and to organic compounds, and could be used on Mars, asteroids, and other planetary surfaces. Grobner C.* Oschutz F. Oberst J. Hiesinger H. Jaumann R. Neukum G. Photometric Analysis of Lunar Surfaces with Complex Topography All earth based observations of the moon have been performed at small phase angles and at controlled incidence and emergence geometry. For this scenario the phase angle alpha is sufficient to approximate the observing geometry. However, if the observing geometry changes in a wide range such as observed during spacecraft flybys, or by the surface topography with increasing resolution, then the simultaneous changes of all observing geometry parameters have to be considered seriously in order to correctly describe the reflectance behaviour of a surface. Hapke B.* Nelson R. Smythe W. Horn L. Gharakanian V. Herrera P. Studies of the Opposition Effect and Negative Polarization with the JPL Photopolarimeters A series of measurements of particulate materials are elucidating the nature of the oppostion effect (OE) and negative branch of polarization (NP) commonly exhibited by surfaces of bodies in the solar system. Both shadow-hiding (SHOE) and coherent backscatter (CBOE) opposition effects have been observed in our materials. However, the strong, broad CBOE's an well-developed negative polarizations that are characteristic of planetary regoliths seem to require abundant particles of size D ~ lambda in regoliths of intermediate albedos. Urquhart M. L.* Jakosky B. M. Can We Use Remote Thermal Emission to Derive Lunar Surface Properties? Grain size, particle density, thermal conductivity, composition and rock abundance all play an important role in the thermal behavior of the lunar surface. Direct investigation of the grain size distribution, particle density, and thermal conductivity of the lunar surface layer is not presently possible, with the exception of the samples returned from the Apollo landing sites. An indirect measurement of lunar surface properties may be possible using remote thermal infrared observations. In order to better understand the interplay between grain size, density, rock abundance, and thermal conductivity a diurnal thermal model for the lunar surface and near subsurface with temperature-dependent specific heat and thermal conductivity was developed. Although particle size, bulk density, and thermal conductivity cannot be investigated completely independently, a clear relationship between these parameters, and their effects on lunar surface temperatures is determined. Thermal conductivity is dependent upon the other parameters of grain size, particle density, composition and rock abundance. To separate the effects each of these parameters have on the thermal conductivity, and hence the nighttime temperatures, laboratory data was used for granulated materials under vacuum conditions which varied only one parameter, while holding the others constant. Since an increase in the bulk density of a material increases the thermal inertia of that substance, an increase in density results in an increase in nighttime temperatures. An increase in the average grain size of the regolith was found to correspond to a decrease in the minimum temperatures predicted by the model. Higher rock; percentages of either vesicular basalt or breccia decrease the overall temperature variation of the models aud increase the nighttime temperatures. Since grain size, density, and rock abundance can each be varied within certain ranges to match an observed minimum temperature, some assumptions regarding the thermal properties of the surface must be made if regolith properties are to be determined remotely. Sunshine J. M.* Towards a More Sophisticated Approach to Compositional Interpretation of Silicate Absorptions in Asteroid Spectra Spectral surveys have shown the surfaces of many asteroids, like most solar system bodies, to be dominated by silicates. Qualitative analyses indicate the presence of both pyroxenes (PYX) and olivines (OLV), and provide first order estimates of composition and relative abundances. However, because both OLV and PYX are members of solid solutions, their spectral properties vary continuously with composition. This, combined with the significant overlap in absorptions in the 1 5m region, complicate our ability to make more detailed compositional interpretations. One approach, the Modified Gaussian Model (MGM) has been successfully used to quantify compositional variations in OLV spectra across its solid solution series and in PYX spectra of both natural and laboratory mixtures. The MGM approach thus holds great promise for accurately resolving spectra with mixtures of OLV and PYX. However, before the MGM can confidently be used on remote spectra, further development of the model and testing with controlled spectra is required. Mitchell D. L.* Ostro S. J. Mineralogical Constraints from Asteroid Radar Observations A new approach to the analysis of asteroid radar observations is expected to dramatically improve constraints on the surface mineralogies of main-belt asteroids (MBAs). In an asteroid radar observation, a circularly polarized signal is transmitted toward the target, and the echo is received in both the same sense of circular polarization as the transmitted signal (the SC sense) and the opposite (OC) sense. The helicity of circular polarization is reversed upon reflection from a surface that is smooth on all scales within about an order of magnitude of the wavelength, but SC echo power can arise from single backscattering from a rough surface, from multiple scattering, or from subsurface refraction. McCord T. B.* Morris J. Persing D. Detection of Meteoroids Entering the Earth's Atmosphere Meteoroids strike the Earth regularly, as can be observed almost every night and sometimes in the day time from the incandescent trail as the meteoroid converts its kinetic energy into radiant energy. The impacting meteoroids are representative of the population of near-Earth objects, and the size, orbit and composition distributions of these objects are indicators of the present state and past evolution of the Solar System as a whole and planetary and satellite surface evolution. These impacting objects have not been observed and reported with any completeness, and the statistical nature of the impacting objects is uncertain to more than an order of magnitude. Defense Department sensors have the capability of providing this near-global and continuous monitoring system to detect and characterize the impacting objects, and as a demonstration project, these authors have been analyzing these DoD data. This effort has resulted in the release of the analysis of one large event occurring over the Central Pacific Ocean on February 1, 1994 (McCord, et al. JGR, in press, 1995). Data from other events are also being analyzed and information leading to a statistical analysis of incoming natural objects seems available. _ Oehler A.* Hanowski N. DLR-Goniospectrophotometer Abundance Estimates from Bidirectional Reflectance Measurements in the UV, VIS and NIR The DLR-Goniospectrophotometer is able to measure high quality multispectral phase curves of regolith-like mineral powders in the UV, VIS and NIR (phase angle 3 -110 , wavelength range 270 nm -1100 nm, spectral bandwidth 10 nm-20 nm, see [Oehler A. et al.] for a description of the method and the instrument). The Hapke model [Hapke B., 1981, and Hapke B., 1986] has been fitted to such phase curves in a straightforward process, where it was not necessary to constrain the parameter space in order to produce unambiguous solutions [Helfenstein et al.]. Analysis of these sets of spectral Hapke-parameters leads to the results listed in a parallel paper [Oehler et al.]. These results are consistent with results obtained by other authors from the analysis of remote sensing data and of data from lunar samples. [Helfenstein et al., Rebhan H., Dummel A.] Having shown that our method to determine sets of spectral Hapke-parameters is accurate, the spectral single scattering albedo (SSA) was used to test the abundance model for intimate mixtures, as proposed by Hapke (equation 17 in [Hapke B., 1981]). This method has been investigated by many research groups and most recently by Mustard and Pieters [Mustard J. F. and Pieters C. M. , 1987, and Mustard J. F. Pieters C. M., 1989]. We can show [Hanowski N.] that converting the spectral SSA to mineral abundances leads to results comparable to those derived from the experiments of Mustard and Pieters. In addition, we can confirm their suggestion [Mustard J. F. and Pieters C. M., 1989], that if high quality photometric data are available it is possible to use the method even for the "most difficult mixtures". Monday, March 13, 1995 SPECIAL PLENARY SESSION 1:30 p.m. Room C Chair(s): D. C. Black D. P. Blanchard Huntoon C.* Welcome Monday, March 13, 1995 1994 LPSC STUDENT PAPER AWARDS 1:30 p.m. Room C Chair(s): D. C. Black D. P. Blanchard James O.* Presentations to the GSA 1994 Stephen E. Dwornik Student Paper Award Winners Monday, March 13, 1995 HAROLD MASURSKY LECTURE SERIES PRESENTATION 1:30 p.m. Room C Chair(s): D. C. Black D. P. Blanchard Drake M. J.* The Moon: What We (Think We) Know About It, How We Know It, and What We Don't Know Monday, March 13, 1995 PRESOLAR GRAINS 2:30 p.m. Room A Chair(s): R. Lewis L. R. Nittler Daulton T. L.* Ozima M. Shukolyukov Y. Radiation-Induced Nano-Diamond Formation and Its Implications for Interstellar Diamonds Interstellar grains of presolar origin are known to exist in the matrices of primitive carbonaceous meteorites; these grains include: aluminum oxide, graphite, silicon carbide, titanium carbide, and diamond. Diamond is unique among all presolar grains because of its assoication with Xe-HL. Several theories have been proposed for the origin of interstellar nano-diamonds, including chemical vapor deposition (CVD) from stellar outflows, impact shock metamorphism driven by supernovae, and UV annealing of carbonaceous grains. Common to all these theories is the problem regarding the incoporation of Xe-HL into the nano-diamonds. However this is not a difficulty if a radiation-induced mechanism is considered. High energy particles emitted during the supernova stage of a massive star (including Xe ions) could have interacted with carbonaceous grains in stellar envelopes. These energetic particles may subsequently have become entrapped within the carbonaceous material when it recrystallized as diamond. Verchovsky A. B.* Pillinger C. T. On the Extraction of Nitrogen and Noble Gases from Presolar Diamonds by Pyrolysis A new technique has allowed us to thermally-desorbing nitrogen completely from nanodiamonds extracted from the Orgueil CI carbonaceous chondrite. We are now able to compare the nitrogen release more directly with noble gases. Nitrogen is > 70% liberated before the release of ^36Ar representative of the HL family of noble gases starts to rise, but there is some overlap of the two components at the tail of the nitrogen profile. Carbon does not significantly react (<10% destroyed) before all the nitrogen and ^36Ar have been evolved. Huss G. R.* Fahey A. J. Wasserburg G. J. Silicon and Carbon Isotopes in SiC from the Qingzhen (EH3) Chondrite Presolar SiC is present in primitive EH chondrites in an abundance comparable to those for CI, CM2, and LL3.0-1 chondrites (on a matrix-normalized basis). The relative abundances of Ne-E(H) and Xe-S in acid residues from Qingzhen and Indarch suggest that the mixture of SiC in these meteorites is similar to those in CI and LL3.0-1 chondrites. (Higher Ne-E(H)/Xe-S ratios in separated SiC from Murchison may reflect loss of small SiC during chemical processing). The literature contains essentially no data on the isotopic compositions of Si, C and trace elements in individual SiC from EH chondrites. In a previous attempt to obtain such data, only one SiC grain was found due to the presence of large amounts of silicon nitride (inferred to be Si3N4) in the acid residue. In this abstract we report Si and C isotopic data for 23 SiC grains from Qingzhen, including 21 "platy" grains ranging in size from ~10^6 km. For obvious reasons, this region was dubbed "Stealth". Figure 1 shows an estimate of the boundary of Stealth. Also shown in that figure is the original estimate of its extent. The reason for the difference is a slightly different selection criterion, and the fact that the original boundary only used the 1988 data. Elsewhere, it has been shown that the most likely explanation for Stealth is that it is a very underdense region with a lack of subsurface scatterers to significant depth, with that depth being >~5 m. So, the question is, what is this thing geologically? Monday, March 13, 1995 SPECIAL SESSION: CLEMENTINE EXPLORES THE MOON 2:30 p.m. Room C Chair(s): G. J. Taylor L. R. Gaddis Nozette S.* The Clementine Mission During the past decade the Departrnent of Defense, and the Ballistic Missile Defense Organization, BMDO (formally the Strategic Defense Initiative, SDIO) of the US Department of Defense (DoD) has invested heavily in space technology, focused on the development of lighter more cost effective components and systems. With the end of the Cold War many of these technologies can be made available to the civilian community. To further these efforts in dual-use application, BMDO and NASA have collaborated on the Clementine mission. Shoemaker E. M.* Robinson M. S. Clementine Observations of Melt Rocks and Volcanic Materials in the Schrodinger Basin The Schroedinger multiring basin (320 km in diameter, centered at 138 degrees E, 75 degrees S) is the second youngest great multiring basin on the Moon. It is the least modified lunar basin of this size. Though a few Orientale secondary craters are superposed on the basin and its ejecta blanket, we have not found any evidence that the terrain at Schroedinger is mantled by a thin veneer of Orientale ejecta. Morphologic details of the basin floor and of the extensive ejecta blanket are crisp and fresh. McEwen A. S.* Robinson M. S. Duxbury T. C. Buratti B. J. Global Albedo Variations on the Moon: Clementine 750-NM Observations Clementine has provided the first global digital color and albedo observations of the Moon. We have processed about 50,000 frames acquired through the 750-nm filter at reduced scale (1 km/pixel). This provides the best albedo map produced to date for most of the far side. No 1-km or larger region of the far side has a very low albedo characteristic of titanium-rich dark mantle deposits or mature mare soils. The brightest extensive region corresponds to the region of highest elevations on the Moon, just north of the South Pole/Aitken basin. This region is clearly bright at least in part due to a concentration of immature soils on bright ejecta blankets of Copernican craters, but an anorthosite-rich composition is also likely. Several far side basins are characterized by intermediate albedos in their interiors, perhaps due to cryptomaria or Mg-suite highland materials. Many new Copernican craters can be identified, and their albedos provide crude age estimates. The brightest crater seen on the Moon at this scale is Giordano Bruno, which must be one of the most recent large craters (> 20 km diameter) on the Moon, probably less than 50 million years old. Lucey P. G.* Taylor G. J. Malaret E. Global Distribution of Iron on the Moon and Its Implications for the Magma Ocean, Crustal Structure and Lunar Origin In a companion abstract we presented a method for derivation of the abundance of the element Fe at high precision (1-2 wt%) from visible wavelength multispectral imaging and applied this method to data obtained recently by the Clementine mission. This mission provides 100% coverage of the lunar surface at 100-300 meter resolution. Of this coverage we have processed data for 93% of the lunar surface between latitudes of 70 degrees S to 70 degrees N at 35 km resolution. These Fe abundance data provide key tests of hypotheses for the early style of differentiation, current compositional structure, and lunar origin. Pieters C. M.* He G. Tompkins S. Staid M. I. Fischer E. M. The Low-Ti Basalts of Tsiolkovsky as Seen by Clementine Tsiolkovsky is a prominent 180 km Late Imbrian crater on the lunar farside (20S, 129E) filled with dark mare deposits. Because maria are not extensive on this side of the Moon, the mare-filled Tsiolkovsky was a notable feature in the earliest Zond images of the farside. The crater was later imaged at higher resolution by Lunar Orbiter 3 as well as by Apollo 15. Tsiolkovsky is located in a 700 km pre-Nectarian basin, Tsiolkovsky-Stark. Although the crust generally is thicker on the farside and is believed to restrict basalt emplacement, the size of the crater and the basin environment may help account for the occurrence of this rare farside mare fill. Crater counts of the mare have indicated that these farside mare deposits are of an age comparable to those of the eastern nearside maria, but until recently little was known of their composition. The darkness of the mare deposits suggested to some that they might be Ti-rich basalts comparable to those of Apollo 11 and 17 of similar age. The multi-spectral images from Clementine provide the first color data for the Tsiolkovsky region. These data indicate that the Tsiolkovsky mare is a low-titanium basalt filling a crater which excavated feldspathic highland materials. McEwen A. S.* Shoemaker M. Zuber M. T. Smith D. E. Two Classes of Impact Basins on the Moon The diameter/depth ratios of impact basins on the Moon group into two classes. Most basins have diameter/depth ratios of less than 300 (class 1), whereas a second class has diameter/depth ratios greater than 400 (class 2). All impact basins that are mostly or entirely within the Procellarum basin fall into class 2; all impacts outside of Procellarum are class 1, except for Australe. Class 2 basins appear to be significantly shallower than class 1 basins primarily due to much thicker fill by mare lavas. Many class 1 basins occur just outside of the circular Procellarum margin, including Humboltianum, Crisium, Nectaris, Schiller-Zucchius, Orientale, Grimaldi, Lorentz, Coulomb-Sarton, and Birkhoff. Mare Australe may bear similarities to the Procellarum basin in terms of age and thermal history. Spudis P. D.* Clementine Laser Altimetry and Multi-Ring Basins on the Moon Altimetry from the Clementine mission allows us to study the distribution and configuration of multi-ring basins on the Moon. In addition to confirming the presence of basins previously mapped from photogeologic evidence alone, the altimetry image shows several depressions that may represent basins that had not been recognized. Both relatively shallow (1-2 km deep) and deep (5-7 km) basins occur on the Moon; there is no correlation between a basin's depth and relative age. The large topographic depression associated with Oceanus Procellarum is only partly circular. A Procellarum trough defined by the 0 km contour appears to be elongate and irregular, suggesting an origin either by internal processes or by a coalescence of multiple, overlapping impact basins, rather than a single large basin-forming impact. Zuber M. T.* Smith D. E. Neumann G. A. Lemoine F. G. Gravity, Topography and the Geophysics of the Moon from the Clementine Mission Global topographic and gravitational field models derived from data collected by the Clementine spacecraft have enabled considerable progress in our understanding of the shape and internal structure of the Moon. The Moon exhibits a 16 kilometer range of elevation, with the greatest topographic excursions occurring on the far side. Lunar highlands are in a state of near isostatic compensation, while impact basins display a range of compensation states that do not correlate simply with basin size or age. A global crustal thickness map reveals crustal thinning under all resolvable lunar basins. The results indicate that the internal structure and thermal history of the Moon are more complex than previously believed, and point to an even greater role of impacts in shaping the Moon than had been appreciated. Neumann G. A.* Zuber M. T. Lemoine F. G. Williams K. K. Crustal Structure of Large Impact Basins on the Moon The early bombardment of the lunar surface dramatically redistributed the newly-formed crust. In order to investigate lunar crustal structure and its implications for the early evolution of the Moon, we have examined Bouguer gravity and isostatic compensation mechanisms using the altimetry from the Clementine lidar and a gravity field derived from Clementine, Lunar Orbiters 2-5, and Apollo subsatellite tracking data. In a previous study we computed a global crustal thickness map by downward continuation of Bouguer gravity to the lunar moho. In the present study we perform more detailed regional modeling of several major impact basins on the lunar near and far sides. We find that typical lunar basins have a central peak of upwarped mantle and are often surrounded by a ring of thickened and/or lighter crust. Blamont J.* Lambert-Nebout C. Clementine: On-Board Image Compression In spaceborne remote sensing the amount of image data collected always increases and the ability to store or to transmit it does not increase so fast. There is then a growing interest in developing on-board compression that could provide both high compression ratios and low degradation. Real-time on-board compression is nowadays possible thanks to the progress of both electronic and data processing : a compression module has been successfully used in 1994 during Clementine mission. We describe first in this paper typical image compression algorithms and present a flexible hardware implementation of a JPEG based compressor. We point out that the more the compression is optimized according to the mission, the more image quality will be preserved. We conclude this paper with some Clementine compression results and analysis and with future prospects in on-board compression. Zook H. A. Potter A. E. Cooper B. L.* The Lunar Dust Exosphere and Clementine Lunar Horizon Glow There is a surprisingly dense cloud of dust, visible to the naked eye, above the lunar surface. This dust exosphere surrounding the moon can not, however, be attributed to ejecta from impacts by interplanetary meteoroids, for two reasons: (1) The spatial density of dust in the cloud is far too high to be caused by meteoroid impact ejecta, and (2) the rate at which dust grains were sensed at the Apollo 17 site peaks sharply at local lunar sunrise or sunset; it is unreasonable to expect such a time variation to be due to meteoroid impacts. Monday, March 13, 1995 OUTER PLANETS/SATELLITES 2:30 p.m. Room D Chair(s): H. F. Levison D. L. Blaney Malhotra R.* The Origin of Pluto's Peculiar Orbit The origin of the highly eccentric, inclined and resonance-locked orbit of Pluto has long been a puzzle, for, in the widely accepted paradigm for the formation of the Solar system, the planets accumulated in a highly dissipative disk of dust and gas orbiting the protosun, and thus formed in near-circular and nearly co-planar orbits. A quantitative theory has been proposed recently which suggests that Pluto's unusual orbital properties may be a natural consequence of the late stages of the formation and dynamical evolution of the outer planetary system. In this picture, Pluto is supposed to have formed in a common near-circular, low-inclination orbit beyond the orbits of the giant planets. During the late stages of the formation of the outer planets, the gravitational scattering and eventual clearing of remnant planetesimal debris by the giant planets (and the concomitant exchange of energy and angular momentum between the planets and the planetesimals) may have caused a significant evolution of the giant planet orbits. In particular, Neptune's orbit may have expanded considerably, and its exterior orbital resonances would have swept through a large region of trans-Neptunian space. During this resonance sweeping, Pluto would easily have been captured into the 3:2 orbital period resonance with Neptune and its eccentricity pumped up to its Neptune-crossing value during the subsequent evolution. The resonance lock would ensure a separation in longitude of the planets at orbit crossing. An analytical calculation given in predicts the relationship between the value of the final eccentricity and the magnitude of the outward radial migration of Neptune. Levison H. F.* Stern S. A. Kuiper Belt Object Encounters with the Pluto-Charon Binary: A Mechanism for Exciting Charon's Eccentricity Recently, Tholen and Buie have reported astrometric evidence for a significant orbital eccentricity of Pluto's satellite Charon, with a likely value near 0.003. Previously, all estimates of Charon's eccentricity were consistent with zero. Below, we demonstrate that this non-zero eccentricity in the Pluto-Charon system can be induced by gravitational interactions with other Kuiper belt objects. Stern S. A.* Buie M. W. Trafton L. M. Flynn B. C. High-resolution HST Images of the Pluto-Charon System We have obtained high-resolution images of Pluto and Charon using the Hubble Space Telescope (HST) Faint Object Camera (FOC). These images were made during the period 20 June to 01 July 1994, and provide the first direct maps of Pluto [Stern S. A.]. These images were made in two color bandpasses, one centered near 410nm and the other centered near 278nm. Images were made in each of the two bandpasses on four dates as Pluto rotated twice on its axis. Together, the four sets of images provide nearly-complete longitudinal coverage of Pluto, with a typical S/N ratio in the 410nm images near 20 per resolution element, and a resolution sufficient to show large scale spots and polar caps on the surface. Roush T. L.* Cruikshank D. P. Pollack J. B. Young E. F. Bartholomew M. J. Near-Infrared Geometric Albedos of Charon and Pluto: Constraints on Charon's Surface Composition The geometric albedos of Charon and Pluto are derived at near-infrared (near-IR) wavelengths (1.4-2.5 um). Comparisons of these to theoretical calculations are used to constrain the identity and relative abundances of surface ices on Charon. These comparisons suggest that widespread regions of pure CH4 ice do not occur on Charon and that if CH4 is abundant on Charon, then it is large grained (about 5 mm) and is likely mixed at the granular level with H2O ice, and possibly CO2 ice. Pappalardo R.* Sullivan R. Rood D. Greeley R. Coon M. Crustal Separation on Europa: New Evidence and a Volcano-Tectonic Mechanism for Resurfacing We report that Thynia Linea, a "gray band" on Europa, is a ~25 km wide and >900 km long region of crustal separation and spreading. Roughly a dozen older features and the ~7 cuspate segments that form its outline apparently have been displaced across its width, with new crustal material forming in between. Displacement azimuths suggest rotation about a pole near 79 degrees S, 200 degrees W in response to NW-SE directed tensile stress, in accord with the stress predictions of non-synchronous rotation. These observations are consistent with a laterally mobile brittle crust, decoupled from ductile or liquid material below. Crustal separation and formation of new crust offers a viable volcano-tectonic scenario for resurfacing Europa. If such is ongoing, resurfacing can be accomplished on a time scale consistent with the satellite's ~10^7-10^8 yr surface age if one gray band forms every ~10^3-10^4 yr and becomes unrecognizable with age, as through continuous deposition of frost onto the surface. Iaquinta-Ridolfi F.* Schenk P. Ejecta Deposits on the Icy Satellites: Deeper Insights into the Cratering Process Recent studies of crater morphology point to fundamental differences in cratering on ice and on rock. To date, no systematic study of ejecta deposits has been made for the icy satellites. Here, we extend crater morphology studies to ejecta deposits with the goal of addressing the importance of composition in cratering. Ejecta morphology and dimensions on Ganymede (where they are well expressed) are compared with those on the Moon, where surface gravity is similar (and not a complicating factor), and with other icy satellites, where composition is roughly similar but gravity varies. This may provide a test of the self-similarity of ejecta blankets, which is predicted theoretically. Ejecta blanket dimensions can also be useful in mapping craters where ejecta can be mapped but crater rims are no longer identifiable, such as palimpsests. Hogenboom D. L.* Kargel J. S. Holden T. C. Buyyounouski M. The Phase Diagram of Ammonia Monohydrate: A New High-Pressure Polymorph In a continuation of previous investigations of the system H2O-NH3, we have modified our high-pressure apparatus with the addition of a new thin-walled flexible teflon sample capsule. The capsule allows us to observe melting and subsolidus transitions between phases and to make precise measurements of relative den-sities. Results are presented for a sample of 50.25 mass-% NH3 (near the ammonia monohydrate composition of 48.6% NH3) at pressures from 10 MPa to 400 MPa and temperatures from 140 K to 205 K. We have found per-suasive evidence of a high-pressure polymorph of ammonia monohydrate. A preliminary phase diagram of ammonia monohydrate includes two branches of the melting transition, corresponding to ammonia monohydrates I and II, and the subsolidus transition between these solids. These data seem to be inconsistent with those reported by Boone and Nicol, who did not observe a high-pressure polymorph. In an accompanying abstract by Kargel and Hogenboom (this volume), we re-examine our and Boone and Nicol's data in an attempt to reconcile the two. Ojakangas G. W.* Malhotra R. Thermal and Orbital History of a Blocky Enceladus Saturn's icy moon Enceladus is enigmatic because, despite its small size (radius about 250 km) its resurfaced terrains of multiple ages indicate a long history of significant internal heating and accompanying volcanic activity. There is even good reason to believe that volcanism continues at present because the satellite appears to be the source of Saturn's E-ring. Recent results suggest that Enceladus has only a very small rocky component, effectively eliminating the possibility of significant radiogenic heating. Enceladus is currently locked in a 2:1 orbital resonance with Dione which enhances the tidal heat budget of these two satellites. We examine the orbital and thermal histories of Enceladus and Dione by means of numerical integration of theequations of motion, coupled through tidal friction to simple heat-balance equations describing the thermal evolutions of the two moons. Mouginis-Mark P. J.* Flynn L. P. Galileo Observations of Volcanic Hot Spots on Io: Predictions from Thermal Data of Hawaiian Eruptions By the end of 1995, the Galileo spacecraft should be in orbit around Jupiter, and will provide the opportunity to observe the thermal characteristics of active volcanism on Io during both the initial high resolution fly-by and the longer-term low-resolution observing phase. While the average heat flow from Io is ~2+-1 W/m^2, active eruption sites may be several orders of magnitude more energetic. Data from the Near Infrared Mapping Spectrometer (NIMS), collected between 0.7 - 5.2 micrometers, should therefore be particularly good for studying elevated surface temperatures. In addition, as we describe below, we believe that nighttime observations with the Solid State Imager (SSI) between 0.6 - 1.1 micrometers will also be useful for observing hotter targets, particularly because of the higher spatial resolution of the SSI compared to NIMS (10 microradians/pixel vs. 0.5 mr/pixel). The availability of these thermal observations for Io would enable many aspects of Ionian volcanism to be interpreted from the same process-related perspective that active terrestrial volcanoes are now studied. High on the list of phenomena that should be studied are the active lava lakes (e.g., Loki), plumes (e.g., Pele), and the large areas of the surface that could be active lava flows (e.g., Lerna Regio). Johnson T. V.* Matson D. L. Blaney D. L. Veeder G. J. Davies A. G. Stealth Plumes on Io Some of the most spectacular expressions of Io's volcanic activity are the large, umbrella shaped eruptive plumes discovered in Voyager camera images (Morabito et al., Science, 204, 972,1979; Smith et al., Ibid., 951-972, 1979). These plumes rise to altitudes of several hundred kilometers, implying ballistic vent velocities of 0.5-1.0 km/s (Strom and Schneider, in Satellites of Jupiter, D. Morrison, ed., U. of Ariz. Press, 1982). At least nine such plumes were observed in eruption by Voyager, eight of them apparently active over the four months separating the Voyager encounters, and there is abundant evidence in the images for surface deposits from similar activity at many other locations. Blaney D. L.* Hanner M. S. Russell R. Lynch D. Hackwell J. The Infrared Distribution on SO2 Frost on Io: The Effect of Thermal Emission on the Estimation of Frost Abundances A persistent problem with the distribution of SO2 frost on Io has been the lack of agreement between the infrared abundance and distribution (4.08 micrometers band depth]) and the uv/visible distribution. In the past, the flux from thermal emission of volcanic regions at 4 micrometers has been considered to be negligible when compared with the solar insolation. Spectra taken in 1993 between 3- 13 micrometers allow for explict modeling of the thermal emission from volcanic hotspots. These results show that thermal emission is important in the estimation of 4 micrometers band depth. With thermal emission removed, the SO2 line depth has increased, indicating that there is even more SO2 frost than calculations without removing thermal emission would indicate. It is possible to reconcile the 1993 measurements with the previous Howell et al. observations if the measurements presented in Howell el al were collected during a period of higher thermal emission than in 1993. The reconciliation of the different IR observations, however, only acerbates the discrepancy between the IR SO2 frost abundances and the uv/visible abundances (i.e. there is now even more SO2 frost than the uv and visible observations would indicate). Davies A. G.* Silicate Lava Models and the Io Thermal Outburst of 9 January, 1990 From observations of thermal outbursts on Io, volcanism appears to be predominantly silicate in nature. An Io Lava Flow Model (ILFM) has been developed to add physicality to the predominantly parametric nature of the analysis of volcanic processes on the ionian surface. A series of simple models has been applied to the complex 1990 outburst data. Morris R. L.* Domingue D. L. Volcanic Color Units on the Surface of Io McEwen did an intensive low resolution study of the global color and albedo variations on Io with a high resolution study of Ra Patera. We have reproduced McEwen's work on Ra Patera and found more detailed color units for this area. Using medium resolution images we have attempted to expand this to other volcanic areas focusing on Mbali, Talus, Maasaw, and Agni Pateras. Our goal was to see to what degree the units defined in the high resolution study of Ra Patera can be distinguished at medium resolution and what is the variation in color units between Ra Patera and other volcanic regions. Preliminary analysis shows that the units seen at medium resolution correspond well with those at higher resolution. Initial examination indicates that the color units defining the Ra flows have similarities to those that describe Mbali and Talus Pateras. Maasaw and Agni Pateras show slightly less structure and there are distinct variations between them. Tuesday, March 14, 1995 CHONDRULE FORMATION 8:30 a.m. Room A Chair(s): I. D. Hutcheon J. P. Greenwood Hood L. L.* Formation of Nebular Shock Waves and Resulting Thermal Histories of Chondrule Precursor Grains Previous work has indicated that gas dynamic shock waves occurring within the protoplanetary nebula could represent a plausible source of transient heating events needed to explain the existence of meteoritic chondrules. Quantitative tests of this proposed mechanism are possible for specific shock wave generation models. In this paper, we explore the consequences of shock wave generation in the nebula due to inhomogeneous disk accretion from the surrounding molecular cloud core. In particular, astrophysical evidence suggests that multiple infalling molecular cloud core clumps may have produced shocks that could have thermally processed a large fraction of dust grains in the nebula. Alternatively, larger-scale inhomogeneities could have produced time-dependent and axially asymmetric structures in the evolving disk as suggested by the FU Orionis phenomenon. Here, numerical calculations of a representative shock wave in the nebula are combined with calculations of the thermal histories of precursor dust grains exposed to such a shock wave. Results allow direct comparisons with meteoritic evidence relating to the heating and cooling rates of chondrules. Wood J. A.* Were Chondrules Formed Early or Late in the Protosolar Nebula? The protosolar nebula is understood to have gone through two very different stages of evolution: First, a brief (~0.5 My) active period during which inter-stellar material fell onto the disk, and most of it accreted to the sun; second, a much longer (~10 My) period when the sun, a T Tauri star, was surrounded by a low-mass (perhaps minimum-mass),relatively quiescent, nebula. It is commonly assumed that chondrules were formed and chondrites accreted in this leisurely second stage of nebular history. However, high-energy processes were needed to create chondrules (and CAIs), and it is important to recognize that ~99.99% of the mechanical energy which was dissipated as heat during solar system formation was expended in the infall stage. The author submits that the infall stage is a much more promising setting for chondrule and chondrite formation than the quiescent nebula stage. Hutcheon I. D.* Jones R. H. The 26Al-26Mg Record of Chondrules: Clues to Nebular Chronology One key to the development of a more quantitative picture of solar system origin is an improved determination of the duration of the solar nebula and the chronology of events during this earliest epoch of solar system history. For the past several years we have pursued an experimental approach to nebular chronology using the ^26Al - ^26Mg system (half-life about 0.7 Ma) in chondrules and refractory inclusions (CAI) in chondritic meteorites. This study has revealed a striking contrast in the ^26Al - ^26Mg record between CAI, plagioclase-olivine inclusions (POI )and ferromagnesian chondrules. Sears D. W. G.* Huang S. Benoit P. H. The Formation of Chondrules Many properties of chondrules (cooling rate, efficiency of production, the large number of compound chondrules, complimentary composition relative to matrix, size-sorting, and the inferred composition of ambient gases) seem to be inconsistent with chondrule formation in a nebula setting. It also seems that the arguments levied against an origin by impact are no longer convincing: (1) Chondrules formed well after the formation of proto-Jupiter so that the relative impact velocities in the asteroid belt (especially near resonances) were high enough to produce melts; and (2) recent theoretical calculations for impact into weak asteroids, prompted by observations of Phobos and Gaspra, suggest that >50% of the ejecta returns to the asteroid surface; (3) arguments based on the lack of chondrules on the lunar surface ignore differences in the sizes of the parent objects, their locations in space and their surface compositions. We suggest that impact on a regolith remains the most likely mechanism for the origin of chondrules. Eisenhour D. D.* Buseck P. R. Radiative Heating and the Size Distribution of Pre-Chondrule Aggregates of Dust The size distributions of chondrules vary among the major chondrite groups, with mean diamters ranging from ~0.15 to 1.0 mm and minima between ~0.03 and 0.25 mm. A possible explanation for the narrow size distribution of chondrules is their formation form aggregates of dust by radiative heating. In the presence of ~0.3 to 8 micrometers EM radiation, smaller dust aggregates are heated less efficently than larger aggregates. This selective heating may be responsible for the paucity of small chondrules within chondrites. Small dust grains and grain aggregates could have existed within chondrule formation regions but escaped melting because of their inability to efficiently absorb light. If chondrule sizes are the result of radiative heating, then the size distributions of prechondrule aggregates of dust can be inferred by deconvolving the effects of radiative heating from observed chondrule size distributions. The dust-aggregate size distributions determined by such deconvolution are found to be in good agreement with those predicted by models of dust agglomeration within the early solar nebula. Huang S.* Sears D. W. G. Gas Flow and Fluidization in a Thick Dynamic Regolith: A New Mechanism for the Formation of Chondritic Meteorites We have previously shown that size and density sorting in a regolith which has been fluidized by the passage of gases from the interior of the body can quantitatively explain metal- silicate fractionation, an important property of ordinary chondrites. Here we discuss whether the flow rates and flux of volatiles expected from a primitive parent body are likely to be sufficient for this mechanism. Many meteorite parent bodies may have contained volatiles. From a consideration of heat diffusion and fluid mechanics, we calculate the gas flow rate of volatiles (e.g., water) in the regolith of an asteroid-sized object heated by 26Al. Our calculations show that the flow velocities and flux of water vapor are sufficient to produce conditions suitable for fluidization. Other heat sources have yet to be considered, but literature work suggests that they may be equally effective. Alexander C. M. O'D.* Formation of Chondrules by Recycling and Volatile Loss Recently, it has been suggested that recycling of chondrule fragments and volatile loss during chondrule formation, rather than nebular condensate precursors, could have produced the inter-element correlations observed amongst lithophile elements in bulk chondrule INAA data. The exploration of recycling-volatility models has been motivated by several observations. Firstly, there is scant mineralogical evidence that "traditional" equilibrium condensation occurred, and the presence of presolar grains in all chondrite groups shows that at least some nebular material did not experience nebular-wide heating. In addition, the rims and matrices of chondrites do not contain the equilibrium condensates expected from the deduced chondrule precursor compositions. Nor do the refractory condensates ever survive unmelted in chondules despite the presence of chondrule derived relict olivine. Secondly, there is petrologic evidence for recycling of chondrules, these include; most relict grains apparently having come from chondrules, compound chondrules with one enclosing the other, reheating of chondrules, and chondrule rims containing chondrule fragments. Yu Y.* Hewins R. Is Non-Linear, Rapid Cooling Plausible for Chondrule Formation? Evidence from Olivine Zoning Profiles Dynamic crystallization experiments with rapid heating and controlled non-linear cooling processes have demonstrated that high cooling rates are needed for chondrules to retain most of their moderately volatile elements, e.g., Na and S. However, the high cooling rate required to retain volatile elements seems to contradict the relatively low cooling rate suggested by a recent study on the compositional zoning in olivines from both natural chondrules and experimental charges. Naturally, the question would be what kind of textures the non-linear, high cooling rate process can produce and whether the olivine zoning profiles observed in natural chondrules can only be produced with low cooling rates. We have conducted a series of experiments under conditions similar to the Na and S retention experiments reported earlier, and performed detailed electron microprobe analyses on the major and minor elements of the olivine phenocrysts from charges with porphyritic olivine textures. The results show that the textures and olivine zoning profiles observed in natural chondrules can also form under the heating and cooling conditions employed in our experiments. These, combined with the volatile loss experimental results, suggest that non-linear cooling with high initial cooling rate is a plausible process that chondrules might have experienced. Greenwood J. P.* Hess P. C. Dissolution and Melting Kinetics: Applications to Chondrules Experiments illustrating the importance of melting and dissolution kinetics in the generation of chondrules have been undertaken and will be discussed. This paper deals with three subjects: 1) experiments investigating the formation of cotectic melts in the forsterite-albite system, 2) experiments studying the flash melting of albite up to 1600 degrees C on the order of seconds, and 3) utilization of these results to gain new insights on the formation of porphyritic chondrules. Jones R. H.* Danielson L. R. A Chondrule Origin for Dusty, Relict Olivine Grains "Dusty", relict olivine grains are commonly observed in FeO-poor chondrules. These grains contain many small, micron-sized blebs of Ni-poor, Fe metal which gives them their dusty appearance in transmitted light. They are interpreted as the products of solid-state reduction of more FeO-rich olivine which was reduced before or during chondrule formation. The source of the original FeO-rich olivine has been suggested to be either primitive condensate material, or derivation from previous generations of chondrules. Here, we examine the possibility that these grains could be derived from previous generations of chondrules, by comparing the compositions of dusty olivines with olivine compositions in FeO-rich chondrules which are the most likely chondrule precursors. Similarities in major and minor element compositions between these two occurrences of olivine argue that dusty relics do indeed originate from a chondrule source. This observation places important constraints on the frequency of chondrule recycling in the chondrule forming region. Brearley A. J.* Layne G. D. Light Lithophile Element (Li, Be, B) Abundances in Microchondrules in CH Chondrites: Insights into Volatile Behavior During Chondrule Formation We have measured the major and minor element compositions of glassy and cryptocrystalline microchondrules in three CH chondrites by broad beam electron microprobe analysis. The chondrules are all SiO2-rich, and pyroxene normative, but fall into two distinct compositional groups based on their FeO content (low FeO and high FeO). The concentrations of the light lithophile elements (LLE: Li, Be, B) have been determined in selected chondrules by ion microprobe. Be is positively correlated with Al showing that its behavior is strongly refractory, and it is also negatively correlated with the most volatile LLE, B. This negative correlation suggests that the volatile abundance of this group of chondrules is related to the volatile content of the precursor, not volatile loss during chondrule formation. Inoue M. Nakamura N.* REE Abundances in Chondrules from Murchison and Yamato-793321 (CM) Meteorites: Constraints on the Formation of CM Chondrules Except a few cases, almost no REE data have been reported for individual chondrules from CM chondrites. In order to obtained the trace element constraints on the formation and evolution of chondrules from Murchison and Yamato-793321 (CM) meteorites, abundances of REE, Ba, Sr, Rb and K were precisely determined by isotope dilution mass spectrometry (IDMS) together with petrographic examinations. The experimental procedures are similar to those previously described. Lofgren G. E.* DeHart J. M. Some Observations on the Petrology of Enstatite Chondrites PCA91020 and EET87746 with Implications for Chondrite Forming Processes Enstatite chondrites provide an important contrast with the ordinary variety because they define a different set of formation conditions. Renewed interest in enstatite chondrites has been generated by the recent increase in the number of type 3 samples that have been collected in Antarctica. We report here on two such meteorites that have important features bearing on the origin of this class of meteorites. Tuesday, March 14, 1995 MARS GEOPHYSICS 8:30 a.m. Room B Chair(s): K. E. Herkenhoff R. A. Craddock Frey H. V.* Bills B. G. Nerem R. S. Roark J. H. The Isostatic State of Martian Topography--Revisited Phillips and Saunders first discussed the isostatic state of martian topography through a plot of free-air gravity versus gravity from topography. Their analysis was limited by the relatively low resolution gravity then available and by the relatively sparse and low quality of the available topography. We repeat that analysis here using the new higher resolution (degree and order 50) GMM-1 gravity field of Smith et al. and the current USGS topography now available as a digital elevation model. The new gravity field fully resolves anomalies associated with individual structures such as volcanic constructs and the Valles Marineris, making possible study of the isostatic nature of localized areas, some of which show significant departures from simple isostatic compensation at shallow depth. Turtle E. P.* Melosh H. J. Martian Elastic Lithospheric Thickness from Flexural and Gravitational Modeling The way in which an elastic lithosphere responds to a load placed upon it depends directly on specific parameters of both the load and the lithosphere. Therefore observations of the Martian lithospheric response to volcanic loading can provide information about Mars' structure. Comer et al. mapped the locations of circumferential graben caused by flexure of the lithosphere under various volcanic loads. Through flexural modeling they were able to determine lithospheric thicknesses for which failure would occur in these regions. In our research we have created topographic and gravitational models for various volcanic loads on the Martian lithosphere and compared the results to the observed topography and gravity for Mars in order to determine the thickness of the elastic lithosphere in these regions. Kiefer W. S.* Zuber M. T. Williams K. K. Modeling Gravity Anomalies at Martian Shield Volcanos: A Reduced Estimate of Elastic Lithosphere Thickness at Olympus Mons An important constraint on models of planetary structure and evolution is the heatflow or near-surface thermal gradient. This can be inferred indirectly from estimates of the thickness of a planet's elastic lithosphere. Estimates of the elastic thickness at Olympus Mons on Mars have varied widely. Both gravity modeling and observations of tectonic features have been used to favor a lithosphere with a thickness of 140 km or more. On the other hand, the topography of a possible flexural moat and bulge surrounding Olympus Mons implies an elastic thickness of no more than 50 km. Here, we use a new, high resolution gravity model to derive a new estimate of the elastic lithosphere thickness of 25 to 50 km. Zuber M. T.* Smith D. E. Robbins J. W. Mars Without Tharsis The significant power in the Martian gravity field due to the Tharsis rise may mask or modify gravitational signatures that contain subtle but important information on martian geophysical processes. In order to isolate gravitational signals in regional areas where the field is affected by Tharsis as well to investigate characteristics of the global field, we have developed a technique to "remove" Tharsis from the martian gravity field. Removing Tharsis facilitates analysis of various geophysical problems relevant to the structure and evolution of Mars. Anderson F. S.* Grimm R. E. Crustal Thickness Variations at Valles Marineris, Mars The thicknesses of the crust and elastic lithosphere at Valles Marineris (VM) are constrained using the gravity and topography data sets of Goddard Mars Model-1 (GMM-1). Derived results useful for modeling formation of the rift include bounds on total extension and the minimum heat flow at the time of rifting. Regional crustal thickness lies between 45-60km, pre-erosional extension between 10-25%, and heat flux is constrained to be > 50 mW/m^2. Under slow extension, the crustal thickness and heat flux results are consistent with early distributed faulting (wide rifting) forming the central parallel troughs. Schultz R. A.* Senske D. A. Relationship Between Uplift, Faulting, and Strain Across Valles Marineris, Mars Topography adjacent to central Melas Chasma and Coprates Chasma is probably related to flexural uplift and unloading due to trough voids, with effective elastic lithosphere increasing in thickness from the eastern terminus to Melas Chasma. Topography farther west is not fit by the uplift profiles and is probably not directly related to trough faulting. These results indicate that the choice of topography for use in strain calculations must be made with some caution given that only certain regions of high topography (i.e., central and eastern troughs only) can be confidently attributed to the flexural signatures of uplifted lithosphere. Golombek M. P.* Franklin B. J. Tanaka K. L. Dohm J. M. Banerdt W. B. Extension Across Thaumasia and Around Tharsis on Mars Thaumasia is a large region, which makes up the southern part of the Tharsis province on Mars. It is made up of ancient Noachian basement and younger Hesperian units that are complexly fractured by a number of different fault sets, including a characteristic fanning set of long, narrow grabens that appear to radiate from a point to the north on Syria Planum. Thaumasia is the only remaining region of Tharsis across which estimates of the extension have not been made. However, because Viking Orbiter images in this region are not high resolution (generally a couple of hundred meters per pixel) and the scarps are narrow (generally around 0.5 km), photoclinometery cannot be used to estimate the vertical relief or throw, which is required to calculate the extension, across individual normal faults. In this abstract, the amount of extension across grabens in the Thaumasia region has been determined by measuring fault scarp widths; results are incorporated into estimates of the total extension around Tharsis, providing the first quantitative estimates of the regional and total strain around Tharsis on Mars. Pruis M. J.* Tanaka K. L. The Martian Northern Plains Did Not Result from Plate Tectonics The formation of the northern lowlands of Mars has remained a long-standing enigma. Impact and tectonic modes of origin have been proposed; the challenge has been to account for (1) the physiographic outline and depth of the northern lowlands and (2) the erosion along the highland/lowland boundary and infilling of the northern lowlands during the Late Noachian/Early Hesperian. Recently, Sleep formulated a plate-tectonic model for the origin of the crustal dichotomy. This model can explain the shape, depth, and relative youth of the northern lowlands by the formation of thin crust and the subduction of thicker, ancient highland crust. However, Sleep did not support his hypothesis with a thorough investigation of geologic evidence. Our examination of Sleep's hypothesis is twofold. First, we determined, both from Sleep's model and the literature on terrestrial plate tectonics, the characteristics and geologic associations of proposed plate-tectonic structures expected to be preserved. Second, we carefully searched for these identifying features at more than thirty key locations using Viking images and topographic data. Because of the many inconsistencies between plate-tectonic predictions and actual observations, we conclude that large-scale, horizontal lithospheric plate movements did not form the northern plains of Mars during Late Noachian/Early Hesperian time. Carruthers M. W.* Igneous Activity and the Origin of the Fretted Channels, Southern Ismenius Lacus, Mars Evidence for igneous activity near and within the fretted channels in southern Ismenius Lacus (30-45N, 315-350W) suggests that interactions between igneous intrusions and ground ice may have been instrumental in the development of the fretted channels in this region. Evidence for igneous activity includes: I ) a maar-like closed depression at the head of the main channel in MCS-SC; 2) a mound of dark material within the main channel in MC5-SC; 3) uplift of, or levee formation on channel flanks; 4) rough, dark material in the lowlands of the fretted terrain; and 5) dark material on the floors of craters exhumed by the fretted channels. Lopes-Gautier R.* Bruno B. Taylor G. J. Kilburn C. Lava Flows on Alba Patera: Analysis of Flow Properties Using Three Complementary Models Morphological data are at present our major source of information for extra-terrestrial lava flows. Lava composition and effusion conditions must therefore be inferred from the final morphology of lava flows. In this study we use three complementary lava flow models to infer emplacement characteristics of several of Alba Patera's younger flows. The results provide new insight into the late effusive history of this unusual volcano. Barlow N. G.* Degradation Studies of Impact Craters in the Mangala Valles Region of Mars The author continues her studies of crater degradation based on quantitative measurements of crater depth changes. Recent studies have focused on two areas surrounding the Mangala Valles region of Mars. Crater depth-diameter ratios for fresh craters in the study area indicate that the craters follow the expected depth-diameter relationship for fresh craters across most of Mars. This allows us to compare the current crater depth to that expected for a fresh crater of approximately the same size to determine the amount of degradation which the crater has experienced. Based on analysis of craters 1-5 km in diameter, we have identified regions of high, moderate, and low degradation within the study areas to the east and to the west of Mangala Valles. Aeolian, fluvial, and volcanic processes appear to be the dominant forces causing the degradation of craters in these study areas. Murray J. B.* Iliffe J. C. Martian Impacts and the Grooves of Phobos: Implications for the Evolution of Phobos' Rotation Axis Recent work has suggested that the grooves of Phobos might be explained as chains of coalesced secondary craters from impacts on Mars. This work examined groove characteristics and orientations on Phobos, and showed that they were consistent with such a hypothesis. We have since looked at the 16 largest impact craters on Mars (i.e. all those greater than 200km diameter), and using standard gravitational models, have traced the paths of the ejecta from these impacts out to the orbit of Phobos. We have chosen large craters because Mars' atmosphere may have been denser than at present, which may have affected trajectories of ejecta from smaller impacts. Using a constant ejection angle, we show that the predicted lines of secondary craters on Phobos from these impacts can be made to match the orientations of most of the grooves on Phobos, simply by varying velocities of ejection at Mars. Those which do not can be matched by assuming that Phobos' orbit was in Cassini state 1 in the past, or with its spin axis highly inclined if it was captured. Tuesday, March 14, 1995 LUNAR HIGHLAND ROCKS AND GEOLOGY 8:30 a.m. Room C Chair(s): J. J. McGee G. Snyder Hawke B. R.* Peterson C. A. Lucey P. G. Taylor G. J. Blewett D. T. Spudis P. D. Remote Sensing Studies of Lunar Anorthosite Deposits In recent years, we have been conducting a variety of remote sensing studies of lunar basin and crater deposits in order to determine the composition of surface units and to investigate the stratigraphy of the lunar crust. Special attention has been given to determining the distribution and modes of occurrence of pure anorthosite (plagioclase >90%) in order to answer the critical question of whether or not the lunar crust is enriched in plagioclase. If the Moon once had a magma ocean, an anorthositic crust should have been produced by plagioclase flotation. In previous studies, we combined telescopic visible and near-infrared spectral observations with Earth-based multispectral imagery in order to determine the lithology of relatively small areas (2-10 km) of the lunar surface. Now, high resolution multispectral images are available from the Galileo and Clementine missions. Numerous deposits of pure anorthosite have been identified, and interesting patterns have emerged. The purposes of this study were 1) to investigate the utility of a variety of image analysis techniques for the rapid identification of anorthosite deposits, 2) to summarize our current understanding of the distribution and modes of occurrence of lunar anorthosites, and 3) to assess the implications for composition, stratigraphy, and origin of the lunar crust as well as for the magma ocean hypothesis. Premo W. R.* Tatsumoto M. Pb Isotopes of Non-Mare Lunar Rocks (>3.9 Ga): Constraints on Early Lunar Evolution We believe the present volume of Pb isotopic data from various samples of non-mare lunar rocks (>3.9 Ga) indicates at least three isotopically distinct reservoirs that produced magma sources for the lunar crustal rocks that formed over the first 500 m.y. of lunar history. An early, presumably magma ocean-type source that existed ~4.42 to 4.44 Ga is characterized by 238U/204Pb values varying between 35 and 100 and probably produced the early plagioclase-rich lunar crust. A later (~4.1 to 4.42? Ga), probably upper lunar mantle reservoir continued to evolve and formed magmas or liquids with progressively increasing micrometer values between ~300 and >1000. After the early reservoirs were exhausted and most of the lunar crust and upper mantle were emplaced, a lower lunar mantle reservoir continued to develop (at least as old as 3.9 Ga) and produced magmas with relatively low-micrometer values between 10 and 50. The fact that most "old" lunar crustal rocks exhibit high 207Pb/206Pb values requires that they were either derived from, mixed with, or contaminated by Pb produced from early-formed, high-micrometers magma sources. The ubiquity of these U-Pb characteristics may also be an artifact of the Apollo and Luna sampling sites, all located on the near side of the Moon that was obviously deeply excavated during the basin-forming event(s), suggesting that the Pb signature in these rocks may be metamorphically emplaced. Snyder G. A.* Taylor L. A. The Growth and Modification of Lunar Crust: KREEP Basalt Crystallization and Precipitation of Mg- and Alkali Suite Cumulates The evolution of the lunar highlands crust, as determined by returned samples from the Moon, involves two distinct stages. The first stage is the formation of anorthositic to leuconoritic crust as flotation cumulates from an incipient lunar magma ocean approximately 4.4-4.5 Ga ago. The second stage involves the modification of this early crust from 4.4 to 3.9 Ga through the crystallization of basaltic melts which have been contaminated by urKREEP -- so-called KREEP basalts. These KREEP basalts could have crystallized within the crust to form cumulate gabbros, norites, anorthosites, monzodiorites, and possibly granites, with the proportion of trapped KREEPy residual liquid determining the large-ion lithophile-element enrichment of the rock. The earliest-formed cumulates represent the magnesian suite, and later cumulates comprise the alkali suite. Generally, the proportion of trapped KREEPy liquid is small (2-20%). The broad age range for the lunar alkali-suite and magnesian-suite rocks indicates that parental KREEP basalt magmatism was not a unique event, but was an important process, possibly repeated several times throughout the first 600 to 700 million years of lunar history. Shervais J. W.* Stuart J. B. Ion Microprobe Study of Lunar Highland Cumulate Rocks: New Results Deciphering the origin and evolution of the lunar highland crust is difficult in the western lunar highlands because of the small size of the highland rock clasts recovered from breccias, and because it is impossible to calculate parent magma compositions from whole rock chemical data on cumulate rocks. The composition of cumulate plutonic rocks, such as those which comprise the lunar highlands, are a function of primocryst composition, the proportions of cumulate phases, the proportion and composition of trapped melt, and subsequent reactions between the trapped melt and cumulate phases. We report here new data from our effort to characterize the parent magmas of lunar cumulates by using the ion microprobe and electron microprobe to analyze their cumulus phases. Parent magma compositions are calculated from the phase chemistry using equilibrium crystal/liquid partition coefficients. This approach eliminates the need for "representative" bulk rock samples, and it allows us to monitor the evolution of the trapped intercumulus melt during closed system crystallization. This approach is also effective for texturally pristine samples which are known only in thin section. Schultz P. H.* Making the Man in the Moon: Origin of the Imbrium Basin The lunar nearside is dominated by the effects of the Imbrium impact from its initial stages of formation to subsequent mare flooding. It is also enigmatic due to asymmetry in the distribution of certain ejecta facies, massif rings, wrinkle ridge pattern, and concentric pattern of crustal failure. Baldwin and Wilhelms previously suggested that Imbrium may have formed by an oblique strike from the northwest. New insights about the oblique impact process allow re-examining such a proposal. We conclude that a 30 degree impact from the northwest would not only account for the asymmetry in the topographic expression and ejecta facies but also for the offset in the geophysical expression. Moreover, the proposed Procellarum basin would be an expected consequence of the trajectory. Haskin L. A.* Moss B. E. McKinnon W. B. How Much Imbrium Material Should be Present at the Apollo 17 Site? Photogeologists have not found universally compelling evidence for disturbance of the Taurus Littrow region by Imbrium ejecta, although that location is only ~4.1 to ~2.8 transient crater radii from the center of the Imbrium Basin based on the extreme estimates of 335 km and 485 km for the Imbrium transient crater). The principal materials at the site are mare basalt plus volcanic ash on the valley floor, surrounded by highland massifs regarded by most investigators as mainly Serenitatis ejecta. Here, we ask what average thickness of Imbrium-derived ejecta we might expect on the basis of modern cratering theory, as adapted by Moss et al. to estimate average thicknesses of basin ejecta (primary fragments, PriFrags) and average proportions of PriFrags in ejecta deposits as a function of distance from a large crater or basin. The calculations provide the average thickness of basin primary ejecta (PriFrags) falling onto a "square of Interest" (SOI) and the depth to which the PriFrags erode. A SOI is the selected member of the set of adjacent (approximate) squares that form a ring at the chosen distance from the center of a basin or crater. The resulting regolith layer will not be uniform over the SOI; the calculations also yield the average distribution of thicknesses and depths. Jolliff B. L.* Korotev R. L. Rockow K. M. Haskin L. A. Apollo 17 Impact-Melt Breccias: In Search of Imbrium Components Although the Apollo 17 landing site on the eastern rim of the Serenitatis basin was chosen in part to sample basin-related material unaffected by the Imbrium event, we might still expect to find some Imbrium ejecta among the Apollo 17 highlands rocks and soils. Impact-melt breccias (IMBs), potentially good time-and-provenance indicators because the impact process tends to homogenize diverse target rocks into a compositionally uniform matrix, are obvious candidates. Although variable clast contents may "dilute" some trace element concentrations, most trace-element ratios are commonly preserved. At the Apollo 17 site, there are two groups of IMBs among the large-rock samples. The "poikilitic" IMBs dominate the samples; they constitute most of the sampled boulders, both at South Massif (light mantle deposit) and at the base of North Massif, and they are the dominant IMB component of the sampled highland soils. The "aphanitic" IMBs are found mainly among South Massif samples (e.g., 72255 and 73215, and we also find them among the lithic fragments of the soils from stations 2 and 3. We find a third set among the soil samples, an incompatible-trace-element(ITE)-rich group, that is not represented among the large rock samples from Apollo 17. Although these small samples have a considerable range of compositions, their ITE concentrations and interelement ratios are similar to IMB groups from the Apollo 14, 15, and 16 sites, suggesting a common ancestry or that they formed by different large (basin?) impacts into similar target rocks. Compositional similarities between the Apollo 17 ITE-rich IMBs and Apollo 15 Gp B IMBs, dated at 3.87 Ga by, may preclude an origin as Imbrium melt breccias if Imbrium is younger than 3.85. Perhaps the similarities in ITE concentrations and interelement ratios between Apollo 17 ITE-rich ITEs and IMB groups from formations at the Apollo 14 (Fra Mauro formation), Apollo 15 (Apennine Front), and Apollo 16 (Cayley Plains) sites clearly affected by the Imbrium event, mean that the Apollo 17 ITE-rich IMBs were also delivered to their present location by Imbrium ejecta. Ryder G.* Stockstill K. Chemical Variation Among Serenitatis Impact Melt Breccia Samples in the Taurus-Littrow Valley, Apollo 17 Landing Site Samples from the Serenitatis melt sheet collected as far apart as 15 km at the Apollo 17 landing site form a reasonably coherent cluster of compositions. In the present study, 3g chips of 17 rocks were analyzed to better assess any variation that exists among samples. Previous analyses were mainly on samples less than 500 mg, and most less than 200 mg, and hence are subject to greater sampling uncertainties. Neutron activation was performed on about 80 mg aliquots of homogenized powder. The analyses show that there are small but real differences among samples at the 3 g level, but that for most elements the dispersion is no wider than for the Manicouagan melt sheet. However, this study confirms that there are some systematic differences between the North and South massif samples as suggested by Rockow et al. James O. B.* Siderophile Elements in Lunar Impact Melts: Nature of the Impactors This paper reports results in a continuing study of siderophile and volatile elements in lunar rocks. The study is a reexamination of previously published radio chemical neutron activation analysis data to reevaluate the relations among meteorite-contaminated rocks and to determine compositions of the impacting bodies. The results of the current study reinforce the conclusions presented previously by James (1993) concerning Apollo 17 melt rocks: (1) both aphanitic and poikilitic melt rocks contain a similar, EH-chondrite, meteoritic contaminant, and (2) the variations in the siderophile-element patterns of the aphanitic melt rocks can be explained by mixing of siderophile-element-rich granulitic breccia clasts with the same EH-chondrite-contaminated melt as in the other melt rocks. Thus, both the poikilitic and aphanitic melt rocks could be products of the same impact. The impacting body was an EH chondrite, and its volatiles appear to have been largely lost. The current study further shows that many melt rocks from the Apollo 14, 15, and 16 sites have siderophile-element patterns indicating that the bodies whose impacts formed these melts were probably type IAB irons. Hawke B. R.* Peterson C. A. Coombs C. R. Lucey P. G. Smith G. A. Taylor G. J. McEwen A. S. Robinson M. S. Spudis P. D. Remote Sensing Studies of the Aristarchus Region of the Moon We combine near-infrared spectra from Earth-based telescopes with Clementine color images to unravel the stratigraphy and compositions of the Aristarchus Plateau, a complex area in the lunar Oceanus Procellarum. The plateau is a complex of both partly exposed highlands terrain, ultimately associated with the ejecta from the Imbrium basin, patchy plateau lavas, and red dark mantling deposits of probable pyroclastic origin. Highlands rock types present include anorthosite, gabbroic rocks, and troctolite and/or dunite. Sinuous rilles such as Vallis Schroteri emplaced, at least in part, the plateau lavas. The Aristarchus impact crater has blanketed the eastern plateau with ejecta, largely of highlands composition; this ejecta is distributed asymmetrically. Correlation of observed compositions with stratigraphically defined geological units continues. Pinet P. C.* Costard F. Chevrel S. Martin P. Bellagh F. Aristarchus Plateau Spectral Mapping from Clementine and Telescopic High Resolution Spectro-Imaging Data 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, revealing that the Plateau contains a widespread mare basalt component. _ Chevrel S. D.* Pinet P. C. Head J. W. Bellagh F. UV-VIS-NIR Spectral Classification in the Gruithuisen Domes Region The Gruithuisen domes d and g are relatively high albedo features located in the northwestern border of Mare Imbrium, about 250 km south of Sinus Iridum. These domical features are topographically distinct from adjacent highland and mare terrain, and show unique surface morphology, different from those of typical domes associated with mare basalts. The morphology of the flows that are on and surrounding the domes clearly indicates that these flows are highly viscous in their emplacement and may also have been explosive in places. The Gruithuisen domes are similar in shape and surface texture to many terrestrial domes of dacitic and rhyolitic composition characterized by extrusions of more viscous lavas at low rates. Principal Component Analysis (PCA) applied to a spectral UV-VIS-NIR dataset of this region permits the derivation of a spectral classification with its related spatial distribution in the image. _ Bell J. F. III* Lucey P. G. Blewett D. T. Hawke B. R. Robinson M. S. Roush T. L. Bregman J. D. Rank D. M. Harker D. Temi P. Imaging Spectroscopy of the Moon in the Mid-Infrared: 8.3 to 13.3 Micrometers Image Cubes of Tycho We report our initial progress on the use of a mid-IR array camera and a 1.5% resolution circular variable filter (CVF) to obtain imaging spectroscopic observations of selected regions of the Moon in the 8 to 14 llm wavelength region. Our specific goals are to detect and spatially map spectral features diagnostic of lunar plagioclase, olivine, pyroxene, and possibly of other rarer silicate phases such as quartz. We have concentrated our efforts on a small number of regions that are well-characterized by other techniques and that have the potential to exhibit high levels of spectral contrast and spatial heterogeneity. Tuesday, March 14, 1995 CALCIUM-ALUMINUM-RICH INCLUSIONS AND THEIR FORMATION PROCESSES 1:30 p.m. Room A Chair(s): S. S. Russell G. J. MacPherson Tsuchiyama A.* Takahashi T. Uyeda C. Formulation of Isotopic Mass Fractionation by Evaporation of Solids and Its Application to Chondritic Materials Isotopic mass fractionation by evaporation of solids was formulated. The fractionation is determined by evaporation rates of solids, R, and diffusion coefficients of elements in solids, D. If the grain size is smaller than the order of D/R, isotopic fractionation is expected. The results were applied to isotopic fractionation occurred in the primordial solar nebula. The evaporation rates of forsterite and metallic iron in the nebula were also estimated based on previous experiments. It is concluded that fractionation is expected only for Mg isotopes and not for Si and O isotopes in forsterite of >= micrometers in size. This indicates that isotopic fractionation observed in CAI's cannot be interpreted by evaporation in the solid state. Mg isotopic fractionation observed in forsterite in Allende could occur at low temperatures (1000-1200 K) for long durations (10-10^6 yr). A large isotopic mass fractionation of Fe is expected by evaporation of metallic iron. Nichols R. H. Jr.* Wasserburg G. J. Grimley R. T. Evaporation of Forsterite: Identification of Gas-phase Species Via Knudsen Cell Mass Spectrometry While equilibrium calculations provide first-order insights into the thermal mechanisms that affected a variety of high-temperature condensates and evaporative residues found in meteorites, it is clear that many of these phases were formed in a non-equilibrium state . Isotopic fractionation observed in CAI's, chondrules and circumstellar condensates indicates that measured species are different from predicted equilibrium species, implying that the formation of many of these objects involved kinetic effects. In an effort to understand more clearly the kinetic (or non-equilibrium) processes associated with evaporation and condensation we have recently built a high-temperature furnace that admits species evaporated from a sample directly into an electron-impact ionization source and quadrupole mass spectrometer, following the experimental technique pioneered by Inghram. The furnace can presently be operated up to temperatures above 2100 K (stable to +-2 K) in either the Knudsen (equilibrium) or Langmuir (non-equilibrium) configurations. We present here our first results of a Knudsen configuration analysis of forsterite. Davis A. M.* Hashimoto A. Clayton R. N. Mayeda T. K. Isotopic and Chemical Fractionation During Evaporation of CaTiO3 There is abundant evidence in refractory inclusions for high temperature isotopic and chemical fractionation events in the early solar system. Large enrichments in the heavy isotopes of oxygen, magnesium, silicon, calcium and titanium have been observed in some calcium-,aluminum-rich inclusions (CAIs). Rare earth element (REE) patterns of CAIs often show evidence for volatility fractionation. CAIs occasionally have negative cerium anomalies, but volatility fractionation of light REE (LREE)from heavy REE (HREE) is much more common. We have previously reported that residues with substantial enrichments in the heavy isotopes of oxygen, magnesium and silicon form from evaporation of liquids of forsterite composition and of chondritic initial composition. We have also shown that evaporation of a liquid of REE-doped chondritic initial composition forms residues whose REE patterns have large negative cerium anomalies. In order to better understand the causes for these fractionations in nature, we have evaporated trace element-doped liquids of CaTiO3 initial composition and measured oxygen, calcium and titanium isotopic compositions and trace element patterns in residues. Floss C.* Kransel G. Zinner E. Palme H. Rammensee W. El Goresy A. Trace Element Fractionations in Hibonite-bearing Evaporation Residues: Comparison with Hal-Type Hibonite Inclusions Diverse processes have been proposed to account for the origin of Ca-Al-rich refractory inclusions (CAIs) from carbonaceous chondrites. For many inclusions multi-stage processes, including condensation, distillation and igneous differentiation, are required to account for all compositional and petrographic features. Among the CAIs is a group of four hibonite-bearing inclusions for which an origin through Rayleigh distillation appears to be well-established. These so-called HAL-type hibonite inclusions (HAL from Allende, DH-H1 from Dhajala, and 7-404 and 7-971 from Murchison) have Ca and Ti isotopic compositions that are mass-fractionated with enrichments in the heavy isotopes; two also have isotopically heavy Mg. Furthermore, they have very low Mg and Ti concentrations compared to other meteoritic hibonites and exhibit large Ce and V depletions. Through evaporation of the Allende carbonaceous chondrite we have produced two distillation residues that contain large crystals of hibonite set in a matrix of Ca-Al-rich glass. Like the HAL-type hibonites, these hibonites have low Ti concentrations and are virtually devoid of Mg. REE and other refractory trace element (RTE) compositions also show remarkable similarities to those of the HAL-type hibonites, including depletions in Ce, V and Ba, indicative of distillation under oxidizing conditions, and a smooth depletion of the HREE relative to the LREE (observed in HAL and DH-H1) produced through igneous fractionation. Eu anomalies in some of our residues and in three of the HAL-type hibonites, together with the Ce depletions, provide evidence for a change from oxidizing to reducing conditions during evaporation and show that multi-stage processes are not required to produce this association; both anomalies can result from a single evaporation episode. _ Paque J. M.* Effect of Residence Time at Maximum Temperature on the Texture and Phase Compositions of a Type B Ca-Al-Rich Inclusion Analog Interpretation of the various models for the formation of Ca-Al-rich inclusions (CAIs) within the context of experimental studies is highly dependent on the parameters selected for the experiments. The majority of the experiments of were performed with a time at maximum temperature (tres) of 3 hr. Based on these experiments concluded that the maximum temperature (Tmax) for the formation of CAIs was near the isothermal crystallization temperature of melilite (~1400 degrees C for an Raverage Type BS CAI composition) with a cooling rate less than tens of degrees per hour. However, many of the models for the formation of chondrules and CAIs imply that tres was brief ("flash heating"). To test the effect of tres on resulting texture and chemistry a series of experiments with t(sub)res=0 min to 18.5 hr were performed. A decrease in tres, with Tmax and cooling rate constant, resulted in textures more consistent with those found in CAIs. Based on textures only this result expands the range of Tmax and cooling rate (CR) that can duplicate the characteristics of Allende Ca-Al-rich inclusions, similar to the preliminary results reported by. However, the experiments with shorter tres do not reproduce the mineralogy and crystallization sequence (particularly with respect to anorthite) inferred from natural CAIs, nor the mineral compositions. Steele I. M.* Peters M. T. Shaffer E. E. Burnett D. S. Minor Element Zoning Revealed by Cathodoluminescence and Substitution of Mg and Na into Synthetic and Natural Anorthite Anorthite is a common phase in high-temperature calcium, aluminum-rich inclusions (CAIs) as well as in other igneous samples, including eucrites and many lunar rocks. Cathodoluminescence (CL) of individual crystals shows intricate zoning which can be correlated with the concentrations of Na and Mg in Allende CAIs [Hutcheon I. D. et al.]. Experimental crystallization of anorthite from melts based on CAI compositions results in similar zoning patterns as well as quantitative similarities in minor element concentrations. Zoning can be interpreted as both sector and oscillatory suggesting two different mechanisms of minor element incorporation during growth. Partition coefficients will not be constant but rather are dependent on crystallographic direction and particular growth zones. Correlations among minor elements provide suggestions for substitution mechanisms within the anorthite structure. Meeker G. P.* Formation of CAIs by Partial Melting and Accretion During Heating in a Gas of Solar Composition A new model for the formation of USNM 5241, a type B1 CAI has been proposed. The model provides an explanation for previously unrecognized large-scale major element zoning patterns in the melilite mantle of 5241. A second inclusion, EGG-6, has now been discovered to contain the same chemical zoning patterns observed in 5241. The existence of two inclusions with identical large-scale chemical zoning supports an open system process of formation involving partial melting and accretion during heating in a gas of solar composition. Fahey A.* Huss G. Wasserburg G. Meeker G. P. REE Abundances in Allende 5241 and EGG-6: Correlations with Wide Area Elemental Mapping Few CAIs have been studied as extensively as the type B1 inclusion, Allende 5241. Three separate origins have been proposed for this inclusion; 1) capture of xenoliths by a liquid of uniform composition followed by crystallization and addition of a second liquid to form the melilite mantle, 2) capture and assimilation of xenoliths followed by fractional crystallization of a liquid droplet during cooling, and 3) partial melting of an aggregate of grains with accretion of a melilite mantle from a gas of solar composition during ambient heating. Simon S. B.* Davis A. M. Grossman L. Crystallization of Compact Type A Refractory Inclusions: Implications from Crystal Zoning and Trace Element Distribution Compact Type A inclusions (CTAs) are a major type of coarse-grained refractory inclusion found in CV3 chondrites, but they have not been thoroughly studied and their crystallization histories are not well understood. Our data show that in most CTAs both melilite and fassaite exhibit zoning trends and interelement correlations quite unlike those of their counterparts in Type B1 inclusions, suggesting that most CTAs did not form simply by crystallization of liquids. While some CTAs may have been molten, others show no evidence of such an origin. Kennedy A. K.* Bevan A. W. Trace Element Constraints on the Formation of the Rim on a Compact Type A Inclusion Compact Type A (CTA) calcium-aluminum-rich inclusions (CAI) crystallised from melts and are composed of >75% low akermanite content (Ak 3 - Ak 35) melilite (Mel) that poikilitically encloses spinel (sp) and perovskite (Pv). Sp and fassaite (Fass), and sometimes hibonite (Hib) and plagioclase (Plag) occur as interstitial material in the cores of some inclusions. CTA often have complex evolutionary histories, having undergone metamorphism, alteration, and deformation. We have examined a large elongate (1.8 cm x 0.9 cm) Allende CTA, hereafter referred to as 134-1, that contains minor amounts of Sp and Pv and that has a very thick (>1 mm) multilayered (up to 6 layers) rim sequence surrounding the interior Mel. We have measured major element compositions of the different phases by SEM, and have used the high resolution and sensitivity of the Western Australia SHRIMP II ion microbe to measure P, K, Sc, Ti, V, Cr, Mn, Sr, Y, Zr, Nb, Ba, Hf, Ta, REE, Ph, Th, and U in Mel, Pv, and clinopyroxene (Cpx). The thick multilayered rim of this CTA allows comparison of TE and REE wihin the rim and interior phases, and this allows us to study the development of the rim. In addition, the distribution of the trace elements in interior Mel and Pv provides a history of the formation and evolution of this CTA. Russell S. S.* Fahey A. J. Huss G. R. Wasserburg G. J. Isotopically Unusual Inclusions from CO3 Meteorites: Links to CAIs from Other Meteorite Classes Isotope studies of CAIs from CO3 meteorites have not been as extensive as studies from CV3s or CM2s, although some inclusions from CO3s (a hibonite bearing microspherule in Lance and a hibonite-hercynite inclusion with an ultra-refractory REE pattern, also in Lance) are known to have atypical elemental and isotopic characteristics. As part of a survey of CAIs from CO3 meteorites, we have identified three more isotopically interesting inclusions, and have observed parallels between these CAIs and previously-described examples from other meteorite classes. Two inclusions are texturally similar to hibonite-bearing microspherules and one inclusion was apparently formed by the same type of process as HAL-type hibonites. A comparison of CAIs from each class suggests that HAL-type inclusions were formed during multiple events in the solar nebula, whereas microspherules probably formed from a single reservoir. Ruzicka A.* Boynton W. V. Fine-grained CAI's in Efremovka and Leoville: In-situ Layer Growth and Confirmation of a Link to Rims on coarse-grained CAIs Fine-grained CAIs (here defined as spinel- or melilite-bearing "Ca- and Al-rich" inclusions that have a characteristic grain size of <50 micrometers) in the Efremovka and Leoville (CV3) chondrites were studied using SEM, microprobe, and optical techniques to provide a basis of comparison to well-studied inclusions in Allende and other chondrites. CAIs in Vigarano, Leoville, and Efremovka were less affected by low-temperature alteration than CAIs in other chondrites, but to date little work has been performed on fine-grained CAIs in these meteorites. Our results 1) confirm that fine-grained CAI in Efremovka and Leoville largely escaped low-temperature alteration; 2) suggest that some (but probably not all) of these inclusions are aggregates with various proportions of olivine (ol) and layered objects with "refractory" (mainly Al-rich) cores; 3) suggest that all of the inclusions experienced high- temperature metamorphism that enabled clinopyroxene (cpx) layers to grow at least partly in situ (after inclusion formation); and 4) indicate that fine-grained CAIs are very similar and probably genetically related to rim layers on coarse-grained CAIs. Hoppe P.* Kurat G. Walter J. Maurette M. Trace Elements and Oxygen Isotopes in a CAI-bearing Micrometeorite from Antarctica We continued our investigation of trace element abundances and oxygen isotopic compositions of spinel-rich CAIs in Antarctic micrometeorites. The particle studied is an unmelted phyllosilicate-rich micrometeorite which contains two spinel-rich CAIs. They consist of irregularly shaped Mg-Al-spinel which contain some small (< 2 micrometers) perovskite grains and are rimmed by a Fe-rich phyllosilicate. Both, spinel and the Fe-rich phyllosilicate rim, show flat chondrite-normalized patterns of the refractory trace elements which are strongly enriched by factors of ~ 200 and ~ 10 with respect to their CI abundances, respectively. The matrix has trace element contents that are chondritic for most elements with somewhat elevated abundances as compared to CI and CM chondrites. The spinel has oxygen isotopic compositionsrich in 16O (up to delta 17,18O of ~ -40 per mil) falling along or slightly to the 18O-rich side of the carbonaceouschondrite anhydrous minerals mixing-line. Tuesday, March 14, 1995 MARS GEOLOGY 1:30 p.m. Room B Chair(s): J. R. Zimbelman N. Cabrol Mellon M. T.* Jakosky B. M. The Distribution and Behavior of Martian Ground Ice During Past and Present Epochs Mars undergoes significant oscillations in its orbit due to gravitational interactions with the sun and other planets. Similar, but smaller, oscillations in the Earth's orbit are known to influence its climate, producing ice ages. Therefore, it is possible that orbitally-induced changes in the martian climate, and in particular the behavior of subsurface water ice, are considerably more impressive. Previous studies have shown that in the current martian climate ground ice is stable poleward of the mid-latitude regions and that the molecular diffusion of water vapor is capable of transporting atmospheric water into the regolith of short time scales. The question remains as to whether during favorable climate conditions the diffusion of water vapor can populate the near-surface regolith (top few meters) with ice before the climate changes toward more unfavorable conditions. To address this question we explore and map the behavior of ice in the near-surface regolith over the past 1 million years. We find that the past behavior differs significantly from that at the present epoch. Clifford S. M.* Mars: The Response of an Ice-rich Crust to Burial by a Volatile-poor Mantle Consideration of the martian valley networks, outflow channels, and various other surface features, suggests that the martian crust is water-rich and that it may host a planetary inventory of outgassed water equivalent to a global ocean 0.5 - 1 km deep. However, the geologic evidence also indicates that Mars has undergone extensive resurfacing -- including the production of up to 2 km of ejecta from impacts, as much as 0.5 km of extrusive volcanics, and an unknown (but potentially large) volumetric contribution of interbedded weathering and sedimentary deposits. Given the potential effect of such resurfacing on the geothermal temperature profile of the near-surface crust, and the strong influence of subsurface temperature on the stability and distribution of ground ice, the response of an initially ice-rich crust to burial by volatile-poor mantles has been considered -- including potential changes in state, distribution, and modes of volatile transport. Metzger S. M.* Martian Sinuous Ridges: Exploring Their Paleoclimate Significance Despite close scrutiny over the past five years, a range of interesting ridges on Mars has defied a convincing explanation. These sinuous ridges may have resulted from one or more geomorphic processes, including some unique to that planet, and as such could reveal important paleoclimate events. Martian sinuous ridges (MSR) comprise a number of variations including Dorsa Argentea's complex, apparently o?erlapping and intersecting features, Utopia's parallel, slightly arcuate and regularly spaced "Thumbprint" Terrain, the singular ridges within southern Utopia channels that have large widths relative to their channels, and the Argyre features that run unbroken for great distances in a moderately dendritic pattern. Proposed formation mechanisms have embraced aeolian, fluvial, glacial, karst, lacustrine, and mass-wasting processes (and combinations thereof), each of which carries major climatic implications. This update will attempt to frame the arguments for and against some of the formation mechanisms offered to date and thereby demonstrate the importance of these discussions in understanding Martian surficial processes. Ruff S. W.* New Clues on the Origin of the Martian Sinuous Ridges The search continues for new evidence to explain the origin of sinuous ridges seen in the Dorsa Argentea region of Mars' south pole. These features have been described in detail in previous papers. Recent work by Clark and Walder seems to weaken the case for the glacial origin supported by Kargel and Strom while evidence from analysis of terrestrial linear dunes is strengthening the dune hypothesis proposed by the author. According to Clark and Walder, terestrial glacial eskers are expected to form over a crystalline rock substrate, a geological setting unlikely to be found in the disrupted and heavily mantled regolith near the south polar region of Mars. But the dendritic appearance of the ridges, which gave rise to the esker hypothesis, can also be explained by the mechanism of lateral migration of linear dunes. New evidence for this idea based on remote sensing of the dunes on the Cactus Plain of Arizona will be given. Additional photographic evidence from various terrestrial sites will be presented in support of the dune hypothesis. Costard F.* Baker V. Thermokarst Landforms and Processes in Ares Vallis, Mars Ares Vallis is a flat-floored, elongate valley, 1500 km long and 25 km wide. The termination of Ares Vallis, located at 28 degrees W and 14 degrees N, exhibits a widening of the valley of 100 km over a length of 220 km. The valley floor of lower Ares Vallis is characterized by irregular terraces and by an unusual number of parallel ridges and attached hollows, ranging in size from 2 km to 5 km. These ridges are up to 20 km long, 1.5 km wide, and on the order of 40 m high. The origin of primary Ares Vallis has not been completely resolved. Despite the morphological evidence for a primary flood outflow origin, several features on the valley floor indicate that other secondary processes may be of considerable importance for explaining the extent valley morphology. Komatsu G.* Ori G. G. House P. K. Baker V. R. Flow Reconstruction for Ares Vallis, Mars The next U.S. Mars mission, Mars Pathfinder, has selected a primary candidate landing site located at the mouth of the Ares Vallis, one of the huge outflow channels debauching into Cryse Planitia. Our studies focus on the geological aspects of this area by employing geological mapping and by estimating paleodischarges for the channel (this report). We hope to provide pre-mission support data and insights which can be examined more thoroughly based on the mission results. Ori G. G.* Komatsu G. Baker V. R. Geological History of the Area Surrounding the Mouth of the Ares Vallis (Mars) Ares Vallis is the eastern most channel of the complex affecting the southern rim of the Chryse Basin. Its position allows investigation of the relationships among channel systems, highlands and basins. Five units have been recognized in the surroundings of the mouth of the Ares Vallis. De Hon R. A.* Hydrogeologic Provinces of Mars In order to examine the occurrence of water on Mars, the surface is divided into 12 hydrogeologic provinces that are defined by closed topographic basins of internal drainage. Provinces may be further subdivided into smaller, closed basins and nested, open basins. A low-elevation plain within each peovince provides a natural sedimentary trap. Moore H. J.* Geology of the Tempe-Mareotis Region, Mars The Tempe-Mareotis region is of interest because (1) the rock units and landforms are the results of a variety of geologic processes that began billions of years ago and continue to the present day, (2) the volcanism contrasts with volcanism elsewhere on Mars, and (3) the volcanism is related to volcanism in Tharsis in space and time. Processes include: (1) fluvial resurfacing of Noachian rocks in the Hesperian (or possibly Late Noachian), (2) northeast-trending graben with zigzag segments produced by extensional stresses after the fluvial erosion, (3) plains volcanism that began in the Late Hesperian and continued into Amazonian time, and (4) deposition of dust sand ongoing surface changes due to winds. The basaltic plains volcanism inTempe-Mareotis is quite different than volcanic styles of the flood basalts of the plana and shields of the montes. Intense fracturing of the rocks in Tempe Fossae preceded the plains volcanism; this zone of fracturing lies along the north eastward extension of the Tharsis bulge and a great circle defined by the volcanoes atop the bulge (Arsia, Pavonis, and Ascraeus Montes and Uranius Patera). Volcanism in Tempe-Mareotis occurred in the Late Hesperian and Early Amazonian when many of the Tharsis volcanoes were forming, but eruptions from most of the great Tharsis shield volcanoes continued into more recent times. Zimbelman J. R.* An Examination of Hypotheses of Formation for the Enigmatic Massive Deposits in Amazonis Planitia, Mars Massive deposits in the Amazonis Planitia region of Mars (12 degrees S to 18 degrees N, 125 degrees to 220 degrees W) cover both the heavily cratered highland terrain south of the equator and the moderate- to lightly-cratered plains north of the highlands. These unusual materials have stimulated many hypotheses for their formation. Here we review the principal hypotheses of formation for these enigmatic deposits and consider some of the implications associated with each proposal. Excellent Viking Survey mission images (2.5 degrees S to 9 degrees N, 137 degrees to 150 degrees W) provide a useful data set for evaluating the hypotheses of formation. Relative stratigraphic positions and detailed geomorphic information can be obtained from geologic mapping of the deposits at a scale of 1:500,000, which can be supported by the available image quality. The mapping information provides tests for most of the proposed hypotheses, as well as establishing a scientific framework for future collection of high resolution data of the deposits. Fuks K. H.* Treiman A. H. Murchie S. Layering in the Upper Walls of Valles Marineris, Mars: A Diagenetic Origin A packet of resistant layers, totaling about 400m thick, is present at tops of the chasma walls throughout Valles Marineris. The packet consists of an upper darker layer (about 50 m), a central brighter layer (about 250 m) and a lower dark layer (about 100 m). The packet appears to be continuous and of nearly constant thickness over the whole Valles system (4000 km E-W and 600 km N-S), independent of elevation (3-10 km) and age of plateau surface (Noachian through upper Hesperian). The genetic mechanism most consistent with the observed properties of the layers is that they formed in situ by diagenesis of pre-existing materials. Dohm J. M.* Tanaka K. L. Geologic Summary of the Thaumasia Region of Mars Based on Detailed Stratigraphy and Crater Statistics We have mapped the geology of the Thaumasia region (lat 15 phi to 50 phi S., long 55 phi to 115 phi) at 1:5,000,000 scale. The region includes the high plains of Syria, Sinai, Solis, and Thaumasia Plana, and the Thaumasia highlands that are fractured by Thaumasia, Claritas, Coracis, Melas, and Nectaris Fossae. The materials of the region have been divided into 39 geologic units. Crater diameter statistics for 28 units covering 96.6% of the total map region were compiled by measuring crater-rim widths from the Viking Mosaicked Digital Image Model (at 231 m/pixel) and by calculating unit areas from digitally scanned contacts using ARC/INFO (Table 1). In addition to stratigraphic relations, we determined the densities of superposed and total craters for many of the units to ascertain both material and resurfacing ages. In turn, these ages were used to define ages of fault sets, channels, and other features. Based on our geologic mapping and relative-age determinations, we present the following geologic summary of the Thaumasia region (province locations from). Cabrol N. A.* McKay C. P. Grin E. A. Was Life Discontinuous on Mars? Geology and Exobiology Arguments Water is defined as a key-element for the inception of Life. Considering that the pre-biotic environments of the Earth and Mars were probably sufficiently similar, the fluctuant parameter for the apparition and development of Life appears to be the duration of a favorable environment. Tuesday, March 14, 1995 LUNAR MANTLE PROCESSES AND MARE BASALTS 1:30 p.m. Room C Chair(s): J. Shervais I. Antonenko Parmentier E. M.* Hess P. C. Spatial and Temporal Scales of Lunar Differentiation: Implications for Subsequent Thermal and Chemical Evolution of the Moon The onset of mare volcanism at about 3.9 Gyr from source depths greater than 400-500 km provide important constraints on mechanisms of lunar internal evolution. Earlier models of lunar internal evolution do not account for these important characteristics of lunar magmatism. We continue to explore a model for the evolution of the lunar interior that explains the origin of mare basalts and possibly the existence of a lunar core. We assume that a magma ocean differentiated into the anorthositic crust and chemically stratified cumulate mantle. The cumulate mantle is gravitationally unstable with dense ilmenite cumulate layers overlying olivine-orthopyroxene cumulates with Fe/Mg that decreases with depth. Primitive lunar mantle rises to its level of neutral buoyancy in the cumulates and the dense ilmenite rich layer sinks to the center, forming a core. After this overturn, radioactivity within the incompatible element-rich ilmenite-cumulate core heats the overlying mantle. Buoyancy due to thermal expansion overcomes the stable chemical stratification in the mantle creating a convectively mixed layer above the core-mantle boundary that is the source region for mare basalts.Predictions of this model for a range of mantle viscosity (1019 to 1020 Pa-s), core radioactivity values (5 to 10 times bulk earth), and several values of the chemical density gradient above the core-mantle boundary are summarized in Table 1. If the content of radioactive elements in the core and the chemical density gradients above the core are sufficiently high, but within a range of values that might have been present in the moon, deep, high pressure melting at the top of the mixed layer is predicted after a reasonable time interval to explain the 3.9 Gyr onset of mare basalt volcanism. Wagner T. P.* Grove T. L. Origin of High-Ti Lunar Magma by Erosion of Ilmenite We have experimentally determined the dissolution rates of ilmenite in Apollo 15 green and Apollo 14 black ultramafic glass compositions to constrain the origin of high-Ti lunar magmas. We find that ilmenite has a dissolution rate that is an order of magnitude faster than the dissolution rate of silicate minerals. We conclude that assimilation of ilmenite-rich material in the shallow lunar interior is a viable mechanism for producing high-Ti magmas. It is generally accepted that high-Ti magmas are related to the late stage cumulates of the lunar magma ocean, but the nature of the relationship is still unknown. Hubbard and Minearproposed that in-situ assimilation of these cumulates by low-Ti primary magmas could generate high-Ti magmas. Ringwood and Kessonrejected this model based on assumptions of the assimilation reaction stoichiometry and thermal energy constraints. They proposed that the Ti-rich cumulate layer sank, enriched deeper source areas and induced melting. Wagner and Groveshowed that assimilation of shallow level cumulates is consistent with thermal energy and mass balance constraints based on experimental studies of the high-Ti black glasses. Agee C. B.* Circone S. Crystal-Liquid Density Inversions in High-TIO2 Lunar Basalt The sinking and floating spheres technique has been previously used to determine the high-pressure compressibility of komatiite and peridotite liquids [Agee C. B. and Walker D., 1993; Agee C. B. and Walker D., 1988; Ohtani E. et al.]. From these studies crystal-liquid density inversions are predicted to exist during deep-Earth melting. Recently we have extended investigations on high-pressure density inversions to the magmatic differentiation of the Moon. Longhi J.* Bertka C. M. A Critical End Point to Pigeonite-Augite Liquidus Equilibria Pyroxene liquidus equilibria play an important role in basalt petrogenesis. One of the most important observations in recent years has been the replacement of high-Ca clinopyroxene (cpx) by low-Ca (or "pigeonitic") cpx on the solidus of lherzolitic (olivine-orthopyroxene-clinopyroxene +- plagioclase/spinel/garnet) mantles with increasing pressure. This observation holds for terrestrial, martian, and lunar compositions. However, the transformation has not yet been tightly constrained by experiments, and the mechanism remains subject to debate. One side proposes that the transition involves a 3-pyroxene reaction (opx+aug=pig) and is thus relatively abrupt. The other side advocates involvement of 2-clinopyroxene critical equilibria and thus the transition is more gradual. We now present experimental data and graphical arguments in support of critical phenomena. Regardless of the mechanism, however, partial melting in the presence of low-Ca cpx has important geochemical implications: because of the well-established positive correlation of REE partition coefficients with the Ca content of clinopyroxenes, the partition coefficients of cpx will be much lower (~opx) at higher pressures than at low pressures where most of the partition coefficients have been measured. Thus many calculations of percent melting and relative proportions of opx and cpx in basalt source regions may need to be reexamined. Colson R. O.* Colson M. C. Chromium, A Dimer in Silicate Melts?: Implications for Redox Equilibria and Partitioning The redox behavior and partitioning of chromium is important in understanding lunar geochemistry. For example, Cr partitioning has been used to infer the oxidation state of the lunar interior. However, work we report here suggests that, under some conditions, trivalent Cr exists in part as a dimer in silicate melts. This means that both the proportions of trivalent and divalent Cr and the partitioning of trivalent Cr depend on chromium concentration for some range of concentrations. This dependence on concentration has not been taken into account in work to date. We find that tetravalent Sn also exists as a dimer in silicate melts. Simple concentrations of elements or oxides in melts cannot succeed in predicting activities of species when those species are dimers or other species more complex than the simple ions previously supposed to exist in silicate melts. Rutherford M. J.* Fogel R. A. A15 Green Glass Volatiles and Oxidation State A15 green glasses are widely accepted as products of lunar fire-fountain volcanism because they are penocyst-free, more primitive than known low-Ti lunar basalts, and because they are variously coated with what appears to be volcanic gas condensates including S, Cl, F, and various volatile metals. The origin of the picritic green glass melts is still not well known, however, and the driving force for the fire-fountaining is still in question. This paper primarily deals with the eruption process because this is the last process to affect the texture and composition of the green glass with the possible exception of post-depositional modifications which can be readily identified. In this project we have studied the green glass samples with a view toward identifying the composition and abundance of the gas present during the fire-fountaining event and the oxidation state of the magma before and after the eruption. The approach used is a combination of microprobe analyses and thermodynamic and diffusion profile analysis. No relicts of S diffusion-out profiles are present in the green glass in contrast to what is predicted by the diffusion data and cooling rates, but there are significant S composition variations; these are attributed to differences that existed at fragmentation. Ni-rich metal grains in the outer margins of glass spherules fix the log f(sub)O2 of the melt at -11.05 at 1350 degrees C. Shearer C. K.* Layne G. D. Papike J. J. Origin of the Apollo 15 Green Glass. Evidence from Ni, Co, Mn, V, and Cr The behavior of elements with siderophile and chalcophile characteristics have historically been used to place constraints on conditions and characteristics of the lunar mantle, the bulk composition of the Moon, the existence, nature, and evolution of a lunar core, and the evolutionary relationship between the Earth and the Moon. The source of some of this data has been primitive volcanic glasses that were returned from the Apollo and Luna missions. However, the understanding of these glasses, within a petrogenetic framework, is open to some interpretive problems. For example, the compositional variability in the Apollo 15 very low Ti picritic glasses has long been debated. Stolper interpreted the major element variability in these glasses to be inconsistent with a volcanic origin. Numerous other studies confirmed a magmatic origin but were unsuccessful in explaining the compositional variability. Galbreath et al documented an unusual positive correlation between Co, generally considered to be compatible in basaltic systems, and incompatible elements such as REE, Ba, and Zr. They interpreted this relation as either a result of source mixing or magma mixing. Unfortunately, some of the subtleties of these relations were lost because of the precision of the ion microprobe techniques (7 to 15%). Further studies by Shearer and Papike of very low Ti picritic glasses from other lunar sampling sites indicated that the mantle sources for these picritic magmas were extremely varied and consistent with the mixing models of Galbreath et al. Steele et al documented similar compatible-incompatible element relations in the Apollo 15 very low Ti glasses based on INAA analyses of individual glass beads. Based on experimental studies, they concluded that this trace element behavior was a result of the incompatible element behavior of siderophile elements such as Ni and Co in basaltic systems at very low oxygen fugacities due to a higher proportion of Ni0 and Co0 in the melt. Subsequent studies have indicated that, although this is a complex experimental and interpretive problem, the observations of Steele et al are probably incorrect. The purpose of this study is to use more precise ion microprobe analyses of these picritic glass beads from the Apollo 15 site to better understand the behavior of Co, Ni, Mn, Cr, and V at low lunar oxygen fugacities and to interpret the relationship among these glasses. Lindstrom D. J.* Martinez R. R. Trace Elements in Small VLT Basalt Clasts from 72235 Basaltic clasts in highlands breccias are the best samples of old, pre-mare lunar volcanics. For example, basalt clasts in boulder 1, station 2 breccia 72235 may be among the oldest mare basalts, yet because the clasts are so small no trace element data are available on these samples, and their relationship to basaltic glasses and lithic fragments in soils and other breccias are unknown. This work utilizes a new micro-coring technique to extract microgram-sized samples of these basalt clasts from thin sections, and micro-INAA techniques to obtain trace element data. Results show that these clasts are similar in composition to LOCo or MECo Apollo 17 VLT basaltic glasses. Blewett D. T.* Hawke B. R. Lucey P. G. Spudis P. D. Exploring the Crisium Region with Multispectral Imagery A recently completed near-IR spectral study has provided new information on the composition of surface units associated with the Crisium basin. The basin massifs are composed of noritic anorthosite or anorthositic norite. A number of units that contrast with the nearby highlands in various older remote sensing data sets (radar, color-difference image, Apollo orbital geochemistry) were also investigated. The spectral analysis shows that these units contain large proportions of mare basalt-like material. In some cases these units represent mare basalt deposits contaminated with highlands debris by lateral impact mixing (cryptomaria). In one location, the morphology of the anomalous unit argues against the existence of areally extensive basaltic lava flows, and an origin of the mare-like component by pyroclastic activity seems to be indicated. Analysis of multispectral images collected with Earth-based telescopes or by the Galileo spacecraft permits the extent and variation in the mare component to be determined over large areas. Suspected pyroclastic deposits have been mapped on telescopic and Lunar Orbiter photographs using photogeological methods; image analysis reveals these deposits to be spectrally distinct. Gaddis L.* McEwen A. Robinson M. The Distribution of Localized Lunar Pyroclastic Deposits as Related to Crustal Thickness The distribution of localized lunar pyroclastic deposits (LLPD) have been examined with regard to crustal thickness as inferred from Clementine altimetry data. These preliminary analyses indicate that LLPD occur in regions of increasing (60-70 km) crustal thickness along the margins of major mare-filled basins (where crustal thicknesses are <60 km). These results suggest that crustal thicknesses of ~60 to 70 km permit lunar magmas to just reach the lunar surface without producing significant lava flows. LLPD emplacement appears to be associated intimately with crustal thinning due to impact and mare infilling of basins on the lunar near side. On the far side, the emplacement of LLPD may be inhibited by inadequate magma supply in underlying mantle source regions. Antonenko I.* Head J. W. Estimates of Cryptomare Thickness and Volume in Schiller-Schickard, Mare Humorum and Oceanus Procellarum Areas Minimum cryptomare thickness estimates in selected areas were refined using the or d(sub)e=.33d (5), and were found to be approximately 500 m. DHC distribution cryptomare were found to be significant, representing 2% of the total known mare conclusions about the duration and timing of mare volcanism in the area. Mustard J. F.* Head J. W. Evidence for Cryptomare in the Northern Light Plains of the Moon The onset, distribution, and volumetric significance of earliest lunar mare basaltic volcanism is an important question in determining the overall thermal evolution of the Moon, but one made difficult to address by the lack of widespread sampling sites and the superposition of impact crater and basin deposits. Detection of pre-basin mare deposits (termed cryptomaria (1)) and development of criteria for their further delineation has led to increased knowledge of their distribution and significance. In this study, we use Galileo CCD data from the Earth-Moon 2 encounter (2) to assess the possible occurrence of cryptomaria underlying the extensive light plains units north of the Imbrium Basin and Mare Frigoris. At the resolution of the Galileo SSI data, we find no evidence for cryptomaria predating the emplacement of Imbrium-aged plains and ejecta. Evidence does exist, however, for cryptomaria of several compositional affinities north and east of Mare Frigoris and apparently postdating the Imbrium event, but predating the time of formation of the Orientale Basin. Although the Orientale basin is several thousand km distant, apparently contributions of highland ejecta from there and elsewhere obscured the basic mare signature to produce the cryptomaria. Sabol D. E. Jr.* Smith M. O. Adams J. B. Pinet P. C. Tephra Deposits at the Apollo 17 Landing Site Using Telescopic CCD Images Band ratios and spectral mixture analysis (SMA) employing image endmembers avoid the need to calibrate an image to reflectance. These approaches have proven to be effective for mapping compositional variability on the lunar surface. However, to assess detection thresholds it is necessary to bring the image data into alignment with the framework of lunar samples, allowing direct comparison with known, "pure" surface components. In this study, we used CCD images collected at the 2-meter telescope (F/D=25) of the Pic du Midi Observatory in France of the Apollo 17 landing site to examine the composition and spatial distribution of the varying backgrounds in the image, and then determined the detectability of the Apollo 17 orange and black glasses for these different backgrounds. Tuesday, March 14, 1995 TEKTITES AND IMPACT STUDIES 1:30 p.m. Room D Chair(s): E. F. Albin T. J. Ahrens Lange J.-M.* Fehr K. T. Grund T. Langenauer M. Meisel T. Palme H. Spettel B. Stoffler D. Moldavites from Lusatia (Germany) II: A Chemical Comparison with Czech Moldavites Chemical analyses of Lusatian moldavites confirm previous suggestions concerning their affinity to Bohemian and Moravian moldavites. Small but characteristic chemical differences exist between moldavites from Bohemia, Moravia and Radomilice area. Lusatian moldavites cannot be assigned unambiguously to any of the three groups. Some Lusatian moldavites have compositions matching the Bohemian sub-set others are similiar to the Moravian group. Thus the origin of Lusatian moldavites cannot be explained Albin E. F.* Roden M. F. New Major-Element Abundances and Interelement Correlations for Georgia Tektites Tektites found in Georgia (georgiaites) are a variety of macro-tektite from the North American tektite strewn field. Although approximately one thousand specimens have been collected, previous studies draw conclusions based on the analysis of only 16 georgiaites. We analyzed eighteen additional specimens by electron microprobe to further document bulk compositions of the tektites and to search for correlations between elements or element ratios. Tektites used in this investigation are part of a very large georgiaite collection (95 specimens) belonging to the Fernbank Science Center in Atlanta, Georgia. Major-element compositions were determined with an automated four spectrometer JEOL 8600 electron microprobe using Bence-Albee matrix correction procedures. Small chips 2-6mm in diameter were separated from each tektite, mounted in epoxy, and polished. Reported abundances are averages of multiple point analyses using a 5 micron diameter beam to avoid Na loss by volatilization. Newman S.* Beckett J. Bashir N. Stolper E. Water in an Indochinite: Implications for the Thermal History of Tektites Tektites are naturally-occurring, aerodynamically shaped silicate glasses generally thought to have been produced by supercooling of impact melts ejected at high temperature into the earth's atmosphere. Tektites can be chemically heterogeneous and the preservation of these variations may provide constraints on thermal histories. Water is of particular interest in this regard because its diffusivity in rhyolitic melts is high and reasonably well-known as a function of temperature. Bulk measurements of water in tektites indicate 70-500 ppm but detailed spatial distributions necessary for kinetic modeling have not been previously available. Wasson J. T.* The Disintegration of Comet Shoemaker-Levy 9 and the Tunguska Object and the Origin of the Australasian Tektites The breakup of comet Shoemaker-Levy-9 during a close encounter with Jupiter called attention to the low strength of comets. About 10% of comets disrupt during their first perihelion passage, and those that make repeat passages have splitting probabilities only a few times lower. Upon entering the Earth's atmosphere, most comets (and low-strength asteroids) probably, like the 1908 Tunguska object, disintegrate and deposit the bulk of their energy in the atmosphere. The deposition and flow of a clast-free melt sheet across a large region in Southeast Asia, as indicated by the presence of layered tektites associated with the 0.76-Ma Australasian event, requires a high-luminosity sky as would result from the accretion of a fragmented comet. The energy deposited during the Australasian tektite event was 10^4 - 10^5 times larger that that deposited by the Tunguska object, and the associated holocaust encompassed a region having an area of ~2x10^5 km^2. Rocchia R.* Robin E. Froget L. Gayraud J. Meon H. Diemer E. The Meteoritic Content of Lybian Desert Glasses The existence of natural glass in the Lybian Desert is known since the nineteenth century. The finding of anomalously high concentrations of siderophilic elements by Murali et al. in some brown-coloured streaks suggests that the Lybian Desert Glass (LDG) contains a small fraction, on the order of 0.1%, of meteoritic material. We report here new results about such coloured parts which confirm the quite frequent occurrence of a chondritic component which, in some cases, represents up to 1% in mass of the samples. This supports an impact origin for LDGs. Horz F.* Cintala M. J. Bernhard R. P. See T. H. Cratering and Penetration Experiments in Aluminum 1100 Targets Using Soda-Lime Glass Projectiles from 1-7 km/s Using Teflon targets to simulate thermal blankets from the Long Duration Exposure Facility (LDEF), we demonstrated that it is possible to obtain unique solutions of projectile diameter from diameter measurements of individual penetration holes. This renders the interpretation of penetration holes totally akin to that of craters in infinite halfspace targets. This new approach seems to be substantial improvement over traditional ballistic-limit considerations, which typically yield a single, cumulative count for all projectiles larger than some threshold impactor capable of physically perforating a target of a given thickness. Davis D. R.* Ryan E. V. Farinella P. On How to Scale Disruptive Collisions Computer simulations of large scale impacts, such as those that occur in the asteroid belt or those that are responsible for disruption of outer-planet satellites to form ring systems, depend upon reliable scaling laws to extrapolate laboratory scale results to bodies that are hundreds of kilometers in diameter. Understanding the scaling laws has been a major thrust in the field of impact physics in recent decades, and particular emphasis has been placed on scaling of catastrophic disruptive several events (as opposed to the smaller scale cratering events) in the past years. In a recent paper, Davis et al used three proposed scaling laws for shattering impacts, namely those impacts energetic enough to fracture at least 50% of the mass of the target body, in a numerical simulation of the collisional evolution of asteroids. O'Keefe J. D.* Ahrens T. J. The Role of Central Peak Oscillation in the Formation of Complex Planetary Craters Baldwin and Van Dorn, suggested that the rings around lunar craters formed as the result of the arresting of giant fluid surface waves driven by an impact event. In the intervening 20 years, complex craters' morphologies are observed on all solid planets and major satellites in the solar system. Baldwin and Van Dorn based their arguments on an analogy to droplet impacts into water and other fluids. These experiments showed the formation of a tall transient central peak that was an order of magnitude larger than the droplet size and that the oscillation of the peak drove a series of concentric surface waves. While these papers were provocative, they did not provide a quantitative description of the threshold for wave production nor a description of the assisting mechanisms for the waves in planetary materials. We have modeled large scale impacts on planets for a range of material strengths and found the development of tall transient central peaks that drive surface waves and flooding waves such as observed on Venus. In these series of calculations we varied the gravity scaling parameter ga/U^2 from 0 to 0.34, and the strength scaling parameter Y/rhoU^2 from 0 to 2400, where a is the impactor radius, U the impact velocity, g is the planetary gravitational acceleration, and Y the planetary crustal strength. We have extended our previous calculations out to very late times so as to model the central peak-surface wave interaction. Pierazzo E.* Vickery A. M. Melosh H. J. A Re-Evaluation of Impact Melt/Vapor Production Impact cratering is a very important process for the evolution of planetary surfaces. Although the concept of impact cratering and its effects in planetary sciences is relatively new, it has been subjected to intensive study from many different points of view, ranging from planetary observations fo laboratory-scale experiments, to computational studies and modeling of the physics of cratering. One of the most complex problems is the production of melt and/or vapor during the impact event. The approach to the problem can be of two types: 1) analytical energy balance models, and 2) numerical solutions of the fundamental balance equations. Experimental approaches, while highly desirable, are still limited by the maximum impact velocities obtained in laboratory that do not reach the magnitude of real impact events. In this work numerical simulations are used to determine the amount of melt and vapor for impact events of different size and magnitude. The results of the simulations are compared to previous work. Schmidt R. M.* Hassig P. J. Asteroid Entry into Venusian Atmosphere: Pressure and Density Fields There is considerable evidence that an ambient atmosphere has an influence on crater formation. The effect is reasonably well quantified for explosive cratering at various depths of burst. However, for impact cratering there is an additional synergistic effect of the impactor traveling through the atmosphere prior to hitting the surface. This creates a strong shock flow field setting up a transient pressure/density variation across the surface. There have been a number of experiments to isolate different aspects of the impact cratering process; in particular, impact at reduced pressure and explosive simulation at increased pressure. What has not been shown is how size and velocity effects can be scaled to full size applications of interest such as the Venusian crater record. Namely, does the transient crater lip lead or lag the dynamic pressure increase? What is the overpressure and density at the ejecta plume leading the excavation? Is there some time scale after which the remaining variation can be approximated by a constant pressure and density; and if so, are they higher or lesser than the initial undisturbed values? Schultz P. H.* Effect of Impact-induced Shear Heating on Vaporization and Melting As impact angle (from the horizontal) decreases, peak pressures decrease as expressed by reduced crater size and impactor fragmentation. Laboratory experiments also indicate, however, that vaporization of easily volatized targets such as dry ice increases as impact angle decreases, a phenomenon that also correlates with enhanced magnetic field generation. Consequently, frictional shear heating may contribute significantly to the observed enhanced vaporization. The present study explores the processes controlling energy partitioned into internal energy during oblique impacts and considers possible implications for volatile release on the planets. Barnouin O. S.* Schultz P. H . Laboratory Clues to the Emplacement of Distal Ejecta Deposits by Atmospheric Processes During an impact, the advancing ejecta curtain displaces atmosphere along its front to create a vortex ring at its top by flow separation. Previous experiments in fine grained targets document how such a ring vortex entrains, carries and deposits ejecta to form the distal edges of the ejecta facies. Such an origin for the distal ejecta at large scales is consistent with observations including striations extending over the inner ejecta. The ring vortex is created mechanically at the time of crater formation. It moves downwards to the target surface and must be distinguished from thermally created vortices that rise buoyantly. A comparison of a model estimating the strength, the size, and the horizontal and vertical position of the ring vortex with experiments in a fine grained target help understand the physical mechanism controlling the creation and decay of the vortex and thereby the run-out of distal ejecta. Experiments with a fine grained target provide an analog to the atmospheric processes at large scale impacts because in both cases ejecta entrainment is extensive. Results suggest that entrainment increases the effective kinematic viscosity of the flow in the ring vortex. A preliminary test of the model indicates that it satisfactorily predicts distal ejecta run-out for two martian craters in an identical target. Tuesday, March 14, 1995 PLANETARY SCIENCE EDUCATIONAL ACTIVITIES AND TECHNOLOGY 6:30 p.m. LPI Dasch E. J. Keffer L. Pruzan K. The NASA Space Grant Program: An Aid in Proposal Planning The NASA National Space Grant College and Fellowship Program--Space Grant--has completed its first five years of operation and conducted an extensive evaluation of the 52 State Consortia and the national program. The purposes of this presentation are: (1) to describe the program and its present configuration; and (2) to illustrate how Space Grant may be of help in providing resources for proposal writing across the spectrum of research, education, and public service. Edgett K. S. Christensen P. R. Wakefield D. A. Rigberg B. M. The Arizona Mars K-12 Education Program: Outreach for Mars Global Surveyor, Mars Pathfinder, and Other Missions The Arizona Mars K-12 Education Program provides opportunities for direct interaction between K-12 teachers, students, and the scientists and engineers who are involved with the Mars Global Surveyor and Mars Pathfinder missions. Student visits to the Mars Global Surveyor facility at Arizona State University (ASU), K-12 teacher workshops, and supplementary education and mission update materials were the primary activities in 1994. Interested educators have witnessed the transition from Mars Observer to the new Mars missions through an on-going workshop series, Internet communications, and a newsletter, TES News. In 1995, the program is continuing the previous themes and expanding to incorporate development of classroom materials and continued exposure to the latest news in planetary science and Mars exploration through the on-going teacher workshop series. The program has a growing list of educators who live/teach outside Arizona. Betts B. H. Bormanis A. Kenealy R. Nash D. B. Planetary Science Education: The SJI Program The San Juan Institute (SJI) has created a successful Planetary Science education program that continues to expand. The SJI education program currently has several active parts: (1) Planetary Science field trips, generally held at the Institute, which teach basic Planetary Science and expose over 3000 third to eighth grade students per year to a working scientific institution; (2) a constant flow of high school and college student science interns; (3) teacher workshops related to science conferences held at SJI; and (4) public lectures on topics in Planetary Science. SJI is currently working with several partners developing ways to more widely disseminate its current field trip programs, including: (1) real time interactive video feeds that will allow students at remote sites to view and participate in field trips held at SJI; (2) developing video tapes of the field trips; (3) and providing portions of the programs over local cable television. SJI also is working on expanding its field trips at the Institute by offering them on more days each week (current booking is two days per week) and by developing a field trip oriented towards advanced (AP and Gate) high school students. Other projects in development include teacher and student workshops, and hypertext field trips available over the World Wide Web (WWW). In addition to teaching a new generation about Planetary Science and exciting them about science in general, SJI hopes that its program will be of use to others in Planetary Science education as a model in whole or in part, and as an established location to implement newly developed education support materials. Alexander W. M. Schaub G. E. Smith S. R. Adventures in Space and Atmosphere Science In the spring of 1991, a program of teacher/student workshops was launched by the Center for Space Science and Engineering Research and Education at Baylor University at Waco, Texas and funded by the Texas Space Grant Consortium (TSGC). The workshops were the outgrowth of the TSGC Committee for Community Service and Outreach, chaired by Dr. W. M. Alexander of Baylor. Objectives of the Space Grant Program relating to secondary education are to use interest in space to increase participation in science and math in the public schools, to foster space-related programs and curricula for public schools and for the public, and to increase the pool of high school graduates who enter college to study science, math and engineering with emphasis on underrepresented minorities and women. The Adventures in Space and Atmosphere Science module aimed at stimulating interest in teachers and students by giving a glimpse of the day-to-day work involved in development, design, and construction of space flight hardware, as well as the interpretation of the data obtained by remote monitoring space probes. Each segment consisted of a relatively brief presentation of conceptual material, an extended period of hands-on activities, followed by a wrap-up period that summarized the objectives and findings of the segment. Four conceptual topics were developed: missions to comets and the outer solar system, missions to the inner solar system, missions to the Earth/Moon system, and communications in space. The lab activities illustrated the design and calibration of space instruments, random sampling of space phenomena, remote sensing, and communicating space data. Central to the development of these activities was the use of inexpensive, easily accessible materials to model actual space science principles. Lindstrom M. Allen J. Fitzmaurice R. 'Exploring the Solar System': A Model for Teacher Workshops and Scientist-Educator Partnerships "Exploring the Solar System" is a teacher workshop to be offered this summer at NASA-JSC. The workshop will provide teachers with scientific knowledge, experiences, and materials to improve their teaching of astronomy and planetary science and to integrate these subjects into their Earth Science curriculum. Murali A. V. Space Science and Technology Educational Program for K-12 Students in Southeast Texas: A Multidisciplinary, Multi-Institutional Consortium Study at Lamar University-Beaumont A three-year multidisciplinary and multi-institutional consortium study is proposed to develop a comprehensive, structured, and classroom-tested Space Science and Technology Educational Program (SSTEP) dedicated to K-12 education in southeast Texas (Region V school district) at Lamar University-Beaumont (LU-B). The goals of the program are to employ space science and technology course work to motivate K-12 students towards excellence in science and mathematics and to enhance the curriculum. A special effort will be made to improve and document the minority student academic achievement levels in the region through the program [minority students constitute ~71% of the total ~20,000 students in Beaumont Independent School district (BISD) and ~36% of the total ~87,000 students in southeastern Texas]. Lebofsky L. A. Lebofsky N. R. Project Artist: Integrating Astronomy and Planetary Sciences into the Elementary and Middle School Curriculum Scientists from the University of Arizona and teachers from southern Arizona are collaborating on ways to make astronomy and planetary science ("space science") accessible and interesting to children from diverse backgrounds at both the elementary and middle school levels.Programs which offer instruction in space science through hands-on experiences, integrated curriculum, and translated materials are described below and will be presented in the session. Edwards M. TekLab: Excitement in the Elementary School The idea behind establishing the technology laboratory (TekLab) in our school was to provide a learning environment that would excite and motivate the students to learn. Unlike earlier generations, children today are very savvy about technology and want to know as much as they can about it. Therefore, in the TekLab we provide access to current technologies that fourth through sixth graders will play and learn by hands-on experience. We notice that the hands-on activity in the Teklab is a good motivator for students to learn mathematics and science. Other factors that enhance learning in the TekLab are: Douglas D. Space Science Education Program for K-5 Students The Space science education program of Sallie Curtis Elementary School started in 1990 with the participation of its first grade students in the NASA sponsored Space Exposed Experiment Developed for Students (SEEDS). This program consisted of planting both the cosmic ray exposed [through Long Duration Exposure Facility (LDEF) of NASA] and the control tomato seed samples in our school garden and growing the tomatoes (to test for possible mutations induced by exposure to higher level of cosmic radiation). We named our project "the great tomato adventure" which received national attention3. The students from the first group saved the seeds for the next years class. Each year students grow the garden and save the seeds for the next year; thus "space tomato garden" has become a tradition at Sallie Curtis Elementary School. Rothery D. A. Getting Planetary Science into the Open University Degree Planetary science is now an integral part of Open University courses in Earth Sciences and Physics. The Open University is distance teaching organisation, and its students study part-time, at home but with local tutorial support, in all parts of the UK, and increasingly in other parts of Europe. In 1995 over 1500 students will be taking at least one of the two courses having a significant planetary science component. Each is about 250 hours study time (one-twelfth of an honours degree), and is at a level equivalent to second year or late first year study in a conventional 3 year full-time degree. Apart from teaching planetary sciences, they revise concepts first learned in other contexts and impart various general skills. The courses are designed for distance learning, and are based on specially-written course texts, supported by video cassettes, audio cassettes, broadcast TV, and approximately 10 hours of tutorial time. Both these courses have proved popular with students. 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 and Venus data on CD-ROM volumes from Magellan, Viking and 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) covenng 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 environrnent 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. Kanefsky B. World-Wide-Web Tools for Locating Planetary Images The explosive growth of the World-Wide Web (WWW) in the past year has made it feasible to provide interactive graphical tools to assist scientists in locating planetary images. The highest available resolution images of any site of interest can be quickly found on a map or plot, and, if online, displayed immediately on nearly any computer equipped with a color screen, an Internet connection, and any of the free WWW browsers. The same tools may also be of interest to educators, students, and the general public. Image-finding tools have been implemented covering most of the solar system: Earth, Mars, and the moons and planets imaged by Voyager. The Marsimage-finder, which plots the footprints of all the high-resolution Viking Orbiter images and can be used to display any that are available online, also contains a complete scrollable atlas and hypertext gazetteer to help locating areas. The Earth image-finder is linked to thousands of Shuttle images stored at NASA/JSC, and displays them as red dots on a globe. The Voyager image-finder plots images as dots, by longitude and apparent target size, linked to online images. Altner B. Kochhar A. Brandt D. The Internet as a Medium for Public Debate on the Issue of Returning to the Moon The current surge of interest in the global Internet at many levels of our society is largely due to the evolution of the World Wide Web (WWW) into an increasingly sophisticated application for discovering and interacting with distributed information resources. In the true spirit of a grassroots movement, we seek to harness the unique capabilities of this medium in order to stimulate public discussion of some of the important and controversial issues facing us today. Mouginis-Mark P. J. Space Science Education Using the World Wide Web Recent developments in the speed, diversity and level of interaction provided by material being offered over the World Wide Web (WWW) enables new approaches in space science education to be developed. No longer is it necessary for a teacher or college professor to prepare slide sets or video tapes for widespread distribution; instead much of the work can be focused on developing a comprehensive data base on a single server and in educating students to access the Web. For planetary sciences, a fine example of the breadth of space science information is provided by the Lunar & Planetary Lab at the University of Arizona. This form of presentation of selected images of the Solar System has numerous advantages, including the timely presentation of new results, and the iterative development of ideas and data bases compared to the longer term task of rewriting a text book or a new CD-ROM. Kanaya A. Development and Use of Multimedia Material in Planetary Science In order to assist secondary school children to understand the evolution of the earth and the moon, multimedia software has been developed. This software is based upon the concept of breaking down the complex structure of the many integrated sciences into components that can be more easily understood by children. It then utilizes hypothetical models to help present the ideas of these natural phenomena in quantities comprehensible by children. The many components are then integrated utilizing electronic multimedia. The integrated material will assist the user in understanding how the earth and moon have evolved as well as how the scientific understanding of their evolution has been developed. In addition, this program will influence the children to apply scientific reasoning to assist them in understanding the natural phenomena that they encounter in their everyday lives. The assistance and collaboration of the scientists in the individual integrated disciplines, as well as database managers and secondary school teachers are required enhance and complete the principal software shell. Condit C. D. Tanaka K. L. Mars Geologic Landform Atlas and Reference Source (MGLARS): A Stand-alone Hypermedia Program Designed for Geologic Research, Education and Map/Image/Data Dissemination The Mars Geologic Landform Atlas and Reference Source (MCGLARS) is an interactive, multimedia, computer-graphical geologic landform atlas, geologic map, and literature database that can be distributed on CD-ROM, diskettes, or by Internet. The atlas is a stand-alone program created using SuperCard software for Macintosh computers; it will run on Macintosh and Windows-based PC systems having at least 8MB of RAM. The atlas is designed to show images of type geologic features on Mars and provides a link between these images and the literature database. A demonstration version of the atlas has been developed that includes the following feature's: (1) A shaded-relief base map including nomenclature, screen buttons and menus to access Viking image data and digital movies; (2) a geologic map and correlation chart of Mars (digitized version of formal 1 :15,000,000-scale Viking-based map); (3) annotated black-and-white and false-color images and movies of Mars, showing scale, landforms, terrains, and landing sites; (4) a comprehensive literature database (with key-word search and, from these, point and click access to associated images.) Strait M. M. The Use of Image Processing in the Classroom The computer has entered the classroom in many ways over the past decade. At Alma College, the computer is an integral part of doing chemistry, from simple word processing and graphing to more complex molecular modeling and spectral interpretation. As early as a student's first course in chemistry, experiments are interfaced directly to a computer to collect data and then allow interactive processing of the data to produce the desired type of output for further calculations. The nature of the chemistry department at Alma College allows us to explore ways of using the computer in many new and different ways, such as the interpretation and analysis of images. Murali A. V. Planetary Rover Unit: A Hands-on Educational Facility to Motivate K-12 Students Towards Excellence in Science and Mathematics We have built a Planetary Rover Unit (PRU) on the Lamar University-Beaumont (LU-B) campus as a part of the hands-on educational facility of Space Science and Technology Educational Program (SSTEP) for K-12 students in the region. The PRU consists of a mobile planetary rover and a base-camp unit which is functionally similar to the mission control room at NASA/Johnson Space Center, Houston. Bindschadler D. L. Moore W. B. The Venus Hypermap: Hypermedia Education and Publication Via the World Wide Web With the emerging technologies represented by networks such as the Internet, hypermedia protocols such as the World Wide Web and the Hypertext Markup Language (HTML), and the workstation capabilities of today's personal computers, numerous opportunities exist to communicate up-to-date scientific findings to a broad audience. The relatively low cost of Web-based publication of information and the existence of relatively simple software tools that are largely platform-independent permit the scientist to abstract his results in a form useful for audiences at a variety of levels. Here we describe an example of such a hypermedia publication, the Venus Hypermap. Targeted at a high school-to-undergraduate level, the Venus Hypermap is a presentation of some of the highlights of the Magellan mission to Venus. Currently under construction, it will contain text and graphics (e.g., images of Magellan radar and topography data) that illustrate features of Venus' highlands, coronae, chasmata, and large, rise-associated volcanoes. The Venus hypermap may be accessed via a WWW browser such as Mosaic or Netscape at: http://artemis.ess.ucla.edu/~bmoore/Vmap/top.html Tuesday, March 14, 1995 CLEMENTINE EXPLORES THE MOON 6:30 p.m. LPI Eliason E. M. Malaret E. R. Woodward G. Clementine Mission: The Archive of Image Data Products and Data Processing Capabilities The Naval Research Laboratory (NRL) and NASA's Planetary Data System (PDS) have jointly prepared an archive of digital image data acquired by the Clementine Mission. The archive, to be available on CD-ROM media and through network services, will support the needs of researchers in NASA's upcoming lunar and asteroid science analysis program. The Clementine image archive contains 1) the raw planetary images, 2) supporting ancillary data, and 3) software for image decompression and simple image display. Future companion archive volumes, to be prepared by the PDS and NRL, will contain software and ancillary data that fully support the data collection. The archive elements will include SPICE kernels, radiometric calibration data and software, experiment manifest, sensor console logs, and mission history. The U.S. Geological Survey and Applied Coherent Technology have developed advanced image processing systems for the systematic processing of the Clementine image collection. Versions of these software systems will be made available to the science community for research applications. The Clementine image archive and the supporting software systems that process the data will provide valuable resources for the lunar science investigator. Acton C. H. The Clementine SPICE Archive Cameras on the Clementine spacecraft returned nearly two million images, offering planetary scientists an abundance of research material. With this quantity of high resolution, multispectral images, having ready access to precision knowledge of the observing geometry for each is important to complete and economical data interpretation. This knowledge will be of further value when future lunar missions use the Clementine image archive as a database for correlative analyses. NASA selected the SPICE ancillary information system as the mechanism for assembling, archiving and providing researchers ready access to the spacecraft trajectory, lunar position and orientation, and camera pointing data needed to calculate pertinent observation parameters such as latitude, longitude, altitude and lighting angles. This poster provides prospective users of the Clementine images a "quick-look" summary of the SPICE ancillary data and software available for their use. McEwen A. Becker T. Bennett A. Bowell J. Edwards K. Eliason E. Gaddis L. Isbell C. Howington-Kraus A. Kirk R. Lee E. Morgan H. Robinson M. New Digital Images of the Moon A selection of recently-processed digital images of the Moon will be shown in this oversize poster display. The majority of the data is from Clementine, but also includes some products consisting of Clementine data merged with or compared with Galileo, Apollo, Mariner 10, and Lunar Orbiter images. Spudis P. D. Stockstill K. R. Ockels W. J. Kruijff M. Physical Environment of the Lunar South Pole from Clementine Data: Implications for Future Exploration of the Moon The polar regions of the Moon have attracted attention as possible targets of exploration for a variety of reasons, including the operationally attractive possibility of permanently shadowed areas containing water ice and permanently sunlit areas and the scientifical of a polar site for lunar based astronomy, and the general geological exploration of the Moon. Recently, international interest in lunar exploration has grown, as shown at the first International Lunar Workshop in Switzerland last year. We have studied images of the south pole of the Moon obtained from the Clementine spacecraft during a single month's orbit of the Moon (late March to late April, 1994) in order to better understand the physical environment of the region. We have identified a small area near the pole (close to areas of permanent shadow) that appears to be exposed to almost constant solar illumination. Kruijff M. Ockels W. J. Lunar Visibilities and Lighting Conditions Communication possibilities, availability of solar energy and the thermal environment are important parameters for site selection, when considering a landing on the lunar surface. They depend on local visibility of the Sun and the Earth. These "environmental parameters" heavily impact mission timing, duration and return, as well as design of subsystems related to thermal loads, telemetry and data handling. Lucey P. G. Taylor G. J. Derivation of Fe Abundance with 1-2 wt% Precision from Clementine Multispectral Imaging We have developed a method for derivation of the abundance of the element Fe at high precision (1-2 wt%) from visible wavelength multispectral imaging and applied this method to data obtained recently by the Clementine mission to the Moon. This mission provides 100% coverage of the lunar surface at 100-300 meter resolution. Of this coverage we have processed Fe abundance data for 93% of the lunar surface between latitudes of 70 degrees S to 70 degrees N at 35 km resolution. Tompkins S. Pieters C. M. Central Peaks of Lunar Impact Craters: New Views from Clementine While the Moon's crust has been shown to be compositionally heterogeneous on a 10's of km scale, a better understanding of the extent and character of this heterogeneity would be valuable to test and modify petrologic models of lunar crustal evolution. The Clementine mission, which has provided digital multispectral images for the entire Moon, will contribute substantially by allowing spectrally distinct units within lunar impact craters to be mapped spatially, thus providing geologic context. Impact craters on the Moon have proved useful as tools for examining the near-surface stratigraphy of the lunar crust. The central peaks of complex craters are believed to have uplifted material from depths of approximately 0.10 - 0.15 times the crater diameter. In part based on the clarity of spectral features, these central peaks are thought to remain lithologically coherent, despite the forces acting upon them during the impact process. Previous studies of craters using telescopic near-infrared (NIR) spectra provided high spectral resolution information for a single location selected by the observer. The recently acquired Clementine UVVIS camera data adds spatial information to this view of central peaks. Initial examination of the data has shown that the central peaks are compositionally far more heterogeneous than previously recognized. McEwen A. S. Robinson M. S. Eliason E. M. Lucey P. G. Hawke B. R. Duxbury T. C. Clementine and Groundbased Observations of the Aristarchus Region of the Moon Multispectral and topographic information acquired by the Clementine spacecraft, supported by groundbased spectroscopy, provide new information on the composition and geologic history of the Aristarchus region of the Moon. Altimetry profiles show the Anstarchus Plateau dipping about one degree to the north-northwest, and rising about 2km above the surrounding mare plains. A multispectral mosaic reveals the distribution of major compositional units. Dark reddish pyroclastic glass covers most of the plateau to depths of 10-30 m as determined from 100-200 m diameter craters that have exposed material below the pyroclastics. Combination of the UVVIS and NIR camera data, calibrated and cross-checked with groundbased spectra of the region, show that the eastern walls of Aristarchus crater expose olivine-rich material. The central peaks of Aristarchus are composed of anorthosite. Compositional parametric images constructed from the data set show complex mineralogic variations within Aristarchus crater and within Aristarchus A, a 15 km crater to the north of Aristarchus. Aristarchus crater itself sits astride a large scarp which is shown to expose anorthositic material similar to portions of the interior of Aristarchus crater. Gillis J. J. Spudis P. D. Clementine Color Mosaics of Procellarum Volcanic Complexes: Evidence for Dome Morphology Linked to Volatile Content and Eruption Rate Variations in lunar dome morphology have been attributed to either variable eruption rates and different volatile content or to the fractionation of magma towards more evolved composition. Clementine color composite images reveal that morphological dome constructs are not distinctive in color and by inference, composition. Therefore, we suggest that changes in dome morphology are attributable to physical processes of eruption. Shkuratov Yu. G. Stankevich D. G. Can Lunar Opposition Spike Measured by Clementine Exist? The brightness opposition spike measured by Clementine is probably an artifact. The spike has too small width, 0.25 deg. It could not be observed because the light source (the Sun) has the angular diameter of 0.5 deg. Pinet P. C. Shevchenko V. V. Chevrel S. Bellagh F. Rosemberg C. Reiner Gamma Formation: Optical, Spectroscopic and Polarimetric Properties from Clementine and Earthbased Data The Reiner Gamma Formation is an unusual high-albedo morphological lunar feature, with no topographic expression, approximately 30x60 km in extent, located in western Oceanus Procellarum. It correlates with the strongest magnetic anomaly detected with Apollo magnetometers and its origin is still unresolved. A detailed remote sensing survey of the region by means of earth-based telescopic and Clementine CCD spectro-imaging and polarimetric imaging techniques has been made in the UV-visible-near infrared domain and the distribution of the main types of materials is proposed on the basis of their spectral/polarimetric characteristics. _ Oberst J. Cook A. Roatsch T. Scholten F. Storl J. Wewel F. Albertz J. Hoffmann H. Jaumann R. Clementine Stereo Data Analysis: Digital Terrain Models and Rectified Color Images Three dimensional morphology may be used to understand the role of lunar surface dynamic processes. Color and photometric imagery can reveal the chemical, mineralogical, and physical properties of the the lunar soil. Insights into geological stratigraphy and material depositional history may be gained by studying the variation of color and/or photometric properties of the surface with topography. Here we describe two approaches of producing digital terrain models (DTMs) using automatically matched stereo Clementine imagery: (1) an initial, but fast, method that relies upon using approximate camera positions and orientation data, for producing relative height DTMs. (2) a more accurate technique of photogrammetric bundle-block adjustment using ground points for producing absolute height DTMs. It is important to incorporate a DTM into the image rectification process when producing ortho images, else topography may cause localized spatial distortions. Although this effect on Clementine nadir colour imagery is small, it should be taken into account for photometric imaging sequences taken at different viewing angles where surface normals and shading effects are of interest. Color ortho image mosaics and DTMs produced from Clementine imagery are presented and analyzed. Lemoine F. G. Smith D. E. Zuber M. T. The Gravitational Field of the Moon: Goddard Lunar Gravity Model-1 Information on planetary gravity fields provides information on the internal structure of the planet, and is essential for accurate navigation by orbiting and landing spacecraft. We have recently re-evaluated the gravity field of the Moon using data obtained from the Clementine mission along with the historic data acquired from the Apollo-15 and Apollo-16 subsatellites, and Lunar Orbiters 1 through 5. In this paper we describe the data used in the model and the techniques employed in the development of the field, and examine the resolution and characteristics of the field. We also characterize spatially the errors associated with the new gravity field solution by evaluating the field in a statistical sense and by examining correlations with visible features over different sections of the planet. Williams K. K. Neumann G. A. Zuber M. T. Lunar Mascon Basins: Analysis of Effective Elastic Lithosphere Thickness Using Gravity Anomaly Models Analysis of data collected by the Clementine spacecraft now provides more accurate global topographic and gravitational fields than were previously available. These data provide a detailed view of the internal structure of the moon, showing marked redistribution of the crust under lunar mascon basins. Thinning of the crust is an important factor for explaining the large mascon gravity anomalies associated with these basins. The gravity anomaly results from upwarp of a higher-density mantle, infilling by dense mare basalts, and crustal topography. The new Clementine lidar data is used to remove the effects of surface topography. The resulting Bouguer gravity anomaly over a basin is modeled assuming flexural isostasy. Such models constrain the effective elastic thickness of the lithosphere at the time of loading. The results of this study will provide information about the thermal history and mechanical structure of these lunar basins. Staid M. I. Pieters C. M. Head J. W. A Multispectral View of Stratigraphy in Mare Tranquillitatis During its two months of lunar mapping in the spring of 1994, the Clementine spacecraft's ultraviolet-visible (UV/VIS) camera acquired high-spatial resolution digital images of the lunar surface in five spectral channels. Clementine data of the Mare Tranquillitatis region have been studied within the context of lower resolution Galileo multispectral data obtained during its second lunar flyby in 1992. Together these data sets provide insight into the stratigraphy of mare units, lateral and vertical mixing processes within the mare, and the underlying structure of the basin. Tuesday, March 14, 1995 OUTER PLANETS/SATELLITES 6:30 p.m. LPI McCord T. B. Coombs C. R. Granahan J. Sunshine J. Interpretation of Spectral Compositional Units on the Icy Galilean Satellites Surface units are an important indicator of a planet's past geologic and geomorphic processes. Icy Galilean satellite surface units, as defined by multispectral maps from Voyager, can provide an indication of their past processes and their original state. Ground-based spectrometry has supplied some material identif1cation on a global scale. Based on these data, distinct spectral units have been found on all the icy satellites. Europa and Ganymede have been studied relatively more than Callisto, which remains poorly understood due to poor Voyager data quality. In this study, we assess the Voyager data for Ganymede and Callisto to look for correlations between spectral units and geology. Our analyses of the Voyager multispectral data and groundbased spectro-photometric data is helping to constrain composition and mineralogy of units, leading to a better basic scientific understanding of their spatial distribution and origin. These new results are immensely helpful in planning the upcoming Galileo Jupiter orbit measurements as well as laying a foundation for interpreting the NIMS and SSI data. Goguen J. D. Investigation of Callisto's Unusual Photometric Properties using a 'Phase Cube' of Voyager Images A new tool combining the spatial and phase angle coverage of the Voyager images was created for investigation of the origin of Callisto's unusual photometric properties. Using PICS software, 50 Voyager clear filter images of Callisto at phase angles < 50 degrees were radiometrically calibrated, projected into cylindircal maps with 1 degree resolution, co-registered, and stacked to create a "phase cube": an "image cube" with 2 spatial dimensions and 1 solar phase angle dimension. Helfenstein P. Veverka J. Hillier J. Callisto's Regolith: A Better Estimate of the Ice/Silicate Ratio A valuable outgrowth from modern planetary photometry is its potential use for estimating mineral abundances in multi-component regoliths. The most frequently studied multi-particle mixing equation represents the single scattering albedo (w) of a particulate mixture as a weighted average of end-members whose single-scattering albedos are presumably known. Estimates w are generally obtained by application of Hapke's photometric model to reflectance measurements of planetary surfaces and/or laboratory analogs. Experimental efforts to verify the mixing model demonstrate that it successfully predicts mixing ratios in simple two component systems when dark (opaque) particles are mixed with other dark (opaque) particles and when bright (transparent) particles are mixed with other bright (transparent) particles. The mixing equation appears to work less well for predicting ratios of dark opaque particles mixed with bright transparent particles--a particle mixing scheme that is especially important for the surfaces of outer planet satellites for which dark (silicate) grains may be mixed with bright (icy) particles. The present study shows that the inadequacy of the mixing model to accurately represent mixtures of opaque with transparent particles is likely due to unreliable estimates of single scattering albedo obtained from Hapke's model when unrealistic particle phase functions are adopted. We followed the approach of earlier workers and modified Hapke's equation to include double Henyey-Greenstein particle phase function that is known to accurately model the scattering behaviors of large irregular particles. We applied this model to detailed photometric observations of the Jovian moon Callisto -- a good planetary example for which regolith may be a mixture of dark (silicate) grains and bright (icy) particles -- and compared our results to earlier estimates that employed a one-term Henyey-Greenstein function. Allowing for the presence of a significant forward-scattering lobe the phase function yields a w that is 73% larger than earlier estimates based on a scattering function that modeled only the backscattering lobe. If we assume that Callisto's surface is a mixture of transparent frost grains (w=1.0, density=0.9g/cc) and dark (silicate) grains (w=0.07, density=2.5 g/cc), then our value of w for Callisto predicts silicate:ice ratio of 26:74 by volume compared to 60:40 from earlier estimates that used an overly simplistic particle phase function. Head J. Murchie S. Collins G. A Pre-Galileo Review of Major Geologic Questions About Ganymede Fifteen years of analysis of Voyager images has led to a broad understanding of several aspects of the geology of Ganymede. These include the gross morphology and distribution of tectonic features in dark and light terrains, the emplacement of light terrain by high albedo cryovolcanism, and variations in the forrn and albedo of impact features as a function of size. However, there is substantial disagreement about the origin and evolution of dark terrain, the detailed evolution of light terrain, and crustal stratigraphy. These issues are all essential to understanding how the evolution of the surface has been influenced by internal thermal evolution and exogenic processes. Here we review the observations and interpretations from Voyager images pertinent to these issues and identify observations from Galileo that will help to resolve them. Schenk P. Iaquinta-Ridolfi F. Geology of Large Impacts Basin on Ganymede and Callisto Impact craters on the icy Galilean satellites are characterized by an astonishing variety of unique varieties, which are clearly related to impact in ice. New stereo images of Ganymede and Callisto have recently been constructed that considerably improve geologic mapping and interpretation. Recent studies of craters on Ganymede have suggested that internal structure or composition may be important in controlling crater morphology. Basin scale structures should probe these satellites to significant depths. In light of these recent advances, as well as the approaching Galileo tour, we have completed the first systematic geologic study of large impact structures on these bodies since 1982. Duxbury N. S. Brown R. H. The Plausibility of Boiling Geysers on Triton Since the discovery of Tritonian cryovolcanic activity in Voyager 2 images in 1989, it was commonly assumed that the geysers on Triton are of condensing type only. In this case the initial state is N(sub)2 vapor, produced by the subliming southern polar cap. Vapor condenses while it cools and accelerates, releasing latent heat which helps acceleration. The energy source for the geysers can be either solar or internal, due to radioactive decay in Triton's extensive core.The corresponding mechanisms of energy supply are the solid-state greenhouses or convection in solid nitrogen. Internal heat source on Triton was estimated to be quite large compared to the absorbed insolation (5-20%). Since the heat conduction coefficient of solid nitrogen is 50-100 times less than that of water ice at the temperature of the mantle-cap interface, the temperature gradient in the cap is significantly increased relative to its value of 0.3 K/km in the mantle. This gives the temperature difference of 15-30 K across the nitrogen layer (not taking phase transitions into consideration) and thus nitrogen can reach its melting temperature of 63.148 K (at zero pressure) at the base of the cap. In our models we assumed a layer of liquid nitrogen only a few centimeters thick at the base. We applied analytical techniques and used the results of experiments with convecting Newtonian fluids having different Prandtl numbers and rigid-rigid boundary conditions with heating only from below. Kargel J. S. Hogenboom D. L. The Ammonia-Water System: An Attempt to Reconcile the LaFayette and UCLA Investigations Lab investigations by Hogenboom and colleagues (the Lafayette group) have produced a large body of data on phase stabilities and densities in the planetologically important system NH3-H2O with the use of a novel piezometer capable of extremely precise volume and temperature measurements at pressures up to 400 MPa. The only other large body of high-pressure data for this system is that of the UCLA group, most comprehensively the diamond anvil studies of Boone and Nicol, who have examined phase stabilities between 300 MPa and 5 GPa. Unfortunately, it has seemed that these datasets are in disagreement. Here we show that the melting curve of ammonia dihydrate produced by Boone and Nicol is thermodynamically inconsistent with other known parameters of the system and is in sharp disagreement with the melting curve that we have generated (and which is thermodynamically consistent). The melting curve of ammonia monohydrate given by Boone and Nicol also disagrees with the melting curve we have generated. Further confirming that there is a problem with either the UCLA group's data or their interpretation, we have found convincing proof of the existence of a high-pressure polymorph of ammonia monohydrate, something that the UCLA group did not find even at pressures up to 10 times what we have investigated. However, it is possible to reconcile the Lafayette and UCLA groups' data if the UCLA group encountered but did not recognize high-pressure polymorphs of ammonia-water compounds. We propose phase diagrams of ammonia dihydrate and monohydrate that are based on a syn-thesis of the Lafayette and UCLA groups' data. We suggest that the ammonia-water compounds have a series of high-pressure polymorphs comparable to the series exhibited by pure ice. _ Goldsby D. L. Kohlstedt D. L. The Transition from Dislocation to Diffusion Creep in Ice The rheology of Ice I has been investigated extensively at both 1 atm and high confining pressures (e.g., Duval et al., 1983; Durham et al., 1992). The rheology of ice can be described by a power-law creep equation of the form epsilon dot=A(o^n/d^m)exp(-Q/RT), where epsilon dot is strain rate, A a material parameter, sigma stress, d grain size, and Q the activation energy; RT has the usual meaning. It is well-established that ice deforms by dislocation creep at sufficiently high stresses and large grain sizes (Duval et al., 1983), with n = 3-4 and an activation energy Q = 78 kJ/mol. Evidence for diffusion creep (n = 1) in ice, however, is ambiguous. A handful of studies have reported creep results consistent with Newtonian flow (eg., Bromer and Kingery, 1968; Colbeck and Evans, 1973), but these experiments were conducted at very low stresses and strain rates, so that transient creep may have dominated the creep response. Yet, diffusion creep should be the dominant deformation mechanism over a wide range of temperatures, tectonic stresses, and grain sizes occurring in icy planetary interiors. We have therefore conducted a series of creep experiments at one atmosphere total pressure on very fine-grained water-ice to determine constitutive laws for both diffusion creep and dislocation creep. Lorenz R. Cassini Mission: Radar Sensing of Craters on Titan Crater density and morphology offers useful insight into the bombardment, atmospheric and resurfacing history of a body. This information for Saturn's moon Titan has been hidden by the thick haze-laden atmosphere, but will be yielded by the forthcoming NASA/ESA Cassini mission. Here I review what we may expect to learn from orbiter radar and optical observations, noting their limitations due to incomplete a real coverage and atmospheric opacity, respectively, and from high-resolution (but localised) measurements from the Huygens probe. Radar mapping from orbit offers, most probably, the best prospects for crater science on Titan. Tuesday, March 14, 1995 ORIGINS OF PLANETARY SYSTEMS 6:30 p.m. LPI Jin L. Clayton D. D. Anomalous-Cosmic Ray Origin of 26Al in Molecular Clouds We propose that the special cosmic rays observed in Orion by their nuclear deexcitation lines (4.43 and 6.13 MeV) are accelerated by the mechanism known in the solar system as "anomalous cosmic rays". These are known to be primarily He and 16O, both singly charged because of their origin as pickup ions following ionization in fast plasma flow (the solar wind). The single charge facilitates their penetration of the increasing magnetic field of cloud cores. These ions are known to dominate the galactic cosmic rays below 20 MeV/nucleon at Earth, and we argue that they dominate at the same energies in star-forming regions. In the clouds they make 4.43 and 6.13 MeV gammas by 16O(p,p'a)12C* and by 16O(p,p')16O* respectively. Associated 26Al is produced by 16O(12C,pn)26Al and by He(23Na,n)26Al and He(24Mg,pn)26Al at low energy. The infrared power required need be only 10^5 Lsun, big but acceptable. Spergel M. S. Source of Nuclei Production in the Protosolar Nebula In order to investigate induced nuclei production in the protosolar nebula, during it active period of condensation and accretion, two sources of primary particles fluxes within the protosolar nebula should be considered, an external source and an internal source. The external source would be from a source such as a nearby supernova or an O and B association of stars, j(sub)SN. The internal source would be from the emerging protosun, j(sub)ES. Calculations indicate that for all reasonable values of density and composition of the protosolar nebula, the internal source will dominate in nuclei production. Bringol L. A. Hyde T. W. Grain Charging in a Dusty Plasma Dusty plasmas are found in planetary rings, cometary tails and the protoplanetary nebula. Dust grains immersed in a plasma become charged. This charge affects the motion of the grain in the electromagnetic field of a planetary magnetosphere and is involved in both the formation of the spokes in Saturn's rings and the erosion of those rings by micrometeorites. Additionally, the charge on the grain affects the coagulation rate of the dust into planetesimals in the protoplanetary nebula. Hyde T. W. Nazzario R. Dust Grain Resonances in Protoplanetary Systems It is generally accepted that planets form by accretion from an initial population of small, solid bodies, or planetesimals in orbit about a star. When the orbits of these planetesimals intersect, collisions and binding by self-gravity produce growth of the planetesimals into larger bodies which eventually reach planetary size. Most current theories take as an initial assumption, the existence of a "cache" of planetesimals with initial sizes between 1 and 10 km as a starting point for planetary accretion calculations. The primary reason for this unwillingness to discuss the origin of the planetesimals is the complexity of the problem in describing the accretion process of dust to planetesimal in any analytical manner. At different times in the growth pattern, the constituent parts (mostly dust) are dominated by forces other than gravity, with many of these being extremely difficult to model. As a result, the coagulation processes involved are not well understood at the present time. KenKnight C. E. Realism in Nebular Simulations A realistic model of the solar nebula must handle transfers of energy, momentum, and solids. Most models make severe simplifica-tions and approximations while trying to treat this nonlinear and nonlocal problem. It is nonlocal because the ability to shed heat at a point depends on how solids are distributed from the point to infinity. It is nonlinear, for example, in temperature fluctuations because components may change phase and markedly change the optical or physical properties of the solids. The growth of agglomerates of solids is nonlinear, proportional to the square of solid concentrations. Realistic models must follow the time-dependence of the condition of the solids in response to the most important movement of gases, their orbit around the grow-ing Sun. We may expect to find heating events driven by supersonic vertical turbulence. Models must include growth of gravity-stabilized rings of sediments in the central plane. Thus we may find a cause for history-dependent variations of water, C, S, and Fe concentrations and thus the great variety of meteorite types.Such rings were places to store CAIs until chondrules began to form. Weidenschilling S. J. Davis D. R. Multi-Zone Modeling of Planetary Accretion: Updated Results We have made improvements and corrections to our multizone accretion model. This model combines continuum size and eccentricity distributions for small bodies in zones of semimajor axis with discrete large bodies having specific masses and orbital parameters. We match the initial conditions of Wetherill and Stewart; our results are in good agreement with theirs for early stages of planetary growth, including "runaway" of a small number of embryos. Later evolution with our model yields higher eccentricities of the large bodies and less tendency for dynamical isolation. We attribute this result to spatial variation of gravitational stirring rates, localized depletion of smaller bodies near orbits of embryos, and stochastic effects that soften the cutoff of interactions when bodies do not fill the range of orbital spacings. Gor'kavyi N. Numerical Study of Formation of Satellite Systems: Prograde and Retrograde Satellites of Jupiter, Saturn and Neptune About 1.1 million of prograde and retrograde trajectories of particles of a protosatellite disk are investigated numerically within the scope of the plane circular three-body problem. It is shown that the solar perturbations inhibit accretion in a prograde disk at the radii greater than 0.4Rh (Rh is the radius of the Hill sphere). The change in surface density of prograde disks of Jupiter, Saturn and Neptune with accumulation of planetesimals from heliocentric orbits is investigated numerically. It is shown that regions of particles with retrograde motion develop in an initially prograde disk. The radii of these regions agree well with the retrograde satellites - Triton, Phoebe and the Pasiphae group. The accumulation zones for prograde particles also well with the main groups of prograde satellites of Jupiter, Saturn and Neptune, including the Himalia group. These results refute the capture hypothesis for the outer satellites of giant planets. Tuesday, March 14, 1995 MARS EXPLORATION 6:30 p.m. LPI Shelfer T. D. Morris R. V. Nguyen T. Q. Agresti D. G. Wills E. L. Backscatter Mossbauer Spectrometer (BaMS) for Solid-Surface Extraterrestrial Mineralogical Analysis We have developed a four-detector research-grade backscatter Mossbauer spectrometer (BaMS) instrument with silmultaneous low-resolution x-ray fluorescence analysis capability. A flight-qualified instrument based on this design would be suitable for use on missions to the surfaces of solid solar-system objects (Moon, Mars, asteroids, etc.). Target specifications for the flight instrument are mass <500 g, volume <300 cm^3, and power <2 W. The BaMS instrument would provide data on the oxidation state of iron and its distribution among iron-bearing mineralogies and elemental composition information. There data are of fundamental importance for the characterization and identification of extraterrestrial surface materials. Wdowiak T. J. Agresti D. G. Mirov S. B. A Laser Raman Spectrometer System Suitable for Incorporation into Lander Spacecraft Raman spectroscopy has significant laboratory utility in the analysis of organic/hydrocarbon and inorganic/mineral substances indicating desirability as an instrument in missions to the surfaces of Earth's moon, Mars, comets and asteroids. Recent developments make possible the integration of current technologies in lasers, optical components such as filters, array detectors, power components, and computers into a laser Raman spectrometer that will meet both the science yield and spacecraft compatibility requirements. As an example we envision the use of fiber optics to sense samples at distances as great as 1 km from the spacecraft in which the core of the instrument resides. Banerdt W. B. Lognonne P. Pike T. Kaiser W. Karczewski J. F. Cacho S. Cavoit C. Striebig N. An Ultra-Broad-Band Seismic System for Exploring the Interior of Mars Determining the structure of the interior of Mars has been a high priority for every Mars exploration strategy developed in the past 20 years. As seismic information is spread over a large frequency band, from tidal frequencies (10(sub)-2 mHz) up to 100 Hz, Ultra_Broad_Band seismometers (UBB), which cover DC-100 Hz, are then the best candidates for future seismic networks on Mars and other terrestrial bodies. Such a high_sensitivity seismometer is presently in development. In order to achieve high sensitivity at both high and low frequencies, it combines a Long Period (LP) 3_axis sensor, developed by IPGP/INSU, and a Short Period (SP) 3_axis sensor, developed by JPL, into a single integrated package with a shared command and data system and environmental control. Kerridge J. F. Carr M. H. Clark B. C. DesMarais D. J. DeVincenzi D. L. Farmer J. D. Hayes J. M. Holland H. Jakosky B. M. Joyce G. F. Klein H. P. Knoll A. H. McDonald G. D. McKay C. P. Meyer M. A. Nealson K. H. Shock E. L. Ward D. M. The Exobiological Exploration of Mars Whether or not life emerged on Mars remains an issue of profound scientific and philosophical importance, bearing on the origin of life on Earth and the distribution of life in the universe. Addressing that issue will require a sequence of increasingly finely focused experiments. Currently planned missions should satisfy most early requirements of that sequence, and adoption of recommended modifications would not impair achievement of traditional planetary-science goals. Cabrol N. A. Brack A. Episodic Oasis for Water-Dependant Life on Mars The experiments carried out by the Viking landers have ruled out the possibility of present day life at the surface of Mars. Since models of martian biology have been proposed to evaluate the possibility of life development in the past or surviving either dormantly or active in specific niches presenting favorable conditions. Generally these models assume that martian life has the same biological features as terrestrial life, but with a high capability to adapt to extreme environmental fluctuations over short geological periods, in order to survive during the hospitable periods of mild climate, the ice ages, and even under current conditions. Allton J. H. A Case for Sterile Sample Return from Mars: Alternative to Quarantine The quarantine of returned planetary samples to prevent back contamination is costly, incompatible with good sample curation practice, and would delay sample return due to political considerations. Present day public debate almost assures a lengthy and expensive quarantine. However, sterilization of the returned sample would eliminate the need for quarantine. Important geoscience information can still be retained in the samples by use of dry heat, radiation or thermoradiation. The choice may actually be between returning a sterilized sample or returning no sample at all. Araghi K. Oxygen Production from Carbon Dioxide by Zirconia Electrolysis Research has been conducted on issues related to the production of oxygen from carbon dioxide and air, using zirconia cell and ceria tube. Both in-house fabricated disk cells and vendor-supplied tube cell are being tested and characterized. These processes are under consideration for a Mars sample return mission which would manufacture their own oxygen from CO2 in the martian atmosphere. Most of the present effort has been concentrated on the development of a reliable test bed that can provide repeatable and accurate measurements of cell and tube performance. Initial experiments, including endurance tests as long as thousands of hours, were conducted with two identical cells and one tube. Allen C. C. Treiman A. H. Who Needs a Few More Mars Samples When We Already Have the SNCs? A spacecraft mission to return samples from Mars has been a long-standing goal of planetary exploration. Recently, the need for sample return has been questioned because some meteorites, the SNCs, are almost certainly from Mars. However, many important uncertainties about Mars require detailed analysis of documented samples, and cannot be resolved by continued investigation of the SNCs and/or robotic spacecraft missions. These uncertainties include: whether the SNC meteorites really are from Mars (requiring trace element and oxygen isotope analyses); absolute age calibration of the martian crater chronology (requiring isotopic analyses of rocks from known locations); and the composition and origin of the highlands and other unsampled units (requiring trace element analyses). Tuesday, March 14, 1995 MERCURY 6:30 p.m. LPI Robinson M. S. Edwards K. A New Digital Image Mosaic of Mercury We have started an investigation of the color and albedo of the planet Mercury utilizing Mariner 10 digital image data in order to more fully understand the regolith, and thus the crust, of this enigmatic planet. The majority of information concerning the geography of Mercury comes from the Mariner 10 image dataset. The three encounters between Mariner 10 and Mercury in 1974-75 incorporated different observation strategies but nearly identical illumination, so that only one hemisphere of the planet was imaged. The hand-laid mosaics from the first flyby are by far the best known, though images from the first and second flybys were later digitally mosaicked into 1:5,000,000 quadrangles. Individual mosaics in this series exhibit average discontinuities of 20 km due to camera-pointing errors, and discontinuities between quadrangles can be worse. To correct these errors we are controlling the best Mariner 10 coverage for all of Mercury observed during the first two encounters. Tuesday, March 14, 1995 REMOTE SENSING 6:30 p.m. LPI Grant J. A. Schultz P. H. Collins W. K. Effectiveness of Ground Penetrating Radar in Argentine Loess: Implications for Future Mars Surface Radar Sounders Ground penetrating radar (GPR) can accurately delineate shallow stratigraphy around a variety of landforms, thereby limiting the amount of accompanying groundtruth required for interpretation. As a proven instrument in the terrestrial environment, GPR is also being developed for inclusion aboard future Russian missions to Mars and is being suggested as a component on future U.S. landers/rovers on Mars. In anticipation of these missions, GPR effectiveness in a variety of potential Mars analog environments is being explored. Because much of the Martian surface is mantled by fine-grained unconformable deposits an understanding of GPR performance in such substrates is crucial. Here, we describe the results of GPR deployment in loess deposits of the Argentine pampas and conclude that the instrument could help place important new constraints on the stratigraphy and origin of possibly analogous fine-grained deposits on Mars. Oehler A. Dummel A. Hanowski N. Rebhan H. Neukum G. Spectral Photometric Parameters (UV, VIS, NIR) for 13 Materials Similar to Lunar and Asteroidal Regoliths Presented here are results of a large set of goniospectrophotometric measurements of regolith analog materials taken with the DLR-Goniospectrophotometer. For each of 13 different materials we took phase curves in 70 spectral channels [Oehler A. and Neukum G., Oehler A., Hanowski N.]. The photometric model developed by Hapke [Hapke B. W., 1981, and Hapke B. W., 1986] was fitted to each single phase curve (a second order Legendre polynomial was used for the phase function). Then the spectral Hapke parameters of all samples were analysed, leading to the following results: 1. The opposition effect amplitude decreases approximately linearly with increasing single scattering albedo. 2. The opposition effect amplitudes observed at low albedos are too high to be consistent with the theory of shadow hiding and (for one laboratory sample and for the lunar data) also too high to be consistent with a theory taking into account shadow hiding and coherent backscattering effects. 3. The microstructure of the samples and not the single scattering albedo is the dominant factor influencing the distinct spectral forward/backward scattering properties of the individual samples. 4. The increasing (g<90 ) and decreasing (g>90 ) redness of lunar soils with increasing phase angle can be simulated with the laboratory data set. 5. The Hapke-parameters measured for the lunar surface lie within the parameter space covered by the laboratory samples.Sample material. The samples analysed here are listed in Tab. 1. We have selected 13 materials relevant for the simulation of lunar and asteroidal surfaces. The grain sizes and porosities were selected to be analogous to the grain size of the upper lunar surface. SEM images were taken of each sample in order to analyse the particle microstructure. Structures range from individual particles with smooth surfaces to fluffy agglomerates, where it becomes difficult to determine what kind of structure makes up a single particle. Wenrich M. L. Christensen P. R. Deriving the Optical Constants of Quartz Using Thermal Emission Spectroscopy When considering using emissivity data for mineral identification of a surface that is composed of fine-grained material such as that of Mars, grain size affects must be understood, because when the wavelength of emitted energy from a mineral is similar to the diameter of the grain, the thermal emissivity spectrum is affected. Through the use of classical dispersion analysis, values for the real (n) and imaginary (k) indices of refraction of minerals may be ascertained. These optical constants are inherent to mineral composition and are thus unaffected by grain size. Values of n and k may be used in accordance with developing radiative transfer models to predict the shape of the emissivity spectra for any grain size. In addition to predicting the appearance of mineral spectra for various grain sizes using known values of n and k, it is ultimately expected that useful values of n and k for minerals that only occur in fine grain sizes, such as clays, may be derived from their complex spectra. The objective of this study is to evaluate the use of vibrational emission spectroscopy for determining n and k via dispersion theory. Hays J. E. Mustard J. F. Effects of Particles <= Lambda of Light on Reflectance Spectra Reflectance spectra of particulate materials generally exhibit regular and systematic changes in intensity and spectral contrast as a function of particle size. For multiple scattering regimes, reflectance increases and contrast decreases with particle size, while reflectance decreases with contrast for single scattering regimes (e.g. restrahlen bands). However, with hyper-fine particles (<25 micrometers) a number of distinct intensity and contrast reversals have been observed in mid-infrared biconical reflectance spectra, principally in the multiple scattering regimes. The most striking change with particle size is seen in the transparency feature. Initially, as particle size decreases, the transparency feature becomes stronger, reaching a maximum for particles between 10-15 micrometers, and then decreases as particle size continues to decrease. This change in brightness is accompanied by a decrease in the lambda of the peak reflectance, and an increasing asymmetry about the peak of the feature. However, biconical reflectance spectra at mid-IR wavelengths may exhibit anomalous behavior due to non-lambertian scattering. To verify the character of these important observations, we have measured the olivine suite in directional-hemispherical reflection (DHR) and in thermal emission, and are preparing a sieved suite of quartz for measurement. The results of the measurements and continued modeling are reported here. Clark P. E. Trombka J. Floyd S. Solar Monitor Design for the NEAR X-Ray Spectrometer The X-ray spectrometer onboard the Near Earth Asteroid Rendezvous (NEAR) spacecraft will contribute to the determination of the composition and structure of Eros. The excitation source for the X-ray lines characteristic of major elements is the sun. The solar spectrum consists of a continuum which is generated by thermal bremstrahlung and radiative recombination, with superimposed major lines which result from emission due to transitions in H- and He-like ions. Harris A. W. Bowell E. L. G. Muinonen K. Evaluation of CCD Systems for NEO Surveys In August 1994 NASA appointed a study group to consider means of detecting as many as possible Near-Earth Objects larger than 1 km in diameter within 10 years. Since the publication of the Spaceguard Survey Report of the NASA International Near-Earth-Object Detection Workshop, there have been several advances in CCD technology and computer detection algorithms that allow for considerably improved performance of telescope systems over that assumed in that report. Thus for the new report, we have reevaluated several xisting or planned telescope systems to assess their ability to achieve the above stated goal. To do this, we have evaluated the performance of various systems, in terms of the limiting magnitude achievable vs. the rate of motion in the sky of a point source target (a moving asteroid). If the target object is moving only very slowly, so that the image does not trail in a given exposure time, the limiting magnitude is the same as for fixed objects (stars).For images that are short trails, image reconstruction can improve the detection threshold, so the limiting intensity of a detectable object falls off only as the square root of rate of motion. Beyond some limit (we assume 100 pixels of trail length), image reconstruction fails or becomes too computer-intensive, so the limiting intensity falls of in proportion to trail length. We plot the performance for the Spacewatch Camera, in itsoriginal configuration as well as the current somewhat improved system and the Spacewatch II 1.8 m system under construction [Rabinowitz, personal communication], a ~0.5 meter Schmidt system being constructed at Lowell Observatory (Lowell Observatory Near-Earth Object Survey = LONEOS), and a CCD system under development by Lincoln Laboratory for retro-fitting into a USAF system of 1 m short-focus telescopes currently in use for tracking Earth satellites (Ground-based Electro-Optical Deep Space Surveillance system = GEODSS). The performance characteristics are obtained by scaling from actual performance obtained by the original Spacewatch camera, to include higher quantum efficiency, faster readout, and lower signal-to-noise thresholds expected for the systems under development. McKay D. S. Wentworth S. J. McBride K. M. Patinas on Lunar Rocks: The Unusual Surface of Apollo 17 Basalt 75075 Space weathering of lunar rocks includes any process which changes their surface properties from those of fresh unaltered material. If the surface is notably different in appearance, the rock is said to have a patina. Patinas on lunar rocks (and also on rocks on asteroids and other small or planetary bodies) may be extremely important because they may modify or even dominate remote sensing data from such rocks. Furthermore, unless patinas are removed, they can also influence data from landers which contain or deploy spectral and chemical analyzers. Rock surfaces are modified by a number of processes. Both erosion and accretion occur because of micrometeorite (and larger) impacts as well as solar wind and flares. Effects of these processes include microcraters, splash glass, vapor-deposited material, implanted noble gases, radiation damage, adhering soil particles, and even loose dust. We are beginning systematic studies of rock and soil grain patinas, and we are classifying them and trying to understand their processes of formation and their effect on surface properties. Keller L. P. Christoffersen R. McKay D. S. The Oxidation State of Altered Rims on Ilmenite from Lunar Soils Ilmenite grains in lunar soils are commonly surrounded by complex rim sequences that result from their exposure to the lunar "weathering" environment. These rims consist of an outer, thin amorphous rind of vapor-deposited silicate material, and a relatively thick, inner layer that is depleted in Fe relative to stoichiometric ilmenite. TEM studies have shown that the inner rim is not amorphous, but consists of a microcrystalline assemblage of ilmenite containing platy precipitates of rutile and probable Fe metal grains. In this study, electron energy-loss spectroscopy (EELS) was used to demonstrate that the altered rims on soil ilmenites contain significant trivalent titanium. Our data indicate that the disordered rims are chemically "reduced" and that oxygen has been lost from the rims. These results have implications regarding the processes responsible for the formation of disordered rims on ilmenite as well as their potential effects on the optical properties of lunar soils. Fischer E. M. Pieters C. M. A Model for Lunar Soil Optical Alteration Due to Space Weathering On atmosphereless bodies such as the Moon, space weathering processes affect the optical properties of surface material. The effects of optical alteration include a reduction of reflectance, a reduction of absorption band strength, and the creation and steepening of a red-sloped continuum. These optical alteration effects, which increase with continuing exposure to the point at which the soil becomes mature, are superimposed on the diagnostic spectral features that characterize mineralogy. Thus, as a result of space weathering, the spectrum of a lunar soil is dependent upon both the degree of exposure at the lunar surface and the composition. This fact complicates the direct spectral comparison and compositional interpretation of soils of different exposure degree -- for example, soils from fresh craters and those from mature surrounding materials, and soils from craters of different ages. On the Moon, soils are characterized by a wide range of exposure age from local to global scales. In order to directly compare spectroscopic data of materials of diverse exposure age, the optical contribution due to exposure must either be normalized or removed. To achieve this, a mathematical model describing the optical alteration of lunar soils was developed. This model allows calculation of the spectrum of a soil of any exposure age from an observed spectrum of a soil of any other exposure age. Fischer et al. (3) presented an initial form of this model based upon spectroscopic study of several Apollo 16 soils. In the present work, we report a refined and improved model, and the application of this model to four evolutionary suites of lunar soils. Lucey P. G. Modeling the Spectral Effects of Microscopic Iron Metal on Glass and Minerals The optical effect of soil maturation is a vexing problem in spectral remote sensing of the Moon. This effect--primarily darkening, reddening and loss of spectral contrast--has been studied by a number of workers partly in an effort allow separation of the spectral effects of composition and maturity. Currently, there is a near consensus that much of the spectral changes observed with maturation is due to the presence of extremely fine grained Fe metal in the lunar soil. This metal is produced by reduction of divalent Fe to Fe metal by micrometeonte impact heating of soil components rich in solar wind hydrogen. The process has been studied empirically in the laboratory by Allen et al in a series of experiments which dramatically illustrate the influence of fine-grained metal. In these experiments powdered minerals and one glass were reduced by heating in a flow of hydrogen gas. This process produced very fine blebs of Fe metal which partially coated the surfaces of mineral grains. The samples before and after reduction were characterized spectrally and the reduced samples were darker, redder, and exhibited lower spectral contrast than the unreduced samples, characteristics consistent with the optical effects of maturation. The spectra of these samples and their careful documentation were used for the testing of models attempting to account for the spectral effects of maturation assuming that the process carried in the laboratory is representative of that occurring on the lunar surface (data graciously provided by R. Morris). As the optical effect of this fine-grained metal, much of which is sub-wavelength in size, cannot be modeled with geometric optics, other approaches must be sought. An approach using electromagnetic wave theory is pursued below. Clark R. N. Swayze G. A. Automated Spectral Analysis: Mapping Minerals, Amorphous Materials, Environmental Materials, Vegetation, Water, Ice and Snow, and Other Materials: The USGS Tricorder Algorithm One of the challenges of imaging spectroscopy is the identification, mapping, and abundance determination of materials, whether they be mineral, vegetable, or liquid. Given enough spectral range, spectral resolution, signal to noise, and spatial resolution, all these identifications are possible. In the past, scientists spent years analyzing single spectra, but new imaging spectrometer data sets rapidly generate millions of spectra. Spectral analysis of such data can produce maps of specific materials, but the analysis must be both fast and robust. A new system is described that satisfies both criteria. Many materials show diagnostic absorption features in the visual and near infrared region of the spectrum. This region is covered by modern imaging spectrometers such as the Cassini VIMS instrument (0.4 to 5 micrometers), or the terrestrial AVIRIS system (0.4 to 2.5 micrometers). The challenge is to identify the materials from absorption bands in their spectra, and determine what specific analyses must be done to derive particular parameters of interest, ranging from simply identifying its presence to deriving its abundance, or determining specific chemistry of the material. Recently, a new analysis algorithm was developed that uses a digital spectral library of known materials and a fast, modified-least-squares method of determining if a single spectral feature for a given material is present. Clark et al. made a another advance in the mapping algorithm: simultaneously mapping multiple minerals using multiple spectral features. This was done by a modified-least-squares fit of spectral features, from data in a digital spectral library, to corresponding spectral features in the image data. The feature analysis can be done on both absorption and emission features. This version has now been superseded by a more comprehensive spectral analysis system called Tricorder. Lucey P. G. A Mixing Model Approach to Obtaining Quantitative Mineralogy from Spectra of Lunar Soils An important goal of lunar spectroscopy is the derivation of quantitative mineralogy from spectra of lunar materials, both in the laboratory and remotely. While many approaches are available, the use of nonlinear mixing models using endmember minerals is an attractive one given the relatively simple mineralogy of the Moon. Attempts at quantitative analysis using Hapke-based theory have been attempted, but none have fully exploited the range of tools available, addressed compositional variation within lunar mineral endmembers, nor included the effects of maturity in any but a rather ad hoc manner. Presented here is a model which, in addition to the relative abundances of endmembers, takes into account the grain size of the material, the powerful effect of Mg-number on the spectra of mafic minerals, and a theoretical approach using optical constants of iron to model the effects of maturity. Holasek R. Self S. Satellite Observations and Measurements of the 1991 Mount Pinatubo Eruption We demonstrate the use of Geostationary Meteorological Satellite (GMS) and NOAA polar orbiting Advanced Very High Resolution Radiometer (AVHRR) satellite images of the June 1991 Mount Pinatubo volcanic eruption plumes in providing details of the dynamics and changing character of this major explosive eruption. Stratospheric sulfate aerosols generated by the Pinatubo eruption plumes have had a far-reaching impact on the Earth's radiation budget, atmospheric and surface temperatures, regional weather patterns, and atmospheric chemistry and optical properties, including environmentally important atmospheric effects such as global ozone depletion. The presence of the Pinatubo stratospheric aerosol veil is responsible for an increased atmospheric optical depth, and resultant global cooling, possibly in excess of 0.5 degrees C in 1992. In order to better model the effects produced by volcanic aerosols it is important to understand the mechanisms responsible for injecting aerosol producing material to stratospheric altitudes. Of particular interest to volcanologists and atmospheric scientists is the altitude and duration attained by eruption plumes as indicators of atmospheric injection levels, quantities of ash and gas erupted and the dynamic conditions necessary for plumes to reach these altitudes. Tuesday, March 14, 1995 DIFFERENTIATED METEORITES 6:30 p.m. LPI Masarik J. Reedy R. C. Production of Cosmogenic Nuclides in SNC Meteorites in the Martian Surface Depth-dependent production rates of ^3He, ^10Be, ^21Ne, ^22Ne, ^26Al, ^38Ar, and ^53Mn were calculated for Shergotty, Nakhla, and Chassigny (the "SNC" meteorites, believed to have come from Mars) for spherical geometries in space and also in the martian surface for cases of no and a 15-g/cm^2-thick atmosphere. The calculations were done with the well-tested LAHET Code System. The martian atmosphere does not seriously affect production rates and profiles, and thus the apparent absence of complex cosmic-ray exposure histories in SNC meteorites, in contrast to frequent complex histories for lunar meteorites,is not due to unusual production rates in the martian surface. Relative nuclide production rates in Mars and SNC meteoroids depend on the nuclide and the meteorite's composition but only slightly on the martian atmosphere's thickness. Mittlefehldt D. W. Lindstrom M. M. Hunting the HED Holy Grail: A (Slightly) Magnesian Basalt Clast from EET 92014 Howardite and a Review of Alleged Primary, Magnesian HED Basalts Eucrites are ferroan basalts, likely from the asteroid 4 Vesta. Associated with eucrites in polymict breccias and on Vesta are magnesian, cumulate orthopyroxenites (diogenites). The existence of diogenites is generally considered to indicate that more magnesian primary melts were formed on Vesta, and the identification and characterization of such melts has taken on some of the aspects of the search for the Holy Grail. Previously suggested candidates, all from howardites, include Kapoeta rho, PE1 from Y-7308, and an unnamed clast from Kapoeta, which have only been studied in thin section. The two Kapoeta clasts are pyroxene-phyric, glassy clasts, while PE1 is a hornfels. Here, we report on the chemical and petrographic characterization of a black, pyroxene-phyric glassy clast from the howardite EET 92014, here affectionately referred to as BC. Like the medieval search for the Holy Grail, we believe the search for primary magnesian HED basalts has been fruitless. Grove T. L. Compositional Variability in Diogenite Pyroxene: Constraints from Experiments on Magnesian Eucrite Parent Magmas Melting experiments were performed on magnesian eucrite parent magmas at 1 bar and 1000 bars buffered at iron-wustite (IW). The compositions of low-Ca pyroxenes have been determined in olivine + low-Ca pyroxene + spinel - saturated liquids over the temperature range of 1190 to 1230 degrees C. The purpose was to test whether these liquids could produce pyroxenes that resemble the compositions of low-Ca pyroxenes preserved in diogenites. The experimentally produced trend parallels that determined for the diogenite group and spans a range of pyroxene Mg# from 0.69 to 0.76 over a temperature interval of 1190 to 1230 degrees C. The experimentally produced pyroxenes overlap with the composition of diogenite pyroxenes at high Mg#s. At an Mg# of < 0.72, the experimental pyroxenes contain higher Wo contents (Wo5 in experiments vs Wo3 in diogenites). The minor element abundance variations for Ti and Al are comparable in the experimentally produced and natural diogenite pyroxenes. Cr abundances are higher in the experimental pyroxenes by up to a factor of 2. The most magnesian pyroxenes in diogenites are found in Manegaon and Tatahouine and have Mg#s of about 0.80. These compositions have not yet been produced experimentally. Best estimates indicate that these high Mg# pyroxenes crystallized from a parental magma with an Mg# of 0.49 and at a temperature of about 1250 degrees C. Jurewicz A. J. G. Jones J. H. Mittlefehldt D. W. Longhi J. Making Melts Having 40%, 50%, or 60% SiO2 from Chondritic Materials: A Synopsis of Low-Pressure, Low-Volatile, Equilibrium Melting Relations It has been shown that by changing the melting conditions, chondrites can be made to produce very different partial melts (e.g., 40 wt% silica -- "angritic"; 50 wt% silica -- "eucritic"; 55-60 wt% silica -- "andesitic"). This work takes a step beyond directly comparing experimentally-produced melts with natural meteorites. Rather, it considers how to use the silicate phase equilibria of chondrites as a tool for predicting the type of the basalt produced at given conditions. Accordingly, we present a phase diagram and general rules for extrapolating the effects of oxygen fugacity and composition on the silicate-melting characteristics of chondrites, so the results can be applied to silicate partial melting at low pressures for a wider variety of chondritic parents and conditions. Hanowski N. Brearley A. J. Pyroxene Microstructures in the Equilibrated Eucrite Juvinas We have examined the compositional and microstructural characteristics of pyroxenes in equilibrated eucrite, Juvinas by SEM, TEM and electron microprobe analysis in order to elucidate the postcrystallization thermal history of this meteorite. These studies show large variations in exsolved augite lamellae in different grains of Juvinas pyroxenes, supporting brecciation and subsequent different thermal histories for these grains. Equilibration of the meteorite was halted relatively early by quenching. Subsequent slow cooling lead to the exsolution of several generations of opaque phases. The rare occurrence of shock related features indicates a rather small influence by impact processes. Domeneghetti M. C. Molin G. M. Palme H. Stimpfl M. Zipfel J. Low Temperature Thermal History of Non-Chondritic Meteorites as Deduced from Fe2+-Mg Ordering in Orthopyroxene New results on the ordering of Fe^2+ and Mg between the M1 and M2 sites in two small orthopyroxene crystals of the Acapulcoite ALHA81261 are reported. Closure temperatures (T(sub)c) for Fe2+-Mg ordering calculated from site occupancies and structural parameters according to the procedure of Ganguly et al. yield 528 +- 50 and 474 +- 50 degrees C for the two crystals. These temperatures are lower than corresponding temperatures for the recently analysed Lodranite FRO90011, but significantly higher than Johnstown orthopyroxene and much higher than closing temperatures in orthopyroxene of Landes silicate inclusion (T(sub)c = 230 degrees C) determined earlier in the Department at Padova. The quality of the data on ALHA81261 is not sufficient to derive reliable cooling rates. The crystals are very small and show exsolved minerals. The sequence of decreasing closure temperatures from FRO90011 to Landes should, however, in a qualitative sense, reflect decreasing cooling rates. Silicates enclosed in iron meteorites and thus involving core formation processes seem to have cooled more slowly (i.e. deeper in the interior of a parent asteroid) than meteorites that are only affected by partial melting events. Mikouchi T. Chikami J. Takeda H. Miyamoto M. Mineralogical Study of LEW88774: Not So Unusual Ureilite LEW88774 is a Cr-rich ureilite and has been proposed to be quite different in mineralogy from all previously known ureilites. It mainly consists of pyroxene, olivine, picrochromite, and carbon. Our mineralogical study indicate that pyroxene is rich in Ca (Bulk Wo: 15-20) and now consists of coarse augite and low-Ca pyroxene lamellae intergrowth each up to 50 micrometers in width. The diffusion profiles of CaO across the lamellae indicate that pyroxene cooled slowly (0.01 degrees C/year) down to 1160 degrees C, corresponding to the depth of 1 km in a parent body. Below this temperature it would have cooled down quickly possibly due to quenching by excavation. This is not contradictory to a formation history of ureilites so far believed, although the lamellae show coarse exsolution features. Unlike other meteorites, chromite in LEW88774 is very rich in Al and poor in Fe, and belongs to the category of picrochromite. High ZnO in chromites is similar to that in chondrites and primitive achondrites. LEW88774 will be the first ureilite including abundant Ca, Al-rich phases generally missing in normal ureilites, which would supports the possibility that ureilites are not so differentiated meteorites from primitive materials presumably a kind of carbonaceous chondrite. Guan Y. Crozaz G. The Quest for the Elusive LREE Carrier in Ureilites: An Ion Microprobe Study The rare earth elements (REEs) provide one of the most important constraints in exploring the enigmatic petrogenesis of ureilites. It is known that ureilites either have LREE-depleted or V-shaped REE patterns. The former patterns have been attributed to the major silicate minerals in ureilites, olivine and clinopyroxene -- the HREE-enriched mineral phases. The simultaneous enrichment of LREEs and HREEs in the V-shaped patterns, however, suggests that component(s) other than olivine and clinopyroxene carry the LREEs in ureilites. Acid leaching experiments indicated that the LREE-enriched component may be associated with a minor phase which is heterogenously distributed in the carbon-rich veins. Although the LREE and HREE enrichments have usually been ascribed to distinct petrogenetic events, recent data have raised the question of whether the LREE-enriched component is indigenous to ureilites or whether it was introduced by terrestrial contamination. The latter alternative would also have important implications for ureilite chronology. To evaluate these two possibilities, we have made in situ REE measurements of various mineral phases in ureilites, using the Washington University ion microprobe. Shimaoka T. K. Shinotsuka K. Ebihara M. Prinz M. Whole Rock Compositions of Aubritic Meteorites: Implications for Their Origin Aubrites (enstatite achondrites) are heterogeneous assemblages of nearly iron- free enstatite and a variety of rare minerals which formed under highly reducing conditions. Considering the similarities in bulk chemistry of the major and minor mineral phases, and an inferred oxygen fugacity, Watters and Prinz concluded that EL6 chondrites may be the most suitable parent material for aubrites. They further suggested that aubrites formed through the fractional crystallization of such materials. Examining the correlations of several pairs of the elements having great differences in volatility, Wolf et al. also inferred that aubrites may have originated by igneous processes in a differentiated parent body. Okada et al. concluded that the Norton County precursor materials were igneous rocks, mostly plutonic orthopyroxenites, which were formed by extensive melting and igneous differentiation. Brett and Keil and Keil argued that aubrites were produced by melting and fractionation, but not from any known enstatite chondrite group. Recently, Lodder et al. analyzed the whole rock and mineral separates, including oldhamite, from the Pena Blanca Spring aubrite and concluded that the distribution of the REE among the minerals could not be the result of equilibrium fractional crystallization processes. They concluded that a short non-equilibrium melting event, rather than a large degree of equilibrium partial melting, is the best explanation for what occurred on the aubrite parent body. In order to contribute to an understanding of the origin of aubrites, we have analyzed, by instrumental neutron activation analysis (INAA), six aubrites (Bishopville, Cumberland Falls, Khor Temiki, Mayo Belwa, Norton Co. and Pena Blanca Spring), which are all observed falls. In addition, we also analyzed three enstatite-rich meteorites (Happy Canyon, Ilafegh 009 and Mt. Egerton). Most of the meteorites in this study were also analyzed by inductively coupled plasma mass spectrometry (ICP-MS) for REE (including Y) and U. Since these meteorites are highly heterogeneous in their mineral abundances and compositions, an effort was made to reduce the bias in sampling small chips of the individual meteorites, so that conclusions as to the petrogenesis of aubrites can be more meaningful. Yang C.-W. Williams D. B. Goldstein J. I. AEM Study of the Metal Phases in Meteorites Three sub-zones in the tetrataenite rim which is the outermost region of the retained taenite were observed in irons, mesosiderites, pallasites as well as chondrites. This sub-zone structure is due to the precipitates in the tetrataenite rim. The island phase in the cloudy zone also contains precipitates. The microstructure and chemical composition of the metallic phases were investigated using the high resolution electron optical instruments. _ Tuesday, March 14, 1995 MARS GEOLOGY/GEOPHYSICS 6:30 p.m. LPI Fori A. N. Schultz R. A. Analysis of Fault Lengths Across West Candor Plateau, Valles Marineris, Mars As part of a larger project on the history of stress and strain across Valles Marineris, an analysis of the lengths of small grabens was conducted on the west Candor Plateau. The lengths of individual grabens from three distinct sets are analyzed by a two-dimensional window-sampling method to investigate scaling relationships and depth of faulting. Length distributions are influenced by fault growth and coalescence processes, image resolution, window size, and the post-faulting geologic history of the region. Cumulative length-frequency distributions can be linear, with slopes (on the area plot) of -2, suggesting self-similar growth of the graben array. Existing measurements of vertical stratigraphic offset (throw), along with newly measured graben lengths, imply either that throw and length are unrelated (if depth of faulting is proportional to graben width and nearly constant) or that throw and graben length scale directly, implying variable depth of faulting. Fault-length statistics are apparently not sensitive to the presence of mechanical layers in the 10 to 100 km depth range of the lithosphere. Anderson R. C. Peer B. J. An Analytical Approach to Identifying Radial Lineaments Associated with the Tharsis Region of Mars The most dominant tectonically controlled region on Mars is the Tharsis bulge. Asymmetrical in shape, this extensive plateau is 6-10 km high and 4000 km in diameter. Associated with the bulge is a vast array of volcanic and tectonic features such as grabens, normal faults, and lunar-like wrinkle ridges. The faults and grabens display a radial pattern which has lead previous researchers to conclude a relationship exists between the bulge and the faulting. Numerous studies on the orientation of the faults, grabens, and ridges have been perforrned for the Tharsis bulge. These studies have concluded that the tectonic features do not display a "pure" radial pattern indicating a single center, but instead may have multiple centers. In this study a quantitative approach has been used to recognize possible centers of radial tectonic features associated with the Tharsis bulge. Lineaments were digitized for the central Tharsis region (30 degrees N and 30 degrees S latitude; 45 degrees W to 157.5 degrees W longitude). A lineament is defined as any "linear" feature that can be identified and mapped. For the Tharsis region this includes faults, grabens, and ridges. Centers were identified by the greatest concentration of radially projected lineaments. Kiefer W. S. Johnson M. S. Physical Properties Controlling the Style of Extensional Deformation in the Tharsis Region of Mars The style and amount of extensional deformation varies considerably around the Tharsis region. Valles Marineris concentrates considerable extension on just a few main structures, whereas a similar total amount of deformation is distributed over numerous graben near Alba Patera and in Tempe Terra; in still other areas, there is little extensional deformation. These regional variations are likely to be controlled by the physical properties of the lithosphere, which we assess using a model for terrestrial rifting developed by Buck. This model was applied to Valles Marineris by Anderson and Grimm; we generalize the approach to the Tharsis region as a whole. We find that concentrated deformation is favored by a thick crust and low moho temperature, whereas distributed deformation is favored by a thin crust and high moho temperature. The lateral variations in crustal thickness and moho temperature required to explain the variations in extensional style around Tharsis are consistent with those inferred from an inversion of long-wavelength gravity and topography data. Crumpler L. S. Head J. W. Aubele J. C. Magma Chambers Associated with Calderas on Mars: Significance of Long-term Magma Replenishment Rates Calderas on Mars are characterized by a wide range of shapes, structural complexity, and interaction with regional stress fields. The presence of calderas implies a fundamental condition influencing their formation on larger planetary bodies. In order to form calderas, the conditions must exist for the formation of relatively shallow magma chambers. Here we show that specific rates are necessary in order to maintain long-lived magma chambers. The observed range of characteristics of the calderas of Mars may reflect these differences in rate. CONCLUSIONS. Analysis of simple models of the steady state thermal conditions for magma chambers associated with Martian calderas imply: (1) Relatively high, long-term magma supply rates must occur to support the presence of calderas. In addition to other factors, the absence of calderas on the smaller terrestrial planets may be a consequence of magma production and ascent rates that are too low for magma chamber formation. (2) The latest volcanic events in each shield volcano and on Mars as a whole are likely to be a result of deformation or magma transport associated with large deep magma chambers. (3) Deep, low temperature magma chambers are easier to maintain at subliquidus temperatures. Increasing crystallinity of subliquidus magmas from deep chambers may account for variations in lava flow morphology without postulating chemical fractionation. And (4), multiple or nested calderas at the summits of many Martian volcanoes are probably a result of short-term higher magma replenishment rates. Their formation requires separate, renewed, shallow magma chamber formation in each case, thus accounting for their generally nested distribution. Maciejak F. Lenat J. -F. Provost A. Volcano-Tectonic Evolution of Tharsis Tholus, Mars We present preliminary results from an on-going project aiming to study physical processes of martian volcanism. In order to obtain the best possible identification and characterization of the volcanic products and landforms, image processing and numerical stereo-photogrammetry techniques are applied to the Viking imagery. Our preliminary work is a study of Tharsis Tholus (13 degrees N 91 degrees) a 120 by 150 km wide volcano which is an old (3.5 -3.8 Ga) dome, partially embayed by surrounding lava plains. Its volcano-tectonic history was briefly described in but have not yet been thoroughly documented. Crown D. A. Stewart K. H. Debris Aprons in the Eastern Hellas Region of Mars Recent geologic analyses and mapping studies have focused on documenting the geologic history of the eastern Hellas region of Mars. Of particular interest has been an assessment of the styles, timing, and geologic context of volcanic activity associated with the eruptive centers at Hadriaca Patera and Tyrrhena Patera. As a consequence of the relationships between volcanic units in the region and the Dao, Harmahkis, and Reull Valles outflow systems, constraints have been placed on the timing and nature of erosional and depositional processes which occurred in the eastern Hellas region. The current study examines a late stage of degradation that is represented by a series of debris aprons located in remnants of the eroded southern highlands of Mars, primarily along Harmahkis and Reull Valles. Crown D. A. Peitersen M. N. Downflow Morphologic Variations in Hawaiian and Martian Lava Flows Determination of the physical properties and emplacement conditions of planetary lava flows is dependent upon observations of flow morphology, measurement of flow dimensions, and the development and use of appropriate dynamical models. Rheologic and thermal models have been used to calculate values for the viscosity, yield strength, and effusion rate of martian lava flows. Understanding the relationship between lava flow morphology and eruption conditions, cooling history, and the physical properties of lavas is complicated by the diversity of flow types and lava compositions, variable source conditions during eruptions, variations along flow in underlying topography and surface roughness, and the complexities introduced by temperature-dependent rheologic properties, crust formation, vesiculation, and crystallization. Results from modeling studies are used to assess the evolution of volcanic processes on planetary surfaces with only limited amounts and types of data. Most models of flow behavior use average values of flow width and/or thickness based on a few measurements and do not consider width and thickness as independent variables, but rather assume a constant cross-sectional area. In planetary studies, width and thickness may be two of few known or measurable parameters. Changes in downflow dimensions reflect variations due to the temperature (or time) dependencies of volcanic processes and lava properties; the nature and magnitude of the response of a flow to changes in underlying slope also provides important but often neglected information regarding its eruption and emplacement history. Utilization of all relevant data is essential for accurate interpretations of lava flow dynamics and styles of planetary volcanism. Fagents S. A. Wilson L. Basaltic Volcanic Activity on Mars: Numerical Modelling of Lava Fountain Eruptions Numerical modelling of explosive basaltic eruptions on Mars indicates that widely dispersed deposits of cooled scoria should be the common result of steady lava fountain eruptions. This raises intriguing questions regarding the formation of martian lava flows, since explosive activity should be more common than on Earth as a result of the low martian atmospheric pressure and gravitational acceleration. Provided that martian flow-forming magmas are not essentially devoid of volatiles, flows may be fed by highly collimated fissure eruptions. Alternatively, unsteady, surging eruptions may create suitable near-vent conditions for lava flows to be formed. In either case, we anticipate that extensive mantles of pyroclastic material should commonly be associated with the source regions of fountain-fed lava flows, but which may not be of a sufficiently great thickness to allow identification from current Viking image data. The model results thus provide some predictions for testing with data from future missions to Mars. Tatsumura M. J. Mouginis-Mark P. J. Eruption Characteristics of Long Lava Flows on the Northern Flank of Elysium Mons, Mars High resolution Viking images of long lava flows on the northern flank of Elysium Mons present an opportunity to examine and constrain eruption characteristics through observations of flow lengths, widths, thicknesses, and general morphologies . Because of the long lengths of the flows (in excess of 300 km for the whole flow field) which is somewhat unusual for Mars, we consider whether the flows represent vigorous and voluminous eruptions or alternatively, were slowly emplaced over years to decades. Previous work has also suggested that the eruptions were "pulsing" so as to produce lava flow segments that emerge from the upslope segment before it. In this work, we present flow data, from which we derive possible eruption characteristics that formed this flow field and conclude that the lavas were emplaced at relatively high effusion rates. Jons H.-P. Large-Scale Emplacement of Young Lavas in the Area of the TaNoVa-Updoming, Mars The large-scale emplacement of the youngest lavas (i.e. parasitic shields in the general sence and youngest Tharsis lavas including the youngest Syria Planum lavas) in the area of the Tharsis, Noctis Labyrinthus, Valles Marineris (TaNoVa) - Updoming has been widely influenced by the spatial distribution of concentrically arranged escarpments, the relief of the Tharsis ridge, and by the rugged landscape of the aureole around Olympus Mons. Murray J. B. Rothery D. A. Digital Elevation Models of the North Polar Regions of Mars We have used recently developed software which creates digitalelevation models (DEM) automatically from stereo images to examine Viking images of the North Polar region of Mars. Three areas were selected for study, all of them in the polar layered terrain and covered by images with approximately 50 m resolution. One of the areas shows a simple stratigraphic succession without interruption; the other two have prominent unconformities, demonstrating two clearly-defined phases of deposition, interrupted by an episode of erosion. The DEMs created from these images can reveal thicknesses and relative inclinations of the strata, if simple assumptions are made. Lucchitta B. K. Kasei Valles, Mars (II): The Ice Story Morphologic features in the Kasei Valles outflow channels resemble those of cataclysmic floods on Earth, an observation that leads to an inferred origin for these channels by similar large outbursts of water . Yet, outflow-channel features in Kasei Valles are generally an order of magnitude larger than those caused by terrestrial catastrophic floods, implying that Kasei floods were much larger, perhaps by as much as one to three orders of magnitude than those on Earth and in other Martian outflow channels. Releasing such gigantic volumes of water from the ground in one sudden event is difficult, and explanations offered for such releases are not entirely satisfactory. As an alternative to flooding, many of the morphologic features in the Kasei Valles possibly were carved by ice and sub-ice water channels, an idea supported by several observations. Parker T. J. Schenk P. M. Viking Stereo of the Martian Crustal Dichotomy in Southern Elysium: Evidence for Extensive Fluvial and Coastal Erosion? Stereo image data is indespensible for the geomorphic interpretation of planetary surfaces. Broad undulations or gentle regional topographic gradients may be completely unrecognizable in monoscopic images, because they are often dominated by albedo variations and shading associated with high-frequency topography, such as channel margins, crater rims, and fault scarps. On Mars, regional-scale features, including the crustal dichotomy, major volcanic constructs and impact basins, are readily identifiable on the current global topographic maps produced by combining S/C radio occultation data, Earth-based radar, and low-resolution stereo image data. But smaller features, such as valley network drainage basins, require higher resolution topography. Cabrol N. A. Grin E. A. Dawidowicz G. Shalbatana Vallis (Mars): Headwater Migrations as an Alternative to Recharge Process The challenge to understand the origin and history of valley erosional morphologies requiring extensive flow rate by water recycling to maintain their hydraulic gradients is not successful met. Matching the morphometry of relic fluvial landforms with the organization of their valley paths may emphasize the range of the genetic process and allows hydrogeologic response to the debated major question of aquifer recharge. The erosional unconformity of the three segments in the middle course of Shalbatana vallis, a Chryse Basin outflow, illustrates a process of successive abandoned valleys. The comparison between their cross section profiles gives clues to understand the flow discharge episodes according to the conformity between each valley section and their relative elevation. Herkenhoff K. E. Geologic Map of the MTM-85000 Quadrangle, Mars The geologic map presented here covers the portion of Mars between latitudes 82.5 degrees S to 87.5 degrees S, longitudes -20 degrees W to 20 degrees W. The Viking Orbiter 2 images used to construct the photomosaic base used for mapping were taken during the southern summer of 1977, with resolutions no better than 140 meters/pixel. A digital mosaic of Mariner 9 images was also constructed to aid in mapping. The Mariner 9 images were taken during the southern summer of 1971-72, and have resolutions as high as 85 meters/pixel. However, usefulness of the Mariner 9 mosaic is limited by incomplete coverage and atmospheric dust opacity. The most common bedrock unit in this quadrangle is the polar layered deposits, first described by Murray et al. and Cutts. The layered deposits are characterized by smooth morphology and gently sloping, asymmetrical troughs. Visibility of layers is limited by the resolution of available images, but layering in the southern part of the map area appears similar to that in other parts of the layered deposits in terms of topographic expression, dip, and thickness. Only one primary impact crater has been recognized in the layered deposits on this map. It is about 17 km in diameter and appears to be partly obliterated by erosion of the layered deposits on its north side. While this crater has clearly been modified, its generally circular shape and the preservation of secondary craters in its vicinity are strong indications that the layered deposits have not been deformed significantly by "glacial" flow. This observation has implications for the rheology of the layered deposits and, therefore, the composition and average temperature of the bulk of the deposits. Socki R. A. Romanek C. S. Gibson E. K. Jr. Cryogenic Weathering as a Mechanism for Extreme 13C Enrichment in Martian Carbonate: Soil from Wright Valley, Antarctic as a Terrestrial Analog The carbon isotope composition of carbonate was measured from a suite of well-characterized polar desert soils of the Dry Valleys of Antarctica to determine the extent of 13C enrichments attributed to cryogenic freezing processes in terrestrial environments. These data are then used to gauge whether cryogenic freezing is a viable aqueous process that can produce the extreme 13C enrichment observed in Martian carbonates. Horai K. Fujii C. Nakagawa H. Matsui T. Thermal Conductivity of Powdered Silicates Under Simulated Martian Surface Conditions Thermal conductivity of powdered silicates was measured in the temperature range between 200 and 300K under the environmental gaseous pressure of CO2 ranging from 10 to 10^5 Pa. The purpose of the study is to provide basic data that are relevant to the project of Martian heat flow measurement that is expected in the near future. The measurement was conducted by the needle probe technique on a series of samples including bentonite that is used as a simulant of the Mars' surface material. Craddock R. A. Maxwell T. A. Howard A. D. Morphometric Analyses of Martian Highland Impact Craters The mechanisms responsible for degrading highland impact craters on Mars have remained a long-standing mystery. Leighton et al. recognized that martian craters are in widely different degrees of preservation, and it was presumed, ages. Based on historical and spacecraft observations of aeolian activity, Hartmann suggested that loose crater ejecta had been removed and redistributed. Alternatively, Jones and Chapman and Jones suggested that water was the major erosive agent during an early martian crater obliteration event. Based on Viking data, Arvidson et al. suggested volcanism as yet another possible process responsible for crater degradation. In recent years a number of other investigators have supported various aeolian mechanisms to explain martian crater degradation. However, based on the range in crater morphology, the timing of degradation, and the estimated amount of erosion, we believe that only a combination of processes can explain the record of resurfacing evidenced by the highland craters. Because the geologic materials which contain degraded impact craters are the oldest exposed units on the planet, the morphology of these craters represents the results of the earliest processes operating on Mars. Obviously both aeolian and fluvial processes have occurred, but which process is principally responsible for martian highland crater degradation? The answer has importance for understanding the martian atmospheric and climatic history, the history of water and aeolian material on Mars, the nature of the highland units, and in assessing the possibility of life having developed on Mars. Because it appears that crater morphometry is highly dependent upon geologic unit, we have constrained our initial analysis to dissected materials (Npld) in the Margaritifer Sinus region. Using radiometrically-calibrated (red filter), moderate resolution (~200 m/pixel) Viking orbiter images, we measured the morphometry of 264 fresh impact craters using the photoclinometric algorithm written for the Planetary Image Cartography System. These measurements are important for establishing a basis for comparison with degraded impact craters. Although we have measured ~4 times the number of complex craters reported by Pike and Davis and restricted the work to dissected highlands materials, our results fall within their 95% confidence interval. Measurements of 28 impact craters in various degrees of degradation are also presented. Depth to diameter ratios suggest that larger diameter craters are proportionally shallower than smaller diameter craters. The low correlation coefficient for the rim height versus diameter measurements (r = 0.029) is most likely due to the inclusion of craters with widely different states of preservation in the data set. Demura H. Kurita K. Regional Variation of Onset Diameter in the North of Elysium, Mars Martian crater is characterized by fluidized craters such as the rampart crater. Onset diameter, which is the critical diameter separating ordinary craters and fluidized craters, is considered to be related to the depth of groundwater/permafrost layer. The excavating depth of crater corresponding to the onset diameter is expected to suggest the discontinuity of volatile distribution which makes fluidized ejecta. We studied the size distribution of fluidized craters in high resolution Viking images at the North of Elysium Region, and found the existence of onset diameter. This indicates that the upsurface depth of volatiles is an order of tens' meters. This value is different from the previous estimation which is an order of hundreds' meters. Mauldin L. C. Grimm R. E. Crustal and Lithospheric Structure of Martian Impact Basins Gravity anomalies from the Goddard Mars Model-1 over the largest impact basins on Mars are consistent with nearly complete isostatic compensation of the original basins. However, thickening of the elastic lithosphere with time can be demonstrated from the isostatic response to later basin infill. Isidis basin in particular shows a marked positive free-air anomaly, indicating development of a thick (> 100 km) elastic lithosphere to flexurally support 2-4 km of volcanic load within several hundred million years of basin formation. The best-fit thickness of highland crust surrounding Hellas is ~80 km. The gravity data are consistent with 50 - 100% complete excavation of the crust during formation of the Hellas basin. Frey H. V. Roark J. H. A Multi-ring Impact Basin in Thaumasia We suggest a previously unreported multi-ring impact basin exists in the Thausmasia area of Mars, which appears to significantly predate the formation of the fossae which dominate this region. The main ring of the basin is 570 km in diameter; there appears to be one interior and perhaps one or two exterior rings. The fractures and graben in this region appear totally unaffected by the presence of the basin, which we take to indicate complete mechanical adjustment of the impact feature prior to formation of the graben. Major compressional features of the South Tharsis Ridge Belt do change orientation from NW-SE west of the basin to SW-NE to the east of the impact. The basin is overlain by a number of large degraded and embayed craters, suggesting it may be one of the oldest relic structures on the planet. Komatsu G. Baker V. R. Johnson J. R. Ares Vallis: Flood Geomorphology of Mars Pathfinder Landing Site The Mars Pathfinder project has selected a candidate location for the landing site. We mapped the local geology of the region in order to identify features which may be imaged by the lander. The landing ellipse encompasses part of the outwash plains of the Ares Vallis, one of the largest outflow channels on Mars. This region contains a variety of flood-related features, and the lander instrument may provide insights to understand Martian paleohydrology. For comparison, we used the experience from terrestrial analogs: the Channeled Scabland of eastern Washington State and flood-modified valleys in Altai Mountains. These landscapes were formed by cataclysmic floods caused by the failures of glacier-dammed lakes in Pleistocene. The scales of the terrestrial floods were probably an order of magnitude smaller than their Martian counterparts, but they provide the best available terrestrial comparisons. Edgett K. S. Physical Properties of the Ares Valles (Primary) and Trouvelot Crater (Back-up) Landing Sites for Mars Pathfinder: Thermal Inertia and Rock Abundance from Viking IRTM Observations Albedo, rock abundance, and thermal inertia derived from Viking Infrared Thermal Mapper (IRTM) observations allow for comparison of the proposed Ares Valles and Trouvelot Crater (back-up) landing sites for Mars Pathfinder with the Viking lander sites of 1976. The Ares site is likely to be about as rocky as the Viking 1 site, but might have some dark sand (missing at the Viking 1 site) and somewhat less bright red dust than the Viking 1 site. The Trouvelot Crater site could be about as rocky as the Viking 1 site, but with more dark, windblown sand than found at the Ares or Viking 1 sites. Assessment of surface properties at these sites using high resolution (2-10 km) IRTM data is underway. Edgett K. S. Rice J. W. Very Young Volcanic, Lacustrine, and Fluvial Features of the Cerberus and Elysium Basin Region, Mars: Where to Send the 1999 Mars Surveyor Lander The Elysium Basin region (10 degrees S - 20 degrees N, 170 degrees W - 230 degrees W), which includes the low albedo Cerberus feature and the Marte Valles channel system, has some of the youngest volcanic and fluviolacustrine landforms on Mars. Viking images show that the Cerberus Rupes fractures were probably the source for some of the water and volcanic material which shaped this region. One lava flow that emanates from the Cerberus Rupes is so young that it still has a low albedo (it is not chemically weathered or covered by dust). The presence of both water and volcanism during the Late Amazonian in this region make it a favorable site for the Mars Surveyor lander being planned for launch in January 1999, provided that the mission emphasis is on the mineralogy and chemistry of sediments of relatively recent lacustrine and/or hydrothermal origin. With the potential for landing sites in mind, specific key features in this region should be targeted for observations by the 1996-launch Mars Global Surveyor camera and thermal emission spectrometer. Golombek M. P. Parker T. J. Moore H. J. Mars Pathfinder Landing Site Characteristics The landing site chosen for Mars Pathfinder is at the mouth of Ares Vallis in southeastern Chryse Planitia (19.5 degrees N, 32.8 degrees W). Ares Vallis is a large outflow channel that drained the highlands to the southeast. The region contains large streamlined islands of older plateau materials and smooth outflow deposits in the channels. Many of the streamlined island are surrounded by terraces that may represent stages of downcutting during the flood or layering. Potential source rocks for the outflow deposits include ancient crustal units-Noachian plateau materials (Npl(sub)1, Npl(sub)2), Hesperian Ridged Plains (Hr), and a variety of reworked channel materials (Hcht, Hch, Hchp). The landing ellipse, 100 x 200 km, is placed on a flat, smooth surface between streamlined islands and knobby terrain to the east, large streamlined islands to the south, large fresh impact craters to the north, and scabby or etched terrain to the west. Scabby or etched terrain is rough-appearing in high-resolution images ( about 40 m/pixel) and may have resulted from fluvial plucking or eolian deflation. Portions of the landing site are peppered with secondary craters that either excavated or are covered by dark albedo material. Morphologies of the secondaries indicate that the source crater is to the south. In this abstract we describe the characteristics of the landing site. Bertka C. M. Fei Y. An Experimental Study of the Internal Structure of Mars Without seismic data, models of the internal structure of Mars are dependent on density profiles determined from the phase equilibrium relations of model mantle compositions and the equations of state of the phase assemblages stable at high pressure and temperature. The density profiles are constrained by the mass and moment of inertia of Mars. Whether a model mantle composition is consistent with the physical properties of Mars depends on the density profiles of the mantle and core and the depth of the mantle-core boundary. The purpose of our study is to provide the experimental data necessary to investigate the internal structure of Mars. Our recent study on structure and density of FeS provides accurate density measurements of FeS at high pressure and temperature, eliminating one of the major uncertainties in determining density profiles of a sulfur-bearing Martian core. In this study we evaluate the relationship between the depth of the Martian core-mantle boundary and the sulfur content of the core using our density measurements of FeS at high pressure and temperature and a mantle density profile obtained by Kamaya et al.(1993) for a Morgan and Anders (1979) model Martian mantle composition. We have also begun an experimental determination of the high-pressure mineralogy of a Dreibus and Wanke model Martian mantle composition (1985) up to core-mantle boundary pressures. The effect of the density profiles of the mantle and core on the internal structure of Mars will be discussed. Jurgens R. F. Slade M. A. Franck C. R. Howard S. D. Standish E. M. Winkler R. Choate D. Cormier R. Dendrenos P. Rose R. Goldstone 3.5-cm Radar Observations of the 1995 Mars Opposition As part of the International Mars Watch, Goldstone radar observations of Mars were performed during the 1995 Opposition (~Dec.'94-March'95). While much of the allocated time was already devoted to observations for Mars Pathfinder landing site validation, a number of other tracks were performed for various other scientific objectives as outlined below. During this opposition, the sub-Earth latitudes are in Mars' Northern hemisphere over terrain not previously examined with current sensitivity. The radar data types possible in 1995 will be described. A good overview can be found in Chapter 20 of the book MARS. Smith D. E. Zuber M. T. Progress in Improving Our Knowledge of the Shape of Mars from Occultations Present knowledge of the topography of Mars is inadequate for addressing a wide range of fundamental problems in geophysics, geology, and atmospheric science. The data acquired to date by several techniques has not provided reliable and consistent values for even the equatorial and polar radii, and large uncertainties exist in the altitudes of many major surface features such as major volcanic shields. One principal shortcoming of the present Mars topographic model is the lack of long wavelength (hundreds to thousands of km) control. However, a dramatic improvement can be made by re-analysis of selected existing data. Specifically, it is possible to derive a topographic field model solely from existing spacecraft occultation data that will have a radial accuracy better than 500 m at all latitudes, as compared to the current Mars Digital Elevation Model (DEM), in which errors at high latitudes range from greater than two to in excess of three km. Tuesday, March 14, 1995 MARS SPECTROSCOPY 6:30 p.m. LPI Hamilton V. E. Christensen P. R. Interpreting the Origins and Evolutions of Martian Basalts from Pyroxene Composition: I. Orthopyroxenes Visible and near-infrared spectroscopic studies [Adams J. B., Singer R. B., Singer R. B. and Roush T. L., Mustard J. F. et al.] have identified pyroxenes on the martian surface ranging in composition from augite to low-Ca orthopyroxene, or pigeonite. Pyroxenes are primary minerals in basaltic rocks; if two or more are present, they may be used to establish pressure and temperature conditions of the basaltic melt. The Thermal Emission Spectrometer (TES) on the Mars Global Surveyor spacecraft will provide mineralogical data about the surface materials of Mars, and will allow for studies of individual minerals such as pyroxenes. Thus, a clear understanding of the thermal infrared (~3 to 50 mm) spectral properties of various pyroxene compositions will be of great value in the interpretation of the origins of martian basaltic rocks. Laboratory spectroscopic studies of pyroxenes will be carried out in order to (1) characterize the spectral effects of compositional variations among pyroxenes, and (2) establish the minimum compositional difference detectable in pyroxenes through vibrational spectroscopy. Rowland C. M. Roush T. L. Sloan G. C. Bell J. F. III Thermal Infrared (7-14 Micrometers) Spectral Imaging of Mars Thermal infrared spectral images of Mars, covering the 7-14 micron wavelength region, were obtained at the Wyoming Infrared Observatory (WIRO) during March 1993 using the Geophysics Laboratory Array Detector Spectrometer (GLADYS). Comparison of these spectra to previous observations indicate the presence of spectral features consistent with martian atmospheric aerosols and dust. Erard S. ISM at Mars and Phobos: The Imaging Spectroscopy Data Base The first imaging spectrometer to be flown on a planetary mission was ISM/Phobos. The instrument was developed at IAS and DESPA (Paris Observatory), and was supported by CNES (French space agency). The data set is comprised of 11 image cubes of Mars, 2 image cubes of Phobos, various deep sky and Martian limb observations. Every pixel within an image cube corresponds to a complete spectrum with high signal-to-noise ratio; the spectra range from 0.76 to 3.15 micrometers in 128 spectral channels. These data were used to study surface mineralogy of both Mars and Phobos, and aeronomy of Mars. The data set is currently being archived in a PDS-compatible format, and will be available from CNES during the first semester of 1995. Agresti D. G. Wdowiak T. J. Wade M. L. Armendarez L. P. Farmer J. D. A Mossbauer Investigation of Hot Springs Iron Deposits We report here on initial measurements by Fe-57 Mossbauer spectroscopy of deposits collected from two iron-containing subaerial thermal springs in Yellowstone National Park (Chocolate Pots and Jim's Black Pool) and one at Manitou Springs, Colorado. Our work has been inspired by two facts: first, the likely occurrence of extensive volcanism and abundant surface water early in Mars history, which may have led to the presence of life in iron-rich hot springs, with a surviving mineral record of such activity; and second, the development of a compact backscatter Mussbauer spectrometer (BaMS) that is capable of in situ measurements on Mars of such mineral indicators. YNP samples were collected while varying distance and orientation from the vent source and at various depths to reflect differing environmental and microbiological factors. Mossbauer investigation of these samples was carried out at temperatures from 300 K down to 12 K. Our preliminary measurements show a rich variety of spectral signatures that exhibit a number of distinctive features, most importantly: a range of ferrous/ferric contributions, reflecting a variation of environmental redox potential, possibly due to microbiological diversity with depth (becoming more aphotic and anoxygenic) and/or distance from the vent source (decreasing temperature); and also differing degrees of magnetic order, including superparamagnetism, which implies the presence of nanophase iron minerals, the deposition of which was possibly also influenced by the microbiological environment and may be associated with ultimate fossilization. Tuesday, March 14, 1995 LUNAR HIGHLANDS, MARE, AND SURFACES 6:30 p.m. LPI Longhi J. Jurewicz S. R. Plagioclase-Melt Wetting Angles and Textures: Implications for Anorthosites The rheological properties of crystal-rich magmatic suspensions and mushes play an important role in partial melting, magma transport, and differentiation. Of particular interest to lunar studies is the role that the rheology of plagioclase-melt mixtures plays in the formation of anorthosites. Accordingly, we have begun a study of plagioclase-melt wetting angles. Results from a melting experiment on a terrestrial gabbroic-anorthosite show a mean wetting (dihedral) angle of 45 degrees a result similar to that obtained for alkali feldspar-melt by. However, the distribution of measured wetting angles is non-ideal reflecting crystalline anisotropy, the development of planar crystalline faces, and heterogenous melt distribution. These results are qualitiatively similar to those obtained for biotite-melt by and suggest: a) that the transition from suspension (liquid rheology) to mush (solid rheology) occurs at lower melt fractions than would be predicited for uniform spheres; and b) that there is a non-zero equilibrium melt fraction that cannot be withdrawn from a plagioclase-rich matrix under hydrostatic stress. Kadik A. A. Lebedev Ye. B. Separating the Melt from the Crystalline Matrix: Simulation in a Centrifuge A high-temperature centrifuge has been used for separating the melt from the crystalline matrix during partial melting of an olivine-pyroxene-plagioclase mixture. It was found that at 1190 degrees C and 25 vol.% of initial amount of the melt the segregation involves multiphase flow of the liquid and of the crystals.It leads to the formation of three layers: a) a crystalline residue at the bottom with intergranular silicate liquid and slight differentiation of the grains; c) a layer of cumulated liquid; d) a layer of plagioclase crystals + melt. Conclusions are drawn on the conditions of plagioclase extraction during partial melting to form the feldspar-rich lunar highland crust. Premo W. R. Evidence for Metamorphic Pb in Ancient Lunar Highland Rocks (>3.9 Ga)?: An Alternative Explantation for 'Ugly' U-Pb Systematics A plot of Mg' vs 207Pb/206Pb using the limited present database for non-mare, probably-pristine, ancient lunar rocks illustrates an inverse correlation between the two parameters; i.e. low-Mg highland rock types (e.g. ferroan anorthosites) have significantly higher 207Pb/206Pb values than high-Mg rocks. The fact that anorthosites contain >90% plagioclase by volume compared to the variable, but notably smaller, proportions of plagioclase in high-Mg rocks (e.g. norites, troctolites, dunite, gabbros) probably has a great deal to do with these statistics. If reduced Mg numbers or MgO and FeO values in anorthositic plagioclase is a metamorphic signature as suggested by, then the correlation implies that high 207Pb/206Pb values are also metamorphic. If this is so, the only presently identifiable source for the high 207Pb/206Pb values is high-micrometers (>1000) KREEP. To this point, high-micrometers magmas were thought to be the source of ferroan anorthosites and high-Mg suite rocks, although alternate explanations had been mentioned. But if this is not the case, then this Pb had to either have been added metasomatically as suggested by or it is metamorphically emplaced as implied by. So the question is: how and when does KREEP Pb get into ancient lunar highland rocks? Grier J. A. Kring D. A. Swindle T. D. Impact Melts and Anorthositic Clasts in Lunar Meteorites Clasts in Lunar Meteorites QUE93069 and MAC88105 QUE93069 is a 21.4g meteorite that has been classified as an anorthositic breccia of lunar origin. We present petrographic and electron microprobe analyses of QUE93069 and a new thin section of MAC88105. In particular, we compare the impact glasses in these meteorites with other lunar meteorites, and Apollo 16 sample 64001 to gain insights into the compositional origin and history of these samples. Our results confirm that QUE93069 is an anorthositic breccia with highland affinities. Hill D. H. Marvin U. B. Boynton W. V. Clasts from the Calcalong Creek Lunar Meteorite The Calcalong Creek lunar meteorite was recovered as a single, 19 gram stone. It is a microbreccia containing submillimeter clasts in a glassy matrix. In thin section, Calcalong Creek resembles "lunar clast-laden impact-melt breccias" of the Stoffler et al. classification. The bulk chips reveal vesicular agglutinates which weld the clasts together. This suggests that Calcalong Creek is a regolith microbreccia with few spherules. Our earlierwork showed that Calcalong Creek contains the highest abundance of the KREEP chemical component among the known lunar meteorites. However, a specific REE carrier had not been identified. We report on our petrologic and trace element survey of representative clasts as well as preliminary INAA analyses of the largest bulk chip analyzed to date. Albrecht A. Herzog G. F. Klein J. Middleton R. Schultz L. Weber H. W. Kallemeyn G. W. Warren P. H. Trace Elements, 26Al and 10Be, and Noble Gases in Lunar Rock 14286 The 4.4-gram lunar rock 14286 is an archetypal sample of lunar metal. It consists mainly of kamacite with 4.7-6.2% Ni and 0.51-0.63% Co. The rest, perhaps 10% by volume, comprises sub equal proportions of maskelynitized plagioclase and pyroxene, both medium-grained, and a partial coating of fine-grained regolith breccia. Warren et al. noted especially the silicate texture in 14286 as evidence for a prolonged period of crystallization, most likely in a melt pool created by a large meteorite impact. The time and circumstances of 14286's arrival on the Moon are of interest. Here we develop constraints on the rock's history through analyses of trace elements in INAA, of ^26al and ^10Be by accelerator mass spectrometry, and of noble gases by static mass spectrometry. Flynn B. C. Stern S. A. On the Stoichiometry of Metal Abundances in the Lunar Atmosphere We present preliminary results of an ongoing effort to determine to what degree metal abundances in the lunar atmosphere are stoichiometric (i.e., reflective of the lunar surface composition). From Apollo sample returns, we know that several species are more abundant at the lunar surface than either Na or K (Taylor S. R.), which are the only atmospheric constituents to have been observed from Earth (Potter A. E. and Morgan T. H.; Sprague A. et al.; Flynn B. C. and Mendillo M.; Stern S. A. and Flynn B. C.). Production of the atmosphere by solar wind sputtering has been inferred from monitoring atmospheric brightness through lunar crossings of the Earth's magnetotail (Potter A. E. and Morgan T. H.). Assuming solar wind sputtering is the dominant source of Na and K then, by simple stoichiometric arguments, one might expect that relatively abundant lunar surface constituents such as Si, Al, Ca, Mg, Fe, and Ti are more abundant in the lunar atmosphere than Na and K. Furthermore, observations of K (Potter A. E. and Morgan T. H.) indicate that K production behaves stoichiometrially relative to Na. Sisterson J. M. Schneider R. J. Jr. Jull A. J. T. Donahue J. Cloudt S. Beverding A. Englert P. A. J. Castaneda C. Vincent J. Reedy R. C. Production Cross Sections for 14C From Elements Found in Lunar Rocks: Implications for Cosmic Ray Studies The solar cosmic ray (SCR) record preserved in lunar rocks can be one of the simplest records to interpret if the elemental analysis of the rock is well understood, all of the cross section data for the interactions of SCR particles with these elements are known, and production rates of these nuclides by galactic cosmic ray (GCR) particles are well known. Although many proton production cross sections have been measured, at least over part of the proton energy range required, there is still insufficient data, particularly for energies near the reaction thresholds, for this record to be fully understood with confidence. New measurements have been made for the cross sections 16O(p,3p)14C, Si(p,x)14C, Mg(p,x)14C, 27Al(p,x)14C, Fe(p,x)14C and Ni(p,x)14C as part of the ongoing effort by this collaboration to provide these needed cross section values, which will help in the interpretation of the SCR record by establishing elemental GCR production rates and by unfolding the depth profile measurements after removing GCR contributions. Masarik J. Reedy R. C. Low-Energy Neutrons and Reaction Products in the Lunar Surface Depth-dependent neutron production rates, the neutron induced fission rate of ^235U, and rates of ^60Co, ^41Ca, and ^36Cl produced by neutron-capture reactions were calculated using the LAHET Code System. Both the absolute magnitudes and the depth profiles for all investigated reactions are in good agreement with experimental data down to 400 g/cm^2.All calculated nuclide-production-rate-versus-depth profiles rise sharply from the surface to a broad maximum from 100-200 g/cm^2 and drop off at greater depths with an e-folding length of about 180 g/cm^2. Snyder G. A. Hall C. M. Taylor L. A. Halliday A. N. 40Ar/39Ar Ages of Apollo 11 Group D Basalts: Evidence of High-Ti Volcanism in the Nectaris Basin and a Probable 2.0 Ga Age for Crater Theophilus? High-Ti basalts from the Apollo collections span a range in age from 3.87 Ga to 3.55 Ga. The oldest of these are the common Apollo 11 Group B2 basalts which yield evidence of some of the earliest melting of the lunar mantle beneath Mare Tranquillitatis. Rare Group D high-Ti basalts from Mare Tranquillitatis have been studied in an attempt to confirm the postulated link with Group B2 basalts. ^40Ar/^39Ar ages have been determined on one known Group D basalt and a second postulated Group D basalt using a continuous laser step-heating procedure. Three separate runs of known Group D basalt 10002,116 yielded ^40Ar/^39Ar plateau ages of 3798 +/- 9 Ma, 3781 +/- 8 Ma, and 3805 +/- 7 Ma (all errors 2s). Furthermore, this sample has apparently suffered significant ^40Ar loss at an event around 2.0 Ga, possibly due to effects of formation of a major early Copernican impact, such as that which formed the nearby crater Theophilus. Thus, Group D basalts from the Apollo 11 landing site may not be indigenous and could be ejecta from the Nectaris basin. Another suspected Group D basalt, 10002,1006, yielded disturbed age spectra on two separate runs, probably due to ^39Ar recoil effects. Using the "reduced plateu age" method of Turner et al., the ages derived from this sample were 3898 +/- 19 and 3894 +/- 19 Ma. Scherer E. E. Beard B. L. Barovich K. M. Johnson C. M. Taylor L. A. An Improved Method for Determining the Hf Isotopic Composition of Lunar Basalts Recently it has been suggested that the source region of high-Ti mare basalts contained residual garnet, if ilmenite was also a residual phase. This may imply that the source region of high-Ti mare basalts was significantly deeper than previously thought. This conclusion is based on modelling of Lu-Hf isotopic and concentration data of lunar basalts reported by Unruh et al., in the most recent Hf isotopic investigation of lunar rocks. Such interpretations highlight the exceptional ability of the Lu-Hf system to reveal the mineral assemblages involved in igneous processes like crystallization and partial melting, which produce strong mineral-dependent Lu-Hf fractionations. However, only 16 lunar samples have been analyzed for Hf isotopic composition to date, partly as a result of the formidable analytical challenges faced in separating Hf from other elements (e.g., Ti and Zr), and the poor ionization efficiency of Hf. Thus there are still large gaps in the Hf isotopic data set for lunar basalts. For example, the 7 high-Ti basalts that have been analyzed define an apparent horizontal trend on a plot of eHf versus eNd. We observe that this trend is defined by high-Ti Apollo 11 samples that have low initial eNd values, and high-Ti Apollo 17 samples that have high eNd values. It is important to note, however, that Apollo 17 high-Ti basalts that have low eNd values (e.g., samples 77516 [eNd = +4.0] and 78586 [eNd = +2.7] and Apollo 11 samples that have high eNd values (e.g., samples 10044 [eNd = +7.1] and 10047 [eNd = +7.4] have not been analyzed to determine if this apparent horizontal trend between initial eHf and eNd is characteristic of individual landing sites. In preparation for new Hf isotope studies of these rocks, we have developed a new analytical technique that has been tailored to determining the Hf isotopic composition and Lu and Hf abundances of high-Ti basaltic rocks (up to 13 wt% TiO2). McGee J. J. James O. B. Mineral-Chemical Variations During Basalt Crystallization Data are presented for two quartz normative basalts (QNBs), 15065 and 15085, and compared with previous data for 15388 and 15384. Sample 15384 , an olivine-normative basalt has a hypidiomorphic-granular intergrowth of pyroxene, plagioclase, and olivine, with local patches of variolitic intergrowths of pyroxene and plagioclase. Sample 15388 has two textural domains: 1) early crystallized, variolitic intergrowths of pyroxene and plagioclase; and 2) later crystallized, hypidiomorphic-granular intergrowths of plagioclase, pyroxene, and interstitial opaque oxides and cristobalite. Locally, there are sparse large pyroxene phenocrysts, some of which contain sparse olivine grains. QNBs 15065 and 15085 are texturally similar to 15388, but with more abundant and larger pigeonite phenocrysts and less abundant and smaller variolitic patches. Compositional variations in the minerals of 15388, 15065, and 15085 show that 15388 crystallized from a distinctive parent magma, richer in TiO2 and Al2O3 and poorer in FeO than the parent magma of the QNBs. The compositional data display variation trends in plagioclase formed by stages of early crystallization of plagioclase-supersaturated liquid and subsequent normal equilibrium crystallization. Positively correlated trends of FeO, MgO and Na2O in early plagioclase that crystallized from a plagioclase-supersaturated melt are similar to those interpreted as igneous in lunar ferroan anorthosites, where brecciation and metamorphism obscure the effects of primary crystallization. Cazzaniga A. M. Hess P. C. Possible Source Compositions for Lunar Picritic Glasses: Ilmenite vs. Karooite A lunar magma ocean would produce unstable stratified layers: a light, Mg-rich olivine layer would settle initially, followed by a progressively less Mg-rich pyroxene layer, eventually leaving an incompatible-rich melt to crystallize into a dense ilmenite layer below the light anorthositic crust. The density contrasts cause a catastrophic overturn, allowing the layers to mix and the ilmenite to sink to form a core (1). A "mantle" layerwith variable amounts of olivine and orthopyroxene (depending on the efficiency of the overturn) and small amounts of ilmenite, or otherTi-oxide, could be formed. Melting of such a layer could create the range of titanium found in the two arrays of medium to high Ti lunar picritic glasses. Coombs C. R. Hawke B. R. Lunar Pyroclastics: Hide'n Seek Our remote sensing analyses confirm the fact that pyroclastic deposits are more numerous and widespread than initially mapped, and that greater volumes of material were erupted than initially interpreted. As a result of our systematic telescopic remote sensing and mapping of portions of the lunar nearside, we have identified and mapped numerous, previously unrecognized, pyroclastic deposits, including new deposits NE of Humorum basin and SW of Crisium Basin. Analysis of the ground-based telescopic spectra and orbital geochemistry data has enabled classification of these pyroclastic deposits into different spectral/compositional groups. Weitz C. M. Head J. W. III Lunar Pyroclastic Deposits: Nature and Distribution of Pyroclastic Glasses at Taurus-Littrow Comprehensive analyses of remote sensing data has shown that a wide variety of pyroclastic deposits and landforms exist on the Moon. Our goal is to relate the range of these features to theoretical predictions of magma disruption and pyroclastic dispersal in the lunar environment. One type of pyroclastic deposit is the volcanic glass sampled at all of the Apollo landing sites, with large concentration of glasses found at the Apollo 15 and 17 sites. Regional (>100 km diameter) and localized dark mantle deposits composed of pyroclastic glasses have been identified on the Moon, with both deposits generally associated with sinuous rilles or irregular depressions. In this analysis, we concentrate on the glasses found at the Taurus-Littrow (TL) region where the Apollo 17 landing site is located. The dark mantle deposit at TL covers over 100 km from north to south while the western portion of the deposit has been buried by younger low-Ti basalts within Mare Serenitatis. A drive tube taken at Shorty Crater showed orange glasses on top of their crystallized equivalents (black glasses). Assuming that the glasses were not inverted by the impact, the stratigraphy indicates that either the black glasses were deposited first or that the glasses cooled slower at the base of the deposit. In this analysis, we have combined petrogenesis, remote sensing, and modelling studies of the glasses found at TL in order to understand their origin, distribution, and mode of emplacement. Yingst R. A. Head J. W. III Spatial and Areal Distribution of Lunar Mare Deposits in Mare Orientale and South Pole/Aitken Basin: Implications for Crustal Thickness Relationships Analyses of high resolution images of the mare surface have documented the geomorphology and the stratigraphy of volcanic structures (D. Wilhelms, J. Head). Petrological analysis of lunar mare basalts has led to a series of mare basalt petrogenetic models (J. Longhi). The interaction between the source regions and those features observed on the surface, however, remains ambiguous, in regard to both conceptual and quantitative assessments of this relationship, and the interpretation of surface features in terms of the dimensions of the source regions and conduits, and the method of transportation to the surface. In recent studies we have been working to establish a basic theoretical and observational framework for considering the ascent and eruption of magma on the Moon. We consider the principles of reservoir development and neutral buoyancy zones at the base of the highlands crust or at rheological boundaries, the overpressurization of reservoirs, and the general properties of dikes that would be propagated toward the surface (J. Head and L. Wilson, 1992, 1994, L. Wilson and J. Head). In this contribution we report on an analysis of the distribution and areal extent of mare deposits, both laterally as a function of areal density, distribution and spacing, and vertically as a function of elevation, and investigate implications for the relationship between crustal thickness and favorability of mare extrusion. O'Keefe J. A. The Earth as a Ringed Planet It is suggested that about 65 million years ago, there was a powerful volcanic eruption on the moon. Some of the ejecta from the explosion was trapped in orbit around the earth. Poincare had previously pointed out that, provided that the collisions between the particles were inelastic, a cloud of this kind would organize itself as a ring or a set of rings like those of Saturn. Tuesday, March 14, 1995 LUNAR EXPLORATION, REMOTE SENSING, SAMPLING, AND ANALYTICAL METHODS 6:30 p.m. LPI Meyer C. An Outline of the Jules Verne Discovery Mission to Explore the Lunar Mantle Via South Pole-Aitken Basin A Discovery-class mission is being designed to analyze a portion of the lunar mantle using specially instrumented rovers to explore an ultramafic 'island' of uplifted mantle within the South Pole-Aitken Basin on the backside of the Moon. The main purpose of this mission will be to check the validity of the 'magma ocean hypothesis' as the igneous process for the original chemical differentiation of the Moon. Planning for such a mission will focus our thinking about the whole Moon and lead to multiple working hypotheses for its internal differentiation. Lamb M. Ma K. B. Cooley R. Mackey D. Meng R. Chu C. W. Chu W. -K. Chen P. C. Wilson T. L. Protoype Superconducting Lunar Telescope Mount A prototype high-temperature superconducting (HTS) bearing mount for a light-weight (<20kg) lunar telescope has been developed in the laboratory. It consists of hybrid superconducting magnetic bearings (HSMB) which are a natural candidate for operation in the extended lunar nights (14.5 Earth-nights) where ambient temperatures are 84 degrees - l02 degrees K, easily within the range of existing HTS technology. These bearings are passive (requiring no active cryogenic thermal control), ceramically stable, light, and frictionless. Kaneko Y. Takizawa Y. Kawazoe T. Lunar Orbiting Observatory Mission The mission objectives of the Lunar Orbiting Observatory Mission (LOOM) are global characterization of the lunar surface and the investigation of the environment of the moon for both scientific and resource exploration benefits. The following disciplines will be addressed by the core instrument payload; geology and mineralogy (imaging VIS-IR spectrometer), geochemistry (gamma-ray spectrometer, fluorescent X-ray spectrometer), topography (laser altimeter), radiation environment (radiation monitor, heavy ion detector). In addition, landing probe is also considered as one of the main payload for both scientific and technological mission. Landing probe has two key objectives. The first is to demonstrate the soft landing technology that is essential for the next lunar mission such as roving vehicle and sample return mission. The second key objective is to perform delta VLBI measurement between the orbiter and the landing probe from the earth tracking stations. It enables to measure lunar gravimetry and libration in high accuracy. Johnson P. E. Pinet P. C. Chervrel S. D. Sabol D. Adams J. B. Multispectral Mixture Modeling of the Apollo 15 Landing Site We apply a new mixture modeling technique to telescopic CCD images of the Apollo 15 landing site. These images were made at ten wavelengths between 0.4 and 1.05 microns and have a spatial resolution of 500 meters. We have selected six endmembers and fit them, three at a time, to the image cube. An x-squared analysis is used to test goodness of fit. We find three mare endmembers and three highland endmembers. The highlands endmembers appear to be mainly related to the maturity of the regolith. Endmembers from the image are compared with a spectral reflectance library obtained from returned lunar soils and rocks. Hiesinger H. Jaumann R. Neukum G. Comparison of Unsampled Lunar Basalts Comparison of spectral characteristics of the Apollo landing sites with spectra of unsampled regions allows the identification and classification of materials similar to the materials exposed at the landing sites. Assuming a strong correlation between geochemistry and spectral behavior it is possible to expand our knowledge of the chemistry of the landing sites to unsampled regions. Starukhina L. V. Shkuratov Yu. G. Simulation of the Albedo-Color Diagrams for the Moon and Asteroids Albedo-color diagrams for the Moon and asteroids are simulated on the base of a theoretical model of light scattering by particulate surfaces. The model enables one to calculate the values of the imaginary part k of refractive index for the surface material from its reflectance. It has been shown that the width of the diagrams cannot be explained by variation of porosity of surface material even in its most wide range and can be accounted for by variations of k at 0.42 micron in the range +/-6% for the Moon and about 18% for S and C-asteroids. The best fit for the lunar diagram form is achieved by variation of the real part n of refractive index in the range +/-0.2 from its average value. Calvin W. M. Kieffer H. H. Soderblom L. A. Sunshine J. M. Lunar Silicate Absorptions Mapped by Galileo NIMS The Galileo spacecraft, carrying the Near Infrared Mapping Spectrometer (NIMS), flew by the Earth--Moon system for the second time in December of 1992. The highest resolution NIMS image of the Moon acquired during the flyby covers primarily the northern hemisphere of the lunar nearside with 204 wavelengths in the spectral region from 0.7 to 5.2~micrometers. Spatial resolutions are approximately 27 km/pixel in the final calibrated image cube. Although many spectral channels of the data set are saturated, as the instrument was designed for the lower flux levels at 5 AU, we have had good success in working with the unsaturated data to map the lunar mineralogy. Noise or jitter in the spectral dimension is introduced by the motion of the spacecraft between grating steps. We have designed simple algorithms that effectively remove this component, resulting in reasonably noise-free spectra. We have used these spectra to map the 1- and 2-micrometers silicate absorption features of the moon, including band center position, asymmetry, and depth. Gaddis L. McEwen A. Becker T. Recalibration of Galileo SSI Lunar Data From EM1: The Effects of Scattered Light Removal The radiometric calibration and systematic processing procedures for Galileo Solid State Imaging (SSI) multispectral data from the first Earth-Moon encounter (1990) have been updated. We applied these procedures to Lunmap14 (L14), a whole-disk imaging sequence of the Moon centered near Mare Orientale. L14 data, which were obtained at a 20 degree phase angle, have a spatial resolution of about 8 km/pixel, and a subspacecraft position of -20.7 degrees latitude and 98.3 degrees longitude. Data from six SSI filters were used in this analysis, and they were obtained at nominal wavelengths of 410 (VLT), 560 (GRN), 660 (RED), 756, 889, and 990 (about 1-micron) nm. A major element of this recalibration is the correction for scattered light, a low-level, wavelength-dependent brightness component. The present correction for scattered light does not account for "stray light" that is also captured by the camera from areas outside the field of view, so the results of this analysis are best shown with whole-disk data such as that of L14. After removal of scattered light, the 1-micron band depths of limb basalts show the greatest change: they are about 3% deeper than those of the previously calibrated data. These results resolve the need for the anomalously low-Fe lithologies of limb basalts as suggested by the shallow 1-micron band depths in the original analyses. Small mare ponds and limb basalts are now shown to have mafic mineral contents comparable to many basalts of the nearside. _ Benoit P. H. Sears D. W. G. The Use of Lunar Soil for Non-Critical Radiation Dosimetry for Robotic Rovers Radiation is arguably one of the most critical hazards in planning and management of long-term space missions and missions to essentially airless worlds like the Moon. Long- term radiation exposure not only presents well-known hazards to living organisms but also poses threats to the longevity and routine operation of equipment, especially to devices which rely heavily on solid-state electronics. Clearly any long-term mission to airless worlds will require extensive environmental radiation monitoring. Such monitoring to protect equipment, however, will be difficult, especially for robotic explorers in areas far from human bases. We here suggest that, in the case of the Moon and possibly other bodies, it is possible to use samples of unprocessed soil as dosimeters and measure the absorbed dose using thermoluminescence dosimetry. In addition to the utility of these measurements for mission planning, the data can be used to address other questions important from an exploration standpoint, such as the on site determination of the degree of maturity of the soil and its approximate composition. Sutter B. Hossner L. R. Ming D. W. Henninger D. L. Phosphorus Adsorption and Desorption Properties of Minnesota Basalt Lunar Simulant In anticipation of a permanent lunar base, research is being conducted to determine the feasibility of growing crops in lunar soil as a component of NASA's Advanced Life Support System (ALSS). In order to reduce resupply costs, astronauts will grow crops to produce oxygen, food, and water (plant transpiration) and recycle carbon dioxide. Therefore, it is important to understand how plant nutrients will interact with lunar soil. However, due to the high value and limited amounts of lunar soil available to researchers, lunar simulants are used in plant growth studies. Because phosphorus (P) is an important nutrient for plant growth, it is important to study its interactions with lunar simulants. The objective of this study was to understand P adsorption and releasing interactions from Minnesota basalt lunar simulant (MBLS). Understanding these relationships will assist in establishing P requirements for wheat growing in MBLS. Data from this study will assist in establishing methods for future lunar soil/P studies and will aid in predicting P interactions with lunar soil. Hayes S. A. Bustin R. M. Allen C. C. Reduction of Iron Oxides in Simulated Lunar Soil Using Carbon as the Reducing Agent One of the first industrial activities on the lunar base will probably be recovering oxygen from lunar minerals such as oxides. Iron(II) oxide occurring in ilmenite is common in some lunar soils. Both hydrogen and carbon monoxide have been used as reducing agents for the iron oxides in simulated lunar materials. Lunar ilmenite has also been reduced with hydrogen. The method described here involves the reduction of iron oxides in lunar simulant using carbon in the form of graphite as the reducing agent. Using MLS-1 simulated lunar soil, we have shown that the iron oxides in the soil can be reduced with finely powdered carbon (200 mesh, 99.9999% pure). Although a trace amount of carbon dioxide is produced during the reduction, the majority of the analyzed gas is carbon monoxide. Elemental iron is produced, as evidenced by its magnetic properties and by X-ray diffraction analysis. Samples analyzed by XRD showed no evidence of iron(II) or iron(III) forms but only elemental iron, indicating that the iron oxides had been completely reduced. Anderson W. W. Ahrens T. J. Scott R. F. Model for Penetration of Surficial Materials by a Coring Projectile One possible method for obtaining samples of surficial materials from planetary bodies is to propel a coring tube toward the surface, allowing the momentum of the tube to drive the penetration process. The mechanics of penetration of a tube are similar to those for a solid penetrator, but there are also significant differences. These differences arise because of the effects of friction against the inside walls and the sudden increase in the effective mass and frontal surface area of the corer that occurs if the corer should become completely filled with target material. We have developed a detailed model for the forces acting on a hollow, open-ended penetrator as it penetrates the surface of a target. This analysis can be used with penetration data for a target to estimate properties such as density and strength for the target, as well as being used to design a corer for optimum performance in a given target. Norman M. D. Laser Ablation ICPMS: A Powerful New Tool for Microbeam Trace Element Analysis A focused laser beam can be used to ablate small amounts of solid material which are then introduced into an inductively-coupled plasma mass spectrometer (icpms) for trace element and/or isotopic analysis. A laser ablation system has recently been installed in our laboratory and attached to a Perkin-Elmer ELAN 5100 icpms. Initial experiments on various operating conditions, and quantitative analyses of fused rock standards are reported. Despite problems with the fractionation of relatively volatile elements such as Pb and Zn during the course of a run, excellent results were obtained for a variety of more refractory elements, such as Sr, REE, Nb, U, and Th. Sutton S. R. Rivers M. L. GSECARS: A National Resource for Earth and Planetary Science Research at the Advanced Photon Source, Argonne National Laboratory The Advanced Photon Source (APS) at Argonne National Laboratory is entering its commissioning phase. Over the next several years, scientific consortia will be constructing beamlines and instrumentation to take advantage of this unique, very brilliant source of hard x-radiation. One such consortium is GeoSoilEnviroCARS (GSECARS), a multi-institutional group of experienced synchrotron users in the earth, planetary, soil and environmental sciences which will construct and operate beamlines at the APS as a national user facility available on a peer review basis. The GSECARS facility will have a major impact on the earth and planetary sciences by providing significant advances in analytical capabilities of high pressure crystallography, x-ray absorption spectroscopy, x-ray fluorescence microprobe and microtomography. Hillgren V. J. Determining the Oxygen Fugacity of Experimental Charges: Don't Believe Everything You Calculate Oxygen fugacity (fO2) is an important variable in many geochemical and petrological experiments, particularly metal-silicate partitioning experiments. However, it is often not possible to directly measure the fO2 during an experiment. In these cases, the oxygen fugacity is calculated after the fact based on a calibration of the iron-wustite (IW) buffer and the composition of the run products. The calculation involves several assumptions, and the purpose of this work was to compare calculated and measured fO2's and determine if the assumptions made are valid. I find that at oxygen fugacities below IW there is a constant offset between calculated and measured fO2's, and at higher oxygen fugacities, the offset steadily increases with increasing fO2. These results demonstrate that there is a need for a calibration curve for calculated oxygen fugacities. Wednesday, March 15, 1995 CHONDRITES 8:30 a.m. Room A Chair(s): M. K. Weisberg M. I. Petaev Bennett M. E. III* McSween H. Y. Jr. An Internal Heating Model for the L Chondrite Asteroidal Parent Body L chondrites contain a larger proportion of high-shock stage meteorites than either H- or LL-group ordinary chondrites. They also have a greater number of gas-poor samples than the other classes of ordinary chondrites. Both of these observations have been hypothesized to be the result of multiple impact events between the L chondrite parent and other asteroid bodies. Heterogeneous post-shock reheating might also explain the lack of coherence seen in L chondrite metallographic cooling rates and in 244Pu fission track data. Despite these observations, not all mineral thermometers appear to have been perturbed by secondary thermal processing. Two-pyroxene thermometry performed on a petrologic suite of 16 low- to high-shock stage L chondrites revealed a strong inverse correlation between petrologic type and peak metamorphic temperature. This correlation is consistent with the formation of an onion-shell internal structure for the L chondrite parent asteroid in which static thermal metamorphism produced the entire sequence of petrologic types within a concentrically zoned parent. Using the constraints of an onion-shell structure that was internally heated by the decay of 26Al, we performed a model calculation for the thermal evolution of the L chondrite parent asteroid. This model was based on equations derived from earlier thermal modeling, but was updated to include more recent information concerning the physical nature of the L chondrite parent. Parameters used to constrain the model and results of the model calculations are presented here. _ Shinotsuka K. Ebihara M.* A Metamorphic Sequence of Ordinary Chondrites Based on the Distribution of REE, Th and U Based on the metamorphic activity meteorites experienced, ordinary chondrites(OC) are divided in two major groups; less metamorphosed, unequilibrated OC (UOC) and highly metamorphosed, equilibrated OC (EOC). Because the degree of metamorphism varies more greatly in UOC than in EOC, UOC are further divided into subgroups, mainly based on several (petrological) parameters, such as an induced thermo-luminescence (TL) and a variation of Fe/(Fe + Mg) ratio in olivine or pyroxene. On the other hand, the bulk contents of volatile elements, such as carbon and noble gases, are known to be sensitive to the metamorphic activity, as indicated by their decrease with increase of petrologic types of OC. These variables also were proposed as (chemical) parameters usable for subclassification of UOC, but classifications deduced from these two different parameters are not always consistent. Petaev M. I.* Breakdown of Primary Na,Al-rich Pyroxene in an Al-rich Object from the Gorlovka H3-4 Chondrite: Evidence for Secondary Heating and Partial Melting An Al, Ca-rich chondrule in the Gorlovka H3-4 chondrite consists of spinel, fassaite and primary Na, Al-rich pyroxene phenocrysts embedded in a plagioclase (An50) ground mass. A secondary heating event has altered phenocrysts to sympletictic aggregates of olivine (Fa18), fassaite, Fe, Al, Cr-rich spinel, and ilmenite, enclosed in Na, Al, Si-rich glass. Krot A. N.* Zolensky M. E. Wasson J. T. Scott E. R. D. Mendybaev R. A. Ohsumi K. Parent-Body Alteration of Type 3 Ordinary Chondrites Various parent-body alteration processes including aqueous alteration, carbonation, oxidation and alkali metasomatism have been previously reported in several type 3 ordinary chondrites (OC). We found that some of the type 3 H, L and LL chondrites experienced carburization and oxidation of metallic Fe-Ni both in matrices and chondrules. The carburization and oxidation reactions, probably by a C-O-H containing fluid (e.g., 3Fe(s) + 2CO(g) = Fe3C(s) + CO2(g); 3Fe(s) +4CO2(g) = Fe3O4(s) +4CO(g)), resulted in formation of Fe-Ni carbides (cohenite, haxonite), magnetite, Ni-rich taenite and Co-rich kamacite. Carbon monoxide could have been released from the chondritic material or formed by carbon (e.g., organic compounds) gasification by water vapor (C(s) + H2O(g) = CO(g) + H2(g)) during shock heating or thermal metamorphism and transported to the surface through zones of high fluid permeability. Nehru C. E.* Weisberg M. K. Prinz M. Parnallee (LL 3.6) Heating Experiments: Implications for the Origin of Type 6 and 7 Chondrites Experiments at one atmosphere, MW oxygen fugacity on Parnallee, LL 3.6, show that it is not affected texturally or chemically at temperatures of up to 1150 degrees C with run durations on the order of a few days to over a week. However prolonged heating for 60 days at 1125 degrees C, resulted in textural recrystallization and partial melting. The residual chondrite retains a few "ghost" chondrule structures as well as relict minerals. All mineral compositions were completely homogenized as was all zoning. The texture of this residue is similar to that of a type 6 chondrite. The newly generated melt is richer in MgO, FeO, CaO and poorer in Al2O3 and Na2O compared to the feldspathic glass in the unequilibrated original meteorite. The total rock, residue plus melt, would resemble a type 7 chondrite after extensive annealing. Flynn G. J.* Thomas K. L. Bajt S. Sutton S. R. Klock W. Clark L. The Chemical Composition of Semarkona Matrix: Implications for Formation and Aqueous Alteration and a Comparison to Hydrated IDPs To investigate the formation and evolution of Semarkona matrix and its relationship to hydrated IDPs we determined the major and trace element abundances and mineralogies of 24 samples of Semarkona matrix, each about 1 microgram in mass. The volatile lithophile elements Na, K, and Rb are enriched to about 3.5xCI, and the Na, K, Rb, and Al contents are generally correlated, while the average volatile chalcophile and siderophile contents (Cu, Ga, Ge, Se, Zn, S, and Br) and carbon are depleted relative to CI. The Semarkona matrix element abundance pattern cannot be explained as simple dilution of material of CI composition by a refractory component, such as chondrule fragments. The addition of lithophiles, possibly during the aqueous alteration, is required. Semarkona matrix is both chemically and mineralogically distinct from the hydrated IDPs, thus previous suggestions of a genetic relationship between the two are not correct. Endress M.* Bischoff A. The Compositional Variability of Dolomites in CI Chondrites: Implications for Physico-Chemical Conditions of Circulating Fluids on the CI Parent Body Carbonate minerals are well known from carbonaceous chondrites which have experienced severe aqueous alteration. Especially CI chondrites contain a variety of carbonates, mainly dolomite and less abundant calcite and magnesite (breunnerite). These minerals are believed to have precipitated from aqueous solutions circulating on the CI parent body. Therefore, the understanding of carbonate formation is an important key in order to unravel the physico-chemical conditions of these solutions during the alteration event. For this study we determined the chemical composition of more than 150 dolomites in the CI chondrites Orgueil, Ivuna, and Alais by electron microprobe technique. In contrast to previous studies we observed not only significant compositional differences between dolomites within one CI chondrite (as reported earlier), but also differences between dolomites among CI chondrites. As a main result of this investigation we will demonstrate that the compositional variability of dolomites between different CIs is not controlled by variations of the Mn^2+ concentration (as it is the case for compositional differences between dolomites within one CI). Rather, the variability is due to variable temperatures and/or pH-, Eh-, and p(sub)CO2-conditions in the fluids. Cassen P.* On the Abundances of Moderately Volatile Elements in Meteorites The abundances of the moderately volatile elements (those which condense or evaporate in the temperature range 650 - 1350 K) in chondritic meteorites deviate from solar (or "cosmic", as defined by CI meteorite composition) in a manner that strongly suggests that they were determined primarily by volatility, independent of chemical affinity. Wasson and collaborators have long argued that the observed correlation of relative abundance with condensation temperature reflects a systematically selective process which favored the accretion of refractory material over volatile material from a nebula cooling from a hot initial state. This view is supported by the detailed examination of chondrules and matrix in carbonaceous meteorites and the contrast between meteorite trace element abundance patterns with those produced in heating experiments. However, the idea that the moderately volatile abundances reflect global condensation has not previously been quantitatively tested or used as a constraint on nebula evolution. We have therefore constructed models of the solar nebula designed specifically for addressing meteoritic data. Calculations so far indicate that the abundance patterns of the moderately volatile elements in chondritic meteorites can be produced naturally in an evolving nebula that cools with the reduction in opacity associated with the accumulation of meteoritic bodies. Dodd R. T.* Wolf S. F. Wang M. S. Lipschutz M. E. Streams of H4-6 Chondrite Falls: A Progress Report In an earlier work [Dodd R. T. et al.], we reported that, from their circumstances of fall, a group of 17 H chondrites ("H Cluster 1") which fall in May over the 40-year period 1855-1895 seem to have a coorbital origin as members of a putative meteoroid stream. Contents of thermally labile trace elements determined by RNAA and treated by multivariate statistical techniques in 13 members of H Cluster 1 prove to be readily distinguishable compositionally from 45 other H4-6 chondrite falls [Dodd R. T. et al]. Thus, a group of meteorites distinguishable by one criterion - similar fall dynamics - proves to be distinguishable from an otherwise similar suite of falls by another criterion - contents of labile trace elements, suggesting a distinguishable, genetic thermal history. Here, we report our progress on an analogous study of additional H4-6 chondrite falls that may sample as many as 4 other putative meteoroid streams. Schultz L.* Weber H. W. Noble Gases in Chondrites from a Proposed Meteoroid Stream Concentrations and isotopic compositions of noble gases have been measured in ten H-chondrites that belong to a group of meteorite falls proposed to be members of a co-orbital meteoroid stream. Exposure ages between about 4 and 70 Ma are found and three chondrites exhibit loss of cosmogenic 3He. This demonstrates a rather continuos loss of meteoroids from the parent object as well as different perihelia of their orbits. Hsu W.* Crozaz G. Negative Sm, Eu, and Yb Anomalies in Enstatite from UECs: The Results of Evaporation or Igneous Fractionation? Enstatites in unequilibrated enstatite chondrites (UECs) exhibit various REE patterns, ranging from relatively flat to extremely HREE-enriched (the latter being characteristic of igneous fractionation). These patterns tend to be mirror images of those observed in oldhamite. The abundances of REEs also show large variations from one enstatite grain to another; these have been found to be associated with the amount of melt inclusions trapped in the enstatite crystals. We have also found that some enstatites from UECs have negative Eu, Yb and Sm anomalies (listed here in the order of decreasing occurrence) and suggested that the REE depletions might be due to evaporative loss during chondrule formation. In order to test this suggestion, we report here Mg and Si isotopic measurements in enstatite from UECs. As will be shown below, the isotopic data do not support an evaporative origin for the REE depletions in enstatite. We considered whether the REE depletions in enstatite might be due to igneous fractionation under the highly reducing conditions at which UECs formed. Weisberg M. K.* Boesenberg J. S. Kozhushko G. Prinz M. Clayton R. N. Mayeda T. K. EH3 and EL3 Chondrites: A Petrologic-Oxygen Isotopic Study A petrologic-oxygen isotopic study of 15 E3 chondrites (10 EH3,4 EL3, and LEW87223) shows that all have sharply defined chondrules, easily recognizable olivine (> 1 vol.%) with a range of FeO and minor element compositions, clear or devitrified glassy chondrule mesostases, and some FeO-bearing (Fs>2) enstatite. Additionally, kamacite in EH3 has lower Ni than that in EH4-5 chondrites, and EL3 kamacite has lower Si than that in EL6 chondrites. EH3 chondrites differ from EL3 chondrites in having higher modal abundances of sulfides, lower abundances of enstatite, and more Si-rich and Ni-poor metal compositions. LEW87223 differs from both EH3 and EL3 chondrites in having the highest abundance of metal and FeO-rich enstatite, and the lowest abundance of sulfides. It is the first member of a new kind of E3, more oxidized than EH or EL chondrites. The increase in sulfide abundance and degree of reduction from LEW87223-EL3-EH3 suggests that sulfidation reactions in the nebula were major processes in establishing the relative oxidation states of enstatite chondrites. EH3 components formed under conditions of lower fO2 and higher fS2 than those of the EL3 and LEW87223 chondrites. If EH4-5 and EL6 chondrites evolved from EH3 and EL3 chondrites, respectively, their parent bodies experienced different metamorphic evolutions. For EL6 chondrites to have evolved from EL3 chondrites reactions in which Si was reduced from silicates and incorporated into the metal phase must have occurred, whereas for EH5 chondrites to have evolved from EH3 chondrites reactions in which Ni was redistributed from perryite into kamacite took place. Oxygen isotopic compositions of EH3 and EL3 chondrites plot along the terrestrial mass fractionation line at relatively lower 18O and 17O compositions than EH5 and EL6 chondrites, indicating addition of heavier oxygen during metamorphism or the initial compositions of the equilibrated chondrites had heavier oxygen than EH3 and EL3 chondrites. Rubin A. E.* Scott E. R. D. Keil K. Shock Metamorphism of Enstatite Chondrites Stoffler et al. developed a shock metamorphism classification scheme for extraterrestrial rocks based on shock effects in olivine and plagioclase; the scheme was applied to ordinary and carbonaceous chondrites [Stoffler D. et al. and Scott E. R. D. et al.]. We have extended this scheme to account for shock effects in orthopyroxene and applied it to 43 enstatite chondrites. Orthopyroxene exhibits the following sequence of progressive shock effects: shock stage S1, sharp optical extinction; S2, undulose extinction; S3, development of polysynthetically twinned clinoenstatite lamellae parallel to (100); S4, weak mosaicism; S5, strong mosaicism. Most EH chondrites have been moderately shocked; several (e.g., Abee, RKPA80259) have been partly to extensively melted by impact events. EL3 chondrites exhibit foliations and are moderately to strongly shocked (S4-S5). All EL6 chondrites are S2, including Atlanta and Blithfield (which contain cm-size kamacite veins) and Jajh deh Kot Lalu (which contains a 1.6-cm-long oldhamite-rich melt vein). Because shock veins in ordinary chondrites typically form only at higher shock stages, it is plausible that these EL6 chondrites were initially shocked to stage S3-S4 and subsequently annealed. Alternatively, these rocks may have been shocked to S2 levels when they were already hot., it is plausible that these EL6 chondrites were initially shocked to stage S3-S4 and subsequently annealed. Alternatively, these rocks may have been shocked to S2 levels when they were already hot. Wednesday, March 15, 1995 METEORITES AND MARS I: SURFACE, VOLATILES, AND ATMOSPHERE 8:30 a.m. Room B Chair(s): A. H. Treiman V. C. Gulick McCoy T. J.* Wadhwa M. Keil K. Zagami: Another New Lithology and a Complex Near-Surface Magmatic History Zagami contains a dark, mottled lithology (DML) which occupies 20% of the rock. Major, minor, and trace element mineral compositions indicate that this lithology represents an intermediate stage in the crystallization history between the widely-known normal Zagami lithology and the highly-evolved Zagami DN lithology. DML also includes pockets of Zagami DN, pyroxene clumps composed of high-FeO pyroxenes, and shock-melt pockets. The heterogeneity within Zagami suggests that it did not form in a homogeneous, thick lava flow. Concentration of evolved melt may have involved pyroxene clumping, crystal settling, flow differentiation and melt migration, possibly in thin injection units within a thick magma body. Friedman R. C.* Taylor G. J. Treiman A. Processes in Thick Lava Flows: Nakhlites (Mars) and Theo's Flow (Ontario, Earth) Nakhlites are medium-grained pyroxenites that may have erupted as lava flows on Mars. To elucidate the crystallization environment of the nakhlites, we have begun to examine a terrestrial analog, Theo's flow.This 120-m thick, Archean lava flow contains a 54-m thick pyroxenite zone which bears a striking resemblance to the nakhlite meteorites in texture, with large euhedral augite grains and a fine-grained mesostasis.To quantify these textural similarities, we used crystal size distribution(CSD) analysis on three nakhlite sections, on a sample of the pyroxenitic zone of Theo's flow, and on two thinner pyroxenitic flows associated with Theo's. The similar numeric results support using Theo's flow as an analog for the nakhlites. Calculations made from CSD data of a 5-m thick pyroxenitic flow gave a pyroxene growth rate of 6.0x10^-10 cm/s, in agreement with estimates for other silicates. Applying this growth rate to the thicker flows and nakhlites, we found that all had experienced steady-state growth periods of about 0.3 yr. Subsequent accumulation and liquid phase sintering, or Ostwald ripening, must then have occurred to produce the final textures observed. Griffith L. L.* Shock E. L. Arvidson R. E. Calculating the Effects of Hydrothermal Alteration on Mars Hydrothermal systems on Mars could have pervasively altered the crust on a regional basis, greatly modified the volatile budget, and generated channels, chaotic terrain, and other surface features. An example of the relevance of hydrothermal systems is the observation that hydrothermal carbonates provide a plausible reservoir for carbon dioxide on Mars. Our model results indicate that hydrothermal carbonate formation is likely under a wide range of possible Martian conditions. Subsurface deposition of carbonate would provide an explanation for the apparent lack of surface expression of carbonates on Mars. Locating and identifying surface manifestations of hydrothermal systems may be more easily accompllshed by looking for a general alteration assemblage, such as quartz-chlorite-tremolite-albite-carbonate-magnetite-augite, rather than restricting a search specifically to carbonates. Swindle T. D.* Burkland M. K. Grier J. A. Lindstrom D. L. Treiman A. H. Noble Gas Analysis and INAA of Aqueous Alteration Products from the Lafayette Meteorite: Liquid Water on Mars <350 Ma Ago Noble gas and INAA analyses of aqueous alteration products from the Lafayette meteorite confirm that the aqueous alteration was pre-terrestrial,and are consistent with the aqueous alteration products being the host of elementally-fractionated martian atmospheric noble gas. Furthermore, K-Arages of 100-300 Ma suggest that aqueous alteration, presumably on the surface of Mars, occurred in that time range. Schaefer M. W.* Paleosols on Mars: Another Look at the Viking Lander Measurements The surface of Mars may be thought of as a paleosol -- a fossil soil -- and as such it reflects the weathering processes that have occurred on Mars throughout its long history. The chemical composition of the Martian paleosols, as measured by the Viking landers, shows the effects of aqueous alteration under conditions of only moderate oxidation. Leshin L. A.* Epstein S. Stolper E. M. The Abundance and D/H of Water Dissolved in EETA 79001 Shocked Glass and Crystalline Host The striking similarity between both the abundance and isotopic composition of noble gases and nitrogen found in shock-produced glass (known as lithology C) in shergottite EETA 79001 and the current martian atmosphere provides the best evidence that the SNC meteorites originated on the planet Mars. The abundance and isotopic composition of CO2 in the glass isalso consistent with this theory. The D/H of water in the currentmartian atmosphere is distinctive with a deltaD(sub)SMOW value of ~+4000. In this work, we report the abundance and D/H of dissolved water in the EETA 79001 glass, as well as the D/H and water content of the crystalline basaltic EETA 79001 host rock (known as lithology A). Stephens S. K.* Stevenson D. J. Rossman G. R. Carbonates on Mars: Experimental Results Experiments exposing crushed basalt glass to carbon dioxide and water vapor at conditions similar to those at the past and present surface of Mars (T = 245-300 K,P(CO2) = 7 mb-1 bar, and P(H2O) equivalent to the vapor pressure of H2O at this T) consistently yield continued uptake of CO2 for t ~ 0.5-100 days. Direct correlation of these results with absorption strengths near 7 microns in reflectance spectra of experimental samples supports the conclusion that carbonates can form on Mars. Rates of carbonate formation correspond to 0.01-1 monolayer per unit of log10 t and are faster for greater H2O abundances. The logarithmic rate is consistent with declining accessibility of chemisorption sites and allows a finite limit to be put on total carbonate formed over geologic time. For a global layer of basalt powder, only a high specific surface area (>1 m2/g),a thick regolith (>100 m), or plentiful H2O vapor (equivalent to films >5 monolayers thick) allow total P(CO2) stored as carbonate to exceed ~10-100 mb. Unless very wet conditions once existed, or unless diffusive transport of CO2 and H2O through a rind becomes significant over geologic timescales, carbonate formation probably could not have removed an early, dense CO2 atmosphere. Porcelli D.* Wasserburg G. J. Origin of Rare Gases in the Terrestrial Atmosphere and Planetary Comparisons We have previously presented a model for the distribution and transport of rare gases within the Earth which explains the available observational data for mantle He. Ne, Ar, and Xe isotope compositions and provides specific predictions regarding the rare gas isotopic compositions of the lower mantle, interactions between rare gas reservoirs, and mantle rare gas concentrations. Here we discuss the constraints on the acquisition and evolution of atmospheric rare gases that are derived from this model, and show that these constraints are compatible with planetary evolution processes put forth by others. Earlt losses of rare gases has occurred on both the Earth and Mars, and a late atmosphere was added to the Earth by accretion of gas-rich materials. Both atmospheres have suffered loss of rare gases, causing elemental and isotopic fractionation, as well as small additions by outgassing of the planetary interior. These processes are reflected in the relationship between the atmospheric and interior rare gases of both planets. Jakosky B. M.* Reedy R. C. Masarik J. 14C Measurements of the Martian Atmosphere: An Indication of Surface-Atmosphere Exchange of CO2 Mars atmospheric C02 can exchange between the atmosphere and non-atmospheric reservoirs in the martian polar caps or regolith. Production of 14C in the martian atmosphere and near-surface regolith due to cosmic-ray bombardment should give rise to a steady-state population of 14C in C02. Exchange of atmospheric C02 with C02 in the regolith, however, will decrease the amount of l4C02 in the atmosphere; this results because the available 14C is spread over a larger total volume of CO2. As a result, a measurement of the 14Co2 mixing ratio in the atmosphere would be a direct indication of the size of the exchangeable reservoir of non-atmospheric C02. As such, it would have important implications for understanding martian volatiles and recent climate change. We estimate the production and steady-state population of 14C in the martian atmosphere, and show that a returned sample of pristine martian atmosphere would provide sufficient CO2 to allow a laboratory determination of the 14C02 abundance. Craddock R. A.* Greeley R. Estimates of the Amount and Timing of Gases Released into the Martian Atmosphere from Volcanic Eruptions It is generally believed that the atmospheres of the terrestrial planets formed during or soon after accretion. Models describing the formation of the martian protoatmosphere are based on estimates of atmophilic species measured by the Viking landers or taken from the SNC meteorites. These models attempt to explain the atmospheric abundances and the apparent isotopic fractionation that has occurred by a variety of processes ranging from total volatile gain or loss (e.g., impact erosion) to mechanisms which fractionate either elements or both element and isotopes (e.g., adsorption of nebular gases). However, an important, often missing component in these models is the amount of volatiles gained by planetary outgassing. Volcanism has been a dominant geologic process on Mars throughout much of its history, but only recently have estimates been made of the total volume of volcanic material erupted through time. We have modeled the total amount of outgassing that may have occurred from volcanic eruptions based on a weight percent of water of 0.5 to 1%. Gulick V. C.* Tyler D. Haberle R. M. McKay C. P. Effects and Lifetime of Ocean Induced CO2 Pulses on Mars: Implications for Fluvial Valley Formation Baker et al. proposed that a variety of anomalous geomorphological features on Mars can be explained by episodic ocean formation triggered by extensive Tharsis volcanism and the associated massive hydrothermal system. The rapid outpourings of ground water from outflow channel discharges are assumed to release CO2 dissolved in the ground water, adsorbed on the regolith and resident in the north polar cap into the atmosphere. These pulses of CO2 relatively late in Mars' climatic history are suggested to be responsible for short greenhouse periods that Baker et al. believe to be evident in the geologic record. Here we investigate the climatic effect of instantaneous pulses of CO2 added to the martian atmosphere at 1 and 2Ga. We find that such pulses can indeed produce short periods of modest greenhouse warming and significantly increase the saturation vapor pressure. Therefore, significantly greater snowfall at higher elevations is possible than under current climatic conditions. This snowfall, in conjunction with melting of snow by localized geothermal heating, might be responsible for late periods of fluvial erosion and glacial activity. Fanale F. P.* Postawko S. E. Mass Loading and the Early Mars Greenhouse: When a Pollutant Becomes the Environment Attempts to model a greenhouse condition on early Mars are fostered by the need to explain clearly fluid dissection of the most ancient terrain despite the presumption that the sun's luminosity was probably significantly lower than today. Such attempts have typically relied on a greenhouse effect produced by a massive (2-4 bar) carbon dioxide atmosphere. A possible fatal flaw in these scenarios is the expectation that such a situation would produce lower temperatures than otherwise owing to carbon dioxide clouds. Altematively trace constituents such as S02 and NH3 (which are intrinsically much better greenhouse gases than ) have been suggested to augment such a greenhouse effect. Perhaps the most prominent model is that suggested by Postawko and Kuhn who pointed out that S02 in a mixing ratio of 10-3 could augment a CO2 greenhouse in a major way. This approach also encounters a serious problem in that the mean residence time of an S02 molecule in the atmospheres of both the present Earth and Mars is extremely short - perhaps a year or less. Thus mixing ratios even as low as 10-3 for S02 could never be approached. Such an evaluation assumes that the mean residence time (M.R.T.) of S02 on early Mars would have been comparably short. That assumption is incorrect. Wednesday, March 15, 1995 METEORITES FROM THE MOON 8:30 a.m. Room C Chair(s): R. P. Harvey Gladman B.* Burns J. A. Lee P. Delivery of Lunar Meteorites to Earth As more and more meteorites from the Moon and Mars are discovered, it is increasingly clear that impact fragments can escape from large bodies more easily than previously believed. For the case of the lunar meteorites Warren has concluded that few, if any, common source craters are plausible; thus at least a half-dozen separate lunar launch events must have occurred in the last million years. Since the terrestrial inventory of lunar meteorites is undoubtedly incomplete, one can argue for a reasonably large injected flux of lunar material into the near-Earth environs. We study the transfer dynamics by means of numerical simulations. We attempt to construct a self-consistent model of lunar meteorite delivery that takes into account the impactor flux, the delivery efficiency, and the statistics of meteorite finds. Kring D. A.* Hill D. H. Boynton W. V. The Geochemistry of a New Lunar Meteorite, QUE93069, a Breccia with Highland Affinities The 1993 ANSMET field team collected a 21.4 g fragment of an ovoid stone, partially covered with a gray-green frothy fusion crust, that has since been classified as an anorthositic breccia or microbreccia of presumed lunar origin. Geochemical analyses of a split of this meteorite indicate that it is indeed of lunar origin. The bulk Fe/Mn ratio in QUE93069 is ~75,which is similar to the values of ~75 +-5 in Apollo samples and in previously identified lunar meteorites, and yet distinct from those in all other groups of meteoritic material. Similarly, the sample has bulk K/La (~90), K/U (~2000), and Fe/Sc (~4600) ratios that are characteristic of lunar samples and distinct from those in other groups of meteorites. The K/U ratio also indicates that QUE93069 is not a terrestrial meteorite excavated from an anorthositic crustal region. Lindstrom M. M.* Mittlefehldt D. W. Morris R. V. Martinez R. R. Wentworth S. J. QUE93069, A More Mature Regolith Breccia for the Apollo 25th Anniversary QUE93069 is the 10th distinct lunar meteorite and the Apollo 11 25th anniversary lunar sample. It is a highlands regolith breccia that is more mature than the other highlands lunar meteorites. Based on partial analyses, its composition is similar to, but distinct from, that of MAC88104/5. Similarities in lithophile elements suggest similar protoliths, while differences in siderophile elements and IslFeo require distinct regolith histories. Warren P. H.* Kallemeyn G. W. QUE93069: A Lunar Meteorite Rich in HASP Glasses We report results from a study of the petrology and geochemistry of the 21.4-g lunar meteorite QUE93069. Our petrographic studies are almost complete, and show the rock to be a relatively mature highland regolith breccia, rich in glassy spherules and spherule fragments. These glasses include a large proportion with HASP (high-alumina, silica-poor) compositional affinity, two "ultra Mg" glasses, and one glass of medium-Ti mare-basaltic composition. Mare basalt is also rare among the breccia clasts, which are mostly very fine grained anothositic types, of impact melt or polymict-granultic origin. Like most other lunar-meteorite regolith breccias (LMRBs), this rock is extraordinarily coherent (resistant to crushing), compared to Apollo regolith breccias. Nishiizumi K.* Caffee M. W. Finkel R. C. Reedy R. C. Exposure History of Lunar Meteorite QUE93069 Most lunar meteorites have complex cosmic ray exposure histories. They have 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 transportfrom the moon to the earth. As is the case for many other meteorites found in Antarctica, their terrestrial ages can be long. Measurement of cosmogenic radio- and stable nuclides can constrain these ages and help to unravel the complex histories of these objects. To unravel the complex history of these objects requires measurement of four or more cosmogenic nuclides in the same sample. We report here cosmogenic ^36Cl (half-life = 0.30 My), ^26Al (0.705My), and ^10Be (1.5 My) results for a new lunar meteorite QUE93069. Wednesday, March 15, 1995 LUNAR SURFACE PROCESSES 9:45 a.m. Room C Chair(s): A. Basu L. A. Taylor Taylor L. A.* Rossman G. R. Qi Q. Where Has All the Lunar Water Gone? One of the first significant findings from study of the lunar rocks was the apparent absence of any minerals containing water. Apatite and amphibole were reported, but the hydroxyl sites were occupied by chlorine and fluorine. Estep et al. studied various lunar minerals, basalts, and glasses by infrared and raman vibrational spectroscopy and found no evidence for hydroxyl (= water), further verifying the anhydrous nature of lunar samples. Subsequently, the mineral akaganeite, FeOOH, was found in many lunar rocks, especially 66095 and other Apollo 16 rocks. But this was conclusively shown to be due to terrestrial contamination. Effectively, the extremely deliquescent mineral lawrencite, FeCl(sub)2, underwent oxyhydration to form the "rust" when exposed to the Earth's atmosphere. However, water has been observed to evolve from all samples of lunar soil upon heating, even up to 400 degrees C. This water is largely a contaminant; however, some of it is possibly lunar. Friedman et al., Zeller et al., Gibson and Moore, and others felt that small amounts of water could be synthesized on the outer few 100 Angstrom of soil particles from solar-wind protons. [Note: protons are effectively hydrogen nucleii.] This still remains the general consensus, but this remains only a theory without positive proof. Kim J. S. Kim Y. Marti K. Kerridge J. F.* The Isotopic Signature of Recently Implanted Solar Nitrogen in 68815 Analysis of N, Xe and Ne in a surface fragment of 68815 has yielded (1) a value of 38 per mil for delta^15 in the recent solar radiation, (2) a trapped N/Xe ratio somewhat lower than that in typical regolith but still enhanced relative to the solar value, and (3) a N surface concentration corresponding to an exposure duration of about 100,000 yr. These findings lead to the following conclusions: (a) The long-term variation in delta^15 is more complex than previously thought; (b) Any mixing-model interpretation of the long-term trend must invoke a minimum of three isotopically distinct components; (c) There is still no clear evidence for major nonsolar N in the lunar regolith; (d) Erosion of the lunar surface limits the lifetime of the solar-wind-implanted layer to about 100,000 yr. Becker R. H.* An Evaluation of Possible Explanations for 'Excess' Nitrogen in the Lunar Regolith It has been known for some time that (a) N/36Ar ratios in lunar regolith samples are a factor of 10 greater than "solar" values, and (b) that nitrogen isotopic ratios in the lunar regolith vary by at least 30%. It has also long been known that (c) C/N ratios in regolith materials are well correlated, with a lunar C/N ratio about 0.5 times solar. Mechanisms for producing the isotopic variation in the sun are lacking, and models that predict fractionations between solar and solar wind elemental ratios, such as that based on first ionization potentials, do not provide the observed N/36Ar and C/36Ar enhancements (10- and 5-fold, respectively). I attempt here to evaluate other possible models for producing these nitrogen effects, in the light of 3 other observations on lunar materials. Wieler R.* Baur H. Solar Xe, Kr, and Ar in Lunar Samples; Do They Get Fractionated in the Sun or on the Moon? Element abundances in solar energetic particles differ from bulk solar or photospheric values as a function of the first ionisation potential (FIP) or a related parameter, elements with FIP less than or equal to 10eV being 4-5 times overabundant. Elements close to the 10eV step are particularly important to learn more about this separation. We concluded that lunar samples retain solar Xe (FIP=12.1eV), Kr (14.0eV) and Ar (15.8eV) unaltered relative to the abundances in the solar corpuscular radiation, a conclusion based mainly on ilmenites analysed by the closed system etch technique. We deduced that Xe in the solar corpuscular radiation is enhanced relative to Kr and Ar and that the Xe-Kr fractionation varied with time. Here we present additional experiments confirming the ilmenite results and discuss other studies in the light of our new data. Geiss and coworkers showed that the Xe overabundance deduced here is expected, if the first ionisation time governs the fractionation at the source. Reedy R. C.* Masarik J. Cosmogenic-Nuclide Depth Profiles in Lunar Rocks Depth-dependent production rates of ^10Be, ^21Ne, ^26Al, ^36Cl and ^53Mn in 2-cm vertical cores in hemispherical lunar rocks of various radii on the Moon's surface were calculated using evaluated cross sections and the LAHET Code System for particle fluxes. Similar calculations for meteorites and lunar soil cores have given good agreement with measurements. The nature of the reactions making a nuclide affects changes in production rates for rock geometries relative to a slab, but production rates for rocks with radii less than about 15 cm tended to be lower, those with radii of about 20-100 cm tend to be higher, and those with radii greater than about 1 meter are similar to those for a slab. The shapes of these profiles near the very surface are not very different from those for a slab. Thus the question of where is the solar-proton-produced ^10Be expected in lunar rocks is not answered by explicitly considering the geometry of a lunar rock on the Moon's surface. Nishiizumi K.* Kohl C. P. Arnold J. R. Finkel R. C. Caffee M. W. Masarik J. Reedy R. C. Final Results of Cosmogenic Nuclides in Lunar Rock 64455 Nineteen samples were ground from the glass-coated lunar rock 64455,82 with an average depth resolution of 50 um. We report here the final detailed depth profiles of cosmogenic nuclides ^10Be (half-life = 1.5 My), ^26Al (0.71My), and ^36Cl (0.30 My) using AMS (accelerator mass spectrometry). Results show clear evidence of SCR (solar cosmic ray) effects for both ^26Al and ^36Cl. The flat ^10Be depth profile and its activity level are consistent with a 2 My exposure history for the rock. These highest SCR profiles, after exposure age correction, indicate that rock 64455 had a very low erosion rate and is a nearly ideal sample for the investigation of SCR history. Symes S. J. K.* Benoit P. H. Sears D. W. G. McKay D. S. The Luminescing Lunar Regolith We have examined three thin sections from the Apollo 16 double drive tube 60009/60010 using cathodoluminescence (CL) petrography. The observed CL intensity, as assessed visually from mosaics of photographic prints covering the entire sections, is governed primarily by maturity. Mature regolith from 4 cm depth has relatively low CL intensity, while immature regolith from 53 cm has relatively high CL intensity and submature regolith from 29 cm depth displays intermediate intensities. We suggest that these differences reflect progressive destruction of the CL phosphor (feldspar) by regolith working and is consistent with our earlier suggestion that thermoluminescence sensitivity variations provide a new quantitative means of evaluating regolith maturity. We found that unshocked monomineralic feldspar grains exhibit green CL, possibly due to Fe^2+ acting as an activator, while recrystallized feldspar in lithic clasts exhibits blue CL, which may result from higher sodium concentrations. Notably, not all recrystallized feldspar in clasts exhibits blue CL. Our previous thermoluminescence (TL) results indicated that the most "primitive" lunar highland material was found in immature soil samples. Our present data suggest that the primitive component is monomineralic feldspar grains which dominate the TL (and CL) signal in immature soil samples. The blue-luminescing clasts, which are larger and thus more resistant to destruction during regolith gardening, dominate the TL signal in mature soils. Basu A.* McKay D. S. Morris R. V. Wentworth S. Variations of the Ferromagnetic Resonance Index (IS/FeO) of Individual Aggultinates The ferromagnetic index Is/FeO has long been used as a relative measure of the maturity of lunar soil samples. This index is the relative concentration of nanophase metallic iron normalized to total iron content calculated as FeO. Furthermore, for most soil and core samples in the Apollo and Luna collections, there exists a strong positive correlation between the abundance of agglutinates in the sample and the sample's Is/FeO. Both indices were shown to relate to the degree of micrometeorite reworking or maturity of the soil samples. Variation of either of these parameters at some Apollo sites exceeds two orders of magnitude. Wednesday, March 15, 1995 CRATERS ON THE EARTH WITH DIAMONDS 8:30 a.m. Room D Chair(s): D. J. Roddy W. U. Reimold Roddy D. J.* Shoemaker E. M. Anderson R. R. Manson Impact Structure Research Program: Summary Through January 1995 The Manson structure is a 74-m.y.-old eroded impact crater buried beneath glacial drift in western Iowa. This feature, about 36.5 km in diameter, was interpreted as an ancient "cryptovolcanic" feature, but identification of shock metamorphic features in core drilled in the central uplift led to the suggestion that Manson was instead formed by a large impact. More recently, preliminary isotopic age studies indicate that the Manson event may have occurred at the time of the K-T boundary, i.e., 65 m.y. ago, suggesting that the Manson event could have contributed to the K-T impact event and its mass extinctions. In 1991, the Manson Impact Structure (MIS) Research Program was initiated as a multi-year study to produce an integrated view of the cratering event, define its relation to the K-T boundary, and determine the effects on the environment. These integrated studies, now reaching completion, confirm that the Manson structure is indeed an impact site but that the impact occurred prior to K-T boundary time by about 9 million years. More specifically, new isotopic age studies based on 40Ar/39Ar dating show (1) an impact age of 73.8 + 0.3 Ma (relative to 513.9 Ma for MMhb-1 hornblende standard) or 74.9 + 0.3 Ma (relative to 520.4 Ma for MMhb-1 hornblende standard), and (2) that far-field ejecta from the Manson crater are still preserved in Upper Cretaceous Pierre Shale over 240-500 km northwest of the impact site. Today, the well-preserved Manson structure offers significant insight into large-body impact cratering processes and effects. Anderson R. R.* Witzke B. J. Hammond R. H. Watkins D. K. Preliminary Investigation of Shocked Grains (Manson Ejecta?), Paleontology, and Depositional Features in the Crow Creek Member, Pierre Shale (Upper Cretaceous) of Southeastern South Dakota and Northeastern Nebraska, and Its Effect on ... The Crow Creek Member is one of several marl units recognized within the Upper Cretaceous Pierre Shale of eastem South Dakota and northeastern Nebraska. The member has been interpreted as a basal transgressive unit of the Bearpaw Cycle, one of several Upper Cretaceous transgressive-regressive cycles. The recent significant discovery of impact shock-metamorphosed mineral grains in the Crow Creek and the new 40Ar/39Ar age (73.8 Ma) for the Manson Impact Structure (northwest Iowa) led to the suggestion that the two were coeval. Shocked grains in the Crow Creek were interpreted as distal impact ejecta derived form the Manson Structure, and it was further suggested that the member displayed evidence of impact-induced tsunami sedimentation triggered by the Manson impact, a tsunami that may account for certain regional ullconformities in the Western Interior Basin. Deposition of the Crow Creek Member and Manson impact are constrained within the span of the late Campanian ammonite zones of Exiteloceras jenneyi or Didymoceras stevensoni in the North American Western Interior Basin. Any biotic crises or extinction events that occurred during the range of these zones may be evident from the extensive published paleontologic database for the Western Interior (both marine and nonmarine taxa). A review of these zones in the Western Interior basin indicates that the Manson impact event produced few recognizable effects on the terrestrial or marine biota in the interior of North America. The evolving lineages in the region, for the most part, continued unimpeded by any catastrophic ecologic changes that may have been brought about by the Manson impact event. Nevertheless, there is some evidence to suggest that portions of the biota in the Western Interior were significantly affected by the impact event, primarily certain large vertebrate animals, both terrestrial and marine. Higher trophic levels may have been more vulnerable to ecologic disruptions created by a regional catastrophic event, but such speculation requires further testing. Although impact events have been of considerable interest in understanding global extinction events, it is likely that the consequences of a Manson-sized impact would largely be evident at a regional, not global, scale. Koeberl C.* Reimold W. U. Brandt D. The Newporte Impact Structure, North Dakota: Shock Metamorphism in Breccias The 3-km-diameter Newporte structure is situated about 2 km south of the U.S.-Canada border in North Dakota. It is covered with about 3 km of rock formations and has first been studied in the course of hydrocarbon exploration in the late 1970s, when it was found to contain significant oil reserves.The structure is situated in Precambrian crystalline bedrocks, which are highly fractured and brecciated. Detailed petrographic and geochemical studies of rock samples from three drill holes show the presence of three types of breccias, a mainly granite-derived breccia, a mainly metasediment-derived breccia, and a mixed breccia type. Quartz grains (as well as a few rare feldspar grains) from all three breccia types show planar deformation features (PDFs) with up to five sets per grain. Orientation measurements show predominantly {10?3} (omega) orientations, which are characteristic of shock metamorphism. These observations confirm the impact origin of the Newporte structure. Schmidt G.* Palme H. Kratz K.-L. The Fractionation of Os, Re, Ir, Ru, Rh, Pd and Au in Impact Melts from European Impact Craters (Saaksjarvi, Mien and Dellen) and the Determination of the Meteoritic Components Lunar highland rocks contain an excess of siderophile elements, which has been attributed to meteoritic influx after the formation of the lunar crust. Siderophile element enrichment has subsequently become a standard method for the identification of terrestrial impact craters. Janssens et al., Grieve and Palme et al. have shown the dominant role of impact melt as the main carrier of meteoritic material at large terrestrial impact craters. Coherent melt sheets always have the highest concentrations of siderophile elements. This has been observed at Clearwater French B. M.* Koeberl C. Gilmour I. Shirey S. B. Dons J. A. Naterstad J. Petrology and Geochemistry of Target Rocks and Breccias from the Gardnos Impact Structure, Norway The Gardnos structure in south-central Norway (60 degrees 39' N, 09 degrees 00' E), an area with unusual breccias about 5 km across, was recently identified as a deeply eroded impact structure about 650 m.y. old, formed in high-grade Precambrian metamorphic rocks at least 1.7 Ga old. Surface exposures along the Dokkelvi River (SW part of the structure) and a 400-m core drilled atBranden (NW part of structure), demonstrate the presence of thicksections of various impact-related rocks, including allochthonous monomict granitic breccia previously termed "GardnosBreccia" (>200 m; Branden core) beneath the crater floor, melt-bearing crater-fill breccias (50 m), and post-impact clastic sediments (>150 m). The melt-bearing breccias contain a wide variety of distinctive petrographic shock effects, including multiple sets of Planar Deformation Features (PDFs) in quartz . Re-Os isotopic analyses show the presence of a meteoritic component in impact melt breccias, which is absent in the target rocks. Our data confirm the identification of Gardnos as an impact structure. Reimold W. U.* Gibson R. L. Colliston W. P. Layer P. W. The Vredefort Dome in the Witwatersrand Basin: New Argon Chronological Data and the Geochronological Record of the Central Kaapvaal Craton Between >3 and <1 Ga Ago The Vredefort Dome, situated centrally in the world's richest goldfield, the Witwatersrand Basin, has recently been confirmed as the erosional remnant of one of the largest impact structures known on Earth. The impact event at ca. 2 Ga ago exposed a > 20 km cross-section through the crust of the Kaapvaal Craton. This provides a unique opportunity to study the crustal evolution in this region. We present a review of geochronological data, together with new results, that indicate over 2 billion years of Earth history preserved in the rocks of the Vredefort Dome. Master S.* Armstrong R. A. Brandt D. Ferraz M. F. F. Gumede T. Koeberl C. Reimold W. U. Robertson D. J. Woldai T. Zeil P. New Geological, Geophysical and Remote Sensing Data From the Highbury Impact Structure, Zimbabwe In September 1993, an International Impact Crater and Meteorite Expedition to Zimbabwe examined several circular features, of which one, the Highbury Structure, situated in c. 1.8 Ga metamorphic rocks of the Magondi Belt, was proven to be an impact structure or astrobleme. Initially, the structure was thought to be about 15 km in diameter, with a possible outer ring. New GIS-integrated computer-enhanced Landsat TM images, aeromagnetic maps, and a digital terrain model, clearly show that the structure is c. 20 km in diameter, and is centred on 30 degrees 06'55" E, 17 degrees 03'53" S. "Gries"-textured authigenic breccias are common in tremolite marbles near the centre of the structure. New data on the distribution of shock-characteristic Planar Deformation Features (PDFs) in quartz (which are mainly parallel to the 1013 and 1012 planes) indicate that the Highbury impact event post-dates a set of NNE-trending dextral wrench faults (including the Mtemwa Fault), which are older than the 1.10+-0.27 Ga Deweras Dyke. Ground magnetic, gravity and resistivity surveys have helped to delineate a poorly-exposed undeformed intrusive granophyre, which is believed to be an arkose-derived impact melt, because it commonly contains xenocrystic rounded detrital zircons. Preliminary SHRIMP U-Pb dating of single euhedral igneous zircon grains from the granophyre has yielded a concordant age of 1034+-17 Ma, which is tentatively interpreted to be the age of the impact event, since older detrital zircons also suffered a major Pb-loss at the same time. Koeberl C.* Masaitis V. L. Langenhorst F. Stoffler D. Schrauder M. Lengauer C. Gilmour I. Hough R. M. Diamonds From the Popigai Impact Structure, Russia Impact diamonds form as a result of shock-metamorphism during impact. Although diamonds in meteorites are relatively well-known, the study of diamonds at terrestrial impact craters has largely been confined to Russia. Masaitis found that diamonds form from carbon in the target rocks, mainly graphite-bearing (e.g., graphitic gneiss) or coal-bearing rocks.The firstcrater, at which abundant impact diamonds have been found since 1971 is the100-km-diameter Popigai crater in Russia. Subsequently, impact diamonds have been recovered from a number of other craters, e.g., Kara, Puchezh-Katunki, and Ries.The distribution of impact diamonds in impactites is a function of the initial distribution of coal- or graphite-bearing rocks among the target rocks, as well as the shock zonation. No impact diamonds are recovered from the central part of the crater (e.g., a hot melt body), because temperatures were too high and led to a combustion of the carbon. However, in the zone immediately following, the temperatures were low enough and the pressures high enough to form abundant impact diamonds. The highest abundances are found in impact melt rocks (tagamites) and suevites. Some diamonds are found in situ in the target rocks. In this report, we discuss preliminary results of our studies of six impact diamonds from the Popigai crater. We have used electron microscopy (SEM, TEM), neutron activation analysis, infrared spectroscopy, x-ray analysis, andisotope analyses for characterizing the samples. Our results show that some of the samples contain the hexagonal diamond modification lonsdaleite. TEM images showmicrocrystalline units of <5 micrometers in size with a layered structure (possibly aremnant of the graphite structure). Infrared (IR) spectroscopy indicates the presence of solid CO2 and water in microinclusions in the diamonds, with CO2 being under a pressure greater than 5 GPa (at room temperature). Hough R. M.* Wright I. P. Pillinger C. T. Gilmour I. Microdiamonds from the Iridium-rich Layer of the Arroyo El Mimbral K-T Boundary Outcrop, N. E. Mexico Nanometre-sized diamonds at the Cretaceous-Tertiary (K-T) boundary were first reported from the Knudsens farm outcrop, Red Deer Valley, Alberta Canada, and subsequently for the Berwind Canyon, New Mexico, and Brownie Butte, Montana, K-T sites. We report here the first discovery of diamonds in samples from the iridium-rich layer at the Arroyo El Mimbral outcrop, N.E. Mexico. The newly found diamonds are up to 30 micrometers in size, a significantly larger grain size than those previously studied which were only 3-6 nm. The combustion temperature of the diamonds durig stepped heating is predominantly between 650-900 degrees C and the carbon isotope composition measured simultaneously indicates the presence of two components with delta 13C values of -15 per mil and -11 per mil, not dissimilar to the isotope compositions of diamonds from Berwind Canyon and Brownie Butte. This provides a positive correlation between the iridium-rich horizon at Arroyo El Mimbral and the upper boundary or "fireball" layers at Berwind Canyon and Brownie Butte, USA. The coarser diamond size in the El Mimbral sample mirrors that for other impact related features such as shocked quartz as one moves closer to the proposed impact site at Chicxulub on the Yucatan peninsula. We suggest that these diamonds are analogous to impact produced diamonds found at known terrestrial impact craters. Shelkov D.* Verchovsky A. B. Pillinger C. T. Hutchison R. Milledge H. J. Carbonado: More Clues to a Common Impact Origin for Samples from Brazil and the Central African Republic Brazilian and Central African carbonados can be matched via their carbon and nitrogen isotopic compositions to suggest a common origin. Features seen at the margins of the stones investigated may be interpreted as indicating vely high temperature processing consistent with an impact ongin. Gilmour I.* Hough R. M. Wright I. P. Pillinger C. T. A Carbon Isotope Study of Refractory Carbon Phases in Impact Melt Rocks from the Nordlinger Ries Crater The 24 km diameter Nordlinger Ries crater in Southern Germany was formed by the hypervelocity impact of an, as yet, unknown type of asteroid some 14.8 Ma ago. It is a typical terrestrial crater consisting of a bowl-shaped depression with an upraised rim. The impact origin of the Ries was long suspected and was confirmed by the discovery of coesite (a high pressure polymorph of SiO2) in the impact breccia, suevite. We have examined the suevite for the presence of refractory carbon phases following extensive acid treatment. Using SEM and TEM we have identified carbon grains with an average size of 3-4 micrometers and an x-ray pattern characteristic of either diamond or its hexagonal polymorph lonsdaleite. The combustion temperature of the residue in a stepped heating experiment is between 500 and 700 degrees C and simultaneous carbon isotope measurements reveal the presence of at least two isotopically distinct components with delta 13C values of -16 per mil and -22 per mil. We propose that the occurrence of diamond at postulated impact sites may be an additional criterion for the determination of an impact origin. Herrick R. R.* Lyons S. N. Inversion of Impact Crater Morphometric Data On all planets, the appearance of impact craters changes dramatically with increasing size; larger craters have central peaks, terraces, low depth-diameter ratios, etc. However, these features have different onset diameters and different shapes on each planet, and these variations provide important clues about the formation process of complex craters. In the past, limited data necessitated a forward modeling approach to interplanetary comparisons. The recent addition of data from Venus and the icy satellites now makes the inverse approach a feasible method for determining the factors controlling interplanetary differences in crater morphometry. Wednesday, March 15, 1995 METEORITES AND MARS II: FOCUS ON ALH 84001 1:30 p.m. Room A Chair(s): D. Mittlefehldt M. Wadhwa Jakosky B. M.* Jones J. H. Mars Volatile Evolution from SNC and Planetary Data Analysis Understanding the martian volatile system and its evolution with time is fundamental to understanding the history of the martian surface: The earliest geology is closely tied in with the extant climate and the nature and distribution of water at or near the surface. Subsequent history, including such effects as weathering of surface materials and evolution of the polar deposits, also closely connects to the behavior of volatiles. We examine the evidence pertaining to the evolution of the volatile inventory and its distribution in the martian system throughout time, based on in situ measurements from spacecraft. earthbased telescopic observations, and terrestrial analyses of the SNC meteorites. Our goals are to try to arrive at a self-consistent (and unique?) view of the system, and to describe measurements that could be made to resolve uncertainties and ambiguities. Knott S. F. Ash R. D. Turner G.* 40Ar-39Ar Dating of ALH84001: Evidence for the Early Bombardment of Mars In a recent study of the diogenites, Middlefeldt demonstrated major differences between them and the Antarctic meteorite ALH84001, principally, the presence in ALH84001 of relatively sodic plagioclase - present as isotropic shock induced maskelynite, pyrite rather than troilite as the sulphide phase, chromite of distinct composition and differences in trace element chemistry. He argued that ALH84001, formerly classified as a diogenite, is a member of the SNC group of meteorites which are relatively well established as originating on the planet Mars. The distinctive composition of the oxygen isotopes in ALH84001 confirmed their link with the SNC group. Unlike the other martian meteorites, which have young crystallisation ages, Sm-Nd systematics indicate that ALH84001 formed early in the planet's history. In this report we present the first evidence from stepped heating and laser probe ^40Ar-^39Ar dating that it was subsequently involved in the early bombardment episode which generated the heavily cratered surfaces of the Moon, the southern uplands of Mars and many of the minor bodies of the solar system. Nyquist L. E.* Bansal B. M. Wiesmann H. Shih C.-Y. 'Martians' Young and Old: Zagami and ALH84001 New isotopic analyses of fine- and coarse-grained portions of the Zagami basaltic shergottite yield Rb-Sr isochrons of 186 +/-5 and 183 +/-6 Ma, respectively, and initial ^87Sr/^86Sr (I[sub]Sr) =0.722266 +/-0.000051 and 0.721661 +/-0.000057, respectively. The difference in I(sub)Sr of about 9 e-units shows that Sr-isotopic heterogeneities were carried into the crystallizing magma, probably by pyroxene phenocrysts. No isotopic heterogeneities were detected in Nd-isotopic analyses of the same samples, and regression of the combined Sm-Nd data for both lithologies gives an isochron age of 180 +/-37 Ma. The data imply Zagami crystallized from a melt about 180 Ma ago. Sm-Nd analyses of "martian" orthopyroxenite ALH84001 give an internal isochron age of 4.50 +/-0.13 Ga. Rb-Sr analyses are consistent with this age andindependently give a (preliminary) age of 4.95 +/-0.40 Ga and initial ^87Sr/^86Sr = 0.698 +/-2. Variations in ^142Nd/^144Nd for the ALH84001 samples correlate with Sm/Nd ratios to give ^146Sm/^144Sm = 0.0011 +/-0.0015, or 0.0022 +/-0.0010 relative to CHUR. Young ages and extreme geochemical fractionation in the SNC meteorites remain a strong argument for their origin on a large, geologically active parent body, consistent with their hypothesized martian origin. However, the interpretation of the about 180 Ma ages of shergottites as magmatic ages is, in our opinion, problematic. In the contextof their hypothesized martian origin, it seems most probable that these meteorites were ejected from Mars in a single large impact on old, varied terrain between 15 and 180 Ma ago. Wentworth S. J.* Gooding J. L. Carbonates in the Martian Meteorite, ALH84001: Water-borne but not like the SNCs The origin of carbonates in ALH84001 is significantly constrained by Fe-sulfate and ZnS accessories. Unlike previously documented occurrences of Ca-Mg-carbonates in shergottites, nakhlites, and Chassigny (SNCs), which indicate paragenesis from cold to warm, highly oxidizing water, the Ca-Mg-Fe-carbonates in ALH84001 apparently formed from water under chemically reducing, and possibly hotter, conditions. Reducing conditions and elevated temperatures might account for the surprising lack of secondary silicates and oxides in ALH84001. Harvey R. P.* McSween H. Y. Jr. Carbonates in the Martian Orthopyroxenite ALH 84001: Evidence of Formation During Impact-driven Metasomatism An enigmatic feature of ALH84001 (A84) is the widespread occurrence of carbonate minerals in this orthopyroxenite member of the "martian" or "SNC" group of meteorites. The presence of these minerals, whether attributed to high- or low-temperature processes, has wide-ranging ramifications for our understanding of Martian crustal fluids. Romanek C. S.* Thomas K. Gibson E. K. McKay D. S. Socki R. A. Petrogenesis of Carbon and Sulfur-bearing Minerals in the Martian Meteorite ALH84001 Unusual carbonate minerals found in the new Martian meteorite ALH84001 provide insights into surficial processes that shaped the early history of Mars, but despite detailed geochemical and isotopic examination carbonate petrogenesis has yet to be fully-characterized. High-resolution TEM and SEM analyses were performed on carbon- and sulfur-bearing textural elements of ALH84001 to better constrain the environment and timing of carbonate precipitation. Thomas K. L.* Romanek C. S. Clemett S. J. Gibson E. K. McKay D. S. Maechling C. R. Zare R. N. Preliminary Analysis of Polycyclic Aromatic Hydrocarbons in the Martian (SNC) Meteorite ALH 84001 Previous work has shown that pre-terrestrial organic compounds exist in interplanetary dust particles (IDPs) and certain meteorites. Polycyclic aromatic hydrocarbons (PAHs) have been found in several IDPs and numerous ordinary and carbonaceous chondrites. We document, for the first time, the occurrence of PAHs in the newest member of the SNC meteorite clan, Alan Hills 84001. Wadhwa M.* Crozaz G. Constraints on the Rare Earth Element Characteristics of Metasomatizing Fluids in the Martian Meteorite ALH84001 ALH84001, a newly identified SNC meteorite, shows clear petrographic and geochemical evidence of multiple phases of infiltration metasomatism which resulted in the formation of late stage minerals, such as maskelynite and apatite, and carbonates. We have analysed the REE compositions of these minerals to constrain the geochemical characteristics of the infiltrating fluids from which they crystallized. We propose that their petrographic and geochemical features suggest that this meteorite underwent at least two major phases of infiltration metasomatism. During the first, a LREE-enriched silicate melt fluxed through it and formed the non-cumulus assemblage comprising minerals such as maskelynite and apatite; during the second, a CO2-rich aqueous fluid, which may have been LREE-depleted, infiltrated it and precipitated the carbonate. Mittlefehldt D. W.* Lindstrom M. M. Gibson E. K. ALH 84001: Trace Element Geochemical Similarites of its Trapped Melt Component to Nakhla, Lafayette and Chassigny Based on a reconnaissance study of one thin section and a whole rock sample, ALH 84001 was described as having some petrologic similarities with the martian basalt-lherzolite suite on the one hand, and some geochemical similarities with the martian clinopyroxenite-dunite suite on the other. In particular, it was suggested that while the major cumulus mineralogy of ALH 84001 seemed to more closely resemble the petrologic features of the basalt-lherzolite suite, the trapped melt component appeared to have a trace element signature more closely akin to that of the clinopyroxenite-dunite suite. Preliminary ion probe measurements suggested that the orthopyroxene in ALH 84001 was not in equilibrium with the interstitial melt component as inferred from measurements on apatite and feldspar-rich glass, and this interstitial melt was suggested to be a foreign melt that infiltrated the cumulus minerals. Here we report on continued petrologic and geochemical characterization of ALH 84001, along with geochemical studies of Chassigny, Lafayette, Nakhla, Shergotty and Zagami. We find a strong incompatible element geochemical affinity of the ALH 84001 trapped melt component with those in the martian clinopyroxenites and dunite. The trapped melt component in ALH 84001 is clearly distinct from melts exemplified by the martian basalts. We find no strong petrologic support for the suggestion that the trapped melt component in ALH 84001 was a later addition, however, neither can we rigorously exclude it. Zoning in TiO2 in pyroxene near the trapped melt material is compatible with igneous zoning associated with the crystallization of a co-genetic trapped melt. Based on the evidence at hand, we believe it is more likely that the trapped melt component in ALH 84001 is co-genetic with the cumulus minerals. Bogard D. D.* Exposure-Age-Initiating Events for Martian Meteorites: Three or Four? Based on cosmogenic He and Ne, it was concluded earlier that at least three, and probably only three distinct impact events (now dated at ~0.6 Myr, ~3 Myr, and ~12 Myr) were required to explain the cosmic ray exposure histories of the nine SNC, or martian meteorites then known. Three groups have concluded that the the tenth recognized martian meteorite, ALH84001, has an exposure age of ~14-16 Myr and that this is longer than any other SNC exposure. This conclusion, if correct, would require a total of four impact events, either in space or on Mars, and would imply that ejection of solid objects from Mars is a more common process than previously imagined. This abstract reexamines requirements for a fourth impact event. Yanai K.* Re-searching for Martian Rocks from Diogenite-Diogenitic Achondrites The Shergottites, Nakhlites and Chassigney are well known as SNC martian meteorites for a long time. Recently Mittlefehldt (1994) reported that ALH84001 originally classified as a diogenite, has been properly classified as a unique orthopyroxenite martian meteorite. ALH84001 is the newly discovered martian meteorite different from all SNC meteorites for its petrography, geochemistry and petrogenesis; therefore, the new martian meteorite will give important information on petrologic evolution of martian crust. Mayeda T. K. Yanai K. Clayton R. N.* Another Martian Meteorite The recognition that the orthopyroxenite ALH 84001 is related to the SNC group rather than being a diogenite, as originally classi_ed, has led to re-examination of other ultramafic achondrites which might also be SNC-related. The oxygen isotopic compositions of SNC meteorites form a coherent group related to one another by mass-dependent fractionation effects, and well resolved from other achondrite groups, such as HED, lodranite/acapulcoite, ureilites, and primitive achondrites. We have determined isotopic compositions of several achondrites. Six samples from the Y 79 collection are con_rmed as diogenites, as originally classified, as is sample Y 23-93. Two previously unclassified samples (Y 28-93 and Y 29-100) have isotopic compositions which fall in the lodranite/acapulcoite field. Sample Y 25-95 has a value of d^17O of +0.21%, compared with a mean of +0.29% for all analyzed SNC meteorites. The d^18O and d^17O values are at the low end of the SNC range, and are most similar to the data for Chassigny (d18O = +3.91; d17O = +2.33). This is consistent with our observation that the dominant mineral in an X-ray powder pattern of the analyzed sample is olivine. Meteorite Y 25-95 appears to be derived from the same parent body as the SNC meteorites. _ Wednesday, March 15, 1995 THE CHICXULUB SYNDROME 1:30 p.m. Room B Chair(s): A. C. Ocampo B. F. Bohor Grant J. A.* Schultz P. H. Campos-Enriquez J. O. Definition of Shallow Subsurface Structure Around the Chicxulub Impact Crater Using Ground Penetrating Radar A growing body of evidence indicates that the buried Chicxulub impact crater in northern Yucatan is closely tied to the global extinctions that marked the Cretaceous-Tertiary boundary. The present topographic expression of the crater is limited, however, to a somewhat circumferential zone of collapse sinkholes or cenotes located 70-90 km from the crater center. Previous workers demonstrated this cenote ring intercepts and channels the generally northward flowing groundwater around the crater center, thereby forming an important regional hydrogeologic boundary. Goals of the present study include better understanding cenote origin and their possible reflection of subsurface structure/stratigraphy. Earlier efforts at resolving cenote origin were impeded by the remoteness of the area, paucity of outcrop, and relied heavily on access to existing wells and satellite data to locate cenotes and map nearby fractures over distances of lO's of meters. Our approach utilized a ground penetrating radar (GPR) as a means of delineating shallow stratigraphy along and adjacent to the cenote ring, thereby helping to place new constraints on its origin. Hildebrand A. R.* Pilkington M. Connors M. Ortiz-Aleman C. Chavez R. E. Chicxulub Crater Size and Structure as Revealed by Horizontal Bouguer Gravity Gradients and Cenote Distribution The size of the Chicxulub crater, Yucatan, Mexico is currently in dispute with diameter estimates ranging from 170 km upwards of 300 km. These size estimates have been largely based on the shape of the Bouguer gravity anomaly over the crater and the distribution of fractures inferred from the cenotes (sinkholes) in the region. To better understand the size of the crater we have acquired additional gravity data in radial profiles across the southwestern quadrant of the crater and have compiled all cenote locations on the northern Yucatan. Analysing the Bouguer data plus the cenotes' distribution reveals distinct concentric features out to ~85 km radius and two rings of cenotes coincident with horizontal gradient maxima, requiring some relationship between the two features. The outer gradient features may be modelled as concentric normal faults within the crater's zone of slumping. Sigurdsson H.* D'Hondt S. Carey S. Espindola J. M. Macias J. L. Geochemistry of the Cretaceous/Tertiary Impact Ejecta Deposit in Mexico and Belize A large meteor impact, such as the one which formed the Chicxulub crater, redistributes a vast amount of crustal material over the planet's surface and forms a radially distributed and extensive ejecta blanket. We have studied the geochemistry and sedimentology of the Cretaceous/Tertiary ejecta deposit at eight localities in southeastern Mexico and northern Belize, including the sites San Manuel, Guayal, Chilil, Bochil, Gabriel Esquinca, Coxquihui and Albion Island, ranging from 350 to 700 km from the center of the impact crater. At most of these sites, the impact deposit overlies directly upper Cretaceous limestones, and consists of a thick polymict breccia, which grades uniformly upward into microbreccia, grits, sandstone, siltstone and finally claystone at the base of the Paleocene. Gerasimov M. V.* Dikov Yu. P. Yakovlev O. I. Wlotzka F. High Temperature Vaporization of Quartz-Calcite-Anhydrite/Gypsum Targets in Relation to Chicxulub Impact Chicxulub crater is considered to be formed by the possible impact event which resulted in K/T mass extinction [Pope K. O. et al. and Hildebrand A. R. et al.]. The presence of thick deposits of anhydrite in the target rocks must have resulted in the liberation of enormous quantities of sulfur gases and the synthesis of noticeable quantities of sulfur acid aerosols which could strongly affect the biosphere [Brett R.]. During a previous set of experiments [Gerasimov M. V. et al.] we investigated the vaporization of pure anhydrite and gypsum and found that a large quantity of the evolved sulfur is efficiently trapped in the forming condensate. Here we have investigated the high temperature vaporization of mixtures of quartz, calcite and gypsum or anhydrite. This experiment showed that isolated and chain calcium silicates are formed in the resulting conden-sate. More sulfur remains in the atmosphere in the presence of silicon in the target, since a certain amount of calcium is bound as calcium silicates and less calcium is available for the formation of sulfates in the condensate. Alvarez W.* Claeys P. Kieffer S. W. Emplacement of KT-Boundary Shocked Quartz from Chicxulub Crater We propose that only high velocity, steep trajectories of shocked quartz grains could explain their occurrence in the upper of the two distinct KT boundary layers found in the US western interior and the higher abundance of shocked quartz at sites in the Pacific Ocean than at European sites the same distance away toward the east. The velocities and angle of launch required are greater than those needed to emplace the ejecta in the lower of the two KT boundary layers in the western interior, the presumed impact-melt ejecta. Such conditions of angle and velocity are difficult to explain by impact cratering dynamics in a homogeneous target medium, and are particularly difficult to reconcile with flow solely in a ballistic ejecta curtain launched at ~ 45 degree angle. We present a model in which the acceleration is provided by expanding gas (CO2 + H2O) from shock devolatilization of the 3 km limestone layer that overlies crystalline basement at Chicxulub. Bohor B. F.* Fisler D. K. Gratz A. J. Distinguishing Between Shock and Tectonic Lamellae With the SEM Much of the controversy over the origin of "planar features" in quartz grains is due to the method of examination. Use of the optical microscope alone for characterization of these planar features is inadequate for distinguishing between lamellar features formed by shock metamorphism and those formed by tectonic deformation. However, application of the scanning electron microscope (SEM) with its greater range of magnification, combined with hydrofluoric-acid (HF) etching of the quartz grains, allows for a positive distinction to be made between these two types of lamellar features. Boslough M. B.* Cygan R. T. Izett G. A. NMR Spectroscopy of Quartz from the K/T Boundary: Shock-induced Peak Broadening, Dense Glass, and Coesite We have obtained solid state 29Si nuclear magnetic resonance (NMR) spectra for quartz samples collected from K/T boundary sites in the Raton Basin, Colorado, as part of a project to evaluate the use of NMR as a quantitative tool to identify and characterize naturally-shocked minerals. This work follows our group's demonstration of a strong correspondence between NMR spectra and Kieffer's classification levels for shocked quartz from Meteor Crater (MC), Arizona. The insoluble residue of a sample from the Clear Creek North (CCN) location was analyzed using both magic-angle spinning (MAS) and cross-polarization magic-angle spinning (CPMAS) methods. The MAS spectrum for CCN consists of a single strong peak with a chemical shift consistent with crystalline quartz. The peak width is 0.96 ppm, significantly greater than would be expected for unshocked natural quartz. The CPMAS spectrum shows three distinct peaks that are consistent with a densified amorphous phase similar to that found in Class 3 quartz sandstone from MC. A sample from Berwind Canyon(BER) that was subjected to strong acid treatment was also analyzed by MAS NMR. The quartz resonance is strongly broadened, and the spectrum is remarkably similar to that of Class 2 quartz from MC. Identification of coesite in the BER sample has been confirmed by x-ray diffraction. There is no evidence for stishovite in either sample. Robin E.* Gayraud J. Froget L. Rocchia R. Spinel from the Cretaceous-Tertiary Boundary: Characteristics, Origin and Implications Numerous spinel crystals with various but unique compositions in the spinel group have now been reported worldwide at the Cretaceous-Tertiary (K-T) boundary. We propose that they are derived from ablation droplets produced during the interaction with the atmosphere of large meteoroids decelerated at low altitudes. Our scenario implies multiple meteoroid impacts all over the Earth, resulting either from the fragmentation of the K-T impactor by an oblique impact or from the encounter with a fragmented comet. Heymann D.* Nazarov M. A. Korochantsev A. Smit J. The Chicxulub Event: Did it Produce a Global Layer of Fullerene-bearing Sediments? When Heymann et al. discovered that C(sub)60 and C(sub)70 fullerenes were present in sediments from the Cretaceous-Tertiary Boundary (KTB) sites at Woodside Creek and Flaxbourne River in New Zealand, only 10 kilometers apart, the issue arose whether these were localized occurrences, or whether the Chicxulub event had produced a global layer of fullerene-bearing sediments. There exist currently two distinct hypotheses about the formation of fullerenes by the Chicxulub event. One posits that they were formed by the wildfires following the impact [Wolbach W. S. et al.] and accompanied the charcoal and soot which were apparently deposited at the KTB worldwide [Heymann D. et al. and Wolbach W. S. et al.]. Obviously this hypothesis predicts that fullerenes should occur in all KTB sediments enriched in iridium, carbon, and soot. The second hypothesis is derived for Chicxulub from the conclusion that fullerenes at the Sudbury impact site were formed in the hot atmospheric plume from carbonaceous matter of the impactor, perhaps by pyrolysis, almost immediately after the impact [Becker L. et al.]. Although it is not simply obvious how the Sudbury scenario could generate a global layer of sediments enriched in iridium, charcoal, soot and fullerenes, one cannot rule out the possibility that this had happened provided that the fullerenes survived in the atmosphere until the charcoal and soot from the fires had appeared there too. Both hypotheses might have to be modified, however, should it turn out that fullerenes at the KTB are, indeed, restricted to the New Zealand sites only. Becker L.* Bada J. L. Bunch T. E. Fullerenes in the K/T Boundary: Are They a Result of Global Wildfires The discovery of fullerenes (a third form of pure carbon besides diamond and graphite consisting mainly of the molecules C60 and C70) raises many interesting questions about the importance of this unique form of carbon on the Earth. Fullerenes have been detected in deposits associated with two separate events involving the impact of a large bolide (asteroid or comet) with the Earth. Fullerenes were detected in shock-produced breccias (Onaping Formation) associated with the 1.85 billion-year-old Sudbury Crater, and in clay sediments within the 65 million-year-old Cretaceous/Tertiary (K/T) boundary. Fullerenes in the Sudbury Crater were apparently synthesized within the impact plume from the carbon contained in the bolide and fullerenes in the K/T boundary clays may have been a product of global wildfires triggered by the impact event. A serious problem for the wildfire hypothesis for the origin of C60 in the K/T boundary clays, is that the atmosphere at this time had the same oxygen content as the present day Earth. In order to test the wildfire theory for the origin of fullerenes in the K/T boundary clays we examined several soot samples using laser desorption (reflectron) mass spectrometry (LDMS). No mass peaks were detected for C60+ and C70+ suggesting that soot production from these burnt materials does not yield significant amounts of fullerenes. _ Premo W. R.* Izett G. A. Meeker G. P. Major-Element and Isotopic Compositions of Relic Tektites and Glass-like Shards from the K-T Boundary Spherule Bed at El Mimbral, Mexico "Glass-like" shards recovered from a spherule bed of the K-T interval at Arroyo El Mimbral have been analyses for their major-element and radiogenic isotopic geochemistry. The results show that the brown glasses are compositionally similar and yield essentially the same age as the brown to black relic tektites from Beloc, Haiti. Other glasses of different colors apparently contain about 12% volatiles and, therefore, may not be glass but rather some other optically isotropic glass-like alteration material. U-Th-Pb, Rb-Sr, and Sm-Nd concentrations and isotopic compositions help define a mixing trend between Chicxulub basement and Cretaceous carbonates, but also suggest a component exhibiting a depleted mantle signature, possibly basaltic crust or material derived from it. Wednesday, March 15, 1995 SPECIAL SESSION: DISCOVERY 1:30 p.m. Room C Speakers to be announced This session will present "Discovery-class" missions with emphasis on those missions that are candidates in NASA's Discovery Program. An overview of the current status of the Discovery Program will be provided by Discovery Program Manager, Mark Saunders, followed by reports on Mars Pathfinder, NEAR, and the new missions selected during the recently completed competition for Discovery missions. There will also be presentations on similar-scale mission concepts under consideration by other space agencies. The detailed schedule for this session will be available in the conference information packet and at the registration desk. Wednesday, March 15, 1995 NASA PROGRAMS UPDATE 5:30 p.m. Room A Speakers to be announced Thursday, March 16, 1995 DIFFERENTIATED METEORITE MELANGE 8:30 a.m. Room A Chair(s): R. O. Pepin H. Haack El Goresy A.* Zinner E. Pellas P. Caillet C. Acapulco's Graphite Menagerie: Diverse Carbon and Nitrogen Isotopic Signatures Six new types of graphite with diverse C- and N-isotopic composition were found in Acapulco. Their isotopic variations are larger than those previously found in spherulitic and feathery graphites. At least three distinct isotopic reservoirs are required to explain the C- and N- isotopic compositions. Kim Y.* Zipfel J. Marti K. Evolutionary Trends in Acapulcoites and Lodranites: Evidence from N and Xe Signatures We present N and Xe isotope abundances in mineral separates of two Acapulcoites and three Lodranites. The comparison of isotopic signatures allows to trace the evolution of Acapulcoites and Lodranites. The light N signatures in metals of Acapulco, ALHA81261 and Lodran reveal primary signatures which were not erased during the high temperature history of these meteorites. Some exchange of N probably occurred between kamacite, taenite and graphite. Trapped Xe in Acapulco and EET84302 is always associated with 129Xe(sub)rad.. High concentrations of trapped Xe were found associated with metal inclusions in Acapulco silicates and associated with silicate inclusions in Lodran metal and probably were introduced during differentiation of these meteorites at silicate melting temperatures in the form of gas bubbles. The large 129Xe(sub)rad component found in the Cl-rich flaky phase on metal grain boundaries in Acapulco was probably introduced by a Cl and I rich melt at low temperatures. Benedix G. K.* McCoy T. J. Keil K. Textural and Mineralogical Variations in Winonaites: Clues to the History of the IAB Iron-Winonaite Parent Body Stony iron meteorites and primitive achondrites provide our best record of processes of partial melting and incomplete differentiation. Primitive achondrites include the acapulcoites, lodranites, winonaites, and silicate inclusions in IAB and IIICD irons. This work is the first comprehensive study of the winonaites (Winona, Pontlyfni, Mt. Morris (Wis.), Tierra Blanca, Yamato 74025, Yamato 75300, and Yamato 75305). Other meteorites previously called winonaites are now known to be acapulcoites (e.g., Acapulco and ALH A77081). The winonaites and IAB's are distinguished from other primitive achondrite groups by oxygen isotopes and their reduced mineral chemistry. The distinction between IABs and winonaites is somewhat arbitrary, but winonaites are those members that are stony and do not have a metallic matrix. Previous work on winonaites included chemical and mineralogical analyses, evidence for high temperature metamorphism and relationship to silicates in IAB irons. The origin of these meteorites involved metamorphism, partial melting, and brecciation on the parent body. Scott E. R. D.* Love S. G. Haack H. Fragmentation and Reaccretion of Differentiated Asteroids: Evidence from Thermal Histories of Meteorites and Theoretical Constraints For asteroids with diameters of 50-500 km, there are significant ranges of specific impact energies which will cause asteroids to be fragmented and reaccreted with scrambled interiors. Scrambling requires that specific impact energies must be ~1/2 the asteroid's gravitational binding energy with flight times around 2 hours. For asteroids >500 km in size (e.g., Vesta), fragmentation and reaccretion events are less likely to scramble asteroid interiors. Thermal histories of the IVA irons and stony irons appear to be broadly consistent with these constraints. Fragmentation and reaccretion occurred when the IVA core was at 1200 degrees C and the asteroid had a mean temperature of 400-700 degrees C. Other fully differentiated asteroids with sizes of 50-500 km may also have been scrambled by impacts to make S asteroids. Haack H.* Ulff-Moller F. Rasmussen K. L. The Thermal Evolution of IVA Iron Meteorites: Evidence from Metallographic Cooling Rates Unlike the other groups of iron meteorites for which a core origin is inferred, group IVA iron meteorites exhibit highly diverse metallographic cooling rates (20-3000 degrees C/My). Furthermore, studies of the silicates in the IVA stony-iron Steinbach suggest that this meteorite cooled very fast (~100 degrees C/hour) at high temperatures. We have determined metallographic cooling rates, and their variation with temperature, for 16 IVA iron meteorites in order to illuminate their thermal evolution. Our results imply a complex thermal evolution of the IVA parent body, possibly a result of a catastrophic fragmentation and reassembly of the parent body after crystallization but prior to Widmanstatten pattern formation. Shen J. J.* Papanastassiou D. A. Wasserburg G. J. Precise Re-Os Determinations on IIA and IVA Iron Meteorites We have pursued our attempts to develop high precision procedures for the determ Os metals as gravimetric standards, a mixed Re-Os tracer, and closed system proc equilibrium between tracer and normal. We obtained reliable calibrations of Re, tracer calibration for chunks of metal from individual Os "buttons" as normals, different, high purity Os metal samples. This indicates the need for further imp have addressed the problem of chemical blanks, including the filament loading bl with TPAI and CHC1(sub)3, and by further purification by heating at 135 degrees procedures, the Cr remains in the Cr^6+ state (as determined at the end by titra CrO(sub)3, respectively. Chloroform can be dirty for Os, but it can be cleaned u blank, we have determined that even the cleanest Pt contains substantial amounts relatively high and variable effective filament loading blanks, which are also h BaSO(sub)4 precipitate in water; the sample Re is loaded on the BaSO(sub)4 after investigated the use of Ni filiments for the analysis of Re. There is an apparen content. The low-purity Ni wire from Johnson Matthey, shows low Re content and t The Ni wire is pressed flat over a 2 mm length, using cleaned sapphire rods. The emitter, ranges to more than 16%. The IE is also affected by the presence of org reduced using a UV source. Smoliar M. I.* Walker R. J. Morgan J. W. Shirey S. B. Comparative Re-Os Isotope Systematics in IIA and IIIA Iron Meteorites For this study we have analyzed 6 IIA irons (10 independent dissolutions, including replicates: samples are Filomena, Gressk, Coahuila, Negrillos, Lombard and Bennett County) and 6 IIIA irons (8 independent dissolutions, including duplicates: samples are Toubil River, Ssyromolotovo, Susuman, Henbury, Costilla Peak and Charcas). Both sets give well-defined lines (Model 1 regressions, 95% confidence). For IIA irons: slope is 0.07851 +- 0.00014 (MSWD=1.2) and initial 187Os/188Os is 0.09544 +- 0.00007. For IIIA irons: slope is 0.07885 +- 0.00028 (MSWD=1.2) and initial 187Os/188Os is 0.09526+- 0.00013. The slope of the IIA isochron agrees within uncertainty of our previous results. There are two problems that preclude a straightforward interpretation of these data. First, uncertainty in the decay constant of 187Re is currently on the order of +- 3%. Absolute age determinations using the Re-Os system are consequently hampered. Second, as discussed previously, there still exist no viable absolute standards for the calibration of Os spikes. The approximately +- 1% uncertainty in the absolute concentration of Os also hampers absolute age interpretations of isochrons. Despite these two limitations, the new data can be used in the comparison of different iron groups and variations within iron groups. Several conclusions can be made from these data. First, the good fit of the data to presumed isochrons suggests that the IIA and IIIA systems crystallized during a short time interval (<5 Ma). Slower crystallization would likely not have resulted in isochronous behavior. Our less precise data for B subgroups also indicate that they crystallized within the same general time interval. Second, the high precision isochrons also indicate that at least the samples we examined have remained closed-systems with respect to Re and Os since their crystallization. Finally, although the slopes and initial ratios for the two iron groups overlap, the overlap is minimal. The results may indicate that the group IIAB system crystallized 5-20 Ma subsequent to the IIIAB system. The differences in the initial ratios therefore may have resulted from the ingrowth of radiogenic Os from a IIAB parent body with chondritic Re/Os prior to core formation. Chen J. H.* Wasserburg G. J. A Comparison Between 107Pd and 53Mn Chronometers in Some Meteorites In previous studies we presented evidence for well resolved excesses (*) of radiogenic 107Ag* in a wide variety of iron meteorites and some pallasites. The linear correlation between Pd/Ag and 107Pd/108Pd for many metal samples of Gibeon and for metal and FeS samples from three meteorites provides unambiguous evidence of the in situ decay of 107Pd in planetary differentiates in the solar system. Olsen E. J.* Davis A. M. Clayton R. N. Mayeda T. K. Clarke R. S. Jr. Wasson J. T. Mbosi: An Anomalous Iron with Anomalous Silicate Inclusions Mbosi is an ungrouped iron with a Ge/Ga ratio >10 (like the IIF irons, Eagle Station trio, and four other ungrouped irons). Mbosi contains silicate inclusions that consist of coexisting glasses. One glass is about 55.18% SiO2, 8.82% Al2O3, 20.94% MgO, 7.20% CaO, 2.05%P2O5, 3.57% FeO (and smaller amounts of other oxides) and is partially devitrified into micrometer size crystallites. The other glass is 91.34% SiO2, 4.77% Al2O3, 2.05% CaO, 1.11% FeO (all other oxides are minor). The weighted whole rock composition is unique. The high P2O5 content suggests a possible relationship with mesosiderites. The bulk REE and refractory siderophile patterns are flat, each at 7xC1. The bulk oxygen isotope composition plots near the anomalous meteorites Tucson, Bencubbin and Weatherford, and not far from Eagle Station. We conclude that Mbosi contained silicate inclusions with a silica polymorph. It underwent melting of both the host metal and the silicate inclusions. The silica polymorph did not completely melt before cooling commenced. Mbosi's position on the 3-oxygen isotope diagram indicates it may be related to other anomalous meteorites such as Tucson, Bencubbin, Weatherford and the Eagle Station trio. _ Klein J.* Middleton R. Xue S. Herzog G. F. Masarik J. Reedy R. C. 59Ni in Canyon Diablo Spheroids Canyon Diablo, an IA iron meteorite of ~15m radius, fell to Earth ~50 ka ago creating Meteor Crater in Arizona, a structure about 1.2 km in diameter and 180 meters deep. Nickel-iron fragments ranging in size from ~100 micrograms to 700 kg have been recovered from a 260-square kilometer area surrounding the crater. Included in these fragments are several thousand tons of sand-grain size Ni-Fe droplets that are believed to have condensed from a cloud of metallic vapor. Most of these droplets are comprised of metallic cores, enriched in Ni compared to bulk Canyon Diablo, surrounded by coatings of iron oxide and siliceous glass. The droplets are roughly spherical, with diameters between 0.5 and 1.0 mm. The mineralogy, petrography, and chemical compositions of these spheroids have been extensively studied. We have been studying these droplets to determine their position in Canyon Diablo before its impact and to look for evidence of isotopic fractionation caused by evaporative loses. Mathew K. J.* Begemann F. Solar Ne in the Brenham Pallasite We have determined the noble gas isotopic composition in grain size fractions prepared from the olivine separates of the Brenham Pallasite and the neon results are presented. We show that the neon isotopic ratios from step-wise heating of the samples define a tie-line joining the solar and the spallation end points. The significance and implications of the result are discussed. Pepin R. O.* Rider P. E. Examination of Kr and Xe Isotopes in Pesyanoe by Acid-Etching Evidence for a solar-like 3He/22Ne component in mantle-derived basalts, diamonds, and well gases has been reported by Honda et al.. An issue of considerable importance for the sources, incorporation mechanisms, and transport of primordial noble gases in the Earth is whether signatures of isotopically solar components also exist in Ar, Kr and Xe. Elemental abundances of these heavy gases relative to 22Ne are ~2-5 orders of magnitude higher than the corresponding solar ratios, attributable in part to addition of atmospheric gases by seawater-magma interaction, subduction, or both. Their nonradiogenic isotope ratios are generally considered to be indistinguishable from those in air. The intent here is to examine the constraints imposed by the current data set of heavy-gas isotopic measurements on the presence and relative magnitudes of solar-composition components in the mantle. Fahey A.* Huss G. Wasserburg G. Lodders K. REE Abundances and Cr Isotopic Composition of Oldhamite and Associate Minerals from the Pena Blanca Spring Aubrite The aubrites, or enstatite achondrites, are a class of highly reduced, brecciated meteorites. They contain sulfur-bearing phases such as oldhamite (CaS), and alabandite ((Mn,Fe)S). In order to better understand the origin of these minerals we have begun a detailed study of REE patterns in several sulfide phases from Pena Blanca Spring. Ion microprobe sensitivity factors for the REE and several other trace elements in sulfides were determined by comparison of INAA data on a single oldhamite inclusion. The measured REE sensitivity factors for sulfides are significantly different than the corresponding factors for oxide minerals. In addition, Cr isotopes were measured in alabandite to determine the level of 53Cr present from the decay of 5lMn. An upper limit on (55Mn/55Mn)(sub)0 in this inclusion of ~3 x 10^-6 was determined. Thursday, March 16, 1995 VENUS RESURFACING AND LITHOSPHERIC PROPERTIES 8:30 a.m. Room B Chair(s): A. T. Basilevsky C. L. Johnson Head J. W. III* Venus: Synthesis of Observations, Outstanding Questions, and Implications for Future Exploration Venus, because of its Earth-like nature in terms of size, density, and position in the Solar System, has always been a subject of intense interest. Earlier observations revealed Earth-like mountain belts, rift zones, highly deformed terrains and vast volcanic plains. These emerging clues to geologic processes and history came in parallel with the exploration of the smaller terrestrial planetary bodies (Moon, Mars, Mercury) and the characterization of them as one-plate planets whose surfaces stabilized early in the history of the Solar System, in contrast to the Earth with its laterally moving plates and constant surface renewal. Was Venus similar to the Earth with a component of plate tectonics, was it like the smaller bodies with early heavily cratered regions and superposed old volcanic plains, or was it something different and perhaps unique? Interest was heightened by the increased awareness of the changes that have occurred in the history of the Earth and the knowledge that the first half of EarLh history was very different from the last half in terms of the nature of the atmosphere, biosphere and lithosphere. Could Venus be a Rosetta stone allowing planetary scientists to decipher the extremely fragmentary early history of the Earth? The global coverage provided by the Magellan measurements has yielded a new set of constraints on the geologic characteristics and history of Venus; a new picture of Venus and its relevance to the evolution of the Earth has emerged, and new questions for future exploration have arisen. Turcotte D. L.* Magellan, Venus, and the Earth The Magellan Mission has provided a wealth of data on Venus. We now have a far better understanding of the tectonics and volcanism on the planet. This is important in terms of the exploration of the earth's sister planet, but it is also important in that it provides insights into how the earth works. Two major questions concerning the earth are: (1) Why does the earth have continents and (2) why does the earth have plate tectonics? In essence, Venus has neither. These differences can probably be attributed to the presence of liquid water (oceans) on the surface of the earth. _ Price M.* Resurfacing History of the Venusian Plains Based on Distribution of Impact Craters Mapping and dating of the Venusian plains using units defined by the relative preservation of lobate flow morphology confirms that long-term weathering of volcanic flow complexes causes increasing degradation of flow boundaries and homogenization of flow backscatter signatures. Progressive deterioration of flow morphology correlates with increasing crater density and decreasing frequency of embayed craters, and therefore age. Plains emplacement lasted at least 100 Ma, and developed progressively into concentrated younger activity in the rifts and large volcanic rises. Simulations confirm that this history is compatible with the randomness of craters. Basilevsky A. T.* Head J. W. Stratigraphic Studies in the Area of Baltis Vallis, Venus Photogeologic mapping of a large area (4,500 x 4,500 km) around Baltis (Hildr) Vallis has led to the identification of six stratigraphic units the relations of which are consistent over the entire mapped region. Most of the mapped area is occupied by wrinkle-ridged plains consisting of two subunits separated by the emplacement of Baltis Vallis. The latter, as other canali-type channels, appears to be formed geologically instantaneously. These relationships support the interpretation that as a stratigraphic unit, plains with wrinkle ridges formed relatively simultaneously over large parts of Venus. Kreslavsky M. A.* Size Distribution of Young and Old Coronae on Venus Topography of small coronae (<300 km in diameter) in planes of Venus was surveyed to assess whether each corona is young or old according to conventional scenario of corona formation. Size fequency distribution of young and old coronae was obtained. It was compared with a model for corona population. The model was based on the supposition that the flux of corona-forming diapirs was steady during population formation. Observations showed shortage of small young coronae in comparison with the model under any reasonable set of parameters. It points that convection style of upper mantle has been changing during formation of apparent surface. Wichman R. W.* Crater Morphometry on Venus: Relative Floor Size and Internal Crater Modification Although venusian craters are rarely embayed (from the outside) by surface volcanism, the lunar record of mare-filled craters indicates that volcanism can also occur inside craters without necessarily affecting surfaces outside the crater rim. Such crater-centered volcanism, however, should embay both the crater walls and any central peak structures, thereby increasing the observed floor size relative to both the pristine crater rim and the central peak complex. Since venusian crater floors show a dichotomy between young, radar-bright albedos and older, darker units resembling the surrounding volcanic plains, crater-centered volcanism also may be common on Venus. This hypothesis can be tested by an analysis of venusian crater morphometry. Based on a preliminary data set of 115 craters, there is no clear evidence for any change in floor size as a function of floor albedo. Nevertheless, the data are widely scattered, which may obscure minor changes in floor size as a function of age. Possible increases in crater floor size, however, are apparent for other classes of crater modification, including the youngest inferred craters on Venus (bright floored with dark halos), those craters embayed by external surface volcanism, craters showing internal modification similar to lunar floor-fractured craters, and craters greatly modified by regional tectonism. Since statistical comparisons of these measurements are hampered by both the wide scatter of the derived data and the small samples for some crater modification styles, a more detailed set of measurements is in progress to expand and refine the trends reported here. Namiki N.* Solomon S. C. Degassing of Argon, Helium, and Water and the Nature of Crustal Formation on Venus The history of volcanic resurfacing and crustal formation on Venus are topics of an important ongoing inquiry. The apparently random distribution of impact craters and the small fraction of modified craters have led to the hypothesis that a catastrophic resurfacing event occurred 300 to 500 My ago. On the other hand, analysis of the cratering records on volcanoes and coronae and Monte Carlo models of resurfacing of craters have shown that volcanic activity of as much as 7 km3 yr-1 has continued over the last 500 My. Such magmatism transfers not only heat, but also magmatic volatiles from the planetary interior. Therefore the present abundance of atmospheric constituents can constrain the nature of postulated catastrophic resurfacing events and of post-catastrophe volcanism. Johnson C. L.* Sandwell D. T. Regional Resolution Analyses of Magellan Gravity Data Many geophysical investigations of Venus are concerned with establishing the mechanism(s) by which topography is supported. Distinguishing between local, flexural, or dynamic compensation requires gravity observations over a wide range of wavelengths. The spatial resolution of the Magellan gravity data depends primarily on the spacecraft altitude. Resolution analyses of some cycle 5 data suggest a maximum resolution of about 400 - 450 km, equivalent to approximately 2 - 2.5 times the spacecraft altitude. Thus, using a combination of all of the available data, we might expect minimum resolvable wavelengths of less than 400 km in a few regions, given a good signal-to-noise ratio. These predictions are consistent with the power spectra of spherical harmonic models for the Venus gravity field. At present, spherical harmonic models are available to degree and order 75, corresponding to a wavelength of 500 km. The power spectrum for model MGN75ISAAP, shows anomalously high power at degrees 74 and 75, indicative of shorter wavelength signal in the gravity field, which is unresolved in current spherical harmonic models. In this study, resolution analyses of cycles 4 - 6 gravity data over several regions of geophysical interest are presented. Both single-taper and multi-taper spectral coherence methods are applied to the line-of-site (LOS) doppler velocity residuals at each region studied. The effects of spacecraft altitude, viewing geometry and noise due to solar effects are examined. The inference of elastic lithospheric thickness from the gravity data (on a regional basis) is discussed in the light of the resolution analysis results. McGovern P. J.* Simons M. Solomon S. C. Estimates of Elastic Lithosphere Thickness and Heat Flux Beneath Large Volcanoes on Venus A fundamental unknown on Venus is the global heat flux. The heat flux cannot be measured directly on Venus, but the thermal gradient may be inferred indirectly from the thickness of the elastic lithosphere. To this end, we calculate gravity/topography admittances in the harmonic domain for nine large volcanoes on Venus. We compare the observed admittances to those calculated from a thin elastic spherical shell flexure model in order to estimate the elastic plate thickness supporting the volcanoes. We convert elastic plate thickness to mechanical plate thickness, and then estimate the thermal gradient and heat flux. The resulting heat flux values are lower than those calculated by scaling Earth's heat flux to Venus. We also note that mechanical plate thickness estimates at volcanoes are generally higher than those found at coronae. Simons M.* Solomon S. C. Hager B. H. Admittance Constraints on the Thickness of the Venusian Lithosphere? Global maps of the spatial and spectral variation of the geoid/topography admittance of Venus are now available from recent Magellan data. At length scales of 1000 to 5000 km, the admittance varies locally between about 0 and 35 m/km. High admittance values calculated spatially as geoid-topography-ratios (GTRs) by others have been used to suggest the existence of an approximately 300-km-thick thermal boundary layer (TBL). However, none of the admittance spectra, when considered for a given geographic area, or when averaged globally, are well explained by primary compensation of topography at a single depth (300 km or otherwise). For the Earth, we have models of interior density constrasts based on seismic tomography and plate tectonics. We do not have this luxury for Venus, and must rely on models of mantle convection to produce self-consistent families of interior density variations. We present the results of numerical models of convection which produce admittance spectra with spectral slopes similar to the observed spectra and which have thermal boundary layer thicknesses ranging from 100 to 300 km. The ambiguity in these results suggests that the magnitude of a single GTR estimate or spectral admittance should not be used to constrain the TBL thickness. This observation, combined with estimates of the mechanical plate thicknesses on Venus and their implications for planetary heat loss, suggest that there is no geophysical reason to reject the hypothesis of a 100- to 150-km-thick thermal boundary layer on Venus. Freed A. M.* Melosh H. J. Long Term Survival of the Topography of Ishtar Terra, Venus Ishtar Terra is a high (3-4 km) and broad (~1000 km) plateau on Venus that has been subjected to a high surface temperature and a low erosion rate. Survival of this topography can be explained by a very strong crust or a young age. However, on the basis of crater density, Ishtar Terra is perhaps 500 m.y. old. Previous flow laws for dry diabase and websterite do not provide the crust with sufficient viscosity to prevent loss of topography due to gravitational spreading. Thus, the maintenance of Ishtar Terra has the appearance of a paradox. A solution to this paradox may lie with a new flow law for Maryland diabase (A(sub)o=4.2 MPa^-ns^-1, Q=505 KJ/mol, n=5.1) that provides the crust with sufficient viscosity to maintain high topography for at least 500 m.y. Brown C. D.* Grimm R. E. Impact Crater Rebound and Lithospheric Structure on Venus This study takes advantage of the relative prevalence and uniform distribution of impact craters on Venus to attempt to place constraints on crustal thickness and temperature gradient using models of viscous relaxation and elastic flexure. We improve on earlier work on this subject by employing a recent dry diabase flow law and using Magellan altimetry coverage. We focus on eight of the largest Venus craters, which have a mean depth of 1.05 +- 0.40 km. By applying conservative assumptions regarding viscosity, relaxation time, and amount of relaxation, we find that the crustal thickness is unbounded by the viscous relaxation model, and the thermal gradient is very poorly bounded. Very little relaxation occurs under the conservative conditions due to the strength of the anhydrous diabase rheology. Since we suspect some relaxation, the true crustal viscosities are probably less than we have assumed. Again applying conservative assumptions on material properties, strain rate, and crater cavity size and depth, the elastic flexure model yields a lower bound on elastic thickness of 7.9 km, and a corresponding upper bound on thermal gradient of 26 K/km. This result is inclusive of previously reported lower temperature gradients. Thursday, March 16, 1995 THE NEAR MISSION AND THE NATURE OF S-CLASS ASTEROIDS 8:30 a.m. Room C Chair(s): S. Murchie M. J. Gaffey Veverka J.* Malin M. Murchie S. Bell J. III Chapman C. McFadden L. Robinson M. Thomas P. Asteroid 433 Eros: High Resolution Imaging and Spectral Mapping from NEAR Primary objectives of the NEAR orbital mission to asteroid 433 Eros include detailed measurements of surface morphology, constraining the internal structure of Eros, and determining the composition of its surface. These will be addressed by an imager-spectrometer instrument package in the payload. The MultiSpectral Imager (MSI) will determine morphology at scales down to 5 m, as well as gross physical properties of the asteroid including size, shape, large-scale roughness, and spin state. From imaging alone the volume will be determined to within 2-5%, providing the basis for a very precise determination of Eros's mean density. High resolution color imaging will map spectral variations and characterize impact, regolith, and other geologic processes. The imaging data will be combined with spectral maps produced by the Near Infrared Spectrometer (NIS) to investigate the mineralogy of the surface and search for correlations between mineralogical variations and geology. Trombka J. I.* Boynton W. V. Bruckner J. Squyres S. W. Bailey S. H. Clark P. E. Evans L. G. Floyd S. R. Starr R. Gold R. Goldsten J. McNutt R. The X-Ray/Gamma-Ray Spectrometer for the NEAR Mission In 1999, the NEAR (Near Earth Asteroid Rendezvous) Mission Spacecraft will rendezvous with the Earth-crossing asteroid, Eros. Eros is the second largest NEA; 13 4 15 4 36 km. In 1975, Eros approached within 0.15 AU of Earth and extensive telescopic and radar observations were made. These and earlier observations have indicated that Eros is an S-type asteroid. The S-type asteroids are believed to be the most common type within the main asteroid belt and may represent either differentiated or undifferentiated mineralogical materials. One of the primary objectives of the NEAR mission is to obtain global elemental composition maps of the asteroid with sufficient accuracy to enable comparison with major mete-orite types. The XGRS (X-ray/Gamma-Ray Spectrometer) instrument will contribute significantly to this objective by measuring the elemental composition of the asteroid. Yeomans D. K.* Konopliv A. S. Barriot J. P. The NEAR Radio Science Investigation The science objectives of the NEAR Radio Science investigation are to determine the mass of asteroid 433 Eros to better than 0.1%, to determine its bulk density to an accuracy level commensurate with the accuracy of the volume determination (~1%), and to investigate its interior homogeneity by comparing high order gravity fields determined from the shape models with gravity fields determined directly from the spacecraft tracking data. Additional science objectives include the determination of the asteroid's moment of inertia matrix, its rotation state, and placing upper limits upon any outgassing that may have occurred during its 100-year observational history. If the decision is made to allow the NEAR spacecraft to fly past asteroid 253 Mathilde enroute to its rendezvous with asteroid Eros, the mass and bulk density of Mathilde will also be determined. Coherent, closed loop, X-band tracking data, in combination with the optical navigation and lidar data will provide the necessary measurements. Acuna M.* Zanetti L. Russell C. Near Earth Asteroid Rendezvous Magnetometer (MAG) Investigation The NEAR Magnetometer Investigation (MAG) is an integrated technical, scientific as well as spacecraft management undertaking to establish the nature of the magnetic field of 433 Eros, operating with minimal resources. The instrument is being implemented as a follow-on to many previous successful undertakings between Johns Hopkins APL and GSFC's Laboratory for Extraterrestrial Physics (MAGSAT, Hilat, AMPTE/CCE, UARS, Viking, Freja, etc.) in the field of space magnetometry. A single triaxial fluxgate magnetometer system of very large dynamic range has been built by APL and GSFC for flight in one year's time. To minimize costs and spacecraft impacts, no booms are used and the sensor is mounted approximately one meter from the main spacecraft bus, on the high gain antenna feed support. This arrangement is almost identical to that used by the NEAR MAG Team Leader in the highly successful GIOTTO mission to Comet Halley in 1986. The primary magnetometer science objectives of the NEAR mission are to determine the intrinsic magnetization of the asteroid EROS. These signals may be of the same order as the nominal NEAR spacecraft magnetic interference level. Using proven methods and the statistics of nearly a year in orbit, even magnetic signals due to EROS that are smaller than the spacecraft noise can be reliably isolated. Zuber M. T.* Smith D. E. Cheng A. F. Cole T. D. The NEAR Laser Ranging Experiment The primary objective of the NEAR Laser Ranging investigation is to obtain high integrity profiles and grids of topography for use in geophysical and geological studies of asteroid 433 Eros. The resolution, surface coverage, and absolute accuracy that will characterize these data will enable detailed analysis of the structure, morphology and evolution of this near-Earth asteroid. Asteroidial radii will be produced in a well-defined reference frame so that topographic data can be readily compared to information from other NEAR sensors, which will further increase the value of these data. Chapman C. R.* Near Earth Asteroid Rendezvous: Eros as the Key to the S-Type Conundrum Gaffey et al. wrote that "perhaps no other question so haunts asteroid science as the identification of the ordinary chondrite parent bodies." This is one phrasing of the larger question of the S-type asteroids and their relationship to meteorites. The Earth-approaching asteroid 433 Eros, target of the NEAR mission, serves both conceptually and specifically as a keystone for the resolution of this problem. There are other problems in asteroid science and others involving NEA's -- e.g. extinct comet nuclei -- but the selection of Eros as the target for NEAR focuses the science objectives of the mission on the S-type conundrum. I will highlight this compositional/mineralogical issue in my review of the imaging/spectral science goals for NEAR, but also mention other goals dealing with the fundamental properties of Eros, its detailed shape and configuration, the morphology of its surface, its impact and collisional history as inferred from its cratering record and structure, and search for any satellites. Gaffey M. J.* Meteoritic Aspects of the S-Asteroid Issue: A Perspective for the NEAR-Mission Fanale F. P.* Granahan J. C. The 'Ordinary Chondrite--S Asteroid Controversy'; Still Going, Still Going The S asteroid-ordinary chondrite controversy encompasses several conflicting hypotheses which, for simplification, may be represented by two arguments: (1) Many S asteroids are the parent bodies of ordinary chondrites but have very different reflectance spectra than ordinary chondrites because of processes which alter their optical surfaces exposed to the space environment. (2) S asteroids are the parent bodies of ordinary chondrites (aggregates from the pre-planetary nebula) but rather mixes of magmatically differentiated cores mantles and crusts of larger proto-asteroids.In this case they would be mixtures of achondritic and metallic material.Space weathering (to whatever extent it occurs) is not sufficiently effective on asteroids to alter chondritic materials to yield S-asteroid spectra.The chondritic parent bodies remained undifferentiated because they were small enough and had such short thermal lag times that heating by short lived nuclides or electromagnetic induction was not sufficient to differentiate them.Further, their small size is the reason that they have not observationally identified. Murchie S. L.* Pieters C. M. The Relation of 433 Eros to Other S Asteroids: Evidence from Reflectance Spectroscopy at 0.33-2.5 Micrometers In 1999 the NEAR spacecraft will perform the first rendezvous with an asteroid, the S-type 433 Eros. Data returned by g-ray, X-ray, and NIR spectrometers will be used to address the questions of Eros's surface composition and the link between S-asteroids and meteorites. In order to properly frame these questions, it is necessary to understand how Eros's spectral properties compare with those of other S asteroids. In this review we integrate previous measurements of Eros to derive a composite spectrum from 0.33-2.5 mcm, and we use this spectrum to assess Eros's mineralogy and the spectral class of S asteroids to which it belongs. Eros exhibits 1-mcm and 2-mcm mafic mineral bands (Bands I and II) centered near 0.96 and 1.96 mcm. For such a Band II position, the center wavelength of Band I is longer than expected for pure pyroxene, suggesting an additional phase with an absorption longward of 1 mcm. The area ratio of Band II/Band I is also somewhat smaller than for pure pyroxene, suggesting an additional absorption near 1 mcm. Both the band centers and areas indicate a mafic mineral composition rich in orthopyroxene with a component of olivine. The values of these attributes place Eros as a member of the S(IV) spectral class of Gaffey et al. The continuum slope and band strength are typical of S-asteroids of comparable size, suggesting that 433 Eros is representative of small S(IV) asteroids. Smith D. E.* Zuber M. T. Rowlands D. D. Deriving the Dynamics of Eros from Gravity Variations One of the objectives of the Near Earth Asteroid Rendezvous (NEAR) mission is to improve our understanding of the dynamics and interior of Eros. One source of information about the interior is the gravity field, and both the gravity field and the dynamics are information that can be extracted from the very precise data that is used to track the spacecraft. Because Eros has an irregular shape its gravity field is non-symmetric and this causes perturbations of the motion of the NEAR spacecraft that can be detected in the tracking data. These perturbations can be used to map the gravity field of the body but they can also be used to determine the orientation of the gravity field and its rate of rotation. It is the irregularity of Eros' s gravity field that permits the estimation of its rotation from the time and spatially varying gravity signal. Bell J. F.* Q-Class Asteroids and Ordinary Chondrites With most asteroid classes, there is general acceptance of the principle that lab spectra of meteorites should resemble telescopic spectra of their parent asteroids. No one seriously questions the association of basaltic achondrites with V asteroids or olivine achondrites with A asteroids, since the spectra in question are both very similar and have a high information content (distinct absorption bands that can be unambiguously associated with specific silicate mineral compositions). For years a similarly strong spectral link has existed between ordinary chondrites and Q asteroids such as 1862 Apollo, but the significance of this remained doubtful because the first known Qs were in short-lived planet-crossing orbits. Recently, a Q asteroid (3628 Boznemcova) has been found in the main belt near the 3:1 Kirkwood gap, ideally placed to deliver material to Earth. In the case of any other asteroid type, this discovery would establish Qs as the parent bodies of ordinary chondrites beyond question. However, this straightforward interpretation is resisted in many quarters because it seemingly contradicts many of the fundamental tenents of asteroid science established over the last 20 years. Many continue to argue that Q asteroids are unimportant or irrelevant, and that the true source bodies of OCs must be found amoung the larger S asteroids, specifically the S(IV) subclass of Gaffey. A variety of rationalizations have been brought forward to argue that the discovery of Boznemcova has not fundamentally altered the terms of the S-asteroid debate. Most of these ideas derive from classical concepts of asteroids evolved in the 1960s and 1970s which do not stand up to more recent data. Geissler P.* Petit J.-M. Greenberg R. Ejecta Reaccretion on Rapidly Rotating Asteroids: Implications for 433 Eros and 243 Ida The molphology of crater ejecta blankets and the distribution of impact-derived regolith on asteroids such as 433 Eros and 243 Ida may vary markedly with surface location, due to their low gravity, nonspherical shape and rapid rotation. We have modelled the processes of ejecta reaccretion and regolith redistribution for the specific cases of Eros and Ida with a 3-dimensional numerical simulation which tracks the orbits of particles launched from the surface of a rotating triaxial ellipsoid. The results predict radically different ejecta distributions for rapidly rotating asteroids, depending upon the assumed initial ejecta launch velocity. Thursday, March 16, 1995 INTERPLANETARY DUST 1:30 p.m. Room A Chair(s): D. F. Blake J. F. Kerridge Liou J. C.* Zook H. A. Dermott S. F. Orbital Evolution of Micron-sized Dust Grains Coming from the Kuiper Belt Since the discovery of the first trans-Neptune object and the still ongoing discoveries of such objects, it has been proposed that micron-sized dust particles produced from those so-called "Kuiper belt objects" may contribute significantly to the interplanetary dust population that constitutes the zodiacal cloud. We report here preliminary results from numerical calculations of the orbital evolution of such dust particles. Due to Poynting-Robertson (PR) drag and solar wind drag, micron-sized dust particles will spiral toward the Sun once they are released from their large parent bodies. This motion leads dust particles to pass by planets as well as encounter numerous mean motions resonances (MMRs) associated with planets. Trapping into exterior MMRs with Neptune and, to a lesser extent, Uranus dominates the orbital evolution of those particles. Gravitational scattering by Saturn or Jupiter usually controls their final fate. Sixteen out of twenty particles in our numerical modeling eventually were scattered out of the Solar System. The remaining four particles were able to complete their journeys all the way to the Sun. Brownlee D. E.* Joswiak D. J. Schlutter D. J. Pepin R. O. Bradley J. P. Love S. G. Identification of Individual Cometary IDP's by Thermally Stepped He Release Interplanetary dust particles (IDPs) collected in the stratosphere provide a moderately representative sampling of the main belt asteroids and the short period comets that are the major dust producers in the solar system. Distinction between typical IDPs from these two sources is of key interest because the samples should contain materials and records of processes that existed both in the inner region of the solar nebula where the asteroids formed as well as the Kuiper belt region beyond 50 AU where most of the SP comets formed. One approach to distinguish between asteroidal and cometary IDPs is to use systematic differences of heating that these particles experience during hypervelocity entry into the atmosphere. Unlike conventional meteorites, asteroidal dust particles reach the Earth by Poynting-Robertson drag and approach the Earth on asteroid-like orbits with low eccentricity and inclination. They typically enter at velocities near 12 km/s while comet particles, with their higher eccentricity and inclination, typically enter at speeds >14 km/s. Entry calculations indicate typical peak temperature differences of 300 degrees C for comet and asteroid particles of the same size and density. The systematically stronger heating of comet dust can in principle be detected by various temperature dependent properties such as track erasure, magnetite rim formation, volatile depletion, mineralogical changes and He depletion. We utilize several of these indicators but rely primarily on the thermally stepped He release method developed by Nier and Schlutter. He trapped in unheated IDPs is released over a range of temperatures because it is sited in different phases, at various depths in grains, in defects and as interstitial atoms or as bubbles. The Nier-Schlutter technique determines the temperature release profile of particles. Low velocity (weakly heated) IDPs contain He that is released at low and high temperature while high velocity (strongly heated) IDPs retain only He released at high temperature. With this technique individual IDPs in the 5æm to 20æm size range are subjected to a series of 5 second pulses of increasing temperature and liberated 3He and 4He is measured after each pulse. The duration and shape of each pulse is similar to the typical heat pulse encountered by micrometeorites entering the atmosphere. The result for a single particle is an S-shaped curve of the fraction of total He released as a function of temperature. The 50% release temperature is usually the inflection point of the release curve and this best defined point of the curve is used as a quantitative indicator of the maximum temperature experienced by a particle during atmospheric entry. This, along with accurate measurement of the particles mass, size and density can be used to calculate the most probable entry velocity. We are using these techniques along with a series of TEM, SEM and reflectance studies to determine the properties of typical cometary samples. Bradley J. P.* Brownlee D. E. Keller L. P. Reflectance Spectra (from 380-800 Nanometers) of Interplanetary Dust Particles Many of the IDPs collected in the stratosphere are believed to be from asteroids. Asteroidal IDPs probably sample a broad range of objects, in contrast to conventional meteorites which mostly originate from a small fraction of the asteroids. Reflectance spectroscopy is the major technique for classifying asteroids and, from comparison of the spectral characteristics of meteorites and asteroids, it has been possible to infer source regions of some classes of meteorites. In principle, it should also be possible to infer the sources of some asteroidal IDPs using reflectance spectroscopy. The first reflectance spectra of (nine) IDPs were reported one year ago. Spectral data have now been collected from more than a hundred IDPs, several polar micrometeorites, and various (particulate) mineral standards . Rietmeijer F. J. M.* Magnesiusm Loss from Unmelted Stratospheric Interplanetary Dust Particles During Atmospheric Entry Atmospheric entry heating of interplanetary dust particles [IDPs] causes evaporative loss of volatile elements such as sulfur. That other elements are similarly affected is not yet established but qualitative indicators of Mg-loss from unmelted IDPs include (1) profiles of decreasing Mg-content in the matrix of type CM IDP L201103 from its interior to the Fe-oxide rim, (2) 3 - 6 wt% Al(sub)2O(sub)3 in Mg,Fe-silicates where its content reaches crystallochemically high proportions, and (3) the presence of nonstoichiometric single-crystal silicates, (Mg,Fe,Ca)(sub)(2-x) Si(sub)(2+ 0.5x)O(sub)6 or (Mg,Fe,Ca)(sub)(2-x) Si(sub)(l+0.5x)O(sub)4, in many IDPs. I explore here the compositions of single-crystal Mg-rich clinopyroxenes [Cpx] (~35 to 60 mole% CaSiO(sub)3), clinoenstatite [CLEN] and Mg,Fe-olivines in the unmelted IDPs L2011K7, L2005T13, and L2005E40 and relict grains in quenched IDP L2005B22. The location of each energy dispersive spectroscope analysis is on photographic record in order to evaluate the quality of the analyses (e.g. contamination). I argue that evaporative magnesium-loss occurs in unmelted IDPs even when the typical Fe-oxide rim due to atmospheric entry heating is poorly developed. Engrand C. Maurette M.* Zolensky M. Kurat G. Walter J. Electron Microprobe Analyses of Antarctic Micrometeorites and Interplanetary Dust Particles Collected in the Stratosphere Both stratospheric IDPs (herein simply IDPs) and Antarctic micrometeorites (AMMs) consist of highly unequilibrated mineral assemblages, usually including a major carbonaceous component, and show similarities with the most primitive carbonaceous chondrites. Very few attempts have been made to compare IDPs and AMMs on the same scale of analyzed volume, with similar instruments. We report here on precise electron microprobe analyses of "polished" flat surfaces of IDPs and AMMs embedded in the same epoxy resin in order to compare their abundances of major elements on a similar scale. On the scale of about 10 micrometers we observe a strong variability of Si-normalized Al, Mg and Fe abundances, from CI bulk values to the average composition of the much smaller grains from comet Halley. A particular depletion of Ca, S and Ni is observed in both types of grains, with depletion in IDPs being less marked. These results further complicate the identification of the most primitive objects of the solar system and characterization of the dominant interactions between the terrestrial environment and IDPs and AMMs. Bajt S.* Flynn G. J. Sutton S. R. Klock W. Comparison of Fe Oxidation States in Interplanetary Dust Particles and Matrix Fragments of Primitive Chondrites The valence state of Fe has been determined in four chondritic IDPs, L2005 J24, L2011 P12, L2011 N1 and W7027 C5, and on 50 micrometers fragments of three primitive chondrites, Murchison, Orgueil and Semarkona, using micro-XANES technique. We have clearly identified 2 distinct types of IDPs, based on differences in their Fe oxidation states. One type, represented by L2005 J24 and L2011 P12, has about 90% of the Fe in the trivalent state and is very similar to Orgueil, while the second type, represented by L2011 N1 and W7027 C5, has 33-43% Fe in trivalent state. All four particles showed similar patterns for major and trace element composition. Stephan T.* Arndt P. Jessberger E. K. Maetz M. Reimold D. Walter J. Multielement Analysis of Antarctic Micrometeorites Using SEM, EDXA, EMPA, TOF-SIMS, and PIXE Two micrometeorites, 10M8 and 7M2, were selected from the 100-400 micrometers size-fractions of dust collections 1994 and 910115B, respectively, from the blue ice near Cap Prudhome, Antarctica, for a geochemical and petrological study using SEM (scanning electron microscopy), EDXA (energy dispersive X-ray analysis), EMPA (electron microprobe analysis), TOF-SIMS (time-of-flight secondary-ion-mass-spectrometry), and PIXE (proton-induced X-ray emission) to investigate major, minor, and trace element compositions as well as lateral element distributions. Thomas K. L.* Keller L. P. Clemett S. J. McKay D. S. Messenger S. Zare R. N. Hydrated Cluster Particles: Chemical and Mineralogical Analyses of Fragments from Two Interplanetary Dust Particles Chondritic interplanetary dust particles (IDPs) are among the most pristine solar system materials known, yet despite their small size they have been intensely studied. Our previous work on 53 fragments from one anhydrous cluster IDP (L2008 #5) showed that the individual fragments display strong chemical and mineralogical heterogeneity. We are currently extending our studies of cluster particles to those consisting primarily of hydrated fragments in order to compare their chemical and mineralogical heterogeneity to anhydrous cluster particles, micrometeorites, and fine-grained chondrite matrix. Flynn G. J.* Bajt S. Sutton S. R. Klock W. Chemical Composition of Large Stratospheric Dust Particles: Comparison with Stratospheric IDPs, Cluster Fragments, and Polar Micrometeorites Interplanetary dust particles (IDPs) larger than 25 microns collected from the stratosphere overlap in size with the smallest micrometeorites collected from polar ices. Comparison of these particles should permit an understanding of polar alteration effects. Element abundances in 6 stratospheric IDPs, all larger than 35 microns, were measured using the synchrotron x-ray microprobe. These IDPs are not dramatically depleted in Ni or Ge, suggesting the depletions of these elements in polar micrometeorites result from weathering on Earth. Lead was undetectable in each IDP, thus the large Pb enrichments in polar micrometeorites are most likely terrestrial contamination. These large IDPs are not enriched in volatiles relative to CI, indicating they are chemically different from the smaller IDPs. Some of the small, volatile-enriched IDPs are fragments of large clusters, suggesting the structurally weaker IDPs may be chemically distinct from those which survive collection intact. The absence of Zn and S depletions and the survival of hydrated phases in these large IDPs are inconsistent with the severe atmospheric entry heating predicted theoretically for large IDPs. Zolensky M. E.* Thomas K. Iron- and Iron-Nickel Sulfides in Chondritic Interplanetary Dust Particles All examined IDPs contain much more pyrrhotite than troilite, suggesting that the former was a common nebular condensate phase. Hydrous IDPs contain pentlandite which is probably of secondary origin. Bradley J. P.* GEMS and Interstellar Silicate Grains A recent paper has described a population of submicrometer-sized "amorphous silicate" grains (GEMS) which are ubiquitous among the fine-grained matrices of some interplanetary dust particles (IDPs). (GEMS is an acronym for Glass with Embedded Metal and Sulfides). Although GEMS could have formed in the Solar System, it is argued that they are most likely preserved interstellar grains for the following reasons. (1) GEMS are abundant in cometary IDPs. (2) They appear to have suffered prolonged exposure to ionizing radiation. (3) The exposure occurred prior to accretion of the IDPs in which they reside. (4) The physical properties of GEMS are similar to those of interstellar (silicate) grains (as inferred from astronomical observations). If the interstellar origin hypothesis for GEMS is correct, then (a) interstellar dust is available for routine collection in the stratosphere using U2 aircraft and, (b) it is possible to recognize some interstellar components in primitive meteoritic materials based on chemical anomalies. Blake D. F.* Jenniskens P. The Metastable Persistence of Vapor-deposited Amorphous Ice at Anomalously High Temperatures Studies of the gas release, vaporization behavior and infrared (IR) spectral properties of amorphous and crystalline water ice have direct application to cometary and planetary outgassing phenomena and contribute to an understanding of the physical properties of astrophysical ices. Several investigators report anomalous phenomena related to the warming of vapor-deposited astrophysical ice analogs. However gas release, ice volatilization and IR spectral features are secondary or tertiary manifestations of ice structure or morphology. These observations are useful in mimicking the bulk physical and chemical phenomena taking place in cometary and other extraterrestrial ices but do not directly reveal the structural changes which are their root cause. The phenomenological interpretation of spectral and gas release data is probably the cause of somewhat contradictory explanations invoked to account for differences in water ice behavior in similar temperature regimes. It is the microstructure, micromorphology and microchemical heterogeneity of astrophysical ices which must be characterized if the mechanisms underlying the observed phenomena are to be understood. Grady M. M.* Sexton A. Wright I. P. Drolshagen G. Morphology and Chemical Analyses of Residues from Impacts into MLI-Blankets on EURECA The Space Shuttle Atlantis (STS-46), carrying the European Retrievable Carrier (EURECA) was launched from Cape Canaveral on July 31st 1992. EURECA was deployed using the remote manipulator system of the Shuttle on 2nd August 1992 at an orbital altitude of ~ 426 km, ascending to its operational orbit of ca. 500 km within 5 days of deployment. After orbiting the Earth for approximately 11 months, EURECA was retrieved by the Space Shuttle Endeavour (STS-57) on June 24th 1993, and returned to the Kennedy Space Centre. In common with other satellites, EURECA was covered with thermal insulation blankets (multi-layer insulation). During exposure in space, the MLI blankets were bombarded by various particles, as witnessed by a record of impact holes. We have examined these holes, with a view to discerning the relative fluence of natural micrometeoroids and space debris, and obtaining an approximate value for the flux of particles at a height of 500 km during 1993. The holes penetrate the top (beta-cloth) layer of the blanket, the kapton light block underneath and several of the Al foil reflective layers (RL) below. Impact holes are generally ellipsoidal in shape, and vary in mean diameter from ca. 60 to 1300 micrometers; the size distribution is approximately bimodal , although the total number of holes is small, at 63, with maxima at 250-350 micrometers and 450-500 micrometers. One of the largest holes, ~ 610 micrometers in diameter. Thursday, March 16, 1995 VENUS GEOLOGY AND SURFACE PROPERTIES 1:30 p.m. Room B Chair(s): R. A. Brackett M. Bulmer Stofan E. R.* Coronae on Venus: Topographic Variations and Correlations Between Morphology and Regional Setting Coronae on Venus display a wide range of surface characteristics and topography [Pronin A. A. and E. R. Stofan; Stofan E. R. et al.; Squyres S. W. et al.]. Previously, a classification scheme for corona morphology based primarily on tectonic features was developed [Stofan E. R. et al.]. Here, a classification scheme for corona topography is presented. The variations in morphology of coronae may result from differences in mode of origin, or they may result from variations in lithospheric structure and/or regional tectonic environment. In order to assess the effects of regional tectonic environment, the global population of coronae is classified according to regional geologic setting. Correlations between the previously identified classes of coronae, topography, and geologic setting are then assessed. Tormanen T.* Kauhanen K. Topography and Morphology of Double-type Coronae on Venus from Magellan Data We have studied the topography of 40 double-type coronae and corona-like features,which were first identified on the basis of their planform and morphology and which can be divided into 4 morphological classes. A typical corona has a raised annular rim and interior lower than the rim but higher than surrounding plains. Some features have a low annular moat exterior to the structural annulus. We examined the topographic characteristics of the annulus and the corona interior, and the presence of the outer annular moat in the double coronae. The double coronae can be divided into 3 classes based on the presence and characteristics of an annular topographic feature. The topography of the interior and the presence of an outer moat also show some systematic characteristics relative to morphology. The ongoing work focuses on the quantitative examination of the topography. Janes D. L.* Squyres S. W. Lithospheric Control of Plateaus Above Venusian Diapirs Models of the formation of coronae on Venus involve a three-stage process: 1) a rising mantle diapir produces a domical uplift and radial extensional fracturing, 2) flattening of the diapir against the base of the lithosphere results in a plateau-shaped uplift, and 3) relaxation of the plateau takes place to form the characteristic central bowl, raised rim, and encircling moat as well as concentric extensional fracturing. The initial uplift and final relaxation processes have been well studied, but the intermediate phase, the development of plateau-shaped topography, has not. Both layered viscous models and elastic/viscous layered models of a rising spherical diapir produce domical uplift and radially oriented extensional fracturing consistent with observed radially fractured domes. Similarly, both purely viscous and visco-elastic modeling of gravitational relaxation produce the final topography of coronae and concentric extensional fracturing on the outer wall of the surrounding moat. However, these relaxation models require that relaxation begin with a flat-topped uplift, formation of which has not been explored. Moreover, neither the initial uplift models nor the relaxation models produce the concentric extensional fracturing observed onthe upraised rim. Bulmer M. H.* Stofan E. R. Guest J. E. Phenomenology of Calderas on Venus A preliminary global examination of the venusian surface imaged by Magellan SAR revealed a large number of calderas and volcano-tectonic depressions of different types and sizes. A more comprehensive study of the data has begun. A database of calderas and large depressions on Venus has been designed to incorporate information on their morphological and morphometric aspects. The global survey has been conducted using Magellan C1 MIDR products. Thirty C1 MIDRS have been examined which extend in latitude between 7.64 degrees N and 22.61 degrees S. Measurements were taken from the digital SAR images and GTDR framelets using NIH Image and on the topographic map from the Pioneer Venus mission (V50M 6/60 RKT 1981 I-1324). Each record in the database has thirteen fields that provide data on the name type 1, latitude, longitude, location, diameter, along and across track length, height, basal altitude, vent distribution, associated lava flows and associated tectonics. A comments field also provides information on the data products and supplementary data on the caldera or large depression as well as its geological setting. Guest J. E.* Lancaster M. G. Copp D. L. Speculations on Lava Flow Textures on Venus Magellan images of Venus reveal that the surfaces of most lava flows within fields of digitate and sheet-like morphology [Lancaster M. G. et al.] may be quite smooth