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