ALBERT

Description: The rocks and soils of the moon will be the raw materials for various engineering needs at a lunar base, such as sources of hydrogen, oxygen, metals, etc. The material of choice for most of the bulk needs is the regolith and its less than 1 cm fraction, the soil. For specific mineral resources it may be necessary to concentrate minerals from either rocks or soils. Therefore, quantitative characterizations of these rocks and soils are necessary in order to better define their mineral resource potential. However, using standard point-counting microscopic procedures, it is difficult to quantitatively determine mineral abundances and virtually impossible to obtain data on mineral distributions within grains. As a start to fulfilling these needs, Taylor et al. and Chambers et al. have developed a procedure for characterization of crushed lunar rocks using x ray digital imaging. The development of a similar digital imaging procedure for lunar soils as obtained from a spectrometer is described.

Description: This paper describes a study conducted by NASA Ames Research Center (ARC) in collaboration with the Jet Propulsion Laboratory (JPL), Pasadena, California on the image acceptability of the Galileo Low Gain Antenna mission. The primary objective of the study is to determine the impact of the Integer Cosine Transform (ICT) compression algorithm on Galilean images of atmospheric bodies, moons, asteroids and Jupiter's rings. The approach involved fifteen volunteer subjects representing twelve institutions involved with the Galileo Solid State Imaging (SSI) experiment. Four different experiment specific quantization tables (q-table) and various compression stepsizes (q-factor) to achieve different compression ratios were used. It then determined the acceptability of the compressed monochromatic astronomical images as evaluated by Galileo SSI mission scientists. Fourteen different images were evaluated. Each observer viewed two versions of the same image side by side on a high resolution monitor, each was compressed using a different quantization stepsize. They were requested to select which image had the highest overall quality to support them in carrying out their visual evaluations of image content. Then they rated both images using a scale from one to five on its judged degree of usefulness. Up to four pre-selected types of images were presented with and without noise to each subject based upon results of a previously administered survey of their image preferences. Fourteen different images in seven image groups were studied. The results showed that: (1) acceptable compression ratios vary widely with the type of images; (2) noisy images detract greatly from image acceptability and acceptable compression ratios; and (3) atmospheric images of Jupiter seem to have higher compression ratios of 4 to 5 times that of some clear surface satellite images.

Description: A Mars Pathfinder landing site in Melas Chasma (Valles Marineris) would yield significant science return, but is outside present mission constraints. In Melas Chasma, Mars Pathfinder could investigate minimally altered basaltic material, sedimentary deposits, chemical weathering, tectonic features, the highland crust, equatorial weather, and Valles mists. Critical issues include the following: (1) nature and the origin of the Valles interior layered deposits, important for understanding water as a sedimentary and chemical agent, and for the past existence of of environments favorable for life; (2) compositions of little-altered basaltic sands, important for understanding magma genesis and weathering on Mars, and the martian meteorites; and (3) structure and composition of the highland crust, important for understanding Mars' early history .

Description: Goldstar radar can provide topography 'profiles', statistical surface roughness, and radar images within a few degrees of the sub-Earth point. Goldstone/Very Large Array (VLA) bistatic radar observations can image the whole disk of Mars with integration times on the order of ten min before pixel smearing occurs. Data from all these radar techniques can be useful for observing the local surface conditions relating to landing safety issues for Mars Pathfinder.

Description: The goal of the ASI/MET experiments is to extend our knowledge of Mars atmosphere structure and meteorology over that established by the Viking mission. The two in situ soundings of Mars atmosphere by Vikings 1 and 2 were highly similar, but radio occultations and infrared soundings have shown large variability in atmosphere structure on Mars with latitude, season, and terrain elevation. It would be of great interest to obtain an in situ sounding showing strong contrast in thermal structure with the Viking profiles. These would be expected to occur in the winter season, in the southern hemisphere, or at polar latitudes. These options are ruled out by Pathfinder Mission constraints, which place the entry in low, northern latitudes in mid summer, with small seasonal difference from the two Viking landers, and small latitude difference from Viking 1.

Description: This site is located at 16 deg N, 177 deg W on the flood plains of Marte Valles, which is perhaps the youngest channel system on Mars. The young age of this channel warrants investigation because of climatic implications for fluvial activities in recent geologic time. The paucity of craters makes this an excellent site in terms of safety requirements. Some of the objectives stated previously for the Maja Valles region would also apply to this site (grab bag of rock types, etc.).

Description: The northwest Isidis Basin offers a unique opportunity to land near a fretted terrain lowland/upland boundary that meets both the latitudinal and elevation requirements imposed on the spacecraft. The landing site lies east of erosional scarps and among remnant massif inselbergs of the Syrtis Major volcanic plains. The plains surface throughout Isidis exhibits abundant, low-relief mounds that are the local expression of the 'thumbprint terrain' that is common within a few hundred kilometers of the lowland/upland boundary. The massif inselbergs are not as numerous nor as massive as those fretted terrains to the northwest, so local slopes are not expected to be steep. Neither feature should pose a serious threat to the lander. Landing on or adjacent to one of these features would enhance the science return and would help to pinpoint the landing site in Viking and subsequent orbiter images by offering views of landmarks beyond the local horizon.

Description: Several of the most fundamental issues about the geology of Mars can be addressed using information on composition and structure of the plateau plains ('highlands') that cover approximately half the planet. The units that compose the highlands are interpreted as a mixture of volcanic, fluvial, lacustrine, and impact ejecta deposits. A more precise inventory of differing of igneous and sedimentary lithologies in highland rock units would not only lead to a better understanding of how the plateau plains formed, but would also clarify the nature of the surface environment during the first 800 m.y. of martian history. Structural features including bedforms, joints, and small faults that are unresolved from orbit record a history of the emplacement and deformation of the highlands. In addition, weathering products present in this very ancient terrain represent a mineralogic record of past climate and of the pathways by which bedrock is altered chemically. Their similarity or dissimilarity to bright soils observed spectroscopically and in situ at the Viking Lander sites will be evidence for the relative roles of regional sources and global eolian transport in producing the widespread cover of 'dust.' Unfortunately, these issues are difficult to address in the plateau plains proper, because bedrock is covered by mobile sand and weathering products, which dominate both surface composition and remotely measurable spectral properties. However, the 'Tartarus Colles' site, located at 11.41 deg N, 197.69 deg W at an elevation of -1 km, provides an excellent opportunity to address the highland geology within the mission constraints of Mars Pathfinder. The site is mapped as unit HNu, and consists of knobby remnants of deeply eroded highlands. It contains rolling hills, but lacks steep escarpments and massifs common in most highland remnants, and is free of large channels that would have removed colluvium from eroded upper portions of the stratigraphic column. These characteristics indicate that variety of bedrock types from throughout the Noachian-Hesperian stratigraphic column may remain at the site.

Description: A strategy for Pathfinder site selection must be developed that is fundamentally different from most previous considerations. At least two approaches can be identified. In one approach, the objective is to select a site representing a key geologic unit on Mars, i.e., a unit that is widespread, easily recognized, and used frequently as a datum in various investigations. The second approach is to select a site that potentially affords access to a wide variety of rock types. Because rover range is limited, rocks from a variety of sources must be assembled in a small area for sampling. Regardless of the approach taken in site selection, the Pathfinder site should include eolian deposits and provisions should be made to obtain measurements on soils. A recommended approach for selecting the Mars Pathfinder landing site is to identify a deltaic deposit, composed of sediments derived from sources of various ages and geologic units that shows evidence of eolian activity. The site should be located as close as possible to the part of the outwash where rapid deposition occurred because the likelihood of 'sorting' by size and composition increases with distance, decreasing the probability of heterogeneity. In addition, it is recommended that field operation tests be conducted to gain experience and insight into conducting science with Pathfinder.

Description: A very important surface component, typically described as 'dark gray material', was not seen at the Viking lander sites, but is common to all low-albedo regions on Mars. Dark material probably includes unaltered mafic volcanic and/or crustal rock and soil not coated by dust, weathering rinds, or varnish. A Pathfinder landing in Cerberus (9 deg N - 16 deg N, 194 deg W - 215 deg W) will guarantee examination of materials that are distinctly different from the two Viking lander sites. In situ study of dark material will provide vital ground truth for orbiter-based observations like those anticipated from Mars '94/'96 and Mars Global Surveyor.

Description: Along a 500 km-wide belt extending between 202 deg and 180 deg W and lying astride the martian equator, moderately low-albedo, uncratered smooth plains exhibit low thermal inertia and potentially favorable conditions for the preservation of near-surface ice. The Cerberus Plains occupy a topographic trough as much as 2 km below the planetary datum, and the denser atmosphere at these altitudes would also favor long residence times for near-surface ice once emplaced. The plains have previously been interpreted as the result of young (late Amazonian) low viscosity lava flows or similarly youthful fluvial deposition. However, the plains are also included in maps of possibly extensive martian paleoseas or paleolakes. Ice emplaced as such seas dissipated could still be preserved under thin (a few tens of centimeters) sedimentary cover. In any case, and if a sea once existed, aqueous-born interstitial cementation, probably including hydrated iron oxides and sulfate minerals, would have been favored and is now susceptible to investigation by the Pathfinder alpha proton x-ray spectrometer and multispectral imager.

Description: The Mars pathfinder MET experiment will make pressure, temperature, and wind measurements on the surface of Mars. The Viking Lander Meteorology Experiment measurements were marked by the presence of variations associated with synoptic weather disturbances throughout the fall and winter season. Numerical simulations of the Mars atmospheric circulation show that the winter midlatitudes are the center of activity for traveling disturbances of planetary scale, disturbances that have their fundamental origin in the baroclinic instability of the wintertime Mars atmospheric circulation. The studies are consistent with Viking observations in that the disturbances decay in amplitude toward lower latitudes. The further north the Mars Pathfinder is located, the more clearly it will be able to detect the signatures of the midlatitude weather system. A landing site close to 15 deg N should allow measurement of the weather disturbances, along with observations of the thermal tides, slope winds, and the relatively steady winds associated with the general circulation - the 'trade winds' of Mars. A landing site near 15 deg N would be significantly further equatorward than the Viking Lander 1 site, and thus would provide more of a view of tropical circulation processes.

Description: The Imager for Mars Pathfinder (IMP) had a single 12-position filter wheel for one of its two 'eyes'. Originally eight, and then nine, of these filters were optimized for surface science, and three narrow-band filters for atmospheric science. Because of some design revisions we will now have filter wheels on both sides. The wheels for right and left eyes are identical, 12 filter positions each, and rigidly linked to the same rotation shaft. There are now 13 surface filters, in addition to 5 for atmospheric observations. Details of all the filter positions are tabulated and approximate gaussian bandpasses for the 13 surface filters are shown.

Description: Four areas fit within the elevation and latitude constraints: Chryse, Elysium, Amazonis, and Isidis. There is geomorphic evidence that all have supported standing water. In some cases it would be difficult to pick a landing site that had no hope of teaching us about the climatic history of Mars. The southeast Elysium Basin provides an optimal target in which a variety of materials may be accessible in a near-shore environment. The albedo of the region is moderately low, and the thermal inertia is indicative of moderate rock coverage or some consolidation of fines, arguing that the site has not been covered with eolian dust deposits.

Description: This candidate landing site is located at 19 deg N, 53.5 deg W near the mouth of a major outflow channel. Maja Valles, and two 'valley network' channel systems, Maumee and Vedra Valles. The following objectives are to be analyzed in this region: (1) origin and paleohydrology of outflow and valley network channels; (2) fan delta complex composition (the deposit located in this area is one of the few identified at the mouth s of any channels on the planet); and (3) analysis of any paleolake sediments (carbonates, evaporites). The primary objectives of the Chryse Outflow Complex region (Ares, Tiu, Mawrth, Simud, and Shalbatana Valles) would be outflow channel dynamics (paleohydrology) of five different channel systems.

Description: The Cerberus Plains in southeastern Elysium and western Amazonis cover greater than 10(exp 5) sq km, extending an east-west distance of approximately 3000 km and a north-south distance of up to 700 km near 195 deg. Crater numbers are 89 plus or minus 15 craters greater than 1 km/10(exp 6) sq km, indicating a stratigraphic age of Upper Amazonian and an absolute age of 200-500 Ma. The material forming the surface is referred to as the Cerberus Formation. The two ideas postulated about the unit's origin are fluvial and volcanic. Regardless of which interpretation is correct, the Cerberus Plains is an important candidate for a pathfinder landing site because it represents the youngest major geologic event (be it fluvial or volcanic) on Mars.

Description: The problems that now confront Mars Pathfinder are much the same as those that confronted Viking, but more and better information exists today. Like Viking, Mars Pathfinder must select a landing site compatible with lander and rover designs as evidenced by available data (Viking images, radar and thermal observations, albedo and color observations, visible-infrared spectra, etc.). Most regions at low elevations probably contain favorable sites, but some sites at low elevations with weak quasispecular echoes and low thermal inertias may be unfavorable.

Description: If Mars Pathfinder were able to land at a site on Mars from which the SNC meteorites were ejected by impact, the Pathfinder mission would essentially represent a very inexpensive sample return mission. Geologic units that contain four potential impact craters from which SNC meteorites could have been ejected from Mars are accessible to the Mars Pathfinder lander. Determining that SNC meteorites came from a particular spot on Mars raises the intriguing possibility of using Pathfinder as a sample return mission and providing a radiometric age for the considerably uncertain martian crater-age timescale. Pathfinder instruments are capable of determining if the rock type at the landing site is similar to that of one or more of the SNC meteorites, which would strengthen the hypothesis that the SNC meteorites did, in fact, come from Mars. Unfortunately, instrument observations from Pathfinder are probably not capable of determining if the geologic unit sampled by the lander is definitively the unit from which a SNC meteorite came from as opposed to Mars in general or perhaps a particular region on Mars. This abstract evaluates the possibility of landing at potential SNC ejection sites and the ability of Pathfinder to identify the landing site as the place from which an SNC meteorite came.

Description: Potential landing sites are confined to latitudes between 0 deg and 30 deg N and surfaces below 0 km elevation. The landing ellipse is 100 x 200 km oriented N 74 deg E. The constraints essentially eliminate the slopes of Elysium Mons, Olympus Mons, Tharsis Ridge, Lunae Plaunum, all the southern highlands, and almost all the Noachian material of Arabia Terra. Those areas that remain as potential landing sites are chiefly lowland plains of Amazonis Chryse, Isidis, and Elysium Planitia. Any attempt to sample highland material further constrains the possible landing sites by eliminating areas of Hesperian or Amazonian lavas and sediments. One possible sampling strategy is to sample materials within those few 'highland' terrains that extend to low elevations. A second strategy is to sample materials at the mouth of an outflow channel that drains from the highlands. Potential landing sites include outflow channel material at the edge of Chryse Planitia and highland materials bordering southern Amazonis Planitia.

Description: The Mars Pathfinder mission (formerly Mars Environmental Survey or MESUR) is scheduled for launch in December 1996 and is designed to place a small lander on the surface of Mars. After impact, the lander unfolds to expose its solar panels and release a miniature rover. Also on board is the Imager for Mars Pathfinder (IMP) binocular camera which is elevated by a deployable mast to obtain a panoramic view of the landing area. The design of this deployable mast is based on similar designs which have a long and successful flight history. In the past when this type of self-deployable mast has been used, a rate limiter has been incorporated to control the speed of deployment. In this application, to reduce weight and complexity, it was proposed to eliminate the rate limiter so that the mast would deploy without restraint. Preliminary tests showed that this type of deployment was possible especially if the deployed length was relatively short, as in this application. Compounding the problem, however, was the requirement to deploy the mast at an angle of up to 30 degrees from vertical. The deployment process was difficult to completely analyze due to the effects of gravitational and inertial loads on the mast and camera during rapid extension. Testing in a realistic manner was imperative to verify the system performance. A deployment test was therefore performed to determine the maximum tilt angle at which the mast could reliably extend and support the camera on Mars. The testing of the deployable mast requires partial gravity compensation to simulate the smaller force of Martian gravity. During the test, mass properties were maintained while weight properties were reduced. This paper describes the testing of a deployable mast in a simulated Martian environment as well as the results of the tests.

Description: The most widely used thermal inertia data for Mars assumes the atmospheric contribution is constant and equal to 2 percent of the maximum solar insolation. Haberle and Jakosky investigated the effect of including a dusty CO2 atmosphere and sensible heat exchange with the surface on thermal inertia. We recently utilized Haberle and Jakosky's coupled surface-atmosphere model to investigate the effects of such an atmosphere on the thermally derived albedo. The thermally derived albedo is the albedo which, together with the thermal inertia, provides model surface temperatures which best match the observed temperatures. New maps are presented of thermal inertia and thermally derived albedo which incorporate dust opacities derived from IRTM data.

Description: In earlier work, we showed using Mossbauer data that the mineralogy of iron-bearing phases in impact melt rocks from Manicouagan Crater (Quebec, Canada) is to a first approximation hematite and pyroxene. The visible and near-IR reflectivity data for these impact melt rocks show a continuous trend in band position from approximately 850 to approximately 1000 nm, which corresponds to the positions for the hematite and pyroxene endmembers, respectively. The oxidation is thought to occur shortly after the impact when oxidizing vapors and/or solutions reacted with the impact melt which was below its solidus temperature but still relatively hot. The reflectance data have important implications for Mars because band positions which occur between approximately 850 and 1000 nm bands observed for Mars can be attributed to hematite-pyroxene assemblages and not necessarily to different ferric mineralogy. Because oxidation of impact melt rocks at Manicouagan is thought to occur subsolidus, precursors for the hematite include oxides such as magnetite and ilmenite and silicates such as pyroxene and olivine. To determine if the Manicouagan impact-melt rocks are related to each other by simple thermal subsolidus oxidation, we calcined in air a relatively unoxidized Manicouagan impact-melt rock (MAN-74-608A). Previous work has shown that np-Hm (nanophase hematite) particles can be derived by calcination of iron-bearing silicates.

Description: Last year we published the GMM-1 (Goddard Mars Model-1) gravity model for Mars. We have completely re-analyzed the Viking and Mariner 9 tracking data in the development of the new field, designated GMM-2. The model is complete to degree and order 70. Various aspects of the model are discussed.

Description: The present Mars atmosphere is relatively thin and cold. It is not at all like that which is presumed to have been responsible for the formation of valley networks and the heavy erosion of craters during the earliest epochs of martian history. An important goal of Mars exploration is to try to understand the properties of the early atmosphere, the initial inventory of volatiles at the planet's surface, the processes by which the atmosphere and climate have evolved over time, and the current location of volatiles presumed to have been in the atmosphere in the earlier times. The current status of understanding of the escape of volatiles to space over geologic time and the resulting fractionation of isotopes of stable atoms remaining in the atmosphere are described, and a scenario for volatile abundance and evolution that is consistent with the available information on the escape and fractionation of each species is constructed. In particular, the evolution of hydrogen, carbon, oxygen, and nitrogen, as contained in atmospheric (and non-atmospheric) water, carbon dioxide, and molecular nitrogen, is examined.

Description: Ozone is a key to understanding atmospheric chemistry on Mars. The O3 abundance has been inferred from UV spectra by several spacecraft, with the most complete coverage provided by Mariner 9. The Mariner 9 UV spectrometer scanned from 2100 to 3500 Angstroms in one of its two spectral channels every 3 seconds with a spectral resolution of 15 Angstroms and an effective field-of-view of approximately 300 sq km. The only atmospheric absorption in the 2000 to 3000 Angstrom region was assumed to come from the Hardey band system of ozone, which has an opacity of order unity. Therefore, the amount of ozone was inferred by fitting this absorption feature with laboratory data of ozone absorption, as shown in Fig 1. Mars O3 shows strong seasonal and latitudinal variation, with column abundances ranging from 0.2 micron-atm at equatorial latitudes to 60 micron-atm over the northern winter polar latitudes (1 micron-atm is a column abundance of 2.689 x 10(exp 15) molecules cm(exp -2). However, the O3 abundance is never great enough to significantly affect atmospheric temperatures or surface temperatures and frost budgets.

Description: One of the most puzzling mysteries about the planet Mars is the hemispherical asymmetry in the polar caps. Every spring the seasonal polar cap of CO2 recedes until the end of summer, when only a small part, the residual polar cap, remains. During the year that Viking observed Mars, the residual polar cap was composed of water ice in the northern hemisphere but was primarily carbon dioxide ice in the southern hemisphere. Scientists have sought to explain this asymmetry by modeling observations of the latitudinal recession of the polar cap and seasonal variations in atmospheric pressure (since the seasonal polar caps are primarily frozen atmosphere, they are directly related to changes in atmospheric mass). These models reproduce most aspects of the observed annual variation in atmospheric pressure fairly accurately. Furthermore, the predicted latitudinal recession of the northern polar cap in the spring agrees well with observations, including the fact that the CO2 ice is predicted to completely sublime away. However, these models all predict that the carbon dioxide ice will also sublime away during the summer in the southern hemisphere, unlike what is observed. This paper will show how the radiative effects of ozone, clouds, airborne dust, light penetration into and through the polar cap, and the dependence of albedo on solar zenith angle affect CO2 ice formation and sublimation, and how they help explain the hemispherical asymmetry in the residual polar caps. These effects have not been studied with prior polar cap models.

Description: Angrite LEW 87051 consists of large olivine crystals set in a fine-grained groundmass that clearly represents a crystallized melt. A few olivines contain Ca-poor, Cr-rich cores that crystallized from a very different melt than the outer part of the crystals constituting the majority of olivine in LEW 87051. We evaluate a model in which the cores formed through fractional crystallization of one melt, then were incorporated into a different melt as xenocrysts, whereupon the original zoning patterns were modified by diffusion. Using a similar approach, we calculate zoning patterns for the cores that would result from perfect fractional crystallization, compare them with the observed zoning, and determine whether the differences could result from diffusive modification consistent with known diffusion rates for Ca, Mn, and Cr. Using distribution coefficients from the 1400 C, IW + 1 experiments, we computed CaO, Cr2O3, and MnO abundances in the hypothetical parent melt by inverting the olivine at the centers of the cores. We further assumed that the primary zoning profile for CaO is essentially unmodified, because the diffusion rate of Ca in olivine is slow. We carried out the fractional crystallization calculation until the calculated Ca content was that observed at break in zoning profiles at the outer edge of the core. We then normalized the distance of this calculated profile to the length of the observed profile in the olivine core and calculated profiles for MnO and Cr2O3. The CaO zoning profile agrees well with the observed profile. The observed MnO profile is slightly higher than the calculated profile near the edge. Diffusion calculations indicate that reversal of the general trend of primary zoning through diffusion would require that diffusion of Cr is 5-10x faster than that of Mn.

Description: A fragment of a carbonaceous chondrite (#53.12, maximal dimension about 2 mm) containing a phyllosilicate-sulfide vein was found during an inspection of small pieces of the Kaidun meteorite. Phyllosilicate veins are apparently rare in carbonaceous chondrites and have so far only been reported from the Y82162 CI chondrite. In hand sample the vein was visible on two perpendicular faces. The polished section prepared from one side displays a complex structure. A single vein, 150 microns in width, bifurcates, and each branch narrows toward a large rounded object (RO). The section contains abundant ROs, most of them less than or equal to 100 microns in diameter. The vein has sharp contacts to the surrounding matrix, whereas the RO contacts are diffuse. The phyllosilicate in the main vein has a massive texture along the contact, which becomes platy toward the vein center where the crystals protrude into an open space. The texture of the largest RO resembles that of a barred olivine (BO) chondrule. Some of the smaller ROs also texturally resemble chondrules. The BO chondrule contains rounded sulfide-silicate objects and small metal grains covered by oxides. Phyllosilicates of the main vein consist mainly of serpentine. The phyllosilicate near the contact with the matrix has low contents of minor elements and a high Mg/Fe ratio. The composition changes in a regular manner toward the center: Al, Na, Ca, Ni, and S increase, indicating increasing amounts of sulfates admixed. The phyllosilicate vein could only have formed after a substantial rock was formed. Mechanical stress probably opened a crack that was subsequently filled by phyllosilicate, pyrrhotite, and finally by a (Fe,Mg)-sulfate. The source of the matter mobilized to form the vein could have been within the rock itself or outside. No compositional or mineralogical zoning is apparent at the vein-rock contacts. The nature of the transporting agent (liquid H2O or vapor) must also remain an enigma. M. Zolensky has recently observed similar phyllosilicate-filled veins in dark, wet clasts in the Al Rais CR chondrite.

Description: The production rates of cosmogenic nuclides depend on the primary cosmic-ray particles, the irradiated-body's bulk composition, size, and shape, and the sample's composition and shielding depth. Although much work has been done on some of these dependencies, more detailed studies still need to be done on others. This work describes the influence of irradiation geometry on nuclide production in lunar rocks. In most cases, computer simulations of cosmogenic nuclide production were restricted to spherical objects irradiated with a 4 pi isotropic flux (meteoroids) or in lunar core samples irradiated by a 2 pi flux incident on semi-infinite layers or cylinders of huge sizes. Many lunar samples are rocks found on top of the lunar surface. For these rocks, neither of the above-mentioned models correspond to the real conditions. We present results of our simulations of cosmogenic nuclide production in models simulating the irradiation of rocks sitting on top of the lunar surface. The Galactic Cosmic Rays (GCR) production profiles in lunar rocks were calculated using the Los Alamos 3-D Monte Carlo LAHET Code System (LCS). The irradiated object was modeled as the union of a sphere with the radius of the Moon and a small hemisphere with radii varying from 10 to 100 g/sq cm simulating the lunar rock. These calculations for the production of cosmogenic nuclides in lunar rocks by GCR particle show that there are important differences between the results obtained by commonly used geometric irradiation models and the lunar-rock models presented. The steeper GCR production profiles for a rock could help to explain the poor agreement for Be-10 in rock 68815, where slab models give GCR profiles flatter than the observed profiles.

Description: Twenty-eight chondrules separated from Chainpur (LL3.4) were surveyed for abundances of Mn, Cr, Na, Fe, Sc, Hf, Ir, and Zn by Instrumental Neutron Activation Analysis (INAA). Six, weighting 0.6-1.5 mg each, were chosen for Scanning Electron Microscopy (SEM)/Energy Dispersive X-ray (EDX) and high-precision Ce-isotopic studies. LL-chondrite-normalized (Mn/Fe)(sub LL) and (Sc/Fe)(sub LL) were found to be useful in categorizing them. Five chondrules (CH-16, -17, -18, -23, and -28) were in the range 0.5 less than (Mn/Fe)(sub LL) less than 1. 4 and 0.5 less than (Sc/Fe)(sub LL) less than 1.4. The sixth (CH-25) had (Mn/Fe)(sub LL) and (Sc/Fe)(sub LL) ratios of 0.40 and 8.1, respectively, and was enriched in the refractory lithophile elements Sc and Hf and the refractory siderophile element Ir by 2.7 and 4.4x LL abundances respectively. SEM/EDX of exterior surfaces of the chondrules showed they consisted of varying proportions of low- and high-Ca pyroxenes, olivine, glass, kamacite/taenite, and Fe-sulfides. Chromium-53/chromium-52 for the six chondrules and bulk Chainpur (WR) are presented. Chromium-54/chromium-52 is close to terrestrial and does not correlate with Mn/Cr. We provisionally ignore the possibility of initial Cr isotopic heterogeneities among the chondrules. Omitting both the CH-25 and WR data, a linear regression gives initial (Mn-53/Mn-55)(sub I) = 8 +/- 4 x 10(exp -6), corresponding to chondrule formation at Delta(t)(sub LEW) = -9 +/- 4 Ma prior to igneous crystallization of the LEW 86010 angrite. If initial (Mn-53/Mn-55)(sub 0) in the solar system were as high as approximately 4.4 x 10(exp -5) when Allende CAI formed, our data suggest Chainpur chondrules formed approximately 9 Ma later, in qualitative agreement with 'late' I-Xe formation ages for most Chainpur chondrules.

Description: Significant advances in the understanding of space weathering processes were recently reported. Submicroscopic iron blebs were produced in lunar simulant glass and natural terrestrial minerals by high-temperature reduction in controlled atmosphere furnaces. These experiments altered the samples' optical properties and microtextures so that they resembled those of extremely mature lunar soil. The results contributed to a revised model for natural reduction in the regolith. Subsequently, supporting results were obtained by reduction of lunar samples. Research to date has focused on reduction of three lunar surface components: basalt, pyroclastic glass, and mare soil. An extensive set of H reduction experiments with simulants has led to a detailed understanding of reaction mechanisms and kinetics. Reduction experiments using lunar basalt were recently conducted by Carbotek. Reduced samples from these test were analyzed. Reduction experiments on lunar glass 74220 were run at temperatures of 900-1100 C. Reduction efficiency of volcanic glass proved to be a function of the sample's FeO abundance and reaction temperature. We also reduced mare soil 75061 at temperatures of 900-1050 C. Partial reduction of FeO in olivine and pyroxene occurred, but was slower and less complete than reduction of ilmenite. Our experiments on simulants and lunar samples have indicated that the most readily reduced phases in the regolith are ilmenite and glass. Based on initial tests with simulants we proposed refinements to the accepted model for space weathering of the regolith. The impact of a micrometeoroid flash heats and melts and ejects from the impact point a small volume of soil that contains trapped solar wind H and C. Reduction occurs rapidly, while the melt volume is still in motion. When a droplet encounters unmelted soil, it envelopes cold mineral grains. The melt is chilled rapidly. Our analyses of experimentally reduced lunar basalt, glass, and mare soil support the proposed model for space weathering.

Description: Several lines of evidence now suggest that Mercury is a planet rich in moderately-volatile elements such as Na and K. Recent mid-infrared spectral observations of Mercury's equatorial and mid-latitude region near 120 degrees mercurian longitude indicate the presence of plagioclase feldspar. Spectra of Mercury's surface exhibit spectral activity similar to labradorite (plagioclase feldspar with NaAlSi3O8: 30-50 percent) and bytownite (NaAlSi3O8: 10-30 percent). These surface studies were stimulated by the relatively large abundance of Na and K observed in Mercury's atmosphere. An enhanced column of K is observed at the longitudes of Caloris Basin and of the antipodal terrain. Extreme heating at these 'hot' longitudes and severe fracturing suffered from the large impact event could lead to enhanced outgassing from surface or subsurface materials. Alternatively, sputtering from a surface enriched in K could be the source of the observed enhancement. Recent microwave measurements of Mercury also give indirect evidence of a mercurian regolith less FeO-rich than the Moon. An anomalously high index of refraction derived from the whole-disk integrated phase curve of Danjon may also be indicative of surface sulfides contributing to a regolith that is moderately volatile-rich. The recent exciting observations of radar-bright spots at high latitudes also indicate that a substance of high volume scattering, like ice, is present in shadowed regions. Other radar-bright spots have been seen at locations of Na enhancements on the atmosphere. All combined, these pieces of evidence point to a planet that is not severely depleted in volatiles or semi-volatiles.

Description: The possibility that condensed volatiles might be stable in the permanently shadowed polar regions of the Moon and Mercury has been anticipated in a number of theoretical studies. In 1992, VLA-Goldstone and Arecibo observations revealed the presence of bright, depolarized radar features near the poles of Mercury that have been widely interpreted as evidence for the presence of polar ice deposits. Recently acquired high-resolution Arecibo radar images show that the anomalous radar features are concentrated in crater-sized regions whose locations can be made to coincide exactly with the locations of known impact craters in the Mariner-10 images. These new Arecibo images provide an unusual opportunity to learn more about the distribution and history of Mercury's polar ice deposits. We have constructed a thermal model that can predict surface and subsurface temperatures within impact craters on Mercury and the Moon. Included in the calculations are the effects of 1-dimensional subsurface heat conduction, direct sunlight, multiply reflected sunlight within the crater, and re-radiated infrared radiation within the crater. We also use realistic crater topographic profiles for larger flat-floored craters as well as smaller spherical bowl-shaped craters.

Description: We investigate the thermal history of an Earth-like planet with the help of a parameterized mantle convection model including the volatile exchange between mantle and surface reservoirs. The weakening of mantle silicates by dissolved volatiles is described by a functional relationship between creep rate and water fugacity. We use flow law parameters of diffusion creep in olivine under dry and wet conditions. The mantle degassing rate is considered as directly proportional to the seafloor spreading rate, which is also dependent on the mantle heat flow. To calculate the spreading rate, we assume that the heat flow under the mid-ocean ridges is double the average mantle heat flow. The rate of regassing also depends on the seafloor spreading rate as well as on other factors like the efficiency of volatile recycling through island arc volcanism. Both mechanisms (de- and regassing) are coupled self-consistently with the help of the parameterized convection model under implementation of a temperature and volatile-content-dependent mantle viscosity.

Description: How the terrestrial planets obtained their original endorsement of volatiles and proceeded to lose them selectively is the theme of this paper. Where good and convincing answers to these questions are still not available, the principal issues are becoming better and better understood, and thus many of the decisive measurements and calculations needed to obtain them can be well defined. In this paper our present inventories of volatiles such as the noble gases, hydrogen compounds and atmophiles such as CO2, N2, and O2 will be presented. Careful attention will be given to isotopic ratios and their significance. Escape processes past and present - blow off impact erosion, thermal and non-thermal loss of neutral and ionic species will be discussed, and the contribution of each to volatile loss assessed. It will be argued that our understanding of these processes is adequate to explain the present level of fractionation of many of the noble gases and their isotopes on all the terrestrial planets. It will also be argued that we are close to understanding loss of hydrogen well enough to assess reasonably the early abundance of water on early Venus and Mars.

Description: The purpose of this study is to develop a standard method for measuring modal abundances of glass in single agglutinates in the lunar regolith. Not only does agglutinitic glass increase in single agglutinates as clasts of older agglutinates get incorporated into newer agglutinates with increasing maturity, but it is in this glassy phase that nanophase superparamagnetic Fe metal originates as a result of reduction reactions during the agglutination process. We report the results of two sets of independent measurements using two different methods to determine the proportion of glass in single agglutinates. We have used polished grain mounts (PGM) of five hand-picked single agglutinates from Apollo 16 soil 61181. The ISI Scanning Electron Microscope (SEM) fitted with a high-resolution Backscattered Electron (BSE) detector was used to collect high-contrast BSE images of the agglutinates. Several images were collected to represent each single agglutinate. The contrast, brightness, and focus were adjusted to optimize each image collected. Histograms of the grayscale range for all images produced four 'peaks' corresponding to epoxy, glass, crystalline phases, and metal grains. We analyzed every grid point for 12 elements with a less than 1 micron electron beam using a CAMECA SX-50 electron probe microanalyzer (EPM). If any analysis was not within about 10% of the stoichiometry of a known lunar mineral, we considered that point to be nonmineralic. Our results show that there is a remarkable correspondence in the glass percentages obtained by the two methods. The EPM method may overestimate glass because secondary fluorescence from dusty clasts in agglutinitic glass can give the appearance of nonmineralic targets. The BSE method may underestimate glass because the diversity of compositions of agglutinitic glass may not be contained under one grayscale 'peak' during image analysis.

Description: For terrestrial planets, atmospheric compositions are not static, but evolve with time, in part due to degassing of the interior. Unfortunately, the evolution is slow enough that it is usually not observable on human timescales, or even on the timescales of rocks that preserve samples of Earth's ancient atmosphere. Preliminary results on a recently recognized Martian meteorite, ALH84001, indicate that it is a very old rock, and has a relatively high noble gas content suggestive of atmospheric incorporation, but with an isotopic composition slightly inconsistent with currently known Martian reservoirs. Hence, this rock may provide a sample of ancient Martian atmosphere, which can be used to test models of volatile evolution (in particular, degassing) on Mars. ALH84001 is a cumulate orthopyroxenite. Although originally classified as a diogenite, its oxygen isotopes, and several chemical and petrographic features, strong suggest that it is, like the SNC meteorites, Martian. A Sm-Nd crystallization age of 4.5 Ga has been reported. The meteorite is rich in noble gases, compared to most SNC's. In many respects the noble gases are typical of SNC meteorites. However, there are some subtle differences. In particular, the Xe isotopes in SNC meteorites can be explained as a mixture of Martian atmospheric Xe (as represented by glass in EETA 79001), the Xe in the dunite Chassigny (usually assumed to be representative of the Martian interior, and with lower (129)Xe/(132)Xe, (134)Xe/(132)Xe and (136)Xe/(132)Xe ratios), and later additions from known processes like fission, spallation and terrestrial contamination. The isotopic composition of ALH84001 is inconsistent (at greater than 2-3 sigma) with any mixture of those components. Even if no accumulation of fission Xe during the age of the rock is assumed, there is too little (136)Xe and (134)Xe for the amount of (129)Xe measured.

Description: Mercury and the Moon have long been considered to be similar objects, but this view was based on limited information at visible wavelengths. It is now known that real differences exist in the atmospheres and in the rock types as deduced from mid-infrared observations. Lunar Ar and perhaps Ne were observed from Apollo landed experiments in spite of a large local background. An ultraviolet spectrometer (UVS) in orbit placed upper limits on a number of gases including H and O. At Mercury, the UVS on Mariner 10 measured H, He, and perhaps O and again a number of upper limits for other gases. Less than a decade ago, emissions of Na and K were observed from the ground, with abundances 2 orders of magnitude smaller on the Moon than on Mercury. It is likely that impacting meteoroids bring a significant amount of water to Mercury, whose atmosphere should therefore contain water vapor. This notion is supported by the recent discovery of radar-bright polar deposits almost certainly composed of ice. A variety of origins is likely for the different components. Impact of meteoroids could supply water, Na, and K, and could also vaporize surface material; Na, K, and Ar could be degassed from the crust. H is probably from the solar wind, and He could reasonably come from degassing or the solar wind. A substantial loss process for all components is photoionization (or dissociation); ions on one side are swept away into the solar wind and on the other are swept back to the surface. This process is complicated at Mercury by its permanent magnetic field, but there is little doubt that both loss and recycling still occur. Some of the recycling probably occurs by way of the magnetotail through night side auroral precipitation; atoms implanted in the surface will diffuse back into the atmosphere shortly after dawn, an effect that seems to be present for K. Finally the light atoms H and He have high enough velocities to escape directly, an effect that may be enhanced by the suprathermal velocity distributions that are likely to be present.

Description: We wish to draw attention to a major controversy that has arisen in the area of CM-chondrite petrology. The problem is important because its resolution will have profound implications for ideas concerning nebular dynamics, gas-solid interactions in the nebula, and accretionary processes in the nebula, among other issues. On the one hand, cogent arguments have been presented that 'accretionary dust mantles,' were formed in the solar nebula prior to accretion of the CM parent asteroid(s). On the other hand, no-less-powerful arguments have been advanced that a significant fraction of the CM lithology is secondary, produced by aqueous alteration in the near-surface regions of an asteroid-sized object. Because most, if not all, CM chondrites are breccias, these two views could coexist harmoniously, were it not for the fact that some of the coarse-grained lithologies surrounded by 'accretion dust mantles' are themselves of apparently secondary origin. Such an observation must clearly force a reassessment of one or both of the present schools of thought. Our objective here is to stimulate such a reassessment. Four possible resolutions of this conflict may be postulated. First, perhaps nature found a way of permitting such secondary alteration to take place in the nebula. Second, maybe dust mantles could form in a regolith, rather than a nebular, environment. Third, it is possible that dust mantles around secondary lithologies are different from those around primary lithologies. Finally, perhaps formation of CM chondrites involved a more complex sequence of events than visualized so far, so that some apparently 'primary' processes postdated certain 'secondary' processes.

Description: The asteroid 4 Vesta, with its unique basaltic crust, remains a key mystery of planetary evolution. A localized olivine feature suggests excavation of subcrustal material in a crater or impact basin comparable in size to the planetary radius (R(sub vesta) is approximately = 280 km). Furthermore, a 'clan' of small asteroids associated with Vesta (by spectral and orbital similarities) may be ejecta from this impact 151 and direct parents of the basaltic achondrites. To escape, these smaller (about 4-7 km) asteroids had to be ejected at speeds greater than the escape velocity, v(sub esc) is approximately = 350 m/s. This evidence that large fragments were ejected at high speed from Vesta has not been reconciled with the present understanding of impact physics. Analytical spallation models predict that an impactor capable of ejecting these 'chips off Vesta' would be almost the size of Vesta! Such an impact would lead to the catastrophic disruption of both bodies. A simpler analysis is outlined, based on comparison with cratering on Mars, and it is shown that Vesta could survive an impact capable of ejecting kilometer-scale fragments at sufficient speed. To what extent does Vesta survive the formation of such a large crater? This is best addressed using a hydrocode such as SALE 2D with centroidal gravity to predict velocities subsequent to impact. The fragmentation outcome and velocity subsequent to the impact described to demonstrate that Vesta survives without large-scale disassembly or overturning of the crust. Vesta and its clan represent a valuable dataset for testing fragmentation hydrocodes such as SALE 2D and SPH 3D at planetary scales. Resolution required to directly model spallation 'chips' on a body 100 times as large is now marginally possible on modern workstations. These boundaries are important in near-surface ejection processes and in large-scale disruption leading to asteroid families and stripped cores.

Description: Compound chondrules are considered to be the product of collisions between molten chondrules during chondrule formation Wasson, J. T. et al. (1994) have argued that some compound chondrules are formed when a chondrule with an accretional rim experienced a flash-melting event similar to a chondrule-forming event. We have designed experiments to investigate the formation of compound chondrules by both methods. Experiments were performed on a Deltech vertical muffle tube furnace to form synthetic chondrules to use as accretion rim material. For our experimental conditions, it is clear that compound chondrules can only be made by a collisional event. Our changes maintain their spherical shape and produce distinct boundaries between charges that are similar to natural compound chondrules. Furthermore, collision event(s) between chondrules will cause nucleation if they are molten and undercooled, thus producing chondrule textures. Flash melting chondrules with accretionary rims will not produce compound chondrules but will produce new chondrules with new textures.

Description: Current abundances of radiogenic noble gases (4-He, 40-Ar, 129-Xe) in the planetary atmospheres provide valuable information on planetary evolution through their degassing histories. Of these, 40-Ar (from 40-K with half life 1.25 Ga) can be used to envisage long-term degassing from their interior. Present amounts of 36 and 40-Ar should have information on timing and degree of degassing from the interior. On the other hand, 129-Xe (from 129-I with half life 17 Ma) is used in discussing early planetary degassing. Because life time of helium in the atmosphere is short, atmospheric 4-He (from 235-U, 238-U, 232-Th) data might be used only for current degassing. The relative abundance of 40-Ar in the Cytherean atmosphere is 0.26 of that in the Earth's atmosphere. We have executed numerical calculation of a 40-Ar degassing model along with mantle evolution and melt migration. Our result shows that the average magma production rate on Venus is about 5 (km(exp 3)/yr). Duration of plate motion on Venus, if existed, would be less than several hundred million years.

Description: We examine the effects of loss of constituents of the Martian atmosphere due to sputtering by solar-wind pick-up ions and photochemical escape during the last 3.8 billion years. Sputtering is capable of efficiently removing species from the upper atmosphere to space, including the light noble gases; nitrogen and oxygen are removed by both sputtering ad photochemical processes. Due to diffusive separation (by mass) above the homopause, removal from the top of the atmosphere will fractionate the isotopes of each species, with the lighter isotope being preferentially lost. This allows current measurements of the isotopic ratios to be used as a measure of the atmospheric evolution as integrated over geologic time. For carbon and oxygen, isotopic fractionation is buffered by exchange of atmospheric species with non-atmospheric reservoirs of CO2 and H2O. This allows us to determine the size of the non-atmospheric reservoirs which are capable of mixing with the atmosphere; these reservoirs can be CO2 absorbed in the regolith and/or H2O in the polar ice caps. Such an exchangeable reservoir is required in order to keep the fractionation of the atmospheric gases as low as is observed.

Description: As described by Grun et al. the dust impact detector on the Ulysses spacecraft detected a totally unexpected series of dust streams in the outer solar system near the orbit of Jupiter. Five considerations lead us to believe that the dust streams emanate from the jovian system itself: (1) the dust streams only occur within about 1 AU of the jovian system, with the strongest stream being the one closest to Jupiter (about 550 R(sub J) away); (2) the direction from which they arrive is never far from the line-of-sight direction to Jupiter; (3) the time period between streams is about 28 (plus or minus 3) days; (4) the impact velocities are very high - mostly around 40 km s(exp -1); and (5) we can think of no cometary, asteroidal, or interstellar source that could give rise to the above four phenomena; such streams have never before been detected.

Description: Shock compression of the materials of planetary interiors yields data which upon comparison with density-pressure and density-sound velocity profiles constrain internal composition and temperature. Other important applications of shock wave data and related properties are found in the impact mechanics of terrestrial planets and solid satellites. Shock wave equation of state, shock-induced dynamic yielding and phase transitions, and shock temperature are discussed. In regions where a substantial phase change in the material does not occur, the relationship between the particle velocity, U(sub p), and the shock velocity, U(sub s), is given by U(sub s) = C(sub 0) + S U(sub p), where C(sub 0) is the shock velocity at infinitesimally small particle velocity, or the ambient pressure bulk sound velocity. Numerical values for the shock wave equation of state for minerals and related materials of the solar system are provided.

Description: We have employed three-dimensional numerical simulations of the impact of Comet Shoemaker-Levy 9 (SL9) on Jupiter and the resulting vapor plume expansion using the smoothed particle hydrodynamics (SPH) method. An icy body with a diameter of 2 km can penetrate to an altitude of -350 km (0 km = 1 bar) and most of the incident kinetic energy is transferred to the atmosphere between -100 to -250 km. This energy is converted to potential energy of the resulting gas plume. The unconfined plume expands vertically and has a peak radiative power approximately equal to the total radiation from Jupiter's disc. The plume rises a few tens of atmospheric scale heights in approximately 10(exp 2) seconds. The rising plume reaches the altitude of approximately 3000 km; however, no atmospheric gas is accelerated to the escape velocity (approximately 60 km/s).

Description: Clementine was a mission designed to test the space worthiness of a variety of advanced sensors for use on military surveillance satellites while, at the same time, gathering useful scientific information on the composition and structure of the Moon and a near Earth asteroid. Clementine was dispatched for an extended stay in the vicinity of Earth's Moon on 25 Jan. 1994 and arrived at the Moon on 20 Feb. 1994. The spacecraft started systematic mapping on 26 Feb., completed mapping on 22 Apr., and left lunar orbit on 3 May. The entire Clementine project, from conception through end of mission, lasted approximately three years. Topographic profiles derived from lidar laser altimetry permitted construction of a global topographic map of the Moon. Clementine also aimed at mapping the color of the Moon in eleven different wavelengths in the visible and near infrared parts of the system. With rock and soil samples of known geological context available from the Apollo and Lunar programs, the Clementine mission offers the data needed to construct a global digital image model of the Moon.

Description: This document is the final report of the MAUDEE (Mars Upper Atmosphere Dynamics, Energetics, and Evolution) consortium. It describes a low cost Discovery mission to investigate the upper atmosphere of Mars and to understand the manner in which Mars has evolved over geologic time. In keeping with the innovative philosophy permeating the Discovery Program and in order to minimize the burden of reading an extensive prose exposition, a new presentation format has been adopted. The format involves a series of view graphs with facing text. The view graphs form the basis of a complete oral presentation of the MAUDEE mission and the facing text provides more detailed, but still brief, explanatory descriptions. Readers can scan the view graphs and/or read the facing text at their discretion. The oral presentation of this study was given to code SL personnel at NASA Headquarters on February 23, 1994. MAUDEE is an essential component of the Mars Exploration Program. It provides the information required to understand the evolution of the planet via the escape of volatiles. It provides the key measurements needed to understand the upper atmosphere of the last of the three terrestrial planets to be so studied. It connects and supplements investigations based on other Mars missions: Mars Surveyor, Planet-B and Mars-96. The MAUDEE mission plan involves a combination of remote and in-situ sensors, housed in three instrument packages. The sensors make measurements of the atmospheric regions between 60-200 km. These instruments are based on extensive heritage from Earth explorers and Pioneer Venus. The mission scenario has several phases and employs aerobraking maneuvers to lower initial apoapsis, thereby reducing fuel requirements. The spacecraft has body-mounted solar cells, enabling deep diving into the Martian atmosphere. The orbital inclination allows for pole-to-pole latitudinal sweeps in an initial elliptical phase, followed by a circular phase affording detailed diurnal measurements. The nominal mission duration at Mars is one Mars year.

Description: Minor absorption bands in the 0.55 to 0.7 micron wavelength range of reflectance spectra of 10 S asteroids have been found and compared with those of spinel-group minerals using the modified Gaussian model. Most of these S asteroids are consistently shown to have two absorption bands around 0.6 and 0.67 micron. Of the spinel-group minerals examined in this study, the 0.6 and 0.67 micron bands are most consistent with those seen in chromite. Recently, the existence of spinels has also been detected from the absorption-band features around 1 and 2 micron of two S-asteroid reflectance spectra, and chromite has been found in a primitive achondrite as its major phase. These new findings suggest a possible common existence of spinel-group minerals in the solar system.

Description: Substantial evidence suggests that a UV Spectrally Absorbing Material (UV-SAM) exists on Triton's surface. This evidence is found in the positive slope in Triton's spectrum from the UV to the near-IR, and the increasing contrast in Triton's light curve in the blue and UV. Although it is now widely-thought that UV-SAM's exist on Triton, little is known about their distribution and spectral properties. The goal of this NDAP Project is to determine the spatial distribution and geological context of the UV-SAM material. We hope to determine if UV-SAM's on Triton are correlated with geologic wind streaks, craters, calderas, geomorphic/topographic units, regions containing (or lacking) volatile frosts, or some other process (e.g., magnetospheric interactions). Once the location and distribution of UV-SAM's has been determined, further constraints on their composition can be made by analyzing the spectrographic data set. To accomplish these goals, various data sets will be used, including Voyager 2 UV and visible images of Triton's surface, IUE and HST spectra of Triton, and a geologic map of the surface based on Voyager 2 and spectrophotometric data. The results of this research will be published in the planetary science literature.

Description: This directory provides information about the scientific investigations funded by the NASA Venus Data Analysis Program (VDAP) during fiscal year 1993. The VDAP Directory consists of summary sheets from the proposals that were selected by NASA for funding in FY 93. Each summary sheet indicates the title, principal investigator, institution of the investigation, and information related to the objectives of the research activities proposed for FY 93. The objective of the VDAP Program is to advance our understanding of the nature and evolution of Venus. VDAP supports scientific investigation using data obtained from the Magellan, Pioneer Venus, and other Venus missions, as well as earth-based observations that contribute to understanding the physical and evolutionary properties of Venus. The program intends to enhance the scientific return from these missions by broadening the participation in the analysis of Venus data. Categories of research funded by VDAP are atmosphere, ionosphere, geology, geophysics, and mapping. The directory is intended to provide the science community with an overview of the research projects supported by this program. Research activities identified in this directory were selected for funding in FY 93 on the basis of scientific peer review conducted by the VDAP Review Panel.

Description: Careful consideration must be given to the simultaneous treatment of the radiative transfer of the CO2 15 micron band and dust calculations for the Martian winter polar region show that a simple sum of separately calculated CO2 cooling rates and dust cooling rates can easily result a 30 percent error in the net cooling particularly near the surface. CO2 and dust hinder each others ability to cool the atmosphere. Even during periods of low dust opacity, dust still reduces the efficacy of CO2 at cooling the atmosphere. At the other extreme, when dust storms occur, CO2 still significantly impedes the ability of dust to cool the atmosphere. Hence, both CO2 and dust must be considered in radiative transfer models.

Description: Kaidun is a remarkable chondrite breccia fall containing lithic clasts that span a wide range of chondrite groups including C and E chondrites, as well as having clasts with characteristics not yet found in existing chondrite samples. The dominant lithology in Kaidun appears to be CR chondritic, consonant with recent O isotope data. The carbonates in Kaidun are presented as one mineralogical basis for comparing it to the other hydrated chondrites and to better understand its relative alteration history. The four polished thin sections of Kaidun studied contained a variety of lithologies that we classified into four groups -- CR, E, CM-like, and dark inclusions (DIs). DIs contain sulfide and magnetite morphologies that superficially resemble CI chondrites, and some of the previously reported CI lithologies in Kaidun may be what we term DIs. Carbonates were found in all lithologies studied. Carbonates in Kaidun are similar in composition to those in CR chondrites. Some of the DIs in Kaidun, previously characterized as CI, have carbonates similar to those in CR chondrites and are unlike those in CI or CM chondrites. Most carbonates in Kaidun and CR chondrites are calcites, some of which formed at temperatures above 250 C. Dolomite is less common and some may be metastable. Alteration temperatures in the Renazzo CR chondrite were estimated to be approximately 300 C, based on O isotope fractionation between phyllosilicates and magnetite. Temperatures of up to 450 C were proposed for the alteration of a CR-like dark inclusion in Kaidun, based on the presence of hydrothermal pentlandite veins. The alteration temperatures for Kaidun and the other CR chondrites are considerably higher than those suggested for CI or CM parent bodies.

Description: The origin of mesosiderites, which consist of approximately equal-weight proportions of Fe-Ni metal and silicates (gabbros, basalts, orthopyroxenites, dunites), remains an interesting and complex problem in meteoritics. There is general agreement that multiple impact events were probably involved in the formation of these brecciated stony-iron meteorites, but given the heterogeneity of mesosiderites, additional processes have been invoked to explain the unique and intricate textural and compositional makeup of mesosiderites. We conducted a series of shock recovery experiments to test the impact event(s) scenario. The results indicated significant similarities between the shocked analogs and many mesosiderites. We have compared our analogs with a suite of thin sections of Barea mesosiderite. I have conducted a series of flash heating experiments in which equal-weight proportions of gabbro and stainless steel (SS304) powders were compressed into small charges and heated under reducing conditions for short times. These experiments were used to bracket localized, peak postshock temperatures in our analog shots and to compare the mixing relations between the silicate and metal. The shock recovery experiments used porous metal-silicate powder starting mixtures, therefore our experiments are most analogous to an impact scenario where the target is an asteroidal regolith surface composed of a loose mixture of Fe-Ni metal and heterogeneous silicates. Analog experiments may really describe a secondary impact process similar to the late-stage, localized impact melting event. This leaves one of the crucial questions about mesosiderite genesis unanswered, namely what is the source of the Fe-Ni metal that is so intimately distributed in these meteorites?

Description: Recognition and characterization of the different CM lithologies as components in all meteorites could reveal details of the nature and chronology of alteration and brecciation events on hydrous asteroids. The CM chondrites are of particular interest, as they are the most common carbonaceous chondrites and are found as clasts within other types of meteorites, which suggests that the CM parent asteroids are (or were) widespread in the sections of the asteroid belt providing samples to Earth. Some CM2s, including EET 90047, ALH 83100, and Y 82042, are more 'extensively' altered, and are distinguished by a high proportion of Mg-rich phyllosilicates and Ca-Mg carbonates, frequently in rounded aggregates, and near absence of olivine or pyroxene. 'Completely' altered CMs, called CM1s, essentially lack olivine or pyroxene; these include EET 83334, ALH 88045, and the CM1 clasts in Kaidun. Cold Bokkeveld and EET 84034, both highly brecciated CMs, consist of both extensively and completely altered lithologies. We describe how these lithologies further cosntrain physicochemical conditions on hydrous asteroids. We conclude that CM chondrites exhibit the petrologic range 2 through 1, and that progressive alteration on the parent hydrous asteroid(s) was accompanied by significant increases in temperature (to a peak of approximately 450 C), fO2, water-rock ratio, and (locally) degree of chemical leaching, all well beyond the conditions recorded by CM2s.

Description: Martian orthopyroxenite ALH 84001 is unusual compared to other martian meteorites in its abundance of Mg-Fe-Ca carbonites. Becasue textural evidence indicates that these carbonates are undoubtedly of martian origin, we have undertaken stable isotopic studies to elucidate their origin by evaluating whether they represent primordial martian C that was outgassing from the mantle of Mars, or volatile additions to the ALH 84001 protolith that equilibrated with the martian atmosphere. If precipitation occurred in a closed system then the isotopic results are compatible with the observed chemical zonation. A unique temperature of formation can be calculated using the difference in C-13 and O-18 between the Fe and Mg carbonates, assuming that precipitation occurred at a constant temperature. Precipitation of approximately one-half of the CO2 reservoir at 320 C can account for the observed values, with the original CO2 reservoir having a delta C-13 of approximately 45% and delta O-18 of approximately 22%. If carbonate precipitated in equilibrium with a large isotopically homogeneous CO2 reservoir (open system), isotopic differences must be attributed to a change in temperature of at least several hundreds of degrees. This temperature change is compatible with a calculated range of temperatures based on carbonate geothermometry. Clearly, carbonate in ALH 84001 is in delta O-18 disequilibrium with orthopyroxene groundmass. Most likely, the carbonate precipitated from a fluid that equilibrated with the martian atmosphere. The deposits or fluids in equilibrium with these deposits were remobilized in the crust producing the carbonate in ALH 84001. This observation establishes a link for the first time between the atmospheric and lithospheric C and O pools that reside on Mars.

Description: ALH 84001, a ferroan martian orthopyroxenite, originally consisted of three petrographically defined components: a cumulus assemblage of orthopyroxene + chromite, a trapped melt assemblage of orthopyroxene(?) + chromite + maskelynite + apatite + augite +/- pyrite, and a metasomatic assemblage of carbonate +/- pyrite. We present the results of Instrumental Neutron Activation Analysis (INAA) study of five bulk samples of ALH 84001, combined with Scanning Ion Mass Spectrometer (SIMS) data on the orthopyroxene, in order to attempt to set limits on the geochemical characteristics of the latter two components, and therefore on the petrogenesis of ALH 84001. The INAA data support the petrographic observations, suggesting that there are at least three components in ALH 84001. We will assume that each of the three geochemically required components can be equated with one of the petrographically observed components. Both trapped melt and metasomatic components in ALH 84001 have higher Na than orthopyroxene based on compositions of maskelynite, apatite, and carbonate. For the metasomatic component, we will assume its Na content is that of carbonate, while for a trapped melt component, we will use a typical Na content inferred for martian meteorite parent melts, approximately 1 wt% Na2O. Under these assumptions, we can set limits on the Light Rare Earth Elements/Heavy Rare Earth Elements (LREE/HREE) ratios of the components, and use this information to compare the petrogenesis of ALH 84001 with other martian meteorites. The above calculations assume that the bulk samples are representative of different portions of ALH 84001. We will also evaluate the possible heterogeneous distribution of mineral phases in the bulk samples as the cause of compositional heterogeneity in our samples.

Description: Exploration of the Moon and planets began with telescopic studies of their surfaces, continued with orbiting spacecraft and robotic landers, and will culminate with manned exploration and sample return. For the Moon and Mars we also have accidental samples provided by impacts on their surfaces, the lunar and martian meteorites. How much would we know about the lunar surface if we only had lunar meteorites, orbital spacecraft, and robotic exploration, and not the Apollo and Luna returned samples? What does this imply for Mars? With martian meteorites and data from Mariner, Viking, and the future Pathfinder missions, how much could we learn about Mars? The basis of most of our detailed knowledge about the Moon is the Apollo samples. They provide ground truth for the remote mapping, timescales for lunar processes, and samples from the lunar interior. The Moon is the foundation of planetary science and the basis for our interpretation of the other planets. Mars is similar to the Moon in that impact and volcanism are the dominant processes, but Mars' surface has also been affected by wind and water, and hence has much more complex surface geology. Future geochemical or mineralogical mapping of Mars' surface should be able to tell us whether the dominant rock types of the ancient southern highlands are basaltic, anorthositic, granitic, or something else, but will not be able to tell us the detailed mineralogy, geochemistry, or age. Without many more martian meteorites or returned samples we will not know the diversity of martian rocks, and therefore will be limited in our ability to model martian geological evolution.

Description: Chondrule J2689, a large (8-mm) RP chondrule from Bachmut (L6) was previously found to be in disequilibrium with its host in a variety of features: (1) It has a fine-grained hornfelsic texture; (2) it contains low amounts of metal with a low-Ni taenite composition not found in the matrix; (3) the Ni/Co ratio of that metal is close to the solar ratio, which is equal to the bulk Ni/Co ratio (23) of the chondrule; (4) the bulk alkali content of the chondrule is high and the Na/K ratio is fractionated with respect to the average L chondrite ratio of 8; (5) Cr is depleted in spite of the high pyroxene content; (6) the siderophile elements are strongly depleted but are fractionated with their abundances increasing with volatility; and (7) the O isotopes of the chondrule and the host are out of equilibrium. However, the (Fe,Mg) silicates, feldspar, and chromite have chemical compositions indistinguishable from that of the host chondrite. We have studied a chip of the chondrite and of adjacent matrix by stepwise heating and by combustion in O for N and Xe isotopic abundances. (1) The chondrule preserved distinct bulk, metal, and O isotopic compositional features. It is therefore unlikely that the 'equilibration' of the major silicates Fe/Mg ratios could have taken place after accretion; (2) the chondrule was well equilibrated before break-up and exposure to cosmic rays; (3) two N signatures in the matrix also indicate that the matrix is not equilibrated; and (4) all data collected so far point toward the presence of unequilibrated Bachmut components. Very few reactions took place after accretion.

Description: Ranging in size from mere grains and palm-size stones to boulders and many-mile- wide hunks of rock, meteorites hold many secrets of our solar system, and indeed of our universe. The 57th Annual Meeting of the Meteoritical Society discussed many aspects of this fascinating 'chunk' of the evolution of the Solar System. Topics covered included: chemical composition, meteorite types, meteorite age determination, meteorite origins, and find locations, as well as a multitude of other important subjects.

Description: Clementine was a mission designed to test the space-worthiness of a variety of advanced sensors for use on military surveillance satellites while, at the same time, gathering useful scientific information on the composition and structure of the Moon and a near-Earth asteroid. Conducted jointly by the Ballistic Missile Defense Organization (BMDO, formerly the Strategic Defense Initiative Organization) of the US Department of Defense and NASA, Clementine was dispatched for an extended stay in the vicinity of Earth's moon on 25 January 1994 and arrived at the Moon on 20 February 1994. The spacecraft started systematic mapping on 26 February, completed mapping on 22 April, and left lunar orbit on 3 May. The entire Clementine project, from conception through end-of-mission, lasted approximately 3 years.

Description: Planetary volatile inventories are products of several factors: (1) condensation-accretion of pre-planetary material which determines the bulk volatile inventory; (2) energy history of a planet, including timing, causes, and mechanisms of degassing; (3) the volatile sinks, including temporary, long term, and permanent; and (4) external processes operating on the volatile inventory. Information regarding the current surface compositions provide insight into both internal and surface-atmosphere evolutionary history. Our discussion focuses upon the surface composition of outer solar system planets and satellites as determined by spacecraft and telescopic spectral observations. We provide a review and an update of the recent work by Cruikshank and Brown that includes more recent observations and interpretations. In the context of formation and evolution of solar system bodies, the interesting ices typically considered are simple molecules formed from elements having high cosmic abundances. These mainly include ices of H2O, NH3, SO2, H2S, CH4, CO, CO2, and N2. In the solid state, these ices have vibrational spectral features, analogous to their gaseous counterparts but rotational transitions are quenched, that lie in the near- and mid-infrared. The overtone and combination modes, occurring in the visible and near-IR region, are of particular importance as standard observational techniques used to identify these ices rely upon reflected solar energy. Table I summarizes the ices found on various bodies in the outer solar system. H2O is most abundant surface material in the inner and middle regions while more volatile species appear to dominate surfaces in the outermost edge of the outer solar system.

Description: No convincing explanation for the anomalous behavior of the Atmospheric Structure Experiment temperature sensors at approximately 13 km altitude has been found. It occurred on all of the widely-spaced probes, in a similar fashion. A preliminary effort has been made to determine atmospheric chemical species which might be present at 13 km. The purpose of this effort is to initiate suggestions of possible chemical interactions and to explore the effects of the presence of possible metal reactants including condensation. Equilibrium fractions of chemical species were calculated at a variety of conditions. Baseline calculations were made for the altitudes near 13 km. For comparison calculations were also made at 13 km but with the introduction of plausible metal atoms.

Description: Continued press coverage of the comet Shoemaker-Levy 9 impact on the surface of Jupiter is presented. This tape covers 21 Jul. 1994.

Description: A broad variety of volcanic edifices have been observed on Venus. They ranged in size from the limits of resolution of the Magellan SAR (i.e., hundreds of meters) to landforms over 500 km in basal diameter. One of the key questions pertaining to volcanism on Venus concerns the volume eruption rate or VER, which is linked to crustal productivity over time. While less than 3 percent of the surface area of Venus is manifested as discrete edifices larger than 50 km in diameter, a substantial component of the total crustal volume of the planet over the past 0.5 Ga is related to isolated volcanoes, which are certainly more easily studied than the relatively diffusely defined plains volcanic flow units. Thus, we have focused our efforts on constraining the volume productivity of major volcanic edifices larger than 100 km in basal diameter. Our approach takes advantage of the topographic data returned by Magellan, as well as our database of morphometric statistics for the 20 best known lava shields of Iceland, plus Mauna Loa of Hawaii. As part of this investigation, we have quantified the detailed morphometry of nearly 50 intermediate to large scale edifices, with particular attention to their shape systematics. We found that a set of venusian edifices which include Maat, Sapas, Tepev, Sif, Gula, a feature at 46 deg S, 215 deg E, as well as the shield-like structure at 10 deg N, 275 deg E are broadly representative of the approx. 400 volcanic landforms larger than 50 km. The cross-sectional shapes of these 7 representative edifices range from flattened cones (i.e., Sif) similar to classic terrestrial lava shields such as Mauna Loa and Skjaldbreidur, to rather dome-like structures which include Maat and Sapas. The majority of these larger volcanoes surveyed as part of our study displayed cross-sectional topographies with paraboloidal shaped, in sharp contrast with the cone-like appearance of most simple terrestrial lava shields. In order to more fully explore the differences between large venusian edifices and volcanoes on the Earth and Mars, we developed a volume scaling algorithm which relies on conservation of volcano morphometry as basal diameter is varied.

Description: The global distribution of multi-kilometer (approx. 9 km) length scale 'roughness' (hereafter mesoscale roughness or MR) on Venus can be estimated from the Magellan global altimetry dataset (GxDR) and then compared with MR data derived for Earth from 5' ETOP5 data and for Mars (from USGS Mars DTM dataset). The mesoscale roughness parameter (MR) represents the RMS variance in meters of the actual planetary surface topography relative to the best fitting tangent plane defined on the basis of a 3x3 pixel sliding window. The best-fit plane was computed using a least-squares solution which minimizes delta H, the sum of the squares of the differences between the 9 local elevation values (H(sub i)), and the elevation of best-fit plane at the same grid location. Using the best-fit plane and delta H, we have computed the RMS 'roughness' var(delta R), where this parameter is always minimized on the basis of its calculation using least squares. We have called this 'ruggedness' parameter the Mesoscale Roughness (MR) because it is directly related to the high-frequency variance of topography after mesoscale slopes and tilts (i.e., for Venus, the baseline over which MR is computed (dx) is approx. 8.8 km and dx for Earth is approx. 9.3 km) are removed. As such, MR represents the degree to which a planetary surface is more rugged than approximately 10 km scale facets or tilts. It should not be confused with the radar 'RMS Roughness' parameter computed at 0.1 to 10 m length scales on the basis of the Magellan radar altimeter echo. We will use our MR parameter to investigate the global ruggedness properties of Venus as they relate to geological provinces and in comparison with the spatial pattern of MR for Earth and Mars.

Description: In 1992, the National Research Council's Space Studies Board charged its Committee on Planetary and Lunar Exploration (COMPLEX) to: (1) summarize current understanding of the planets and the solar system; (2) pose the most significant scientific questions that remain; and (3) establish the priorities for scientific exploration of the planets for the period from 1995 to 2010. The broad scientific goals of solar system exploration include: (1) understanding how physical and chemical processes determine the major characteristics of the planets, and thereby help us to understand the operation of Earth; (2) learning about how planetary systems originate and evolve; (3) determining how life developed in the solar system, particularly on Earth, and in what ways life modifies planetary environments; and (4) discovering how relatively simple, basic laws of physics and chemistry can lead to the diverse phenomena observed in complex systems. COMPLEX maintains that the most useful new programs to emphasize in the period from 1995 to 2010 are detailed investigations of comets, Mars, and Jupiter and an intensive search for, and characterization of, extrasolar planets.

Description: This report proposes a method that will allow lunar vertical-shaft mining. Lunar mining allows the exploitation of mineral resources imbedded within the surface. The proposed lunar vertical-shaft mining system is comprised of five subsystems: structure, materials handling, drilling, mining, and planning. The structure provides support for the exploration and mining equipment in the lunar environment. The materials handling subsystem moves mined material outside the structure and mining and drilling equipment inside the structure. The drilling process bores into the surface for the purpose of collecting soil samples, inserting transducer probes, or locating ore deposits. Once the ore deposits are discovered and pinpointed, mining operations bring the ore to the surface. The final subsystem is planning, which involves the construction of the mining structure.

Description: We have examined the Venus Balloon concept in order to further develop the ideas and concepts behind it, and to creatively apply them to the design of the major Venus Balloon components. This report presents our models of the vertical path taken by the Venus Balloon and the entry into Venusian atmosphere. It also details our designs of the balloon, gondola, heat exchanger, power generator, and entry module. A vehicle is designed for a ballistic entry into the Venusian atmosphere, and an atmospheric model is created. The model is then used to set conditions. The shape and material of the vehicle are optimized, and the dimensions of the vehicle are then determined. Equipment is chosen and detailed that will be needed to collect and transmit information and control the mission. A gondola is designed that will enable this sensitive electronic equipment to survive in an atmosphere of very high temperature and pressure. This shape and the material of the shell are optimized, and the size is minimized. Insulation and supporting structures are designed to protect the payload equipment and to minimize mass. A method of cooling the gondola at upper altitudes was established. Power needs of the gondola equipment are determined. Power generation options are discussed and two separate thermoelectric generation models are outlined.

Description: The design of an unmanned Martian polar exploration system is presented. The system elements include subsystems for transportation of material from earth to Mars, study of the Martian north pole, power generation, and communications. Early next century, three Atlas 2AS launch vehicles will be used to insert three Earth-Mars transfer vehicles, or buses, into a low-energy transfer orbit. Capture at Mars will be accomplished by aerobraking into a circular orbit. Each bus contains four landers and a communications satellite. Six of the twelve total landers will be deployed at 60 deg intervals along 80 deg N, and the remaining six landers at 5 deg intervals along 30 deg E from 65 deg N to 90 deg N by a combination of retrorockets and parachutes. The three communications satellites will be deployed at altitudes of 500 km in circular polar orbits that are 120 deg out of phase. These placements maximize the polar coverage of the science and communications subsystems. Each lander contains scientific equipment, two microrovers, power supplies, communications equipment, and a science computer. The lander scientific equipment includes a microweather station, seismometer, thermal probe, x-ray spectrometer, camera, and sounding rockets. One rover, designed for short-range (less than 2 km) excursions from the lander, includes a mass spectrometer for mineral analysis, an auger/borescope system for depth profiling, a deployable thermal probe, and charge coupled device cameras for terrain visualization/navigation. The second rover, designed for longer-range (2-5 km) excursions from the lander, includes radar sounding/mapping equipment, a seismometer, and laser ranging devices. Power for all subsystems is supplied by a combination of solar cells, Ni-H batteries, and radioisotope thermoelectric generators. Communications are sequenced from rovers, sounding rockets, and remote sensors to the lander, then to the satellites, through the Deep Space Network to and from earth.

Description: This paper describes the techniques developed for measuring stagnation-point radiation in NASA's cancelled Aeroassist Flight Experiment (AFE). It specifies the need for such a measurement; the types and requirements for the needed instruments; the Radiative Heating Experiment (RHE) developed for the AFE; the requirements, design parameters, and performance of the window developed for the RHE; the procedures and summary of the technique; and results of the arc-jet wind tunnel experiment conducted to demonstrate the overall concept. Subjects emphasized are the commercial implications of the knowledge to be gained by this experiment in connection with the Aeroassisted Space Transfer Vehicle (ASTV), the nonequilibrium nature of the radiation, concerns over the contribution of vacuum-ultraviolet radiation to the overall radiation, and the limit on the flight environment of the vehicle imposed by the limitations on the window material. Results show that a technique exists with which the stagnation-point radiation can be measured in flight in an environment of interest to commercial ASTV applications.

Description: Presented are results of some studies to develop tools useful for the analysis of Venus surface shape and its roughness. Actual work was focused on Maxwell Montes. The analyses employ data acquired by means of NASA's Magellan satellite. The work is primarily concerned with deriving measurements of the Venusian surface using Magellan stereo SAR. Roughness was considered by means of a theoretical analyses based on digital elevation models (DEM's), on single Magellan radar images combined with radiometer data, and on the use of multiple overlapping Magellan radar images from cycles 1, 2, and 3, again combined with collateral radiometer data.

Description: Continued press coverage of the comet Shoemaker-Levy 9 impact on the surface of Jupiter is presented. This tape covers 16 Jul. 1994.

Description: The objectives of this program are to further the understanding of the upper atmospheres of Titan, Triton, and the Earth in terms of the observed emissions of the 13-14 eV states of N2. These states are generated at quite high rates, yet very little emission is observed from them. The reasons are complex, involving resonance trapping and predissociation, and it is desired to quantify the effects of predissociation, particularly on the c(sub 4)' 1 Sigma(sub u),(sup +) state of N2. Earlier experiments had indicated that predissociation of the c(sub 4)' state was of little importance, yet over the last two years a growing body of evidence has shown that for levels above v = 2, predissociation is in fact a major process. It is the v = 0 level for which production by electron bombardment and photoexcitation is highest, and so it has been most important to evaluate the effects of predissociation on this particular level. The goal has been to target c(sub 4)' (v = 0) for a thorough analysis, in which both the extent of predissociation as a function of rotational level and the atomic product branching ratio, where the only possible products are N(4S) + N(4S) and N(2D) + N(4S), are determined. For the first year of funding, the intention was to demonstrate two-photon excitation of the intermediate N2(a(sup 1) Pi(sub g)) state, so that the gap to the 13 eV energy region could be bridged, and then use a second laser to reach the c(sub 4)' state itself.

Description: This is the first analysis of the high frequency data from the Viking lander and spectra of wind, in the Martian atmospheric surface layer, along with the diurnal variation of the height of the mixed surface layer, are calculated for the first time for Mars. Heat and momentum fluxes, stability, and z(sub O) are estimated for early spring, from a surface temperature model and from Viking Lander 2 temperatures and winds at 44 deg N, using Monin-Obukhov similarity theory. The afternoon maximum height of the mixed layer for these seasons and conditions is estimated to lie between 3.6 and 9.2 km. Estimations of this height is of primary importance to all models of the boundary layer and Martian General Circulation Models (GCM's). Model spectra for two measuring heights and three surface roughnesses are calculated using the depth of the mixed layer, and the surface layer parameters and flow distortion by the lander is also taken into account. These experiments indicate that z(sub O), probably lies between 1.0 and 3.0 cm, and most likely is closer to 1.0 cm. The spectra are adjusted to simulate aliasing and high frequency rolloff, the latter caused both by the sensor response and the large Kolmogorov length on Mars. Since the spectral models depend on the surface parameters, including the estimated surface temperature, their agreement with the calculated spectra indicates that the surface layer estimates are self consistent. This agreement is especially noteworthy in that the inertial subrange is virtually absent in the Martian atmosphere at this height, due to the large Kolmogorov length scale. These analyses extend the range of applicability of terrestrial results and demonstrate that it is possible to estimate the effects of severe aliasing of wind measurements, to produce a models which agree well with the measured spectra. The results show that similarity theory developed for Earth applies to Mars, and that the spectral models are universal.

Description: The efficacy of the UV spectroscopy technique used by Mariner 9 to remotely measure ozone abundance at Mars is discussed. Previously-inferred ozone abundances could be underestimated by as much as a factor of 3, and much of the observed variability in the ozone abundance could be due to temporal and spatial variability in cloud and dust amount.

Description: A computer model is used to study the radiative transfer of the martian winter-polar atmosphere. Solar heating at winter-polar latitudes is provided predominately by dust. For normal, low-dust conditions, CO2 provides almost as much heating as dust. Most heating by CO2 in the winter polar atmosphere is provided by the 2.7 micron band between 10 km and 30 km altitude, and by the 2.0 micron band below 10 km. The weak 1.3 micron band provides some significant heating near the surface. The minor CO2 bands at 1.4, 1.6, 4.8 and 5.2 micron are all optically thin, and produce negligible heating. O3 provides less than 10 percent of the total heating. Atmospheric cooling is predominantly thermal emission by dust, although CO2 15 micron band emission is important above 20 km altitude.

Description: The Cold War's end opened an opportunity for greater cooperation in planetary exploration for the United States and Russia. Two study groups were formed, Mars Together and FIRE and ICE. The Mars Together team developed a concept for a flight in 1998 that merged one U.S. Mars Surveyor 98 mission with the former Russian Mars 96 mission to further understanding of the Mars surface and atmosphere. The FIRE and ICE team developed concepts for a dual-spacecraft mission to the solar corona and for a mission to Pluto. The missions, scientific potential, and open issues are described.

Description: Infrared spectroscopy provides unique insights into the chemistry and dynamics of the atmospheres of Jupiter and Saturn -- and of Titan, the enigmatic satellite of Saturn. The 5 micron spectral region of these objects is transparent to deep levels, and is therefore particularly useful for the identification of molecules in the deep atmosphere at very low (parts per billion) concentrations. In 1991 we obtained data at J, H, K, and M and made repeated observations of Titan's albedo as the satellite orbited Saturn. The J albedo is 12 + -3% greater than the albedo measured in 1979; the H and K albedos are the same. There was no evidence for variations at any wavelength over the eastern half of Titan's orbit. We also obtained low resolution (R = 50) spectra of Titan between 3.1 and 5.1 microns. The spectra contain evidence for CO and CH3D absorptions. Spectra of Callisto and Ganymede in the 4.5 micron spectral region are featureless and give albedos of 0.08 and 0.04 respectively. If Titan's atmosphere is transparent near 5 microns, its surface albedo there is similar to Callisto's. In 1992 and 1993 we obtained further spectroscopic data of Titan with the UKIRT CGS4 spectrometer. We discovered two unexpected and unexplained spectral features in the 3-4 micron /spectrum of Titan. An apparent emission feature near the 3 micron (nu3) band of methane indicates temperatures higher than known to be present in Titan's upper stratosphere and may be caused by unexpected non-LTE emission. An absorption feature near 3.47 microns may be caused by absorption in solid grains or aerosol's in Titan's clouds. The feature is similar, but not identical to organics in the interstellar matter and in comets.

Description: The existing radiative transfer and inversion programs will be modified for application to the atmospheres of Uranus, Neptune, and Jupiter. The programs will be employed for analysis of KAO planetary observations in order to develop far infrared photometric calibration standards. This work will be carried out on MSFC computers. The expected end product of this task is a working program for analysis of the observations, and numerical data representing the planets as far-infrared calibration sources.

Description: A large population of discrete volcanic edifices on Venus has been identified and cataloged by means of Magellan SAR images, and an extensive database describing thousands of such features is in final preparation. Those volcanoes categorized as Intermediate to Large in scale, while relatively small in number (approx. 400), nonetheless constitute a significant volumetric component (approx. 13 x 10(exp 6) cu km) of the total apparent crustal volume of Venus. For this reason, we have focused attention on the morphometry of a representative suite of the larger edifices on Venus and, in particular, on ways of constraining the eruptive histories of these possibly geologically youthful landforms. Our approach has been to determine a series of reproducible morphometric parameters for as many of the discrete volcanoes on Venus that have an obvious expression within the global altimetry data acquired by Magellan. In addition, we have attempted to objectively and systematically define the mathematical essence of the shapes of these larger volcanoes using a polynomial cross-section approximation involving only parameters easily measured from digital topography, as well as with simple surface cylindrical harmonic expansions. The goal is to reduce the topological complexities of the larger edifices to a few simple parameters which can then be related to similar expressions for well-studied terrestrial and martian features.

Description: Highly structured Langmuir waves, also known as electron plasma oscillations, have been observed in the foreshock of Venus using the plasma wave experiment on the Galileo spacecraft during the gravity assist flyby on February 10, 1990. The Galileo wideband sampling system provides digital electric field waveform measurements at sampling rates up to 201,600 samples per second, much higher than any previous instrument of this type. The main Langmuir wave emission band occurs near the local electron plasma frequency, which was approximately 43 kHz. The Langmuir waves are observed to shift above and below the plasma frequency, sometimes by as much as 20 kHz. The shifts in frequency are closely correlated with the downstream distance from the tangent field line, implying that the shifts are controlled by the electron beam velocity. Considerable fine structure is also evident, with time scales as short as 0.15 milliseconds, corresponding to spatial scales of a few tens of Debye lengths. The frequency spectrum often consists of beat-type waveforms, with beat frequencies ranging from 0.2 to 7 kHz, and in a few cases, isolated wavepackets. The peak electric field strengths are approximately 1 mV/m. These field strengths are too small for strongly nonlinear processes to be important. The beat-type waveforms are suggestive of a parametric decay process.

Description: This catalog collects and describes science payloads considered for future robotic and human exploration missions to the Moon and Mars. The science disciplines included are geosciences, meteorology, space physics, astronomy and astrophysics, life sciences, in-situ resource utilization, and robotic science. Science payload data is helpful for mission scientists and engineers developing reference architectures and detailed descriptions of mission organizations. One early step in advanced planning is formulating the science questions for each mission and identifying the instrumentation required to address these questions. The next critical element is to establish and quantify the supporting infrastructure required to deliver, emplace, operate, and maintain the science experiments with human crews or robots. This requires a comprehensive collection of up-to-date science payload information--hence the birth of this catalog. Divided into lunar and Mars sections, the catalog describes the physical characteristics of science instruments in terms of mass, volume, power and data requirements, mode of deployment and operation, maintenance needs, and technological readiness. It includes descriptions of science payloads for specific missions that have been studied in the last two years: the Scout Program, the Artemis Program, the First Lunar Outpost, and the Mars Exploration Program.

Description: The design for the Mars Analysis and Return Vehicle with In-Situ Resource Utilization (MARVIN) project is outlined. The MARVIN mission is designed to collect samples of the Martian environment; to produce fuel from local Martian resources; and to use the fuel produced to return the samples to earth. It uses only existing technologies. Exploratory Technologies' mission-design efforts have focused on methods of orbit determination, sample collection, fuel production, power, communications, control, and structural design. Lambert Targeting provided Delta-V's, launch dates, and travel times. The landing site is the Tharsis Plateau, to the southeast of Olympus Mons, chosen for its substantial scientific value. Samples of soil, dust, and atmosphere are collected with lander-based collection devices: the soil sample, with a robotic arm similar to those used in the Viking missions; the atmospheric sample, from a bleed line to the compressor in the fuel-production facility; a dust sample, from the dust-collection container in the fuel-production facility; and a redundant dust sample, with a with a passive filter system, which relies upon neither a power source nor other collection methods. The sample-return capsule (SRC) houses these samples, which are triply contained to prevent contamination. Proven technology can be used to produce methane and oxygen for fuel with relative ease at the landing site: the Sabatier reactor produces methane and water by combining carbon dioxide and hydrogen (brought from earth); the Reverse Water-Gas Shift unit combines carbon dioxide and hydrogen to form carbon monoxide and water; a water-electrolysis unit splits the water into hydrogen and oxygen. The Mars-lander vehicle (MLV) transports the equipment from earth to Mars. The Mars-ascent vehicle (MAV) contains the SRC and the engine, which is the same for both the MLV and the MAV. All equipment that is unnecessary for the Mars-Earth trajectory remains on Mars. This report presents detailed sizing information, for which a spreadsheet has been developed. The trends suggest possibilities for expansion, and suggestions for future work in these areas are offered.

Description: The pneumatic soil sampler concept was successfully demonstrated by penetrating a Martian simulant soil to a depth of 2 meters. Working gas pressure, composition, and pulsing were evaluated with the objective of minimizing gas usage. Also, the probe penetration force was investigated with the objective of minimizing probe weight. Gas and probe penetration force, while not yet optimized, are within the range which make the soil sampler concept feasible. While the tests described in this report did not answer all the questions and address all the variables associated with pneumatic soil sampling, valuable data experience and knowledge were gained which can be used to further develop the concept.

Description: The final report summarizes the results from three research areas: (1) window design for the radiometric measurement of the forebody radiative heating experienced by atmospheric entry spaceraft; (2) survey of the current understanding of chemical species on selected solar system bodies and assess the importance of measurements with regard to vehicle environment and with regard to understanding of planetary atmospheres with emphasis on Venus, Mars, and Titan; and (3) measure and analyze the radiation (VUV to near-IR) from the shock heated gas cap of a blunt body in an Ames arc Jet wind-tunnel facility.

Description: Continued press coverage of the comet Shoemaker-Levy 9 impact on the surface of Jupiter is presented. This tape covers 17 Jul. 1994.

Description: Silicon nitride Si3N4, has previously been observed to be a common constituent of acid residues of Qingzhen (EH3) and Indarch (EH4). Ion probe analysis of the Si, N and C isotopic compositions of individual Si3N4 grains from Qingzhen and Indarch acid residues suggest most, if not all, grains are Solar System in origin. A few grains have isotopically anomalous C but this is probably due to small presolar SiC grains adhering to them. In situ observations of the Si3N4 in Qingzhen show that it is only present within, and probably exsolved from, host phases which contain elemental Si in solid solution. Thermodynamic calculations suggest that the Si3N4 probably formed during metamorphism and not in the nebula. Thermodynamic calculations also show that sinoite (Si2N2O) and not Si3N4 should be the stable phase during metamorphism. It appears that kinetic factors must have inhibited the formation if sinoite in Qimgzhen and Indarch.

Description: The ten specimens of the paired Acfer 059/El Djouf 001 CR2 chondrite contain abundant lithic fragments which we refer to as dark clasts. Petrological and mineralogical studies reveal that they are not related to the CR2 host meteorite but are similar to dark clasts in other CR2 chondrites. Dark clasts consist of chondrule and mineral fragments, phyllosilicate fragments and clusters, magnetite, sulfides and accessory phases, embedded into a very fine-grained, phyllosilicate-rich matrix. Magnetite has morphologies known from CI chondrites: spherules, framboids and platelets. Average abundances of major elements in the dark clasts are mostly in the range of both CR and CV chondrites, but strong depletions in Na and S are apparent. Oxygen isotopic compositions of two dark clasts suggest relationships to type 3 carbonaceous chondrites and dark inclusions in Allende. The dark clasts are clearly different in texture and mineralogical compositions from the host matrix of Acfer 059/El Djouf 001. Therefore, these dark clasts are xenoliths and are quite unlike the Acfer 059/El Djouf 001 CR2 host meteorite. We suggest that dark clasts accreted at the same time with all other components during the formation of Acfer 059/El Djouf 001 whole rock.

Description: An inclined secondary orbiting in a disk will launch bending waves from resonance sites where the Doppler shifted forcing frequency matches the disk's natural frequency for vertical oscillations. These vertical resonances are of two types: external resonances falling interior and exterior to the perturber's semimajor axis that excite its inclination and coorbiting resonances that fall at the perturber's orbit and damp its inclination. We show that torques from coorbiting resonances dominate the bending wave interaction for a constant density disk. In this case the inclination ultimately decay and an estimate of the characteristic time scale for this process is made.

Description: On October 5 and 6, 1991, three dual-frequency ingress radio occultation experiments were conducted at Venus during consecutive orbits of the Magellan spacecraft. The radio signals probed a region of the atmosphere near 65 deg N, with a solar zenith angle of 108 deg, reaching below 35 km at 3.6 cm, and below 34 km at 13 cm (above a mean radius of 6052 km). The high effective isotropic radiated power (EIRP) of the Magellan spacecraft and highly successful attitude maneuvers allowed these signals to probe deeper than any previous radio occultation experiment and also resulted in the most accurate thermal and sulfuric acid vapor abundance profiles ever obtained at Venus through radio occultation techniques. The performance of the spacecraft and the experiment design are discussed in an accompanying paper. Average electron density profiles retrieved from the data possess peaks between 2600 and 6000/cu cm, well below typical values of 10,000/cu cm retrieved in 1979 by Pioneer Venus at similar solar zenith angles. Other basic results include vertical profiles of temperature, pressure, and density in the neutral atmosphere, 13- and 3.6-cm absorpttivity, and H2SO4 (g) abundance below the main cloud layer. H2SO4 (g) becomes significant below 50 km, reaching peaks between 18 and 24 ppm near 39 km before dropping precipitously below 38 km. These sharp decreases confirm the thermal decomposition of sulfuric acid vapor below 39 km. Since the Venus atmosphere rotated approximately 10 deg between experiments, the data contain information about the horizontal variability of the atmosphere. All derived profiles exhibit significant variations from orbit to orbit, indicating the presence of dynamical processes between 33 and 200 km. In particular, the orbit-to-orbit variations in temperature and in H2SO4 (g) abundance appear to be correlated, suggesting that a common mechanism may be responsible for the observed spatial variations.

Description: H2S opacity may significantly affect the brightness temperatures of Uranus and Neptune due to possible depletion of ammonia in the tropospheres of those planets (de Pater et al. 1991). Though the rotational line centers of H2S are in the millimeter wavelengths region, significant absorption is also present at centimeter wavelengths due to pressure broadening of the lines. Accordingly, the properties of H2S under Jovian conditions have been measured in order to constrain further the constituents' abundances on these planets. These absorptivity measurements show values that are significantly greater than values predicted by the Van Vleck-Weisskopf models traditionally used at centimeter wavelengths. In order to better model the opacity due to H2S under Jovian conditions a Ben-Reuven lineshape formalism has therefore been developed and is presented. This formalism provides a possible constraint on the relative abundances of H2S and NH3 on Neptune based on Voyager 2 radio occultation results (Lindal 1992).

Description: 108 +/- 11 photons of the martian He 584-A airglow detected by the Extreme Ultraviolet Explorer (EUVE) satellite during a 2-day exposure (January 22-23, 1993) correspond to the effective disk average intensity of 43 +/- 10 Rayleigh (Ra). Radiative transfer calculations, using a model atmosphere appropriate to the conditions of the observation and having an exospheric temperature of 210 +/- 20 K, result in a He mixing ratio of 1.1 +/- 0.4 ppm in the lower atmosphere. Nonthermal escape of helium is due to electron impact ionization and pickup of He(+) by the solar wind, to collisions with hot oxygen atoms, and to charge exchange with molecular species with corresponding column loss rates of 1.4 x 10(exp 5), 3 x 10(exp 4), and 7 x 10(exp 3)/sq cm/s, respectively. The lifetime of helium on Mars is 5 x 10(exp 4) years. the He outgassing rate, coupled with the Ar-40 atmospheric abundance and with the K:U:Th ratio measured in the surface rocks, is used as input to a single two-reservoir degassing model which is applied to Mars and then to Venus. A similar model with known abundances if K, U, and Th is applied to Earth. The models for Earth and Mars presume loss of all argon accumulated in the atmospheres during the first billion years by large-scale meteorite and planetesimal impacts. The models show that the degassing coefficients for all three planets may be approximated by function delta = delta(sub 0) x (t(sub 0)/t)(exp 1/2) with delta(sub 0) = 0.1, 0.04, and 0.0125 Byr for Earth, Venus, and Mars, respectively. After a R(exp 2) correction this means that outgassing processes on Venus and Mars are weaker than on Earth by factors of 3 and 30, respectively. Mass ratios of U and Th are almost the same for all three planets, while potassiumis depleted by a factor of 2 in Venus and Mars. Mass ratio of helium and argon are close to 5 x 10(exp -9) and 2 x 10(exp -8) g/g in the interiors of all three planets. The implications of these results are discussed.

Description: Absorption features centered near 0.60-0.65 and 0.80-0.90 micrometers have been identified in the spectra of five low-albedo main-belt and outer-belt asteroids. These absorption features are attributed respectively to the (6)A(sub 1) goes to (4)T(sub 2)(G) and (6)A(sub 1) goes to (4)T(sub 1)(G) charge transfer transitions in minerals such as goethite, hematite, and jarosite that are products of the aqueous alteration of anhydrous silicates. A shoulder near 0.63 micrometers has also been identified in the absorption feature centered near 0.7 micrometers attributed to oxidized iron in phyllosilicates found predominantly in C- and G-class asteroids reflectance spectra. The coexistence of iron oxides with phyllosilicates in asteroids believed to have undergone aqueous alteration would be expected based upon analogy with terrestrial aqueous alteration and the observed mineralogy of carbonaceous chondrites. The number of low-albedo asteroids having only iron alteration absorption features compared to the number of low-albedo asteroids having spectral characteristics indicative of phyllosilicates is small. Either the conditions under which these asteroids formed are rare, or the iron alteration minerals could be formed in the interiors of objects where phyllosilicates dominate the surface mineralogy.

Description: The lunar Mg suite, which includes dunites, troctolites, and norites, could make up 20-30% of the Moon's crust down to a depth of 60 km. The remainder is largely anorthositic. This report focuses on norites because we have found that the chemical characteristics of orthopyroxene are effective recorders of their parental melt compositions. Many of the samples representing the Mg suite are small and unrepresentative. In addition, they are cumulates and thus are difficult to study by whole-rock techniques. Therefore, we decided to study these rocks by SIMS techniques to analyze a suite of trace elements in orthopyroxene. The 12 norite samples were selected from a recent compilation by Warren who attempted to select the best candidate samples from the standpoint of their pristine character. Our present database includes greater than 300 superior Electromagnetic Pulse (EMP) analyses and greater than 50 scanning ion mass spectroscopy (SIMS) analyses for 8 Rare Earth Elements (REE), Zr, Y, and Sr. The Mg#s for the parental melts calculated from Mg#s in orthopyroxene show that most melts have Mg#s in the range of 0.36-0.60. This compares with a range of Mg#s for lunar volcanic picritic glass beads of 0.4-0.68. Therefore, although the cumulate whole-rock compositions of the Mg suite can be extremely magnesian, the calculated parental melts are not anomalously high in Mg. A chemical characteristic of the Mg-suite norites that is more difficult to explain is the high KREEP content of the calculated parental melts. The REE contents for the calculated norite parental melts have REE that match or exceed the high-K KREEP component of Warren. Therefore, mixing of a KREEP component and a picritic melt cannot, by itself, explain the high estimated REE contents of the melts parental to norites. Advanced crystallization following KREEP incorporation, especially of plagiclase, may also be required.

Description: We have been quantitatively measuring C abundances in hydrated interplanetary dust particles for the past few years, but in general, we have had to infer the distribution and nature of the C-bearing materials within these particles because of the complex microtextures of hydrated IDPs. Aside from rare carbonate grains, other C-bearing phases are difficult to distinguish from the fine-grained, poorly crystalline phyllosilicates that comprise the bulk of these particles. We know that carbonates alone cannot account for the high C abundances observed in most hydrated IDPs and that additional C-bearing phases must be present. We have recently applied the technique of electron energy-loss spectroscopy (EELS) in the transmission electron microscope (TEM) to identify and form the distribution of C-bearing phases in hydrated IDPs. These preliminary data show that several C-rich hydrated IDPs contain a mixture of two major forms of C, Mg-Fe carbonate and amorphous C. The near-edge structure in the C k-edges from these IDPs shows no evidence for the development of graphite or even poorly graphitized C. We conclude that the 'elemental' C in these IDPs is either very poorly ordered or is exceedingly fine-grained (we refer to this C as 'amorphous C'). The amorphous C is intimately intergrown with the fine-grained phyllosilicates and is evenly distributed within three of the four IDPs analyzed (only G1 contains discrete 'hot spots' of amorphous C). Not all hydrated IDPs contain carbonates.

Description: Recent analysis analytical electron microscope study of lunar soils showed that the approximately 60-nm-wide amorphous rims surrounding many lunar soils grains exhibit distinct compositional differences from their hosts. On average, the amorphous rim compositions reflect the local bulk soil composition with the exceptions of Si and S, which are enriched relative to the bulk soil. These chemical trends led us to propose that the amorphous rims were in fact deposits of impact-generated vapors produced during regolith gardening, a hypothesis that runs contrary to the generally accepted view that the rims are produced through amorphization of the outer parts of mineral grains by interaction with the solar wind. Analytical data are reported for amorphous rims on individual minerals in lunar soils in order to show that the magnitude of the chemical differences between rim and host are so great that they require a major addition of foreign elements to the grain surfaces. The average composition of amorphous rims is listed as a function of host mineralogy as determined in microtone thin sections using energy-dispersive X-ray spectrometry in the transmission electron microscope. As the host mineral becomes chemically more complex, the chemical differences are not as clear. The average rim compositions are remarkably similar and are independent of the host grain mineralogy. Whether there are 'sputtering' or radiation effects superimposed on the vapor-deposited material can be debated. We do not explicitly exclude the effects of radiation damage as a contributing factor to the formation of amorphous rims; we are merely emphasizing the major role played by condensed vapors in the formation of amorphous rims on lunar soil grains.

Description: The Nova 001 (= Nuevo Mercurio (b)) and Nullarbor 010 meteorites are ureilites, both of which contain euhedral graphite crystals. The bulk of the meteorites are olivine (Fo79) and pyroxenes (Wo9En73Fs18, Wo3En77Fs20), with a few percent graphite and minor amounts of troilite, Ni-Fe metal, and possibly diamond. The rims of olivine grains are reduced (to Fo91) and contain abundant blebs of Fe metal. Silicate mineral grains are equant, anhedral, up to 2 mm across, and lack obvious preferred orientations. Euhedral graphite crystals (to 1 mm x 0.3 mm) are present at silicate grain boundaries, along boundaries and protruding into the silicates, and entirely within silicate mineral grains. Graphite euhedra are also present as radiating clusters and groups of parallel plates grains embedded in olivine; no other ureilite has comparable graphite textures. Minute lumps within graphite grains are possible diamond, inferred to be a result of shock. Other shock effects are limited to undulatory extinction and fracturing. Both ureilites have been weathered significantly. Considering their similar mineralogies, identical mineral compositions, and identical unusual textures, Nova 001 and Nullarbor 010 are probably paired. Based on olivine compositions, Nova 001 and Nullarbor 010 are in Group 1 (FeO-rich) of Berkley et al. (1980). Silicate mineral compositions are consistent with those of others known ureilites. The presence of euhedral graphite crystals within the silicate minerals is consistent with an igneous origin, and suggests that large proportions of silicate magma were present locally and crystallized in situ.

Description: Constraints on the recent geological evolution of Venus may be provided by quantitative estimates of the rates of the principal resurfacing processes, volcanism and tectonism. This paper focuses on the latter, using impact craters as strain indicators. The total postimpact tectonic strain lies in the range 0.5-6.5%, which defines a recent mean strain rate of 10(exp -18)-10(exp -17)/s when divided by the mean surface age. Interpretation of the cratering record as one of pure production requires a decline in resurfacing rates at about 500 Ma (catastrophic resurfacing model). If distributed tectonic resurfacing contributed strongly before that time, as suggested by the widespread occurrence of tessera as inliers, the mean global strain rate must have been at least approximately 10(exp -15)/s, which is also typical of terrestrial active margins. Numerical calculations of the response of the lithosphere to inferred mantle convective forces were performed to test the hypothesis that a decrease in surface strain rate by at least two orders of magnitude could be caused by a steady decline in heat flow over the last billion years. Parameterized convection models predict that the mean global thermal gradient decreases by only about 5 K/km over this time; even with the exponential dependence of viscosity upon temperature, the surface strain rate drops by little more than one order of magnitude. Strongly unsteady cooling and very low thermal gradients today are necessary to satisfy the catastrophic model. An alternative, uniformitarian resurfacing hypothesis holds that Venus is resurfaced in quasi-random 'patches' several hundred kilometers in size that occur in response to changing mantle convection patterns.

Description: Helium and neon were extracted from individual lunar ilmenite grains, approximately 100 micrometers in diameter, using a pulsed step-heating technique. Grains from lunar samples 71501 and 79035, believed to have been exposed to solar corpuscular radiation at greatly different times, were studied. The results found were consistent with the hypothesis that in addition to solar-wind-implanted gas, a second more deeply implanted component was present in both species of grains. Average isotopic ratios were determined giving equal weight to each of the particles. As found in depth studies employing chemical etching, both the He-3/He-4 and Ne-20/Ne-22 ratios were lower in the more deeply implanted gas than in the solar wind component. The He-3/He-4 ratio in the solar wind component of the more ancient grains was lower than that in the more recently exposed ones, whereas no difference was found for the more deeply embedded He. In the deeply embedded component of the ancient grains, the He-4/Ne-20 ratio was approx. 2x that found in the more recently exposed grains. In the shallowly implanted component, the ratio varied greatly from grain to grain, preventing comparison with the solar wind elemental composition.