ALBERT

Description: Present understanding of planetary atmospheres is surveyed. The formation of the planets and their atmospheres is briefly reviewed, and attention is given to the compositions of the atmospheres of earth, Venus, and Mars, the outer planets, and Titan. Lists of the individual atmospheric gases and their concentrations are included.

Description: This design course is directed to studying problems related to mobile exploration of the surface of Mars. Constraints on the vehicles considered are set by the payload and performance currently envisioned by mission analysis carried out previously at the Jet Propulsion Laboratory. The students are given full flexibility to examine those aspects which suit their interests and background. There are no regularly scheduled class lectures. Weekly review meetings are held with personnel from the Jet Propulsion Laboratory, and students use JPL resources as required.

Description: New radar observations of the moon in 1981-1984 were made using the 430 MHz (70 cm wavelength) radar at the Arecibo observatory, Puerto Rico. The new observations have produced a high resolution lunar radar map with radar cell-sizes near 2-5 km. This new resolution is a three-fold improvement over the previous mapping done in the late 1960's. Since the Arecibo radar antenna beam is only ten arc-minutes (about one-third of the width of the lunar disk), this new map is a mosaic of some eighteen observations. A radarmetric control between the various pieces of the mosaic was obtained via a 'beam-swing', limb-to-limb calibration. When the limb-to-limb calibration was combined with the mosaic, there were significant radar scattering differences across the maria. Eastern Mare Tranquillitatis and western Oceanus Procellarum have weaker echoes than other maria, while the central portion of Mare Serenitatis and northern Mare Imbrium have stronger echoes. There is a radar scattering difference across the southern terra as areas nearer Mare Orientale have stronger echoes than areas further from Mare Orientale.

Description: The configuration and morphology of the plasma clouds in the ionotail of Venus (revealed by the Pioneer Venus Orbiter) are studied, and the rate of planetary ion escape, which may be associated with the dissipation and removal of the ionospheric plasma, is estimated. The data supplied by the Orbiter's instruments, the Orbiter electron temperature probe, the ion mass spectrometer, the neutral mass spectrometer, the magnetometer, and the plasma analyzer, are analyzed, and the results of the observations are discussed.

Description: Continued analysis of the pressure and temperature data returned by the two Vega mission balloons has revealed an apparently significant difference in mean atmospheric static stability between the two data sets. Furthermore, the stability is time dependent within each data set. The 6.5 K temperature contrast between the two balloons remains, and appears to have a counterpart in the contrast between two of the Pioneer Venus probes at these levels, which has been attributed to planetary scale waves. Comparisons of the Vega 2 lander data with those of the Pioneer Venus Large Probe shows relatively close agreement in the state properties and in the atmospheric static stability profiles as well.

Description: Instruments aboard the gondolas of the two Vega balloons obtained in situ measurements of pressure, temperature, vertical velocity relative to the balloon, cloud particle backscatter, lightning and the ambient light level. Atmospheric motions at the balloon float altitudes were also determined from earth-based tracking results. To illustrate the history of the balloon flights and to facilitate comparisons between some of the different observed quantities, measurements of pressure, temperature and backscatter are presented as time series for the entire lifetime of each balloon. Both long and short period variations have been detected. In addition, the environmental entropy encountered by each balloon will be discussed.

Description: Backscatter nephelometer measurements obtained during the Vega 1 balloon flight are reported. During periods of minor convective activity and initial balloon ascent, the data generally agree with those obtained from other Venus mission descent probes. However, during the period of greatest convective activity experienced by the Vega 1 balloon, the signals were about a factor of two greater than any previously obtained. Although the clouds appear to be unbroken, deviations in the behavior of the detailed backscatter signals with time from those of the ambient pressure or temperature signals, especially during periods of minor convective activity, indicate much small-scale variability in cloud structure.

Description: During the Vega Venus balloon mission, data on the thermodynamic state of the atmosphere were obtained as well as wind and cloud information. Explanations are provided for the following: (1) the large amplitude atmospheric vertical winds encountered by the Vega balloons, (2) the observed 6.5 K temperature difference consistently measured between the two Vega balloons, and (3) the apparent influence of surface topography on atmospheric motions seen by the Vega-2 balloon as it flew over mountainous terrain (known as Aphrodite). It is believed that vertical winds of the magnitude encountered by the Vega-2 balloon over Aphrodite may be the result of surface-induced gravity waves.

Description: A global array of 20 radio observatories was used to measure the three-dimensional position and velocity of the two meteorological balloons that were injected into the equatorial region of the Venus atmosphere by the Vega spacecraft. Initial analysis of only radial velocities indicates that each balloon was blown westward about 11,500 kilometers (8000 kilometers on the night side) by zonal winds with a mean speed of about 70 meters per second. Excursions of the data from a model of constant zonal velocity are generally less than 3-meters per second; however, a much larger variation is evident near the end of the flight of the second balloon. Consistent systematic trends in the residual for both balloons indicate the possibility of a solar-fixed atmospheric feature.

Description: At Uranus, the Voyager 2 plasma wave investigation observed very significant phenomena related to radio emissions, dust impacts, and magnetospheric wave-particle interactions. On January 19, 1986 (R = 270 R-sub U) the plasma wave investigation detected an intense radio burst at 31 and 56 kHz, and this provided the first indication that Uranus had a magnetosphere. During the encounter, more of these sporadic bursts were observed along with relatively continuous radio emissions extending down to 10 kHz, and a sporadic narrowband radio signal with f near 5 kHz. As Voyager passed through the ring plane, the plasma wave investigation recorded a large number of dust impacts. The Voyager 2 plasma wave instrument also detected many strong electromagnetic and electrostatic plasma waves, with intensity peaks in the region within 12 Uranus radii. These waves have characteristics that can interact strongly with the local plasma and with the trapped energetic particles, leading to precipitation into the atmosphere, charged particle acceleration, and charged particle diffusion. In addition, strong wave activity was detected in the region of the bow and shock and moderate levels in the magnetic tail.

Description: The hardware of the Planetary Radio Astronomy Experiment aboard Voyager 2 and the results of the measurements of radio emissions from Uranus are described. Strong 40-kHz to 850-kHz radio emissions were detected after closest approach on the day-side of Uranus. The time variations of these emissions were periodic, with a period of 17.24 h closely matching that of Uranus's rotation and evidently being controlled by the strength and shape of its magnetic field. The instrument also recorded possible Uranian electrostatic discharges, vertex early arcs occurring in sequences of more than a dozen events with approximately 10-min period, and very intense isolated bursts lasting tens of minutes.

Description: The ionospheres of the major planets Jupiter, Saturn, and Uranus are reviewed in light of Pioneer and Voyager observations. Some refinements to pre-Voyager theoretical models are required to explain the results, most notably the addition of significant particle ionization from 'electroglow' and auroral processes and the need for additional chemical loss of protons via charge exchange reactions with water. Water from the Saturn rings has been identified as a major modifier of the Saturn ionosphere and water influx from satellites and/or meteorites may also be important at Jupiter and Uranus as well, as evidenced by the observed ionospheric structure and the identification of cold stratospheric carbon monoxide at Jupiter.

Description: Results obtained over the last 21 years from satellite measurements of the neutral upper atmospheres of Venus and Mars are discussed. Particular attention is given to the chemistry of the neutral upper atmospheres of the two planets, the exospheric temperatures, and the sensitivity of their atmospheric temperatures to solar activity. Various theoretical models used to interpret the planetary atmospheric data are discussed.

Description: The major characteristics of the neutral upper atmospheres of outer planets are discussed, with special attention given to the Uranus upper atmosphere, probed by Voyager 2. The composition, thermal structure, photochemistry, and vertical mixing of the Uranus atmosphere are compared with the respective features of other outer planets. Unlike the atmospheres of Jupiter and Saturn, which reflect the solar ratios of the elements, the Uranus atmosphere was found to have only few constituents, including NH3, CH4, H2, He, C2H2, and C2H6. The eddy diffusion coefficient of Uranus, determined from occultation experiments, was found to be in the range 10,000-100,000 sq cm/sec, the lowest value amongst the major planets; this implies relatively sluggish vertical mixing. Another major difference from Saturn and Jupiter is in the fact that stable hydrocarbon products (C2H2 and C2H6) in the Uranus atmosphere begin to condense at around 5-10 mb level, resulting in the production of haze in the lower stratosphere.

Description: Measurements of radio waves that have propagated through planetary atmospheres have provided exploratory results on atmospheric constituents, structure, dynamics, and ionization for Venus, Mars, Titan, Jupiter, Saturn, and Uranus. Highlights of past results are reviewed in order to define and illustrate the potential of occultation and related radio studies in future planetary missions.

Description: Aspherical harmonic model of the planetary magnetic field of Uranus is obtained from the Voyager 2 encounter observations using generalized inverse techniques which allow partial solutions to complex (underdetermined) problems. The Goddard Space Flight Center 'Q3' model is characterized by a large dipole tilt (58.6 deg) relative to the rotation axis, a dipole moment of 0.228 G R(Uranus radii cubed) and an unusually large quadrupole moment. Characteristics of this complex model magnetic field are illustrated using contour maps of the field on the planet's surface and discussed in the context of possible dynamo generation in the relatively poorly conducting 'ice' mantle.

Description: Results of an analysis of the Voyager 2 plasma-science-experiment electron measurements made during the Uranus encounter are presented. The energy coverage is in the range of 10 to 5950 eV. The large day-night asymmetry together with the spin axis alignment with the solar direction and the large tilt of the planetary magnetic dipole suggest that solar-wind-driven time-dependent magnetospheric convection will be an important transport mechanism within the Uranian magnetosphere. The steady state convection time of the plasma is estimated to be between 1 and 3 days.

Description: Uranus' magnetosphere has a well-developed, thick magnetopause that was fully traversed twice by Voyager 2, once inbound to the planet and once outbound. This boundary appears to resemble earth's magnetopause in approximate shape and even to the extent of supporting surface waves which were observed on the inbound pass at a distance of 18.3 Uranus radii. There were apparently eight partial transitions from the magnetosheath into the current sheet of the magnetopause at this time, followed by a final complete transition to the magnetosphere. Six of the estimated normal vectors to the local boundary show clear evidence of oscillations in the slope with typical angular excursions, from one partial transition to the next, of about 90 deg. The vectors oscillated approximately in a plane that was severely tilted by about 49 deg with respect to Uranus' orbital plane.

Description: On Jan. 24, 1986, during the inbound trajectory to Uranus, the planetary radio astronomy instrument on Voyager 2 detected left-hand-polarized emission for several hours at frequencies near 200 kHz. Unlike the dominant source observed on the nightside of the planet, only a single episode of this inbound emission was seen, and the emission was extremely weak. Generating less than 500 kW, it is probably the weakest freely propagating planetary radio emission thus far observed by Voyager. This weak emission is associated with a source region in the vicinity of the north (dayside) magnetic pole of Uranus, where magnetic-field lines point approximately in the direction of the spacecraft when the emission is observed. Therefore the wave normal angle relative to B is probably less than 90 deg in the generation region. It is concluded that direct wave generation in the magnetoionic ordinary mode is most likely. The source location for this emission is consistent with a region magnetically conjugate to the dominant nightside source, which is located in the vicinity of the south magnetic pole. The ordinary mode emission is discussed in the context of the electron-cyclotron maser mechanism.

Description: Homogeneous and core-differentiated silicate/ice models of the Uranian satellites Miranda, Ariel, Umbriel, Titania, and Oberon are examined in the light of imaging observations and mass and density determinations obtained during the Voyager 2 encounter with Uranus in January 1986. The data and model predictions are compared in extensive tables and graphs and discussed in detail. The mass fractions of silicates in Oberon and Titania are found to be between 0.42 and 0.65, about the same as the average for the satellites of Jupiter and Saturn but significantly higher than that for the smaller Saturnian satellites or that predicted by current solar-nebula models. It is suggested that the satellites formed by accretion of material from their primary planets' outer envelopes. The observed rock/ice fractions are attributed to solar-nebula CO and solid-organics abundances and to preferential dissolution of H2O in outer-envelope planetesimals.

Description: Internal (mineral) Rb-Sr isochrons were determined for six Apollo 14 mare basalts of lunar breccia 14321. The ages of these fragments, which represent the time of crystallization of the respective mafic lavas, range from about 4.05 to 4.24, and, possibly, to 4.33 Ga and are among the oldest dates reported for mare basalts. For three of the fragments, internal Sm-Nd isochron data were collected and initial Nd isotopic composition was determined. The results are discussed with reference to the original source of these different basaltic fragments.

Description: Ground-based infrared photometry of Deimos at 4.8, 10, and 20 microns is reported. The observed fluxes are significantly brighter than predicted by the 'standard' thermal model. Recent recalibrations that modify the model beam pattern of the infrared emission are marginally consistent with the observations at 10 and 20, but not at 4.8 microns.

Description: Criteria are described by which refractory interplanetary dust particles (IDPs) can be differentiated from the products of spacecraft debris. These criteria have been used to discover and characterize IDPs that are composed predominantly of refractory phases. Two of these particles contain hibonite, perovskite, spinel, refractory glass, and a melilite; only hibonite was identified within a third. The grain size for all particles ranges from 0.05 to 1 micrometer, so that they are much finer grained than the refractory calcium- and aluminum-rich inclusions in meteorites. The glass-containing refractory IDPs may be primitive nebular condensates that never completely crystallized and thus have been preserved extant.

Description: Voyager-2 PLS low-energy plasma data and the magnetometer data are combined with the the LECP ion data (E greater than 28 keV) for the distant magnetotail observations (R = 5000-9000 Jupiter radii). A definite enhancement of LECP fluxes within the core regions (where the PLS densities and magnetic-field pressure are lower than in the surrounding regions) is shown, indicating that this hot tenuous plasma is present within the core regions. In general there is a strong anticorrelation between PLS density and LECP fluxes, while a less pronounced anticorrelation between magnetic-field pressure and LECP fluxes is observed. Estimates of LECP pressures suggest that this hot plasma can provide the previously described missing pressure in the core if heavy ions dominate the ion composition. The angular dependence of the LECP data indicates a flow of this hot plasma in the anti-Jupiter direction. This outflowing plasma could be the remnant of the magnetospheric wind observed near Jupiter by LECP. On the basis of this preliminary study, the core regions are found to have similarities to a plasma sheet.

Description: The recovery of polished 600-micron-thick 7-mm-diameter disk samples and powder samples of the Leedey L6 chondrite from 29-70-GPa experimental shocks is investigated experimentally under vacuum conditions, with a focus on the possible direct shock loss of radiogenic Ar. The results of petrographic examination of the shocked samples and Ar isotope measurements are presented in tables, graphs, and micrographs and characterized in detail. The principal damage mechanisms are identified as mechanical disaggregation and increasing cataclasis, with little melting. The degassing of radiogenic Ar is found to be relatively modest and to increase with shock pressure and sample porosity; it is attributed to a combination of mechanical grain disruption and feldspar melting. It is inferred that Ar loss in naturally shocked materials (especially those showing few signs of melting) probably occurs during extended exposure to high temperatures during impact formation.

Description: The first simulations of lightning in planetary atmospheres by laser-induced plasmas are reported. These simulations show that the fraction of the energy in lightning discharge channels that is radiated in the visible spectrum is similar for earth, Venus, and Titan, but quite different for Jupiter. One implication of these results is that the amount of trace gases produced by lightning in the Jovian atmosphere must be larger than previously estimated.

Description: Lunar radar images have been investigated in order to understand how various radar parameters affect landform identification. The high-resolution (1-2 km cell size) 3.8-cm images of Zisk et al. (1974) were used along with the low and high resolution 70-cm mosaics of Thompson (1974 and 1987). The results indicate that radar cell size is the single most important radar parameter and that lunar features are likely to be correctly identified if they are longer than an 'identification resolution' of five times the radar cell size. The moon is considered here as an analog of Venus, since the two planets have similar mean scattering behavior.

Description: The capability of an orbital gamma ray spectrometer to address presently unanswered questions concerning H2O and CO2 on Mars is investigated. The gamma ray signal produced by the Martian atmosphere and by several simple models of Martian surface materials is calculated. Results are reported for: (1) the production of neutrons in the atmosphere and in the subsurface material by cosmic ray interactions, (2) the scattering of neutrons and the resultant neutron energy spectrum and spatial distributions, (3) the reproduction of gamma rays by neutron prompt capture and nonelastic scatter reactions, (4) the production of gamma rays by natural radionuclides, (5) the attenuation of the gamma ray signal by passage through surface materials and the Martian atmosphere, (6) the production of the gamma ray continuum background, and (7) the uncertainty in gamma ray line strengths that results from the combined signal and background observed by the detector.

Description: A statistical comparison of data on the composition of the Venusian surface obtained with Venera and Vega lander X-ray fluorescence spectrometers and data on the composition of the terrestrial ocean floor indicates that the Venera 14 data correspond to certain thoeiitic basalts of the Kane fracture zone in the Middle-Atlantic ridge (22-25 deg N). It is suggested that deep magma storage regions exist on Venus and that the separation of tholeiitic and alkali magmas from a single primitive parent magma is an important process on Venus.

Description: The application of the systems engineering process to the planning of the Voyager spacecraft mission is described. The Mission Planning Office prepared guidelines that controlled the use of the project and multimission resources and spacecraft consumables in order to obtain valuable scientific data at an acceptable risk level. Examples of mission planning which are concerned with the design of the Deep Space Network antenna, the uplink window for transmitting computer command subsystem loads, and the contingency and risk assessment functions are presented.

Description: An introduction is presented to the chemistry of the solar system. The qualitative and quantitative elemental analysis of the solar system is reviewed, and the elemental synthesis processes that led to the formation of the solar system are discussed. The chemical processes of the primordial mixture from which the solar system formed are examined, and the resulting chemical composition of meteorites, asteroids, comets, and planets is described.

Description: Numerical simulations of Keplerian systems consisting of 200 mutually colliding rotating particles show that friction and surface irregularity reduce the equilibrium velocity dispersion and transfer some of the energy of random velocities to rotational velocities. The results confirm the theoretical predictions of Salo (1987). Simulations are performed for identical and different particle sizes, and for a power-law distribution of sizes. Taking rotation into account is not found to alter the previously observed Rayleigh distribution of eccentricities and inclinations, while the components of the rotational velocities are found to follow a Gaussian distribution. Application to the rarefied regions of Saturn's rings suggests that friction is able to reduce the equilibrium geometric thickness by about one-half.

Description: The results of simulations of the Venusian surface environment involving windblown grains are presented which show that significant chemical and physical changes may occur even in the slow-moving winds recorded on Venus. The edges of grains beome worn and shed comminuted debris, which collects on weathered surfaces and grains alike. The resulting transfer of material from loose grains to bedrock surfaces (and vice versa) could yield misleading results on rock composition; moreover, the generation of comminuted debris would enhance chemical reactions that could affect the composition of the atmosphere. The results are thus relevant in assessing rates of surface degradation, the evolution of small-scale surface features as seen in images returned from the Soviet Venera missions, and in the interpretation of compositional data for surface materials.

Description: Thermal emission models indicate that Charon contributes a significant amount of the infrared radiation detected by IRAS during the observation of mutual eclipse events. The IRAS observations also show that the most probable diameters for Pluto and Charon are 2200 and 1300 (+ or - 150) km. These results are consistent with there being some atmosphere on Pluto.

Description: Voyager 2 UV-photopolarimeter occultation observations toward Gamma Peg, obtained on the nightside of Uranus at planetocentric latitude 68.9 deg N on January 24, 1986, are used to probe the structure of the Uranian atmosphere. The data are presented graphically and compared with the predictions of model atmospheres. The temperature profile for an aerosol-free atmosphere ranges from 85 + or - 2.3 K at 2.7 mbar to 96 + or - 13 K at 370 microbar, and the 1-mbar radius is found to be 25,219 + or - 6.3 km. The extinction coefficient for an aerosol haze layer at 1 mbar or higher is shown to be less than or equal to about 0.0001/km, but it is suggested that a well-mixed haze layer consisting of meteor or ring dust and/or photochemical condensates may well be present below 3 mbar.

Description: Voyager radio-occultation and IR spectroscopy measurements are combined to infer an He mole fraction in the upper troposphere of Uranus of 0.152 + or - 0.033; the corresponding mass fraction is Y = 0.262 + or - 0.048. This value is in agreement with recent estimates of the solar He abundance, suggesting that He differentiation has not occurred on Uranus. Comparisons with values previously obtained for Jupiter and Saturn imply that migration of He toward the core began long ago on Saturn and may also have recently begun on Jupiter. The protosolar He abundance inferred from the Uranus measurements and from recent solar evolutionary models is used along with an assumed primordial He mass fraction of 0.23-0.24 to estimate a 3-4-percent enrichment of He in the interstellar medium between the big bang and the origin of the solar system. The result is in agreement with galactic chemical evolution models which include a substantial decrease in D during the evolutionary process.

Description: The Uranian atmosphere is investigated on the basis of S-band and X-band occultation observations (including measurements of Doppler frequency perturbations) obtained during the Voyager 2 encounter with Uranus in January 1986. The data are presented in extensive tables and graphs and characterized in detail. The atmosphere is assumed to have an H2/He abundance ratio of about 85/15, but also to contain small amounts of CH4 at above-cloud relative humidity 30 percent, cloud-base relative humidity 78 percent, and below-cloud mixing ratio 2.3 percent by number density. Other parameters estimated include magnetic-field rotation period 17.24 h, 1-bar equatorial radius 25,559 + or - 4 km, polar radius 24,973 + or - 20 km, equatorial acceleration of gravity 8.69 + or - 0.01 m/sec sq, and atmospheric temperature 76 + or - 2 K (assuming 85 + or - 3 percent H2).

Description: The temperature structure of the Uranus atmosphere is investigated on the basis of 325/cm and 225/cm Voyager 2 IRIS observations of a layer between 60 and 200 mbar (including the tropopause). The data are presented in graphs and analyzed in detail. The latitudinal variations of the temperature near the tropopause and the inferred thermal winds are shown to be in good agreement with the findings reported for lower altitudes by Hanel et al. (1986), although greater in amplitude. A linear zonally symmetric circulation model with no solar and condensation heating, a frictional damping time 1-2 times the radiative damping time, a subrotating atmosphere at low latitudes, and zonal winds decaying with altitude is proposed to account for the observed structures.

Description: Crater size-frequency data for Umbriel, Titania, and Oberon are presented, and the implications of those data are discussed in terms of the geologic histories of these bodies and the populations of objects that have cratered them. The surfaces of Oberon and Umbriel are old and are inferred to date to a period early in their histories when the cratering rate was significantly higher than at present. No significant endogenic resurfacing appears to have occurred on either body after that inferred period of intense cratering. Titania exhibits the youngest surface of these three and appears to have undergone almost complete endogenic resurfacing. Among the Uranian satellites the surfaces of Oberon and Umbriel are interpreted to be the oldest, that of Titania intermediate, and those of Ariel and parts of Miranda the youngest. The size-frequency distributions for these satellites have an average slope of about -3, indicative of a steep crater production function. The cumulative size-frequency data for these Uranian satellites may be interpreted to indicate that parts of their surfaces are saturated with craters at small diameters.

Description: It is shown that the enhanced abundance of siderophile elements and chromium in tektite-like glasses from the two impact craters of Zhamanshin and Aouelloul cannot be explained as a result of contamination of the country rock by meteorites nor, probably, comets. The pattern is, however, like that found in certain Australasian tektites, and in Ivory Coast tektites. It is concluded, in agreement with earlier suggestions by Campbell-Smith and Hey, that these craters were formed by the impact of large masses of tektite-like glass, of which the glasses which were studied are fragments. It follows that it is necessary, in considering an impact crater, to bear in mind that the projectile may have been a glass.

Description: The total mass incident on the moon during the period of heavy bombardment is estimated on the basis of recent compilations of the lunar impact record, together with a mass-scaling law for crater diameters in the large-body regime. Using only those craters and basins extant, a lower limit is obtained that is independent of cometary flux models as well as of extrapolations of the lunar impact record or assumptions about the cometary mass spectrum. The results obtained indicate that the earth would have acquired an exogenous ocean of water 4.5-3.8 Gyr ago, if cometary mass constituted more than about 10 percent of the impacting population.

Description: Computational results are given for Titan atmosphere aerosol electrical conductivity and charge at altitudes up to 400 km, together with a consideration of ionization from such sources as galactic cosmic rays and electron precipitation from the Saturnian magnetosphere. Predicted conductivity shows the existence of substantial electron concentrations up to the Titan surface. At altitudes of more than 100 km, and aerosol concentrations greater than 10/cu cm, electron/positive ion-recombination is found to be controlled by the recombination of the aerosols' surfaces rather than by the gas-kinetic recombination rate.

Description: Pursuant to the past attribution of many geomorphic features on Mars to the movements of water- or ice-lubricated debris, experiments have been conducted for water freezing in wet, sand-like basaltic substrates. It is found that substantial undercooling can be achieved under Martian conditions, independently of freezing-point depressions due to soluble salts. Attention is given to results for a clay-poor soil with negligible salinity from Mauna Kea, Hawaii, which demonstrate that the degree of undercooling is essentially independent of both soil particle size and water/soil mass ratio, albeit with cooling rate variations.

Description: The tectonics of the Tharsis and adjoining areas is considered to be associated with the convection in the Martian mantle. Convection and mantle plume have been responsible for the primary uplift and volcanism of the Tharsis area. The radial compressional forces generated by the tendency for downslope movement of surface strata, vertical volcanic intrusions, and traction of mantle spreading beneath Tharsis were transmitted through the lithosphere to form peripheral mare ridge zones. The locations of mare ridges were thus mainly controlled by the Tharsis-radial compression. The load-induced stresses then contributed to further ridge formation over an extended period of time by the isostatic readjustment, which was responsible for long-term stresses in the adjoining areas. Extrusions, changes in internal temperature, and possible phase changes may also have caused changes in mantle volume, giving rise to additional compressional forces and crustal deformations.

Description: Silicon carbide has been identified in two separates from the Murray carbonaceous chondrite that are enriched 20,000-fold in isotopically anomalous neon and xenon. The SiC is present in the form of crystalline grains 0.1-1 micron in size. Cubic and 111-plane-twinned cubic are the most common ordered polytypes observed so far. The anomalous isotopic composition of its carbon, nitrogen, and silicon indicates a presolar origin, probably in the atmospheres of red giants. An additional silicon- and oxygen-rich phase shows large isotropic anomalies in nitrogen and silicon, also associated with a presolar origin.

Description: Spectrophotometric observations of the atmosphereless planetary satellites obtained with the IUE since its launch in 1978 are reviewed. The value of IUE data in complementing the images obtained by planetary probes is indicated; the techniques used to interpret IUE images of the satellites and extract textural information are discussed; and data on the Gallilean satellites of Jupiter and the satellites of Saturn are presented in extensive tables and graphs and characterized in detail. Consideration is given to the broadband geometric albedos, the orbital phase curves, hemispherical spectral asymmetries, spectral orbital phase variations, and the prospects for further observations with the Space Telescope and other earth-orbiting platforms.

Description: Chemical equilibrium models used currently to interpret observations of Jupiter are reexamined using new data defining thermal profiles, which are substantially different from those used in the previous models. A model is developed for the chemical reactions controlling the composition of the upper troposphere on Jupiter, specifically the cloud-forming region from 10 bar to 0.1 bar, which includes, for the first time, the effects of aqueous chemistry on the composition and the vertical distribution of many measurable species in the atmosphere, identifying the factors influencing their abundances above the H2O cloud. The thermodynamic data for potential condensates on Jupiter, i.e., NH3(s), NH4SH(s), (NH4)2S(s), and H2S(s), are reexamined, recognizing the lack of data on sulfides for the temperature range of interest on Jupiter. Vertical profiles of mixing ratios for CO2, H2S, NH3, and H2, obtained for several assumed bulk abundances with respect to solar, are presented.

Description: The rings of Uranus are not all circular; nor do they all lie in the plane of Uranus' equator. The epsilon ring of Uranus, which is the largest and outermost one, ranges in width from 20 to 96 km in proportion to the distance of the ring material from Uranus. Typical rings can be modeled by assuming that its edges are defined by a combination of smooth elliptical orbits and random 'wiggles'. A transition from a few large moons in outer regions to many small moons just outside ring-filled inner regions is a feature noted to be held in common by the ring and moon systems of Saturn, Uranus, and Neptune.

Description: Estimates are presented of the amount of water vapor and/or CO2 released from specific volcanic deposits on a relatively young Martian volcanic construct, Alba Patera. Estimates for the corresponding release rates are also given. Values for the constraints on water-supply-driving eruptions are presented and discussed.

Description: Plasma and magnetic field observations performed onboard the Voyager 2 spacecraft have been used to investigate Jupiter's foreshock. Large-amplitude waves have been detected in association with the quasi-perpendicular structure of the Jovian bow shock, thus proving that the upstream turbulence is not a characteristic signature of the quasi-parallel shock.

Description: The anomalous radar echoing properties of three ice-clad moons of Jupiter appear to be due to glory-like backscattering from buried craters. The enormous glare from these sources would impair geologic studies based on standard methods of radar mapping. It is not known whether similar or different problems will arise in the radar study of other icy surfaces in the outer solar system, or of the unseen surface of Titan. In any event, the results from the moons of Jupiter illustrate the role of exploratory measurements and the importance of possible bistatic radar-mapping techniques based on the use of separated transmitters and receivers.

Description: The planetary surface thermal models used in the present study are composed of particles which, while bright and optically thin in the visual, are dark and opaque in the thermal IR. It is assumed that insolation is absorbed over a finite regolith distance whose scale length relative to diurnal skin depth for thermal diffusion can be substantial. Attention is given to the lower daytime and higher nighttime temperatures predicted by comparison with models assuming absorption only at the surface. It is noted that, with sufficiently deep penetration of insulation and high thermal IR particle opacity, a solid-state greenhouse can result.

Description: Vacuum sublimation effects on solid sulfur yield a form of the element that is white at room temperature, is fluffy in texture, and forms on frozen sulfur in vacuum through differential evaporation of molecular species in the solid. This vacuum sulfur should exist in large quantity on Io, if the solid free sulfur there has solidified from a melt; a sulfur volcanism model for Io is accordingly developed on this basis which implies that the color and spectra of different sulfur regions of Io could indicate their relative crystallization ages and cooling histories. The flux of sublimating hotspot sulfur appears consistent with estimated turnover rates of the Io surface.

Description: New spectra of the Pluto-Charon system taken just before and during a total eclipse of the satellite are presented. The spectrum of Charon extracted from the data reveals the signature of water ice. There is no evidence for any methane or ammonia frost on the surface of Charon. The significance of these findings for the evolution of the Pluto-Charon system are discussed.

Description: The scientific aims and proposed scenarios of sample-return missions (SRMs) to Mars, Venus, comets, and asteroids are reviewed. SRMs are currently being evaluated as natural follow-ons to the NASA remote-sensing orbiter missions of the 1990s. The technological advantages of ground laboratory analysis of returned samples are discussed; and it is argued that SRMs to large evolved bodies can provide a record of (1) the composition of the solar system at different heliocentric distances and (2) the nature of the processes that led to the accretion of small objects (preserved as comets and asteroids) into large planetary bodies. Also considered are NASA research and analysis programs needed to support studies of solar-system origin. It is recommended that planning of SRMs be begun immediately, although they may not be feasible before the year 2000.

Description: It has been suggested that nonthermal Na atoms with velocities in excess of 2.1 km/sec in the Mercury atmosphere can be accelerated off the planet by solar radiation pressure; Na abundance may accordingly be expected to decrease with increasing radiation pressure. While this is confirmed by the present measurements, high resolution line profile measurements on Na emission indicate that very little, if any, of the Na is nonthermal, while the bulk is at a temperature approaching that of the planetary surface. Attention is given to explanations for the observed variation.

Description: Errors that emerged in the Hapke et al. (1980) geological analysis reproduction of the Mariner 10 orange/UV color ratio map of Mercury are presently noted, and it is judged that the relationships that emerge between the color and the terrain are nonlunar in that they lack Mercurian analogs of high Fe-Ti lunar maria basalts. The crust is found to be low in Fe(2+) and Ti(4+) in view of three major considerations: (1) rays and ejecta blankets are exceptionally blue; (2) the Fe(2+) band of the reflectance spectrum of Mercury is weak; and (3) Mercury albedo contrasts are less pronounced than the lunar ones. The extrusive origin of the smooth plains is supported by these observations.

Description: The polarization-time variations noted in the present linear polarization observations of the integrated Mercury disk, with filters covering the 0.3-1.0 micron bandpass and between 53-130 deg of phase angle, may be accounted for in terms of longitude dependence through the variation of brightness or other properties over the surface. Kepler's equation for the eccentric anomaly was solved for each observation, and the true anomaly was used to find the subsolar longitude. The difference found in the polarization-albedo relations of Mercury and the moon indicates a difference in surface texture and/or composition.

Description: The distribution of total and polarized intensities from Mercury's subsurface layers have been mapped using VLA observations. The first detection of a hot pole along the Hermean equator is reported and modeled as black-body reradiation from preferential diurnal heating. These observations appear to rule out any internal sources of heat within Mercury. Polarized emission from the limb of the planet is also found, and is understood in terms of the dielectric properties of the Hermean surface.

Description: The results published by U.S. scientists during 1983-1986 from studies related to the magnetospheres of Jupiter, Saturn, and Uranus are discussed. Consideration is given to the magnetic fields of these planets, charged particle environments, the interactions between the planetary rings and planetary satellites, the solar wind interactions, radio emissions, and auroras. Special attention is given to observations of (1) a small flux of energetic electrons and protons in the otherwise radiation-free environment in the magnetosphere under the rings of Saturn (interpreted as interactions of Galactic cosmic rays with the rings), (2) spokes, and (3) Saturn ring erosion.

Description: The geology, geomorphology, and atmosphere of Venus are characterized on the basis of observations obtained with the Soviet Venera 15 and 16 spacecraft (including two international Vega balloon experiments), the Pioneer Venus Orbiter, and the Arecibo radio telescope during the period 1983-1986. Features discussed include impact craters, evidence of tectonic and volcanic activity, the high average age of the Venusian surface (apparently over 1 Gyr, indicating resurfacing rates much lower than on earth), and atmospheric temperature differences across the equator.

Description: Observations and theoretical investigations of the Uranus (U) system from the period 1983-1986 are reviewed, with an emphasis on the Voyager 2 encounter with U on January 26, 1986. Topics addressed include the bulk U composition, structure, and heat flux; the U atmospheric composition, structure, and circulation; the U rings; the major and minor U satellites; the U magnetosphere; and the Lyman-alpha 'electroglow' observed on the sunlit hemisphere of U.

Description: While photoelectric lightcurves obtained for 532 Herculina in 1984 exhibit two maxima and two minima, the lightcurve has shown only one maximum and one minimum over the same rotation period in some other oppositions. The use of photometric astronomy yields a sidereal period of 0.3918711 + or 0.0000001 day, with a retrograde rotation for the north pole at 276 deg longitude and +1 deg latitude. A model consisting of a sphere with two dark regions that are each about 0.13 times the brightness of the surrounding surface is developed for Herculina, and it is shown that its generated lightcurves are consistent with both the observed amplitudes and the timings of extrema over the 28,630 sidereal rotations of 30 years. The lightcurves for the next four oppositions are predicted on the basis of the photometric astrometry pole and the two dark region model.

Description: The Na, K, Rb, Cs, and P abundances in eucrites, diogenites, basaltic clasts from polymict eucrite, howardites, and mesosiderites are examined, and compared with an average of highly incompatible refractory (AHIR) elements normalized to cosmic abundances. It is observed that basaltic eucrites and basaltic clasts show a positive correlation between K, Rb, and Cs, and alkali element/AHIR ratios; the volatile loss of the alkali elements from the basalt affects the parent body inventory of volatile elements. The data reveal that for diogenites, the alkali /AHIR ratios are 1.4-2 times greater than in basaltic eucrites and are more variable; and the negative relation between K, Rb, Cs, and the alkali/AHIR ratio correlate with progressive alkali loss through volatile outgassing during crystallization of one or more magmas resulting in a greater than 90 percent loss of the volatile element inventory from the parent body. It is also detected that P displays volatile loss from the basaltic eucrites and elevated P/AHIR ratios in diogenites.

Description: The topographies of the major moons of Uranus are described, and reasons for the appearances of the surfaces are given. Oberon and Titania, the two outermost major moons, have similar bulk properties but different appearances. Oberon appears to have been a largely passive structure for incoming projectiles, while Titania's surface shows extensive resurfacing. Umbriel and Ariel also have similar bulk properties and contrasting appearances, the former being totally bland except for two unexplained bright spots and the latter having a extensive network of surface faults. Miranda, the smallest and innermost of the major moons, has three large, remarkable ovoid regions on its surface. It is suggested that these ovoids are due to an uncompleted differentiation process.

Description: The meteoroid impact-seismic activity data recorded by the Apollo lunar seismic network is examined. The study investigates the difference in temporal distribution between large and small impacts, clustering of impacts in a two-dimensional space of the time of the year and the time of the month, and the relationship of these observations with terrestrial observations. Several distinct families of meteoroids impacting the moon are identified. Most meteoroids producing small impact-seismic events, including ones associated with cometary showers, appear to approach from retrograde heliocentric orbits. In contrast, most meteoroids associated with large impact-seismic events appear to approach from prograde orbits; the observation is consistent with a hypothesis that many of them represent stony asteroidal material. It is suggested that the previously reported discrepancy between lunar and terrestrial meteoroid-flux estimates may be due to the differences in lunar and terrestrial detection efficiency among various families of meteoroids.

Description: Laboratory simulations of geological processes on the terrestrial planets are described, summarizing results published during the period 1983-1986. Included are studies of wind-driven processes on Mars and Venus (using the special wind-tunnel facilities at NASA Ames); simulations of shock-induced loss of volatiles from solids; equation-of-state determinations; impact experiments simulating cratering, spallation, regolith formation, and disruption; fluid-flow simulations of channel formation on Mars; and dust studies. The use of the microgravity environment of the Space Station for planetary-geology experiments is briefly considered.

Description: A semiquantitative model is proposed for the narrow, moist convective plumes of the Jupiter troposphere which appears capable of explaining the apparent H2O vapor depletion in the 5-micron window without requiring a global depletion of oxygen from solar abundance. Chemical dessication is held to be unlikely, in virtue of its requirement for the many-fold enhancement of ammonia. Arguments based on mixing length theory are presented to justify a small filling factor, or ratio of the area occupied by updrafts to the total area.

Description: Primitive meteorites contain up to 400 ppm of a very fine-grained type of carbon tentatively called C-delta. Evidence is presented here that part of all of the C-delta is primary, not shock-produced, diamond formed by stellar condensation as a metastable phase. It appears that interstellar dust contains diamond.

Description: Mineralogy of a basaltic clast, Ba-2, found on the cut surface of a new slab of lunar highland regolith breccia 60019 was studied and compared with that of lunar meteorites ALHA81005 and Y791197. The coarse-grained Ba-2 clast consists of Na-bearing plagioclase, high-Ca clinopyroxene, ilmenite, silica, and mesostasis. The mineralogy and chemical zoning trends in the Ba-2 clast suggest that this basalt is similar to the 60639,1 basalt in the Apollo 16 rake samples, and to Luna 16 high-alumina moderate-TiO2 mare basalts; however, olivine is not present in Ba-2. On the basis of these and other findings, and assuming the early formation of the Apollo 16 regolith breccia (as proposed by McKay et al., 1986), it is suggested that B-2 could represent a post-Nectaris lava flow older than the formation of 60019.

Description: Measurements of the rate of hydrous alteration of amorphous Mg-SiO smokes are reported as a function of temperature, as inferred by observing changes in the infrared spectra of these materials. It is shown that under the conditions reported for the nucleus of Comet Halley, based on measurements made by the Vega and Giotto missions, amorphous, anhydrous Mg-SiO smokes would become hydrated within several weeks in the dusty regolith observed on the surface. However, if such grains were released in 'jets' or from loose ice fragments, then previously amorphous grains would retain their anhydrous nature. Similarly, brief periods of aqueous activity on meteorite parent bodies would convert amorphous, fine-grained material to hydrated phyllosilicates much more rapidly than coarse mineral grains. A kinetic model might therefore be developed to explain the observed textural relationships in the matrices of carbonaceous chondrites such as Mokoia, where amorphous phyllosilicates are intimately associated with coarse anhydrous grains.

Description: Broadband colorimetry, visual photometry, near-infrared photometry, and 10 and 20 micron radiometry of the near-earth asteroids (NEAs) 1986 DA and 1986 EB are used to show that these objects belong to the M class of asteroids. The similarity among the distributions of taxonomic classes among the 38 NEAs to the abundances found in the inner astoroid belt between the 3:1 and 5:2 resonances suggests that NEAs have their origins among asteroids in the vicinity of these resonances. The implied mineralogy of 1986 DA and 1986 EB is mostly nickel-iron metal; if this is indeed the case, then current models for meteorite production based on strength-related collisional processes on asteroidal surfaces predict that these two objects alone should produce about one percent of all meteorite falls. Iron meteorites derived from these near-earth asteroids should have low cosmic-ray exposure ages.

Description: The results of an analysis of the energetic particle and magnetic field data acquired by the Voyager 2 spacecraft at distances of about 40-70 Jupiter radii on the nightside of the planet are reported. As in a previous study of similar data at distances of greater than about 80 Jupiter radii, the energy densities of ions (primarily protons) is found to be sufficient to provide the diamagnetic depressions measured in the magnetic field intensity as the spacecraft made successive encounters with the nightside plasma sheet. There is some evidence that the percent contribution of the protons to the energy balance decreases with increasing distance from the planet over this radial interval, although this conclusion is dependent upon the assumption that the proton and heavier ion (oxygen) energy spectra are similar.

Description: The surfaces of the principal Uranian satellites are characterized on the basis of UV and IR geometric albedos, phase curves, and phase coefficients obtained in full-disk photopolarimetric observations during the Voyager 2 encounter with Uranus in January 1986. The data are presented in tables and graphs and found to be consistent with a heavily cratered terrain and loosely packed regolith. The Bond albedos are calculated as 0.22 + or - 0.1 for Ariel, 0.07 + or - 0.05 for Umbriel, 0.16 + or - 0.12 for Titania, and 0.19 + or - 0.22 for Oberon. The characteristics of the Uranian satellites indicate compositions (and probably formation conditions and surface-modification mechanisms) distinct from those of the Saturnian and Jovian satellites.

Description: On the basis of the location of the source field lines of the smooth nightside component of Uranus kilometric radiation, the most likely free energy source is the outer radiation belts. As the terminator sweeps over the magnetic north polar region, precipitation of electrons generated by solar heating of the upper atmosphere and submergence of the electron mirror points deeper in the atmosphere will create a backscattered electron distribution with an enhanced population at large pitch angles. The clocklike radio emission turns out to be a direct consequence of the terminator's control of the emission process.

Description: The broad-bandwidth radio emission detected by Voyager 2 over the nightside of Uranus is examined. It is concluded that the source location of the smooth component is consistent with emission originating near the electron gyrofrequency from a small set of field lines whose foot points lie near the Uranomagnetic southern (dark) pole. The source centroid is at L = 11.5, and extends in latitude between about L = 8 and L = 25. This deduced source region is primarily on closed field lines that pass through the outer radiation belt and have their opposite foot points near the Uranomagnetic northern pole (near the present epoch terminator). The source location of the bursty component is less well defined but is consistent with the set of open field lines which map down to the region surrounding the planet's south magnetic dipole tip.

Description: Two aluminous mare basalt clasts of high K abundances from Apollo 14 breccia 14304 (the 14304,127 and 14304,128 samples) were characterized with respect to the Rb and Sr concentrations and isotopic compositions, and the Rb-Sr, K-Ar, and Sm-Nd isotopic age determinations were carried out. The results suggest that these high-K basalts were melts derived from mantle material and that they have experienced about ten-fold Rb/Sr and K/La enrichments at approximately the time of crystallization.

Description: In the present gravity analysis of Mariner 10/Deep Space Network radio Doppler and range data for Mercury encounters in March 1974 and March 1975, a combined least-squares fit to the Doppler data has determined two second-degree gravity harmonics that are referred to a 2439-km equatorial radius. It is noted that the 1-sigma error limits on the gravity results encompass the possibility that harmonics other than J2 and C22 significantly differ from zero. The Deep Space Network radio range data obtained with Mariner 10 are primarily applicable to such improvements of Mercury's ephemeris as the more precise determination of perihelion precession.

Description: High-signal-to-noise-ratio observations of the Pluto-Charon system at 25, 60, and 100 microns using IRAS are combined with visual-magnitude and mutual-eclipse constraints to evaluate thermal models of Pluto and Charon. These models are consistent with eclipse observation by Dunbar and Tedesco (1986) but not with Reinsch and Pakull (1987). The most likely model for Charon is the standard asteroid model, typical for the icy Galilean and Saturnian satellites. Charon models with a significant atmosphere can be ruled out. Based on currently available radius and albedo constraints, no significant numerical distinction is possible between Pluto models ranging from isothermal spheres with surface emissivity between 0.4 and 0.9. Concerns regarding the viability of an emissivity as low as 0.4 favor the higher-emissivity models. The globally uniform surface temperature of Pluto may thus at present be as low as 45 K, with a methane column abundance of 6.7 cm atm. The most likely models are centered on radii of 1180 and 747 km and albedos of 0.47 and 0.26 for Pluto and Charon, respectively.

Description: Aqueous chemistry on Uranus affects the atmospheric abundances of NH3 and H2S below the methane cloud base. Here a complete thermochemical equilibrium model for the H2O-NH3-H2S system is presented. Inclusion of H2S increases the aqueous removal of NH3 to 20-30 percent, but aqueous chemistry alone cannot account for the depletion of NH3 in the 150-200-K region of the atmosphere required to fit microwave observations. Formation of NH4SH clouds can account for the observed depletion provided the H2S/NH3 ratio is enhanced by a factor of 4 relative to solar. Perturbations to the chemical balance between N and S, for example by the general circulation on Uranus, would then produce regions with either NH3 or H2S aloft.

Description: This paper presents detailed evidence in support of the earlier suggestion by Taylor et al. (1985, 1986) that the 100-Hz noise (or LW events) detected by the Pioneer Venus Orbiter across the nightside of Venus should not be unconditionally interpreted as evidence of 'lightning' at Venus. It is shown that the statistical behavior of the electric field noise is consistent with the interpretation that the noise source is local to the spacecraft; the LW events result from the plasma instabilities through which the Orbiter passes. It is also shown that the occurrence of the LW events attributed to lightning decreases significantly at altitudes below about 180 km. The noise is most frequently observed near 180-200 km; the irregularities in this region are interpreted as a result of the nightside interaction of the solar wind/IMF with the ionosphere.

Description: Low-density aggregates composed of submicron clay aerosols were formed experimentally as the sublimation residues of masses of dust-nucleated ice. These ice-dust mixtures are considered to be possible analogs of materials of Martian north polar deposits. Low-density (0.002 g/cu cm) spheroidal pellets formed from these materials in wind tunnel experiments are studied as possible candidates for the formation of dunes in the north polar region of Mars. It is shown that these particles move like sand grains under conditions of saltation and, taken in sufficient quantities, can form the dunes observed in the north circumpolar erg.

Description: The techniques and data used in previous determinations of the orbital evolution of the Galilean satellites of Jupiter are reviewed, and a new analysis is developed on the basis of Lieske's (1986) collection of 15,711 eclipse observations from the period 1652-1983, photographic observations, and Voyager optical-navigation images (Synnott et al., 1982). The data and results are presented in tables, and the secular changes in the satellite mean motions are found to be smaller than previously determined: e.g., (-7.4 + or - 8.7) x 10 to the -10th/yr for Io and (+8 + or - 42) x 10 to the -9th/yr for the relative evolution between Io and Europa.

Description: The early structural history of the Martian Tharsis bulge was studied in terms of a compensation model proposed by Froidevaux and Isacks (1984) proposed for terrestrial plateau uplift. The analysis was based on the regional topographic data on the bulge and the distribution of compressional tectonic features and major volcanoes. The results indicate that the initial phases of the Tharsis formation were dominated by the mantle plume doming, followed by extrusional dome raising; the volcanism became the most important bulge building factor only after thickening of the crust. The formation of the Tharsis bulge and the partial relaxation of this bulge have led to significant horizontal stresses and compression within the peripheral areas. The compressional peripheral zone around the bulge gave rise to dorsa formation while the high bulge crests have been in tensional state.

Description: In the present investigation of Martian volcano-ground ice interaction processes, a numerical model is developed that encompasses conductive heat transport, surface radiation, heat transfer to the atmosphere, and H2O phase-changes in an ice-rich permafrost over which lave flows erupt, followed by the intrusion of sills. An examination is made of the two large scale interaction regions formed near Aeolis Mensae and near the volcano Hadriaca Patera, northeast of Hellas. The inferred channel discharges are compared to discharge rates calculated for lava-ground ice interactions, showing that meltwater (probably accumulated under the surface) was rapidly released at a discharge rate that was limited by soil permeability. The volcano-ground ice interactions have been an important Martian geologic process, and could account for the palagonites constituting the Martian dust.

Description: A two-dimensional model of the ionosphere of Venus has been constructed by expanding pertinent quantities in Legendre polynomials. The model is simplified by including only a single ion species, O(+). Horizontal plasma flow velocity and plasma density have been calculated as a coupled system. The calculated plasma flow velocity is found to be in good agreement with observations and the results of earlier studies. Solar zenith angle dependence of plasma density, particularly on the nightside, shows some features which differ from results of earlier studies and observed values. Effects of raising or lowering the ionopause height and changing the nightside neutral atmosphere have been discussed.

Description: A qualitative model of lithospheric flexure and faulting is proposed which accounts for certain rille fault features around mare regions of the moon. It is suggested that the subsidence of lunar mare basins may be due to the existence of their mascons and their volcanic fills as loads that flexed the lithosphere, in addition to the effects of drying up and cooling of internal hot volumes. Differences in the internal attack against the lunar crust from above and below the mean surface are considered. The rille formation is shown to be characterized by peripheral bulging and bending, and it is suggested that asthenosphere-related effects in lunar tectonics may have been very important.

Description: Hameen-Anttila's (1984) theory of self-gravitating collisional particle disks is extended to include the effects of particle spin. Equations are derived for the coupled evolution of random velocities and spins, showing that friction and surface irregularity both reduce the local velocity dispersion and transfer significant amounts of random kinetic energy to rotational energy. Results for the equilibrium ratio of rotational energy to random kinetic energy are exact not only for identical nongravitating mass points, but also if finite size, self-gravitating forces, or size distribution are included. The model is applied to the dynamics of Saturn's rings, showing that the inclusion of rotation reduces the geometrical thickness of the layer of cm-sized particles to, at most, about one-half, with large particles being less affected.

Description: The Neptune moon Triton may have an appreciable atmosphere; the preliminary calculations of Delitsky (1983) have suggested that there should be a significant resultant chemistry in a possible N2 ocean with dissolved CH4, given charged-particle radiolysis of gaseous mixtures and gamma radiolysis of N2-CH4 solutions. The latter will yield substantial quantities of organic products which will be partially soluble in any N2-CH4 liquid present. Attention is presently given to the energy sources available for Triton, in order to estimate rates of synthesis and ascertain the possible history of such simple and complex organic products on Triton's surface.

Description: An erosional model of Saturn's rings is proposed based on theoretical studies of the high charge-to-mass ratio particles in Saturn's ring plane, and assuming that the B and C rings were initially formed as one ring with the optical thickness of the present B ring. The erosion rate is calculated using data from observed micrometeorite fluxes, and a ring age of 4.4-76 Myr is determined which is inconsistent with the 4.5-Gyr ring lifetime required by the cosmogonic ring hypothesis. The sharpness of the transition between the B and C rings suggests that the principal mass loss is through particles moving at a few m/sec with respect to the parent bodies from which they were eroded.

Description: It is investigated whether Uranus models with higher values of the total planetary ice to rock ratio (I/R) can be made consistent with the rotation period of 17.24 hr. measured by Voyager 2, and with the 16-pole moment of the gravitational field (J4) derived from the observations of French et al. (1986). An atmospheric enhancement of H2O, NH3, and CH4 of not more than about 30 times the solar value, and an I/R value of greater than 16, are implied by these models. A planetary accretion scenario whereby such I/R values are possible for a large range of planetesimal radii and initial velocities is discussed.

Description: As a preliminary step in the development of a general circulation model for general planetary use, a simplified vesion of thef GISS Model I GCM has been run at various rotation periods to investigate differences between the dynamical regimes of rapidly and slowly rotating planets. To isolate the dynamical processes, the hydrologic cycle is suppressed and the atmosphere is forced with perpetual annual mean solar heating. All other parameters except the rotation period remain fixed at their terrestrial values. Experiments were conducted for rotation periods of 2/3, 1, 2, 4, 8, 16, 64 and 256 days. The results are in qualitative agreement with similar experiments carried out previously with other GCMs and with certain aspects of one Venus GCM simulation. As rotation rate decreases, the energetics shifts from baroclinc to quasi-barotropic when the Rossby radius of deformation reaches planetary scale. The Hadley cell expands poleward and replaces eddies as the primary mode of large-scale heat transport. Associated with this is a poleward shift of the baroclinic zone and jet stream and a reduction of the equator-pole temperature contrast. Midlatitude jet strength peaks at 8 days period, as does the weak positive equatorial zonal wind which occurs at upper levels at all rotation periods. Eddy momentum transport switches from poleward to equatorward at the same period. Tropospheric mean static stability generally increases in the tropics and decreases in midlatitudes as rotation rate decreases, but the global mean static stability is independent of rotation rate. The peak in the eddy kinetic energy spectrum shifts toward lower wavenumbers, reaching wavenumber 1 at a period of 8 days. Implications of these results for the dynamics of Venus and Titan are discussed. Specifically, it is suggested that the extent of low-level convection determines whether the Gierasch mechanism contributes significantly to equatorial superrotation on these planets.

Description: A laboratory facility proposed for the Space Station to investigate fundamental aspects of windblown particles is described. The experiments would take advantage of the environment afforded in earth orbit and would be an extension of research currently being conducted on the geology and physics of windblown sediments on earth, Mars, and Venus. Aeolian (wind) processes are reviewed in the planetary context, the scientific rational is given for specific experiments to be conducted, the experiment apparatus (the Carousel Wind Tunnel, or CWT) is described, and a plan presented for implementing the proposed research program.

Description: An understanding of impact processes in low- and microgravity environments would be advanced significantly by the construction and use of an impact facility on the Space Station. It is proposed that initial studies begin as soon as possible in ground-based impact laboratories, on the NASA KC-135 Reduced-Gravity Aircraft, and in existing drop towers. The resulting experience and information base could then be applied toward an experiment package designed for use on Shuttle orbiters to support pilot studies in orbital environments. These experiments, as well as the first efforts made on the IOC Space Station, should involve the impact of various free-floating targets; such studies would yield a substantial scientific return while providing valuable experience and engineering information for use in refining the design of the dedicated Space Station Impact Facility. The dedicated facility should be designed to support impact experimentation, including but not limited to cratering, asteroid and ring-particle dynamics, and accretional processes.

Description: Analysis of high spatial resolution (down to 2 to 5 km) thermal data that shows the presence of aeolian debris mantling in topographic traps such as some channels is presented. In cases where this occurs, even if the mantle is thin, it can prevent remote sensing analysis of actual compositional differences that may characterize the true floors of the features.

Description: For most estimates of available regolith and initial degassed CO(sub 2) inventories, it appears that any initial inventory must have been lost to space or incorporated into carbonates. Most estimates of the total available degassed CO(sub 2) inventory are only marginally sufficient to allow for a major early greenhouse effect. It is suggested that the requirements for greenhouse warming to produce old dessicated terrain would be greatly lessened if groundwater brines rather than rainfall were involved and if a higher internal gradient were involved to raise the water (brine) table, leading to more frequent sapping.

Description: Viking XRF analyses are compared with those for terrestrial and lunar basalt samples, and eucritic meteorites (of possible Mars origin). The comparison indicates depletion of Ca relative to Si in the Mars regolith. It is suggested that carbonate formation during a warmer, wetter epoch early in Mars' history could have been responsible.

Description: It is reported that the SNC meteorites are a factor of 2 richer in moderately volatile elements (Na, K, P, etc.) and some volatile elements (Cl, Br, I, S) compared to the Earth and Mars. It is proposed that Mars accreted homogeneously, with the result that all the water except trace amounts (on reaction with metallic Fe) was converted to H(sub 2) and escaped. This model predicts a total Mars H(sub 2)O inventory of 18 ppm (equivalent to an ocean 80 m deep on the planet), and a value for Earth of 300 ppm, close to the actual abundance.

Description: A model study of the distribution of ozone on Mars is presented. It is showed that knowledge of the vertical ozone distribution, such as could be obtained from ultraviolet measurements from an orbiter, could be used to infer vertical transport rates at various levels in the atmosphere. The dependence of the vertical distribution of O(sub 3) on the height variation of eddy diffusion coefficient is illustrated. Ozone abundance is a valuable diagnostic for other climatological parameters. In addition, the sensitivity of O(sub 3) distribution to eddy diffusion may aid in determining the role of surface oxidation and recombination processes and lead to a better understanding of the volatiles released or adsorbed cyclically in the Martian regolith.

Description: It is showed that periods of enhanced quasitidal pressure oscillations occur during the season preceding the global dust storm season during every Martian year. Periods of these oscillations often differ slightly from the diurnal and semidiurnal, and the oscillations may correspond to atmospheric normal modes excited by solar heating. They may also contribute to the initiation of some global dust storms.