ALBERT

Description: During the final, low solar activity phase of the Pioneer Venus (PV) mission, the Orbiter Ion Mass Spectrometer (OIMS) measurements found all ion species, in the midnight-dusk sector, reduced in concentration relative to that observed at solar maximum. Molecular ion species comprised a greater part of the total ion concentration as O(+) and H(+) had the greatest depletions. The nightside ionospheric states were strikingly similar to the isolated solar maximum 'disappearing' ionospheres. Both are very dynamic states characterized by a rapidly drifting plasma and 30-100 eV superthermal O(+) ions.

Description: In October, 1992 the Pioneer Venus Orbiter entered the atmosphere of Venus, ending nearly 14 years of observations at Venus. Prior to the entry into the atmosphere and subsequent loss of the spacecraft careful management of spacecraft resources had allowed the acquisition of much low altitude data over the nightside of the planet. The long duration of the Pioneer Venus mission has enabled us to study the ionosphere and atmosphere of Venus under different levels of solar activity.

Description: Atmospheric drag measurements from the orbital decay of the Pioneer Venus Orbiter and Magellan spacecraft have recently been obtained of the Venus dayside and nightside atmosphere between 130 and 210 km during a period of low solar activity. These new measurements, combined with the earlier Pioneer Venus drag measurements (1978-80) obtained near the maximum of the 11-year solar cycle, have allowed the detection of the detailed response of temperature, atomic oxygen and carbon dioxide to solar variations. We have found a weak but detectable temperature response on the dayside which is in accord with the response predicted by Keating and Bougher when they assumed very strong CO2 radiative cooling resulting from atomic oxygen exciting CO2 into 15 micron emission. This same radiative process may cause strong cooling in the Earth's upper atmosphere with the doubling of CO2 in the future. With decreasing solar activity, the O/CO2 ratio in the lower thermosphere is found to decrease, apparently due to decreased photodissociation of CO2 and lower temperatures. The percent decrease in atomic oxygen with decreasing solar activity on the dayside is found to be approximately the same as the percent decreases of atomic oxygen transported to the nightside. A very weak response of nightside temperatures to solar activity variations has also been detected.

Description: We have combined the most recent Pioneer Venus Orbiter (PVO) and Magellan (MGN) data with the earlier 1978-1982 PVO data set to obtain a new 60th degree and order spherical harmonic gravity model and a 120th degree and order spherical harmonic topography model. Free-air gravity maps are shown over regions where the most marked improvement has been obtained (Ishtar-Terra, Alpha, Bell and Artemis). Gravity versus topography relationships are presented as correlations per degree and axes orientation.

Description: We report infrared heterodyne spectroscopy (lambda/delta lambda is approximately 10(exp 6)) of C2H6 emission at 11.9 microns from the northern Jovian auroral region, in observations conducted over December 2-7, 1989. Accurately measured line shapes provide information on C2H6 abundance as well as temperature and permit retrieval of the source pressure region. Enhanced emission was observed in the longitude range approximately 150-180 deg at approximately 60 deg north latitude, approximately corresponding to the CH4 7.8 micron hot spot and the region of brightest UV aurora. Significant brightness variations were observed in the hot spot emissions on a time scale of approximately 20 hours. Analysis of the brightest hot spot spectra indicates C2H6 mole fractions of approximately (6.3-6.8) x 10(exp -6) at temperatures of approximately 182-184 K at 1 mbar, compared to mole fractions of (3.8 +/- 1.4) x 10(exp -6) averaged over spectra outside the hot spot at a temperature of approximately 172 K at the same pressure. Fixing the mole fraction to the lower limit retrieved in the quiescent (non-hot spot) region allows the temperature at 1 mbar to be as high as approximately 200 K within the hot spot. These results provide upper limits to the temperature increase near the source of the C2H6 thermal infrared emission. Combined with results from similar measurements of ethylene emission probing the approximately 10-microbar region (Kostiuk et al., this issue), altitude information on the thermal structure of the Jovian auroral stratosphere can be obtained for the first time.

Description: Doppler tracking data of three orbiting spacecraft have been reanalyzed to develop a new gravitational field model for the planet Mars, Goddard Mars Model 1 (GMM-1). This model employs nearly all available data, consisting of approximately 1100 days of S band tracking data collected by NASA's Deep Space Network from the Mariner 9 and Viking 1 and Viking 2 spacecraft, in seven different orbits, between 1971 and 1979. GMM-1 is complete to spherical harmonic degree and order 50, which corresponds to a half-wavelength spatial resolution of 200-300 km where the data permit. GMM-1 represents satellite orbits with considerably better accuracy than previous Mars gravity models and shows greater resolution of identifiable geological structures. The notable improvement in GMM-1 over previous models is a consequence of several factors: improved computational capabilities, the use of otpimum weighting and least squares collocation solution techniques which stabilized the behavior of the solution at high degree and order, and the use of longer satellite arcs than employed in previous solutions that were made possible by improved force and measurement models. The inclusion of X band tracking data from the 379-km altitude, nnear-polar orbiting Mars Observer spacecraft should provide a significant improvement over GMM-1, particularly at high latitudes where current data poorly resolve the gravitational signature of the planet.

Description: A series of surface-modified clays containing nanophase (np) iron/oxyhydroxides of extremely small particle sizes, with total iron contents as high as found in Mars soil, were prepared by iron deposition on the clay surface from ferrous chloride solution. Comprehensive studies of the iron mineralogy in these 'Mars-soil analogs' were conducted using chemical extractions, solubility analyses, pH and redox, x ray and electron diffractometry, electron microscopic imaging specific surface area and particle size determinations, differential thermal analyses, magnetic properties characterization, spectral reflectance, and Viking biology simulation experiments. The clay matrix and the procedure used for synthesis produced nanophase iron oxides containing a certain proportion of divalent iron, which slowly converts to more stable, fully oxidized iron minerals. The noncrystalline nature of the iron compounds precipitated on the surface of the clay was verified by their complete extractability in oxalate. Lepidocrocite (gamma-FeOOH) was detected by selected area electron diffraction. It is formed from a double iron Fe(II)/Fe(III) hydroxyl mineral such as 'green rust', or ferrosic hydroxide. Magnetic measurements suggested that lepidocrocite converted to the more stable meaghemite (gamma-Fe203) by mild heat treatment and then to nanophase hematite (aplha-Fe203) by extensive heat treatment. Their chemical reactivity offers a plausible mechanism for the somewhat puzzling observations of the Viking biology experiments. Their unique chemical reactivities are attributed to the combined catalytic effects of the iron oxide/oxyhydroxide and silicate phase surfaces. The mode of formation of these (nanophase) iron oxides on Mars is still unknown.

Description: The 3-5 micrometer thermal emission of the nightside of Venus, recorded by the Near-Infrared Mapping Spectrometer (NIMS) instrument at the time of the Galileo flyby of Venus, is analysed to infer the properties of the upper cloud boundary. From the global maps of Venus at fixed wavelengths, the limb darkening of the flux is measured at several latitudes, within each infrared channel. By using the nominal Pioneer Venus thermal profile, these data give access to two parameters: the cloud deck temperature and the cloud scale height. It is verified independently, from the NIMS spectra, that this thermal profile is consistent with all the NIMS observations, and that the thermal structure does not vary significantly in the latitude range (25 deg S, 30 deg N). Within this range, the cloud scale height is found to be constant with latitude, and is H = 5.2 km, with an accuracy of about 15%, taking into account the various sources of theoretical and observational uncertainties. At higher latitudes, the temperature profile becomes more isothermal and the presented method to retrieve H is no longer valid.

Description: A large number of i.r. spectra of Venus was obtained using the Near-Infrared Mapping Spectrometer (NIMS) on the Galileo spacecraft, during the February 1990 encounter. Preliminary results show an apparent increase in the tropospheric CO volume mixing ratio (vmr) in the northern polar region. Other possible explanations of the observations are examined and rejected and an increase of the CO abundance north of 47 deg N of (35 +/- 15)% is inferred. Some possible causes of this enhancement are suggested.

Description: The spectroscopic data of the Near-Infrared Mapping Spectrometer (NIMS), recorded during the Galileo flyby of Venus, are analysed to retrieve the water vapor abundance variations in the lower atmosphere of Venus at night. The 1.18 micrometer spectral window, which probes altitude levels below 20 km, is used for this purpose. Constraints on the CO2 continuum and far-wing opacity from existing ground-based high-resolution observations are included in the modelling of the NIMS spectra. The NIMS measurements can be fitted with a water vapor mixing ratio of 30 +/- 15 ppm, in agreement with analyses of ground-based nightside observations. The water vapor abundance shows no horizontal variations exceeding 20% over a wide latitude range (40 deg S, 50 deg N) on the nightside of Venus. Within the same selection of NIMS spectra, a large enhancement in the O2 fluorescence emission at 1.27 micrometer is observed at a latitude of 40 deg S, over a spatial area about 100 km wide.

Description: Using Venus nightside data obtained by the Galileo Near-Infrared Mapping Spectrometer (NIMS), we have studied the correlation of 1.74 and 2.30 micrometer radiation which is transmitted through the clouds. Since the scattering and absorption properties of the cloud particles are different at these two wavelengths, one can distinguish between abundance variations and variations in the properties of the cloud particles themselves. The correlation of intensities shows a clustering of data into five distinct branches. Using radiative transfer calculations, we interpret these branches as regions of distinct but different mixes of Mode 2' and 3 particles. The data and calculations indicate large differences in these modal ratios, the active cloud regions varying in content from nearly pure Mode 2' particles to almost wholly Mode 3. The spatial distribution of these branches shows large scale sizes and both hemispheric symmetries and asymmetries. High-latitude concentrations of large particles are seen in both hemispheres and there is banded structure of small particles seen in both the North and South which may be related. The mean particle size in the Northern Hemisphere is greater than found in the South. If these different branch regions are due to mixing of vertically stratified source regions (e.g. photochemical and condensation source mechanisms), then the mixing must be coherent over very large spatial scales.

Description: The Radio and Plasma Wave Science (RPWS) experiment being built for the Cassini spacecraft will study a wide range of plasma and radio wave phenomena in the magnetosphere of Saturn and will also make valuable measurements during the cruise phase and at other encounters. A feature of data from wave receivers is the capability of producing vastly more data than the spacecraft telemetry link is capable of transmitting back to the Earth. Thus, techniques of on-board data compression and data reduction are important. The RPWS instrument has one processor dedicated to data compression tasks.

Description: The interaction of the thermosphere and ionosphere is largely governed by collisions between ions and neutral particles. On Venus and the Earth, O(+) is a dominant ion, and atomic O dominates throughout much of the thermosphere; therefore an accurate O(+)-O cross section is an important prerequisite for understanding the dynamics of planetary upper atmospheres. The cross section and momentum transfer collision frequency are calculated with a quantum mechanical code which includes resonance charge exchange, polarization, and charge-quadrupole effects. Our results agree well with earlier calculations of Stubbe (1968) and Stallcop et al. (1991).

Description: An overview of the history and current status of research on planetary noble gases is presented. The discovery that neon and argon are vastly more abundant on Venus than on earth points to the solar wind rather than condensation as the fundamental process for placing noble gases in the atmospheres of the terrestrial planets; however, solar wind implantation may not be able to fully reproduce the observed gradient, nor does it obviously account for similar planetary Ne/Ar ratios and dissimilar planetary Ar/Kr ratios. More recent studies have emphasized escape rather than accretion. Hydrodynamic escape, which is fractionating, readily accounts for the difference between atmospheric neon and isotopically light mantle neon. Atmospheric cratering, which is nearly nonfractionating, can account for the extreme scarcity of nonradiogenic noble gases (and other volatiles) on Mars.

Description: The study points out three difficulties with the hypothesis of Hartung (1993) that the Corvid meteor system, observed only once in 1937, may be the return of the clump of ejecta from the formation of a lunar crater, specifically, an event recorded in the chronicles of Gervase of Canterbury on June 25, 1178, which Hartung (1976) previously suggested may be an eyewitness account of the formation of the lunar crater Giordano Bruno. On the basis of this, he predicts that another Corvid shower may be observed in 2003 or 2006. This hypothesis is rejected on the contention that it is implausible that a clump of ejecta could be launched into heliocentric orbit with a low enough dispersion in velocity among separate pieces that it would produce a meteor shower in just one year and not others. Subsequent perturbations by the earth on parts of the clump passing near the earth but not impacting would destroy the coherence of the clump on a time scale much shorter than the 759-yr interval proposed. There are so many orbits that could yield a shower after 759 yr that it is unlikely that a prediction of a return in a specific year would be correct.

Description: A new gravity field determination that has been produced combines both the Pioneer Venus Orbiter (PVO) and the Magellan Doppler radio data. Comparisonsbetween this estimate, a spherical harmonic model of degree and order 21, and previous models show that significant improvements have been made. Results are displayed as gravity contours overlaying a topographic map. We also calculate a new spherical harmonic model of topography based on Magellan altimetry, with PVO altimetry included where gaps exist in the Magellan data. This model is also of degree and order 21, so in conjunction with the gravity model, Bouguer and isostatic anomaly maps can be produced. These results are very consistent with previous results, but reveal more spatial resolution in the higher latitudes.

Description: Results of a comprehensive morphological analysis of the dayside superthermal ion presence in the dayside ionopause region of Venus made using the Orbiter Ion Mass Spectrometer measurements are presented. There is a strong inbound-outbound difference in both the number and intensity of the superthermal ions with most events detected on the inbound crossing of the ionopause. The ambient energy of the detected ions is comparable to the ram energy of cold ions relative to the moving spacecraft. A comparison of the locations of the superthermal O(+) ion occurrences with respect to other parameters shows that the superthermals are a characteristic of the ionopause transition separating the essentially stationary ionosphere plasma from the flowing ionosheath plasma as deduced by Taylor et al. (1980) from individual examples.

Description: Compared to most other Yamato polymict eucrites, Yamato Y792769 eucrite includes fewer and smaller eucritic clasts with homogenized pyroxenes, and its fine-grained matrix is shock-compacted and sintered. In this work, the relationships between the Antarctic eucrite Y792769, monomict eucrites, polymict eucrites, and isotopic ages are investigated, using results of Ar-39/Ar-40 method to date the time of the major thermal event on the Y792769 body and the Rb-Sr and Sm-Nd methods to determine whether relict older ages might have been preserved in some of the breccia materials. The Ar-39/Ar-40 time of the last thermal event which produced the Y792769 texture is 3.99 +/- 0.04 Ga. The complete resetting of the Ar-39/Ar-40 age is consistent with the texture of Y792769 observed in TEM, suggesting that shock compaction converted part of the matrix plagioclase to maskelynite. The Sm-Nd data give an age of 4.23 +/- 0.12 Ga, reflecting partial resetting of the Sm-Nd system during breccia formation. The 3.9 Ga Ar-39/Ar-40 age probably reflects a period of intense meteoroid bompardment which affected the entire inner solar system.

Description: In three-dimensional numerical simulations of a rapidly rotating Boussinesq fluid shell, thermally driven convection in the form of columns parallel to the rotation axis generates an alternately directed mean zonal flow with a cylindrical structure. The mean structure at the outer spherical surface consists of a broad eastward flow at the equator and alternating bands of westward and eastward flows at higher latitudes in both hemispheres. The banded structure persists even though the underlying convective motions are time-dependent. These results, although still far from the actual motions seen on Jupiter and Saturn, provide support for theoretical suggestions that thermal convection can account for the remarkable banded flow structures on these planets.

Description: A review of the planetary system is presented from a uniform perspective, taking advantage of the data obtained during the planetary space missions. A comparative approach is used, which makes it possible to consider each planet or a satellite in context, focusing both on their common origin and on the processes that have influenced their evolution. The discussion focuses on the Jovian planets, the moon and the Mercury, Venus and Earth, and small bodies in the outer solar system. Attention is also given to comets, asteroids, and cosmic impacts.

Description: An electron probe microanalyzer is used to measure the Mn, Fe, and oxygen zoning profiles of olivines in the ALHA 77257 ureilite. This is done to study the effects of reduction on the Mn-Fe value, as ureilite olivines exhibit thin reduced rims. Since the Mn content gradually increases toward the rim of ureilite olivines, while the Fa (= 100 x Fe/(Mg + Fe), mol percent) component decreases, the Mn-Fe content of olivine is likely related to redox conditions. The results of melting experiments suggest that the Mn-Fe positive correlation is related to temperature and that the negative correlation of Mn-Fe in olivine and low-Ca pyroxene is related to reduction.

Description: Voyager IRIS observations of the NEB reveal longitudinal variability of 5-micron brightness temperatures of order 100 C. An anisotropic multiple scattering radiative transfer model is used to calculate synthetic spectra for comparison with the IRIS observations. Mie theory is used to model the spectral dependence of cloud extinction from 180 to 2300/cm. Cloud base locations within the model vary with assumed gas abundances according to thermochemical equilibrium. It is found that spatial variations in the abundance profiles of the condensible species, parahydrogen profiles and cloud optical depths can be used as tracers of the local and large-scale dynamics. The variation of cloud opacity is strongly correlated with the variation of relative humidity, which suggests that dynamic depletion of water vapor above the cloud forming level is the most plausible model to explain the spatial variation in the water profile within the NEB.

Description: Io's brightness was monitored following emergence from eclipse by Jupiter on 14 occasions during the Jupiter apparitions from 1981 to 1989 and no instance is found of what has been called posteclipse brightening. If all the observations are averaged, a 2 percent effect cannot be ruled out; however, this effect is also the size of the rms errors of the summed data set. If condensation followed by sublimation of SO2 frost is hypothesized to be the mechanism which causes posteclipse brightening, then solar insolation alone may not be sufficient to remove an optically thick layer of SO2 frost in the time required to conform with the positive reports of posteclipse brightening in the literature. An additional source of energy is required, such as warming caused by the impact of magnetospheric particles as they dissipate the energy of their motion into Io's surface.

Description: For a fixed heat flow, the surface flow velocity of a convecting layer is not strongly sensitive to the variation of viscosity as a function of depth. Thus, the inferred absence of a low viscosity asthenosphere on Venus can not account for the limited surface motions there. The surface velocity is dependent on the convective geometry. Cartesian geometry convection can produce large surface velocities if the high viscosity surface layer is broken in places by weak zones. On the other hand, a high viscosity surface layer may inhibit the development of large surface velocities in axisymmetric convection.

Description: The paper characterizes the mineralogy and spectral properties of three slightly palagonitized basaltic tephra samples collected near the summit of Mauna Kea in order to contribute to the basis for inferring mineralogy and processes for Martian surface materials. The mineralogy of size fractions of these samples is examined by diffuse reflectance and FIR spectroscopy, optical microscopy, X-ray diffraction, Moessbauer spectroscopy, magnetic analysis, EMPA, TEM, and SEM. For the 20-1000 micron size fraction, sample HWMK11 (red) is essentially completely oxidized and has a hematite (Ti-hematite) pigment dispersed throughout the silicate matrix. Sample HWMK12 (black) has the lowest proportion of ferric-bearing phases and is thus least weathered. For HWMK11, the amount of hematite is essentially constant, and mica is present only in the coarse clay-sized fraction; smectites are low in structural Fe.

Description: Recent laboratory polarimetric measurements of light scattering by layers of alumina grains of different size are used to examine the relevance of the Wolff & Dollfus (1990) theory of negative polarization, which has been developed to describe light scattering by regolithic grains of arbitrary size. It is demonstrated that, while reproducing satisfactorily the main polarization characteristics of light scattering by grains much greater than the wavelength, this theory does not describe the negative polarization of light measured by Geake & Geake (1990) for subwavelength-sized grains. The smallest grains exhibit a polarization feature that may be called the 'polarization opposition effect'. This feature requires an alternative explanation and may be due to the so-called weak localization of photons in discrete random media.

Description: It is shown that N2 may be present in the troposphere of Neptune in an amount difficult to evaluate but which could easily be as high as 0.003, while there is no evidence that it is present in the atmosphere of Uranus. The estimate of the helium abundance depends on the assumed value for N2. If there is no N2 in the observed region of the atmosphere of Uranus and an N2 mole fraction of 0.003 on Neptune, the central value of the estimates of the helium abundance are equal to 0.26 by mass in both planets, which is close to the protosolar value of 0.28. This would imply that the He/H2 ratios measured in the outer atmospheres of Uranus and Neptune are representative of the ratio in the primitive solar nebula and thus were not modified during planetary formation.

Description: The GISS GCM is used here to examine the hypothesis that equatorial superrotation on slowly rotating planets is sensitive to the nature of the vertical radiative heating profile and can exist in the absence of diurnally varying forcing. The general circulation, diabatic heating, and thermal structure produced in the experiments are described and the heat and angular momentum budgets and energy cycles are analyzed to understand the factors conducive to equatorial superrotation. The implications of the results for future planetary missions are addressed.

Description: The spins of the terrestrial planets likely arose as the planets formed by the accretion of planetesimals. Depending on the masses of the impactors, the planet's final spin can either be imparted by many small bodies (ordered accretion), in which case the spin is determined by the mean angular momentum of the impactors, or by a few large bodies (stochastic accretion), in which case the spin is a random variable whose distribution is determined by the rms angular momentum of the impactors. In the case of ordered accretion, the planet's obliquity is expected to be near 0 or 180 deg, whereas, if accretion is stochastic, there should be a wide range of obliquities. Analytic arguments and extensive orbital integrations are used to calculate the expected distributions of spin rate and obliquity as a function of the planetesinal mass and velocity distributions. The results imply that the spins of the terrestrial planets are determined by stochastic accretion.

Description: The initial results of a NIR spectral imaging study conducted from Mauna Kea Observatory during the 1990 opposition are presented. The study's main goals were to determine whether it is possible to map and monitor subtle variations in surface and atmospheric components on Mars from ground-based observations and to constrain Mars surface and airborne dust composition. Small variations near 2.3 microns include both atmospheric (CO) and surface/dust mineral absorption, although the data cannot yet accurately discriminate these components or assess their relative contributions. The present data set, even after preliminary calibration steps, demonstrates the potential of the Mars imaging spectroscopic observations in the near-IR. Focused ground-based telescopic studies in selected wavelength regions accompanied by detailed high spectral resolution atmospheric modeling will allow the mapping and monitoring of surface minerals, atmospheric aerosols, and atmospheric trace gases.

Description: The evolution of solid particles in the solar nebula (or other circumstellar disk) is described. Motions of bodies less than about 1 km in size were dominated by gas drag rather than gravity. An original population of microscopic grains had to produce greater than km-sized planetesimals before gravitational accretion of planets could begin. Planetesimals probably formed by coagulation of grain aggregates that collided due to differential settling, turbulence, and drag-induced orbital decay. Growth of such aggregates depended on sticking mechanisms and their mechanical properties, which are poorly understood. Their growth was aided by concentration of larger bodies toward the central plane of the disk. The nebula could remain optically thick during this process. It is unlikely that a particle layer formed by settling would undergo gravitational instability, as a small amount of turbulence would keep the particle layer from reaching the critical density. This conclusion is independent of the particle size, as even large bodies do not effectively decouple from the gas. Even in a laminar disk, shear in the particle layer would generate enough turbulence to keep it stirred up.

Description: This chapter presents a review of the structure and composition of the giant planets and the theory of their formation and growth. All of the giant planets have heavy-element cores, and have envelopes which contain large amounts of high-Z material in addition to hydrogen and helium. The planets most probably formed through the core instability mechanism. This is a much more complex mechanism than was previously thought, depending, as it does, on several time-dependent parameters. We present the results of new, more detailed, simulations. Towards the end of accretion, the transfer of angular momentum to the outer layers of the contracting protoplanet should lead to the formation of a disk. This disk may be the site of satellite formation. Some recent simulation results are shown.

Description: The nature of the unusually narrow photometric and polarization opposition effects exhibited by Saturn's A and B rings is examined using a theory and results of laboratory measurements. It is pointed out that the small angular width of both phenomena makes it difficult to explain them on the basis of the commonly used shadowing models. On the other hand, it is known from laboratory experiments and theoretical studies, that a strong and very narrow opposition peak in the reflected intensity can be produced by coherent backscattering of light from powderlike layers of small regolithic grains. Using Ozrin's (1992) rigorous vector theory of coherent backscattering, it is shown that, for subwavelength-sized regolithic particles, the photometric opposition effect is accompanied by a polarization opposition effect of the same angular width. This suggests that the polarization opposition effect of the Saturn's ring has the same origin as the photometric opposition effect and is due to coherent backscattering of light from the regolithic layer composed of the submicrometer-sized ice grains.

Description: Polar projections of 50 images of Saturn at 889 nanometers and 25 images at 718 nanometers taken by the HST in November 1990, as well as three images at each wavelength taken in June 1991, have been examined. Among them, 31 show the north polar spot, which is associated with Saturn's polar hexagon, in locations suitable for measurement. The movement of the spot with respect to Saturn's system III rotation rate was studied. During the period of observation, the polar spot had first a short-term westward movement and then a long-term eastward drift. The rate of the long-term drift was -0.060 +/- 0.008 deg/day with respect to system III, approximately 50 percent greater than previously determined from Voyager. The original 1980 and 1981 Voyager data were combined with the new Hubble images to form an 11-yr baseline. The eastward drift over the longer period was -0.0569 degrees per day. The long-term drift could be due to uncertainty in the standard value of the internal rotation period. The short-term movement in November 1990 has a rate that is greater in magnitude but opposite in sign and probably represents a real, transient motion of the spot relative to the internal rotation system.

Description: Recently, Harris et al. (1989) have observed a strong and unusually narrow (HWHM approximately 0.8 deg in the yellow) opposition effect for high-albedo asteroids 44 Nysa and 64 Angelina. In this paper, we apply the theory of coherent backscattering of light from discrete random media to interpret this remarkable opposition brightening. It is shown that coherent backscattering of sunlight from a regolithic layer composed of submicrometer-sized grains with an index of refraction close to that of the mineral enstatite can be a reasonable explanation of the observed opposition effect.

Description: A spherical harmonic model of the gravitational field of Venus complete to degree and order 50 has been developed using the S-band Doppler tracking data of the Pioneer Venus Orbiter (PVO) collected between 1979 and 1982. The short wavelengths of this model could only be resolved near the PVO periapse location (about 14 deg N latitude), therefore a priori constraints were applied to the model to bias poorly observed coefficients towards zero. The resulting model has a half-wavelength resolution of 400 km near the PVO periapse location, but the resolution degrades to greater than 1000 km near the poles. This gravity model correlates well with a degree 50 spherical harmonic expansion of the Venus topography derived from a combination of Magellan and PVO data. New tracking data from Magellan's gravity mission should provide some improvement to this model, although a complete model of the Venusian gravity field will depend on tracking of Magellan after the circularization of its orbit using aerobraking.

Description: The observability of minor species in Titan's atmosphere in its infrared thermal range is systematically studied and modeled to generate synthetic spectra. The model results on methane, water vapor, benzene, allene, and other heavier trace molecules are used to illustrate the capabilities of instruments aboard the Infrared Space Observatory, in particular a high-resolution composite infrared spectrometer, to determine vertical distributions of the molecules in a few hours of integration time.

Description: The flow characteristics of the light ions H(+) and He(+) have been studied in the midnight region of the ionosphere of Venus. Measurements of ion composition, electron and ion temperatures and magnetic fields by instruments onboard the Pioneer Venus Orbiter have been used in rite electron and ion equations of conservation of mass and momentum to derive the vertical flow velocities of H(+) and He(+). When average height profiles of the measured quantities were used, H(+) was found to flow upward, accelerating to speeds of almost 1 km/s at the ion-exobase. In a similar fashion, He(+) was found to flow downward into the neutral atmosphere where it is readily quenched by charge transfer reactions. The polarization electric field played an important role in forcing H(+) upward, but did not contribute enough to the He(+) force balance to produce upward flow. At the ion-exobase, the outward electric polarization force on H(+) was shown to be five times the gravitational force. Using an analogy with the terrestrial ion-exosphere, H(+) was inferred to flow upward into the ionotail of Venus and accelerate to escape speeds. A planet averaged escape flux of 1.4 x 10 exp 7/sq cm/s was calculated, which is comparable to hydrogen loss rates estimated by other investigators.

Description: Near-infrared 10/cm resolution spectra of methane obtained at various temperatures, pressures, and abundances are fit to a quasi-random narrow-band model. Exponential-sum absorption coefficients for three temperatures (112, 188, and 295 K), and 20 pressures from 0.0001 to 5.6 bars, applicable to the cold environments of the major planets, are then derived from the band model for the 230 wavelengths measured from 1.6 to 2.5 microns. RMS deviations between the laboratory and the exponential-sum synthetic transmissions are reported for the best fitting 50 wavelengths. Deviations relevant to broadband, 1-percent spectral resolution observations are also presented. The validity of exponential-sum coefficients derived from broadband (10/cm) transmission data is demonstrated via direct comparison with line-by-line calculations. The complete atlas of coefficients is available from the Planetary Data System-Planetary Atmospheres Discipline Node.

Description: Microwave spectral measurements have been performed on pure room-temperature gaseous ammonia at frequencies from 1.75 to 18 GHz (1.7-17 cm), at 50-, 100-, and 300-torr pressures. These measurements are part of a laboratory program to measure the microwave absorption spectrum of ammonia, under conditions applicable to giant planet atmospheres, now in progress at the Jet Propulsion Laboratory. The pure ammonia data reported here agree well with previous data by Bleaney and Loubser (1950) at 100 and 300 torrs, and with predictions of the absorptivity formalism published by Berge and Gulkis. Success with pure ammonia but failure with mixtures of ammonia in hydrogen and helium (Spilker, 1990) indicates that the Berge and Gulkis formalism does not correctly handle foreign-gas effects on ammonia inversion lines. This may require modifying conclusions of radio astronomical and radio occultation studies that used this formalism. Notably, a suggested depletion of ammonia and superabundance of hydrogen sulfide may have been exaggerated as a result of inaccuracies in the Berge and Gulkis formalism.

Description: Results of a spectral analysis of the ULF wave turbulence in the Neptunian polar cusp are presented. The activity is characterized as broadbanded, extending up to a maximum frequency of about 0.5 Hz, and having maximum wave amplitudes as large as 6 percent of the dc magnetic field. Activity in the cusp region was particularly intense at its frontside and backside, associated with the magnetopause and cusp/magnetosphere boundaries, respectively. The turbulence, particularly that above f(ci), is tentatively identified as whistler mode. It is argued that such whistler mode turbulence should resonate with electrons having energies in the tens of kiloelectron volts. Observations indicate a very strong correlation of the ULF turbulence with the energetic electrons between 22 and 35 keV measured by Voyager's low-energy charged particle experiment. A vigorous interaction between the two is inferred. ULF wave turbulence in the cusp may represent a significant but not complete power source for the magnetosphere.

Description: The explosion over Tunguska, Central Siberia, in 1908 released 10 to 20 megatons (high explosive equivalent) of energy at an altitude of about 10 km. This event represents a typical fate for stony asteroids tens of meters in radius entering the Earth's atmosphere at common hypersonic velocities. Comets and carbonaceous asteroids of the appropriate energy disrupt too high, whereas typical iron objects reach and crater the terrestrial surface.

Description: The amount and rate of release of volatiles (H2O, CO2, etc.) from recent volcanism in Elysium, Mars, are estimated. Possible implications of these volatiles on the climate, weathering, and surface morphology are discussed. Total eruptic volcanics may amount to about 4 x 10 exp 5 cu km and would have released large quantities of volatiles into the Martian environment. Assuming that the lavas contained 1.0 wt pct water, about 7.6 x 10 exp 15 kg of the water, or about 1000 times the present atmospheric water inventory, would have been released. Release amounts of other volatiles are estimated to be 10 exp 15 kg of S, 10 exp 13 kg of Cl, and 10 exp 13 kg of F. The short-term effect of the SO2 gas would be to warm the climate due to its greenhouse properties. Conversion to sulfate aerosols might have resulted in a net surface cooling due to scattering of sunlight. As the sulfate aerosols settled from the atmosphere, the climate could have returned to its preeruption equilibrium.

Description: The present examination of the historical record of HED and mesosiderite basaltic meteorite falls attempts to determine whether that record is consistent with completely random fall times. Strong evidence for nonrandomness emerges, in view of a significant cluster of falls for the 1924-1939 period. The causes of the cluster appear to have acted only on basaltic meteorites, and in a way that was of only 15-year-duration.

Description: Stereographic images of the surface of Venus which enable geologists to reconstruct the details of the planet's evolution are discussed. The 120-meter resolution of these 3D images make it possible to construct digital topographic maps from which precise measurements can be made of the heights, depths, slopes, and volumes of geologic structures.

Description: The data recorded by the dual vector helium/fluxgate magnetometer flown onboard the Ulysses spacecraft during the flyby of Jupiter in February 1992 are analyzed with the aim of identifying the presence of field-aligned current signatures. Field-aligned current flow is expected wherever stress is being transmitted electromagnetically along the magnetic field direction. Sources of such currents at Jupiter are departures within the magnetosphere from corotation, momentum transfer from the solar wind, or centrifugally driven magnetospheric outflow. It is pointed out that the azimuthal field component provides a simple first-order means of monitoring the presence of currents, the currents occurring in regions where the azimuthal component changes significantly. The data from both inbound and outbound passes show evidence of 'leading' and 'lagging' azimuthal field signatures where the field bends out of the meridian, and which are signatures symptomatic of current systems associated with departures from corotation. On the outbound pass, the most intense signatures are found where the field switches from a configuration symptomatic of the field lagging corotation to a configuration representing the field leading. The latter configuration also corresponds to a tail-like displacement of the field and, indeed, the magnetometer data alone cannot distinguish the source of the current system, which could be due to solar wind magnetosphere coupling or which may arise from internal stress imbalance.

Description: A novel theory is presented for the Venus boundary layer which encompasses the effects of topography and uses the mixing-length hypothesis to preclude the unknown eddy viscosity. The maps of mass-flux and erosion/deposition rate presented are based on Pioneer Venus orbiter relief measurements. The typically 19 cm/sec friction speeds associated with the present theory are several times greater than those estimated on the basis of Venera 9 and 10 anemometry, and mean aeolian transport is generally away from the equator, contrary to Magellan orbiter windstreak directions.

Description: The paper considers the question of the formation of the outflow channels and valley networks discovered on the Martian northern plains during the Mariner 9 mission. Parker and Saunders (1987) and Parker et al. (1987, 1989) data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains. It is suggested, that of the geological processes capable of producing similar morphologies on earth, lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on Mars.

Description: Global dust opacity mapping for Mars has been carried forward using the approach described by Martin (1986) for Viking IR Thermal Mapper data. New maps are presented for the period from the beginning of Viking observations, until Ls 210 deg in 1979 (1.36 Mars years). This range includes the second and more extensive planet-encircling dust storm observed by Viking, known as storm 1977b. Improvements in approach result in greater time resolution and smaller noise than in the earlier work. A strong local storm event filled the Hellas basin at Ls 170 deg, prior to the 1977a storm. Dust is retained in equatorial regions following the 1977b storm far longer than in mid-latitudes. Minor dust events appear to raise the opacity in northern high latitudes during northern spring. Additional mapping with high time resolution has been done for the periods of time near the major storm origins in order to search for clues to the mechanism of storm initiation. The first evidence of the start of the 1977b storm is pushed back to Ls 274.2 deg, preceding signs of the storm in images by about 15 hours.

Description: This paper considers impact erosion as the agent of the early Martian atmophere escape, by examining xenon-related constraints imposed on the impact erosion. It is found that, if impact erosion of a planetary atmosphere is important, the remnant atmosphere is likely to fall into one of two qualitatively distinct categories: (1) cometary xenon has an unexpected isotopic composition or (2) factors other than impacts, such as outgassing or recycling, are needed. Neither of these categories can readily account for the present Martian atmosphere.

Description: A mathematical theory is presented which explains the increase of the mean obliquity of Mars over geologic time due to the so called 'climate friction' (i.e., the climatic changes associated with obliquity oscillations of Mars). The theory is compared with a 10 m.y. numerical integration of the equations performed for a hypothetically large amount of climate friction for two cases of the obliquity oscillations: (1) a single sinusoid and (2) a sum of three sinusoids. The theory and numerics agree for both cases within about 12 percent on the size of the secular increase in obliquity. One possible mechanism of climate friction investigated is 'postglacial rebound' on Mars. According to this theory, giant polar caps form when the obliquity is low, and slowly squeeze out an equatorial bulge. When the obliquity is high, the caps disappear, but the bulge takes some time to collapse, due to mantle viscosity, causing it to oscillate out of phase with the obliquity oscillations. This causes a secular increase in the average obliquity.

Description: The behavior and possible past evolution of fully coupled atmosphere and ocean model of Titan are investigated. It is found that Titan's surface temperature was about 20 K cooler at 4 Gyr ago and will be about 5 K warmer 0.5 Gyr in the future. The change in solar luminosity and the conversion of oceanic CH4 to C2H6 drive the evolution of the ocean and atmosphere over time. Titan appears to have experienced a frozen epoch about 3 Gyr ago independent of whether an ocean is present or not. This finding may have important implications for understanding the inventory of Titan's volatile compounds.

Description: A 1D numerical model of the exchange of H2O between the atmosphere and subsurface of Mars through the PBL is employed to explore the mechanisms of H2O exchange and to elucidate the role played by the regolith in the local H2O budget. The atmospheric model includes effects of Coriolis, pressure gradient, and frictional forces for momentum: radiation, sensible heat flux, and advection for heat. It is suggested that in most cases, the flux through the Martian surface reverses twice in the course of each sol. The effects of surface albedo, thermal inertia, solar declination, atmospheric optical depth, and regolith pore structure are explored. It is proposed that higher thermal inertia forces more H2O into the atmosphere because the regolith is warmer at depth.

Description: The major physical aspects of the equatorial electrojet of Earth and the dayside ionospheric current layers of Venus are compared, viz., the electric current intensity and total current, roles of electric field, pressure and gravity, diffusion time scales, and the Bernouille effect. The largest potential differences, of the order of 10 volts, horizontally across the dayside ionosphere of Venus, have important implications for possible dynamo action in the Venus ionosphere and the application of an electric field from the lower atmosphere or from the solar wind. An upper limit to the horizontal scale of vertical magnetic fields in the Venus ionosphere is estimated thereby for the first time. New upper limits on the velocity in, and thickness of, a possible S layer at Venus are presented. If an S layer exists, it is only for extreme conditions of the solar wind. A mechanism for formation of magnetic ropes in the Venus ionosphere is also proposed.

Description: Observational results are presented for two rotational periods of Titan which exhibit the albedo difference noted by Lemmon et al. (1993) between this moon's positions at eastern and western elongation relative to Saturn. The persistence of this difference indicates that this heterogeneity is unlikely to be associated with transient features, and must be intrinsic to the surface. The results presented also indicate that Titan is locked in a synchronous orbit around Saturn.

Description: Laboratory spectra of the first overtone band (2.1480 microns, 4655.4 reciprocal cm) of solid nitrogen show additional structure at 2.1618 microns (4625.8 reciprocal cm) over a limited temperature range. The spectrum of Neptune's satellite Triton shows the nitrogen overtone band as well as the temperature-sensitive component. The temperature dependence of this band may be used in conjunction with ground-based observations of Triton as an independent means of determining the temperature of surface deposits of nitrogen ice. The surface temperature of Triton is found to be 38.0 +2.0 or -1.0 K, in agreement with previous temperature estimates and measurements. There is no spectral evidence for the presence of alpha-nitrogen on Triton's surface, indicating that there is less than 10 percent carbon monoxide in solid solution with the nitrogen on the surface.

Description: Triton's polar caps are modeled as permanent nitrogen deposits hundreds of meters thick. Complex temperature variations on Triton's surface induce reversible transitions between the cubic and hexagonal phases of solid nitrogen, often with two coexisting propagating transition fronts. Subsurface temperature distributions are calculated using a two-dimensional thermal model with phase changes. The phase changes fracture the upper nitrogen layer, increasing its reflectivity and thus offering an explanation for the surprisingly high southern polar cap albedo (approximately 0.8) seen during the Voyager 2 flyby. The model has other implications for the phase transition phenomena on Triton, such as a plausible mechanism for the origin of geyser-like plume vent areas and a mechanism of energy transport toward them.

Description: Observations of the 1.4- to 2.4-micrometer spectrum of Pluto reveal absorptions of carbon monoxide and nitrogen ices and confirm the presence of solid methane. Frozen nitrogen is more abundant than the other two ices by a factor of about 50; gaseous nitrogen must therefore be the major atmospheric constituent. The absence of carbon dioxide absorptions is one of several differences between the spectra of Pluto and Triton in this region. Both worlds carry information about the composition of the solar nebula and the processes by which icy planetesimals formed.

Description: The near-infrared spectrum of Triton reveals ices of nitrogen, methane, carbon monoxide, and carbon dioxide, of which nitrogen is the dominant component. Carbon dioxide ice may be spatially segregated from the other more volatile ices, covering about 10 percent of Triton's surface. The absence of ices of other hydrocarbons and nitriles challenges existing models of methane and nitrogen photochemistry on Triton.

Description: We present a new method for determining the abundance of sulfur dioxide below the clouds of Venus. Absorption by the 3nu3 band of SO2 near 2.45 microns has been detected in high-resolution spectra of the night side of Venus recorded at the Canada-France Hawaii telescope in 1989 and 1991. The inferred SO2 abundance is 130 +/- 40 ppm at all observed locations and pertains to the 35-45 km region. These values are comparable to those measured by the Pioneer Venus and Venera 11/12 entry probes in 1978. This stability stands in contrast to the apparent massive decrease in SO2 observed at the cloud tops since these space missions. These results are consistent with laboratory and modeling studies of the SO2 destruction rates in the lower atmosphere of Venus. The new spectroscopic technique presented here allows a remote monitoring of the SO2 abundance below the clouds, a likely tracer of Venusian volcanism.

Description: New observations of Jovian narrow-band kilometric (nKOM) radio emissions were made by the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses-Jupiter encounter in early February 1992. These observations have demonstrated the unique capability of the URAP instrument for determining both the direction and polarization of nKOM radio sources. An important result is the discovery that nKOM radio emission originates from a number of distinct sources located at different Jovian longitudes and at the inner and outermost regions of the Io plasma torus. These sources have been tracked for several Jovian rotations, yielding their corotational lags, their spatial and temporal evolution, and their radiation characteristics at both low latitudes far from Jupiter and at high latitudes near the planet. Both right-hand and left-hand circularly polarized nKOM sources were observed. The polarizations observed for sources in the outermost regions of the torus seem to favor extraordinary mode emission.

Description: L shell values along the Voyager 2 encounter trajectory and those associated with the N1 through N6 moons and N1R through N6R rings of Neptune are computed numerically on the basis of a simplified description of the Neptunian magnetic field derived from the Goddard Space Flight Center/Bartol Research Institute I8E1 model, which includes internal terms up to and including the octupole (but no external terms). Like Uranus, the large tilt between the dipole term and the rotation axis causes the moons and rings to sweep a very large range of L shells. Their orbital motion introduces additional periodicities, causing the maxima and minima in L space to vary systematically with time.

Description: The Magellan data on the Venus landscape which revealed volcanoes, shining mountains, relaxing plateaus, and craters on the surface of about 500 C, with an atmospheric pressure 90 times that of earth, are discussed. Venus is considered to be a planet that is both incredibly similar and dissimilar to earth. Venus might not exhibit plate tectonism but it might be dominated by catastrophes. The greenhouse around Venus has operated for at least the last 500 million years. The Magellan data revealed channels extending for thousands of kilometers, beautiful outflows surrounding impact craters, and odd volcanic constructs like the steep-sided domes.

Description: Computer generated surfaces have been created to aid in imaging, landing and rover studies for Mars and the moon. They are also being applied to the study of cratering histories. The surfaces are generated in steps which attempt to mimic geologic episodes. Surface roughness is realized fractally, while craters and other specific features have shapes and distributions dictated by observation. Surface materials are assigned appropriate albedos, making the images more realistic. With the inclusion of correlations between crater and rock distributions, the simulations are beginning to acquire a predictive capability.

Description: Northern Jovian hectometric (HOM) radio emissions, detected from high Jovian latitudes by the Unified Radio and Plasma Wave experiment on the Ulysses spacecraft, were observed at all Jovian longitudes. This emission was observed to be predominantly right-hand circularly polarized, but some left-hand circular polarization was observed implying the presence of O mode emissions from the northern Jovian hemisphere. Intense HOM emissions, with well-defined directions and polarizations, were often confined to similar longitudinal regions where intense HOM emissions were previously observed at low latitudes. The present analysis confirms that these northern HOM sources lie in the Jovian polar regions on magnetic field lines that pass through the Io plasma torus. The observations may be consistent with emission from either a filled cone beam or a longitudinal distribution of thin hollow cones.

Description: Thermal emission spectra recorded by the Mariner 9 IR interferometer spectrometer (IRIS) are used to investigate the temperature structure and dust loading of the Martian atmosphere. The analysis is restricted to a subset of the IRIS data consisting of approximately 2400 spectra in a 12-day period extending from LS of 343 to 348 deg, corresponding to late southern summer on Mars. The largest column-integrated 9-micron dust optical depths (about 0.4) occur over the equatorial regions. The highest atmospheric temperatures (greater than 260 K) are found at low altitudes near the subsolar latitude (about 6 deg S), while the coldest temperatures (less than 150 K) are found at levels near 1.0 mbar over the winter pole. The existence of a net zonally averaged meridional circulation with rising motion at low latitudes, poleward flow at altitudes above 40 km, and subsidence over the poles is suggested.

Description: The relationships between near-surface winds and the distribution of wind-related features are investigated by means of a general circulation model of Mars' atmosphere. Predictions of wind surface stress as a function of season and dust optical depth are used to investigate the distribution and orientation of wind streaks, yardangs, and rock abundance on the surface. The global distribution of rocks on the surface correlates well with predicted wind stress, particularly during the dust storm season. The rocky areas are sites of strong winds, suggesting that fine material is swept away by the wind, leaving rocks and coarser material behind.

Description: Coronae are large quasi-circular geologica features that are common on Venus. They appear to be the surface tectonic and volcanic expressions of mantle diapirs that have impinged on the underside of the venusian lithosphere. We have investigated the spatial distribution of 335 coronae and related features identified in Magellan radar data. It is more clustered than a Poisson distribution, with a statistical certainty of more than 99%. It is dominated by a single large cluster centered near the equator at about 245 deg longitude. The features are preferentially found at elevation and geoid values close to the planetary mean, with a paucity at both the highest and lowest levels of topgraphy and geoid. Some coronae appear aligned in quasi-linear chains. We attribute the clustering of coronae and related features to preferential formation of these features above regions of broad-scale mantle upwelling, and suggest that a major mantle upwelling underlies the one large cluster. We suggest that coronae are rare at the lowest elevations because these may be regions of mantle downwelling. The shortage of coronae at the highest elevations may result both from obscuration by other intense tectonism there and from suppression there of their formation by an unusually thick crust. Corona chains may be produced by enhanced passive mantle uplift below failed or incipient rifts.

Description: Since the Voyager encounters in 1979, it has been known that Jupiter's cloud-top zonal winds violate the barotropic stability criterion. A vortex-tube stretching analysis of the Voyager wind data indicates that the more general Charney-Stern stability criterion is also violated. On the other hand, the zonal winds determined by tracking cloud features in Hubble Space Telescope images taken in 1991 precisely match the zonal winds determined by tracking cloud features in Voyager images, and it is hard to understand how a complicated zonal wind profile like Jupiter's could be unstable and yet not change at all in 12 years. In fact, there are at least two unknown ways to violate the Charney-Stern stability criterion and still have a stable flow. The better known of these is called Fjortoft's theorem, or Arnol'd's 1st theorem for the case of large-amplitude perturbations. Although the Fjortoft-Arnol'd theorem has been extended from the quasi-geostrophic equations to the primitive equations, the basic requirement that the potential vorticity be an increasing function of streamfunction is opposite to the case found in Jupiter, where the Voyager data indicate that the potential vorticity is a decreasing function of streamfunction. But this second case is precisely that which is covered by Arnol'd's 2nd stability theorem. In fact, the Voyager data suggest that Jupiter's zonal winds are neutrally stable with respect to Arnol'd's 2nd stability theorem. Here, we analyze the linear stability problem of a one-parameter family of sinusoidal zonal wind profiles that are close to neutral stability with respect to Arnol'd's 2nd stability theorem. We find numerically that the most unstable mode is always stationary, which may help to explain the slowly moving mode 10 waves observed on Jupiter. We find that violation of Arnol'd's 2nd stability theorem is both necessary and sufficient for instability of sinusoidal profiles. However, there appears to be no simple extension of Arnol'd's 2nd stability theorem to the primitive equations. Nevertheless, the primitive growth rates are small, and the primitive system is still governed by the quasi-geostrophic neutral-stability configuration.

Description: Natural glasses occur not only on Earth but also in lunar rocks and in some meteorites. On geological grounds, natural glasses could be expected on the surface of Mars and their presence would have important implications for the weathering and alteration of the near-surface layer. Ultramafic and mafic magmatism rich in iron and some volatiles appears to have been important on Mars as has been impact cratering, volcanism, and possibly hydrothermal alteration. Available data indicate that we can presume the past or present existence of volcanic basaltic glasses, impact glasses formed by thermal fusion of meteorite-impact targets, and diaplectic (mainly maskelynite) glasses. Conditions appear unsuitable for the formation of tektites on the Martian surface, and the production of measurable amounts of fulgurite or combustion glass seems improbable. Prospects for the remote and in-situ identification of Martian glasses and their subsequent weathering products are discussed and include orbital reflectance and thermal emission spectroscopic measurements and Mossbauer spectroscopy from a surface lander.

Description: The spectral image cubes obtained by the Near-Infrared Mapping Spectrometer (NIMS) on Galileo as it flew by Venus have been analyzed to constrain the vertical structure of the clouds, the nature of the aerosol particles, and the location and particle properties of the opacity variations responsible for high-contrast features observed in the near-infrared windows at 1.7 and 2.3 micrometers. A radiative transfer program was used to simulate mid-latitude curves of limb darkening at 3.7 micrometers. Best-fit models to these curves demonstrate that the upper clouds are dominated by mode 2 particles (r-bar = 1.0 micrometers), with a contribution of approximately 15% of opacity from mode 1 particles (r-bar = 0.3 micrometers). The low-latitude upper cloud is well represented by a dual scale-height model, with a particle scale height of approximately 1 km from an altitude of 61-63 km, and a scale height of approximately 6 km above this, up to the level where tau = 1 at approximately 71 km. This model also successfully simulates limb-darkening curves at 11.5 micrometers from the Pioneer Venus Orbiter Infrared Radiometer. Successful simulations of correlation plots of 1.7 vs 2.3 micrometers intensities reveal that mode 3 particles (r-bar = 3.65 micrometers) represent the dominant source of opacity in the lower and middle clouds, and that variation in total cloud opacity reflects chiefly the addition and removal of mode 3 particles near the cloud base. We find that the full spectrum of brightnesses at 1.7 and 2.3 micrometers implies that the total cloud optical depth varies from approximately 25 to approximately 40.

Description: During the Earth-1 Galileo flyby (December 1990), the Near-Infrared Mapping Spectrometer (NIMS) experiment investigated the illuminated side of the Earth in the spectral range 0.7-5.2 micrometers. Mosaics of the entire terrestrial globe were recorded with a spatial resolution ranging from 100 to 500 km. From these spectra, information is retrieved upon the large-scale temperature structure in the stratosphere and in the mesosphere (0-70 km altitude range) from the inversion of the CO2 bands at 4.3 and 4.8 micrometers. These data also permit monitoring of the cloud temperatures, and derivation of the abundances of several minor atmospheric constituents (H2O, CO, N2O, CH4 and O3). These observations constitute a continuation of the study of the atmospheres of the three planets (i.e. Venus, the Earth and Jupiter) targeted by the Galileo spacecraft during its mission. Observing these atmospheres with the NIMS instrument in the near-infrared will provide a unique data set, useful for comparative planetary studies.

Description: Wave-like perturbations have been observed in the nightside neutral density data acquired for He, N, O, N2 and CO2 by the Pioneer Venus Orbiter Neutral Mass Spectrometer (ONMS) during entry in late 1992. The data cover an altitude range of 133-200 km from 0.5-4.5 hours local solar time and occur at medium solar activity (F(sub 10.7) = 120). The perturbations, with an effective wavelength along the orbit of about 100 to 600 km, have similar amplitudes for the various species and helium is out of phase with respect to the heavier mass species. The measurements are comparable to those observed in 1978-80 at solar maximum activity (F(sub 10.7) = 200) above 145 km. Between 133 and 160 km the rms amplitudes grow with altitude at a rate of about (1.6, 2.1, 2.7, 4.4) x 10(exp 3)/km for N, O, N2, and CO2, respectively. The average rms amplitudes above 145 km of 0.08 for N and O and 0.1 for N2 are comparable in magnitude to those observed in the earlier 1978-80 data of 0.06, 0.08, and 0.095 respectively. CO2 is an exception for which the entry value is 0.17 compared to 0.09 earlier. By combining the two overlapping data sets there is a suggestion that the CO2 amplitudes grow in value with altitude up to about 140-170 km and then decrease in amplitude. Like the earlier data, the entry data are consistent with the interpretation that the neutral density perturbations are due to gravity waves propagating upward from the lower thermosphere.

Description: The return of periapsis to low altitudes during the Summer and Fall of 1992 provided a unique opportunity for the Pioneer Venus Orbiter (PVO) to make in situ measurements in the Venusian ionosphere at much lower levels of solar activity (F10.7 approx. equals 120) than existed when periapsis was at low altitudes in 1979 and 1980 (F10.7 approx. equals 220). We present the observations of electron density (N(sub e)) and temperature (T(sub e)) made by the Orbiter Electron Temperature Probe (OETP) during the Entry Period. Empirical models of the N(sub e) and T(sub e) height variations are presented and compared with similar models based on OETP measurements made at solar maximum. The median N(sub e) at the ionospheric peak (approx. 140 km) was essentially unchanged from its solar maximum value, but the ionosphere was increasingly depleted at higher altitudes, reaching a factor of 7 lower densities at 200 km. T(sub e) was lower by almost a factor of 2 at 140 km but was rather significantly enhanced at higher altitudes; exceeding its solar maximum values by a factor of 1.3 at 200 km and a factor of 2 at 500 km. In general these results support the earlier conclusions that the nightside upper ionosphere is depleted at lower levels of solar activity by a reduction of the nightward ion flow. The lack of N(sub e) variation near the peak (between solar maximum and entry) suggests that nightward ion transport does not play as large a role in the peak formation as does local ion production by energetic particles. The decrease does local ion production by energetic particles. The decrease in T(sub e) at low altitudes suggests that the low densities of the upper ionosphere at the time of PVO entry could no longer support the conduction of heat from the dayside ionosphere, thus allowing the lower nightside ionosphere to cool by collisions with ions and neutrals, and by heat conduction to the cooler regions below.

Description: The present overview of available data regarding Uranus and its planetary structures encompasses ground- and space-based observations of the planet with specific attention given to the interpretation of Voyager 2 data. A brief examination of historical observations is given which includes its discovery, position determination, and the related discoveries of Uranus' five large satellites and rings. The observational data preceding the Voyager 2 mission are reviewed in terms of the planetary interior, atmosphere, and magnetosphere. The Voyager 2 mission is given detailed treatment with descriptions of NASA's development and deployment of the spacecraft as well as detailed data from the Uranus encounter. Reference is given to structure and composition of Uranus' atmosphere, valid models of the interior, and wave-particle interactions in the magnetosphere. The structures of the rings and satellites are examined with reference to specific observational requirements from future missions.

Description: A novel method of producing mixtures of glass or minerals with iron metal is presented. A portion of the Fe(2+) in basaltic glass and minerals can be reduced to metal in a few hours at 1100 C and an oxygen fugacity well below the iron-wustite buffer. Part of the iron metals forms rounded submicrometer blebs on the surfaces and in some cases within the grains. A concentration of such blebs equivalent to 20-30 percent of a grain's surface area can totally dominate the reflectance spectra of basaltic glass, pyroxene, and olivine. The production of optically opaque iron metal blebs, combined with the decline in Fe(2+), affects the glass and mineral reflectance spectra in three ways: by lowering the overall reflectivity, reducing the spectral contrast of absorption features, and producing a continuum with a general rise in reflectivity toward longer wavelengths.

Description: Speculation with regard to a permanent lunar base has been with us since Robert Goddard was working on the first liquid-fueled rockets in the 1920's. With the infusion of data from the Apollo Moon flights, a once speculative area of space exploration has become an exciting possibility. A Moon base is not only a very real possibility, but is probably a critical element in the continuation of our piloted space program. This article, originally drafted by World Space Foundation volunteers in conjuction with various academic and research groups, examines some of the strategies involved in selecting an appropriate site for such a lunar base. Site selection involves a number of complex variables, including raw materials for possible rocket propellant generation, hot an cold cycles, view of the sky (for astronomical considerations, among others), geological makeup of the region, and more. This article summarizes the key base siting considerations and suggests some alternatives. Availability of specific resources, including energy and certain minerals, is critical to success.

Description: Chondrules contain higher concentrations of volatiles (Na) than expected for melt droplets in the solar nebula. Recent studies have proposed that chondrules may have formed under non-canonical nebular conditions such as in particle/gas-rich clumps. Such chondrule formation areas may have contained significant Na vapor. To test the hypothesis of whether a Na-rich vapor would minimize Na volatilization reaction rates in a chondrule analog and maintain the Na value of the melt, experiments were designed where a Na-rich vapor could be maintained around the sample. A starting material with a melting point lower that typical chondrules was required to keep the logistics of working with Na volatilization from NaCl within the realm of feasibility. The Knippa basalt, a MgO-rich alkali olivine basalt with a melting temperature of 1325 +/- 5 C and a Na2O content of 3.05 wt%, was used as the chondrule analog. Experiments were conducted in a 1 atm, gas-mixing furnace with the fO2 controlled by a CO/CO2 gas mixture and fixed at the I-W buffer curve. To determine the extent of Na loss from the sample, initial experiments were conducted at high temperatures (1300 C - 1350 C) for duration of up to 72 h without a Na-rich vapor present. Almost all (up to 98%) Na was volatilized in runs of 72 h. Subsequent trials were conducted at 1330 C for 16 h in the presence of a Na-rich vapor, supplied by a NaCl-filled crucible placed in the bottom of the furnace. Succeeding Knudsen cell weight-loss mass-spectrometry analysis of NaCl determined the P(sub Na) for these experimental conditions to be in the 10(exp -6) atm range. This value is considered high for nebula conditions but is still plausible for non-canonical environments. In these trials the Na2O content of the glass was maintained or in some cases increased; Na2O values ranged from 2.62% wt to 4.37% wt. The Na content of chondrules may be controlled by the Na vapor pressure in the chondrule formation region. Most heating events capable of producing chondrules are sufficient to volatile Na. Sodium volatilization reaction rates will be reduced to varying degrees from melt droplets, depending on the magnitude of the P(sub Na) generated. A combination of Na vapor during, and Na diffusion back into chondrules after, formation could maintain and/or enrich Na concentrations in chondrules.

Description: The Ulysses spacecraft encountered Jupiter in February 1992, passing within 6.31 radii of the planet. For approximately 8 days it was inside the Jovian magnetosphere, and for several days before and after that, Ulysses was in the interaction regions formed by the solar wind (the magnetosheath and boundary layer). The inbound trajectory was at an approximately 1000 LT and the outbound trajectory was at 1800 LT, that is, dusk, a unique feature of the flight path. Three regions interior to the magnetosphere were identified as on previous missions both inbound and outbound. In addition, the spacecraft twice penetrated a cusplike region at high latitude in the inner magnetosphere. Following closest approach, Ulysses traversed the Io plasma torus in basically a north-south direction. Although Ulysses is a heliospheric mission, the experiments were suited to an investigation of Jupiter's magnetosphere and have returned much new information. This introduction to the accompanying articles by the Ulysses investigators provides basic information on the experiments, the spacecraft, and the trajectory. In addition, the scientific context of the encounter is reviewed on the basis of the preliminary analyses of the Ulysses observations and a rudimentary comparison with the earlier Pioneer and Voyager results. Some important scientific questions raised by the encounter, along with some tentative answers, are presented.

Description: Individual emission lines of ethylene (C2H4) near 10.5 micron were measured from the equatorial and north polar regions of Jupiter. Observations were made at a spectral resolution of 0.00083/cm using infrared heterodyne spectroscopy at the NASA Infrared Telescope Facility of Mauna Kea, Hawaii. The line shape information possible with this resolving power permitted the retrieval of quiescent ethylene abundances and the investigation of abundance and thermal structure variability in the polar auroral region. A rough distribution of the north polar emission as a function of longitude was obtained with an instantaneous field of view (full width at half maximum) of approximately 1 arc sec on the planet. The greatest C2H4 emission was observed near the nominal north polar methane hot spot (60 deg N, 180 deg longitude, System III, 1965). It was found to be confined to less than 10 deg longitude on the planet. Using a Voyager-derived thermal profile, retrieved ethylene mole fractions for equatorial and north polar quiescent (non-hot spot) regions were consistent with results from existing photochemical models. At the hot spot an 18-fold increase in abundance was required near the 10-microbar level to reproduce the data. Alternatively varying the stratospheric thermal profile, a 67-137 K increase in temperature was required at the approximately 10-microbar level to satisfy the observed emission, if the C2H4 mole fraction is fixed to the quiescent value. These results provide the first direct probe of the upper stratosphere of Jupiter and give upper limits to the temperature increase near the source of the north polar thermal infrared aurora. Combined with results from similar measurements of auroral ethane emission (Livengood et al., this issue) probing the 1-mbar region, altitude information on the thermal structure can be obtained for the first time. The ethylene line emission region extends to the few microbar pressure level and may overlap the region where the H2 ultraviolet aurora is formed; thus it can be used as a probe of the coupling between the ultraviolet and thermal infrared phenomena.

Description: Jupiter is the source of a large number of independent nonthermal radio sources, all of which vary with time. The known causes of the variations include planetary rotation modulation, modulation by Io and/or its torus, and influence by the solar wind which can reach suprisingly deep into the Jovian magnetosphere. However, a significant number of radio variations, both short-term and long-term, are not currently explained by any known mechanism.

Description: It is difficult to imagine a group of planetary dynamos more diverse than those visited by the Pioneer and Voyager spacecraft. The magnetic field of Jupiter is large in magnitude and has a dipole axis within 10 deg of its rotation axis, comfortably consistent with the paleomagnetic history of the geodynamo. Saturn's remarkable (zonal harmonic) magnetic field has an axis of symmetry that is indistinguishable from its rotation axis (mush less than 1 deg angular separation); it is also highly antisymmetric with respect to the equator plane. According to one hypothesis, the spin symmetry may arise from the differential rotation of an electrically conducting and stably stratified layer above the dynamo. The magnetic fields of Uranus and Neptune are very much alike, and equally unlike those of the other known magnetized planets. These two planets are characterized by a large dipole tilts (59 deg and 47 deg, respectively) and quadrupole moments (Schmidt-normalized quadrupole/dipole ratio approximately equal 1.0). These properties may be characteristic of dynamo generation in the relatively poorly conducting 'ice' interiors of Uranus and Neptune. Characteristics of these planetary magnetic fields are illustrated using contour maps of the field on the planet's surface and discussed in the context of planetary interiors and dynamo generation.

Description: Direct confirmation that low-frequency Jovian hectometric (HOM) radio emissions centered near 0 deg central meridian longitude consist of distinct, oppositely polarized northern and southern beams has been achieved using data from the Unified Radio and Plasma Wave (URAP) experiment on the Ulysses spacecraft during the Ulysses-Jupiter encounter in early February 1992. Distinct northern and southern beams were observed in the frequency range from approximately 300 kHz to 1 MHz for at least eight Jovian rotations during the Ulysses inbound pass at distances from 100 to 40 R(sub j). The radiation from the two magnetic hemispheres was measured from different Jovigraphic longitudes and magnetic (or centrifugal) latitudes. Observed temporal variations in the radio intensities, with time scales on the order of 30 min, may result either from longitudinal variations of the HOM sources or from longitudinal density variations in the Io plasma torus. Using the URAP direction-finding capabilities and assuming a tilted dipole planetary magnetic field model, the three-dimensional HOM source locations, the L shell through these source locations, and the beam opening angles were independently deduced. The HOM sources were found to originate at approximately 3 R(sub j) and on low L shells (L approximately 4 to 6), with beam opening angles ranging from 10 to 50 deg.

Description: On February 8, 1992, the Ulysses spacecraft passed through Jupiter's Io plasma torus, where rich spectra of narrow-band resonances were stimulated by the relaxation sounder of the Ulysses unified radio and plasma wave (URAP) instrument. Since the gyrofrequency f(sub g) is comparable to the plasma frequency f(sub p) in the Io torus, it was predicted that the general classification of stimulated ionospheric D(sub n) resonances, developed for 1 is less than or equal to f(sub p)/f(sub g) is less than or equal to 8 in the Earth's topside ionosphere, should apply in the Io torus as well as the Earth's magnetosphere (Osherovich, 1989). The URAP plasmagrams (sounder spectra) in the portions of the Io torus satisfying these plasma conditions are dominated by the D(sub n) resonances for frequencies below f(sub p). On most of these plasmagrams the f(sub p) resonance is also present, but it is seldom the dominant resonance. Neither upper hybrid nor nf(sub g) resonances have been found on these plasmagrams. The identification of D(sub n) resonances has allowed both the electron density and the magnetic field amplitude to be calculated. The derived densities on the outbound pass agree well with a Voyager model of Bagenal (1992). The derived magnetic field values are close to the Goddard Space Flight Center O(sub 6) magnetic field model.

Description: The objectives of the Mars Observer global mapping mission are to determine the global elemental and mineralogical character of the Martian surface material, define globally the topography and gravitational field of Mars, establish the nature of Mars's magnetic field, determine the time and space distribution, abundance, sources, and sinks of volatile Martian material and dust over a seasonal cycle, and explore the structure and aspects of the circulation of the Martian atmosphere. The experiments and instruments to be used in this mission are described, and the operations and data analysis are briefly considered.

Description: On June 11 and 15, 1985 two packages with balloons have been inserted in the atmosphere of Venus from the Soviet VEGA landing modules. This paper summarizes the pressure, temperature, wind illumination and backscattering data from the balloons.

Description: The evolution of the Venus small-scale waves as they propagate into the nightsite is examined, and the small-scale structures are compared with the waves in the three components of the magnetic field, magnetic dip angle, and neutral density. It is demonstrated that the small-scale fluctuations evolve between the transterminator and antisolar regions. It is shown that atmospheric gravity waves may also be producing some of the fluctuations observed at longer wavelengths. The electron temperature and density are shown to be approximately 180 deg out of phase and exhibiting the highest correlation of any pair of variables. Waves in the electron and neutral densities are found to be correlated moderately on most orbits, while the average electron temperature is higher when the average magnetic field is more horizontal.

Description: The major and trace elements were analyzed in samples of LEW86010, LEW87051, and Angra dos Reis angrites were analyzed using INAA and fused-bead EMP techniques. Results suggest that all three angrites originated on a volatile-depleted (Na-poor) but oxidized parent body or on several bodies. The combination of low Na and high FeO/MnO found in these samples suggests that the extreme Na depletion was caused by parent-body outgassing during differentiation, rather than by nebular processes. It was also found that the refractory-element abundances observed in LEW86010 and LEW87051 are related via olivine control; it is suggested that LEW86010 may be a residual melt from a LEW87051-like precursor or that LEW87051 formed from a LEW86010-like precursor via olivine accumulation. On the other hand, the Angra dos Reis displayed an unusual refractory element pattern suggesting that either the angrite parent body was heterogeneous or that Angra dos Reis was formed on a parent body different from that of the other two angrites.

Description: Global digital maps of Jupiter's upper-tropospheric temperature have been generated at the 270- and 150-mb pressure levels, together with IR cloud optical depths at 5 and 45 microns and the ammonia abundance near the 680-mb pressure level, on the basis of Voyager IRIS north-south mapping sequences. Attention is given to the 270-mb and 45 micron data; global digital maps are presented for violet and orange reflectivities. The dominant upper tropospheric thermal structures move at a rate far different from that of the cloud indicators, and are noted to remain stationary relative to the planet's bulk rotation. Strong stationary features are found at a zonal wavenumber of 9 near 15 deg N latitude and of 11 near 20 deg latitude.

Description: The problem of stress, strain, and breakup in solid satellites and stray bodies subject to tidal perturbations is presently addressed in view of three novel considerations. After presenting a new analytic solution for the stress tensor in a homogeneous and compressible elastic sphere, where the inclusion of compressibility alters stresses by several percent, realistic failure criteria are noted to demonstrate the general failure of such ductile bodies as iron meteoroids by plastic shear, while brittle ice bodies fail by either tensile or shear fracture. A reexamination of crack propagation after initial failure allows the diverse breakup criteria to be reconciled.

Description: Results are presented on the analysis of the Pioneer Venus Orbiter Cloud Photopolarimeter (OCPP) UV images, covering 80-day periods from 1979 to 1985. The analysis provides a method for determining wind speeds at the cloud level, from the motions of cloud UV features. Data used for tracking the cloud feature motions are described along with the analysis method. The results identified four wave modes: a diurnal solar tide, a semidiurnal solar tide, a '4-day equatorial' wave, and a '5-day midlatitude' wave. The features of these wave modes are described.

Description: The (Ar-39) (Ar-40) chronologies were determined for 14 different mesosiderites representing the full range of classification according to recrystallization, and these chronologies were compared with analogous data for other meteorite types and for lunar highland rocks. Results of Ar-Ar chronologies indicate the history of a degassing of Ar due to a major thermal event that occurred less than 3.9 Ga ago; this event is not the metal-silicate mixing event, which is known to have occurred earlier than 4.4 Ga ago. It is suggested that a major collisional disruption-reassembly event less than 3.9 Ga ago took place, leaving the metal-silicate breccias buried under tens of kilometers of rubble, where they cooled slowly through the Ar closure temperatures.

Description: Results are presented on major- and trace-element abundance analyses of Apollo 15 pyroclastic green glasses from groups A, B, C, D, and E, carried out using electron- and ion-microprobe techniques. The diagrams depicting Sr, Zr, Ba, and Nd vs Co variations indicate the presence of a high-Co trend in groups A and D and a low-Co trend in groups B and C. Group-E glasses were found to be significantly enriched in Sr, relative to the other four glass groups. Chemical data of this study were integrated with previous data to evaluate various magmatic processes that have been proposed in the past to explain chemical variations in the lunar green glass. Results of calculations using a source mixing model suggest that the Apollo 15 green glasses represent multiple eruptive events from three chemically distinct but compositionally variable source regions.

Description: Four geyser-like plumes were discovered near Triton's south pole in areas now in permanent sunlight. Because Triton's southern hemisphere is nearing a maximum summer solstice, insolation as a driver or a trigger for Triton's geyser-like plumes is an attractive hypothesis. Trapping of solar radiation in a translucent, low-conductivity surface layer (in a solid-state greenhouse), which is subsequently released in the form of latent heat of sublimation, could provide the required energy. Both the classical solid-state greenhouse consisting of exponentially absorbed insolation in a gray, translucent layer of solid nitrogen, and the 'super' greenhouse consisting of a relatively transparent solid-nitrogen layer over an opaque, absorbing layer are plausible candidates. Geothermal heat may also play a part if assisted by the added energy input of seasonal cycles of insolation.

Description: Analysis of the preliminary results from the Voyager mission to the Neptune system has provided the scientific community with several methods by which the temperature of Neptune's satellite Triton may be determined. If the 37.5 K surface temperature reported by several Voyager investigations is correct, then the photometry reported by the imaging experiment on Voyager requires that Triton's surface have a remarkably low emissivity. Such a low emissivity is not required in order to explain the photometry from the photopolarimeter experiment on Voyager. A low emissivity would be inconsistent with Triton having a rough surface at the about 100-micron scale as might be expected given the active renewal processes which appear to dominate Triton's surface.

Description: The location of active geyser-like eruptions and related features close to the current subsolar latitude on Triton suggests a solar energy source for these phenomena. Solid-state greenhouse calculations have shown that sunlight can generate substantially elevated subsurface temperatures. A variety of models for the storage of solar energy in a subgreenhouse layer and for the supply of gas and energy to a geyser are examined. 'Leaky greenhouse' models with only vertical gas transport are inconsistent with the observed upper limit on geyser radius of about 1.5 km. However, lateral transport of energy by gas flow in a porous N2 layer with a block size on the order of a meter can supply the required amount of gas to a source region about 1 km in radius. The decline of gas output to steady state may occur over a period comparable with the inferred active geyser lifetime of 5 earth years. The required subsurface permeability may be maintained by thermal fracturing of the residual N2 polar cap. A lower limit on geyser source radius of about 50 to 100 m predicted by a theory of negatively buoyant jets is not readily attained.

Description: One model for the mechanism driving the plumes of the four active geyser-like eruptions observed by Voyager 2 on Triton is a heating up of nitrogen ice in a subsurface greenhouse environment, where nitrogen gas pressurized by solar heating explosively vents to the surface carrying clouds of ice and dark particles into the atmosphere. A temperature increase of less than 4 K above the ambient surface value of 38 + or - 3 K suffices to drive the plumes to 8-km altitude. Each eruption may last a year or more, over the course of which 0.1 cu km of ice is sublimed.

Description: Various events surrounding Magellan's orbit of Venus are recounted. Significant events include the successful firing of a solid rocket motor while the spacecraft was behind Venus to transfer it from a solar-centered trajectory to an orbit around the planet. The spacecraft orbits Venus every 3.26 hours at a maximum altitude of 8500 km and minimum altitude of 291 km in an elliptical orbit. The successful August 16 test of the synthetic-aperture radar system is discussed, noting that it produced two strips, each about 20 km x 16,000 km, revealing details as small as 120 m. Two anomalies causing a delay in the start of mapping operations and subsequent breaks in the communication link with earth for 14.5 hours and 17.7 hours are discussed. Protective measures directed from the spacecraft's ROM during breach of contact are listed, and possible causes of the anomalies are suggested, such as solar activity or hardware or software elements, although the actual cause is not yet known.