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We have combined spectral reflectance data from the Solid State Imaging (SSI) experiment, the Near-Infrared Mapping Spectrometer (NIMS), and the Ultraviolet Spectrometer (UVS) in an attempt to determine the composition and implied genesis of non-H2O components in the optical surface of Europa. We have considered four terrains: (1) the “dark terrains” on the trailing hemisphere, (2) the “mottled terrain,” (3) the linea on the leading hemisphere, and (4) the linea embedded in the dark terrain on the trailing hemisphere. The darker materials in these terrains exhibit remarkably similar spectra in both the visible and near infrared. In the visible, a downturn toward shorter wavelengths has been attributed to sulfur. The broad concentrations of dark material on the trailing hemisphere was originally thought to be indicative of exogenic sulfur implantation. While an exogenic cause is still probable, more recent observations by the UVS team at higher spatial resolution have led to their suggestions that the role of the bombardment may have primarily been to sputter away overlying ice and to reveal underlying endogenic non-H2O contaminants. If so, this might explain why the spectra in all these terrains are so similar despite the fact that the contaminants in the linea are clearly endogenic and those in the mottled terrain are almost certainly so.

Galileo Solid State Imaging (SSI) data taken during the E6 orbit on February 19, 1997 offer a brief, high-resolution glimpse of Jupiter's White Ovals. These data can be used to infer relative cloud heights and systematic rotations over a small area of latitude and longitude (approximately 20° by 30°) around the White Ovals. To fully understand the behavior of these cloud systems, however, a longer time span of global data is necessary. In this paper, we utilize ground-based Voyager and Hubble Space Telescope data to interpret the Galileo images. We find that the White Ovals' translation rates have slowed from 0.39 deg day−1during the Voyager era to 0.13 deg day−1in 1996 and they, along with three smaller anticyclonic systems, have coalesced into a system of alternating cyclonic and anticyclonic systems that has not been observed before. The largest of the ovals, BC, is now impeding the eastward motion of the system, causing more rapidly translating anticyclonic cells to catch the system and increase the westward extension of the equally spaced features. Rotational velocities for BC and a small anticyclonic system at 42°S have also been measured and peak at 120 m sec−1for both systems, comparable to the Mitchellet al. (J. Geophys. Res.86,8751–8757) measurements of BC and the Great Red Spot (GRS) from Voyager data. The motion of individual features and of the whole system in the ambient wind flow is discussed.

A control network containing 19 points identified on Galileo images obtained during the October 1991 flyby was used to determine the direction of Gaspra's north pole. We find the right ascension and declination (in J2000 coordinates) to be αo= 9.5° ± δo= 26.7° ± 1.2°.This result agrees with earlier determinations based on telescopic observations of Gaspra's lightcurve. We confirm that the rotation is prograde, but we lack the time base to improve the value of the rotation period of 0.2934197 days determined by lightcurve analysis (Magnusson et al. 1992). We propose a prime meridian (0° longitude) that passes through the center of the 630-m-diameter crater Charax, a selection that places the prime meridian close to Gaspra's long axis.

The Galileo Orbiter examined several impact features on Europa at considerably better resolution than was possible from Voyager. The new data allow us to describe the morphology and infer the geology of the largest impact features on Europa, which are probes into the crust. We observe two basic types of large impact features: (1) “classic” impact craters that grossly resemble well-preserved lunar craters of similar size but are more topographically subdued (e.g., Pwyll) and (2) very flat circular features that lack the basic topographic structures of impact craters such as raised rims, a central depression, or central peaks, and which largely owe their identification as impact features to the field of secondary craters radially sprayed about them (e.g., Callanish). Our interpretation is that the classic craters (all <30 km diameter) formed entirely within a solid target at least 5 to 10 km thick that exhibited brittle behavior on time scales of the impact events. Some of the classic craters have a more subdued topography than fresh craters of similar size on other icy bodies such as Ganymede and Callisto, probably due to the enhanced viscous relaxation produced by a steeper thermal gradient on Europa. Pedestal ejecta facies on Europa (and Ganymede) may be produced by the relief-flattening movement of plastically deforming but otherwise solid ice that was warm at the time of emplacement. Callanish and Tyre do not appear to be larger and even more viscously relaxed versions of the classic craters; rather they display totally different morphologies such as distinctive textures and a series of large concentric structural rings cutting impact-feature-related materials. Impact simulations suggest that the distinctive morphologies would not be produced by impact into a solid ice target, but may be explained by impact into an ice layer ∼10 to 15 km thick overlying a low-viscosity material such as water. The very wide (near antipodal) separation of Callanish and Tyre imply that ∼10–15 km may have been the global average thickness of the rigid crust of Europa when these impacts occurred. The absence of detectable craters superposed on the interior deposits of Callanish suggests that it is geologically young (<108years). Hence, it seems likely that our preliminary conclusions about the subsurface structure of Europa apply to the current day.

We present the results of numerical simulations for the long-term evolution of rotational states of near-prolate cometary nuclei over many orbits under sublimation-induced torques. We derive plausible evolutionary paths for the rotational states and show that they strongly depend on the location of active areas on the nucleus. We conclude that for acceptable locations of active regions on the nucleus, the rotational state of the nucleus maintains nearly constant precessional angles and nongravitational forces over many orbits as suggested by observations of Comet P/Halley. In these cases, the strongest active area should be near cometographic midlatitudes when the equatorial plane is defined to contain the intermediate and small axes of inertia. A Halley-like nucleus with active areas and a near-commensurability (≈2) of precessional and rotational periods and an assumed density near 0.4 g cm^-3 is capable of maintaining a consistent 4.1-day delay in the perihelion passage for each apparition as observations suggest. However, the exact amount of delay (or advance) depends on the precise details of the rotational state, including the direction of the rotational angular momentum vector, the details of the active areas such as locations, tilts, and strengths, the secular water production curve, and the mass (or density) of the nucleus. We predict a spin-up of the nucleus provided that the same set of active areas are present over many orbits. This spin-up is presumably the major cause for nuclear splittings. Copyright 1995, 1999 Academic Press

Active volcanism on Io has been monitored during the nominal Galileo satellite tour from mid 1996 through late 1997. The Solid State Imaging (SSI) experiment was able to observe many manifestations of this active volcanism, including (1) changes in the color and albedo of the surface, (2) active airborne plumes, and (3) glowing vents seen in eclipse.

During its first two encounters with Ganymede, the Galileo spacecraft obtained images of a 16,500 km2portion of Galileo Regio, a large expanse of dark terrain, at high resolution (76–86 m/pixel). Through mapping of the G1 and G2 target sites within Galileo Regio, we are able to characterize geological units based on their morphology and relative albedo. We find three generally low albedo units: an intermediate albedo plains unit, a lower albedo plains unit, and the lowest albedo unit which is found on furrow and crater floors. We also find high albedo units which include crater rims, furrow rims, and isolated knobs and massifs. Other features include an intermediate albedo lobate feature interpreted to be a palimpsest and a hummocky unit interpreted to be impact ejecta. Several processes are interpreted to have occurred within Galileo Regio. These include tectonic deformation, mass wasting, sublimation, resurfacing by impact ejecta, and possibly cryovolcanism and isostatic adjustment. We observe that the NW–SE trending furrows (Lakhmu Fossae) in Galileo Regio are degraded and are crosscut by the younger N–S trending furrows (Zu Fossae). We also find several other tectonic features which may be minor faults or fractures related to one or other of these systems. Through mapping and crater size–frequency distributions, we are able to propose a stratigraphy for the Galileo Regio target site. The oldest features in the area are high albedo knobs and massifs, which are interpreted to be remnants of early impact-related features and furrow rims. These may have formed at approximately the same time as the intermediate and low albedo plains units and the furrow systems. The lowest albedo unit of furrow floors probably subsequently evolved through sublimation and mass wasting. Much of the northeast portion of the target area was subsequently obscured by one of the youngest units, ejecta from an impact just to the north. We use our mapping of the high-resolution images of Galileo Regio to evaluate three end-member models for the formation of dark terrain: (1) the crust is dark throughout, (2) material on the surface is the result of a low albedo cryovolcanic layer over a higher albedo crust, and (3) dark material is distributed in small quantities throughout the crust, and geological processes have acted to concentrate low albedo material on the surface. Although it is possible that elements of more than one of these models are present within the dark terrain, we find that the third model, that of a thin veneer of low albedo material, best fits observations of Galileo Regio

We present an analysis of disk-resolved images of Io at 232.5, 260, and 285 nm taken with the FOC (Faint Object Camera) of the Hubble Space Telescope. The images at 232.5 and 260 nm were acquired in 1993 in an effort to separate the surface and SO2atmosphere contributions to the observed UV albedo. We combine these images to make UV color maps of the regions centered on the leading and trailing hemispheres. Between latitudes +60° and −60° we find that the UV colors are dominated by three distinctive components,B0,B1, andB2, one more than was found to be required to fit visible wavelength Voyager data at these latitudes. The Voyager component “B” (McEwen, A. S., T. V. Johnson, D. L. Matson, and L. A. Sonderblom 1988.Icarus75, 450–478) appears to be a combination of two distinct spectral components:B1andB2. We find thatB1, the darker component, represents either a new compositional unit or patches of SO2vapor overlying compositional unitB(=B2). To distinguish between these two possibilities, we propose two simple models of surface reflectivities and SO2vapor curve of growth designed to allow a crude separation between the effects of the absorptions by surface materials and SO2vapor.

New, high-quality Galileo images of Io's dayside and Jupiterlit nightside, examined for diurnal changes in violet albedo patterns and diurnal changes in hemispheric color, do not provide any evidence for significant nighttime condensation of the satellite's thin SO2atmosphere. In particular, the Galileo images do not show the major change in Io's average hemispheric color between day and night that was reported by Burattiet al.(1995,Icarus118, 418–422) in a study of Voyager data. The lack of detectable nighttime frost deposition in the Galileo images is not surprising, given the tenuous, “patchy” nature of Io's SO2atmosphere and the associated difficulty in producing optically thick nighttime frost layers.

Galileo-based models for the shape of 951 Gaspra and the global-average photometric behavior of its surface have been used to model a representative subset of the asteroid's telescopic lightcurves. Fitting the synthetic lightcurves to the observed timing of lightcurve extrema, and knowing the orientation of Gaspra's axes at the time of the Galileo flyby, leads to a sidereal rotation period for the asteroid of 7.042024 ± 0.000020 hr, a slight change from the period reported by Magnusson et al. (1992, Icarus 97, 124-129). Initially, the shapes, amplitudes, and absolute photometry of the synthetic and observed lightcurves agree with each other to within 0.05-0.1 mag. Small modifications to the Gaspra shape model on sides of the asteroid poorly imaged by Galileo (changes of 700 m or less in the southern hemisphere at longitudes 90°-270°W) reduce the typical discrepancies to ≈0.05 mag in lightcurve shape and ≤0.03 mag in absolute photometry. The result demonstrates that Earth-based lightcurves can be used to refine the shape of a spacecraft-imaged irregular object in areas that are poorly constrained by the spacecraft observations. The consistency in absolute photometry validates both the absolute brightness and phase-angle dependence of the Galileo-based model for Gaspra photometry, supports the accuracy of the absolute calibration of the Galileo SSI camera, and confirms the Earth-based determination of the V-filter geometric albedo of the asteroid (0.22 ± 0.03; Tholen et al., submitted for publication). Remaining discrepancies between the synthetic and observed lightcurves show no indication of systematic latitudinal variations in albedo and also cannot be explained entirely by isolated albedo spots. These discrepancies are most likely caused by (1) small, remaining, hard-to-constrain errors in the Gaspra shape model and/or (2) moderate variations in macroscopic roughness across the asteroid's surface, in particular making longitudes 130°-300°W moderately rougher than the opposite hemisphere. Copyright 1995, 1999 Academic Press

We show by numerical simulations that the CO distribution in the coma of Comet P/Halley as measured by neutral gas mass spectrometer aboard Giotto spacecraft (P. Eberhardt et al., 1987, Astron, Astrophys, 187, 481-484) can be explained by an extended jet source originating from vent 1 in the rotational model of M. J. S. Belton et al. (1991, Icarus 93, 183-193). This is an alternative interpretation for the origin of the CO distribution where Eberhardt et al. (1987) explained the observations in terms of a spherically symmetric extended source function. We still find nearly 13 of CO is due to CO sublimating directly at the nucleus, possibly from the vent itself. Approximately 50% of CO that originates in the extended jet source may be formed via photolysis of H₂CO while the rest can be due to CO trapped in grain mantels and other C=O-bearing molecules. The overall production rates of CO in the two models differ by less than 29%. The prediction of the jet source model compares satisfactorily with the distribution of CO observed in the coma by the IUE satellite but unfortunately does not allow one to discriminate between the two models. Copyright 1994, 1999 Academic Press

Imaging of coma morphology of Comet Hale-Bopp from pre-perihelion through perihelion to post-perihelion is presented. Broad band images from 1996 and late 1997 show nearly radial jets streaking out from the nucleus. During both 1996 and late 1997, the brightest jets are approximately in a northern/northeastern direction. The slight curvature present in these radial jets is consistent with radiation pressure effects. Narrow band images around perihelion show two distinctive pictures of the CN and the continuum coma morphology. Spirals are clearly seen in the CN images but not in the continuum where structure is confined to the sunward side. The CN structure is consistent with continuous outgassing of the source of CN from the nucleus during both day and night.

During the first six orbits of the Galileo spacecraft's prime mission, the Solid State Imaging (SSI) system acquired multispectral image mosaics of Jupiter's Great Red Spot, an equatorial belt/zone boundary, a “5-μm hot spot” similar to the Galileo Probe entry site, and two of the classic White Ovals. We present mosaics of each region, approximating their appearance at visible wavelengths and showing cloud height and opacity variations. The local wind field is derived by tracking cloud motions between multiple observations of each region with time separations of roughly 1 and 10 hr. Vertical cloud structure is derived in a companion paper by Banfieldet al. (Icarus135, 230–250). Galileo's brief, high-resolution observations complement Earth-based and Voyager studies and offer local meteorological context for the Galileo Probe results. Our results show that the dynamics of the zonal jets and large vortices have changed little since Voyager, with a few exceptions. We detect a cyclonic current within the center of the predominantly anticyclonic Great Red Spot. The zonal velocity difference between 0° S and 6° S has increased by 20 m sec−1. We measure a strong northeast flow approaching the hot spot. This flow indicates either massive horizontal convergence or the presence of a large anticyclonic vortex southeast of the hot spot. The current compact arrangement of two White Ovals and a cyclonic structure greatly perturbs the zonal jets in that region.

Dactyl was discovered in solid state imaging (SSI) data on February 17, 1994, during the long playback of approach images from the Galileo spacecraft's encounter with the asteroid 243 Ida. Forty-seven images of the Ida–Dactyl pair were obtained. A detailed search for other satellites was made. No confirmed detections were made, all other candidate features being consistent with radiation hits. We deduce a manifold of osculating two-body orbits that approximate Dactyl's motion over the observed orbital arc depending on the assumed mass of Ida. At the time of Galileo's encounter, Dactyl was found to be 85 km from the center of Ida, moving at ∼6 m·sec−1in the same rection as Ida's retrograde spin. The inclination of its orbit is ∼172° in Ida's equatorial system (IAU definition). It was not possible to obtain a definitive orbit or measure of Ida's mass from the observed motion even though supplemental techniques (search for Dactyl's shadow on Ida, changes in angular diameter and brightness, and attempts to determine the spin of Dactyl) were explored. The influence of Ida's irregular gravitational field and solar perturbations on two-body motion are evaluated and found to be undetectable in the observed orbital arc. These effects may, however, strongly influence the motion over orbital time scales. Limits to the value of Ida's gravitation parameter,GM, are derived. A robust lower limit,GM> 0.0023 km3·sec−2, is obtained by requiring Dactyl's orbit to be bound. Hubble Space Telescope observations, which show no evidence of Dactyl on a hyperbolic orbit, excludes values ofGMin the range 0.00216

We provide an overview of the execution, data, and results of the solid state imaging (SSI) experiment at the encounter of the Galileo spacecraft with the asteroid 243 Ida. Ninety-six images of the asteroid, representing 18 time samples during a rotation period (4.633 h), were transmitted to Earth as a result of the UT 1993 August 28.70284 encounter. This provided coverage of ∼95% of the surface and achieved ground resolutions as high as 25 m/pixel. Coverage of most of Ida's surface is available in four colors, with limited regions in five colors, at resolutions up to 105 m/pixel. A natural satellite of Ida, called Dactyl, was discovered in a prograde (with respect to Ida's spin), near-equatorial, orbit moving slowly (∼ 6 m/sec) with a separation of 85 km from Ida.s

A control network containing 64 points identified on Galileo images obtained during the August 1993 flyby was used to determine the direction of Ida's north pole. We find the right ascension and declination (in J2000 coordinates) to be

Galileo flyby images of 951 Gaspra show a crater population dominated by fresh craters several hundred meters in diameter and smaller. They must represent a production population because their spatial density is low (few overlaps) and because degraded craters are underabundant; equilibrium may be attained at diameters near to or below the resolution limit of the best image. We have counted, measured, and classified craters from the highest resolution, “high phase” image, which shows >600 craters in 90 km2. The differential population index (0.2–0.6 km) for the fresh, obvious craters is very “steep” (−4.3 ± 0.3). It probably reflects the index of asteroidal projectiles; it is much steeper than the theoretical value of −3.5 for collisional equilibrium. Gaspra's crater population differs from that observed on Phobos but resembles those observed on the Moon and Mars at these sizes (consistent also with the near-Earth asteroid population). Gaspra's fresh craters are superposed on a landscape that appears “smoothed” at a vertical scale of hundreds of meters. Some “soft,” subdued crater-like features, commonly ∼>500 m across, are visible. Some of these are associated with the linear grooves on Gaspra and may be endogenic features. Many others are probably pre-existing impact craters deeply blanketed or otherwise much degraded.

Models for the shape and global photometric behavior of 243 Ida derived from Galileo images have been used to model the asteroid's telescopic lightcurves. We find excellent agreement between the calculated and observed times of lightcurve extrema, confirming the sidereal rotation period of 4.633632 ± 0.000007 hr determined by Binzelet al. (1993.Icarus105, 310–325.). The shapes and amplitudes of the synthetic and observed lightcurves agree with each other to within ≈0.05 mag, and the average offset in absolute photometry between model and observation is only 0.01 mag. This consistency confirms the accuracy of the shape model derived for Ida by Thomaset al. (1996.Icarus120, 20–32.) and supports the model of the asteroid's photometric behavior developed by Helfensteinet al. (1996.Icarus120, 48–65.).

Seventeen positive relief features ∼45–150m across are identified as probable blocks in Galileo high-resolution images of Ida. Their presence provides direct evidence for regolith retention on asteroids. The spatial distribution, maximum size, and integrated volume of the blocks are consistent with those of blocks associated with craters on the Earth, the Moon, Phobos, and Deimos. The concordance suggests that the features are impact ejecta fragments and that cratering mechanics on Ida, an object of average diameter ∼32 km, are similar to those applying on previously studied rocky bodies. The blocks that lie within or near the rims of craters Lascaux and Mammoth were likely mobilized in the low-velocity tail portion of the excavation flow that formed those craters. A few blocks located near smaller craters may have migrated some distance away from their source, possibly by impact-induced spallation, hopping, rolling, and/or sliding. Some blocks on Ida could be surviving fragments from the Koronis parent body, accreted after its breakup. The lifetime of 102-m sized boulders against collisional disruption is estimated to be in the 108–109year range, consistent with ages considered for the largest and oldest craters on Ida. Extrapolation of successful ejecta scaling laws to other asteroids suggests that blocks ∼15 and 70m across could be present on Dactyl and Gaspra, respectively (in both cases too small to be identified in available Galileo images). Blocks 100m in size could be present on 433 Eros, and km-sized megablocks on 4 Vesta.