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The recently measured dimensionless moment of inertia (MoI) factor for Callisto of 0.3549 [+ or -] 0.0042 (Anderson et al., 2001, Icarus, 153, 157-161) poses a problem: its value cannot be explained by a model in which Callisto is completely differentiated into an ice shell above a rock shell and an iron core such as its neighboring satellite Ganymede nor can it be explained by a model of a homogeneous, undifferentiated ice-rock satellite. We show that Callisto may be incompletely differentiated into an outer ice-rock shell in which the volumetric rock concentration is close to the primordial one at the surface and decreases approximately linearly with depth, an ice mantle mostly depleted of rock, and an about 1800 km rock-ice core in which the rock concentration is close to the close-packing limit. The ice-rock shell thickness depends on uncertain rheology parameters and the heat flow and can be roughly 50 to 150 km thick. We show that if Callisto accreted from a mix of metal bearing rock and ice and if the average size of the rocks was of the order of meters to tens of meters, then Callisto may have experienced a gradual, but still incomplete unmixing of the two components. An ocean in Callisto at a depth of 100-200 km is difficult to obtain if the ice is pure [H.sub.2]O and if the ice-rock lithosphere is 100 km or more thick; a water ocean is more plausible for ice contaminated by ammonia, methane or salts; or for pure [H.sub.2]O at a depth of 400-600 km. Keywords: Callisto; Interiors; Satellites of Jupiter; Thermal histories

Palimpsests are large, circular, low-relief impact scars on Ganymede and Callisto. These structures were poorly understood based on Voyager-era analysis, but high-resolution Galileo images allow more detailed inspection. We analyze images of four Ganymedean palimpsests targeted by Galileo: Memphis and Buto Faculae, Epigeus, and Zakar. Ganymedean craters and Europan ring structures are used as tools to help better understand palimpsests, based on morphologic similarities. From analysis of Galileo images, palimpsests consist of four surface units: central plains, an unoriented massif facies, a concentric massif facies, and outer deposits. Using as a tie point the location in these structures where secondary craters begin to appear, outer deposits of palimpsests are analogous to the outer ejecta facies of craters; the concentric massif facies of palimpsests are analogous to the pedestal facies of craters; and the unoriented massif facies and central plains are analogous to crater interiors. These analogies are supported by the presence of buried preexisting structure beneath the outer two and absence of buried structure beneath the inner two units. Our observations indicate that palimpsest deposits represent fluidized impact ejecta, rather than cryovolcanic deposits or ancient crater interiors. Keywords: Ganymede; Europa; Impact processes; Cratering

Perturbations of the external magnetic fields around Europa and Callisto, Jupiter satellites, are reported. They are believed to arise from induced magnetic fields, generated in response to the varying plasma environment. It is likely that there significant layers of electrical conductivity just under the surfaces of both moons, and that the conducting layers can be explained by the presence of salty liquid-water oceans.

Estimates of standard errors in the gravitational coefficients J2 and C22 of Europa, Ganymede and Callisto are obtained from the Galileo Orbiter spacecraft's simulated radio tracking data. The simulated radio tracking data are obtained during two flybys of Callisto and Ganymede and one flyby of Europa. The errors in J2 are 44, 8.5 and 12 and errors in C22 are 12, 1.7 and 2.1 for Europe, Ganymede and Callisto. The values indicate that the rock and ice in these satellites are mixed or separated uniformally in the interiors.

An impact-generated atmosphere model is applied to the atmospheres of Titan, Ganymede and Callisto. This model postulates that atmosphere accumulates on a satellite or a planet based on the competition between impact erosion and impact supply of stray bodies. Based on the model, Titan has an atmosphere because the frequency of impact bodies is less than that of the satellites of Jupiter since the planet is closer to the sun's gravity well.

Crater chains an Callisto due to tidal split will be formed on the side facing Jupiter and the formation will be visible from Ganymede. A model was constructed to analyze future possibilities of crater formation on Callisto and Ganymede due to tidal spliting. Tensional axis, parallel to the radius vector, causes tidal stress in ellipsoidal and spherical bodies. A distinct line, unique to this model, was formed by the pieces of the split comet. In a model where the comet bursts due to rotational movement and in which the total stress immediately splits the rotating comet, the pieces form a long loop.

A group model light curves, where luminosity is a function of the satellite's location in the penumbral shadow, are obtained from the light curve of a partial solar eclipse of Callisto. The satellite Callisto was eclipsed by the south pole of the planet Jupiter on Dec 6, 1992. The radius of Jupiter's pole is determined by the Callisto ephemeris information, combined with a photometric solution. The radius of the Jupiter's pole is 67,111 kilometers. The polar flattening is 115 kilometers and is less flatter than the values obtained by hydrostatic equilibrium methods.

Galilean satellite astrometry has been derived from CCD photometry of nearly 200 eclipses in the shadow of Jupiter between the years 1990 and 2000. The root-mean-square differences as compared to predictions from J. H. Lieske's E5 ephemeris (1998, Astron. Astrophys. Suppl. 129, 205-217), expressed as along-track distances, are Io, 62 km; Europa, 267 km; Ganymede, 142 km; and Callisto, 146 km. [C] 2000 Academic Press Key Words: eclipses; Io; Europa; Ganymede; Callisto.

Magnetic field perturbations measured during Galileo flybys of Europa and Callisto are consistent with dipole fields induced by the temporal variations of the ambient jovian magnetospheric field. These fields are close to those expected for perfectly conducting moons. We investigate the implications of these observations for the electrical structure of the moon's interiors using a simple shell model. It is found that Europa and Callisto must possess regions where the conductivity exceeds 0.06 and 0.02 S/m at a depth of less than 200 and 300 km below the surface, respectively. This conductivity is unattainable in ice or silicates, unless the ice layer is at least partially molten or very large temperature gradients can be maintained below the ice. An ionosphere or a cloud of pick-up ions are probably also insufficiently conductive. Global Earth-like oceans under the surface of both moons could account for the observations provided they are at least a few kilometers thick. [C] 2000 Academic Press Key Words: Europa; Callisto; interiors; magnetic fields; Jupiter, magnetosphere.

There is strong evidence that oceans exist not far below the surfaces of Europa and Callisto, Jupiter's Galilean moons. Evidence comes from observations of electromagnetic induction signatures by the Galileo magnetometer experiment, indicating that the satellite interiors are electrically conducting. The likely conductor is liquid water containing some electrolyte, and the conducting volume may extend almost to the surface of the satellites.

Radio Doppler data generated by the Deep Space Network (DSN) from five encounters of the Galileo spacecraft with Callisto, Jupiter's outermost Galilean satellite, have been used to determine the mass (GM) and unnormalized quadrupole gravity coefficients in Callisto's external gravitational field. The results are GM= (7179.292 [+ or -] 0.009) [km.sup.3] [s.sup.-2], [J.sub.2] - (32.7 [+ or -] 0.8) x [10.sup.-6], [C.sub.22] = (10.2 [+ or -] 0.3) x [10.sup.-6], [S.sub.22] = (-1.1 [+ or -] 0.3) x [10.sup.-6], [C.sub.21] =(0.0 [+ or -] 0.3) x [10.sup.-6], and [S.sub.21] = (0.0 [+ or -] 1.6) x [10.sup.-6]. Also, four spacecraft images of Callisto have been used to determine its mean radius. The result is R = (2410.3 [+ or -] 1.5) km, with no detectable deviation from sphericity. Derived parameters are Callisto's mean density of (1834.4 [+ or -] 3.4) kg [m-.sup.3] and axial moment of inertia C/M[R.sup.2] = 0.3549 [+ or -] 0.0042. While the mean density indicates that Callisto is a mixture of rock and ice, the moment of inertia is too small for a homogeneous mixture. Accordingly, we present a suite of possible two- and three-layer interior models that satisfy the given constraints for radius, density, and moment of inertia. While not unique, these models show that Callisto cannot be entirely differentiated, and that there must exist a region of mixed ice and rock-metal, possibly extending to the center of the satellite.

The Galileo mission has revealed remarkable evidence of mass movement and landform degradation on the icy Galilean satellites of Jupiter. Weakening of surface materials coupled with mass movement reduces the topographic relief of landforms by moving surface materials down-slope. Throughout the Galileo orbiter nominal mission we have studied all known forms of mass movement and landform degradation of the icy galilean satellites, of which Callisto, by far, displays the most degraded surface. Callisto exhibits discrete mass movements that are larger and apparently more common than seen elsewhere. Most degradation on Ganymede appears consistent with sliding or slumping, impact erosion, and regolith evolution. Sliding or slumping is also observed at very small (100 m) scale on Europa. Sputter ablation, while probably playing some role in the evolution of Ganymede's and Callisto's debris layers, appears to be less important than other processes. Sputter ablation might play a significant role on Europa only if that satellite's surface is significantly older than [10.sup.8] years, far older than crater statistics indicate. Impact erosion and regolith formation on Europa are probably minimal, as implied by the low density of small craters there. Impact erosion and regolith formation may be important on the dark terrains of Ganymede, though some surfaces on this satellite may be modified by sublimation-degradation. While impact erosion and regolith formation are expected to operate with the same vigor on Callisto as on Ganymede, most of the areas examined at high resolution on Callisto have an appearance that implies that some additional process is at work, most likely sublimation-driven landform modification and mass wasting. The extent of surface degradation ascribed to sublimation on the outer two Galilean satellites implies that an ice more volatile than [H.sub.2]0 is probably involved. Key Words: Europa; Ganymede; Callisto; satellites of Jupiter; satellite surfaces.