Voyager imaging observations of Ganymede and Callisto have been reduced and combined with ground-based telescopic data to produce complete solar phase curves of the satellites' leading and trailing hemispheres. The curves, along with disk-revolve measurements, were fit to scattering models to derive hemispherical values of the following physical parameters describing the surface: the single scattering albedo, the single particle phase function, the compaction state of the optically active portion of the regolith, and the mean slope angle of macroscopically rough features. We find that the leading side of Callisto is composed of particles that are more strongly backscattering and less highly compacted than the trailing side. For Ganymede,no hemispheric differences were detected in the compaction state, the surface roughness, or the single particle phase function. We attribute the difference in Callisto to enhanced meteoritic erosion on the leading side. No such difference is observed for Ganymede because the effects of magnetospheric interactions, which are enhanced closer to the primary and on the trailing side, counterbalance the effects of meteoritic erosion predominating on the leading side. For Europa, which was previously shown to have a less compacted trailing side, magnetospheric alterations predominate. Laboratory measurements of the phase functions of simple surface analogues for Callisto, and thermal eclipse measurements showing that the thermal inertias of the outer three satellites are similar, suggest that differences in the measured solar phase function are best attributed to intrinsic properties of the particles (such as size and shape) rather than primarily to the compaction state of the surface. © 1991.
