PHOTOMETRY AND SURFACE-STRUCTURE OF THE ICY GALILEAN SATELLITES

Observations of the icy Galilean satellites from both spacecraft and ground-based telescopes have been fit to radiative transfer models describing the surfaces in terms of their roughness, compaction state, and single-particle phase function and albedo. The macroscopically rough nature of Ganymede and Callisto is similar to the Moon's, while Europa is significantly smoother. The sharply peaked opposition surge on Europa may be due to an extremely uncompacted upper regolith. However, if the peak is attributed to coherent backscatter, its surface may be the most compacted of the Galilean satellites. The compaction state of Callisto is similar to that of the Moon, while Ganymede's surface is more compacted. Callisto and possibly Europa exhibit textural differences in their leading and trailing hemispheres. This dichotomy is due to enhanced erosion by meteorites on the leading side.