Ice in the interiors of Ganymede and Callisto

The low mean densities of Ganymede and Callisto, 1.93 and 1.79 g/cm^3 respectively (Smith et al., 1979) insure that these bodies contain substantial amounts of water. However, the physical state of the water and its distribution in the satellite interiors is uncertain. We propose that the interiors of Ganymede and Callisto consist of three major regions: 1) a silicate core, 2) an undifferentiated ice-silicate lower mantle, and 3) a predominantly ice upper mantle (Fig. 1) (Schubert et al., 1980).