The Icy Galilean Satellites

Ganymede and Callisto, the icy Galilean satellites, are about the size of Mercury (Appendix 2, Sections C and D). Their low mean densities, less than 2000 kg m−3, indicate that they are composed of silicates with very substantial amounts of water. The fraction of water contained in these bodies depends on the density of the silicates; as much as 50% of the satellites could be water. Multiple flybys of Ganymede and Callisto by the Galileo spacecraft have provided us with knowledge of the satellites’ densities and moments of inertia from which we can infer the internal structures of the bodies. Ganymede is fully differentiated into a metallic core, silicate mantle, and thick (many hundreds of kilometers) outer ice shell. In contrast, Callisto is only partially differentiated. Most of the satellite consists of a primordial ice–rock mixture; only in the outer few hundred kilometers can the ice and rock have separated.

The Galileo spacecraft found that Ganymede has its own magnetic field while Callisto does not. Ganymede is the only moon in our solar system known to have an intrinsic global magnetic field at present. Ganymede’s magnetic field is so large that the field must be generated by dynamo action in a liquid metallic core just as a dynamo in the Earth’s outer core produces our magnetic field. Observations of Ganymede by the Galileo spacecraft provide strong support for the complete differentiation of the satellite and for the existence of a liquid metallic core in its interior. A major unsolved question is why Ganymede is fully differentiated while Callisto is only slightly differentiated; both bodies are about the same size and are made up of about the same proportions of ice and rock. One possible explanation is that Ganymede was tidally heated in the past whereas Callisto was not. The appearances of Ganymede and Callisto are consistent with a differentiated interior for Ganymede and a relatively primordial interior for Callisto.