Callisto (globe)
Global view of Callisto.
Diameter 4,806 km
2,986 mi.
Distance from primary 1883,000 km
1170,041 mi.

Callisto is the outermost of Jupiter's four planet-sized moons and is dominated by impact craters. Despite this, a few more interesting features are also visible, including gigantic impact basins, suggestions of ancient volcanism, and a few ancient tectonic features. Among the most interesting features on Callisto are impact scars from tidally disrupted comets. Callisto is nearly as large as the planet Mercury. Its bulk density, however, is only ~1.8 grams per cubic centimeter, or only twice that of water ice. This indicates that the interior is approximately half water ice as well. Callisto is very similar in bulk properties to Ganymede, but apparently had a much simpler geologic history. The different geologic histories of these two bodies has been an important problem for planetary scientists, which may be related to the orbital and tidal evolution of Ganymede.[1]