Callisto is the most heavily cratered satellite in the solar system. In fact, impact craters and associated concentric rings are about the only features to be found on Callisto; there are no large mountains. This is probably due to the icy nature of its surface, with the largest craters and mountains being erased by the flow of the icy crust over geologic time. Two enormous concentric ring impact basins are found on Callisto; Valhalla is the largest with a bright central region that is 600 kilometers in diameter and rings extending to 3000 kilometers in diameter, and the second-largest impact basin is Asgard measuring about 1600 kilometers in diameter. Another interesting feature is Gipul Catena, a long series of impact craters lined up in a straight line across Callisto's surface. This was probably caused by an object that was tidally disrupted as it passed close to Jupiter (much like Comet Shoemaker-Levy 9) before it impacted. Callisto's crust is thought to be approximately 4 billion years old, dating back almost to the formation of the solar system.
Its battered surface lies on top of an icy layer that is about 200 kilometers thick. Beneath the crust lies a salty ocean in excess of 10 kilometers thick. The ocean was discovered from studies of the [magnetic field]?s around Jupiter and its moons. It was found that Callisto's magnetic field varies (flows in various directions at different times) in response to the background magnetic field generated by Jupiter; this suggested a layer of highly conductive fluid within Callisto.
Beneath the ocean, Callisto seems to have a strange interior that is not entirely uniform and does not vary dramatically. [Galileo probe]? data suggests that the interior is composed of compressed rock and ice with the percentage of rock increasing with depth due to partial settling of its constituants. Callisto has the lowest density of the four Galilean moons of Jupiter, only 1.86 gm/cm3, and is about 40% ice and 60% rock/iron. Titan and Triton are probably similar in composition.
Callisto has a very tenuous atmosphere composed of carbon dioxide. The [Galileo probe]? has detected evidence of a weak [magnetic field]? which may indicate some sort of salty fluid below the surface, possibly liquid water.
Unlike neighbouring Ganymede with its complex terrain, there is little evidence of tectonic? activity on Callisto. While Callisto is very similar in bulk properties to Ganymede, it apparently has a much simpler geologic history. The different geologic histories of the two has been an important problem for planetary scientists. "Simple" Callisto is a good reference for comparison with other more complex worlds and it may represent what the other Galilean moons were like early in their history.
Callisto orbits just outside Jupiter's main radiation belt.
Statistics for Callisto:
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