Possibly the most highly cratered body in the solar system, Callisto is similar to Ganymede in its composition of water ice and rock. Unlike Ganymede, however, the satellite reveals no evidence of any internal geologic activity. Although Callisto is only the second largest of the Galilean satellites, it is still larger than the planets Mercury and Pluto.
A far encounter scene from Voyager 1 shows that Callisto superficially resembles Ganymede with areas of light and dark terrain. Bright craters have ejected fresh water and ice with little rock material, and are the youngest features on the satellite.
The large circular feature, named Valhalla, is the largest impact structure in the solar system. In some ways it is similar to multi-ringed basins on the Moon and Mercury, but there is no depression in the center of the structure. The icy crust of the moon, much weaker than rock, is not able to support either mountains or depressions over long periods of time (millions of years). Elevation differences gradually adjust to the average level of the surface.
Callisto as seen by the Galileo spacecraft
