The image on the right
shows the moon Callisto, the fourth Galilean satellite. Callisto, like Ganymede
and Europa, is covered by a frozen water ocean, but unlike the other Galilean
satellites, Callisto is saturated with craters, suggesting very old terrain.
There is no evidence for plate tectonics.
Maximal Cratering Density
The image above left shows a closeup with more detail of the heavy cratering. The cratering density on Callisto is essentially maximal, meaning that no higher density of craters could be formed because the formation of a new crater would on the average destroy one old crater.
The Valhalla Impact Crater
The image on the left shows an enormous impact feature on Callisto called Valhalla. This gigantic meteor impact basin appears to have concentric waves around it with a diameter of about 3000 km. It is likely that the waves were formed when the surface ice was melted by a gigantic meteor impact and then quickly refroze, capturing the waves in a refrozen ice surface.
History of Callisto
Callisto, the most distant Galilean satellite, is only weakly influenced by Jupiter-induced tides. The evidence suggests that the crust of Callisto probably froze as it was formed, and essentially nothing has happened geologically on Callisto for 4.5 billion years except for the influence of large and small meteor impacts.
Information supplied by: http://csep10.phys.utk.edu