Callisto is Jupiter’s moon with the most heavily cratered surface in the Solar System. Its ancient landscape is almost completely covered with impact marks, preserving a long record of collisions.
Callisto is the outermost of Jupiter’s four large Galilean moons. Galileo Galilei observed it in 1610, and the moon is large enough to be seen from Earth with ordinary binoculars under suitable conditions. Unlike Io, Europa, and Ganymede, Callisto shows relatively little evidence of active volcanism, plate tectonics, or large-scale resurfacing, so old craters have remained visible.
Its crater density is close to saturation: new impacts can overlap or erase older ones. Callisto’s most prominent features include huge multi-ring basins, bright impact craters, and chains of craters called catenae. The moon is made of roughly equal amounts of rock and ice, and observations by the Galileo spacecraft also suggested that a salty subsurface ocean may exist beneath its battered exterior.