The icy, dirt-laden surface of Callisto appears to be very ancient and
heavily cratered. The large concentric rings indicate the remains of
several enormous impact basins, created by huge meteors crashing into
the surface, and since erased by the flow of the crust. Callisto’s
density (less than twice that of water) is very close to that of
Ganymede, yet there is little or no evidence of the crustal motion and
internal activity that is visible on Ganymede.