Callisto ("ka LIS toh") is the eighth of Jupiter 's known satellites and the second largest. ... Discovered by Galileo and Marius in 1610. ... Unlike Ganymede , Callisto seems to have little internal structure; however there are signs from recent Galileo data that the interior materials have settled partially, with the percentage of rock increasing toward the center. ... Callisto's surface is covered entirely with craters. ... Like Ganymede, Callisto's ancient craters have collapsed. ... Jupiter .. ...