This image shows the heavily cratered surface of Callisto taken by Voyager 2 on July 7, 1979. An enormous impact basin with concentric rings is located near the top and slightly left of center. Callisto and Ganymede both have densities that indicate a rocky core with an icy mantle and crust. Callisto has by one terrain, highly cratered, with no evidence of evolution after formation apart from heavy bombardment. Its dark color is due to dark chondritic material falling on its surface.
Fresh craters excavate white ice from below making light spots on the dark surface.
Entire hemisphere of Ganymede. The prominent dark region, called Galileo Regio, is about 3,200 km in diameter. The bright spots are relative recent impact craters. Part of the Galileo Regio may be covered with a bright frost. The surface displays numerous impact craters many of which have extensive bright ray systems.