The high density of craters demonstrate that patches of Ganymede are indeed billions of years old. This photo is one of a series released by NASA two days ago from the Galileo spacecraft orbiting Jupiter . The large impact crater on the left is 19 kilometers across, while dark indentations in Ganymede 's crusty surface-ice run diagonally. Ganymede is composed of half rock and half water-ice. Io , Europa , Ganymede , and Callisto .
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