This image of Jupiter's giant moon Ganymede was taken by NASA's Galileo spacecraft during its first encounter. The dark areas in the crust represent older regions while the lighter areas represent younger crust formed by more recent geological activity. The brownish gray areas are formed by a mixture of rock and ice. The brightest spots are formed when impact craters eject icy material from deeper inside the crust. Ganymede is Jupiter's largest moon and is also the largest moon in the Solar System. It is 50% larger than Earth's moon and is larger than the planet Mercury.