This is a map of Ganymede, one of the moons of Jupiter. With a diameter of more than 3,000 miles, Ganymede is three-quarters the size of Mars and the largest satellite in the solar system. This is the first moon-wide, geologic map of the moon. The map's different colors indicate different materials on Ganymede's surface. Those yellowish circles everywhere are craters, young (pale yellow) and old (dark yellow).