Ganymede is the largest moon in the solar system, even larger than the planet Mercury. (At one time Titan, Saturn's moon, was thought to be larger, but this was a mistake.) It is the second-furthest from Jupiter of the 4 Galilean moons, closer than Callisto but further than Io and Europa. Its mean distance from Jupiter is about 15.1 Jupiter Radii, and it orbits every 7.2 days. The same side of the moon always faces Jupiter. Ganymede is mostly water-ice, but with more impurities (silicates) than the ice of Europa. The surface gravity is about 1/7th of Earth's.

(Original Caption with Image):Natural color view of Ganymede from the Galileo spacecraft during its first encounter with the satellite. North is to the top of the picture and the sun illuminates the surface from the right. The dark areas are the older, more heavily cratered regions and the light areas are younger, tectonically deformed regions. The brownish-gray color is due to mixtures of rocky materials and ice. Bright spots are geologically recent impact craters and their ejecta. The finest details that can be discerned in this picture are about 13.4 kilometers across. The images which combine for this color image were taken beginning at Universal Time 8:46:04 UT on June 26, 1996.

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