One of the Galilean satellites, and the largest jovian satellite; it is bigger than Mercury and Pluto. It is believed to have a rock and metal core surrounded by a large mantle of water or water ice, 800–900 km thick, and the surface is of ice, with two types of terrain, one very cratered and dark, the other rather lighter, with many grooves and ridges. These terrains result from tectonic activity, but the details are not known. Both terrains are extensively cratered, the craters being flat, with no ring mountains and central depressions, and suggest the surface is about 3–3.5 Ma old. Ganymede has a magnetic field, embedded within that of Jupiter. Ganymede was discovered in 1610 by Simon Marius and Galileo. Its diameter is 5268km; mass 1.48 × 1023kg; mean density 1940kg/m3; visual albedo 0.42; mean distance from Jupiter 1.07 × 106 km.
Subjects: Astronomy and Astrophysics — Earth Sciences and Geography.