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Ganymede is a Jovian moon, Jupiter's largest satellite, and the largest satellite in the solar system. With a diameter of 5262 km, it is larger than Mercury, although with only half Mercury's mass. Along with Io, Europa, and Callisto, Ganymede is a Galilean moon, meaning it was discovered by Galileo Galilei in 1610 with one of the earliest telescopes. Ganymede is named after the cup-bearer of the Greek gods.
Ganymede has two types of surface geography — dark and old, and light and grooved. Like the other Galilean moons, Ganymede is made out of about equal parts silicate rock and various ices. Some parts of its surface look like a skating rink that has been smashed with a hammer, the hammer in this case being impact asteroids. Overall Ganymede looks light brown, like a sandstone.
As the third visible satellite of Jupiter, Ganymede was historically referred to as Jupiter III. Ganymede has a tenuous oxygen atmosphere. It also has its own magnetic field, the only moon in the solar system that has one. This is likely caused by conducting material slowly circulating in its core, like Earth's. Sufficiently large celestial bodies have an internal temperature necessary to melt iron and convection dynamics that cause its circulation.
Like other Galilean moons, Ganymede has a thin subsurface ocean. It is thought less likely to contain life than Europa, as Europa's ocean is closer to the surface. Ganymede orbits 1,070,000 km from Jupiter, about three times the distance between the Earth and the moon.
Ganymede's largest surface feature is the Galileo Regio, a cracked concentric ring structure similar to Valhalla on Callisto. The heavily cratered regions on Ganymede have a surface over four billion years old, but the lighter regions are somewhat younger. Ganymede's craters lack the well-defined central domes and edges of craters on Mercury and the Moon because Ganymede's crust is made of relatively weak ice which has a tendency to flow.