Ganymede (moon)
Ganymede (Jupiter III) is the largest moon of Jupiter and in the Solar System, and the only moon known to have a magnetosphere. It is the seventh satellite outward from Jupiter and third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. Ganymede has a diameter of 5,268 km (3,273 mi), 8% larger than the planet Mercury, but its mass is only 45% that of Mercury. Ganymede is 2% larger than Saturn's Titan, the Solar System's second-largest moon. At 2.02 times the mass of the Moon, it is the most massive planetary satellite. It is the ninth-largest object in the Solar System, and the largest without a substantial atmosphere.
Ganymede is composed of approximately equal amounts of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans together. Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity brought about by tidal heating.
