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 Galilean moons, the first group of objects discovered orbiting another planet. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. With a diameter of 5,268 km (3,273 mi), it is 8% larger than the planet Mercury, but has only 45% as much mass. Its diameter is 2% larger than that of 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 9th largest object in the Solar System, and the largest without a substantial atmosphere.