Ganymede

Ganymede is the third-closest of Jupiter's large Galilean moons. It orbits Jupiter at an average distance of 665,000 miles (1,070,000 km) over a period of 7.15 days. With the inner two Galilean moon, Ganymede participates in a 1:2:4 orbital resonance. In other words, for each orbit of Ganymede, Europa orbits Jupiter twice, and Io orbits Jupiter four times.

Ganymede is the largest moon in our solar system, with a diameter of 5,262 km (3,280 miles). If Ganymede orbited the Sun, it would be classified as a planet.

Ganymede's surface is mottled by light and dark regions. The dark regions are heavily cratered, indicating an ancient origin. However, Ganymede also has long, bright grooves, mountain ranges, and "lava" flows of water ice, implying a complex geological history. Ganymede is the only moon in the solar system known to possess a weak magnetic field. For this reason, Ganymede is thought to have a molten metallic core and/or an electrically-conducting saltwater ocean some 120 miles (200 km) below the surface.