Ganymede, also known as Jupiter III, is the largest and most massive moon of the planet Jupiter. It is the largest moon in the Solar System. The moon has a diameter of 5,268 km (3,273 miles) and is around 8% larger than the planet Mercury, although it only has about 45% of its mass. The moon has a metallic core and has the lowest moment of inertia factor of any solid body in the Solar System. Ganymede also has its own magnetic field, and is the only known moon in the Solar System to possess one. It is the third of the Galilean Moons, the first group of objects discovered orbiting another planet, and the seventh satellite outward from Jupiter. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.
Ganymede is composed of approximately equal amounts of silicate rock and water ice. It has 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 combined. 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 due to tidal heating.