Ganymede

Ganymede is a natural satellite of Jupiter and the largest moon in the Solar System. It has a diameter of about 5,268 kilometers and was discovered by Galileo Galilei in 1610. The moon is named after the cupbearer of the Greek gods in Homeric myth. It orbits Jupiter at an average distance of roughly 1.07 million kilometers and completes an orbit in about 7.15 days. Ganymede is thought to be partially differentiated, with a silicate-ice crust and mantle surrounding a metallic core.

Ganymede possesses a magnetic field and an intrinsic magnetosphere, the only moon known to have its own. Its interaction with Jupiter’s strong magnetosphere also produces auroral effects at high latitudes. The surface features two main terrains: an older, heavily cratered region and a younger, complex network of grooves and ridges known as the grooved terrain. The atmosphere is extremely tenuous, consisting mainly of oxygen, with surface pressures far below those on Earth.

Evidence from gravity and magnetic measurements suggests a subsurface salty ocean beneath an ice crust, possibly 60 to 150 kilometers thick, in contact with the rocky mantle. If confirmed, this would place Ganymede among bodies considered to host habitable conditions beneath an ice shell, even though the surface environment is inhospitable due to radiation.

Exploration history includes flybys by Voyager 1 and 2 and extensive study by the Galileo spacecraft. The European Space Agency’s JUICE mission, launched in the early 2020s, will conduct detailed studies of Ganymede and other Jovian moons, with a planned arrival in the late 2020s.