Ganymede is the largest moon in the Solar System. It orbits Jupiter and has a diameter of about 5,268 kilometres, making it larger than the planet Mercury, although Ganymede has much less mass.
The moon was discovered by Galileo Galilei in January 1610, along with Jupiter’s other three large Galilean moons: Io, Europa, and Callisto. Ganymede is composed largely of silicate rock and water ice and has a differentiated interior with a metallic core.
Ganymede is also the only moon known to possess its own intrinsic magnetic field. Observations indicate a subsurface saltwater ocean beneath its icy crust, making it an important target in the search for potentially habitable environments beyond Earth. The European Space Agency’s JUICE mission launched in 2023 to study Jupiter and its icy moons, including Ganymede.