Ganymede is the largest moon in the Solar System and is larger in diameter than Mercury.
Ganymede orbits Jupiter and has a diameter of about 5,268 kilometres. Mercury is denser and more massive, but its diameter is about 4,879 kilometres, so Ganymede is physically wider even though it contains less mass.
The moon was discovered by Galileo Galilei in January 1610, along with Jupiter's other large satellites Io, Europa, and Callisto. These four moons are called the Galilean moons and provided early evidence that not every celestial body revolves around Earth.
Ganymede is also the only moon known to possess its own intrinsic magnetic field. Spacecraft observations indicate an icy outer shell, a subsurface saltwater ocean, and a differentiated interior with a metallic core. ESA's JUICE mission is studying the Jovian system and is scheduled to enter orbit around Ganymede in the 2030s.