Ganymede is the largest and most massive moon in the Solar System — larger in diameter than Mercury (though only ~45% of its mass). It is the only moon known to generate its own intrinsic magnetic field, created by circulation in its liquid iron core. This small magnetosphere is nested inside Jupiter’s much larger one, producing auroral phenomena that helped Hubble confirm the existence of Ganymede’s subsurface ocean in 2015 by measuring the way Ganymede’s auroral belts rock in response to Jupiter’s shifting field.

Ganymede is fully differentiated, with an iron-rich core, silicate mantle, a layer of ice that includes high-pressure ice phases, and a liquid water ocean estimated at 100 km thick sitting ~150 km below the icy surface.

Its surface shows two major terrain types: dark, heavily cratered regions roughly 4 billion years old, and bright, younger grooved terrain cut by extensive tectonic features. The European Space Agency’s JUICE (Jupiter Icy Moons Explorer) mission — launched 14 April 2023 — will arrive at Jupiter in July 2031 and in 2034 will become the first spacecraft ever to orbit a moon other than Earth’s when it enters orbit around Ganymede.