Ganymede

The largest moon in the Solar System — bigger than Mercury

Ganymede is the biggest moon of all. It is even bigger than the planet Mercury! Ganymede has its own magnetic field — the only moon that does. Its surface has dark old patches and bright young stripes, like a patchwork quilt. It probably has an ocean under its ice too.

Ganymede (radius 2,634 km) is the largest moon in the Solar System, bigger than Mercury (but lighter, since it is half ice). It orbits Jupiter every 7.15 days. It is the only moon with its own magnetic field — its iron core is liquid. The surface comes in two kinds: dark, ancient, cratered terrain (4 billion years old) and bright, young grooved terrain. Hubble data suggest a salty ocean 100 km deep beneath 150 km of ice. Ganymede, Io, Europa and Callisto are the Galilean moons (1610). ESA's JUICE spacecraft (launched 2023) will enter orbit around Ganymede in 2034 — the first craft to orbit another planet's moon.

Fun facts

• Ganymede is bigger than Mercury, but it's not a planet — because it orbits Jupiter.
• Ganymede has auroras, thanks to its own magnetic field.
• If Ganymede orbited the Sun instead of Jupiter, it would count as a planet.