Ganymede

Jupiter's moon Ganymede measures 5,268 km across, which makes it wider than the planet Mercury. It is the only moon known to have its own magnetic field.

Mercury is wider but Ganymede is less massive, because Mercury is far denser.

Context

Ganymede was first seen by Galileo Galilei in January 1610, along with Jupiter's other three large moons. Its diameter of 5,268 km makes it larger than Mercury (about 4,880 km) and also larger than Pluto, yet it holds only about half of Mercury's mass. The reason is composition: Ganymede is roughly half rock and half water ice, while Mercury is mostly metal and rock.

NASA's Galileo spacecraft flew past in the mid-1990s and detected a magnetic field around the moon, which implies a partly liquid metallic core generating it. Later magnetic and Hubble observations of auroral activity supported the idea of a salty ocean buried beneath the icy crust, possibly sandwiched between layers of ice. Estimates of its depth and thickness vary and remain debated.

The moon matters for the search for habitable environments. ESA's JUICE mission, launched in 2023, is designed to study Ganymede in detail and eventually enter orbit around it in the 2030s, making it the first spacecraft to orbit a moon other than our own. Ganymede orbits Jupiter in about seven days and is locked so that one face always points toward the planet, as our Moon does toward Earth.