Ganymede, bigger than Mercury, may hold more water than Earth
Jupiter's moon Ganymede is the largest moon in the Solar System, wider than the planet Mercury. Beneath its icy crust lies an ocean that may contain more water than all of Earth's seas together, and it is the only moon known to generate its own magnetic field.
Ganymede is roughly half silicate rock and half water. It has separated fully into layers around an iron-rich liquid metal core, making it the most centrally concentrated solid body in the Solar System. That churning core probably drives its magnetic field, which is also shaped by Jupiter's far stronger one. A very thin atmosphere of oxygen surrounds it. Being less dense than Mercury, Io or our own Moon, it has slightly weaker surface gravity than any of them despite its size.
It circles Jupiter in about seven days, locked in a 1 to 2 to 4 rhythm with Europa and Io. Its surface mixes two kinds of ground. Lighter terrain, crossed by long grooves and ridges and covering two-thirds of the moon, formed a little under 4 billion years ago, possibly through tectonic activity powered by tidal heating. Darker regions are packed with impact craters and are older still.
Galileo Galilei spotted it in January 1610, at first taking Jupiter's companions for stars; within days he saw them move and realised they orbited the planet. Simon Marius observed it that same year and named it after Ganymede, the Trojan prince Zeus carried off to be cupbearer to the gods. Galileo preferred to honour his patron Cosimo II de' Medici. The mythological names fell out of favour, and for centuries astronomers called the moon Jupiter III until the mid-20th century. A Chinese record from 365 BC describes the astronomer Gan De seeing a small reddish star beside Jupiter, possibly Ganymede, though the colour is puzzling.
Spacecraft from Pioneer 10 onward have visited. The Voyagers refined its size, and the Galileo probe found the hidden ocean and the magnetic field. The European Space Agency's Juice mission, launched in 2023, plans to fly past the three icy Galilean moons and then settle into orbit around Ganymede.
Source: Ganymede (moon)
