Ganymede

Ganymede is the largest moon in the solar system — a world 5,268 km across, wider than the planet Mercury and about three-quarters the diameter of Mars. It circles Jupiter every 7.15 days at a distance of roughly 1.07 million km, the third of the four great Galilean moons counting outward from the planet. If Ganymede orbited the Sun on its own rather than Jupiter, there would be little argument about calling it a planet.

What sets it apart from every other moon is not just its size but its interior. Ganymede is the only moon known to generate its own magnetic field, a discovery that turned a large ball of rock and ice into one of the more intriguing worlds in the outer solar system — and a prime destination for the spacecraft now on its way there.

Discovery and a borrowed name

Galileo Galilei first recorded Ganymede on 7 January 1610, among the four points of light he saw shifting from night to night beside Jupiter — the observation that proved not everything in the heavens circled the Earth. Galileo numbered the moons rather than naming them, and called the group the Medicean Stars after his Florentine patrons.

The name we use today came from Simon Marius, a German astronomer who observed the moons at nearly the same time and, at Johannes Kepler's suggestion, proposed names drawn from the loves of Zeus. Ganymede was a Trojan prince carried off by the king of the gods to serve as cupbearer on Olympus — the only Galilean moon named for a male figure. Marius's names sat largely unused for centuries, with astronomers preferring 'Jupiter III', and were not widely adopted until the mid-20th century.

Two faces of ice

Ganymede's surface is a study in contrasts, split between two very different kinds of terrain. About 40 percent is dark, ancient, and heavily cratered — most prominently Galileo Regio, a vast dusky plain in the northern hemisphere marked with concentric furrows that may be the scar of an enormous early impact. This dark terrain is among the oldest surface in the Jupiter system, battered for some four billion years.

The remaining 60 percent is brighter and younger, cut by long systems of parallel ridges and grooves that run for thousands of kilometres and rise up to several hundred metres. These grooved lanes appear to record episodes when Ganymede's icy crust was stretched and fractured by tectonic forces, cleaner ice welling up along the faults. The result is a globe crisscrossed by pale bands slicing through darker patches — a frozen map of a moon that was geologically restless long ago.

A hidden ocean and a magnetic field

Beneath the ice, Ganymede is layered like a small planet: a metallic iron core, a rocky mantle, and a thick outer shell of ice and water. Circulation in that liquid-metal core is the leading explanation for the moon's magnetic field, detected by NASA's Galileo orbiter in 1996 — the only such field ever found around a moon. It carves out a small magnetosphere inside Jupiter's own, complete with polar aurorae.

Those aurorae became the key to another discovery. In 2015, astronomers using the Hubble Space Telescope watched how Ganymede's auroral belts rocked back and forth as Jupiter's magnetic field swept past. The muted rocking pointed to a buried, salty, electrically conducting layer — a subsurface ocean. Estimates place it under roughly 150 km of ice, perhaps 100 km deep, holding more water than all of Earth's oceans combined, though it lies far deeper and colder than the ocean suspected inside neighbouring Europa.

Locked in a cosmic rhythm

Ganymede does not orbit alone. It is bound to the inner two Galilean moons in a precise gravitational lockstep called the Laplace resonance: for every single orbit Ganymede completes, Europa makes two and Io makes four. This 1:2:4 clockwork keeps the moons' orbits slightly stretched, and the resulting tidal flexing helps heat their interiors — spectacularly so at volcanic Io, more gently at Ganymede, where past tidal heating may have helped drive the tectonics that shaped its grooved terrain. Like all four Galileans, Ganymede is tidally locked, keeping one hemisphere permanently facing Jupiter. Its surface is bitterly cold, with daytime temperatures ranging from about 90 to 160 kelvin (roughly −180 to −110 °C) between polar and equatorial regions.

The missions that reached it

Ganymede was first seen close-up by the Pioneer and Voyager flybys of the 1970s, but the moon's real reconnaissance came from NASA's Galileo orbiter, which studied the Jupiter system from 1995 to 2003 and found the magnetic field and evidence of a possible ocean. NASA's Juno spacecraft made a fast, high-resolution pass about 1,040 km above the surface on 7 June 2021, returning the sharpest images of Ganymede in two decades.

The moon's future belongs to Europe. ESA's Jupiter Icy Moons Explorer, JUICE, launched in April 2023 and is due to arrive at Jupiter in 2031. After a series of flybys of all three icy Galileans, it is planned to settle into orbit around Ganymede in December 2034 — the first time any spacecraft will have orbited a moon other than our own — to probe the ice shell, map the surface, and characterise the ocean below.

How to spot it yourself

You don't need a spacecraft to find Ganymede. At around magnitude 4.6 it would, in principle, be visible to sharp eyes if it weren't drowned in Jupiter's glare. A pair of steady binoculars is enough to reveal it as a tiny star-like point strung out in a line with Io, Europa, and Callisto on either side of the planet. Watch across a few nights and the moons visibly shift position — the same wandering dance Galileo tracked in 1610. A small telescope shows the line-up more cleanly, and Ganymede, as the brightest of the four, is usually the easiest to pick out.