Callisto

Callisto is the outermost of Jupiter’s four large moons. It is 4 821 kilometres across, almost exactly as wide as Mercury, but it has only about a third of Mercury’s mass, because it is roughly half water ice. Its surface is the most heavily cratered in the Solar System: craters of every size, packed so close that new ones can only land on old ones. For a long time it was the moon nothing happened on. It may still hide an ocean.

The quiet moon

Io, Europa and Ganymede are locked into a rhythm of orbits that keeps flexing them with Jupiter’s tides, and the flexing heats their insides. Callisto takes no part in it. It circles Jupiter every 16.7 days at 1.9 million kilometres, far enough out to be left alone, and it has never been heated much by tides. Without that heat, its surface has barely changed. The European Space Agency says it has not been geologically active for a billion years or so.

What is left is a record of impacts. The largest is Valhalla, a bright centre about 600 kilometres across surrounded by rings that reach about 4 000 kilometres across, one of the largest impact features in the Solar System. Bright frost clings to crater rims and ridges, while darker, ice-poor dust collects in the low ground, so the moon looks like a dark globe sprinkled with white.

“A dead and boring moon”

That was how Kivelson described what scientists had thought of Callisto, when the Galileo spacecraft surprised them in 1998. As Callisto moves through Jupiter’s magnetic field, Galileo’s magnetometer picked up a magnetic signal induced inside the moon. The best explanation was a layer of salty, electrically conducting water beneath the ice.

The case has gone back and forth since. In 2017 Hartkorn and Saur showed that Callisto’s thin, electrically charged upper atmosphere could produce much of the signal, so the ocean might be deeper than thought or not exist at all. In 2025 Cochrane and colleagues analysed all eight of Galileo’s close flybys together and found that an ocean plus that upper atmosphere fits far better than the atmosphere alone. The ocean would lie under tens to hundreds of kilometres of ice.

A new inside

Callisto was long described as a moon whose rock and ice never fully separated. In August 2026 Cascioli and colleagues reanalysed Galileo’s tracking data, including a flyby never studied before, and concluded that it is more separated into layers than believed, with a large rocky core and an interior closer to Titan’s. In their model the ocean could be about 300 kilometres thick, though that figure depends on the model.

The answer should come in the 2030s. Europa Clipper will use nine flybys of Callisto to steer itself, and ESA’s Juice will pass it 21 times from June 2032, as close as 200 kilometres.