Callisto's battered surface has barely changed in billions of years
Jupiter's moon Callisto wears the oldest, most crater-packed surface in the Solar System, with almost no sign of volcanoes or shifting plates. It orbits too far out to be kneaded by Jupiter's tides like its sibling moons. Yet under that frozen shell may lie a salty ocean, and its calm radiation makes it a candidate base for human explorers.
Callisto is Jupiter's second-largest moon and the third-largest in the Solar System, nearly as wide as Mercury at 4,821 kilometres but with only about a third of its mass. Simon Marius and Galileo spotted it independently in 1610, and today it can be seen through ordinary binoculars. Marius named it after a nymph companion of Artemis courted by Zeus, crediting the suggestion to Johannes Kepler, though astronomers mostly called it Jupiter IV until the mid-twentieth century.
Its distance shapes everything. Orbiting about 1.88 million kilometres out, Callisto stays outside the rhythmic orbital lock that binds Io, Europa and Ganymede, so it escapes appreciable tidal heating. It keeps one face permanently toward its planet, so Jupiter seems to hang overhead on the near side, and a day there lasts about 16.7 Earth days. Calculations suggest it may be drawn into that orbital lock in about 1.5 billion years.
Roughly half rock and half ice, Callisto has the lowest density of Jupiter's big moons, and it may be only partly differentiated, with rock gradually concentrating toward the centre rather than forming neat layers. Its surface mixes bright specks of frost on high points with smooth dark blankets, probably left as ice slowly vaporises and wears small features away. Unusually, its leading hemisphere is darker than its trailing one, the reverse of the other Galilean moons.
The most intriguing clue came from magnetism. The Galileo spacecraft found that Callisto responds to Jupiter's shifting magnetic field like a perfect conductor, implying a layer of electrically conductive fluid at least 10 kilometres thick, probably a salty ocean beneath an icy crust 80 to 150 kilometres deep. Conditions for life there are thought less promising than on Europa. Its surface radiation, around 300 times lower than Europa's, is still just over ten times Earth's background level, which is lower than in low Earth orbit and on Mars.
Source: Callisto (moon)
