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CALLISTO is the outermost of Jupiter’s largest moons. Among the planet’s 63 known moons, it is ranked second in terms of size. However, it is the third largest moon in the Solar System.

Along with Io, Europa and Ganymede, Callisto was discovered by the Italian, Galileo Galilei, in January 1610. In honour of his Medici patrons, he called them the Medicean Planets and designated them as: I (Io), II (Europa), III (Ganymede), and IV (Callisto). Their current names were not formally adopted until the mid-1800s.

It is thought that Callisto has a small rocky core surrounded by a large icy mantle. Observations made by the Galileo space probe (eight close encounters from 1994 to 2003) hinted at the possibility of a liquid water ocean beneath Callisto’s surface, at depths greater than 100 km.

Callisto is the most heavily cratered moon in the Solar System. Its largest crater has been named Valhalla. The impact site is surrounded by a series of concentric rings and stretches some 3,000 km in diameter.

Virtually geologically inactive, Callisto is the only body in the Solar System, greater than 1,000 km in diameter, to show little signs of extensive resurfacing.

Callisto’s surface has scarcely changed since the first impacts that moulded it. Estimated to be 4 billion years old, Callisto’s surface is the oldest in the Solar System.

Callisto

Photo taken by Voyager II on July 7, 1979

Average distance from Jupiter: 1,883,000 km (219,965 miles)

Diameter: 4,821 km (2,985 miles)

Orbital period: 16.69 days

Rotational period: 16.69 days

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