CALLISTO

The most cratered world

Mass: 1.1 x 1023kg (2.4 x 1023lbs) Diameter: 4,821km (2,996 miles) Parent planet: Jupiter Discovered: 1610, Galileo Galilei

The outermost of Jupiter’s Galilean moons, Callisto is the third-largest moon in the Solar System and is only slightly smaller than Mercury. Its main claim to fame is the title of most heavily cratered object in the Solar System; its dark surface is covered in craters down to the limit of visibility, the deepest of which have exposed fresh ice from beneath and scattered bright ‘ejecta’ debris across the surface.

Callisto owes its cratered surface to its location in the Jupiter system – the giant planet’s gravity exerts a powerful influence, disrupting the orbits of passing comets and often pulling them to their doom, most spectacularly demonstrated in the 1994 impact of Comet Shoemaker-Levy 9.

Jupiter’s larger moons are directly in the firing line and end up soaking up more than their fair share of impacts, but Callisto’s inner neighbours – influenced by greater tidal forces – have all experienced geological processes that wiped away most of their ancient craters. Callisto’s surface, however, has remained essentially unchanged for more than 4.5 billion years, developing its dense landscape of overlapping craters across aeons.