|The outermost of the four Galilean Moons of Jupiter circles its
parent planet at a distance of a little under two million kilometres, making it nearly twice as distant from
Jupiter as any of its three bright companion moons,
Ganymede, Europa and Io. It has a dark surface speckled with bright
craters, often surrounded by rings of material. Its largest surface feature is a wide circular
region about 600 kilometres in diameter, Valhalla, which is surrounded by circular patterns
extending outward across Callisto's surface.