Callisto: A Moon of Jupiter
Unlike the other three inner moons, Calisto’s surface is heavily cratered
Callisto is the outermost of Jupiter’s four “Galilean moons” — so called because they were first seen in 1610 by Galileo.
Callisto is the second largest of the four, having a diameter of 4800 kilometres (3000 miles), which makes it appreciably larger than our own Moon (3500 kilometres, 2200 miles) and about the same size as the planet Mercury. It is the third-largest moon in the Solar System, after Ganymede (Jupiter) and Titan (Saturn).
It orbits Jupiter at a distance of about 1.9 million kilometres (1.2 million miles) and takes nearly 17 days to complete one orbit.
Callisto is different from its inner neighbours in that it has a dark surface that is scarred by a multitude of craters. These will have remained unchanged ever since their creation, due to Callisto not having been subject to gravitational tides in the way that Io, Europa and Ganymede have, which would create volcanic activity that in turn would alter surface features.
However, Callisto is subject to Jupiter’s gravity in the same way that our own Moon is governed by that of Earth, so that the same side always faces the host planet.