The final of the four large Galilean moons is Callisto. It is the third largest moon in the Solar System, about 4890 kilometres wide, just over the width of Mercury (by only 10 kilometres!) but more than twice the size of Pluto! It is a more familiar-looking moon than the three other large moons of Jupiter, resembling Earth's moon, and has a surface more cratered than any other moon in the Solar System.
It is believed that Callisto has a large rocky core and is made up mainly of water ice and rock. The bright spots on its surface are rings around craters where water has spread and frozen, reflecting brightly in the sunlight. The moon is unlikely to have changed for millions of years, with impact craters on the surface remaining from the formation of the Solar System four billion years ago!