Callisto

Callisto is the outermost of the Galilean moons. It is roughly the same size as Mercury but has a very different history. It is often described as the most “battered” world in the solar system because its surface is completely covered in impact craters.

A Frozen Record

Callisto’s surface is extremely old—about 4 billion years old. Since the moon has no volcanoes or tectonic activity to erase them, every asteroid or comet that has ever hit Callisto has left a mark. This makes its surface a frozen record of the solar system’s history of bombardment.

One of its most prominent features is Valhalla, a massive multi-ring impact structure that stretches 3,800 kilometers across the moon.

A Dead World?

For a long time, scientists thought Callisto was a geologically dead world. However, data from the Galileo spacecraft suggests it too might hide a salty ocean beneath its thick, icy crust. Unlike Io, Europa, and Ganymede, Callisto is not heated significantly by tidal forces, so any ocean would have to be kept liquid by radioactive heat from the moon’s core or anti-freeze chemicals like ammonia.