The insides of most of the moons and planets separated while they were forming out of the primitive solar nebula.

Measurements by the Galileo spacecraft have been shown that Callisto is the same inside from the center to the surface, and did not form a core. This means that, unlike Ganymede, Callisto probably never warmed inside enough to allow for the materials to separate. Planets gain heat by many means. The fact that Callisto never warmed could mean one of many things:

• insufficient warming from forces leading to tides
• insufficient abundance of radioactive elements

The lack of warmth prevented any movements of the crust which would have changed the appearance of the surface over time.