This drawing shows the evolution of Callisto.
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Evolution of Callisto
Most of the moons and planets formed by accretion of rocky material and volatiles out of the primitive solar nebula and soon thereafter they differentiated.
Measurements by the Galileo spacecraft have been shown that Callisto is completely homogeneous inside, and never differentiated. 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:
The lack of warmth prevented any movements of the crust which would have changed the appearance of the surface over time.
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