This drawing shows the evolution of Callisto.
Click on image for full size
NASA
Evolution of Callisto
Measurements by the Galileo spacecraft have been shown that Callisto is the same inside from the center to the surface. This means that Callisto does not have a core at the center.
This means that, unlike Ganymede, Callisto probably never warmed up enough while it was forming to allow for the materials inside to separate.
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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