Ganymede and Callisto compared 9.16
Ganymede (left) and Callisto (right) seen at the same relative scales. The distinct pale and dark terrains on Ganymede are obvious. Callisto has no such distinction, and is dark all over (its brightness has been exaggerated here to help it show up). The bright spots on both bodies are relatively young impact craters, made prominent by their surrounding blankets of pale ejecta.
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Image Credit: NASA, David A. Rothery (text)