Exploratour - Evolution of the Solar System
The temperature difference which played a role in the formation of the planets seems also to have played a role in the formation of these moons. Just as the planets became more and more icy the further from the sun, moons seem to be more and more icy the further from the planet they orbit. For example, Jupiter's moon Io (close to Jupiter) is very rocky, but Europa (farther from Jupiter) has more ice, Ganymede and Callisto (further still from Jupiter) have lots more ice.
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