This is a composite image of three of the Galilean satillites.
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Evolution of Icy Moons

The three moons to the left illustrate three possible stages in the evolution of an icy satellite. The satellite can be cold and so have no internal activity. In that case, the surface is unchanged, old, and heavily cratered. This case may be illustrated by Callisto, the top moon in the picture. The craters are left over from the formation of the moon itself, 4 billion years ago. Nothing has happened to this moon to ever change its surface appearance.

If there was some internal warming, then the surface may show some changes and will not be as heavily cratered. This case may be illustrated by Ganymede, the moon in the middle of the picture. Ganymede has many craters but also trenches and grooves which indicate that the surface flowed at some point in time.

If there was prolonged warming, then the surface may show many changes, and in fact may still be evolving. This case may be illustrated by Europa, the third moon in the picture. The surface of Europa is lightly cratered with evidence of cracks and fractures.

Many moons in the solar system exhibit features of evolution somewhere between those of Ganymede and those of Europa. Examples of these moons include Dione, Rhea, Enceladus, Tethys, Ariel, Umbriel, Miranda, Titania, Oberon, and Triton.

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