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

The three moons in this picture show three possible stages in the history of an icy moon. The moon can be cold. In that case, the surface is unchanged and very cratered. This case may be like Callisto, the top moon in the picture.

If there was some heat inside, then the surface may change and will not be as cratered. This case may be like that of Ganymede, the moon in the middle of the picture. Ganymede has many craters, but also grooves which indicate that the surface flowed at some point in time.

If the heating took place for a long time, then there may be many changes on the surface. This case may be like that of Europa, the third moon in the picture. The surface of Europa has lots of cracks and almost no craters.

Many moons in the solar system are somewhere between Ganymede and Europa in their history. Examples of these moons include Dione, Rhea, Enceladus, Tethys, Ariel, Umbriel, Miranda, Titania, Oberon, and Triton.

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