This is a composite image of three of the Galilean satillites.
Click on image for full size
NASA
Evolution of Icy Moons
The three moons to the left illustrate three possible stages in the
history 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 heating, 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 the heating took place over a long time, 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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