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
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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The surface of Dione does not have many craters, which is an indication of changes to the surface in the past. Instead it has wispy white streaks similar to those found on Rhea extending for many kilometers
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The surface of Rhea is typical of an icy moon. Rhea is as heavily cratered as Mimas on its leading hemisphere. Rhea's trailing hemisphere is dominated by unusual white streaks extending for many kilometers
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The surface of Enceladus does not have many craters. Instead it has grooves similar to those found on Ganymede. These grooves extend for many kilometers over the surface. The presence of grooves indicates
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The surface of Tethys is heavily cratered, which means that the surface is very old and hasn't been changed. Tethys is more lightly cratered in one region, however. This may be a sign of some internal
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The surface of Umbriel seems to be like many icy moons. It appears to have many craters, but is not as heavily cratered as Callisto.
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The surface of Miranda is very unusual. It is not like any other moon in the solar system. Miranda has many craters but also very big grooves. These indicate that there has been activity inside Miranda
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