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4. Callisto

Callisto has been called "boring" since its features are not as remarkable as the other three main moons. Callisto is also the darkest of the moons, but is in fact brighter than our Moon (by reflection - called Albedo).

The surface of Callisto is best described as "dirt ice" and is covered with an unknown dark mineral deposit.

Callisto is 4,800 km in diameter and is 1,883,800 km away from Jupiter.

Like Ganymede, Callisto does have a tenuous atmosphere composed of mostly carbon dioxide. It is believed the gas emanates from the poles - where its cold enough for carbon dioxide ice to form. This is a surprise since the Voyager data suggested Callisto is a "dead world" meaning there is no magnetic field, no geological activity and no atmosphere.

The feature above is called Valhalla Basin, and is one of the larger impact craters discovered. The multi-ring appearance is a result of the shock of impact.

While geologic activity is not evident, what is present is a small magnetic field. Because its believed Callisto has a very small non-metallic core, a liquid ocean under the crust might be responsible.

The image above demonstrates a large number of impacts - evidence that little or no geologic activity is present.

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