IAPETUS

A world with two faces

Iapetus is a medium sized moon of Saturn being 1118 km in diameter. Like most other moons, Iapetus keeps one face toward Saturn. That means one hemisphere always faces forward, in the direction of rotation.

The trailing face of Iapetus is bright icy material, like many of the other moons. The leading hemisphere however, is black as tar. Astronomers believe that there are two possibilities for this blackness. It is possible that black material is being blasted off Phoebe, the next outermost moon, by meteoritic impacts. The material spirals inward and coats Iapetus' leading face.

The other explaination is that some materials darken in color when exposed to cosmic rays. Whatever the reason, either the black material is very thick, or it gets replenished often. There are no bright spots or craters visible on that portion of Iapetus.