Iapetus may be Saturn's most enigmatic moon, with an unknown dark material splattering most of the surface and a strange ridge that makes one side of the moon resemble a walnut. Astronomers hope the Cassini spacecraft can unlock its mysteries.

Iapetus travels in a tidally locked orbit with Saturn, always showing the same face to the planet. When Cassini Regio, the moon's dark hemisphere, faces the Earth, however, it nearly disappears from view, coated entirely in a mysterious black substance that is gradually creeping across the sphere. Astronomers have long speculated on the nature of the substance, which gives the moon an unusually uniform surface, and while many believe that it is composed of the dirt left behind when ice on Iapetus sublimates, they hope that new data from the Cassini spacecraft will offer more concrete clues, as well as help them understand the nature of the equatorial ridge that travels across Cassini Regio, an odd phenomenon in a surface otherwise marked by impact craters.