The excitement of scientists upon Cassini-Huygens entering the Saturnian System was reserved mostly for Titan, Saturn itself, and its rings. That the other moons might be something more than cratered and dead ice bodies was hardly expected.
Enter Iapetus. This strange moon between Phoebe and Titan helped write the exciting new chapter of Saturn moon exploration. Upon the side of Iapetus facing toward its motion around Saturn is a coating of debris “as dark as asphalt,” according to the official NASA Cassini-Huygens page for the moon. The other half of the moon is “bright as snow.”
Perhaps even more surprising was the discovery of a mountain range that neatly straddles the equator of the moon, a ring around Iapetus’ middle that makes little sense at all. Some scientists believe that the formation of this mountain range may provide an explanation for the dark debris, although others believe the debris falls onto the surface from an outside sources.