Iapetus

Iapetus (Saturn VIII) was discovered by Cassini in 1671. Its period of revolution is 79 days 8 hours.

One hemisphere of Iapetus is very dark and the other very light. It is the front face of the satellite which, similarly to Tethys, is the darkest.

Image of the surface of Iapetus taken by the Cassini probe on 10 September 2007. The bright face is very visible Credit: JPL/Nasa

The fact that Iapetus has two such different hemispheres is not yet well understood, and is one of the many puzzling things about this moon of Saturn.

On the equator of Iapetus there is a mountain chain of unknown origin. One theory is that it is the remains of an old ring. According to another theory, it is the trace of an ancient epoch when Iapetus may have intersected the rings of Saturn. But these two theories seem very unlikely now.

Image of the mountain chain on Iapetus on the dark hemisphere.. Credit: JPL/Nasa

The bright face of Iapetus is mostly water ice but another component was detected by Cassini during its close fly-by on 10 September 2007.

Detail of the white face. Credit: JPL/Nasa

Iapetus is mythical for fans of Arthur C. Clarke's, 2001, Space Odyssey, because it is where David Bowman meets the second black monolith that actually contains a wormhole.