The Mystery of the Two Faces of Iapetus

Although Saturn's moon Iapetus was first discovered in 1671 by Giovanni Cassini, its behavior was extremely odd. Cassini was able to regularly find the moon when it was to the west of Saturn, but when he waited for it to swing around to Saturn's east side, it seemed to vanish. It wasn't until 1705 that Cassini finally observed Iapetus on the eastern side, but it took a better telescope because the side Iapetus presented when to the east was a full two magnitudes darker. Cassini surmised that this was due to a light hemisphere, presented when Iapetus was to the west, and a dark one, visible when it was to the east due to tidal locking.

With the advances in telescopes, the reason for this dark divide has been the subject of much research. The first explanations came in the 1970?s and a recent paper summarizes the work done so far on this fascinating satellite as well as expanding it to the larger context of some of Saturn's other moons.

The foundation for the current model of Iapetus' uneven display was first proposed by Steven Soter, one of the co-writers for Carl Sagans Cosmos series. During a colloquium of the International Astronomical Union, Soter proposed that micrometeorite bombardment of another of Saturn's moons, Pheobe, drifted inwards and were picked up by Iapetus. Since Iapetus keeps one side facing Saturn at all times, this would similarly give it a leading edge that would preferentially pick up the dust particles. One of the great successes of this theory is that the center of the dark region, known as the Cassini Regio, is directly situated along the path of motion. Additionally, in 2009, astronomers discovered a new ring around Saturn, following Phoebe's retrograde orbit, although slightly interior to the moon, adding to the suspicion that the dust particles should drift inwards, due to the Poynting-Robertson effect.…

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