Observatory - Cassini Data Solves Mystery of Iapetus's Two-Tone Surface - NYTimes.com Researchers have solved what may be the oldest mystery in planetary science: the two-tone surface of Saturn's moon Iapetus. The odd feature - the moon's trailing side is about 10 times brighter than its leading side - has been a mystery since it was first observed by Giovanni Cassini in 1671. In two papers published online by Science, researchers have unraveled the mystery, using images and data from instruments aboard the spacecraft named for Cassini. The studies confirm an earlier idea that dust, most likely from another of Saturn's moons, falls on the leading side of Iapetus as it orbits the planet. "It's just like a motorcyclist, who only gets the flies on the leading side of the helmet rather than the trailing side," said Tillmann Denk of the Free University of Berlin, an author with John R. Spencer of the Southwest Research Institute of one of the papers and lead author of the other. But the pattern of the surface features - the dark area extends to the trailing side at the equator, for example - is not fully explained by the deposition dust. Rather, the researchers say, the reason has a lot to do with the moon's rotation on its axis, which takes 80 earth days. Such a slow rotation ("midday" lasts for a couple of weeks) allows the distant Sun to warm the dark dust-covered areas enough that water ice becomes vapor. The vapor migrates elsewhere, freezing to ice again when it reaches colder areas. The areas where the ice was lost become darker, and those that gained ice become brighter.