Take a good look at Saturn's moon Iapetus and it has a few striking features that set it apart from every other object in the solar system. For one thing, it seems to have two faces: one white, like freshly fallen snow, and the other dark like volcanic rock. But even stranger, Iapetus has a seam. Right at the equator, and going halfway around the planet, it's probably 20 km (12 miles) high - as if the moon was cut in half and then smashed back together. Planetary geologists have assumed this seam is volcanic in origin, but Paulo C.C. Freire of the Arecibo Observatory has another suggestion. In the distant past, Iapetus gobbled up one of Saturn's rings.