Although Stanley Kubrick's 1966 film version only went as far as Jupiter,  the novel upon which it was based - 2001: A Space Odyssey - took the spacecraft Discovery all the way to Saturnian moon of Iapetus.

Iapetus has long mystified scientists because one side was brighter than the other. In Arthur C. Clarke's novel, the "bright" side was described as a long, smooth white oval, obviously artificial. Photos from the Cassini have shown that the mysterious bright spot definitely is not the product of extra-terrestrial engineering. It has, however, revealed a new feature: a narrow ridge around its middle.

About 13 miles  (20 km) wide and  800 miles (1300 km) long, it stretches along Iapetus' equator like the ridge on a walnut. At the moment, scientists can only guess at what it is. Current theories suggest that it is either a fossil ring system that fell to the surface, or a pile up of crustal rocks formed as the satellite changed its shape. In either case, analysis of impact craters indicate that the ridge is nearly as old as the rest of the solar system, thought to have reached its present configuration about 4.5 billion years ago.

One school of thought states that the ridge formed as the result of the slowing of Iapetus' rotation, which is estimated to have been 10 hours in the beginning, but has since slowed to 80 days. Because of its initial rapid spin while the body had not yet solidified, strong centrifugal force might have caused the equator to bulge outwards. (This actually happens on Earth, but the bulge is so slight - a few miles around the equator - that it not noticeable to observers.)

According to Julie Castillo of NASA's Jet Propulsion Laboratory, the initial rate of rotation would have caused the young moon to be about 1.5 times wider across the equator than from pole to pole. Eventually, Saturn's tidal forces would have slowed the spin, making Iapetus became more spherical. This would have caused the surface area to shrink, leaving the moon with an excess of solid crust.  Castillo says the big ridge is leftover crust that has piled up along the equator.

Wing-Huen Ip of the National Central University of Taiwan puts forth an alternative: the ridge might be the result of falling debris from rings that once circled the moon, which would have fallen to the surface in a narrow strip along the equator.