Iapetus is one of the moons of the planet Saturn. It is the third-largest in Saturn's moon system (of 62 moons, at last count), and is best known for the bizarre ridge which rings the moon along its equator.
Saturn, with 62 moons, has far more companions than any other planet except Jupiter (with 63 known moons). However, only a small number of relatively large moons, like Iapetus, are of particular interest to astronomers - and all of them, in recent years, have tended to be relegated to one side as scientists focus on Titan, the only moon in the solar system to have its own thick methane atmosphere. Iapetus has a long history of observation, however. It was first discovered by European astronomers in the 17th century. Like Saturn's other moons, it was given a name from the Titans of Greek mythology - in this case, the father of Atlas and Prometheus.
Iapetus has a radius of about 735 kilometres (460 miles), large enough that it has been compressed into a spherical shape by the force of its gravity - although, unlike other relatively large moons, in Iapetus's case this transformation was only partial, so that the moon is not truly spherical. It is relatively lightweight, suggesting that much of the interior of the moon consists of ice rather than rock. It orbits farther from Saturn than the larger Titan.
Since its discovery, there have been several strange features of Iapetus that puzzled astronomers. The first was the strange realization that while they could see Iapetus as it orbited on one side of Saturn, they could not see it on the other. This appeared to violate the basic principle that celestial objects follow roughly circular or elliptical orbits around larger bodies. The theoretical answer - eventually confirmed - was that Iapetus was tidally locked, meaning that the moon's day and orbital period last the same amount of time. The same process affects Earth's Moon; in both cases, it means that one face of the moon always faces its host planet. One side of Iapetus, astronomers reasoned, must be darker than the other - and this side always faced Earth during part of the moon's orbit, effectively making it almost invisible for astronomers using the early telescopes of the period.
Subsequent images of Iapetus were able to confirm this theory, but they led to another mystery. In 2004, some of the first high-quality images of Iapetus were taken by the Cassini space probe. They turned up something unexpected: a massive ridge, in many parts more than twice as high as Mt. Everest, running all the way around the moon along its equator. Scientists still have no agreed-upon explanation for Iapetus's ridge, although there are some theories that it resulted from changes in ice formations below the surface, or as a result of some unique way in which the moon formed.