Equatorial ridges are a feature of at least three of Saturn's moons: the large moon Iapetus and the tiny moons Atlas and Pan. They are ridges that follow closely the moons' equator. They appear to be unique to the Saturnian system, but it is uncertain whether the occurrences are related or a coincidence. All three were discovered by the Cassini probe in 2005.
The ridge on Iapetus is nearly 20 km wide, 13 km high and 1,300 km long. The ridge on Atlas is proportionally even more remarkable given the moon's much smaller size, and distorts the moon's shape into an odd, UFO-like appearance. Images of Pan are less clear, but show a structure similar to that of Atlas.
It is not certain how these ridges formed, or whether there is any connection between them. Because Atlas and Pan orbit within the rings of Saturn, a likely explanation for their ridges is that they sweep up ring particles as they orbit, which build up around their equators. This theory is less applicable to Iapetus, which orbits far beyond the rings. One scientist has suggested that Iapetus swept up a ring before being somehow expelled to its current, distant orbit. Others think it was stationary and it is the rings that have been pulled away from it, falling into Saturn's gravity field. But most scientists prefer to assume that Iapetus's ridge was produced by some kind of internal source and is unrelated to the ridges on Atlas and Pan.