Equatorial ridges are a feature of at least three of the moons of Saturn : the largest moon Iapetus and the tiny moons Atlas and Pan . They are ridges that closely follow the equators of the moons. They appear to be unique to the Saturn system, but whether the occurrences are related or coincidental is unknown. All three were discovered by the Cassini probe in 2005. Daphnis also appears to have such a ridge.
The Lapetus ridge is almost 20 km wide, 13 km high and 1,300 km long. The ridge in Atlas is proportionally even more remarkable given the much smaller size of the moon, giving it a disk shape. Pan’s images 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 Saturn’s rings, one likely explanation for their ridges is that they sweep the particles from the ring as they orbit, which accumulate around their equators. This theory is less applicable to Lapetus, which orbits far beyond the rings. A scientist has suggested that Japetus swept a ring before it was ejected into its current and distant orbit.1 Others think it was stationary and it is the rings that have withdrawn, falling into Saturn’s gravity field. [Quote needed] Perhaps more likely is the theory that because Japet has an unusually large Hill sphere compared to other moons in the Solar System, he may once have had his own ring, or even a minor moon that slowly approached, broke into a ring, and then gradually increased at the equator of Japet. But most scientists prefer to assume that the Iapetus ridge was produced by some kind of internal source and is unrelated to the Atlas and Pan ridges.
Another suggested theory is that low-speed collisions between moons could have formed the bulge in the center, although the circumstances for such an event to occur are scarce.