Equatorial ridge

Lastly, the very small moon Daphnis, discovered by the Cassini in 2005, also appears to have such a ridge.

The ridge on Atlas is proportionally even more remarkable given the moon's much smaller size, giving it a disk-like shape.

One scientist has suggested that Iapetus swept up a ring before being somehow expelled to its current, distant orbit.

[citation needed] Perhaps more likely is the hypothesis that because Iapetus has an unusually large Hill sphere compared to other moons in the Solar System, it could once have had its own ring, or even a moonlet that was slowly pulled in closer, torn up into a ring, and then gradually accreted onto Iapetus' equator.

[citation needed] Another hypothesis suggested is that low velocity collisions between moons could have formed the bulge at the centre, although the circumstances for such an event to happen are slim.