Atlas

Atlas travels just beyond the sharp outer edge of Saturn's A ring. Its name once seemed especially apt because the moon was thought to hold that boundary in place like the mythological Atlas holding up the sky. The ring edge is now known to be maintained mainly by an orbital resonance with Janus and Epimetheus, but Atlas remains physically tied to the ring environment beside it.

What makes Atlas unmistakable is its enormous equatorial ridge. Cassini images show a rounded central body surrounded by a smooth band of material, giving the moon a flattened, flying-saucer profile. The leading explanation is that Atlas swept up particles from Saturn’s rings and accumulated them preferentially around its equator, where the rings and moon share nearly the same plane.

The ridge is already close to the limit that Atlas's weak gravity can retain. Near its equator, rotation and centrifugal force almost overcome the pull of the moon, so additional particles may be lost instead of settling permanently. Atlas is therefore a small world whose shape records an active balance between accretion, rotation, and the ring material around it.