Equatorial ridge

An equatorial ridge is a raised landform that runs along a moon's equator. In the Saturn system, at least three moons have such ridges: the large moon Iapetus and the small moons Atlas and Pan. These features appear to be found only around Saturn, though it is unclear if they share a common cause or are simply a coincidence. The tiny moon Daphnis also seems to possess a similar ridge.

Iapetus was first seen by astronomer Giovanni Domenico Cassini in October 1781. Atlas was discovered in images from Voyager 1’s November 1980 flyby of Saturn, and Pan was found by Showalter that same year. Daphnis was discovered later, in 2005, by the Cassini spacecraft. All three equatorial ridges were observed by Cassini.

The ridge on Iapetus measures roughly 20 kilometers wide, 13 kilometers high, and 1,300 kilometers long. On Atlas, the ridge is proportionally even more striking because the moon itself is much smaller, giving it a flattened, disk-like shape. Pan shows a similar structure. Iapetus’s ridge wraps around most of its equator.

Structure

It is as heavily cratered as the surrounding terrain, suggesting the ridge is about the same age as its environment. Because the feature is so large, whatever formed it likely operated on a global scale, not a local or regional one. The ridge is usually a single structure, but in some places it appears double or triple. Its flanks are steep, with slopes exceeding 30 degrees in certain sections.

The volume of material in the ridge is enormous, spanning at least half the equator and extending well over roughly 1,600 kilometers. This estimate does not account for any underlying roots or folds. The ridges on Atlas and Pan were first spotted in images from the Voyager spacecraft in the early 1980s. Pan orbits inside the Encke Gap of Saturn’s A ring, while Atlas orbits just outside the A ring.

Both moons are shaped like oblate ellipsoids: Pan has an equatorial radius of about 16.5 kilometers and a polar radius of about 10.5 kilometers; Atlas has an equatorial radius of roughly 19.5 kilometers and a polar radius of about 9 kilometers. More surprisingly, both have a prominent equatorial ridge. These ridges are roughly symmetrical around each moon’s equator, giving them the appearance of a flying saucer. How these ridges formed—and whether they are related—remains uncertain.