Equatorial ridge
Equatorial ridges on Saturn's moons Iapetus, Atlas, Pan, and Daphnis.
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.
**Structure**
Iapetus’s ridge wraps around most of its equator. 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.
**Formation**
How these ridges formed—and whether they are related—remains uncertain.
- Discoverer of iapetus
- Giovanni Domenico Cassini
- Discovery year of iapetus
- October 1781
- Discoverer of atlas
- Voyager 1 images
- Discovery year of atlas
- November 1980
- Discoverer of pan
- Showalter
- Discovery year of pan
- 1980
- Discoverer of daphnis
- Cassini spacecraft
- Discovery year of daphnis
- 2005
- Ridge width on iapetus
- nearly 20 km
- Ridge height on iapetus
- 13 km
Lore & Background
The ridge on Iapetus wraps most of the way around its equator and is cratered as much as the surrounding terrain, suggesting it is about the same age. The ridge generally appears single but in some places is double or triple, with steep flanks exceeding 30 degrees in some sections. The volume of material involved is very large, spanning at least half the equator and well developed over about 1,600 km. Atlas and Pan's ridges were discovered from Voyager images in the early 1980s. Both moons are shaped like oblate ellipsoids with prominent equatorial ridges that give them a 'flying saucer' appearance. Pan orbits inside the Encke Gap of Saturn's A ring, while Atlas orbits just outside the A ring.
Reader's Guide
The article notes that it is not certain how these ridges formed or whether there is any connection between them. For Atlas and Pan, a likely explanation is that they sweep up ring particles as they orbit, building up around their equators. This hypothesis is less applicable to Iapetus, which orbits far beyond the rings. One scientist suggested Iapetus swept up a ring before being expelled to its current orbit; others think the rings were pulled away from it. A more likely hypothesis is that Iapetus's unusually large Hill sphere allowed it to have its own ring or a moonlet that was torn up and accreted onto its equator. Some scientists prefer an internal source for Iapetus's ridge, unrelated to the others. Other proposed ideas include despinning, volcanic activity, or tectonic warping. For Atlas and Pan, the ridges are thought to be made of ring particles accreted onto a preexisting moonlet, forming an 'equatorial ornament.' The particles likely developed differently due to Pan's nearly circular orbit versus Atlas's eccentric orbit, affecting which hemispheres accrete inner or outer ring particles.
Did You Know?
- Iapetus was discovered by Giovanni Domenico Cassini in October 1781.
- Atlas was discovered from Voyager 1 images in November 1980.
- Pan was discovered by Showalter in the same year as Atlas.
- The ridge on Iapetus is nearly 20 km wide, 13 km high, and 1,300 km long.
More in Moons of Saturn, Part 2 1-24
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