Equatorial ridges are a feature of at least three of Saturn's

moons A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...

: the large moon Iapetus and the tiny moons

Atlas An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth. Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geograp ...

and Pan. They are ridges that closely follow the moons' equators. They appear to be unique to the Saturnian system, but it is uncertain whether the occurrences are related or a coincidence. All three were discovered by the '' Cassini'' probe in 2005.

Daphnis In Greek mythology, Daphnis (; grc, Δάφνις, from , ''daphne'', "Bay Laurel") was a Sicilian shepherd who was said to be the inventor of pastoral poetry. Family According to tradition, he was the son of Hermes and a nymph, despite which ...

also appears to have such a ridge. The ridge on Iapetus is nearly 20 km wide, 13 km high and 1,300 km long. The ridge on Atlas is proportionally even more remarkable given the moon's much smaller size, giving it a disk-like shape. Images of Pan show a structure similar to that of Atlas.

Formation

It is not certain how these ridges formed, or whether there is any connection between them. Because Atlas and Pan orbit within the

rings of Saturn The rings of Saturn are the most extensive ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometers to meters, that orbit around Saturn. The ring particles are made almost entir ...

, a likely explanation for their ridges is that they sweep up ring particles as they orbit, which build up around their equators. This theory is less applicable to Iapetus, which orbits far beyond the rings. One scientist has suggested that Iapetus swept up a ring before being somehow expelled to its current, distant orbit. Others think it was stationary and it is the rings that have been pulled away from it, falling into Saturn's gravity field. Perhaps more likely is the theory that because Iapetus has an unusually large

Hill sphere The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sph ...

compared to other moons in the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

, it could once have had its own ring, or even a

moonlet A moonlet, minor moon, minor natural satellite, or minor satellite is a particularly small natural satellite orbiting a planet, dwarf planet, or other minor planet. Up until 1995, moonlets were only hypothetical components of Saturn's F-ring ...

that was slowly pulled in closer, torn up into a ring, and then gradually accreted onto Iapetus' equator. But most scientists prefer to assume that Iapetus's ridge was produced by some kind of internal source and is unrelated to the ridges on Atlas and Pan. Another theory 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.

References

External links

'Flying Saucers' Around Saturn Explained
- Atlas and Pan born largely from clumps of icy particles in the rings themselves, ''Space.com'', 6 December 2007 {{Saturn Surface features of Saturn's moons

Formation

See also

References

External links