Equatorial ridges are a feature of at least three of Saturn

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's moons

Natural satellite

A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

: the large moon Iapetus

Iapetus (moon)

Iapetus ), occasionally Japetus , is the third-largest moon of Saturn, and eleventh in the Solar System. It was discovered by Giovanni Domenico Cassini in 1671...

 and the tiny moons Atlas

Atlas (moon)

Atlas is an inner satellite of Saturn.Atlas was discovered by Richard Terrile in 1980 from Voyager photos and was designated '. In 1983 it was officially named after Atlas of Greek mythology, because it "holds the rings on its shoulders" like the Titan Atlas held the sky up above the Earth...

 and Pan

Pan (moon)

Pan is the innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn's A Ring. Pan acts as a ring shepherd and is responsible for keeping the Encke Gap free of ring particles.It was discovered by Mark R...

. They are ridges

Ridge

A ridge is a geological feature consisting of a chain of mountains or hills that form a continuous elevated crest for some distance. Ridges are usually termed hills or mountains as well, depending on size. There are several main types of ridges:...

 that follow closely the moon's equator

Equator

An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

. 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.

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, and distorts the moon's shape into an odd, UFO

Unidentified flying object

A term originally coined by the military, an unidentified flying object is an unusual apparent anomaly in the sky that is not readily identifiable to the observer as any known object...

-like appearance. Images of Pan are less clear, but show a structure similar to that of Atlas.

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

Rings of Saturn

The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...

, 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.http://www.universetoday.com/am/publish/iapetus_consume_saturn_ring.html Others think it was stationary and it is the rings that have been pulled away from it, falling into Saturn's gravity field. 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.

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