Rings of Rhea

The Saturnian moon **Rhea** may have a thin **ring system** with three narrow bands in a disk of solid particles. These would be the first rings seen around a moon. The discovery was announced in the journal *Science* on March 6 2008.

In November of 2005 the *Cassini* orbiter found that Saturn's magnetosphere near Rhea did not have energetic electrons. According to the discovery team, this is best explained by assuming they were absorbed by solid material in the form of an equatorial disk having denser rings or arcs, with particles perhaps many decimeters to about a meter in diameter.

## Detection
*Voyager* 1 saw an area without as many energetic electrons trapped in Saturn's magnetic field downstream from Rhea in 1980. These measurements, which were never explained, were made at a farther distance than the *Cassini* data.

On November 26, 2005, *Cassini* made the one targeted Rhea flyby of its primary mission. It passed within 500&nbsp;km of Rhea's surface, downstream of Saturn's magnetic field, and saw the resulting plasma wake as it had with other moons, such as Dione and Tethys. In those cases, there was a cutoff of energetic electrons as *Cassini* crossed into the moons' plasma shadows (the regions where the moons themselves blocked the magnetospheric plasma from reaching *Cassini).* However, in the case of Rhea, the electron plasma started to drop off at eight times that distance, and decreased gradually until the expected sharp drop off as *Cassini* entered Rhea's plasma shadow. The extended distance corresponds to Rhea's Hill sphere, the distance of 7.7 times Rhea's radius inside of which orbits are dominated by Rhea's rather than Saturn's gravity. When *Cassini* emerged from Rhea's plasma shadow, the reverse pattern occurred: A sharp surge in energetic electrons, then a gradual increase out to Rhea's Hill-sphere radius.

These readings are similar to those of Enceladus, where water coming out from its south pole absorbs the electron plasma. However, in the case of Rhea, the absorption pattern is symmetrical.

In addition, the Magnetospheric Imaging Instrument (MIMI) saw that this gentle gradient was punctuated by three sharp drops in plasma flow on each side of the moon, a pattern that was also nearly symmetric. with a targeted flyby scheduled for March 2, 2010.

However, not all scientists are convinced that the seen signatures are caused by a ring system.  No rings have been seen in images, which puts a very low limit at least on tiny dust-sized particles.  Furthermore, a ring made of boulders would be expected to generate dust that would likely have been seen in the images.

## History
Simulations suggest that solid bodies can stably orbit Rhea near its equatorial plane over astronomical timescales. They may not be stable around Dione and Tethys because those moons are so much closer to Saturn, and therefore have much smaller Hill spheres, or around Titan because of drag from its dense atmosphere.

Many suggestions have been made for the possible origin of rings. An impact could have brought material into orbit; this could have happened as recently as 70 million years ago. A small body could have been disrupted when caught in orbit about Rhea. In either case, the debris would eventually have settled into circular equatorial orbits. Given their long-term orbital stability, however, it is possible that they survive from the formation of Rhea itself.

For different rings to exist, something must separate them. Suggestions include moonlets or clumps of material within the disk, similar to those seen within Saturn's A ring.

## References
## Other websites
* NASA podcast