Rings of Rhea

The Saturnian moon Rhea may have a tenuous ring system consisting of three narrow, relatively dense bands within a particulate disk. This would be the first discovery of rings around a moon. The discovery was announced in the journal "Science" on March 6 2008.cite journal
last = Jones
first = Geraint H.
coauthors = "et al."
title = The Dust Halo of Saturn's Largest Icy Moon, Rhea
url = http://www.sciencemag.org/cgi/content/short/319/5868/1380
journal = Science
volume = 319
issue = 5868
pages = 1380–1384
publisher = AAAS
date=March 2008| doi = 10.1126/science.1151524]

In November 2005 the "Cassini" orbiter found that Saturn's magnetosphere is depleted of energetic electrons near Rhea. According to the discovery team, the pattern of depletion is best explained by assuming the electrons are absorbed by solid material in the form of an equatorial disk, which contains within it several denser rings or arcs, of particles perhaps several decimeters to approximately a meter in diameter.

Detection

"Voyager" 1 observed a broad depletion of energetic electrons trapped in Saturn's magnetic field downstream from Rhea in 1980. These measurements, which were never explained, were made at a greater distance than the "Cassini" data.

On November 26 2005, "Cassini" made the one targeted Rhea flyby of its primary mission. It passed within 500 km of Rhea's surface, downstream of Saturn's magnetic field, and observed the resulting plasma wake as it had with other moons, such as Dione and Tethys. In those cases, there was an abrupt 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)."cite web
last = Lakdawalla
first = E.
title = A Ringed Moon of Saturn? Cassini Discovers Possible Rings at Rhea
work = [http://www.planetary.org/home/ The Planetary Society web site]
publisher = Planetary Society
date = 2008-03-06
url = http://planetary.org/news/2008/0306_A_Ringed_Moon_of_Saturn_Cassini.html
accessdate = 2008-03-09] However, in the case of Rhea, the electron plasma started to drop off slightly 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 venting 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) observed 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 symmetrical.

In August 2007, "Cassini" passed through Rhea's plasma shadow again, but further downstream. Its readings were similar to those of "Voyager" 1.

There are no images or direct observations of the material thought to be absorbing the plasma, but the likely candidates would be difficult to detect directly. Further observations are planned for "Cassini"'s first mission extension, [cite news |url=http://www.newscientist.com/article/dn13421-saturn-satellite-reveals-first-moon-rings.html |first=Jeff |last=Hecht |title=Saturn satellite reveals first moon rings |work=New Scientist |date=2008-03-06 |accessdate=2008-03-06 ] with a targeted flyby scheduled for March 2 2010.

Interpretation

"Cassini"'s flyby trajectory makes interpretation of the magnetic readings difficult.

The obvious candidates for magnetospheric plasma-absorbing matter are neutral gas and dust, but the quantities required to explain the observed depletion are far greater than "Cassini"'s measurements allow. Therefore the discoverers, led by Geraint Jones of the "Cassini" MIMI team, argue that the depletions must be caused by solid particles orbiting Rhea:

An analysis of the electron data indicates that this obstacle is most likely in the form of a low optical depth disk of material near Rhea’s equatorial plane and that the disk contains solid bodies up to ~1 m in size.

The simplest explanation for the symmetrical punctuations in plasma flow are "extended arcs or rings of material" orbiting Rhea in its equatorial plane. These symmetric dips bear some similarity to the method by which the Rings of Uranus were discovered in 1977.cite news
title = Saturn's Moon Rhea Also May Have Rings
url = http://www.nasa.gov/mission_pages/cassini/media/rhea20080306.html
work = NASA
date= 2008-03-06
accessdate = 2008-03-08] The slight deviations from absolute symmetry may be due to "a modest tilt to the local magnetic field" or "common plasma flow deviations" rather than to asymmetry of the rings themselves, which may be circular.

Not all scientists are convinced that the observed signatures are caused by a ring system. No rings have been seen in images, which puts a very low limit on dust-sized particles. Furthermore, a ring of boulders would be expected to generate dust that would likely have been seen in the images.cite journal
last = Kerr
first = Richard A.
title = News of the Week: Electron Shadow Hints at Invisible Rings Around a Moon
url = http://www.sciencemag.org/cgi/content/summary/319/5868/1325
journal = Science
volume = 319
issue = 5868
pages = 1325
publisher = AAAS
date=March 2008| doi = 10.1126/science.319.5868.1325]

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.

Several suggestions have been made for the possible origin of rings. An impact could have ejected 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 the possibility of long-term orbital stability, however, it is possible that they survive from the formation of Rhea itself.

For discrete rings to persist, something must confine them. Suggestions include moonlets or clumps of material within the disk, similar to those observed within Saturn's A ring.

References

External links

* [http://www.jpl.nasa.gov/multimedia/podcast/cassini20080306/ NASA podcast]