Rings of Rhea
Encyclopedia
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,...
ian moon Rhea
Rhea (moon)
Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...
may have a tenuous ring system
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
consisting of three narrow, relatively dense bands within a particulate disk. This would be the first discovery of rings around a moon
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 potential discovery was announced in the journal Science
Science (journal)
Science is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
on March 6, 2008.
In November 2005 the Cassini orbiter found that Saturn's magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
is depleted of energetic electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s 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. Subsequent targeted optical searches of the putative ring plane from several angles by Cassinis narrow-angle camera failed to find any evidence of the expected ring material, and in August 2010 it was announced that Rhea was unlikely to have rings, and that the reason for the depletion pattern, which is unique to Rhea, is unknown. However, an equatorial chain of bluish marks on the Rhean surface suggests past impacts of deorbiting ring material and leaves the question unresolved.
Detection
Voyager 1Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
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
Dione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...
and Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...
. 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). 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
Hill sphere
An astronomical body's Hill sphere 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 sphere of its own...
, 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
Enceladus (moon)
Enceladus is the sixth-largest of the moons of Saturn. It was discovered in 1789 by William Herschel. Until the two Voyager spacecraft passed near it in the early 1980s very little was known about this small moon besides the identification of water ice on its surface...
, 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.
In October 2009, it was announced that a set of small ultraviolet-bright spots distributed in a line that extends three quarters of the way around Rhea's circumference, within 2 degrees of the equator, may represent further evidence for a ring. The spots presumably represent the impact points of deorbiting ring material.
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 during Cassinis targeted flyby on March 2, 2010 found no evidence of orbiting ring material.
Interpretation
Cassinis 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 Cassinis 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
Rings of Uranus
The planet Uranus has a system of rings intermediate in complexity between the more extensive set around Saturn and the simpler systems around Jupiter and Neptune. The rings of Uranus were discovered on March 10, 1977, by James L. Elliot, Edward W. Dunham, and Douglas J. Mink...
were discovered in 1977. 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.
| Ring | orbital radius (km) |
|---|---|
| disk | < 5900 |
| 1 | ≈ 1615 |
| 2 | ≈ 1800 |
| 3 | ≈ 2020 |
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.
Physics
Simulations suggest that solid bodies can stably orbit Rhea near its equatorial plane over astronomical timescales. They may not be stable around DioneDione (moon)
Dione is a moon of Saturn discovered by Cassini in 1684. It is named after the titan Dione of Greek mythology. It is also designated Saturn IV.- Name :...
and Tethys
Tethys (moon)
Tethys or Saturn III is a mid-sized moon of Saturn about across. It was discovered by G. D. Cassini in 1684 and is named after titan Tethys of Greek mythology. Tethys is pronounced |Odysseus]] is about 400 km in diameter, while the largest graben—Ithaca Chasma is about 100 km wide and...
because those moons are so much closer to Saturn, and therefore have much smaller Hill sphere
Hill sphere
An astronomical body's Hill sphere 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 sphere of its own...
s, or around Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
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.