|
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 potential discovery was announced in the journal ''Science'' on March 6, 2008. 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 of particles perhaps several decimeters to approximately a meter in diameter and that contains several denser rings or arcs. 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.〔(【引用サイトリンク】 accessdate=2010-10-07 )〕 ==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).''〔 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. Two years later, 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.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rings of Rhea」の詳細全文を読む スポンサード リンク
|