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Planetary News: Cassini-Huygens (2008)A Ringed Moon of Saturn? Cassini Discovers Possible Rings at RheaBy Emily LakdawallaMarch 6, 2008
For the first time, scientists believe they have uncovered evidence for a moon to have its own ring system. During a flyby of Saturn's second-largest moon Rhea on November 26, 2005, Cassini's Magnetospheric Imaging Instrument (MIMI) found that something unseen was blocking the flow of electrons in Rhea's neighborhood. Detailed analysis of the data suggests that the unseen something is a sparse, rocky disk of material orbiting Rhea's equator -- a system of rings orbiting a moon that itself orbits the ringed planet Saturn. The discovery was unexpected, said Geraint Jones, the first author on a paper on the discovery published in Friday's issue of the journal Science. "I'm not aware of anyone suggesting recently that an extant [ring] system could be surrounding one of the moons. We didn't look through the various flyby datasets for the signatures of rings. But having seen electrons absorbed by Enceladus's plume in a similar manner, it's something we should have considered doing." During the November 2005 flyby, Cassini was performing observations designed to investigate whether Rhea has a tenuous atmosphere and to study Rhea's interaction with Saturn's magnetosphere. Cassini did not detect any atmosphere at Rhea. However, as Cassini approached the icy moon, MIMI detected a surprising drop in the number of energetic electrons. The drop in electrons was only found within a distance of about 6,000 kilometers (3,500 miles) from Rhea. This distance coincides with Rhea's Hill sphere -- the region of space where the force of gravity from Rhea is stronger than the force of gravity from Saturn. Clearly, something gravitationally bound to Rhea was interfering with the flow of electrons around Saturn. The cluttered environment of the Saturn system means that all of Saturn's moons orbit within a halo of particles, stuff ejected from the moons' surfaces as a result of micrometeoroid impacts. Several of Cassini's other instruments, including the Cosmic Dust Analyzer (CDA) and the Radio and Plasma Wave System (RPWS), picked up telltale signals from this dust halo at Rhea. Both detected an increase in the amount of large (bigger than 1 micrometer) dust particles that peaked at Cassini's closest approach. But the particle densities measured by CDA and RPWS were not high enough to explain the low number of electrons that were getting through Rhea's Hill sphere to be detected by MIMI. "The broad electron absorption is clearest in MIMI data," Jones explains. "Around 70 percent of electrons in the key energy range disappear between the boundary of the Hill sphere and Rhea; we wouldn't expect any to disappear. The broad decrease is also present in CAPS [Cassini Plasma Spectrometer] data, slightly more muted, but that's due to the different orientations of the instruments, and the signatures don't appear in as wide an energy range as they do in MIMI." Cassini's flyby of Rhea happened at a southern latitude. So one explanation for the inconsistency between the low numbers of electrons measured by MIMI and CAPS and the amounts of particles detected by CDA and RPWS is that there might be a much denser population of particles that orbit around Rhea's equator. This equatorial disk of material would act as an obstacle to the flow of electrons, preventing them from getting through to MIMI's detector, but it wouldn't have been detected by CDA and RPWS because Cassini didn't fly close enough to the equatorial region to sample it. Jones and his coworkers analyzed the MIMI data and determined that the "disk contains solid bodies up to 1 meter in size."
Could such a disk of material be stable in orbit around Rhea when it is so close to the much more massive bulk of Saturn and the possible gravitational perturbations of Saturn's largest moon, Titan? "Dynamical studies show that such bodies could orbit persistently near [Rhea's] equatorial plane," Jones wrote in the Science article. A closer examination of the MIMI data reveals an intriguing detail. As Cassini flew by, the electron flux dropped, was lowest at closest approach, and then increased again on the other side of Rhea. But superimposed on that broad pattern are six very narrow dips in electron flux. The dips are very nearly symmetrical around Rhea -- that is, narrow dips at 1,991, 1,784, and 1,615 kilometers before closest approach were matched by mirror-image dips 2,077, 1,841, and 1,615 kilometers after closest approach. One way to explain this near-symmetry is if Rhea's disk contains three local concentrations of electron-blocking material. In other words, Rhea's disk may contain three rings. (Jones did develop models that can explain the slight asymmetry.) If Rhea has rings, why haven't we seen them in images? Cassini's cameras did point off the edge of Rhea's disk during a set of observations on June 11, 2007, and found nothing. Jones explains in the article that the rings, if they really are there, "would not be easy to detect." An unseeable population of meter-sized bodies orbiting Rhea may sound like a hazard to the spacecraft, and indeed if Cassini were to encounter a rock even a few centimeters in size, it would be catastrophic to the mission. However, the limited extent of the ring system means that Cassini is unlikely ever to encounter it, if it's there. There is only one more close encounter with Rhea planned, on March 2, 2010. Cassini's lead mission planner, David Seal, explains that "that encounter is polar, with a closest approach latitude of 81 degrees. Therefore, we don't expect to get any closer to these proposed rings, and will be safe." As Cassini approaches the end of its primary mission and the beginning of its extended mission, the pace of discoveries continues unabated. Each one of Saturn's moons is proving to be fascinating, with unique geology. Rhea, with its battered surface, has long been overlooked as one of the least interesting of Saturn's moons; it doesn't have Enceladus' geysers, or Titan's methane rivers and seas, or Hyperion's Swiss-cheese texture, or Iapetus' two-toned surface. But it may now have a claim to equal fame: the first moon discovered to have its own ring system. Cassini will continue its study of the Saturn system for at least another two years, searching for more evidence of Rhea's rings. |
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