spacetoday.net: Cassini detects evidence for rings around Saturn moon

Posted: Fri, Mar 7, 2008, 6:09 AM ET (1109 GMT)
Rhea ring illustration (NASA)

A moon of the planet Saturn may have a faint ring around it, possibly composed of the remnants of a collision with a comet or asteroid, scientists announced Thursday. In a paper published in the current issue of Science, researchers reported the discovery of a broad debris disk and at least one ring around Rhea, a moon about 1,500 kilometers in diameter. The disk and ring were not imaged directly, but instead detected by monitoring drops in electron flux by two instruments on the spacecraft as it flew past the moon in November 2005. The sudden, brief drops in electrons matched up on both sides of Rhea, leading scientists to conclude that one or more rings exist within the debris disk. The most likely origin for the rings is a comet or asteroid collision with Rhea that threw up a cloud of gas and dust that went into orbit around the moon; the gas quickly dissipated but the debris remained.