Rhea's dusty halo
BY EMILY BALDWIN
Posted: March 13, 2008
NASA's Cassini spacecraft has found evidence of a “dusty-halo” orbiting Rhea, Saturn's second largest satellite after Titan, the first time rings may have been found around a moon.
Rhea is roughly 1500 kilometres in diameter and its debris disc appears to extend several thousand kilometres from end to end. Particles ranging in size from small pebbles to boulders are thought to make up the disk, as detected by a suite of six instruments on Cassini specifically designed to study the atmospheres and particles around Saturn and its moons.
The new findings have been published in the March 7 issue of Science by a 35-strong research team, but the Cassini spacecraft actually made the discovery in November 2005, when its instruments observed the environment around the moon and sampled the dust directly. The existence of some debris did not come as a huge surprise because a rain of dust constantly hits Saturn’s moons, including Rhea, knocking particles out into space around them. Evidence of a ring structure came from a gradual drop on either side of Rhea in the number of electrons detected by two of Cassini’s instruments, where material appeared to be shielding Cassini from the usual rain of electrons. Observations showing how the moon was interacting with Saturn’s magnetosphere ruled out the possibility of an atmosphere.
"Until now, only planets were known to have rings, but now Rhea seems to have some family ties to its ringed parent Saturn," says Geraint Jones, Cassini scientist and lead author of the Science paper. "Like finding planets around other stars, and moons around asteroids, these findings are opening a new field of rings around moons," adds colleague Norbert Krupp, a scientist on Cassini's Magnetospheric Imaging Instrument.
Now scientists are trying to figure out the origin of the rings. One possible explanation is that they are remnants from an ancient asteroid or comet collision with Rhea, which would have expelled large quantities of gas and solid particles from the satellite’s surface. The gas would have dissipated, leaving the solid particles trapped in a ring around the satellite. Evidence for violent collisions in the outer Solar System is abundant; Rhea is littered with impact scars, and on another moon of Saturn, Mimas, a giant impact is thought to have almost totally disrupted the moon.
Since the new discovery of Rhea’s ring system, Cassini scientists have performed numerical simulations to determine if the satellite can maintain the rings. The models show that Rhea’s gravity field, in combination with its orbit around Saturn, could allow rings to remain in place for a very long time. Once again, the Solar System has unveiled another secret: it’s no longer the case that only the giant planets exhibit rings, the smaller satellites are capable of maintaining ring structures too. The next step is to target Rhea again, in order to learn more about the composition of the ring particles.