Saturn's moon Rhea may have rings, too, Cassini discovers (3/8/2008)

Artist concept of Rhea Rings

UK scientists and international colleagues using NASA's Cassini spacecraft which is currently orbiting Saturn have found evidence of material orbiting Rhea, Saturn's second largest moon. This finding is the first time rings may have been found around a moon. Their results are reported the 7th March issue of the journal Science.

A broad debris disk and at least one ring appear to have been detected by a suite of six instruments specifically designed to study the atmospheres and particles around Saturn and its moons.

"Until now, only planets were known to have rings, but now Rhea seems to have some family ties," said Dr Geraint Jones, Cassini scientist, and the lead author. Dr Jones began this work while at the Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany, and is now at the Mullard Space Science Laboratory, University College London.

Rhea is roughly 1,500 kilometres (950 miles) in diameter. The apparent debris disk measures several thousand kilometres from end to end. The particles that make up the disk and any embedded rings probably range from the size of small pebbles to boulders. An additional dust cloud may extend up to 5,900 kilometres (3,000 miles) from the moon's centre, almost eight times the radius of Rhea.

Professor Michele Dougherty of Imperial College London says "Cassini has made great strides in identifying new features in Saturn's system, from rings around the planet to new moons - this is the first evidence for a ring around a moon and provides a new opportunity for exploring how rings form, evolve and disperse."

Since the discovery, Cassini scientists have carried out numerical simulations to determine if Rhea can maintain rings. The models show that Rhea's gravity field, in combination with an apparent "sweet-spot" orbit around Saturn, could allow rings to remain in place for a very long time.

The revelation was a result of a Cassini close flyby of Rhea in November 2005, when six instruments on board Cassini observed the environment around the moon. Three of the instruments sampled the dust directly. The existence of some debris was expected because a rain of dust constantly hits Saturn's moons, including Rhea, knocking particles into space around them. Other instruments' observations showed how the moon was interacting with Saturn's magnetosphere, and ruled out the possibility of an appreciable atmosphere.

Evidence for an additional debris disk came from a gradual drop on either side of Rhea in the number of electrons detected by two of Cassini's instruments; material near Rhea appeared to be shielding Cassini from the usual rain of electrons. Cassini's Magnetospheric Imaging Instrument also detected sharp, brief drops in electrons on both sides of the moon, suggesting the presence of rings within the disk of debris. The rings of Uranus were found in a similar fashion, by NASA's Kuiper Airborne Observatory in 1977, when light from a star blinked on and off as it passed behind Uranus' rings.

"Seeing almost the same signatures on either side of Rhea was the clincher," added Jones. "After ruling out many other possibilities, we said these are most likely rings. No one was expecting rings around a moon."

One possible explanation for these rings is that they are remnants from an asteroid or comet collision in Rhea's distant past. Such a collision may have pitched large quantities of gas and solid particles around Rhea. Once the gas dissipated, all that remained were the ring particles. Other moons of Saturn, like Mimas, for example, show evidence of a catastrophic collision that almost tore the moon apart.

"The sensitivity of the particles and fields instruments on Cassini, together with in-situ dust measurements, have been used here to discover something special. Signs of rings around a moon are truly exciting findings which are a new part of the evolving jigsaw of solar system dynamics" said Professor Andrew Coates, co-author and Cassini scientist on the Cassini Plasma Spectrometer at University College London's Mullard Space Science Laboratory.

These ring findings make Rhea a prime candidate for further study as the Cassini mission continues. Initial observations by the imaging team when Rhea was near the Sun in the sky did not detect dust near the moon remotely. Additional observations are planned to look for larger particles.

UK participation in the Cassini mission is funded by the Science and Technology Facilities Council.

Note: This story has been adapted from a news release issued by the Science and Technology Facilities Council

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