Posted on 03/06/2008 2:11:24 PM PST by Pyro7480
PASADENA, Calif. - New observations by a spacecraft suggest Saturn's second-largest moon may be surrounded by rings.
If confirmed, it would the first time a ring system has been found around a moon.
The international Cassini spacecraft detected what appeared to be a large debris disk around the 950-mile-wide moon Rhea during a flyby in 2005. Scientists proposed that the halo likely contained particles ranging from the size of grains to boulders.
The finding was described in a study published in the March 7 issue of the journal Science.
Unlike the rings around Saturn, Jupiter, Uranus and Neptune, the apparent arcs around Rhea remain invisible and have not been directly seen. Scientists inferred their existence based on measurements by Cassini, which detected a drop in electrons on both sides of the moon, suggesting the presence of rings was absorbing the electrons.
It's unclear where the rings would have originated, but one explanation is they may be the result of an ancient asteroid or comet collision that spewed material around Rhea.
"Rings may even have survived since Rhea's formation," wrote lead author Geraint Jones, a space physicist from University College London.
Until now, only planets were known to have rings, said Jones, who began the research while at the Max Planck Institute for Solar System Research in Katlenburg-Lindau, Germany.
The Cassini mission, funded by NASA and the European and Italian space agencies, was launched in 1997 and reached Saturn in 2004. The mission is managed by NASA's Jet Propulsion Laboratory in Pasadena.
Ping!
Cool. bkmarking// for later. Thanks.
IBTRAUJ?
"You can call me RHEA..."
Good one ... My thoughts exectly!
LOL. it’s In Before The Rings Around Uranus Jokes.
I know ... I was dying from laughter because I was about to make a smart comment about ‘Uranus and Rings’ but thought that IBTRAUJ was much better.
Now it’s even funnier.
In accordance with unwritten FReeper posting rules, if you use a FReepers name in a post they must be pinged.
You can call me Johnny, but don’t call me .....
I’d have otherwise expected Rhea to be around Uranus, along with Diar...
I didn’t know the rings were made out of debris. This stuff is pretty much way over my head. In honor of Rhea, here’s a video link from Feetwood Mac
Feetwood = Fleetwood!
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A fascinating article in the March 7th issue of Science magazine describes how, when NASA’s Cassini spacecraft skimmed within 300 miles of the Saturnian moon Rhea in November 2005, several instruments detected what appear to be a tenuous dust disk around it containing three thin, sparse rings.
NASA / JPL / JHU-APLI say “detected” the disk and rings because Cassini’s cameras didn't see them. Instead, the encircling matter selectively absorbed electrons whizzing in Saturn's magnetosphere, causing “particle shadows” that the instruments recorded.
NASA and the European Space Agency jointly issued a press release about the Cassini result, but it doesn't nearly tell the whole story.
Instrument readings suggest that a thin disk of dust extends out to 4,000 miles (6,000 km), within which Rhea's gravity dominates that of Saturn. There'd been hints that something odd was happening near Rhea back in 1980, when Voyager 1 went by.
The 35-member team for the Science write-up argues that there's far too much dust to have simply been blasted from Rhea's surface by meteoritic impacts (some of which they expected). Nor can it be due to an extended gas atmosphere — none was detected. And nothing like this was seen during similarly close flybys of neighboring Tethys and Dione.
One instrument recorded a trio of brief, sharp dips in magnetospheric electrons as the spacecraft approached Rhea and again as it sped away. These dropouts imply that Rhea has thin rings located roughly 1,000, 1,200, and 1,260 miles from its center (Rhea itself has a radius of 475 miles or 765 km.)
The team believes these rings could have existed stably around Rhea for many millions of years — long enough to have been created by a major impact with its icy surface some tens of millions of years ago or conceivably enduring since Rhea itself formed.
(For the record, this isn't the first time that charged-particle measurements have betrayed the presence of rings. Similar phenomena, recorded by Pioneer 11 as it swept close by Saturn in 1979, led to the discovery of the planet's F and G rings.)
But what I find most fascinating about this paper is that these ring particles are likely big — their mean diameter could be basketball size or larger — and the team estimates that the total mass involved is of order 10 million tons. Had Cassini passed through Rhea's equatorial plane (where the disk and rings most likely lie) rather than skirting just 125 miles south of it, this particular flyby might have had a much different, potentially catastrophic outcome.
After all, notes lead author Geraint Jones, “No one was expecting rings around a moon.”
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