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.
I 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."