Saturn's moon Rhea could be a mini version of its ringed
parent and the first moon known to have rings of its own.
Scientists detected hints of the rings when the Cassini spacecraft flew by the moon,
Saturn's second largest, in November 2005.
Surprisingly, instruments aboard Cassini measured an absence
of electrons around the moon where astronomers expected the charged particles
to swarm.
"This showed that there was something unique going
on," said Geraint Jones, a Cassini scientist. "We haven't seen
anything like this at any of the other moons. The only thing we can come up
with that fits what we see is that maybe there is some debris around Rhea. If
it is correct this would be the first moon where we have evidence of
rings."
Since Rhea lies within Saturn's magnetosphere, which traps
ions and electrons, scientists expected the moon to be awash in these
particles. Instead, they measured a gap in electrons in a swath of space
surrounding Rhea.
A set of rings
could explain the disappearing electrons because the material making up the
rings — most likely chunks of water ice up to a centimeter or meter in diameter
— would absorb electrons.
Rhea, named after the classical Greek titan goddess, the
mother of Zeus, Hades, Hera and other Olympian deities, is a barren, icy world
covered in craters. Its diameter is a little less than half that of our moon.
So far, Cassini has not been able to see the rings. If the
spacecraft's mission is extended, the researchers hope it might glimpse them on
a future close flyby of Rhea.
Saturn has dozens of moons; to date, 52 of them have been
named.
Before now, the Saturnian moons Enceladus,
Titan and Iapetus
have stolen most of the fame, Jones said. "Rhea seemed to be one of the
least interesting of the moons, but now it looks like it might be interesting
for its own special reasons," he told SPACE.com.
Jones and a team of Cassini scientists detail the findings
in the March 7 issue of the journal Science.
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