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NEWS RELEASE: 2008-039 March 6, 2008
Saturn's Moon Rhea Also May Have Rings
PASADENA, Calif. -- NASA's Cassini spacecraft has found evidence of material orbiting Rhea,
Saturn's second largest moon. This is the first time rings may have been found around a moon.
A broad debris disk and at least one ring appear to have been detected by a suite of six
instruments on Cassini 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 to its ringed parent Saturn," said Geraint Jones, a Cassini scientist and lead author on a paper
that appears in the March 7 issue of the journal Science. 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 kilometers (950 miles) in diameter. The apparent debris disk measures
several thousand miles 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 kilometers (3,000 miles) from the moon's center, almost eight times the radius
of Rhea.
"Like finding planets around other stars, and moons around asteroids, these findings are opening
a new field of rings around moons," said Norbert Krupp, a scientist with Cassini's
Magnetospheric Imaging Instrument from the Max Planck Institute for Solar System Research.
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 its orbit
around Saturn, could allow rings that form to remain in place for a very long time.
The discovery was a result of a Cassini close flyby of Rhea in November 2005, when
instruments on the spacecraft observed the environment around the moon. Three instruments
sampled 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 atmosphere.
Evidence for a debris disk in addition to this tenuous dust cloud 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 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, such as Mimas, show evidence of a catastrophic collision that almost
tore the moon apart.
"The diversity in our solar system never fails to amaze us," said Candy Hansen, co-author and
Cassini scientist on the Ultraviolet Imaging Spectrograph at NASA's Jet Propulsion Laboratory,
Pasadena, Calif. "Many years ago we thought Saturn was the only planet with rings. Now we
may have a moon of Saturn that is a miniature version of its even more elaborately decorated
parent."
These ring findings make Rhea a prime candidate for further study. 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 the larger particles.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency
and the Italian Space Agency. JPL, a division of the California Institute of Technology in
Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate,
Washington. The Cassini orbiter was designed, developed and assembled at JPL. The
Magnetospheric Imaging Instrument was designed, built and is operated by an international team
led by the Applied Physics Laboratory of the Johns Hopkins University, Laurel, Md.
For information on the Cassini mission, visit: http://www.nasa.gov/cassini and
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