[meteorite-list] Saturn's Moon Rhea Also May Have Rings

[Ron Baalke]

March 6, 2008

Dwayne Brown
Headquarters, Washington
202-358-1726
dwayne.c.brown at nasa.gov

Carolina Martinez 
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-9382
carolina.martinez at jpl.nasa.gov

RELEASE: 08-074

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, 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 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 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 on 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 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 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 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, 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. JPL manages Cassini for NASA. "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. The 
Magnetospheric Imaging Instrument was designed, built and is operated 
by an international team led by the Applied Physics Laboratory, Johns 
Hopkins University, Laurel, Md. For information on the Cassini 
mission, visit:

http://www.nasa.gov/cassini

	
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