Oct. 24, 2007

NEW CU-BOULDER STUDY DETECTS FIRST-KNOWN BELT OF MOONLETS IN SATURN RINGS

A narrow belt harboring moonlets as large as football stadiums
discovered in Saturn's outermost ring probably resulted when a larger
moon was shattered by a wayward asteroid or comet eons ago, according
to a University of Colorado at Boulder study.

Images taken by a camera onboard the NASA Cassini spacecraft revealed
a series of eight propeller-shaped "wakes" in a thin belt of the
outermost "A" ring, indicating the presence of corresponding
moonlets, said CU-Boulder Research Associate Miodrag Sremcevic, lead
author of the study published in the Oct. 25 issue of Nature. The
propeller wakes highlight tiny areas of the belt where ring material
has been perturbed by the gravitational forces caused by individual
moonlets, Sremcevic said.

The team calculated that there likely are thousands of moonlets
ranging in size from semi-trailers to sports arenas embedded in the
"A" ring's thin moonlet belt that circles the planet. At about 2,000
miles across, the belt of moonlets is only about 1/80th the diameter
of Saturn's total ring system, which at roughly 155,000 miles across
would stretch about two-thirds of the way from Earth to the moon.

"This is the first evidence of a moonlet belt in any of Saturn's
rings," said Sremcevic of CU-Boulder's Laboratory for Atmospheric and
Space Physics. "We have firmly established these moonlets exist in a
relatively narrow region of the "A" ring, and the evidence indicates
they are remnants of a larger moon that was shattered by a meteoroid
or comet."

Co-authors of the Nature study include Juergen Schmidt, Martin Seiss
and Frank Spahn of the University of Potsdam in Germany, Heikko Salo
of the University of Oulu in Finland, and Nicole Albers of CU-
Boulder's LASP. The images were taken by the Narrow Angle Camera
onboard the NASA Cassini spacecraft, which was launched in 1997 and
has been orbiting the Saturn system since July 2004.

Each propeller feature is about 10 miles long, said Sremcevic, who
with Spahn first predicted the existence of such propellers in
Saturn's rings as an undergraduate at the University of Belgrade in
2000. While four propellers were discovered in the "A" ring in 2006
by a team led by Cornell University, Sremcevic and his colleagues
looked at a much larger image sequence, allowing them to extrapolate
statistically and confirm the presence of thousands of small objects
in the "A" ring's moonlet belt.

The moonlets may be the result of the break-up of a ring-moon similar
to Pan -- Saturn's innermost 20-mile diameter moon -- that was
smashed by a comet or meteor, the team concluded. The team
calculated the mass of the unseen moonlets in the belt greater than
50 feet in diameter to arrive at the estimated size of the moon
involved in the collision creating the belt.

The finding supports the theory that Saturn's rings initially were
created in a "collisional cascade" of ring debris begun by a
catastrophic break-up of an even larger moon in the Saturn system
first proposed by CU-Boulder planetary scientists Larry Esposito and
Joshua Colwell in 1987. The moonlets in the newly discovered belt
may have formed after Saturn's rings already were in place, which
planetary scientists speculate could have been hundreds of millions
or even billions of years ago.

"It seems unlikely that moonlets are remainders of a single
catastrophic event that created the whole ring system, because in
this case a uniform distribution would emerge," the researchers wrote
in Nature. "Instead, the moonlet belt is compatible with a more
recent body orbiting in the A ring."

Esposito, who was not involved in the study, said the propellers
"show a striking demonstration of the lingering effects of the
gravity from these small, embedded moonlets." Esposito is the chief
scientist on the NASA Cassini mission's $12.5 million Ultra-Violet
Imaging Spectrograph designed and built at LASP.

Sremcevic said the discovery of the moonlet belt is another piece in
the puzzle regarding the formation and evolution of Saturn's rings.
"We believe future studies of ring evolution will need to incorporate
the findings and implications from this study."

The NASA Cassini mission, formerly called the Cassini-Huygens
mission, is a cooperative project of NASA, the European Space Agency
and the Italian Space Agency. The Jet Propulsion Laboratory, a
division of the California Institute of Technology in Pasadena,
manages the NASA Cassini mission for NASA's Science Mission
Directorate in Washington, D.C.

For more information about NASA Cassini-Huygens visit http://
saturn.jpl.nasa.gov. To listen to a podcast of Sremcevic describing
his findings visit: http://www.colorado.edu/news/podcasts/.