Cassini Images Ring Arcs Among Saturn's Moons

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Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
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Space Science Institute, Boulder, Colo.
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Julia Maddock +44 (0)1793 442 094
Science and Technology Facilities Council
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News release: 2008-172 Sept. 5, 2008

Cassini Images Ring Arcs Among Saturn's Moons

PASADENA, Calif. -- NASA's Cassini spacecraft has detected a faint, partial ring
orbiting with one small moon of Saturn, and has confirmed the presence of another
partial ring orbiting with a second moon. This is further evidence that most of the
planet's small, inner moons orbit within partial or complete rings.

Recent Cassini images show material, called ring arcs, extending ahead of and behind
the small moons Anthe and Methone in their orbits. The new findings indicate that the
gravitational influence of nearby moons on ring particles might be the deciding factor in
whether an arc or complete ring is formed.

Both Anthe and Methone orbit Saturn in locations, called resonances, where the gravity
of the nearby larger moon Mimas disturbs their orbits. Gravitational resonances are also
responsible for many of the structures in Saturn's magnificent rings. Mimas provides a
regular gravitational tug on each moon, which causes the moons to skip forward and
backward within an arc-shaped region along their orbital paths, according to Nick
Cooper, a Cassini imaging team associate from Queen Mary, University of London.
"When we realized that the Anthe and Methone ring arcs were very similar in
appearance to the region in which the moons swing back and forth in their orbits due to
their resonance with Mimas, we knew we had a possible cause-and-effect relationship,"
Cooper said.

Scientists believe the faint ring arcs from Anthe and Methone likely consist of material
knocked off these small moons by micrometeoroid impacts. This material does not
spread all the way around Saturn to form a complete ring, because of the gravitational
resonance with Mimas. That interaction confines the material to a narrow region along
the orbits of the moons.

This is the first detection of an arc of material near Anthe. The Methone arc was
previously detected by Cassini's Magnetospheric Imaging Instrument, and the new
images confirm its presence. Previous Cassini images show faint rings connected with
other small moons either embedded within or near the outskirts of Saturn's main
ring system, such as Pan, Janus, Epimetheus and Pallene. Cassini had also previously
observed an arc in the G ring, one of Saturn's faint, major rings.

"This is probably the same mechanism responsible for producing the arc in the G ring,"
said Matthew Hedman, a Cassini imaging team associate at Cornell University in Ithaca,
N.Y. Hedman and his Cassini imaging team colleagues previously determined that the
G-ring arc is maintained by a gravitational resonance with Mimas, much like the
new, small moon arcs. "Indeed, the Anthe arc may be similar to the debris we see in the
G-ring arc, where the largest particles are clearly visible. One might even speculate that
if Anthe were shattered, its debris might form a structure much like the G ring," Hedman
said.

Additional analysis by scientists indicates that, while the gravitational influence of Mimas
keeps the Anthe, Methone and G-ring arcs in place, the material that orbits with the
moons Pallene, Janus and Epimetheus is not subject to such powerful resonant forces
and is free to spread out around the planet, forming complete rings without arcs.

The intricate relationships between these ring arcs and the moons are just one of many
such mechanisms that exist in the Saturn system. Cassini Imaging Team Member and
Professor Carl Murray, also from Queen Mary, University of London, said, "There are
many examples in the
Saturn system of moons creating structures in the rings and disturbing the orbits of
other moons. Understanding these interactions and learning about their origins can help
us to make sense of what we are seeing in the Cassini images."

Images of Anthe and Methone with their ring arcs are available
at: http://www.nasa.gov/cassini, http://saturn.jpl.nasa.gov and http://ciclops.org.

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 Cassini-Huygens mission
for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its
two onboard cameras were designed, developed and assembled at JPL. The imaging
team is based at the Space Science Institute, Boulder, Colo.

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