Cassini Images Ring Arcs Among Saturn's Moons
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Cassini Images Ring Arcs Among Saturn's Moons
Jet Propulsion Laboratory
September 05, 2008
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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Carolina Martinez 818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.
carolina...@jpl.nasa.gov
Preston Dyches 720-974-5859
Space Science Institute, Boulder, Colo.
me...@ciclops.org
Julia Maddock +44 (0)1793 442 094
Science and Technology Facilities Council
julia....@stfc.ac.uk