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:
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.
Jet Propulsion Laboratory, Pasadena, Calif.
Preston Dyches 720-974-5859
Space Science Institute, Boulder, Colo.
Julia Maddock +44 (0)1793 442 094
Science and Technology Facilities Council