NASA's Cassini images ring arcs among Saturn's moons

Washington, September 6 : NASA's Cassini spacecraft has detected a faint, partial ring orbiting with one small moon of Saturn and confirmed the presence of another partial ring orbiting with a second moon, which provides 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. Cassini's Magnetospheric Imaging Instrument previously detected the Methone arc, and the new images confirm its presence.

Cassini has also previously observed an arc in the G ring, one of Saturn's faint, major rings.

"This is the 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, New York.

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.

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.

ANI