New ring arcs orbiting Saturn
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: September 09, 2008

NASA’s Cassini spacecraft has spied two separate arcs of material extending ahead of and behind the orbits of Saturn’s small moons Anthe and Methone, likely formed by gravitational disturbances from neighbouring satellite Mimas.

Arcs of material are seen co-orbiting with Saturn's moons Anthe (top) and Methone (bottom). Image: NASA/JPL/Space Science Institute.

Both Anthe and Methone orbit Saturn in locations called resonances, where the gravity of nearby moon Mimas is strong enough to provide 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. The presence of the so-called ring arcs trailing or leading individual satellites are also thought to be controlled by local gravitational influences, which determine whether an arc or a complete ring is formed.

"When we realised 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," says Nick Cooper, a Cassini imaging team associate from Queen Mary, University of London.

The material surrounding Anthe and Methone – which is likely knocked off the moons themselves by the action of micrometeorite impact – does not spread all the way around Saturn to form a complete ring, because the gravitational resonance with Mimas confines the material to a narrow region along the orbits of the moons.

In this image, most of the visible material in the arc lies ahead of Anthe as it orbits Saturn. Over time, the moon drifts slowly back and forth with respect to the arc as a result of gravitational interactions with larger moon Mimas. Image: NASA/JPL/Space Science Institute.

Similar gravitational resonances between other moons are also known to be responsible for many of the structures in Saturn's rings, such as an arc in the G-ring, which is also maintained by a gravitational resonance with Mimas. "Indeed, the Anthe arc may be similar to the debris we see in the G-ring arc, where the largest particles are clearly visible,” says Matthew Hedman, a Cassini imaging team associate. “One might even speculate that if Anthe were shattered, its debris might form a structure much like the G ring."

Although 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 confirmed in the new images, while other images also show complete faint rings associated with several small moons embedded within or near the outskirts of Saturn's main ring system, including Pan, Janus, Epimetheus, and Pallene. But 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, once again showing how dynamic the system is, and how much there is still to learn.