WIREDGOV

Cameras on NASA's Cassini spacecraft in orbit around Saturn have detected faint arcs of material in the orbits of the small Saturnian moons Anthe and Methone, providing further evidence that most of the planet’s small, inner moons orbit within partial or complete rings.

The faint ring arcs from Anthe and Methone likely consist of material knocked off these small moons by micrometeoroid impacts from outside the Saturn system. This material does not spread all the way around Saturn to form a complete ring due to gravitational influence or resonance from the nearby moon Mimas, which acts to confine the material in a narrow region along the moons’ orbits.

The credit for the findings goes to the Cassini Imaging Team and key analyses were made by the members of the Cassini group at Queen Mary, University of London. A joint study with fellow Cassini scientists at Cornell University has shown that gravitational resonances may be the deciding factor in determining whether an arc or complete ring is seen. "This is probably the same mechanism responsible for producing the arc in the G ring," said Dr. Matthew Hedman, a Cassini Imaging Team associate at Cornell University in Ithaca, N.Y. "Indeed, the Anthe arc may be analagous to the debris we see in the G-ring arc, where the largest particles are clearly visible. One may even speculate that if Anthe was shattered, its debris might form a structure much like the G ring," he said.

The recent Cassini images show the arcs of material extending ahead of and behind Anthe and Methone in their orbits. Detailed analyses of the orbits of these bodies have confirmed a close relationship between the moons and their surrounding material.

Both Anthe and Methone experience gravitational effects from the nearby moon Mimas. Each moon is locked in a dynamical resonance with Mimas that produces periodic oscillations in its orbital motion, according to Dr. Nick Cooper, a Cassini Imaging Team associate from Queen Mary, University of London. "When we saw that the sizes of these resonant oscillations were consistent with the observed extents of the Anthe and Methone ring arcs, we knew we had a possible causal relationship," Cooper said.

The intricate relationship between these ring arcs and the moons is just one of many such mechanisms that exist in the Saturn system. Cassini Imaging Team member Professor Carl Murray from Queen Mary, University of London, commented, "Saturn’s system of rings and moons has many varieties of resonance. Understanding all these dynamical interactions and learning about their origins can help us to make sense of what we are seeing in the Cassini images -- everything from ring structure to satellite surfaces can be influenced by resonance."

This is the first detection of an arc of material surrounding Anthe. Hints of the Methone arc were previously detected by Cassini’s Magnetospheric Imaging Instrument, or MIMI, and the new images appear to confirm that detection. Earlier Cassini images have shown faint rings coincident with other small moons either embedded within or near the outskirts of Saturn’s main ring system, such as Pan, Janus and Epimetheus, and Pallene.

As well as improving the understanding of ring-moon interactions, these discoveries are useful to the Cassini mission planners who have to consider potential collision hazards to the spacecraft.

Professor Keith Mason, Chief Executive of the Science and Technology Facilities Council which funds UK involvement in Cassini-Huygens, said, "Cassini has been incredibly successful and continues to make many exciting discoveries. This latest find is another triumph for the Cassini team including the UK scientists at Queen Mary. It will provide us with even more detailed information about this intriguing planet and its many moons."

Notes to editors

The Cassini group at Queen Mary consists of Professor Carl Murray, Drs Kevin Beurle, Nick Cooper and Mike Evans.

Images

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