CASSINI FINDS POSSIBLE ORIGIN OF ONE OF SATURN'S RINGS

Cassini scientists may have identified the source of one of Saturn's

more mysterious rings. Saturn's G ring likely is produced by relatively

large, icy particles that reside within a bright arc on the ring's inner

edge.

The particles are confined within the arc by gravitational effects from

Saturn's moon Mimas. Micrometeoroids collide with the particles,

releasing smaller, dust-sized particles that brighten the arc. The

plasma in the giant planet's magnetic field sweeps through this arc

continually, dragging out the fine particles, which create the G ring.

The finding is evidence of the complex interaction between Saturn's

moons, rings and magnetosphere. Studying this interaction is one of

Cassini's objectives. The study is in the Aug. 2 issue of the journal

Science and was based on observations made by multiple Cassini

instruments in 2004 and 2005.

"Distant pictures from the cameras tell us where the arc is and how it

moves, while plasma and dust measurements taken near the G ring tell us

how much material is there," said Matthew Hedman, a Cassini imaging team

associate at Cornell University in Ithaca, N.Y., and lead author on the

Science paper.

Saturn's rings compose an enormous, complex structure and their origin

is a mystery. The rings are labeled in the order they were discovered.

From the planet outward, they are D, C, B, A, F, G and E. The main

rings--A, B and C--from edge-to-edge, would fit neatly in the distance

between Earth and the moon. The most transparent rings are D--interior

to C--and F, E and G, outside the main rings.

Unlike Saturn's other dusty rings, such as the E and F rings, the G ring

is not associated closely with moons that either could supply material

directly to it--as Enceladus does for the E ring--or sculpt and perturb

its ring particles--as Prometheus and Pandora do for the F ring. The

location of the G ring continued to defy explanation--until now.

Cassini images show that the bright arc within the G-ring extends

one-sixth of the way around Saturn and is about 250 kilometers (155

miles) wide, much narrower than the full 6,000-kilometer (3,700-mile)

width of the G ring. The arc has been observed several times since

Cassini's 2004 arrival at the ringed planet and thus appears to be a

long-lived feature. A gravitational disturbance caused by the moon Mimas

exists near the arc.

As part of their study, Hedman and colleagues conducted computer

simulations that showed the gravitational disturbance of Mimas could

indeed produce such a structure in Saturn's G ring. The only other

places in the solar system where such disturbances are at work are the

ring arcs of Neptune.

Cassini's magnetospheric imaging instrument detected depletions in

charged particles near the arc in 2005. According to the scientists,

unseen mass in the arc must be absorbing the particles. "The small dust

grains that the Cassini camera sees are not enough to absorb energetic

electrons," said Elias Roussos of the Max-Planck-Institute for Solar

System Research, Germany, and member of the magnetospheric imaging team.

"This tells us that a lot more mass is distributed within the arc."

The researchers concluded that there is a population of larger,

as-yet-unseen bodies hiding in the arc, ranging in size from that of

peas to small boulders. The total mass of all these bodies is equivalent

to that of a 100-meter (328-foot) wide, ice-rich small moon.

Joe Burns, a co-author of the paper from Cornell University and a member

of the imaging team, said, "We'll have a super opportunity to spot the G

ring's source bodies when Cassini flies about a thousand kilometers (600

miles) from the arc 18 months from now."

Movies showing the bright arc in the G ring can be found at

http://ciclops.org, http://www.nasa.gov/cassini and

http://saturn.jpl.nasa.gov