Cassini finds possible origin of Saturn's G ring

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.
This is a movie sequence of Saturn's G ring over a full orbital revolution captures its single bright arc on the ring's inner edge. The movie is composed of 70 individual narrow-angle camera images taken during a period of just over 20 hours while Cassini stared at the ring. The orbital period for particles in the center of the G ring is about 19.6 hours. At the beginning of the sequence, the ring arc, a site of concentrated ring particles, is seen rounding the ring edge. The images in this movie were taken on Sept. 19 and 20 at a distance of approximately 2.1 to 2.2 million kilometers (1.3 to 1.4 million miles) from Saturn and at a Sun-G ring-spacecraft, or phase, angle that ranged from 167 to 164 degrees. Image scale is about 13 kilometers (8 miles) per pixel in the radial (outward from Saturn) direction. Credit: NASA/JPL/Space Science Institute

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 are 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 155 miles wide, much narrower than the full 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.

Image Credit: NASA/JPL/Space Science Institute
High resolution image

The arc orbits at a distance of 167,496 kilometers (104,080 miles). It is about 180 kilometers (110 miles) wide in radius and subtends about 30 degrees of orbital longitude. The classical position of the G ring is about 172,600 kilometers (107,250 miles) from Saturn, and the arc blends smoothly into this region. Scientists suspect that bodies trapped in this remarkably bright feature may be the source of the G ring material, driven outward from the arc by electromagnetic forces in the Saturn system.

The arc itself is likely held in place by gravitational resonances with Mimas of the type that anchor the famed arcs in Neptune's rings. There is an obvious narrow dark gap in the G ring beyond the arc. This feature is close to yet another resonance with Mimas, but no arcs are present at this locale.

This view looks toward the unlit side of the rings from about 10 degrees above the ringplane. Imaging artifacts jitter within the scene, a result of the high phase angle and faintness of the G ring. Stars slide across the background from upper left to lower right.

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 known to exist are in 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 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 600 miles from the arc 18 months from now."

Source: Jet Propulsion Laboratory
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