Imaging scientists on Cassini have spent their summer vacations havingmore fun than kids at a carnival. Analyzing fantastic new results fromCassini's first season of prime ring viewing, they are announcing someof their unexpected findings on Saturn's rings, including newstructures in Saturn's diffuse rings, clumps and knots in the F ring --some of which may be small moons -- and a completely unexpected spiralring around the planet in the vicinity of the F ring.
The findings are illustrated in processed images and movies being released today and found at http://ciclops.org, http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini.
First in the line of new discoveries is that parts of the D ring(Saturn's innermost ring) have relocated and dimmed. Images show one ofthe major discrete ring structures in the D ring has changed inbrightness and moved inward towards Saturn by as much as 200 kilometers(124 miles). A change over the 25 years since the NASA Voyagerspacecraft flybys indicatesvery short evolutionary lifetimes in the D ring and is of greatinterest to ring scientists who have been hoping that Cassini wouldyield information about ring ages and lifetimes.
Dr. Matt Hedman, an imaging team associate at CornellUniversity, Ithaca, N.Y. said, "I think our Cassini images of the Dring are providing new information about the dynamics and lifetimes ofring particles in a new regime, very close to the planet."
The delicate G ring encircles the planet at about 170,000kilometers (106,000 miles) from Saturn's center. Cassini scientistshave now found a discontinuous bright ring segment, or'arc', in this ring that bears at least a fleeting similarity to thoseimaged around Neptune in 1989 by NASA's Voyager 2 spacecraft.Scientists think that long-lived arcs may be created ormaintained by a nearby hidden moon. Another thought is that they formedas a result of a meteoroid impact.
Saturn's tenuous D and G rings contain very little material, and the tiny, icy particles are the size of dust or smoke.
In examining the intriguing, knotted F ring, imaging teamscientists have also discovered that the ghostly ringlets flanking thering's core are arranged into a spiral structure wound like aspring around the planet. Other spiraling structures seen in the mainrings of Saturn, the density and bending waves, are initiated by thegravitational influence of an orbiting moon. Density and bending wavesmove across the rings because of the way that relatively massive ringparticles exert a gravitational influence on each other and can allmove together. In contrast, the spiral structure contains very littlemass and appears to originate from material somehow episodicallyejected from the core of the F ring and then sheared out due to thedifferent orbital speeds followed by the constituent particles.
"It is a big surprise to see a spiral arm in Saturn's rings,"said Dr. Sebastien Charnoz, imaging team associate at the University ofParis. "It is very possible that the spiral is a consequence of moonscrossing the F ring and spreading particles around, and may be tellingus that the F ring might be a very unstable or even an ephemeralstructure."
In the same region, scientists continue to spot small,clump-like features that may be loosely-bound clumps of material ortiny moonlets. Some of them have been sighted for the better part of ayear. The solid-or-not nature of these mysterious F ring objects may bedetermined by repeated sightings:moons will persist, while clumps are expected to dissipate with time.
"We have long suspected that small moons were hiding among theF ring's strands and producing some of the structures that we see,"said Imaging Team Member Dr. Carl Murray of Queen Mary, University ofLondon. "But now the problem is that we are detecting objects that maybe either solid moonscontrolling the ring, or just loose clumps of particles within thering, and it's hard to tell the difference. It is like trying todistinguish sheep dogs from sheep in a very large flock."
A puzzling characteristic of at least two of the clumps/moonsis that they ppear to cross the F ring periodically. One of them, anobject that was discovered last year (S/2004 S6), may be responsiblefor forming the spiral.
"If the orbit that we have computed for S/2004 S6 is correct,then it must periodically plow through the core of the F ring," saidDr. Joseph Spitale, an imaging team associate at the Space ScienceInstitute in Boulder, Colo. "The details of that interaction are notunderstood, but there probablyare observable consequences, and maybe the F ring spiral is one ofthem."
These ring results were acquired over the summer as Cassini wasin a prime ring-viewing period where the spacecraft's orbit was raisedto look down on the rings. The discoveries began almost immediately,with the discovery in May of a tiny moonlet orbiting within the narrowKeeler Gap in Saturn's outer A ring.
These and other results were resented in a press briefing atthe 37th Annual Meeting of the Division for Planetary Sciences meetingheld this week in Cambridge, England.
The Cassini-Huygens mission is a cooperative project of NASA,the European Space Agency and the ItalianSpace Agency. The Jet Propulsion Laboratory (JPL), a division of theCalifornia Institute of Technologyin Pasadena, manages the Cassini-Huygens mission for NASA's ScienceMission Directorate, Washington. The Cassini orbiter and its twoonboard cameras were designed, developed and assembled at JPL. Theimaging team consists of scientists from the U.S., England, France, andGermany. The imaging operationscenter and team leader (Dr. C. Porco) are based at the Space ScienceInstitute in Boulder, Colo.
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