From: Particle Physics and Astronomy Research Council
Posted: Wednesday, September 7, 2005
Cassini scientists today (5th September 2005) announced a host of fantastic new results from the spacecraft's first season of prime ring viewing, including some unexpected findings on Saturn's rings. These include new structures in Saturn's diffuse rings, clumps and knots in the F ring - some of which may be small moons - and a completely unexpected spiral ring around the planet in the vicinity of the F ring.
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 of the major discrete ring structures in the D ring has changed in brightness and moved inward towards Saturn by as much as 200 kilometres (124 miles). A change over the 25 years since the NASA Voyager spacecraft flybys indicates very short evolutionary lifetimes in the D ring and is of great interest to ring scientists who have been hoping that Cassini would yield information about ring ages and lifetimes.
Dr. Matt Hedman, an imaging team associate at Cornell University, Ithaca, N.Y. said, "I think our Cassini images of the D ring are providing new information about the dynamics and lifetimes of ring particles in a new regime, very close to the planet."
The delicate G ring encircles the planet at about 170,000 kilometres (106,000 miles) from Saturn's centre. Cassini scientists have now found a discontinuous bright ring segment, or 'arc', in this ring that bear at least a fleeting similarity to those imaged around Neptune in 1989 by NASA's Voyager 2 spacecraft. Scientists think that long-lived arcs may be created or maintained by a nearby hidden moon. Another thought is that they formed as 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 team scientists have also discovered that the ghostly ringlets flanking the ring's core are arranged into a spiral structure wound like a spring around the planet. Other spiralling structures seen in the main rings of Saturn, the density and bending waves, are initiated by the gravitational influence of an orbiting moon.
Density and bending waves move across the rings because of the way that relatively massive ring particles exert a gravitational influence on each other and can all move together. In contrast, the spiral structure contains very little mass and appears to originate from material somehow episodically ejected from the core of the F ring and then sheared out due to the different 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 of Paris. "It is very possible that the spiral is a consequence of moons crossing the F ring and spreading particles around, and may be telling us that the F ring might be a very unstable or even an ephemeral structure."
In the same region, scientists continue to spot small, clump-like features that may be loosely-bound clumps of material or tiny moonlets. Some of them have been sighted for the better part of a year. The solid-or-not nature of these mysterious F ring objects may be determined by repeated sightings: moons will persist, while clumps are expected to dissipate with time.
"We have long suspected that small moons were hiding among the F ring's strands and producing some of the structures that we see," said Imaging Team Member Professor Carl Murray of Queen Mary, University of London. "But now the problem is that we are detecting objects that may be either solid moons controlling the ring, or just loose clumps of particles within the ring, and it's hard to tell the difference. It is like trying to distinguish sheep dogs from sheep in a very large flock."
A puzzling characteristic of at least two of the clumps/moons is that they appear to cross the F ring periodically. One of them, an object that was discovered last year (S/2004 S6), may be responsible for 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," said Dr. Joseph Spitale, an imaging team associate at the Space Science Institute in Boulder, Colo. "The details of that interaction are not understood, but there probably are observable consequences, and maybe the F ring spiral is one of them."
These ring results were acquired over the summer as Cassini was in a prime ring-viewing period where the spacecraft's orbit was raised to look down on the rings. The discoveries began almost immediately, with the discovery in May of a tiny moonlet orbiting within the narrow Keeler Gap in Saturn's outer A ring.
These and other results were presented in a press briefing at the 37th Annual Meeting of the Division for Planetary Sciences meeting held this week in Cambridge, England.
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