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The Rings

Saturn and its ring system
Saturn and its ring system (Click on the image for a larger view)

Perhaps the greatest surprises and the most perplexing puzzles the two Voyagers found are in the rings.

Voyager 1 found much structure in the classical A-, B- and C-rings. Some scientists suggest that the structure might be unresolved ringlets and gaps. Photos by Voyager 1 were of lower resolution than those of Voyager 2, and scientists at first believed the gaps might be created by tiny satellites orbiting within the rings and sweeping out bands of particles. One such gap was detected at the inner edge of the Cassini Division.

Voyager 2 measurements provided the data scientists need to understand the structure. High-resolution photos of the inner edge of the Cassini Division showed no sign of satellites larger than five to nine kilometers (three to six miles). No systematic searches were conducted in other ring gaps.

Voyager 2's photopolarimeter provided more surprises. The instrument measured changes in starlight from Delta Scorpii as Voyager 2 flew above the rings and the light passed through them. The photopolarimeter could resolve structure smaller than 300 meters (1,000 feet).

The star-occultation experiment showed that few clear gaps exist in the rings. The structure in the B-ring, instead, appears to be variations in density waves or other, stationary, forms of waves. Density waves are formed by the gravitational effects of Saturn's satellites. (The resonant points are places where a particle would orbit Saturn in one-half or one-third the time needed by a satellite, such as Mimas.) For example, at the 2:1 resonant point with 1980S1, a series of outward-propagating density waves has about 60 grams of material per square centimeter of ring area, and the velocity of particles relative to one another is about one millimeter per second. Small-scale structure of the rings may therefore be transitory, although larger-scale features, such as the Cassini and Encke Divisions, appear more permanent.

The edges of the rings where the few gaps exist are so sharp that the ring must be less than about 200 meters (650 feet) thick there, and may be only 10 meters (33 feet) thick. In almost every case where clear gaps do appear in the rings, eccentric ringlets are found. All show variations in brightness. Some differences are due to clumping or kinking, and others to nearly complete absence of material. Some scientists believe the only plausible explanation for the clear regions and kinky ringlets is the presence of nearby undetected satellites.

Two separate, discontinuous ringlets were found in the A-ring gap, known as Encke's Gap, about 73,000 kilometers (45,000 miles) from Saturn's cloud tops. At high resolution, at least one of the ringlets has multiple strands.

Saturn's F-ring was discovered by Pioneer 11 in 1979. Photos of the F-ring taken by Voyager 1 showed three separate strands that appear twisted or braided. At higher resolution, Voyager 2 found five separate strands in a region that had no apparent braiding, and surprisingly revealed only one small region where the F-ring appeared twisted. The photopolarimeter found the brightest of the F-ring strands was subdivided into at least 10 strands. The twists are believed to originate in gravitational perturbations caused by one of two shepherding satellites, 1980S27. Clumps in the F-ring appear uniformly distributed around the ring every 9,000 kilometers (6,999 miles), a spacing that very nearly coincides with the relative motion of F-ring particles and the interior shepherding satellite in one orbital period. By analogy, similar mechanisms might be operating for the kinky ringlets that exist in the Encke Gap.

The spokes found in the B-ring appear only at radial distances between 43,000 kilometers (27,000 miles) and 57,000 kilometers (35,000 miles) above Saturn's clouds. Some spokes, those thought to be most recently formed, are narrow and have a radial alignment, and appear to corotate with Saturn's magneticfield in 10 hours, 39.4 minutes. The broader, less radial spokes appear to have formed earlier than the narrow examples and seem to follow Keplerian orbits: Individual areas corotate at speeds governed by distances from the center of the planet. In some cases, scientists believe they see evidence that new spokes are reprinted over older ones. Their formation is not restricted to regions near the planet's shadow, but seems to favor a particular Saturnian longitude. As both spacecraft approached Saturn, the spokes appeared dark against a bright ring background. As the Voyagers departed, the spokes appeared brighter than the surrounding ring areas, indicating that the material scatters reflected sunlight more efficiently in a forward direction, a quality that is characteristic of fine, dust-sized particles. Spokes are also visible at high phase angles in light reflected from Saturn on the unilluminated underside of the rings.

Another challenge scientists face in understanding the rings is that even general dimensions do not seem to remain true at all positions around Saturn: The distance of the B-ring;s outer edge, near a 2:1 resonance with Mimas, varies by at least 140 kilometers (90 miles) and probably by as much as 200 kilometers (120 miles). Furthermore, the elliptical shape of the outer edge does not follow a Keplerian orbit, since Saturn is at the center of the ellipse, rather than at one focus. The gravitational effects of Mimas are most likely responsible for the elliptical shape, as well as for the variable width of the Huygens Gap between the B-ring and the Cassini Division.

 

 
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