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Unraveling a mystery: Saturn's magnificient rings

By Robert C. CowenNatural science editor of The Christian Science Monitor / May 6, 1981



One of the most startling discoveries of Voyager 1's close encounter with Saturn last November was the magnificient complexity of that planet's ring system. Half a year later, planetary scientists still boggle at its intricacy.

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Nevertheless, some new insights have begun to emerge from the profusion of data.

For one thing, there no longer is any doubt of an inner ring, the so-called D-ring, next to the planet and inside the ring system as seen from Earth. There had been considerable controversy over whether or not such a feature had been detected by Earth-based observers. Voyager pictures show it clearly. But the Voyager Imaging Team says the ring material is so thin it is unlikely that ground observers could actually have seen it.

What may be more intriguing, the F-ring --with a third ring along side -- now may be explained as an intricate gravitational interplay between ring particles and a pair of small satellites that flank them.When discovered, these satellites were suspected of keeping the rings in existence, herding them like a pair of sheep dogs. Recently, Stanley F. Dermott of Cornell University showed that they could account for the apparent braiding and clumpiness seen in this ring complex , too.

As noted in a recent report in Science by the Imaging Team -- the group of 27 scientists immediately responsible for collecting, organizing, and analyzing the data -- Voyager pictures "revealed hundreds of components of the ring system, perhaps [only] six of which were previously known." This is the complexity which has awed the experts.

Those known elements consisted mainly of the classic rings -- A, B, C, and D (now confirmed) -- and two apparently "empty" gaps called the Cassini and Encke divisions. There were also signs of faint rings farther out. These elements still form the framework for talking about what now is known to be a system of many hundreds of rings with scarcely any really empty gaps in the classical region.

The rings appear to begin where the planet ends. The Saturn team takes the outer edge of the planet to be at about 60,330 kilometers from its center. Voyager 1 passed within 126,000 km of that so-called "cloud-top level" last Nov. 12.The D-ring, wh ich may actually extend down to this level, now has been traced to a radial distance of 66,500 km from the center of Saturn, the reference point for ring measurements.

This ring is what scientists call "optically thin," meaning it is fairly transparent, which is why it is unlikely to have ever been seen from the ground. Yet it does contain considerable material organized into many narrow features varying in width from several hundred kilometers down to 35 kilometers, the smallest feature the pictures can resolve.

This material extends out to the inner edge of the C-ring, the first of the well-known rings, at 73,200 km. This ring, which shines so brightly as seen from Earth, shows broad bands of nearly transparent material with narrow, much more opaque features within them.

Their structure butts against the B-ring at a distance of 92,200 km. The boundary between the two is sharp, although it shows no gap, and their material seems to be distinctly different. The B-ring is the largest, brightest, generally most opaque of them all. It extends outward some 25,000 km, showing hundreds of bright ringlets and dark gaps ranging in width from 100 km downward to the limits of what the images can resolve.