Unraveling a mystery: Saturn's magnificient rings

By , Natural science editor of The Christian Science Monitor

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.

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.

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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.

Beyond this, beginning at 117,500 km, is one of the most famous classical features --the Cassini division which appears to be an empty gap as seen from Earth. However, as seen by Voyager, it shows five broad, sharp-edged, evenly spaced bands each with detailed structure down to the limits of resolution which , the Imaging Team says, still begs explanation.

Farther out still, the A-ring, which contains the narrow gap of the Encke division within it, begins at 121,000 km and extends out to 136,500 km. This, too, shows many narrow, well-ordered features, while the supposedly empty Encke division at 133,500 km has at least two narrow, clumpy ringlets.

Where the A-ring ends, at 136,200 km, is the edge of Saturn's classical ring system. But three more small, faint rings now are known --Pioneer 11 flyby, G (at 170,000 km), and E (extending roughly from 210,000 km to 300,000 km).

Planetary scientists tend to explain the structure and persistence of the rings in terms of the gravitational influences of one or more of Saturn's many large and small moons. Applying this theory to the intricate detail that now is known has turned out to be a challenge. Indeed, other factors may have to be identified to help account for some of the features. However, one of the more puzzling things -- the braided F-ring structure -- now may be accounted for.

Professor Dermott recently outlined in Nature how a pair of small satellites could impose such a pattern. These tiny satellites, now called 1980S27 and 1980 S26, orbit respectively 500 km inside and between 500 and 2,000 km outside the ring complex. Each of them acts to repel particles in the ring thus confining them to a narrow band.

The farther an orbiting body is from the center of its planet, the slower it goes around. Thus the inner satellite moves faster than the ring particles which, in turn, move faster than the outer moon. When a ring particle is near the inner satellite, it speeds up slightly and moves farther out from the planet. Conversely, a ring particle interacting with the outer satellite slows down a litte and moves inward. That is how the moons repel the particles even though gravity is basically an attractive force.

This much was suggested last November. Since then, Dermott has worked out a simple theory showing how the interaction could also set up waves within the ring material. These would make the rings form loops that could look like braiding. Shock waves could also be set up which would compress some of the ring particles together temporarily thus accounting for the clumps that also show up in the F-ring.

This doesn't explain the third ringlet feature which is neither looped nor clumpy, as Dermott himself points out. Perhaps, he suggests, this ring consists of particles sputtered out of the main F-ring by meteor impact and maintained by electro-magnetic forces. In any event, he says, he considers it quite likely that the outer satellite, especially, is responsible for the braiding.

Now planetary scientists are looking forward to Voyager 2's follow-up encounter with Saturn next August. They hope both for new revelations and for supplementary data. The F-ring, for example, has not yet been photographed from enough angles to allow it to be characterized in three dimensions nor do scientists yet know how its fatures evolve over time.

The Imaging Team calls the Voyager 1 harvest "the greatest leap forward since the 17th century in knowledge about the Saturn system." Now its members are preparing for a second flood of knowledge -- their last close look at a major planet for half a decade until Voyager 1 reaches Uranus or the Galileo mission arrives at Jupiter.

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