The rings: gasp of surprise, glimmer of understanding
Pasadena, Calif. — Saturn is better than a three-ring circus . . . by far. In fact, it has thousands of rings and such a complicated structure that planetary scientists studying it feel they may need years to figure it all out.
Saturn's rings, in fact, make one of the greatest shows provided yet by the US National Aeronautics and Space Administration's (NASA) waning planetary exploration program.
"It's funny, because after the sensational Jupiter encounter [by the Voyager spacecraft two years ago] we were afraid Saturn would be a disappointment," confesses Richard Terrile of the Jet Propulsion Laboratory (JPL) where the spacecraft is being controlled.
From Earth, Saturn's rings look smooth and homogenous. They appear divided into three parts, prosaically dubbed "A," "B," and "C," from the outside in. But when the Pioneer 11 spacecraft flew by Saturn in 1979, it saw some evidence that things were more complicated than previously thought. But Pioneer's camera was too poor to reveal the rings in all their splendor.
That privilege was reserved for the twin Voyager spacecraft. The first passed Saturn nine months ago and found a number of totally unexpected features. The spacecraft's television pictures found a number of additional rings which were labeled "D" through "G". Instead of a bland disk, the spacecraft revealed something that looked a little like a giant phonograph record, a record over 200 ,000 miles in diameter. One of the outer rings was made of several strands that appeared to be braided. And, most unexpectedly of all, the spacecraft's pictures showed strange, radial features that looked vaguely like spokes, appearing and disappearing on the B ring.
At that time, about all the scientists back on Earth could do is gasp in wonder and make slightly wild-eyed guesses as to the reasons for these strange features.
In the interval between Voyager 1 and Voyager 2, however, he researchers spent long hours studying the hundreds of new pictures they had of the rings. The spacecraft flight team reprogrammed the second spacecraft to look far more intensively at the rings than they had planned.
One of the key experiments conducted bt Voyager 2 was a stellar occultation of the rings. In this, the spacecraft's most sensitive instrument (called a photopolarimeter) measured the intensity of the light of a star as it passed through the rings. As a result, scientists got a measure of the ring's transparency, or lack thereof, along a single slice.
"This allows us to see features as small as a city block," explains Arthur L. "Lonne" Lane of JPL.
Although they have only scratched the surface in the analysis of their data, Dr. Lane and his colleagues have some preliminary results. They estimate that the rings are extremely thin, only 100 meters or so thick. If one constructed a model of the rings the thickness of a phonograph record, it would be miles in diameter.
The stellar occultation also reveals evidence of sharp gaps that may be cut by large rocks or "moonlets" a few miles across. Also, the researchers see what may be "gravity waves" -- spiral waves caused by the gravitational pull of Saturn's many moonlets and moons. And there are a few places where they think they actually may have caught the shadow of one of the large ring particles.
The JPL scientists can tell this only by inference, however. All they can directly measure is the optical thickness of the entire ring. To get an estimate of the size of the icy chunks that make them up a different approach is required: passing radio waves through the rings.
Len Tyler of Stanford is the researcher in charge of this experiment. He finds that the average size of ring particles varies from 10 meters in the outer reaches of the rings to 2 meters in the inner. "This grading was a total surprise," says Dr. Tyler. However, Peter Goldreich and Scott Tremaine of the California Institute of Technology (Caltech) believe they understand this. They think it is because smaller particles can move outward and inward more readily than larger, more massive particles. cise in gravity. It is difficult enough to figure out what three different chunks are going to do in outer space, let alone millions. Even with the power of today's computers, it is not possible to model such a complex situation, says Dr. Goldreich.
Voyager's cameras also have revealed a number of unexpected ring features, although they cannot see details as fine as the photopolarimeter. Perhaps the most significant is evidence of the dynamic nature of the rings: They appear to change and wobbe in ways that are poorly understood here.
"When we look at features on opposite sides of the rings, or at the same place at different times, things don't seem to line up," says Dr. Terrile. For instance, when Voyager 1 photographed the outer "F" ring, it recorded several strands that appeared to be braided. But when Voyager 2 looked for the same thing, it was not found.
Then, of course, there are the "spokes." Voyage 2 made a series of spoke movies which show these sharp-edged smudges appear and disappear in the thick B ring. The leading theory for these strange features is that they are fine dust particles levitated above the plane of the rings by static electricity.
This theory is supported by the fact that the spacecraft's instruments have detected radio bursts characteristic of lighting. From the way these vary, the scientists estimate they comes from an area squarely in the middle of where the spokes are seen.
"This is 10,000 to 100,000 times more powerful than terrestrial lighting," estimates Joseph Romig of Radiophysics Inc. He believes this can best be explained by a large moonlet six miles in diameter building up large static electricity charges from ring dust. But the scientists studying the rings feel that they probably would have seen the gap cut by such a body if it were present.
Another remaining question is the composition of the ring material. The scientists know it is mostly ice. Computer processed pictures of the rings show subtle variations in color. The A and C rings are bluer, while the B ring is redder in color.
There is some controversy over the meaning of these color differences. However, a number of the scientists believe they represent subtle differences in chemical composition of the ring material. The most intriguing aspect of these color differences are quite sharp in some places.
"If the ring material were left over from the formation of the planet, you would expect the color changes to be gradual," says Jeffrey Cuzzi of NASA's Ames Research Center. Also, scientists have thought that the continual process of break up and reformation of particles would have smeared out differences of this sort.
"These differences may be evidence that the rings were formed from several larger satellites that broke up," says Eugene Shoemaker of Caltech.
Whether this was the case will require careful analysis and consideration, like many of the other puzzle presented by Saturn's rings. And some probably will not be solved until future spacecraft are dispatched to Saturn to study it further.