Saturn's magnificent rings are turning out to be one of the major puzzles of the solar system. This is the recurring theme as mission team scientists comment daily on the increasingly detailed images coming across 1.5 billion kilometers of interplanetary space from the Voyager 1 spacecraft.
As view from Earth, the rings have seemd to be only a few well-defined broad bands. Now, team scientists are finding a complex system of a hundred or more individual ringlets. What had seemd empty gaps between rings are filled with material unseen from Earth. And there now appear to be two distinctly different kinds of ring material.
Scientists at NASA's Jet Propulsion Laboratory here have difficulty even speculating about the rings in a meaningful way, let along coming up with theories about how the rings are formed and maintained. Moreover, every time they try to grapple with the puzzle, new details appear to challenge them.
As viewed from Earth, Saturn appears to have three main rings, named in order of their discovery. the innermost, or "C" ring, begins 72,600 kilometers (42, 500 miles) from the planet's center and extends out to 91,800 kilometers (54,080 miles). It is immediately succeeded by the "B" ring, the brightest, which extends to 117,000 kilometers (70,200 miles). Then there is an apparent gap, the so-called Casini division, 4,800 kilometers (2,800 miles) wide. The outermost main ring, the "A" ring, extends from 121,800 kilometers (73,080 miles) to 137,400 kilometers (82,440 miles), interrupted near its outer edge by a thin space called the Encke division. Saturn itself has a radius of 60,300 kilometers (36,180 miles).
There are two fainter rings further out. One, which has been photographed from Earth, is the "E" ring, extending as far as 480,000 kilometers (288,000 miles). When the Pioneer Saturn spacecraft drifted past Saturn 14 months ago, it found a thin ring, now called the "F" ring, inside that, just outside the "A" ring. A sixth ring, tentatively named "D," that some observers thought they saw close to the planet, no longer is believed to exist.
This neatly defined ring system now appears amazingly complex. the A, B, and C rings consist of many dozens of distinct ringlets, looking like grooves on a phonograph record.
The Casini division is not empty, but consists of series of rings about 500 kilometers (300 miles) wide, separated by dark edges. And as Voyager nears Saturn, more detail is showing up. A moderately dark ringlet has appeared at the inner edge of the Casini division.
It is getting hard to tell where the division is at all, Dr. Bradford A. Smith of the University of Arizona comments. The more detailed the pictures become, the more the gap seems to be filled with material. Even more striking, the material in the C ring and in the Casini division appears to be distinctly different from that of the A and B rings. It is too early to say whether the division and the C ring have the same kind of material, Dr. Smith says. But there is no doubt that this material has different reflective properties from that of the A and B rings.
To add to the complexity, a curious clumpiness has appeared in the F, the next to the outermost ring. Hints of this were observed by the Pioneer Saturn spacecraft.Scientists don't know what the clumps are, how they form, or how long they last. They appear to persist for at least several days and may represent an actual accumulation of ring particles or a dispersion of larger bodies.
Saturn has a number of small moons, a hundred kilometers or os in diameter, that are intimately associated with the rings. Now this curious clumping has appeared. How, scientists here wonder, do orbiting particles interact with the larger bodies to maintain the rings, which are Saturn's most distinctive features?
All this is perplexing enough. But then there are dark spokes, or fingers of material, tht spread out across the B ring. Weeks of theorizing have not produced even a glimmer of understanding of their nature, Smith says. they may be zones relatively free of particles, or they may be rays of darker material. Moreover, because the rings are believed to consist of individual particles orbiting Saturn, it is hard to see how the spoke-like features could arise as organized patterns that lie right across the particles' orbit.
Saturn is giving planetary scientists one of their most important challenges. Until they can explain how planetary rings arise and persist, they cannot claim to understand how the solar system was formed. They must account for the absence of rings around Mercury, Venus, Earth, and Mars and the prevalence of rings around the outer giant planets. Jupiter and Uranus also are known to have rings. It will not be surprising if Neptune someday is found to have them, too.