Of all the majestic mysteries in the heavens, none is more majestic or mysterious than the rings of Saturn.
Reaching out almost a half million miles from the surface of saturn, the bright rings are the most beautiful sight in the solar system. In their reach, the rings dwarf the planets. In their composition, they are the most ephemeral bodies in the sky. As bright and beautiful as they are, they have almost no substance. The rings are as ghostly as commets in the sky.
The rings of Saturn were first seen in 1610 by Galileo, who said they looked like "cup handles" and didn't even know they were rings. It was not until 1659 that Christian Huygens identified the first ring, not until 1675 that Jean Dominique Cassini found the second ring and not until 1848 that William Bond at Harvard and William Dawes at Cambridge realized there was a third ring.
The three rings of Saturn, that's what they were called for almost 100 years. Then, a fourth ring was found, then a fifth and a little over a year ago, a sixth was discovered. Were these even more? Back came the answer last week, when the Voyager spacecraft swept beneath the rings for the first time. Saturn was circled by at least 300 rings, which have probably been there since the dawn of time. "We're tired of counting," Dr. Bradford Smith of the University of Arizona said last Thursday at the Jet Propulsion Laboratory where the flight of Voyager is directed. "I expect that by the time we finish counting the rings their number will be somewhere between 500 and 1,000."
Numbers like these only deepen the mystery of the rings, whose origins are only a part of their mystery. What are the rings made of? Why are they still there? Why don't all the planets have rings like Saturn does? There are the questions scientists have asked since Huygens found the first ring more than three centuries ago.
Some scientists think all the planets once had bright and beautiful rings, but Mercury and Venus were so close to the sun their rings would have melted. Earth and Mars would have sucked their rings to the surface. Uranus still has nine rings but they are so dark they were found only four years ago. The single ring around Jupiter is so thin it wasn't discovered until Voyager flew by Jupiter more than two years ago.
"Jupiter grabbed so much of the early solar system's hydrogen that it got very hot, trying to be a little star on its own," said Dr. Jeffrey Cuzzi of the Ames Research Center in Mountain View, Calif. "Its luminosity alone may have melted away its rings early in time."
That leaves the rings of Saturn, a planet which didn't get as hot as Jupiter, is far enough out in space to keep the sun from melting the rings and is light enough (Saturn would float if the ocean were big enough) not to pull the rings down to its surface.
That might answer why rings circle Saturn but it doesn't answer why the rings of Saturn formed in the first place. Why didn't the rings get together to form moons? That's the way the other planets treated their excess baggaged. Could Saturn have captured an icy moon early in time and broken it up into the tiny fragments that appear to make up the rings today?
The passage of Voyager under the rings last week suggests a different story. If a moon had broken up to form the rings, it would have come apart in large enough chunks to still be visible inside the rings.
"If you break up a moon, you don't crush it into bread crumbs, you break it into big pieces," Cuzzi said the other day. "What Voyager has shown us inside the rings are nowhere near that big."
Voyager has found three new moons around Saturn but they're outside the main body of the rings. One lies just beyond the outermost of the three large rings, the other two are on either side of the most distant of the six biggest rings. There are no moons visible anywhere inside the three largest rings or anywhere in the gaps between the rings.
"If there were other moons to be seen inside the rings, we would have seen them by now," Cuzzi said last week. "All we've seen are new gaps and a lot more rings."
To scientists, the more important question is not why the rings formed around Saturn but why the rings are still there. Says Voyager Project scientist Edward C. Stone of the California Institute of Technology: "When you look at them, you get a feeling of impermanence, that nothing this flimsy could last this long. But they've been there for 4.6 billion years and I'm sure they'll go on being there for another 4.6 billion years."
What holds the rings together? The prevailing theory is that Saturn's 15 moons keep the rings together, that their positions and different rotating speeds outside the three largest rings serve to keep the rings fenced in, that without the moons the rings would all break up and lose whatever it is they're made of to interplanetary space.
There's little question the two moons just inside and outside the sixth large ring act as shepherders to the material inside the rings. The moon inside the ring moves around a little faster than the ring while the moon outside the ring moves a little more slowly, both of them acting to keep the ring material from falling in toward Saturn or slipping out toward space. There seems to be a harmony in their resonance that keeps the ring intact.
But this doesn't explain the permanence of the other rings, which are a lot thicker than the sixth ring and presumably need more shepherding to keep them in place. Some of this attention comes from the larger moons outside the rings like Mimas, Iapetus and Titan, but not enough to explain why the rings are still there, hanging in space by not even a thread.
"To be honest, we're in a quandary to explain it," Cuzzi says. "I don't think any of us believes the rings are transient but a solid explanation for their permanence just isn't there yet."
One missing piece in the puzzle is the composition of the rings. What are the rings made of? Radar bounced off the rings by the world's most powerful radio telescopes suggests they're made of some kind of exotic metal, rock covered with ice or just ice.Few scientists believe they are made of metal. No matter how light, any metal would be at least 10 times heavier than the rings appear to be. Where would the metal come from? Saturn is no more than a big grassy bell. Whatever metal it contains is so far and deep inside the planet it may be locked up there forever.
That leaves ice. Most scientists agree the rings are made of some rocky ice but they are in fierce disagreement over the kinds of ice that may have formed the rings. Some think it's ammonia ice, others believe they're made of methane ice. The trouble with ammonia and methane is they evaporate rather quickly at the temperatures seen so far in the rings, which is about 87 degrees (minus 369 degrees Fahrenheit) above absolute zero.
Probably, the rings are little more than water ice, which evaporates much more slowly than methane and ammonia at ring temperature. The trouble with that is the rings have a distinct reddish color which cannot be explained by water ice.
"Ice doesn't do that," Cuzzi says. "If the rings are water ice they're contaminated with some kind of cosmic dust or radiation or maybe even the metals the radar people keep talking about."
Cuzzi believes the riddle of the rings lies in whatever scheme made Saturn the second largest planet in the solar system and placed it in the position it's in, sixth planet out from the sun, more than 1 billion miles away from the sun. Cuzzi says that instead of seeking the origins of the rings we should ask ourselves why the planets grow so dramatically in size at Jupiter, then begin to shrink again at Saturn.
"If Saturn were located where Jupiter is and Jupiter where Saturn is," Cuzzi said, "Saturn would still have the rings and Jupiter would not."
Whatever their origins, the rings still capture the fancy of scientists everywhere just the way they did in the times of Galileo, Huygens and Cassini. Said Andre Brahic of the University of Paris: "It's like waiting for a beautiful woman you have never seen before. She is not what you expected but she is quite all right just the same."