Secrets of the Rings

Twisted, knotted, polka-dotted; here today, gone-cosmologically speaking--tomorrow. The finely sculpted adornments of the giant planets are a mystery we're just beginning to solve.

By Dava Sobel|Friday, April 01, 1994
RELATED TAGS: SOLAR SYSTEM
Galileo mistook Saturn's strange markings for a pair of moons. Other early astronomers thought the planet spat out giant clouds of vaporous breath that clung close by; still others suggested that Saturn was shaped like an egg and that two black blotches on its surface made it look like a double-handled cup.

It was Christiaan Huygens who first realized that Saturn's odd "appendages" might in fact be a ring, which he supposed was a solid, shiny strip. Still cautious about this conclusion, however, he hedged his bets by publishing his idea in the form of an anagram. At the end of his 1656 pamphlet announcing his discovery of Saturn's moon Titan, Huygens added a coded message that, unscrambled, read: "It is surrounded by a thin flat ring, nowhere touching, and inclined to the ecliptic." Not until the end of the nineteenth century did James Clerk Maxwell correctly suggest that a solid ring would shatter under the gravitational strains of orbit; he concluded that the ring must consist of a dense collection of countless separate small particles, all orbiting Saturn the way the planets orbit the sun.

Today the sprawling, sparkling Saturnian rings continue to surprise astronomers lured into their beguiling maze. During the past ten years, for example, many scientists have been forced to abandon the long- established notion that the rings of Saturn are as ancient and enduring as the solar system itself. It now appears that the rings could not have formed along with the planet 4.5 billion years ago. Rather, they are a recent addition no more than 100 million years old. Furthermore, the same processes that created them are already sowing the seeds of their destruction. This makes the rings a passing fancy that will disappear before the next 100 million years go by. In all likelihood Saturn has fathered several generations of rings over the course of its lifetime.

Once considered unique, Saturn's rings now represent merely the most spectacular specimens of a cosmic species known to circle every giant world--from Jupiter's diffuse, dusty halo to the narrow, dark hoops of Uranus to the demi-rings that appear to trace a line of dashes around Neptune. The presence of so many different systems suggests that rings sprout as a normal part of a giant planet's life cycle.

"I'm interested in all the ring systems," says Jeffrey Cuzzi of NASA's Ames Research Center in Mountain View, California, who was drawn to his work by the confounding beauty of Saturn's rings. "I like to think of them as a family. Everybody is an individual, yet because of the resemblances, you can understand each one better by knowing something about the others."

To decipher the mysteries encoded in the rings, astronomers are currently scrutinizing these planetary sidekicks with every available means. They view them through ground-based, airborne, and space telescopes- -both by the light that rings reflect from the sun and in the silhouettes created whenever a ringed planet passes against the backdrop of a bright star. Some are planning future spacecraft encounters with Jupiter and Saturn for close-up study, while others content themselves with mental modeling, such as programming supercomputers to mimic ring behavior, or trying to account for the origin and intricate structure of planetary rings with nothing more than "a pencil attached to a brain," as one theoretician quipped.

For now, the ice-bright rings of Saturn reign as lord of all the ring systems. Though Huygens took them for a single, shining body, within 20 years Jean-Dominique Cassini, using a superior telescope, found a gap. There were two rings, he insisted in 1675, with a dark lane between them that is still known today as Cassini's division. Even now the most conspicuous features of Saturn's ring system, seen from a distance, remain the bright A ring, the even brighter, broader B ring that lies inside, just across Cassini's division, and the dim C ring, first seen in 1850, which reaches from the B ring's inner margin almost to the cloud tops of the planet. More recently the Voyager and Pioneer missions confirmed the existence of three additional outlying rings (E, G, and F) and the vanishingly faint innermost D ring.

In fact, close scrutiny reveals at least 100,000 individual ringlets, each a little different from the next, shaped by the pulling of the planet and its many moons on the rich fabric of the ring particles. Picture the system as a spinning superhighway, divided into myriad narrow lanes. With modern radar and spacecraft, the ring particles resolve into red-tinged snowballs that range in size from sand grains to enormous boulders. Large satellites that lie beyond the ring system carve features within it, such as the scimitar-sharp edges and scalloped hems. Tiny moonlets embedded within the rings may knot them, braid them, clump them, and cut slices out of them.

Rings, most researchers agree, are moons gone to pieces--or captured comets, caught on the fly and then torn to shreds by competing gravitational forces. If you scooped up all the scattered particles of ice and dust in the glittering ring system of Saturn and packed them together, you could mold a moon about the size of Saturn's moon Mimas--a little under 250 miles in diameter. Such a satellite probably existed quite close to the planet 100 million years ago. Then along came a comet or another big body on a collision course and blasted the moon to bits.

This ill-fated moon lay within Saturn's Roche limit--named for the nineteenth-century French mathematician Edouard Roche and defined as the region close to a planet where competing gravitational forces are strong enough to shatter unstable satellites or prevent them from forming in the first place. Within the Roche limit, destructive tidal forces dominate other effects. Tidal forces pull those parts of an orbiting body that are relatively close to the planet more strongly than the parts farther away; as a result, the satellite--held together only by the weak glue of its own gravity--may literally be pulled to pieces. (Not all orbiting bodies must submit to tidal forces. The space shuttle, for example, orbits well within Earth's Roche limit yet does not come undone, because its constituent parts are held together by nuts, bolts, and the fierce crystal cohesion of the molecules in its metal parts.)

Outside the Roche limit, particles of a pulverized moon may regroup themselves bit by bit until they eventually fashion a born-again moon. Just such a series of calamitous events may have befallen Uranus's moon Miranda and could explain why this particular satellite looks like an accident victim whose parts have been haphazardly sewn together. Miranda, some astronomers say, apparently formed, broke apart on impact, reassembled itself, suffered a subsequent impact, pulled itself together again, and so on--perhaps as many as five or six times since the solar system was young. Other ring researchers have reason to discount this scenario, but like all aspects of ring evolution, the real history of Miranda remains a matter for informed speculation.

The moon that became Saturn's rings was too close to the massive planet to pull itself back together. Once it was smashed to bits, the pieces--large and small--all went into orbit in a cloud of debris. These particles eventually fell into a disk around the planet's equator. Orbits of particles at angles to the disk plane inevitably crossed, leading to frequent collisions. Only ring lanes more or less parallel to one another were able to survive for long. And even then, the particles continued (and continue) to bump and jostle one another, spreading themselves into concentric rings of varying diameters, like the grooves of a phonograph record.

Today the great circle spreads out into a disk more than 180,000 miles wide but scarcely 60 feet high, making it many orders of magnitude flatter than a pancake. Ring researchers compare it to a sheet of tissue paper spread across a football field.

"You could never get another massive ring like this one around Saturn by bashing up a moon," observes Carolyn Porco of the Lunar and Planetary Laboratory at the University of Arizona in Tucson. "The only satellite left that's big enough to do it is Mimas, and Mimas lies far outside the Roche limit. But at Uranus and Neptune you still have plenty of food for the ring machine. If you broke up all the satellites within the Roche limit of Neptune, you'd get a ring system that would not look too terribly different from Saturn's." That hasn't happened yet because nothing has smashed into one of these moons, but nothing precludes such an encounter in the future.

"These rings just wouldn't be as bright," says Porco, "because the stuff in the outer solar system tends to be darker." The Saturnian rings are lit by reflections from the water ice that predominates there. In the outer reaches of the solar system, carbon prevails; the Uranian rings are dark as soot.

No one is certain, of course, that Saturn's rings were formed by a single large moon. They could just as well have formed from a group of three small satellites that were smashed, creating the current three-main- ring design. It's even possible that the rings hail from a large comet that passed close to Saturn and got ripped apart by tidal forces.

It could happen again, even today. Catastrophic events are hardly relegated to the solar system's distant past. In mid-1992 a comet known as Shoemaker-Levy split into at least a dozen pieces after a close brush with Jupiter; it now looks like an orbiting string of pearls. These strung-out pieces are expected to crash-land on Jupiter next July in a great burst of fireworks. But had the comet's path been a little different, its debris might have remained in Jupiter orbit, perhaps developing into a stunning, substantive new ring. Some researchers are predicting that the smallest particles might yet produce a diffuse ring within ten years.

"Mechanisms that are going on in the solar system today can explain how we got rings in the first place," notes Porco. "This is one of the arguments supporting the idea that the rings could be young."

The dazzling brightness of Saturn's rings is another convincing indication of youth. Even though the rings of Saturn glisten with water ice and are therefore intrinsically brighter than the coal black boulders that encircle Uranus, they still look eerily new, almost like a brand-new pair of tennis sneakers. And that strikes astronomers as odd.

"We think the rings would look much darker if they were very old," says Cuzzi. "In computer models we start off with nice clean rings of water ice, then dirty them up with comet crud--like a snowfall in New York." It doesn't take very long, on planetary time scales, for this dark, computer-generated cometary dirt to scatter among the ice particles and turn the simulated rings the same color as Saturn's--about 100 million years.

Cuzzi's current research is focused on interpreting the varying shades of red in the Saturnian rings. Most "comet crud"--the pervasive debris that eventually dirties almost everything in space--is reddish. Scientists think it's either organic material or iron-bearing minerals, or some combination of both. In Earth's atmosphere the sprinkling of space dust causes meteors, or shooting stars. Saturn probably comes in for an even heavier pelting since the planet's greater mass attracts more material its way. And the rings make the grandest dust collector imaginable.

Whatever the nature of the reddish dust, Cuzzi can account for the gradations in shade by the effects of meteoroid bombardment. "Once dirt gets into the rings, it stays there and mixes in with the other material," he says, in a process he calls pollution transport. Dark dirt, distributed throughout the broad but thin ring plane, never disappears from view. The rings just grow dingier as time goes by. In the latest-generation computer model, just as in actual images of Saturn's rings, color deepens in a broad swath throughout the ring span. The areas with the least amount of material--the gaps, such as Cassini's division, and the tenuous areas, such as the C ring--have the fewest places for the dirt to hide and tend to get darkest fastest.

Aside from causing color changes, the rain of debris could also add enough mass to the ring system to topple it before it reached an advanced old age. The new material landing among the ring particles lacks the angular momentum of the already orbiting particles; since the amount of angular momentum in a system always stays the same, other particles must sink to lower orbits, picking up speed, to compensate (just as a spinning ice skater's speed increases as he pulls in his arms). The effect is that ring particles slowly but inexorably begin to fall from the sky. Since we still see the rings, however, we can assume that they're too young to have accumulated an overdose of extra mass.

"If our estimate of the frequency of meteoroid hits is correct," Cuzzi says, "it's hard to understand how the rings could be more than 100 million years old." That estimate will be checked when the next outer- planet mission mounted by NASA and the European Space Agency, scheduled for launch in 1997, reaches Saturn in 2004. The double spacecraft, named for Huygens and Cassini, will measure the gross amount and individual particle size of infalling meteoroid material, as well as its velocity and electric charge. Once Huygens separates to land on Titan, Cassini will sail for at least four years among the rings and moons of Saturn; the craft is expected to send back some 500,000 images defining the stark geometry of the Saturn system and perhaps revealing the forces that shape it.

Close-ups of the rings will certainly aid the continuing investigation of their complex, convoluted structure. Saturn boasts braided rings, scalloped rings, wide rings, and narrow rings--even partial rings that don't wind all the way around the planet. Any feature that distinguishes other ring systems--the perfect margins of Uranus's thin rings, say, or the unfinished look of Neptune's--can be found somewhere in the broad ring plane of Saturn, and then some.

Porco, who leads the Cassini imaging effort, wouldn't be at all surprised to stumble on a few new moons that are too tiny or too embedded in the ring system to be detected from afar. Saturn's rings are not only the ruined remains of former moons (or comets); they also owe some of their fancy structure to the action of existing moons--at least 18 at last count- -that orbit around and among the ring particles. Obvious ring features, such as the crisp margins and the dark Cassini division, which are readily seen through an amateur's small telescope, look sculpted by an outside hand. In a way, they are.

"You wouldn't expect a bunch of orbiting particles to create such sharp features," says Caltech theoretician Peter Goldreich, explaining why he and Scott Tremaine, now director of the Canadian Institute for Theoretical Astrophysics in Toronto, first took on the ring margins as a thorny problem in dynamics. Something had to be confining particles, preventing them from following their natural tendency to spread out.

Physicists had long suspected that an important clue to the identity of that "something" lay in the respective positions of the Cassini division and the moon Mimas, which orbits about 60,000 miles outside the ring. The satellite travels at a slower pace than the ring particles owing to its greater distance from Saturn. As Kepler showed in his laws of planetary motion, orbital speed depends on distance, with the closest bodies circling the fastest. Thus, at Saturn, the ring particles race ahead of the outlying moons. And the ring particles near the Cassini division travel almost precisely twice as fast as Mimas. Every time the moon orbits once around the planet, the particles shoot around twice. They fall directly under the moon's influence twice each go-around--once when Mimas is halfway through its appointed round, and once at the finish. At both these points Mimas pulls on the particles harder than usual. Since the pull always comes at the same two points in the orbit, it gains strength over time--just as a child on a swing goes higher faster if always pushed at exactly the same place in the swing's arc. Even very gentle pushes in the right place can, over time, create a huge momentum, sufficient to propel the child right over the top. Similarly, Mimas has shoved the particles out of Cassini's division.

The orbits of the particles and Mimas are said to be in 2:1 resonance. Other examples of whole-number harmonics abound in the Saturnian system and likewise create distinctive ring features. For example, when spacecraft flybys revealed that the A ring had a scalloped outer edge, like a stylized celestial flower, Porco was able to explain the shape as the work of resonances between the ring particles and two nearby moons. The moons, Janus and Epimetheus, share roughly the same orbit and keep roughly the same pace. The ring particles, however, regularly overtake the moons in a 7:6 resonance. The particles in the A ring complete exactly seven orbits around Saturn for every six circuits the moons make. The complex pattern of periodic encounters carves the seven rounded lobes on the margin of the A ring. The B ring, sculpted by the same mechanism, has two lobes.

"Other investigators had considered the interactions of satellites with individual particles," says Goldreich, "but those effects would be too small to open gaps or maintain edges." Goldreich and Tremaine realized that the satellites were resonating instead with vast collections of particles that moved together fluidly, like molecules of water in an ocean wave. The particles, held together by their own mutual gravitational attractions, would not only feel the rhythmic tugging of the moons but also transmit the disturbance outward through the ring plane, from the resonant orbits toward the outer fringes of the rings.

Indeed, Goldreich and Tremaine predicted that if they could get close to Saturn's rings, they would actually see these disturbances moving across the rings in spiral waves of compression and rarefaction not unlike sound waves. This bold prediction was verified when Voyager II photographed more than 30 spiral density waves in the A ring that looked like the alternating light and dark stripes of a spinning pinwheel. The waves begin at the resonant orbits, then spiral out through thick and thin regions of the ring; some die out, others crash on its scalloped shores. "A large fraction of the structure seen in the A ring of Saturn is created by these density waves," Goldreich says.

It's no accident that this pattern shares an uncanny resemblance with the spiral arms of the Milky Way. The rings may be built from the same blueprints, although in the case of our galaxy the sources of the gravitational tugging remain unknown. Planetary rings also provide a handy model for the protoplanetary disk that circled our sun and condensed into planets 4.5 billion years ago; the current planetary orbits would correspond to density concentrations of ages past. Even such exotica as the accretion disks observed around neutron stars and white dwarfs, as well as theoretical disks presumed to orbit quasars and black holes, may turn out to be patterned on this same Saturnian model.

Still other complicated resonance effects are created by satellites whose orbits are tilted--orbits, that is, that swing above and below the ring plane instead of tracing a larger concentric circle around it. These satellites pull particles not only sideways but upward and downward too. As a result, the particles are whipped into "spiral bending waves" that mimic the three-dimensional ridges in corrugated cardboard.

Other features in the rings demand different explanations. For example, two tiny Saturnian moons, discovered during a spacecraft flyby, orbit on either side of the planet's thin, outlying F ring. These newfound moons define the ring's shape by keeping the particles, which would otherwise tend to spread out, confined between them in a narrow strip--the way a couple of yapping dogs hold a flock of sheep together. The moons' names are Pandora and Prometheus, but they are more often called shepherd moons. Goldreich and Porco showed that two other shepherd moons, Cordelia and Ophelia, serve as ring bearers to the outermost ring of Uranus--which looks, in Porco's description, "as though someone took a brush and carefully painted between the lines."

The mechanism behind the herding involves both gravitational forces and complex exchanges of angular momentum between moons and ring particles. In brief, it works like this: The shepherd moon outside the ring moves more slowly than the ring particles, which in turn are outpaced by the fast inner moon. The outer moon pulls on the particles as they whiz by, producing a bulge in the ring; but since the particles are moving faster, the bulge quickly gets ahead of the moon and pulls it forward. This increase in speed boosts the satellite's angular momentum, pushing it into a higher orbit. Since the ring particles must lose as much momentum as the moon gains (to conserve angular momentum), they fall into lower orbits. If you could watch this action from above the ring plane, it would appear that the outer moon and the particle ring were repelling each other. Meanwhile, on the inside track, the moon's gravity raises a bulge in the ring, which lags behind the moon. The bulge slows the moon down, robbing it of angular momentum. That same momentum is gained by the ring particles, which respond by jumping into higher orbits. Caught between the inner moon and the outer moon, the particles are focused into a narrow band.

When Neptune's rings were spotted by Voyager II in 1989, they looked like scattered arcs, like broken lines that didn't wholeheartedly embrace the planet. Closer investigation, however, revealed that these arcs are very dense regions of quite tenuous rings that do, in fact, make full circles around Neptune. Porco speculates that the inclination of the orbit of the moon Galatea, tilted slightly in relation to the orbit of the ring particles, might be pulling them into clumps, creating the rich clusters that appear to be isolated ring arcs from afar.

Jupiter's rings are more ephemeral still--so insubstantial that many ring investigators scorn the system as "fluff" too tenuous to trifle with. The largest planet seems to have the frailest ring system of all. Over time, encounters with space debris have pulverized the ring particles to a powder barely detectable except by the closest scrutiny. Jupiter's thin main dust ring is flanked on the inside by a thick, cloudlike halo that reaches halfway to the planet, and on the outside by a faint "gossamer ring," discovered and named by Joseph Burns of Cornell.

With this faint ring, as with perfume, Burns argues, a small amount of material can raise a high level of interest. The amount is so small, in fact, that gravity seems to play a rather small role in the Jovian ring dynamics. Instead the rings seem organized around electromagnetic effects: the minuscule bits of dust dance to electric forces created within Jupiter's enormous magnetic field. Conspicuous by their absence are any ring features that could be attributed to gravitational resonances or shepherding. Although Jupiter has a large retinue of satellites, they can't seem to get a grip on the gossamer ring.

Somewhere buried in the dust, Burns believes, are the remains of the moons that spawned the system. If the rings were merely dust through and through, with no larger bodies shrouded within, the dust would dissipate quickly--in less than three years. Something keeps replenishing the dust particles, and the most likely candidates are small moons under constant meteoroid bombardment. Burns christened these hypothetical bodies "mooms" because they were part moon and part mom, or parent body, to the ring. The concept is generally accepted by other ring researchers, although the name has not stuck.

In this regeneration process, with crumbling remnant moons feeding a waning ring, the Jovian system shows its only family resemblance to the other systems. Old or young, all the rings consist of ancient materials, recycled through the solar system's tumultuous activity.

"The ring systems are a little bit like poppies on the hill," muses Cuzzi. "You come back next year to the same place, and you'll still see poppies on the hillside, but they're not the poppies you saw last year. In the same way, the rings of the planets may not be the same rings that were there a million years ago--or 10 million or 100 million years ago. They're just the most recent incarnation. And the process just keeps on going."
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