June 27, 1989

New Clues Emerge in Mystery of Planetary Rings

WITH Voyager 2 closing in for a rendezvous with Neptune in late August, fascinated scientists are anticipating new insights into one of the solar system's most spectacular and puzzling phenomena: the multitude of rings around the giant outer planets.

Scientists fully expect the spacecraft to discover rings around Neptune and to find that they are unlike those around Jupiter, Saturn and Uranus.

"We're prepared for anything," said Dr. Carolyn C. Porco, a specialist in planetary rings at the University of Arizona who is a member of the team that interprets Voyager's photographs. "There are still so many things we don't know about rings."

If the spacecraft detects rings around Neptune, it will be a fitting climax to 12 years of dramatic discoveries of the number and variety of rings that have coincided with, and often benefited from, Voyager's interplanetary travels. It will also test current speculation about what produces and maintains the rings.

Observations from the ground in recent years have served up tantalizing clues that swarms of icy particles are orbiting Neptune. But they seem to form incomplete rings, with clumps of material in a broken circle. These are described as ringlets or ring arcs. The evidence is indirect so far, and more often than not, astronomers straining for a telescopic glimpse have seen nothing at all.

View Back Toward the Sun

"Something's there," said Dr. Tobias Owen, an astronomer at the State University of New York at Stony Brook who is another member of the Voyager science team. "Nobody's actually seen these ring arcs or understands their physics. But the chances are very good that when Voyager passes Neptune and looks back toward the Sun, its cameras will detect them."

For Voyager 2, the robot spacecraft launched in 1977, the encounter with Neptune will be the culmination of the most far-ranging reconnaissance of the space age. The robot spacecraft flew by and photographed Jupiter in 1979 and Saturn in 1981 and then, continuing to operate after its planned mission had been fulfilled, cruised on to Uranus in 1986 for the first close-up look at that planet. Now, conceding nothing to age and distance, it is racing in at 42,000 miles an hour for the first visit to Neptune. It will make its closest approach on Aug. 24.

Neptune is the fourth-largest planet, after Jupiter, Saturn and Uranus, and usually the eighth one from the Sun. Because Pluto's elongated orbit sometimes brings it closer to the Sun, Neptune now is the ninth and most distant known planet in the solar system.

Project scientists at the Jet Propulsion Laboratory in Pasadena, Calif., announced last week that Voyager, still 60 million miles from its destination, had made its first major discovery. Photographs revealed a 6,200-mile-wide dark spot in the thick atmosphere of the planet's southern hemisphere. The feature reminds scientists of Jupiter's Great Red Spot, although it is about one-third the size. Jupiter's spot is a hurricane-like storm system.

Neptune's spot is darker than the surrounding blue-greenish atmosphere, so it is being called the Great Blue Spot, even though the color has not been determined.

Until 1977, astronomers knew nothing of rings around any planet other than Saturn. Now, they have been seen elsewhere, from the Voyager spacecraft and the ground, in a variety of configurations: a single faint ring at Jupiter, a number of narrow, dark ones at Uranus, and the resplendent broad bands at Saturn. Some are accompanied by tiny moons, while some are circular and others elliptical. Most of them lie in the planet's equatorial plane, except for those tilted at Uranus.

Accidental Discovery

"In the last dozen years, our conception of rings has undergone a revolution," Dr. Andre Brahic and Dr. William B. Hubbard wrote in the June issue of Sky and Telescope magazine. Dr. Brahic, an astronomer at the Paris Observatory, and Dr. Hubbard, a University of Arizona astronomer, are investigating the Neptune ring arcs.

The importance of studying planetary rings goes beyond an attempt to understand the dynamics of the major planets and their moons. Astrophysicists see the rings as an ideal "laboratory" for examining the physical forces that account for the behavior of such celestial objects as spiral galaxies.

The first surprise came in March 1977. A team of astronomers, headed by Dr. James L. Elliot of the Massachusetts Institute of Technology, was making telescopic observations of the passage of a faint star behind Uranus, seeking data about the planet and its atmosphere. They saw the star flicker five times before it was blocked by Uranus and five times after. This betrayed the presence of some intervening material. In this way, astronomers eventually discovered nine extremely narrow, dark rings around Uranus.

Two years later, Voyager 1 and then its companion, Voyager 2, found that Jupiter is circled by a thin ring of icy material, despite some widely held theories that Jupiter had been too hot in its early development for formation of such a ring.

These discoveries set off a flurry of theoretical speculation that continues to this day. Why were the ring systems present at the giant planets and absent at the smaller inner planets like Earth and Mars? Why were the rings at each planet so strikingly dissimilar in configurations?

Work of Tidal Forces Seen

"Although we are still a long way from understanding their origin and behavior," Dr. Brahic and Dr. Hubbard wrote, "it is generally recognized that planetary rings are a natural consequence of tidal forces and collisions among particles."

As early as 1850, in trying to account for the Saturnian rings, the French mathematician Edouard Roche concluded that tidal forces produced by a planet's gravity can break up a satellite or a passing body if it comes close enough to the planet. These forces also can prevent material too close to the planet from coalescing into a larger body. Most of the known rings lie inside this theoretical boundary, known as the Roche limit, an area of gravitational stability where rings may form and persist.

The mass of Saturn's entire ring system, scientists now estimate, is equivalent to that of a 200-mile-wide moon. The icy material in Uranus's rings would be sufficient to form a body no more than 20 miles in diameter.

Ringless Planets

In the inner solar system such debris cannot survive to form rings, Dr. Owen said, because of the proximity of the Sun and the relative nearness of the planets to each other. Solar radiation bombarding the particles causes them to lose orbital velocity and eventually fall into the nearest planet. And the interacting gravitational forces of the planets, especially in the Earth-Moon system, would also tend to sweep the region clean.

More puzzling is the existence of narrow rings. As Dr. Porco said: "No ring in orbit about a planet should remain narrow. Particle collisions, solar radiation and atmospheric drag forces would disperse a narrow ring and cause it to spread."

Peter Goldreich, a specialist in solar system dynamics at the California Institute of Technology, and Scott Tremaine, a scientist at the Institute for Advanced Study in Princeton, N.J., proposed a theory that is now widely accepted. They said the narrow rings were being shaped and preserved by the gravitational forces of small, undetected moons on either side of each ring.

Sure enough, the Voyagers photographed two shepherd moons, Prometheus and Pandora, on the inner and outer boundaries of Saturn's narrow F-ring. Complex interactions of these moons also produced irregularities in the rings that scientists described as an apparent "braiding" of strands of particles. Other undetected moons are suspected of creating partial rings in gaps within Saturn's main ring complex. A similar phenomenon, scientists said, could explain the apparent ring arcs expected to be found at Neptune.

'Elusive Shepherd'

When Voyager 2 reached Uranus, it discovered an additional narrow ring, raising the number to 10, and two tiny moons straddling one ring.

"At least for one ring, we found the elusive shepherd," Dr. Porco said. "Any other shepherding satellites must have diameters smaller than 12 miles, or Voyager 2 would have detected them."

In their most recent attempts to explain the origins of planetary rings, scientists have rejected the idea that the rings are composed of material left over from the time the planets were forming. They propose, instead, various "steady-state" theories in which the rings may be losing particles falling to the parent planet but are being replenished with material scattered by colliding objects.

Wing-Huan Ip, a scientist at the Max Planck Institute for Astronomy in West Germany, has presented evidence that a collision between a small comet and a 60-mile-wide moon could have formed the rings of Saturn in the last 100 million years. Other astronomers, however, questioned whether such a collision is statistically likely.

Researchers at the University of Colorado reported last week that an analysis of Voyager 2's findings at Uranus indicated that the rings around that planet could be no more than 600 million years old, unless they are being continuously supplied with new material. They calculated that rings and their shepherding moons can stay in orbit no longer than that before Uranus's extensive hydrogen atmosphere slows them down and causes them to fall into the planet.

Meteroids and Moons

"Unless we are seeing Uranus at a special time in its history, this requires a continuing process to create the ring material," the researchers, Dr. Larry W. Esposito and Dr. Joshua E. Colwell wrote in the journal Nature.

They suggested, therefore, that collisions between meteoroids and small moons were the source of new ring material. They cited Voyager 2's observations of dust bands near Uranus as the only visible evidence of the moonlets, which are too small to be detected by the spacecraft cameras, and the particles released in the meteoroid collisions. Dr. Esposito said he would not be surprised if Voyager 2 detected similar dust bands around Neptune.

Even though no spacecraft is scheduled to visit Saturn again soon, astronomers will take advantage of a rare event on the night of July 2 and early morning of July 3 to examine the planet's rings in greater detail. At that time, Saturn will pass almost directly in front of a bright star. With infrared telescopes, the astronomers expect to see the rings in an unusually brilliant display, and they will be tilted to present a full view.

"With luck," Dr. Porco said, "we will be able to see any variations in the ring system between Voyager and now."



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