he rings of Saturn may have formed when an ancient celestial body was caught by the planet and broken into moons that kept colliding and grinding each other down until they became the magnificent rings around the sixth planet from the sun.
"What we see now is a finely ground residue of the stuff that made it into the rings when the heavy bombardment of the solar system slowed down 4 billion years ago," Dr. Eugene Shoemaker of the U.S. Geological Survey at Flagstaff, Ariz., said today at the Jet Propulsion Laboratory, where the flight of Voyager II past Saturn is directed.
Earlier, the Voyager II flight directors said the camera platform they unjammed last Wednesday was working so well that they planned to return it to normal operation this Thursday when the spacecraft encounters the outermost moon of Saturn on its way out from the planet. That moon, called Phoebe, is moving backward around Saturn and scientists believe it is the nucleus of a captured comet.
"Each movement we make of the platform frees up the platform a little more," Voyager project director Esker K. Davis said. "It's like a rusty hinge that you keep moving back and forth and right now we are seeing essentially normal movement in our platform."
Normal operation of the camera platform means that Voyager II's onboard computer will again take control, moving the platform in a pre-programmed fashion to compensate for the rapid motions it sees as the spacecraft flies toward Uranus at 25,000 miles an hour. When the platform jammed this week, flight directors at the Jet Propulsion Laboratory took over operation of the platform, commanding it to move at a speed 20 times slower than normal.
The photographs being sent back by the spacecraft as it speeds by Saturn are giving clues to scientists as to what they might find five and eight years hence, when Voyager II reaches Uranus and Neptune.
Shoemaker said that he believes the outer planets Uranus and Neptune went through even more collisions over time than Saturn did. The farther scientists go in the solar system, he said, the more likely it is they will find more signs of chaos early on in time.
"My prediction is we'll find a cluttered moon system at Uranus, full of the regular shapes that suggest a history of collision," Shoemaker said. "We know of six moons at Uranus. I think we'll find 25 or 30, and when we get to Neptune I think we'll see even more." Voyager II is scheduled to arrive at Uranus in 1986 and to reach Neptune in 1989.
Shoemaker's remarks were made in an interview after fresh pictures taken obliquely of the rings by Voyager II suggested that the finest ring particles are in the innermost rings and the largest and coarsest in the outer rings. Small ice particles appear to dominate the inner rings, which show up in Voyager photographs in brilliant purples and blues.
Large pieces of rock appear to make up the outer rings, which are revealed in bright gold and red.
"The fine particles in the inner rings backscatter blue light, just as the fine particles in our own sky make our sky look blue," Shoemaker said. "The yellows and reds of the outer rings are typical of rocky materials like silicates or even carbonaceous chondrites that meteors and even comets are made of."
Most of the 17,000 pictures taken by Voyager so far have been of the thousands of brightly colored rings circling the planet that so fascinate the scientists here studying them. Slowly but surely, a picture of the rings has been emerging that traces their origins back to the breakup of a single large object that was caught by Saturn as it came from deep space near the dawn of time.
"It's difficult for me to imagine how you can avoid mixing the rings early on and making them all uniform," said Dr. Laurance Soderblom of the U.S. Geological Survey. "To me, the ring material chemically and compositionally looks everywhere the same."
Like Shoemaker, Soderblom said he believes that the different colors in the rings result from different sizes in the ring particles.
Everywhere around Saturn, there are signs of collisions. No fewer than eight of Saturn's 17 moons are small irregular-shaped objects that look like broken pieces of rock and ice that may even today still be running into each other. One pair of tiny moons are in the same orbit and obviously were pieces of the same body that broke apart millions or even billions of years ago.
"What you have here around Saturn is a system where bodies at randomized, eccentric and inclined orbits were constantly running into each other," Soderblom said. "Saturn's is a history of worlds in collisions."
The moon of Saturn called Mimas has all the appearance, Soderblom said, of a moon that has been broken apart and reassembled many times. Mimas is located in a position that Soderblom described as a "gravitational well" around Saturn where bodies flying in from space would naturally fall when they arrived at Saturn.