Saturn may be circled by brilliant rings because it is also circled by as many as 200 moons hidden inside the rings and so small they are invisible to Earth-bound telescopes, Voyager Project scientists said today.
"So far, we have not detected any of these moonlets but we think they're there," scientist Edward C. Stone said at the Jet Propulsion Laboratory, where the Voyager II spacecraft's flight by Saturn is being directed. "If we don't find any of these moons imbedded inside the rings we're in trouble."
The 1,700-pound spacecraft should be able to spot at least a few of these moons between the hundreds of dazzling rings that reach out almost a half million miles from Saturn. Now less than 1.7 million miles from the planet, Voyager II will fly as close as 5,000 miles from the rings at 11:25 p.m. EDT Tuesday.
The prevailing theory of Saturn's rings is that there are scores of tiny moons imbedded in the rings, sweeping up planetary dust like little vacuum cleaners, keeping the spaces open between the rings and thus preventing them from forming a single ring.
Saturn is known to have 17 moons in its orbit, five of which were found by Voyager I when it flew by the planet last November. Voyager scientists are convinced that the largest of the theoretical "imbedded" moons lies at the inner edge of the Cassini division, the large gap between the two largest rings that is named for the 17th century Italian astronomer who first decided Saturn was encircled by rings.
"There is a gap here of about 500 kilometers 310 miles , which is much wider than expected," Stone said today. "If you put a moon in there that's about 20 or 30 kilometers across, you'd get a gap that's the same size as the one we observed."
Stone said this hidden moon appears to be in resonance with the moon called Mimas which lies just outside the outermost ring and circles the planet at 40,000 miles an hour. Scientists theorize that Mimas, outside the rings, and the moon inside the Cassini division act as "shepherds" keeping the entire ring system around the planet.
"We're possibly talking about hundreds of these imbedded satellites," said the University of Arizona's Dr. Bradford C. Smith, leader of the team of photographic scientists studying the thousands of pictures being sent back by Voyager more than 1 billion miles away. "Even though we haven't seen any of them yet, these imbedded satellites are the best explanation we have for the existence of so many rings."
The discovery of even a single moon imbedded in Saturn's rings could answer the puzzling mystery of why Saturn is surrounded by as many as 1,000 majestic rings. The biggest question about Saturn's rings is not their existence but what holds them together.
Again, the prevailing theory is that Saturn's many 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 break up and lose whatever it is they are made of to interplanetary space.
There is little question that two moons just inside and outside the sixth large ring act as shepherds to the material inside the ring. The moon inside the ring moves around a little faster than the ring while the moon outside the ring moves a little slower, both of them acting to keep the ring material from falling in toward Saturn or slipping out into space.
Photographs returned today to the Jet Propulsion Laboratory of the 16th moon called Iapetus showed a world that is half brilliant white and half deep and puzzling black. The only moon of its type in the solar system, Iapetus looks like a world that is made half of ice and half of coal.
"The dark side is darker than anything else in the solar system," scientist Smith said. "We're talking about a moon whose dark side is three or four times darker than the darkest region of our own moon."
Scientists today were puzzling over whether the black-and-white moon is made of what they jokingly call either "foofoo dust" or "foofoo goo." If it is made of "foofoo dust," scientists believe Iapetus is coated on one side by dust coming off the outermost moon of Saturn called Phoebe. If Iapetus is made of "foofoo goo," is is being coated in on its dark side by a dark material erupting from inside the moon and brought to the surface by high-speed jets of methane gas.
"We don't know why Iapetus is colored the way it is, but recent photographs taken from earth suggest that Iapetus and Phoebe are made of different materials," said Dr. Eugene Shoemaker of the U.S. Geological Survey at Flagstaff, Ariz. "This suggests that the dark spots are flooded crater floors, a peculiar thing that is unique to Iapetus in the solar system."