News Release

The observations confirmed a 30-year-old prediction that frozen water in the rings is vaporized and then broken into its constituents, creating a tenuous "atmosphere" of gas around the rings.

Researchers have known for decades that Saturn's rings are composed of many particles, each covered at least in part by frozen water. Scientists don't know how the ring system formed, but one theory is that a sizeable moon containing a large quantity of water orbited too close to the planet and was torn apart by gravitational tidal forces. The satellite's debris, including its water, would then have been dispersed around Saturn, evolving into the colorful ring system.

"The question is, how long ago did this happen, and how long do the rings survive?" said Doyle Hall, an associate research scientist in The Johns Hopkins University Department of Physics and Astronomy, who led a team of investigators for the Hubble Space Telescope observations.

The findings are being reported in the April 26 issue of the journal Science, in a paper written by Hall and astronomers Paul Feldman of Johns Hopkins, Melissa McGrath of the Space Telescope Science Institute and Jay B. Holberg of the University of Arizona.

Bombardment by solar radiation, charged particles, dust-size meteorites or other ring particles all tend to erode the icy ring particles, creating small plumes of water vapor. The water vapor molecules then split into hydrogen atoms and hydroxyl molecules, which are made of oxygen and hydrogen.

By analyzing the Hubble data, astronomers learned that the ring system is losing up to 3,000 kilograms (6,600 pounds) of frozen water per second. But even at this rate, the researchers estimate that the rings probably will survive at least another billion years before they are dehydrated into dried-out remnants. Because the erosion rate is relatively slow, the rings might have formed a billion or more years ago, Hall said. But astronomers can't be certain of that because they don't know what the erosion rate was in the past, he said.

Ever since the icy nature of Saturn's rings was discovered, astronomers have wanted to search for the hydroxyl molecules. They got their chance in August 1995, when Saturn's rings appeared edge-on from the Earth. Because the rings are so thin, the edge-on view makes them almost invisible from Earth, just as a sheet of paper would be invisible if viewed perfectly edge-on from a distance. Without the usual bright glare of reflected sunlight from the rings, astronomers were able to study the tenuous envelope of gas that surrounds the rings. The astronomers used the Hubble telescope's Faint Object Spectrograph to observe the region directly above the rings, and detected the signature of hydroxyl molecules in ultraviolet light.

Astronomers won't soon have another chance to spy Saturn's rings edge-on; the orbital configuration that made the rings appear edge-on occurs roughly every 15 years. But the next two times it happens, Saturn will be nearly on the other side of the Sun and out of Earth's view.

"We won't have an opportunity to do this kind of experiment from Earth's perspective until the year 2038," Hall said.

Although ring systems surround all four of the solar system's giant planets, Jupiter, Saturn, Neptune and Uranus, none is as spectacular as Saturn's. The ring system surrounding Uranus, for example, appears to be dried-out and dark compared to Saturn's colorful rings. Perhaps that's because those rings were subjected to the same sort of dehydrating erosion astronomers are now seeing on Saturn, Hall said.

Knowing how fast Saturn's rings are losing water is valuable information that may enable scientists to uncover secrets about the nature of the other planetary ring systems, he said. The findings also could lead to new insights into the workings of Saturn's magnetosphere, which surrounds the planet and its ring system. After the water is broken down into its constituents, the hydroxyl molecules and hydrogen atoms eventually are stripped of electrons, turning into positively charged ions that become trapped in Saturn's magnetic field. The addition of those charged particles may have an important effect on the magnetosphere, which exerts profound influences on Saturn's atmosphere.

EMBARGOED FOR RELEASE AT 5 P.M. EDT ON THURSDAY, APRIL 25, 1996