New Saturn moons or only transient debris?

Rather than discovering several new satellites of Saturn recently, as some astronomers originally reported, the Hubble Space Telescope may only have spied clouds of ice and dust orbiting the ringed planet. Some of that debris, however, may represent remnants of tiny, shattered moons.

On May 22 and again on Aug. 10, Earth crossed the plane in which Saturn's rings orbit the planet. On Nov. 19, the sun followed suit. During such events, which occur only once every 15 years, the rings lose their brilliance, allowing Saturn's faint moons to become visible. Many of the planet's known moons were discovered under these conditions.

Last summer, Amanda S. Bosh of Lowell Observatory in Flagstaff, Ariz., and Andrew S. Rivkin of the University of Arizona in Tucson announced that during the May 22 ring-plane crossing, Hubble detected two objects never seen before. Both appeared to be moons of Saturn (SN: 8/5/95, p. 87). But after analyzing the data further, Bosh has now determined that only one of the bodies is a new moon. The other, dubbed S3, has an orbit so similar to one of Saturn's outer rings, the F ring, that it in fact represents a small clump of material from the ring.

From Hubble images taken during the Aug. 10 crossing, Philip D. Nicholson of Cornell University and his colleagues detected two never-before-seen bodies as well as S3. The team has concluded that none is a moon. Nicholson says the objects must have been created during the last 14 years, because they are too big and too bright for the Voyager craft to have overlooked them during Saturn flybys in the early 1980s. In addition, the brightest of the three bodies is elongated, unlike a satellite.

All three objects seem to have disappeared by Nov. 21, when Hubble again viewed Saturn, Nicholson notes. Bruno Sicardy, an astronomer at the Paris Observatory in Meudon, France, suggests that the objects, thought to lie near the F ring, disappeared precisely because of their location.

In the neighborhood of the F ring, Saturn's tidal pull-the difference in gravitational tug from opposite sides of the planet-could break off a piece of a small moon. This loose package of material would briefly form a cloud of debris bright enough to be seen by a telescope.

Sicardy says the August and November observations reveal that material near the F ring breaks loose and is destroyed in a matter of weeks. The Voyager craft found evidence of such activity, but the new images mark the first time that observations from Earth's vicinity have documented it.

"This is even more interesting than finding new satellites," Sicardy declares, noting that similar processes probably occurred when many of the planets first formed moons and rings.

Earth will again cross Saturn's ring plane on Feb. 11, but the sun will lie too close to Saturn for Hubble to view the event.