Saturn: ring ripple suggests 19th moon.

Saturn: Ring ripple suggests 19th moon

Clues to an unseen moon have hidden since 1980 in the spiky, weed-like patterns of brightness recorded by Voyager 1 as it flew past Saturn's F-ring. By mathematically parting those weeds, astronomers have found five gentle ripples of ring brightness, each with its own periodicity. One ripple, they say, suggests that an unidentified moon, less than 10 kilometers in diameter, traces an eccentric orbit around the giant planet.

Scientists have long puzzled over irregular density variations in Saturn's F-ring, and have suspected that the gravity of nearby moons plays a role. To explore that possibility, Robert A. Kolvoord and Joseph R. Burns of Cornell University, working with Mark Showalter of Stanford University, indirectly examined the density variations by analyzing the ring's shine. Clumping and spreading of F-ring particles, they reasoned, would increase and decrease the ring's reflectivity.

As expected, the analysis showed one undulation due to Prometheus, the nearest satellite to the ring at a distance of 832 km. However, the smaller and more distant moon called Pandora -- 1,520 km from the ring -- causes no apparent perturbation, they report in the June 21 NATURE.

One of the four remaining ripples hints at the gravitational influence of an elusive moon located 1,180 km from the ring, the researchers say. But a final ruling on the proposed moon -- which would represent the planet's 19th known satellite -- must await the scrutiny of the Cassini orbiter, scheduled to begin its tour of Saturn in 2002.