Mihaly Horanyi of CU-Boulder's Laboratory for Atmospheric and Space Physics said the dust particle bursts grew more frequent and intense as the Cassini spacecraft closed in on the planet from 40 million miles away in early 2004. Horanyi, also a CU-Boulder physics professor, said the dust might have escaped into space from the planet's outer "A" ring or perhaps its icy moons, Dionne and Rhea.

A paper on the subject was published in the Jan. 20 issue of the journal Nature. In addition to Horanyi, authors include Sascha Kempf, Ralf Srama, Stefan Helfert, Georg Moragas-Klostermeyer and Eberhard Grun of Germany's Max Planck Institute, and Marcia Burton and Mou Roy of NASA's Jet Propulsion Laboratory.

Prior to 1993, researchers believed such interplanetary dust particles were made up of primitive material originating from interstellar space, asteroid collisions or from comets whizzing through the solar system, Horanyi said.

But in 1993, Horanyi and colleagues found that peculiar bursts of microscopic dust observed streaming from the Jupiter region by the Ulysses spacecraft were emanating from the volcanic plumes of Io, a moon of Jupiter. "That was the first indication that dust can escape from planetary systems," he said.

The dust particles escaping from the Saturn system also are microscopic in size, ranging from one-tenth of a micron to one-hundredth of a micron, he said.

"Grains larger than that would be dominated by the gravity of the planet, while smaller grains would be dominated by the electromagnetic fields. Only those within these size limits can escape the system."

The dust detector aboard Cassini collects electrical signals from each grain of dust it traps, allowing scientists to infer the mass and speed of the particles, he said. "The energetics of the dust particles indicate that the A ring of Saturn is a good candidate for the origin of the dust," Horanyi said.

The burst-like phenomenon of dust grains -- which were traveling at an astounding 60 miles per second -- was due to the planet's magnetic field and the solar wind "bending" the dust trajectories as they streamed from the Saturn system, he said. "The dust is probably coming from the Saturn system at a fairly constant rate, but the spacecraft was not always in the right place to detect it," he said.

Horanyi also is a member of a CU-Boulder team that has designed and built a dust detector to fly on NASA's 2006 New Horizons mission to Pluto. The New Horizons instrument, designed and built primarily by students, is expected to provide new information on the structure and early formation of the solar system.

An identical instrument also built by LASP will be launched in September 2006 on the Aeronomy of Ice in the Mesosphere, or AIM, mission to monitor the dust influx into Earth's atmosphere, he said.