Galileo Finds Jupiter's Rings Formed By Dust Blasted Off Small Moons

The findings were announced today by scientists from Cornell University and the National Optical Astronomy Observatories (NOAO), Tucson, Ariz., at a Cornell news briefing. "We now know the source of Jupiter's ring system and how it works," said Joseph Burns, Cornell professor of astronomy and the Irving Porter Church Professor of Engineering, who reported on the first intimate analysis of a planet's ring system, along with Cornell research associate Maureen Ockert-Bell, Joseph Veverka, professor of astronomy and planetary sciences at Cornell, and Michael Belton of NOAO.

"Rings are important dynamical laboratories to look at the processes that probably went on billions of years ago when the solar system was forming from a flattened disk of dust and gas," Burns explained. Furthermore, similar faint rings probably are associated with many small moons of the solar system's other giant planets. "I expect we will see similar systems at Saturn and the other giant planets" Burns said.

In the late 1970s, NASAs two Voyager spacecraft first revealed the structure of Jupiter's rings: a flattened main ring and an inner, cloud-like ring, called the halo, both composed of small, dark particles. One Voyager image seemed to indicate a third, faint outer ring. New Galileo data reveal that this third ring, known as the gossamer ring because of its transparency, consists of two rings. One is embedded within the other, and both are composed of microscopic debris from two small moons, Amalthea and Thebe.

"For the first time we can see the gossamer-bound dust coming off Amalthea and Thebe, and we now believe it is likely that the main ring comes from Adrastea and Metis," Burns said. "The structure of the gossamer rings was totally unexpected," Belton added. "These images provide one of the most significant discoveries of the entire Galileo imaging experiment."

Galileo took three dozen images of the rings and small moons during three orbits of Jupiter in 1996 and 1997. The four moons display "bizarre surfaces of undetermined composition that appear very dark, red and heavily cratered from meteoroid impacts," Veverka said. The rings contain very tiny particles resembling dark, reddish soot. Unlike Saturn's rings, there are no signs of ice in Jupiter's rings.

Scientists believe that dust is kicked off the small moons when they are struck by interplanetary meteoroids, or fragments of comets and asteroids, at speeds greatly magnified by Jupiter's huge gravitational field, like the cloud of chalk dust that rises when two erasers are banged together. The small moons are particularly vulnerable targets because of their relative closeness to the giant planet.

"In these impacts, the meteoroid is going so fast it buries itself deep in the moon, then vaporizes and explodes, causing debris to be thrown off at such high velocity that it escapes the satellite's gravitational field," Burns said.

If the moon is too big, dust particles will not have enough velocity to escape the moon's gravitational field. With a diameter of just five miles (eight kilometers) and an orbit that lies just at the periphery of the main ring, tiny Adrastea is "most perfectly suited for the job."

As dust particles are blasted off the moons, they enter orbits that are much like those of their source satellites, both in their size and in their slight tilt relative to Jupiter's equatorial plane. A tilted orbit wobbles about a planet's equator, much like a Hula-Hoop twirling around a person's waist. This close to Jupiter, orbits wobble back and forth in only a few months.

Because the swarm of dust grains all have very similar orbits, varying only in the orientation of the Hula-Hoop, all the dust particles orbiting together form a disc shaped like a washer. This explains why, in Galileo's close-up, edge-on view the ring-tip profiles are rectangular rather than the familiar elliptical arc seen on Saturn's rings and Jupiter's main ring.

The top and bottom edges of the gossamer rings are about two to three times brighter than the body of the rings. As Amalthea and Thebe move up and down in their orbits around Jupiter's equatorial plane, they pause as they reverse directions at the top and bottom of their paths, Ockert-Bell explained. Particles coming off these moons have similar tilts and are more visible at the upper and lower reaches of the satellites' orbits. "It's amazing that these satellites and ring edges correspond to the satellites' positions," she said.

The innermost halo ring appears to contain escaped particles from the main ring, Burns said. The particles are electrically charged and are pushed by Jupiter's enormous electromagnetic force, which causes clouds of particles to "bloom" into a vast cloud slowly drawn down into the planet.

Jupiter's diameter is approximately 86,000 miles (143,000 kilometers). The ring system begins about 55,000 miles (92,000 kilometers) from Jupiter's center and extends to about 150,000 miles (250,000 kilometers) from the planet.

Galileo has been orbiting Jupiter and its moons for 2-1/2 years and is currently in the midst of a two-year extension, known as the Galileo Europa Mission. JPL manages the Galileo mission for NASA's Office of Space Science, Washington, D.C. JPL is a division of The California Institute of Technology in Pasadena.

The new images, and further information on this discovery and the Galileo mission, are available on the Internet at the Galileo web site: http://www.jpl.nasa.gov/galileo. A streaming video copy of the entire press conference is available for viewing on the web at http://www.odl.cornell.edu/jupiter/. Viewing this presentation requires Microsoft Netshow software, which can be downloaded at http://www.dl.cornell.edu/download.htm (requires a PC running Windows 95/98).