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Jupiter's Dusty Doughnut Spins Backward

Astronomers have detected a new ring of tiny dust particles circling Jupiter. The ring is unique in the solar system, because most of the dust seems to be orbiting backward--in the opposite direction of Jupiter's rotation. The large doughnut-shaped ring, described in the current Science, is no match for Saturn's glorious halos, however, for its particles are so far apart that the ring is invisible to telescopes.

Jupiter is hardly a tidy place. Small meteorites crashing into its moons throw up dust, and the volcanic moon Io also spews particles into orbit. Some of this flotsam settles within 5000 kilometers of Jupiter's surface in a narrow ring, first spotted by Voyager 1 in 1979. Further out, new dust continually arrives from passing comets, collisions between asteroids, and interstellar debris that wafts through the galaxy. Now, NASA's Galileo spacecraft has found evidence that Jupiter's intense magnetic field captures some of this external dust into large looping orbits that linger for a century or more before spiraling into the planets or moons--or being eroded away by small impacts.

Within the last 2 years, a detector aboard Galileo has registered impacts from dust grains hitting the probe about 1 micrometer across, slightly bigger than the particles in cigarette smoke. The dust extends as far as 1,100,000 kilometers from the planet's surface, within the orbits of Jupiter's four largest moons. When a team of planetary scientists led by Joshua Colwell of the University of Colorado in Boulder modeled the most likely orbits of the dust, they concluded that 80% of the particles move in "retrograde" orbits--counter to Jupiter's spin and the orbits of its major moons. "There are lots of different planetary rings, but this is the first retrograde one we've seen," Colwell says. There's no hope to take a picture of the ring, he notes, because it contains just 10 particles per cubic kilometer of space--too tenuous for Galileo's cameras to spot.

The reason for the backward orbits is not clear, Colwell says. His team suspects that the retrograde motion of the dust ring may be a consequence of the direction from which the dust approaches Jupiter. Other objects in the solar system have similar retrograde orbits, including four of Jupiter's outermost moons--which astronomers believe are captured asteroids. However, these moons are too distant and too small to account for the dust ring.

The research "adds to the many interesting particles near Jupiter we must try to explain," says space scientist Herbert Zook of NASA's Johnson Space Center in Houston, Texas. Similar magnetic fields around Saturn make Zook suspect that researchers will unveil another dusty doughnut there after the Cassini probe arrives in 2004.