Mooning over the dust rings of Jupiter

 The deep impacts that killed the dinosaurs or excavated our moon's vast craters count among the most spectacular examples of collisions in the solar system. Even little crashes, however, can make a big difference.

 Images taken by the Galileo spacecraft reveal that the dust kicked up by scraps of interplanetary debris plowing into four of Jupiter's tiniest moons are the source of the giant planet's dust rings. Mars' moon Phobos also has been pummeled and its surface pulverized into powder perhaps a meter deep.

 Faint rings encircling Jupiter's equator between the planet and its large moon Io were discovered by the two Voyager craft in the late 1970s. The craft revealed a flattened main ring, along with a puffier, inner ring called the halo. The observations also hinted at a third, wispy, outer ring. Galileo images, taken in 1996 and 1997 and released this week, show that the outer ring is in fact two rings, dubbed gossamer rings.

 Jupiter's tiny moon Adrastea, only 20 kilometers across, skims the main ring's outer edge, while another small satellite, Metis, lies within the ring. The Galileo pictures confirm that the ring's densest part is the outer edge, adding weight to earlier suspicions that Adrastea feeds the ring. As one of the smallest Jovian moons, Adrastea has weak gravity and stands to lose great amounts of dust during any impacts.

 The new pictures show that two other moons, Thebe and Amalthea, each orbit the outer edge of a different gossamer ring and provide the material for them. The halo appears to be made of charged dust particles that are lifted out of the main ring by electromagnetic forces, says Joseph Veverka of Cornell University.

"For the first time we understand why Jupiter has rings and how the rings actually work," he says. Veverka and his colleagues unveiled the Galileo images at a Cornell press briefing.

 All four moons appear dark, red, and heavily cratered, indicating that they have been bombarded by meteoroids, which are fragments of asteroids and comets. Correspondingly, the rings contain tiny, reddish particles that resemble dark soot. Galileo viewed the rings almost edge-on, lit from behind by the sun, an arrangement that made micrometer-size particles highly visible.

 The angles at which the satellites orbit Jupiter, relative to the planet's equatorial plane, correlate with the vertical extent, or height, of the rings. Adrastea and Metis orbit Jupiter almost exactly in the equatorial plane. Such orbits do not wobble, and dust lifted from the surface of these moons forms a flat ring. 

 In contrast, the paths of Amalthea and Thebe tilt. Over several months, their orbits precess about the equator like gyrating hula hoops. Dust from these moons create rings with significant height, like the gossamer rings.

 Because the ring particles eventually spiral into Jupiter, the rings would vanish without a supply of new material, Veverka says. The moons provide this reservoir as long as debris continue to pelt them. In a similar fashion, he adds, the small moons orbiting Uranus and Neptune may be the source of the faint dust rings surrounding those planets.

 Saturn's famous rings, which are much more massive and contain larger, icy particles, are thought to have a different origin-either the breakup of a giant, frozen body or collisions between several large, icy moons. The Cassini mission, expected to reach Saturn in 2004, will fly through the ice rings. In December 2000, the craft will swing past Jupiter and is scheduled to view the dust rings from a different angle than Galileo did, notes Carl D. Murray of Queen Mary and Westfield College in London.

 Closer to home, the Mars Global Surveyor has found another example of a body that appears to have been pounded by debris. In this case, the surface is pulverized into dust that stays on the moon.

 Temperature measurements of the Martian moon Phobos reveal that it rapidly loses heat after sunset. Although Phobos completes one revolution in just 7 hours, its sunlit side has an average temperature of -4@C, far higher than the night side's average of -112@C.

 Solid rock or boulders retain heat, but powder cannot, explains Philip R. Christensen of Arizona State University in Tempe. The large temperature variation on Phobos and the moon's lack of jagged surface features suggest that its topmost layer has been ground into a fine powder that might be as deep as 1 meter, he says. NASA announced the finding last week.

 The powder "could be the future source of a dust ring around Mars," says Murray.