Jovian comet crash: puzzles and insights.

More than 2 months after the fragments of Comet Shoemaker-Levy 9 plowed into Jupiter, astronomers continue their struggle to understand exactly what the impacts have revealed about the inside of the gaseous planet. But if much of the Jovian interior remains a mystery, the debris carried aloft by the collisions have dredged up new insights about the planet's upper atmosphere.

Minutes after several of the larger fragments hit Jupiter, plumes of dark debris shot through the Jovian cloud tops. Over days to weeks, as the dusty material slowly settled, high-altitude winds set some of the debris adrift.

Just as a colored dye traces the swirling motion of water, the dark blotches in the upper atmosphere have enabled scientists to trace the motion of Jovian winds at new heights. "This is the first time we have measured the wind speed of Jupiter in the upper atmosphere," says John T. Clarke of the University of Michigan in Ann Arbor.

Using the Hubble Space Telescope to observe in the far-ultraviolet -- wavelengths that detect the highest-altitude debris -- Clarke's team measured how fast the wind blows in Jupiter's upper atmosphere and thermosphere, some 100 to 400 kilometers above the planet's visible clouds. They find that at these altitudes the Jovian wind moves at roughly 1 km per second.

Clarke and other Hubble researchers presented their findings about the Jovian collisions last week at a press briefing in Washington, D.C.

Clarke previously reported Hubble's detection of two short-lived auroral arcs in Jupiter's northern midlatitudes. The telescope observed the features in the ultraviolet about 45 minutes after the K impact, one of the larger fragments of Shoemaker-Levy 9, struck the planet's southern hemisphere.

Auroras form when charged particles spiral along Jupiter's magnetic field lines and crash into the atmosphere above the planet's polar regions. But these arcs lasted only an hour and resided at latitudes lower than any northern aurora ever observed on Jupiter, Clarke notes. Models indicate that the arcs appeared in a region magnetically linked to the K impact site in the south.

Data from the X-ray satellite ROSAT, which detected an X-ray burst in Jupiter's northern midlatitudes minutes after the K impact, may shed light on the auroras. The intensity of the burst suggests that highly energetic charged particles generated the two arcs.

Clarke speculates that the K impact disturbed ions and electrons normally confined to radiation belts that encircle Jupiter. (Similar bands, the Van Allen radiation belts, surround Earth.) The disturbance could have caused a substantial number of the particles to leak into the planet's northern hemisphere, creating the X-ray burst and auroral arcs, he says.

Clarke suggests that the K impact may have created an electromagnetic wave that propagated up through Jupiter's magnetic field, altering the path of charged particles in the belt. ROSAT scientist J. H. Waite Jr. of the Southwest Research Institute in San Antonio, Texas, notes that the X-ray burst appeared rapidly -- even before a plume from the K impact became visible from Earth. This suggests, he says, that the fastest particles in the belt -- electrons -- generated the arcs.