Shoemaker-Levy: sorting the debris.

When they collided with Jupiter last July, the fragments of Comet Shoemaker-Levy 9 generated soaring plumes of hot material. These fireballs--amalgams of Jovian gases and debris from the comet-- rocketed some 3,500 kilometers above the cloud tops before falling back onto the planet.

Which ingredients of the fireballs came from the comet and which from Jupiter? The answer may shed light on the composition of Jovian material normally hidden beneath the cloud tops as well as on the nature of the comet.

Scientists haven't been optimistic about tracing the origin of the debris they detected. But new findings reported at a May meeting of the International Astronomical Union in Baltimore suggest that distinguishing cometary material from Jovian isn't a hopeless task. "The opportunity to separate the comet from Jovian material probably does exist," says David Crisp of NASA's Jet Propulsion Laboratory in Pasadena, Calif.

According to some models, the cometary component of the plumes moved considerably faster than material from Jupiter and therefore rose higher above the planet. The cometary debris would also have taken longer to fall back onto Jupiter than gases ejected from the planet's own atmosphere.

Crisp and his colleagues, using the Anglo-Australian Telescope in Coonabarabran, now believe they have evidence for two distinct kinds of plume material. In observations following several of the impacts, they saw emissions resulting from gases slamming back into the planet. About 6 minutes after impact, they detected methane and ammonia--gases plentiful in Jupiter's upper atmosphere. In contrast, they didn't see oxygen, carbon monoxide, or water for 12 minutes.

The second batch of material could have taken longer to fall back, either because it came from deeper layers of the Jovian atmosphere or because it originated from the comet, Crisp notes. Another team's data, he says, favor the cometary origin.

Viewing an impact from the Canary Islands, Spain, researchers detected several metals found in comets but unknown in Jupiter's upper atmosphere. These detections came 12 minutes after the impact, just like the later detections by Crisp's group. To test more rigorously whether the gases indeed came from fragments of Shoemaker-Levy 9, Crisp plans to measure the isotopic ratios of oxygen, carbon, and hydrogen, which have characteristic values in comets.