Jupiter Probe Arrives Just in Time to Study Mess That Comet Left

Planetary scientists have a unique opportunity. Analysis of the impacts of comet fragments that smacked into Jupiter a year ago this week has yielded new insight into the giant planet's atmosphere. It also raises questions about the atmosphere's composition. The probe released last week by the Galileo Jupiter-explorer should soon resolve some of the puzzle.

The scientists probably couldn't have arranged things better if they had known in advance about the Shoemaker-Levy 9 comet when they began planning Galileo's mission nearly two decades ago. The probe will reach the planet Dec. 7. This has allowed enough time for scientists to thoroughly study the impacts. Yet it is soon enough after the event for interest to remain high.

Fragments of the comet hit Jupiter one after the other from July 16 to 24 last year. They struck just over the Jovian horizon from Earth. This gave observers on Earth a profile view of the upper part of the plumes the impacts ejected just before Jupiter's rotation brought the impact sites themselves into view. The now sharp-eyed Hubble Space Telescope helped the observation. Meanwhile, the Galileo craft had a clear view of the impacts as they occurred.

Putting all the observations together, scientists have come up with the following scenario:

As a comet fragment came into Jupiter's atmosphere, a meteor shower from small debris accompanying the fragment caused a faint glow. Then, as the fragment itself hit, the brightness increased rapidly. A plume of hot glowing material shot back up the tunnel the infalling object made, reaching heights of about 3,500 kilometers (2,200 miles) above Jupiter's cloud tops. That was high enough for observers on Earth to see the jet shoot up above Jupiter's horizon. Ten seconds later, the material in the plume fell back into the atmosphere. Scientists consider this great splat the ''main event'' that caused the dark spots marking the impact sites.

Many of those spots showed rings expanding outward like ripples caused by a stone thrown into water. Large areas of dark material appeared. Since there is no consensus as to what this is, scientists simply call it ''brown stuff.'' However, data taken at many wavelengths ranging from infrared (heat) radiation to X-rays have revealed a variety of chemical substances, including some metals, that the impacts stirred up.

Now come the questions. What substances are native to Jupiter? What came from the comet? What were created during the impact? One of the more striking findings for scientists was an abundance of sulfur-containing molecules. Does Jupiter have that much sulfur or did the comet bring some of it? Galileo's probe may answer this question as it samples Jupiter's normal atmosphere unperturbed by a comet.

Then there are the ripples that spread out from some of the impact sites. They might be acoustic (sound) waves. They might be what geophysicists call gravity waves, such as waves on water or in our own atmosphere. Meteorologists Andrew Ingersoll and Hiroo Kanamori at the California Institute of Technology, in Pasadena, have made a prediction for the Galileo probe to test.

They suggest that the ripples were gravity waves trapped in a water-rich zone of the Jovian atmosphere. They therefore predict that the Galileo probe will find a deep, stable layer in Jupiter's atmosphere extending down to a level where the atmospheric pressure is about 20 times that at sea level on Earth. They also suggest that the probe may find relatively high levels of heavy elements like oxygen, carbon, and nitrogen.

''The comet impacts told us a lot. But they didn't measure anything,'' observes comet specialist Heidi Hammel of the Massachusetts Institute of Technology. The probe will measure things to help scientists sort out what material came from the comet and what was already on Jupiter from the spectacular mess the impacts created.

Scientists couldn't have arranged things better if they had known about the comet when they began planning Galileo's mission nearly two decades ago.