Jupiter's Bruises

By MICHAEL D. LEMONICK/BALTIMORE

Mountains have finally stopped hailing down on Jupiter, and the debris from their catastrophic impacts has started to settle. Here on Earth, the information superhighway is coming unclogged as Internet users relax their manic electronic search for comet-crash pictures. And except for an observing session next week and another in late August, the Hubble Space Telescope is moving on to view other heavenly objects.

But amateur astronomers are still peering intently through their backyard telescopes to get a glimpse of the bruises that Shoemaker-Levy 9 left on Jupiter -- the most prominent features ever seen on the giant planet -- and others are thronging to observatories and planetariums to see what all the fuss was about. Scientists who last week barely had time to sleep, let alone think, are finally turning their attention away from spectacular pictures and starting the long, difficult process of seeing what they can learn from the great comet crash of 1994. Says Keith Noll, an astronomer at the Space Telescope Science Institute in Baltimore: "We're going to be arguing over this stuff for months."

One stunning question that may never be answered: Was Shoemaker-Levy 9 really a comet, or was it an asteroid instead? Comets tend to be a mixture of ice, rock and dust, along with substances, like carbon monoxide, that evaporate easily to form a halo and a tail. Scientists studying the chemical composition of the spots on Jupiter where S-L 9 hit thought they might see evidence of water and oxygen, two of the expected products when an icy comet vaporizes. But except for one unconfirmed report,researchers have found only ammonia, hydrogen sulfide and sulfur gas.

Asteroids are rockier than comets. Yet it is possible for an asteroid to have a halo or a tail, made mostly of dust. Says Hal Weaver of the Space Telescope Institute: "The only real evidence that ((S-L 9 was)) a comet is that it broke apart, and we've never seen that in an asteroid. But maybe this was a fragile asteroid."

Amateur astronomer David Levy, who with Eugene and Carolyn Shoemaker discovered S-L 9, points out that comets were originally distinguished by their appearance. They are objects that look like fuzzy stars with tails, and in any previous century astronomers would have called this discovery a comet. On that basis, argues Levy, "S-L 9 is a comet, period."

The apparent absence of water at the impact sites provides a clue about how far the S-L 9 fragments penetrated Jupiter's atmosphere before exploding. Theorists think that a layer of water vapor lies some 60 miles below the visible cloud tops; above the vapor layer, about 30 miles down, are clouds believed to consist of ammonium hydrosulfide, a sulfur compound. Since no water seems to have been stirred up, the explosions probably took place in the presumed sulfide layer. If researchers confirm that the sulfur rose up from Jupiter, it will be "a major discovery," says University of Arizona astronomer Roger Yelle. "We've always believed that much of the color in Jupiter's clouds comes from sulfur compounds, but we've never detected them."

No one understands why the points of impact are so dark -- a mystery that may take months to solve -- but it's clear that they are very high up in Jupiter's atmosphere, since the planet's familiar stripes can be seen through them. Astronomer Heidi Hammel of M.I.T. says the collisions will provide an opportunity to study the winds above Jupiter's cloud tops. "The mark left by the first impact is already starting to be spread around," she observes. There are also hints of seismic waves -- ripples thatmay have traveled all the way to a dense layer of liquid hydrogen thousands of miles down and then bounced back up to the surface, creating rings half the size of the planet's visible face. These waves may offer clues to Jupiter's internal structure.

The spots that were made by the collisions will undoubtedly blow away eventually -- though no one knows how long it will take -- but it's much too soon to tell whether there will be any permanent changes in Jupiter. There is still every chance that the impacts, especially from the four fragments that hit in nearly the same place last Wednesday and Thursday, will destabilize the atmosphere and create a new, permanent cyclone like Jupiter's Great Red Spot.

It's also possible that the show isn't quite over. Theorists at NASA'S Jet Propulsion Laboratory argue, on the basis of a computer model of S-L 9, that debris -- including boulders several hundred feet across -- has lagged behind the original 21 major fragments. These stragglers, they predict, will keep hitting Jupiter through September. Unlike the participants in last week's show, however, some of the later pieces may smash into the near side of the planet, giving astronomers a chance to watch some strikes directly. Is the theory plausible? Says Weaver: "These are the same guys who said the pieces were big and that the impacts would be huge. They were right, and lots of others were wrong. We've had so many surprises from Shoemaker-Levy 9 already that I wouldn't rule anything out."