Comet impact suggests big Jupiter water supply


Ten months after a shattered comet called P/ Shoemaker-Levy 9 plowed into the planet Jupiter, scientists say they have begun to get a clearer picture of the event from the vast and varied array of data they collected in July.

"We have put the elephant together," said Lowell Observatory astronomer Eugene Shoemaker, a co-discoverer of the comet.

At the conclusion of a four-day conference on the collision at the Johns Hopkins University yesterday, the 200 scientists from around the world seemed ready to agree on some key findings. Among them:

* Shoemaker-Levy 9 (SL-9) really was a comet, broken into more than 23 fragments by a close encounter with Jupiter's gravitation in July 1992. Early observations failed to reveal any water in the comet or its debris, suggesting that SL-9 might have been a rocky asteroid. But more careful analysis turned up the water.

* The most dramatic releases of energy detectable from Earth after major fragments of the comet fell into Jupiter's deep atmosphere did not come from the intensely hot fireballs that erupted from the impact sites. Instead, they came from the "splashback" when the gas plumes that rose thousands of miles above the cloud tops fell back onto them a minute later.

* An atmospheric wave formation observed to move away from major impact sites like a ripple in a pond moved at a speed that suggests there may be 10 times more water hidden deep in Jupiter's atmosphere than anyone had thought. A probe to be dropped into Jupiter's atmosphere from the Galileo spacecraft next fall could provide confirmation.

Dr. Shoemaker said analyses of comet SL-9's orbital history and chemistry indicate it probably was formed early in the solar system's history from water ice, rock and metals that fell together in a loose heap in a region beyond the orbit of Pluto.

At some point, its own orbit was disturbed and it began a long series of loops toward the sun that eventually allowed it to be captured in an orbit ranging between Jupiter and the sun. About 65 years ago, it was drawn into a chaotic orbit that whirled once around the planet every two years.

Finally, on July 7, 1992, it passed too close to Jupiter and was pulled apart. The breakup continued until there were at least 23 visible chunks. Over the next two years, the swarm of debris would stretch out over 4.3 million miles.

Infrared observations from the Galileo spacecraft in July showed that invisible dust ranging out ahead of each visible comet fragment within the swarm fell into the atmosphere first, creating a kind of meteor trail -- a channel of hot gas -- that lasted one to two minutes.

Eight seconds later, Galileo's ultraviolet instruments would record an intense explosion as one of the 23 large fragments shot into the atmosphere and heated nearby portions to tens of thousands of degrees.

The explosions occurred just beyond the Jovian horizon as seen from Earth. They became visible to Earth observers only about 50 seconds later -- after that super-heated atmospheric gas expanded into a fireball, rose thousands of miles above the cloud tops and cooled.

At that point, "it has already expanded so much, it was . . . colder than liquid nitrogen," said Dr. Heidi Hammel, principal research scientist at the Massachusetts Institute of Technology and leader of Hubble's imaging team.

The dramatic "fireballs" photographed from Earth were actually reflections of sunlight from extremely cold particles in the plumes.

But scientists were puzzled by observations that showed yet another, even more brilliant energy release that followed the collapse of the plumes.

They finally concluded that the bursts, with temperatures of several thousand degrees, were created when gases in the collapsing plumes fell back on top of the Jovian atmosphere. The "splash" heated the gases, causing them to expand and rise up again in a sort of a "bounce" that repeated itself at least twice.

After plunging into the planet, most debris from the comet fragments appears to have been intensely heated and "squirted" back out, falling back onto cloud tops in a dark ring or crescent shape.

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