VACETS Regular Technical Column

"Science for Everyone"

"Science for Everyone" was a technical column posted regularly on the VACETS forum. The author of the following articles is Dr. Vo Ta Duc. For more publications produced by other VACETS  members, please visit the VACETS Member Publications page or Technical Columns page.

The VACETS Technical Column is contributed by various members , especially those of the VACETS Technical Affairs Committe. Articles are posted regulary on [email protected] forum. Please send questions, comments and suggestions to [email protected]

Mon, 16 Jan 1995

By Jove: Comet Crash Puzzles

It has been exactly half a year since the fragments of Comet Shoemaker-Levy 9 plowed into Jupiter. There is no better time to re-visit and re-live the excitement of the once-in-a-lifetime event. As of today, six months after the crash, scientists are still continuing their struggle to understand exactly what the impacts have revealed about the inside of the gaseous planet. But even if much of the Jovian interior remains a mystery, the collisions have shed some new insights about the giant planet's upper atmosphere.

Just weeks after Eugene and Carolyn Shoemaker and David H. Levy discovered the comet at California's Mount Palomar Observatory on March 24, 1993, further observations revealed it to consist of 20 or so fragments, eventually labeled A through W, spread out in space like a strand of diamonds. The scientists calculated that the comet had broken up during a previous approach near Jupiter and that it would plunge into the planet for good in July 1994.

On July 16, 1994, for over six days, fragments of Comet Shoemaker-Levy 9 plowed into Jupiter, leaving behind a necklace of dark bruises. The Jovian show was big and dazzling. The Jovian fireworks kept electronic mail buzzing for weeks. How big were the fragments? How large a punch did they pack? How deeply did they penetrate into Jupiter's atmosphere? Shoving these related riddles will help reveal how much of the planet's hidden interior the collisions exposed.

The upper reaches of Jupiter's clouds appear to consist of three distinct layers. Visible clouds of ammonia lie at the top. Beneath this may be a layer of ammonium hydrosulfide. The deepest layer is thought to contain water. How deep did the fragments go? Scientists have not agreed on the subject. To observers marvelling over the first fireballs and dark bruises in Jupiter's atmosphere, it seemed that the comet's fragments must have been big, solid projectiles, plunging deep into the atmosphere. But as the week wore on and astronomers analyzed the impact sites, doubts set in. By the time the curtain had come down, some researchers were arguing that the impact debris and scars didn't look as if they came from deeply penetrating wounds. Now, half a year later, what are the conclusions of the collisions?

The intensity and duration of light from each event offer important clues. Rather than producing a single burst, the larger fragments staged an extended light show: an initial flash as a chunk entered Jupiter's upper atmosphere, a glowing plume of debris shooting thousands of kilometers above the clouds, and the radiation generated when the plume crashed back down with a high-velocity splash.

Telescopes recorded these emissions at a variety of wavelengths, producing a characteristic set of light curves. The light curves traced the progress of chunks of ice and rocks. Of all the instruments staring at Jupiter in July, only those on the Galileo spacecraft had a full view of the fireworks. Heading toward a 1995 rendezvous with Jupiter, the craft was in the right position to directly observe the collisions, which occurred on the back side of the planet just out of direct sight of ground-based and Earth-orbiting telescopes. Unfortunately, a programming error led to the loss of some data coinciding with the collisions and a crippled main antenna limits the spacecraft's ability to transmit information. It hasn't been able to finish transmitting its data yet. In the meantime, scientists have begun to make sense of the information already received.

When Galileo first sighted the fireball from fragment G, believed to be the largest, the fireball appeared to have a diameter of about 10 km and a temperature of 7,500 kelvins. Five seconds later, the craft's near-infrared mapping spectrometer first saw the explosion, recording the rising fireball's expansion and cooling for 90 seconds, until it was thousands of km across and only 400 kelvins. For the K impact, one of the larger fragments, the craft's solid-state imaging camera saw a bright flash lasting about 5 seconds. Over the next 10 seconds, the light dimmed and then brightened again, fading away after another 30 seconds. For N, one of the smaller impacts, the camera recorded a similar pattern.

It is thought that the initial flashes probably reveal the fragments as they streaked through Jupiter's upper atmosphere and started to glow as meteors. As they tunneled through thicker atmosphere, the fragments heated material, which exploded in a a fireball. This produced the second, longer-lasting glow in visible light as well as the extended emission in the near infrared. If this interpretation proves correct, it could help indicate how deeply the fragments plumbed Jupiter. The events viewed by Galileo's camera suggest the fragments didn't penetrate far. This, in turn, indicates that the chunks were not larger than 1 km in diameter. Had the fragments plowed deeper, into thicker layers of atmosphere, Galileo's camera would have recorded several blank frames until the fireball re-emerged above these light-absorbing regions. However, some other scientists are skeptical of this interpretation. Also, did Galileo really see the meteor flashes? The meteor flash provides a key point in time -- signaling when a fragment first entered the Jovian atmosphere. By knowing the elapsed time between a fragment's entry and the time when ground-based telescopes first glimpsed a plume of material above Jupiter's limb, researchers hope to calculate the energy delivered by the impact. If the craft did record some flashes, it may not have captured the most spectacular part of this light show. Even if Galileo did miss the meteor flashes, the craft still captured much of the light emissions, since the exploding fragments radiated most of their energy during the fireball phase.

Several telescopes on Earth recorded intriguing near-infrared light curves. The light curve of the R impact shows three separate rises and dips. First came an abrupt flash of about 10 seconds. After a 1-minute gap, a signal three times as bright appeared and lasted for about 30 seconds. Finally, 5 minutes after the initial flash, the telescopes recorded another rise in light intensity that took 5 minutes to peak and didn't fade completely for half an hour. It is speculated that the initial flash seen from ground-based telescopes, like the flashes detected by Galileo, represent the R fragment streaking into Jupiter's upper atmosphere. But how could a telescope on Earth detect an event on the back side of Jupiter? Note that it took about 10 minutes for the impact sites to rotate into view from Earth. It was suggested that the telescopes may have detected the streak in reflection from light scattering off dust deposited in Jupiter's atmosphere by the cometary fragments. Alternatively, the meteor flash may have begun high enough in Jupiter's atmosphere for it to be visible above Jupiter's darkened limb.

The second rise in intensity occurred when a plume of hot material generated in the explosion shot up through the Jovian atmosphere. Data from the Hubble Space Telescope indicate that this plume rose about 3,500 km above the cloud tops. The third and longest rise represents the violent shock generated when the giant plume crashed back into the atmosphere. Forming a dark spot the diameter of Earth, the falling plume apparently heated the atmosphere around it to some 500 kelvins, creating an infrared glow.

This scenario agrees with a model in which fragments no larger than 1 km in diameter explode relatively high in Jupiter's atmosphere. This model gains support from the presence and absence of specific molecules in the Jovian atmosphere soon after the fragments struck.

The answer to the question how deep did the fragments penetrate the Jovian atmosphere can be obtained by analyzing an entirely different phenomenon. Rings, like ripples on a pond, were detected moving outward from several crash sites. The rings were measured to travel at a speed of about 450 m/s which probably represent gravity waves. Some researchers argue that gravity wave at this speed would have originated from the water cloud layer, indicating that some fragments penetrated to this depth. In contrast, some other scientists maintain that the gravity wave at the speed measured would lie higher up, in the stratosphere.

So what are the answers to all the questions relating to the crash? At the moment, there is no confirmed answer to any of those question. Researchers aim to resolve many questions by next May, when they will gather at a special meeting of the International Astronomical Union in Baltimore. Even if some puzzles still remain by then, scientists may not have to wait long to solve them. Next December, a Galileo probe will parachute into Jupiter's atmosphere. The crash of '94 may be just a prelude for the Jovian exploration of '95.

References: "After the Crash...", R. Cowen, Science News, Vol. 146, pp. 412-414 (December 17, 1993). "By Jove! ...", J. Horgan, Scientific American, Oct 1994, pp. 16-20. "Comet collision", D. Graham, Popular Science, July 1994, pp. 43-45, p.71. And TOO many other references.

Duc Ta Vo, Ph.D.
[email protected]

For discussion on this column, join [email protected]

Copyright © 1996 by VACETS and Duc Ta Vo


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