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The Comet Collision: Caltech Scientists Eagerly Await Celestial Event


For Immediate Release June 17, 1994

The impending crash of the twenty-odd fragments of comet Shoemaker-Levy 9 into Jupiter from July 16 through 22 promises to be one of the most-watched celestial events ever, and planetary scientists and astronomers around the globe are eagerly anticipating the results. At Caltech some scientists are doing more than just anticipating, they are creating detailed models that predict in considerable detail how the Jovian calamity will play out, and are helping plan observations to catch the event at its peak. Following are the Caltech scientists who are most involved, and a description of their work.

Thomas Ahrens, Professor of Geophysics
John O'Keefe, Visiting Associate in Planetary Science

Ahrens and his collaborators, graduate student Toshiko Takata, O'Keefe, and the Jet Propulsion Laboratory's Glenn Orton, have created a detailed computer model of what may happen as the shards of Shoemaker-Levy 9 plunge into Jupiter. The icy chunks, streaking towards the planet at 60 kilometers per hour, may create bright flashes as each of them first hits the atmosphere, just as meteors do on Earth, and then disintegrates as the thickening atmosphere slows it down. The calculations predict that, after comet fragments between 0.4 and 10 kilometers in diameter have pierced the clouds to depths of several hundred kilometers, the tremendous kinetic energy of the pieces will have compressed and heated Jupiter's gaseous hydrogen clouds to nearly the temperature of the surface of the sun. This superheated gas will then expand rapidly and form a plume on the surface, much like the mushroom cloud from a hydrogen bomb detonated on Earth, that may be visible from Earth over the limb, or edge, of Jupiter. This gas plume is predicted to increase the brightness of Jupiter by approximately a factor of two.

Andrew Ingersoll, Professor of Planetary Science

Ingersoll is a member of a team which will be looking for evidence of waves caused by the impacts in Jupiter's atmosphere. Using the Hubble Space Telescope at visible and near-infrared wavelengths, the team hopes to detect subtle changes in the surface clouds, especially changes creating a circular pattern around the impact point, like ripples in a pond. As a wave passes through a point in the atmosphere, the local pressure will rise and then fall, causing the temperature to cool and then warm very slightly, by about one degree Fahrenheit. In areas where gas is just on the edge of freezing, the passing wave's small pressure change should be enough to cause ice crystals to form. Ammonia droplets are the most likely to freeze out, which would create new cloud patterns of white crystals.

Ingersoll has also worked with Hiroo Kanamori, the Smits Professor of Geophysics and director of Caltech's seismological laboratory, and Tim Dowling at MIT, to model what kinds of waves may jostle Jupiter as a result of this impact. One intriguing idea is that the biggest waves will be confined to a narrow altitude range, where the pressure is about 3 to 5 bars, and where there is a layer of gaseous water encircling the planet. (For comparison, the sea-level air pressure on Earth is about 1 bar, and the pressure at Jupiter's cloud tops is about 0.5 bar.) The temperature and density of this layer should have just the right properties so that if the wave travels too high, it will be refracted, or bent, downward. And if the wave travels too low, it will be refracted upward. The net effect is to create an internal wave channel, which should allow the waves to spread a very long distance horizontally. These long-distance waves would act as a probe of Jupiter's interior, much like seismic waves on Earth, and would provide information about different atmospheric levels that cannot be observed optically. Ingersoll and his colleagues hope to detect the water cloud by these observations.

Gerry Neugebauer, Howard Hughes Professor and Professor of Physics
Keith Matthews, Member of the Professional Staff, Division of Physics, Mathematics, and Astronomy

Neugebauer and Matthews are part of a team, led by Phil Nicholson from Cornell University, which will be observing Jupiter in the near- to mid-infrared wavelength (1-20 microns) with the 5-meter Hale Telescope atop Palomar Mountain, near San Diego. They will be looking for evidence of impact flashes, some of which may be bright enough to cause noticeable reflections from the faint rings around Jupiter, or from the halos of gas and dust around the trailing comet fragments. It's possible the plume of superheated gas may be visible over the limb, or edge, of the planet, and that this plume may also reflect the flash.

The team will also be looking for the faint temperature changes expected to result from the outwardly spreading waves in the atmosphere, which would be directly visible in the infrared. And they will be making spectroscopic observations to check for chemical changes. That is, they will study the light from Jupiter by breaking it into a spectrum, or rainbow, in order to examine the bright and dark lines characteristic of different molecules. The impact should create considerable vertical mixing, dredging up from deep in the planet chemicals that are rare on the surface, and bringing them into view.

Duane Muhleman, Professor of Planetary Science

Muhleman will be working at Caltech's Owens Valley Radio Observatory (OVRO) with research fellow Tony Phillips and graduate student Mark Gurwell. The trio will be observing near the 3-millimeter wavelength, looking for aftereffects of the impact. This radio wavelength allows them to see what's happening dozens of kilometers below the tops of the visible clouds, at a level where disturbances caused by the comet are likely to be noticeable. Based on their observations, Muhleman hopes to make a good estimate of the size of the disturbance and of how much energy went into creating it.

Jonas Zmuidzinas, Assistant Professor of Physics
Geoffrey Blake, Associate Professor of Cosmochemistry

Zmuidzinas and Blake are collaborating with principal investigator Peter Wannier of the Jet Propulsion Laboratory on Southern Hemisphere observations to be conducted aboard NASA's Kuiper Airborne Observatory, a plane that flies in the upper reaches of our own atmosphere. Zmuidzinas has built an instrument called a spectrometer which can detect radiation characteristic of various molecules, including water vapor. To avoid interference from water molecules in the air, the instrument will be carried above most of the atmosphere on the Kuiper Observatory. And to have a better, Southern Hemisphere view of the action, the plane will fly out of Melbourne, Australia. Water on Jupiter is usually buried beneath the upper cloud layers, but the comet may mix the upper and lower layers of the atmosphere enough to churn detectable amounts of water vapor to the surface. The spectrometer should be able to determine both the height and the temperature of plumes that contain water vapor, plumes that are expected to result from the collisions.

Contact: Jay Aller          or	  Max Benavidez
         (818) 395-3631	          (818) 395-3226

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