Consider what happens when a child and adult jump into a pool: the adult makes a bigger splash, right?
But that's not what happened in an analogous situation on Jupiter last July when fragments of comet Shoemaker-Levy 9 bombarded the planet. The plumes of material that erupted from Jupiter after each impact rose to about the same height, regardless of how big the comet chunk was.
"Identical plume heights for explosions of different energies were not predicted and have yet to be explained," MIT scientists and colleagues wrote last Friday in the journal Science. Their paper reported this and other observations of Jupiter's atmosphere during and after the collision.
"This is the first paper to really quantify the results we got, though there's still a lot of interpretation left to do," said Heidi B. Hammel, a principal research scientist in the Department of Earth, Atmospheric and Planetary Sciences (EAPS). Dr. Hammel was a team leader for Hubble Space Telescope observations of the crash.
Authors of the paper are Dr. Hammel; Professor Timothy E. Dowling of EAPS; Joseph Harrington (who has since received his PhD from EAPS); Jennifer R. Mills, a senior in physics; Charles M. Santori, a sophomore in physics; and scientists from New Mexico State University, California Institute of Technology, the University of Michigan and the University of Arizona.
Other exciting observations reported in the paper involved the atmospheric waves that spread out from each impact site. MIT scientists led by Professor Dowling had specifically predicted the occurrence of gravity waves, "or the waves you get when you throw a rock into a pond," Professor Dowling said. Their pre-impact simulation showing waves moving at a speed of about 400 meters per second came very close to the measured speed of about 454 meters per second.
Nevertheless, Professor Dowling noted that there are still a variety of questions that remain to be answered. For example, though the scientists know that these waves occurred very high in Jupiter's atmosphere, they don't know if-or how deeply-they penetrated into lower layers of the atmosphere. If it turns out that they did penetrate into the atmosphere, the scientists could learn more about what's below the clouds.
Professor Dowling also said that while his group's gravity-wave simulation looks like ripples emanating from a site, the actual waves captured by Hubble "look more like one discrete black ring followed by another."
On the most basic level, the scientists are still not sure why these rings were visible at all. Their guess is that debris from the explosion helped expose the waves, much as chalk dust can expose a laser beam. "If you clap two chalk erasers together and send the beam through the dust, you can see the beam," Professor Dowling said. Similarly, "it's possible that the only reason we saw the [gravity] ring was because it was passing through the black material resulting from an impact."
More specifically, "we think that as the wave passed through, lifting the atmosphere and cooling it, the ice and the dirt condensed and you see [a ring]," he said. "Then as the wave continues through and pushes the atmosphere back down again, the atmosphere warms up, the ice vaporizes, and you can see right through it" resulting in the clear areas between the discrete rings.
Professor Dowling noted that Mr. Santori "was indispensable" to analyzing the waves. Among other things, said Mr. Santori, "I was working on processing the rings-they came in as raw images and needed to be mapped into grids." He also worked on a different program "to accurately measure the width of each ring."
The overall experience "was pretty interesting," Mr. Santori said. "A comet crashing into a planet is ordinarily something you only read about in science fiction books, so actually seeing it happen and analyzing what happened after it hit, was kind of neat."
Ms. Mills was also an integral part of the MIT team. She worked with Dr. Hammel and wrote half the software for converting raw data from Hubble into clear images. Among other things, during the impact period she created a photographic press release of the first plume that showed, via a sequence of images, how the plume rose, expanded then fell into a flat pancake.
Shoemaker-Levy 9 has come and gone, leaving many surprises and unexplained phenomena in its wake-"Which is good," said Dr. Hammel. "That's how you learn things."
The work reported in the Science paper was supported by NASA. Drs. Dowling, Hammel and Harrington are also authors of a second paper in the same issue that describes infrared observations of the crash. Dr. Hammel is an author of a third paper that describes the chemistry resulting from the crash.
A version of this article appeared in MIT Tech Talk on March 8, 1995.