Forecasting a Big Splash

In collaboration with Kevin Zahnle of NASA Ames Research Center, Mac Low ran three types of simulations: (1) a comet fragment entering Jupiter's atmosphere until it exploded, (2) the initial fireball from the explosion, and (3) what happened in the atmosphere after the initial fireball. The researchers used ZEUS, a program developed at the Laboratory for Computational Astrophysics of the National Center for Supercomputing Applications for modeling the gas dynamics of astrophysical phenomena such as the violent shock waves from supernovae.

For the last six years, Mac Low has used ZEUS to study interstellar gas dynamics, and he realized he could apply the same method with relatively minor changes to simulate a comet crashing into Jupiter's atmosphere -- basically by shifting the scale from light years to kilometers. "The physics," says Mac Low, "is virtually identical. It's only the details of the composition of the atmosphere that change, and of course the length scales, time scales and density changed -- by 20 orders of magnitude in some cases."

Their simulations predicted that the flash from the explosion would last about a minute, with about as much brightness as the sunlit side of Jupiter. Unfortunately for Earth observers, the comet crashed into the back side of Jupiter. Mac Low's calculations suggested, nevertheless, that the fireball would be bright enough to be seen from the NASA spacecraft Galileo or with Earth telescopes as a reflection off one of Jupiter's moons.

The strongest prediction from the simulations, the one Mac Low was most confident of, had to do with how deep the comet would dive into Jupiter's atmosphere before exploding. Other models showed it going in hundreds of kilometers, so that its energy is absorbed relatively slowly -- what one researcher called a "soft catch." Mac Low's results showed, however, that impact with Jupiter's atmosphere would rip the comet apart more quickly and violently, with a fierce explosion, after penetrating only about 110 kilometers below the cloud tops. The resulting plume of superheated debris, according to Mac Low's study, would shoot hundreds of kilometers above Jupiter's layered clouds, giving astronomers a good chance to observe the after-effects and, in the process, learning more than we know now about the composition of Jupiter's atmosphere. Inferences from observational data indicate that this prediction was essentially accurate.

Mac Low's simulations used a computational grid finer than the other models, suggesting that his results more closely approximated reality. As a check on this surmise, Mac Low ran his code at much lower resolution and got a result similar to the other models. "At low resolution we got one result," says Mac Low, "and at high resolution we got another, and as we continued increasing resolution the result stayed the same." The high degree of detail in Mac Low's study -- made possible by the CRAY C90 -- gave a reasonable basis for astronomers to be optimistic that they would have a good show to watch in July.

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