ZEUS code simulates comet crashing into jupiter

As shattered remnants of a comet orbiting Jupiter hurtle towards the giant planet, researchers are trying to understand--through computer simulation--how the comet fragments will affect Jupiter's atmosphere. In a MetaCenter collaboration, Mordecai-Mark Mac Low is using ZEUS software created by NCSA's Laboratory for Computational Astrophysics (LCA) running on the Pittsburgh Supercomputing Center's CRAY C-90 system.

Simulation of the fireball resulting from a comet, 1 kilometer in diameter and made of solid ice, hitting Jupiter. Times shown are 30, 50, and 90 seconds after impact. Temperatures are on a log scale with blue a few hundred degrees Kelvin (K); red, close to 10,000 K--hotter than the surface of the sun. Darkest blue stripe represents the top of the clouds. (Courtesy of Mac Low)

Fragments of the comet Shoemaker-Levy 9 are going to dive into the gaseous planet Jupiter between July 16 and 22 and vaporize just behind the limb of the planet out of view from Earth, scientists say. Astronomers hope to see the explosions reflected off Jovian moons or perhaps through cameras of the crippled Galileo Orbiter. About 20 minutes after each collision, the impact site will rotate into view giving researchers an opportunity to view the aftermath from Earth or from orbiting telescopes, such as the Hubble.

"It is a real disappointment. Something like this only happens every couple of hundred years," says Mac Low, an astronomy research associate at both the UIUC and the University of Chicago. "It is the biggest solar system event of our lifetimes, and it is happening on the wrong side of the largest planet."

Mac Low's experience using ZEUS codes gave him a headstart on the comet simulation. "Mordecai is our primordial ZEUS user," says LCA Director Mike Norman. "He has been riding the ZEUS technology curve since 1987."

Mac Low was--and still is--using ZEUS codes to examine superbubbles, which form when many stars become supernovas in a small region. He was able to quickly retool this and another related project in order to study the comet crash. Mac Low was also using ZEUS to simulate a cloud of interstellar gas getting hit by a supernova shock wave. By treating the comet as a gas--because of its speed and density compared to the Jovian atmosphere-- and using his superbubble models to study the fireball resulting from the impact, Mac Low was able to get preliminary results quickly.

Mac Low's comet research has stirred some debate because it disagrees with the results of other computations of the altitude of the comet explosion. Some of his competitors found that fragments would simply be absorbed by Jupiter's gaseous atmosphere. Mac Low's simulation was computed using 200 gridpoints across the comet, while the other simulations used only eight grid-points across the comet. According to Mac Low, the low resolutions do not show the comet's explosion and fragmentation.

If the calculations are supported by observations, Mac Low and his collaborator, Research Scientist Kevin Zahnle of NASA Ames Research Center, will apply their theory to the Earth. "The big question is: How big a rock will the Earth's atmosphere protect us from?" Mac Low says. If their model is right about Jupiter, it could be adapted to make predictions about asteroids that may one day head for Earth.