Simulation explains mystery of giant planets' tiny moons

November 30, 2001

NEW ORLEANS - In what could be the ultimate in fast-forward, Cornell University planetary scientists have used one of the world's most powerful computing clusters to simulate motions of the small moons of Jupiter over a one billion-year epoch. From this, the researchers have learned how the tugs and pulls of the sun and planets -- even from hundreds of millions of miles away -- shake out the permanent moons of the giant planets from those that get tossed away.

In a three-month computing marathon, the Velocity I cluster at the Cornell Theory Center was able to mimic cosmic conditions over eons that would cause physical perturbations in the moons of Jupiter. The calculations were produced by entering orbital data from hypothetical moons of the planet. As a result, the astronomers now have an explanation for the unusual orbits of 12 confirmed small, eccentric moons of Jupiter.

Joseph Burns, Cornell professor of astronomy and engineering, and Valerio Carruba, Cornell graduate student in astronomy, will detail their research in a talk, "On the Orbital Distribution of Irregular Satellite Systems," at the American Astronomical Society's Division for Planetary Sciences meeting today (Nov. 30) at the Hyatt Superdome in New Orleans. Joining Carruba and Burns on the research were Philip D. Nicholson, Cornell professor of astronomy; Brett J. Gladman, Observatoire de la Côte d'Azur, Nice, France; and Matthew J. Holman, Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass.

"The big moons are the ones you know and love, and their orbits are circular and they are always in the planets' equatorial plane," says Burns. "The small moons, about 10 to 100 miles in diameter, have been captured by the large planets and they have distant, elongated, elliptical orbits that are highly inclined. We wanted to know why." None of the irregular moons (that is, those with non-circular orbits) has an inclination -- the angle relative to the planet's orbital plane -- between 47 degrees and 141 degrees. Thus, there is an area of Jupiter's sky free from moons of any sort. The astronomers discovered that any tiny moons that might once have orbited well off Jupiter's orbital plane, have smashed into the planet or have been tossed into a perpetual orbit around the sun, says Carruba. Below the 39-degree orbital plane, the eccentricities of the moons' elongated-elliptical orbit change little.

In other words, an observer positioned on Jupiter's equator would see the four large Galilean moons grouped directly overhead and the tiny satellites (the 12 confirmed plus a dozen other recently discovered moons) scattered as much as 40 degrees away. Far to the north and south there would be no moons.

To try to explain this phenomenon, the astronomers turned to the Cornell Theory Center's Velocity I cluster. The 256-processor cluster consists of 64 Dell PowerEdge servers, each with four Intel Pentium III Xeon 500 Mhz processors and running Microsoft Windows 2000 operating system. The astronomers "installed" hypothetical moons around Jupiter, programmed in the physical perturbations that would likely occur in a simulated scenario and mimicked cosmic conditions for a period of one billion years.

In addition to finding how the sun's gravity pulls the moons from their orbits, the researchers are studying why the orbits of the tiny moons are tightly clumped together. The astronomers have deduced that the moons were once larger objects broken apart by cometary or asteroidal collisions.

Burns says this research is an early step to understanding how the giant planets were formed. "This research is similar to how archaeologists -- by investigating what remains -- reconstruct the birth and death of civilizations," says Burns. "As planetary scientists, we have a comparable opportunity to decipher the origin of giant planets by interpreting the orbital distribution structure of irregular satellites that still orbit their planets. We hope to use the observed distribution to start to unravel the formations of the planets themselves."
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Cornell University

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