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

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."