Missing Molecule Holds Clues to Comet's Origin

Dr. Tony L. Farnham, a planetary scientist at The University of Texas at Austin studied comet C/1999 S4 along with collaborators Drs. David G. Schleicher and Laura M. Woodney of Lowell Observatory in Flagstaff, AZ; Dr. Peter V. Birch of Perth Observatory in Western Australia; Dr. Clara A. Eberhardy of the University of Washington in Seattle; and Dr. Lorenza Levy of Northern Arizona University in Flagstaff.

The researchers used telescopes at UT Austin’s McDonald Observatory, Lowell Observatory, and Australia’s Perth Observatory to observe it both before and after break-up. They determined that the comet is deficient in the molecule carbon-2, which indicates that the comet formed near Neptune, billions of years ago when our solar system was forming.

"We usually get a look at the surface of a comet, but this time we got to look inside," Farnham said. Farnham is the Harlan J. Smith Planetary Post-Doctoral Researcher at UT Austin’s department of astronomy. Because the opportunity to study the inside of a comet is so rare, it’s essential that astronomers know what type of comet it is, so they can use the information in their models of solar system formation.

Comets are sometimes referred to as "dirty snowballs," because they are made up of dust and rocks held together by ice. Astronomers know that comets generally formed in two places in the early solar system: in the region around Neptune, and the region around Jupiter. Generally, Neptune-origin comets ended up in the Kuiper Belt, a band of comets just beyond that planet’s orbit. Jupiter-origin comets ended up in the Oort Cloud, a halo of comets surrounding our solar system far beyond the orbits of the planets.

Different lines of evidence may indicate another history for the comet. Other researchers found that 1999 S4 is missing other carbon-chain molecules, Farnham said, indicating that the comet may have formed near Jupiter.

The discrepancy in results "may be telling us is that it has a surface material different from what’s inside," Farnham said. "That may be an explanation of what happened, that the comet formed around the Jupiter region, and other materials formed on the surface as it migrated out," into the outer solar system. "We don’t have any proof of that," he cautioned.

Farnham also calculated a lower limit for the length of the comet’s nucleus before break-up: about 0.4 kilometers.

They are useful for studying the history of our solar system because they are believed to be relatively unchanged since the solar system formed, unlike planets, whose geological processes continually re-write their surfaces.

"Comets have been in storage, especially dynamically new comets, like this one," Farnham said. A dynamically new comet is one that has never been near enough to the Sun to have any of its materials vaporize off its surface.

Comet C/1999 S4 was never visible to the naked eye.

For more information, contact

Dr. Tony L. Farnham in the UT Austin department of astronomy at 512-471-1483.