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Missing carbon-2 molecule holds clues to comet’s origin

Astronomers seeking to chart the solar system’s evolution reaped a windfall of information when a comet disintegrated just as it made its closest approach to the sun and they were able to observe the comet’s contents and, possibly, its origin.

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AUSTIN, Texas—Astronomers seeking to chart the solar system’s evolution reaped a windfall of information when a comet disintegrated just as it made its closest approach to the sun and they were able to observe the comet’s contents and, possibly, its origin.

Their work will be published Friday (May 18) in the journal Science — a special issue devoted to studies of the comet, called C/1999 S4 (LINEAR), that disintegrated last July.

Dr. Tony L. Farnham, a planetary scientist at The University of Texas at Austin, and his colleagues discovered a deficiency in the molecule carbon-2 in Comet C/1999 S4 (LINEAR). Farnham, lead author of the paper, is the Harlan J. Smith Planetary Post-doctoral Researcher in the UT Austin department of astronomy. (LINEAR refers to the Lincoln Near Earth Asteroid Research project funded by the U.S. Air Force to study near earth objects.)

Co-authors include Dr. David G. Schleicher and Dr. Laura M. Woodney of Lowell Observatory in Flagstaff, Ariz.; Peter V. Birch of Perth Observatory in Western Australia; Clara A. Eberhardy of the University of Washington in Seattle; and Lorenza Levy of Northern Arizona University in Flagstaff.

"We usually get a look at the surface of a comet, but this time we got to look inside," Farnham said. The group observed the comet both before and after it broke up, using telescopes at UT Austin’s McDonald Observatory, Lowell Observatory and Perth Observatory.

Comets are sometimes referred to as ‘dirty snowballs,’ because they are made up of dust and rock held together by ice. When they venture close to the sun, the ice vaporizes and elements inside are released.

The carbon-2 deficiency discovered by Farnham and his colleagues indicates that the comet formed near Neptune, probably billions of years ago. Most comets were formed during the solar system’s earliest years in two regions: near Jupiter and Saturn, and farther out, near Neptune.

They didn’t stay in those regions, however, because the force of gravity of those giant planets catapulted the comets away, and created two comet habitats: the Oort Cloud (a halo of Jupiter-origin comets enveloping the solar system) and the Kuiper Belt (a belt of Neptune-origin comets orbiting in the plane of the solar system along with the planets, beyond Neptune’s orbit).

Different lines of evidence may indicate another history for C/1999 S4 (LINEAR). Other researchers found the comet to be lacking in other carbon-chain molecules, pointing to a Jupiter-region origin, Farnham said. He said the discrepancy "may be telling us that it has a surface material different from what’s inside. It’s possible that the comet formed near the Jupiter region, and other materials formed on the surface as it migrated out," into the outer solar system. But Farnham cautioned that there is no proof of this.

Understanding the origin of comets is important because they carry key information about the history of the solar system that cannot be derived from studying planets. Comets have changed very little since the solar system formed. They are relics of an earlier time. Geological processes (like volcanic eruptions and the movements of glaciers) have rewritten the surfaces of most of the planets since their formation.

"Comets have been in storage, especially dynamically new comets, like this one," Farnham explained. He said comets remain in the cold realms of the Oort Cloud and Kuiper Belt until some large body like a planet or a passing star gives them a gravitational kick into the inner solar system. A ‘dynamically new’ comet is one that has never been in the inner regions of the solar system before.

Farnham also calculated a lower limit for the radius of the comet’s nucleus before break-up: about 0.4 kilometers. In addition, he observed an ‘outburst’ from the comet, at the same time other researchers using the Hubble Space Telescope photographed a piece of the comet breaking away. Comet C/1999 S4 (LINEAR) was not visible to the naked eye.

For more information, contact Dr. Tony L. Farhnam at (512) 471-1483, or via e-mail at <farnham@astro.as.utexas.edu> or see: <http://www.ll.mit.edu/LINEAR>.