ROCHESTER, N.Y. - When Comet Hale-Bopp flew past Earth in 1997, it carried a rare cargo in its frozen heart - the element argon.
The presence of argon, which vanishes at even the slightest warmth, suggests that Hale-Bopp was born in the icy reaches of the outer solar system, said planetary scientist Alan Stern of the Southwest Research Institute in Boulder, Colo. The comet apparently formed near Uranus and Neptune, not closer to the sun in the vicinity of Jupiter, as scientists had once suspected.
The work is the first time argon has been found in a comet and is the strongest evidence yet of a comet's birthplace, Dr. Stern reported last week in Rochester, N.Y., at a meeting of the American Astronomical Society.
"This argon thermometer gives us an opportunity to determine how much a comet has been heated over its lifetime," said Lucy McFadden, an astronomer at the University of Maryland who was not involved in the research.
When scientists first began to study Hale-Bopp, they discovered that it had visited the inner solar system only a few times before. Most of the time, they thought, the comet hung out in the Oort cloud, a vast sphere of small celestial bodies that orbit the sun at distances far beyond the orbit of Pluto. Every 30 centuries or so, calculations showed, the gravitational nudge of other objects sent Hale-Bopp zipping from the Oort cloud into the inner solar system.
Dr. Stern, with other researchers from the Southwest Research Institute, the Observatoire de Midi-Pyrenees in France, and the University of Maryland, arranged a rocket flight to study Hale-Bopp further.
Just after twilight on March 29, 1997, a rocket thundered into the skies above White Sands, N.M., for a five-minute flight. That was all the time scientists needed for an instrument on the rocket to capture ultraviolet light from Hale-Bopp. Telltale signs of argon gas appeared when the light was analyzed.
"The real key was that Hale-Bopp was such a boomer of a comet," Dr. Stern said. The comet was so big and so bright that the researchers could record its ultraviolet light with unprecedented detail.
If Hale-Bopp had ever been exposed to temperatures higher than 30 to 35 degrees above absolute zero, the argon in the comet would have essentially boiled off, he said. Earlier studies had looked for - but not found - neon gas, which boils off at temperatures of about 20 degrees above absolute zero. So the presence of argon, and the absence of neon, suggest that Hale-Bopp has lived its life between 20 degrees and 35 degrees above absolute zero.
That's too cold for the comet to have formed near Jupiter, Dr. Stern reported at the meeting. "This comet most likely formed in the Uranus-Neptune zone, or possibly farther out," he said.
Scientists once believed that comets were born in the Oort cloud, Dr. McFadden said, but recent research suggests they could form anywhere near Jupiter, Uranus or Neptune - and then get kicked out to the Oort cloud, a banishment from which they occasionally can return.
The researchers want to look for more argon, as well as other rare gases, in comets scheduled to appear in 2002 and 2003. The team is also building a scientific instrument to fly on a European mission, called Rosetta, to Comet Wirtanen in 2003.
The scientists plan to submit their work for publication in Astrophysical Journal Letters.