Hale-Bopp. Photo: NASA

Astronomers announced Monday at the American Astronomical Society meeting in Rochester, New York, that they have detected argon, a scientifically valuable noble gas, in comet Hale-Bopp. This discovery constitutes the first-ever detection of a noble gas in a comet. Noble gases provide valuable tracers of the thermal history and, therefore, clues to the origins of comets.

The discovery was made by a team of four astronomers from Southwest Research Institute (SwRI), based in San Antonio, Texas, collaborating with three colleagues from the University of Colorado, the University of Maryland, and the Observatoire de Midi-Pyrenees in France. The study was supported by NASA.

The data on comet Hale-Bopp were obtained in the form of ultraviolet spectra during a NASA high-altitude suborbital research rocket flight on the evening of March 29, 1997, just as comet Hale-Bopp made its closest approach to the sun. According to team leader and Principal Investigator Dr. Alan Stern, director of the SwRI Space Studies Department, "The argon signals are weak, but unmistakable. We had previously suspected their presence, but were able to recently confirm the result when we cross-compared two independent spectra obtained by our rocket instrument back in 1997."

Adds co-investigator Dr. David Slater, a senior research scientist at SwRI, "Hale-Bopp was among the brightest comets ever witnessed, and surely the brightest comet in modern times. The detection of argon would not have been possible except for Hale-Bopp's unusually high brightness."

Neptune. Photo: NASA/JPL

Because noble gases do not interact chemically with other elements and because noble gases are easily lost from icy bodies like comets at very low temperatures through processes much like evaporation, their presence or absence provides a way of measuring the thermal history of comets. University of Maryland astronomer and team member Dr. Michael A'Hearn explains, "That's the reason cometary astronomers have wanted to detect noble gases for so long. The advance of technology combined with the brightness of Hale-Bopp made this goal a reality."

Interestingly, the team's spectra showed that the argon abundance in Hale-Bopp was so high that it indicates the comet has always been quite cold and likely formed in the deep outer reaches of the solar system, far beyond its once-suspected birthplace in the somewhat warmer Jupiter zone. "Our results indicate that Hale-Bopp was likely formed in the Uranus-Neptune zone," says Stern. The high argon abundance of Hale-Bopp may also help explain the unexpected findings by the Galileo Jupiter entry probe, which found that Jupiter has an argon abundance similar to comet Hale-Bopp. "Perhaps Jupiter was seeded with extra argon by the impact of many comets like Hale-Bopp early in the history of the solar system," remarks Stern.

The detection of argon in Hale-Bopp has whet the scientists' appetite for more noble gas data on comets. The team is preparing an instrument called the ALICE Ultraviolet Spectrometer for NASA to fly to comet Wirtanen aboard the European-U.S. Rosetta comet orbiter mission to be launched in 2003. The team has proposed a series of additional NASA rocket launches in 2002 and 2003 to search for argon and other noble gases, even before the Wirtanen orbiter mission is launched. Stern says, "Using this even more sensitive generation of instruments, we look forward to comparing different comets to one another to learn about the diversity of cometary birthplaces."