AMHERST, Mass. - A chemical in comets thought to reveal secrets of the early days of the solar system may be only the result of a chemical reaction caused by the sun’s radiation. A report by University of Massachusetts astronomer William Irvine cautions that materials in the atmosphere of a comet could have been caused by chemical reactions related to heat from the sun, and may have little direct bearing on what happened when the sun and planets were formed. Details will be published in tomorrow’s issue of the prestigious journal Nature. Besides Irvine, co-authors include UMass astronomy professor Peter Schloerb; former UMass graduate student Edwin Bergin; postdoctoral researcher Matthew Senay; doctoral candidates James Dickens and Amy Lovell; and University of Hawaii astronomers David Jewitt and Henry Matthews.
The report expands on the researchers’ previous discovery of the chemical hydrogen isocyanide (HNC) in the Comet Hyakutake, which passed by the Earth in 1996. HNC is too unstable to exist on Earth, but is commonly found in interstellar space, Irvine said. The discovery raised the question of whether the chemical was interstellar matter that was preserved when the solar system was first formed. "If so, that would suggest that the formation was cooler and gentler than many astronomers currently theorize. The traditional view is that, during formation of a solar system, everything gets so hot that any existing molecules are destroyed," Irvine said. But another possibility, according to Irvine, is that HNC is produced by chemical reactions in a comet’s atmosphere, which astronomers call its "coma."
The more recent findings, reported in tomorrow’s Nature, focus on Comet Hale-Bopp, which passed by the Earth last year. The team tracked levels of HNC as Hale-Bopp neared the sun. Researchers determined that HNC was being formed photochemically in the comet’s coma, Irvine said.
Irvine adds an intriguing caveat: not all comets are the same. Some comets, such as Hale-Bopp, are quite active chemically; others are less so. Comet Hyakutake produced too little gas to allow the chemical reactions that are necessary to produce HNC as it approached the sun. "We don’t know why there are these differences among comets," Irvine said. "The HNC in Hyakutake may be primordial."