By Ron Cowen
When Comet Hale-Bopp passed near the sun 3 years ago, it dazzled skywatchers with brilliant tails of dust and gas. The comet also vented traces of a precious cargo: the inert gas argon.
Scientists had never before detected argon or any other noble gas in a comet, notes S. Alan Stern of the Southwest Research Institute in Boulder, Colo. The presence of argon, he and his colleagues report, suggests Hale-Bopp was born in the outer solar system, between the orbits of Uranus and Neptune. Scientists had previously suspected that the comet arose from material in Jupiter’s vicinity, which is closer to the sun.