In studying how an icy comet has evolved, one clue can be found in the amount of carbon monoxide it gives off as it nears the Sun. Relatively less carbon monoxide would mean that the comet formed in the warmer confines of the ancient disk of gas and dust that swirled around and eventually formed the planets. More carbon monoxide would mean the comet was born in the colder gas cloud that preceded the disk. But astronomers had never had the chance to get detailed information about cometary carbon monoxide until Hale-Bopp appeared in 1997.
Now, researchers from the National Aeronautics and Space Administration and several American universities report that carbon monoxide emissions from Hale-Bopp are consistent with its having been formed in the protoplanetary disk, in an area between Jupiter and Neptune.Researchers, writing in Nature, also report that Hale-Bopp's carbon monoxide comes from two sources, evenly split: ice in the nucleus, and the coma, where more complex molecules may be being are broken down by the Sun's warmth. -- Henry Fontain