CHEMICAL-PROCESSING IN THE COMA AS THE SOURCE OF COMETARY HNC

The discovery of hydrogen isocyanide (HNC) in comet Hyakutake with an abundance (relative to hydrogen cyanide, KCN) similar to that seen in dense interstellar clouds raised the possibility that these molecules might be surviving interstellar material(1). The preservation of mater ial from the Sun's parent molecular cloud would provide important cons traints on the processes that took place in the protostellar nebula. B ut another possibility is that HNC is produced by photochemical proces ses in the coma, which means that its abundance could not be used as a direct constraint on conditions in the early Solar System. Here we sh ow that the HNC/HCN ratio determined for comet Hale-Bopp varied with h eliocentric distance in a way that matches the predictions of models o f gas-phase chemical production of HNC in the coma, but cannot be expl ained if the HNC molecules were coming from the comet's nucleus. We co nclude that HNC forms mainly by chemical reactions in the coma, and th at such reactions need to be considered when attempting to deduce the composition of the nucleus from observations of the coma.