Identification of two sources of carbon monoxide in comet Hale-Bopp

The composition of ices in comets may reflect that of the molecular cloud i n which the Sun formed, or it may show evidence of chemical processing in t he pre-planetary accretion disk around the proto-Sun. As carbon monoxide (C O) is ubiquitous in molecular clouds(1,2), its abundance with respect to wa ter could help to determine the degree to which pre-cometary material was p rocessed, although variations in CO abundance may also be influenced by the distance from the Sun at which comets formed(3-5). Observations have not h itherto provided an unambiguous measure of CO in the cometary ice (native C O). Evidence for an extended source of CO associated with comet Halley was provided by the Giotto spacecraft(6-9), but alternative interpretations exi st(10). Here we report observations of comet Hale-Bopp which show that abou t half of the CO in the comet comes directly from ice stored in the nucleus . The abundance of this CO with respect to water (12 per cent) is smaller t han in quiescent regions of molecular clouds, but is consistent with that m easured in proto-stellar envelopes(11), suggesting that the ices underwent some processing before their inclusion into Hale-Bopp. The remaining CO ari ses in the coma, probably through thermal destruction of more complex molec ules.