Distributed coma sources and the CH4/CO ratio in Comet Halley

Early analyses of the Ion Mass Spectrometer (IMS) data from the Giotto flyby of Comet P/Halley indicated significant abundances of CHn+ (n=1 to 4). The source of these ions was assumed to be frozen CH4 in the nucleus. An abundance of about 2% CH4 was consistent with this interpretation, resulting in a ratio of CH4/CO that is greater than the predicted limits for interstellar clouds or the solar nebula. However, subsequent analyses of data from the Giotto Neutral Mass Spectrometer (NMS) indicate distributed sources of CO and H2CO in the coma that are most likely associated with organic (CHON) particles, rich in CH‐bearing compounds that decay and produce CHn+ species. We present a model that qualitatively accounts for the measured spatial distribution of CO and H2CO and indicates that most of the CH4+ deduced from the Giotto IMS data may originate from organic compound in the coma dust and not from volatiles released directly from the nucleus. As a consequence, the absence of CH4 in the icy phase of the nucleus is consistent with the observations with an upper limit of about 0.5% to the ratio CH4/CO. This is an important criterion for theories of comet formation.