From interstellar dust to comets: the extended CO source in comet Halley

Some simplemolecules in comet comae like CO, C2, C3, CN, H2CO appear to be distributed in such a way that they are neither directly emitted from the nucleus surface nor created as daughter molecules from more complex gas phase species. The only remaining possible source is the organic component in comet dust. The requirements imposed on the comet dust grains by the distributed CO emission are that they be heated sufficiently to evaporate a large fraction of the more volatile fraction of the complex organic refractory molecules and that a large fraction of these contain CO groups. Inferring the dust/gas ratio within the mass limits from the comet dust size (mass) distribution obtained by theGiotto spacecraft for cometHalley, and assuming that the refractory organics remaining on the silicate cores are the heating agent by solar radiation in fluffy aggregates of interstellar core-mantle particles, the upper limits of the total amount of CO provided by dust can be approximately determined as a function of porosity. The resulting maximum CO production rate predicted by the comet dust model is significantly less than the observed distributed CO abundance. A possible solution lies in the fact that the dust to gas ratio has been underestimated in the dust size distribution employed here, by neglecting efficient dust fragmentation and sublimation in the innermost coma.On the other hand, itmaynot be impossible that the extended CO abundance was overestimated due to the crossing of dust jets, the time variation of the comet nucleus activity, and the anisotropic outgassing nature of the nucleus. At this point we can certainly say that, in order to obtain the observed distributed CO production rate from comet dust, it requires not only the organic refractory mantles and very high porosity, but also it seems that more heatable dust must be available than has been deduced from the space observation.