FROM INTERSTELLAR DUST TO COMETS - THE EXTENDED CO SOURCE IN COMET HALLEY

Some simple molecules in comet comae like CO. C-2, C-3, CN, H-2 CO app ear to be distributed in such a way that they are neither directly emi tted from the nucleus surface nor created as daughter molecules from m ore complex gas phase species. The only remaining possible source is t he organic component in comet dust. The requirements imposed on the co met dust grains by the distributed CO emission are that they be heated sufficiently to evaporate a large fraction of the more volatile fract ion of the complex organic refractory molecules and that a large fract ion of these contain CO groups. Inferring the dust/gas ratio within th e mass limits from the comet dust size (mass) distribution obtained by the Giotto spacecraft for comet Halley, and assuming that the refract ory organics remaining on the silicate cores are the heating agent by solar radiation in fluffy aggregates of interstellar core-mantle parti cles, the upper limits of the total amount of CO provided by dust can be approximately determined as a function of porosity. The resulting m aximum CO production rate predicted by the comet dust model is signifi cantly less than the observed distributed CO abundance. A possible sol ution lies in the fact that the dust to gas ratio has been underestima ted in the dust size distribution employed here, by neglecting efficie nt dust fragmentation and sublimation in the innermost coma. On the ot her hand, it may not 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 t he nucleus. At this point we can certainly say that, in order to obtai n the observed distributed CO production rate from comet dust, it requ ires not only the organic refractory mantles and very high porosity, b ut also it seems that more heatable dust must be available than has be en deduced from the space observation.