Symposium on the Diversity and Similarity of Comets [antikvár]

DIVERSITY AND SIMILARITY OF COMETS Armand H. DelsemmeDepartment of Physics and Astronomy, The University of Toledo Toledo, Ohio 43606 U.S.A.Although many cornets appear superficially very different, it is suspected (but not yet completely demonstrated) that they were originally very simiVar. The différences seem to resuit from their évolution and decay, or in some cases from the non linearity of the observed phenomena. New, young and old cornets can be statistically classified by their orbital dynamics; they show photometric différences interpreted by volatile fractionation during their progressive decay. The dust-to-gas apparent ratio, as well as the different types of dust tails, may be linked to an artifact, namely differential grain welding and clustering during crust or mantle decay. Spectral différences concentrate around "normal" ratios (like C3/CN = 0.2 or C2/CN = 1.5) except for a few rare cornets. The ionization efficiency varies much with the size of the cornet, and with a high power of heliocentric distance, because of the variable pénétration of the solar wind. Extreme cases like the C0+ in cornets Humason and Morehouse may reveal dynamically young objects. The "average" bright cornets of the 1970'ies and 1980'ies had a rather homogeneous chemistry with all elements in about cosmic (or solar) proportions, except two typical depletions in the gaseous phase: a factor of 1000 for hydrogen, a factor of 3 to 4 for carbon. Other non "average" cornets showed a large range of CO or C02, suggesting that carbon in gaseous phase may reach up to cosmic proportions. The Giotto analysis of the Halley dust grains suggests that the carbon missing in the gaseous phase of an old cornet could be entirely in rather stable organic molecules, up to 2055 of the dust; one could speculate that this comes from the polymerization of more volatile organic molecules (like formaldehyde) during crust decay with aging.All circumstantial arguments suggest that pristine cornets are radially homogeneous and more or less similar, with molecules of the interstellar type.