Physical chemistry of comets: Models, uncertainties, data needs

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Abstract
Some recent results or discussions pertaining to the physical chemistry of comets are reviewed, with a special interest in those aspects which illustrate the close interaction between laboratory work and astronomical observations or theories. An overview is given of the present state of knowledge on the chemical composition of comets, both from the point of view of molecular abundances and as regards the relative abundances of the elements and of some isotopic species. Error bars are, unfortunately, uncomfortably large in many instances and we point out the various difficulties encountered in the determinatio and interpretation of production rates or abundances. Assessing the true meaning of the concentrations measured in the atmosphere, or coma, involves the construction of elaborate cometary physico‐chemical models. These models and their uncertainties are only sketched. Rather, the emphasis is laid upon the needs which appear as the most important as far as the relevant fundamental physico‐chemical data are concerned: spectroscopic identifications, radiative transition probabilities, photolysis data (rate coefficients, branching ratios, energetics), collisional cross‐sections, or rates of various types of chemical reactions which play a role in the interpretation of the structure and composition of the coma or in the models constructed to infer the primordial composition of comets. Specific examples are given in each of these areas.

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