A general physicochemical model of the inner coma of active comets. 1: Implications of spatially distributed gas and dust production

A multipurpose physicochemical model of the circumstellar coma of a water-dominated comet is presented. Its essential goal is to obviate the nearly complete lack of direct experimental data concerning this region by performing quantitative simulations, using a number of alternative physicochemically plausible schemes. The present paper describes in detail the dusty gasdynamics algorithms of the model; the chemical and radiactive algorithms will be presented in companion publications. As an illustration of the capabilities of the model, we compare two solutions that are compatible with the in situ measured P/Halley gas and dust properties. In the first one, the coma is formed by classical nucleus surface sublimation of dusty gas; in the second one, it is formed in a two-step process: surface ejection of 'active' fragments made of water ice mixed with size-dispersed mineral grains, followed by sublimation of these fragments. The distributed production solution allows a closer fit to the measured H2O velocity profile and is qualitatively compatible with properties of P/Halley that seem hard to reconcile with the pure surface production assumption, viz., the near-nucleus dust brightness decrease law, the weak mass dependence of the tail dust velocities, and the activity at large heliocentric distance. While a definite proof of the occurrence of distributed production must await both a larger set of experimental data and a three-dimensional generalization of the present model, the present results demonstrate that the usual postulate of pure surface production of the coma material is neither the only acceptable one nor the most satisfactory one.