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 circumnuclear coma of a wa ter-dominated comet is presented. Its essential goal is to obviate the nearly complete lack of direct experimental data concerning this regi on by performing quantitative simulations, using a number of alternati ve physicochemically plausible schemes. The present paper describes in detail the dusty gasdynamics algorithms of the model; the chemical an d radiative algorithms will be presented in companion publications. As an illustration of the capabilities of the model, we compare two solu tions that are compatible with the in situ measured P/Halley gas and d ust properties. In the first one, the coma is formed by classical nucl eus surface sublimation of dusty gas; in the second one, it is formed in a two-step process: surface ejection of ''active'' fragments made o f water ice mixed with size-dispersed mineral grains, followed by subl imation of these fragments. The distributed production solution allows a closer fit to the measured H2O velocity profile and is qualitativel y compatible with properties of P/Halley that seem hard to reconcile w ith the pure surface production assumption, viz., the near-nucleus dus t brightness decrease law, the weak mass dependence of the tail dust v elocities, and the activity at large heliocentric distance. While a de finite proof of the occurrence of distributed production must await bo th a larger set of experimental data and a three-dimensional generaliz ation of the present model, the present results demonstrate that the u sual postulate of pure surface production of the coma material is neit her the only acceptable one nor the most satisfactory one.