Cometary Gas-Phase Chemistry Taking into Account Homogeneous and Ion-induced Water Recondensation

A chemical model applicable to the circumnuclear region of water-dominated active comets is presented. The model self-consistently considers the vacuum outflow of coma gas and dust, the development of gas-phase photochemical reactions, and the formation of neutral and positively charged water clusters. Solutions are presented using simple boundary conditions consistent with P/Halley in situ gathered data. The results illustrate the strong propensity of such a medium toward partial recondensation, either by homogeneus or by ion-induced clustering. In particular, the dominance of heavy proton hydrates in the innermost coma positive charge density is predicted. The results are compared to similar effects observed in a closely related natural medium: the terrestrial mesopause region. Their limits are carefully outlined, and directions for future developments are indicated.