Oxygen/Hydrogen Chemistry in the Inner Comae of Active Comets

Abstract

We have constructed a concentric-shell, one-dimensional kinetic model that examines the chemistry of hydrogen and oxygen species in detail. We have studied the effects of the reactions of the reactive OH, O(3 P), and O(1 D) species with themselves and with the abundant stable molecules in the inner coma of moderately and highly active comets. We find that the reactions (1) Oð 1 DÞ þ H 2 O ! 2OH and (2) Oð 3 PÞ þ OH ! O 2 þ H play important roles in the inner comae of active comets. Inclusion of reaction (2) predicts the formation of significant amounts of molecular oxygen. As the densities of O 2 may be as high as 1% those of water in some cases, the possibility of detection exists. We suggest the possibility that the ion O þ 2 may contribute to some previously unassigned features in the optical ion-tail spectra of comets. We also consider the role that reactions of the reactive species might play in the destruction of CO, NH 3 , and organic molecules in the inner coma of the active comet. We find that destruction of formaldehyde, for example, by reaction with OH has a small but essentially negligible effect on the predicted production rate of formaldehyde. Finally, we examine the significance of the reaction of OH with CO in the dense inner coma.