Chemistry of O(¹D) atoms in the coma: implications for cometary missions

Abstract

The forbidden red oxygen lines at 6300 and 6364 Å, which results due to ¹D→³P transition, provide an important diagnostic tool in the study of comets. These lines cannot be produced by resonance fluorescence excitation of the ground-state oxygen atom, and therefore are mainly produced due to dissociation of H₂O and other O-containing species in comets (e.g., OH, CO, CO₂, H₂CO etc.) by photon and electron impact and in other collision reactions. Since the lifetime of ¹D state is quite long (~110 sec) collisional de-excitation processes are important. We have used a coupled chemistry-transport model in conjunction with an efficient emission production code to study the chemistry of O(¹D) atoms and the production of OI 6300 Å emission in comets. The model calculations are made for comet 46P/Wirtanen: the target of the ROSETTA mission. It is found that in the inner coma the density profile of O(¹D) is controlled dominantly by the H₂O. The model predicts ~300 R of OI 6300 Å brightness on comet Wirtanen.