AN ANALYSIS OF CO PRODUCTION IN COMETARY COMAE: CONTRIBUTIONS FROM GAS-PHASE PHENOMENA

Understanding the sources of CO in cometary comae is important for understanding comet chemistry and the roles comets have played in the development of the solar system. Among comets sampled to date, the CO abundances vary widely and no direct correlation of CO abundance with other known comet properties has been identified. The picture is complicated further by the discovery of CO production in the comae of some comets, most notably comets Halley and Hale-Bopp. In this study, we investigate the conditions under which CO can be produced in the coma via gas-phase phenomena. We include photochemistry of several parent molecules, as well as two-body chemical reactions that involve the parents and their photodissociative daughter and granddaughter products. We also consider the level of excitation of 'hot' hydrogen (H*) and O(1D) in the network, because the level of excitation of these reactants strongly influences reaction rates. Our results suggest that the dominant gas-phase contributor to CO formation is the photodissociation of H2CO. Even though typical abundances of H2CO are at ∼1% relative to water in the coma, it produces more CO than other processes due to its relatively short photodissociation lifetime. Because other studies have shown H2CO to have a distributed source as well, it suggests that at least some CO formation in the coma is connected to the H2CO distributed source. We take the time to examine the CO2/CO ratio and note that while the CO2/CO ratio in comets Halley, Hale-Bopp, and Hyakutake are noticeably different when only native CO is considered, the CO2/CO ratios show greater similarity when total CO is considered. Although this sample is relatively small, should the relatively similar CO2/COTotal ratio of ∼0.25 indeed be constant for comets with distributed CO sources, it suggests that the extended CO source of these comets is tied directly to the overall C, H, O chemistry of comets, as is likely to happen if hydrogenation of CO occurred on icy grains.