Formation and Destruction of Carbon Monoxide in Cometary Comae
A'Hearn, Michael F.
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This dissertation examines the potential impact of chemical reactions and outflow behavior on the chemical development of the coma within one of the most challenging problems in cometary chemistry - identifying the source of extended CO as observed in the comae of some comets. Could chemical reactions and/or outflow behavior contribute significantly to the CO abundance? In these studies, the impact of multiple photochemical processes and two-body chemical reactions are examined within a variety of cases designed to examine the effects of nuclear chemical composition and coma physics, including the dynamics of gas emanating from a region of large-scale negative relief topography. The results show that two-body chemical reactions can contribute as much as 30% to the formation of CO and as much as 60% of the loss of CO within the inner coma, depending on the production rate. Furthermore, the fractional contribution of chemical reactions to CO formation and destruction depends on the outflow behavior. However, the overall result suggests that chemical reactions only contribute a small net gain of CO (~5%) over the CO produced solely by photochemistry. The effects of chemical reactions and outflow behavior are more important, however, to secondary species formed exclusively in the coma. The results also indicate that H<sub>2</sub>CO is the primary contributor to the extended CO source in comet Halley. Observations of H<sub>2</sub>CO in comet Halley suggest that it most likely comes from an extended source as well. The extended source problems of CO and H<sub>2</sub>CO may be linked, in which case the precursor to H<sub>2</sub>CO would gradually produce H<sub>2</sub>CO after its release from the nucleus for it to form CO on the observed spatial scale. Such precursors could be large organic molecules, or grains rich in formaldehyde polymers that thermally dissociate upon heating. More difficult to explain, however, is the CO abundance in comet Hale-Bopp, for which the observed H<sub>2</sub>CO abundance is insufficient to explain the observed CO abundance.