Using a Physico-Chemical Coma Model to Understand Observations of Comets

Analyses of observations of the comets, Hale-Bopp and Hyakutake, are performed using a global coma model. The relevant physico-chemical processes are identified in order to provide the framework within which observations of these comets can be understood and inferences about their composition can be made. The latest results detailing these processes are discussed concentrating on the collision-dominated inner coma and including the temperature and velocity structure and photo- and gas-phase chemistry (e.g., gas and electron energetics, electron impact reactions). Special consideration is given to the determination of parent species (e.g., C₂H₂, C₂H₆, C₃H₄) from the plethora of molecules and atoms seen in recent comets, concentrating on observations of C₂, C₃, S₂, and NS. The model is useful for identifying important physical and chemical processes for analyzing observations and in situ measurements of comets, to plan spacecraft encounters with these objects, and to learn lessons from past investigations for improving future models. We acknowledge funding from the NSF Planetary Astronomy and SwRI Internal Research programs.