Physico-Chemical Models of Cometary Comae

Physico-chemical modeling is important to understand the nature and processes relevant to comets. Photochemistry is the major source of radicals and plasma that further initiate key gas-phase reactions, leading to the plethora of molecules and atoms seen in their comae. The effects of photoelectrons that react via electron impact reactions are important to the overall ionization. Within this modeling framework, important physico- chemical processes can be identified to interpret observations and \it in situ measurements of comets and to provide valuable insights into the intrinsic properties of their nuclei. Details of these processes are presented, from the collision-dominated inner coma to the solar wind interaction region. The results include temperature and velocity structures, and photo- and gas-phase chemistry, composition, gas and electron energetics throughout the cometary atmosphere. This model successfully accounted for the Halley water-group chemistry and composition. Prior model results are generally consistent with \it in situ measurements of the PEPE instrument onboard the Deep Space 1 Mission to Comet Borrelly, S2 in Comet Hyakutake, and observations of C2, C3, and NS in Comet Hale-Bopp. This extensive modeling effort to investigate these important cometary processes is highly relevant to past, on going, and future spacecraft missions comets and Earth-based observations of these primitive objects. Acknowledgements: We acknowledge funding and support from the NASA Discovery Data Analysis Program and the NSF Planetary Astronomy Program.