Photochemical Escape of O, C and N from Mars

We discuss the photochemical processes that lead to escape of the atoms O, C, and N from the Martian atmosphere. For molecules containing these species, photodissociation and photodissociative ionization may be important, as well as exothermic ion-molecule reactions, and dissociative recombination of molecular ions. We have estimated the magnitudes of escape arising from these processes for O and C. For O, dissociative recombination (DR) of O _{2 ⁺} is much more important than other processes. For C, there are contributions from photodissociation of CO and DR of CO ⁺ , as well as photodissociative ionization and electron-impact dissociative ionization of CO. At one time DR of CO _{2 ⁺} was thought to produce energetic C atoms. We show that escape of C atoms via this mechanism is unimportant even if the reaction produces energetic C atoms. Escape of N is by similar processes, although the dominant process has not been identified. DR of N _{2 ⁺} and photodissociation of N ₂ are possibilities, but the rate of photodissociation must be computed with high resolution cross sections and similarly high resolution solar fluxes. We determine the escape rates of atoms using a Monte Carlo code that tracks the energies and directions of an energetic atom from its production altitude from collision to collision in spherical geometry until its energy falls below the escape energy or it reaches an altitude that we have currently chosen as 700 km with enough energy to escape. Our first calculations for escape of O were "proof of concept" calculations for which we introduced some approximations, which are not made in our current calculations. We discuss these briefly.