Isotope fractionation in the photochemical escape of O from Mars

The production of energetic and escaping neutral O atoms at the current epoch in the martian thermosphere is thought to be dominated by the O2+ dissociative recombination process:O2++e→O∗+O∗+ΔEIn this equation, O∗O∗ represents a fast O atom and ΔEΔE is the energy released. There are five energetically allowed channels of this reaction, with exothermicities, and thus O kinetic energies, that depend on the electronic energies of the O atoms produced, on the vibrational and rotational states of the initial O2+ ions, and on the ion and electron velocities. We have recently reported the escape probabilities and rates for 16O16O produced in dissociative recombination of 16O16O+16O16O+ for 60° solar zenith angle low and high solar activity models of the martian thermosphere/ionosphere (Fox, J.L., Hać, A. [2009]. Icarus 204, 527–544). Because the isotope ratios of atmospheric species, including 18O/16O18O/16O, contain information about the evolution of the atmosphere, we investigate here the probabilities for escape of energetic 18O18O produced in dissociative recombination of the isotopomer 18O18O16O+16O+. We first predict the altitude dependent vibrational distribution of 18O16O+(v)18O16O+(v), and we compute the nascent energy distributions of the fast 18O18O atoms as functions of vibrational excitation, of the electron and ion temperatures, and of the rotational distributions of the ions; we compare these energy distributions to those of 16O16O produced in DR of 16O16O+(v)16O16O+(v). We then use a 1-D spherical Monte Carlo code to determine escape probabilities for 18O18O as functions of energy and altitude. In this code, energetic atoms are followed in spherical geometry from their altitude of production from one collision to the next through the background atmosphere until their energies fall below the escape energy, or they reach an altitude of 700 km with energies above the escape energy. We convolve these escape probabilities with the nascent energy distributions to determine the probabilities of escape of 18O18O produced in DR of 18O16O+18O16O+. We find that there is a substantial altitude dependent isotope effect, which operates in addition to the effects of diffusive separation of the neutral species from which the O2+ ions are created.