Isotope fractionation in the photochemical escape of O from Mars

Details Isotope fractionation in the photochemical escape of O from Mars Isotope fractionation in the photochemical escape of O from Mars

Jul 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010icar..208..176f&link_type=abstract

Icarus, Volume 208, Issue 1, p. 176-191.

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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∗+ΔE In this equation, O∗ represents a fast O atom and ΔE 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 16OO16O 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 18OO, contain information about the evolution of the atmosphere, we investigate here the probabilities for escape of energetic 18OO16O. We first predict the altitude dependent vibrational distribution of 18OO+(v), and we compute the nascent energy distributions of the fast 18OO produced in DR of 16OO+(v). We then use a 1-D spherical Monte Carlo code to determine escape probabilities for 18OO produced in DR of 18OO+. 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.

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