On the Escape of Oxygen and Hydrogen from Mars

Escape rates of oxygen atoms from dissociative recombination of O ₂ ⁺ above the Martian exobase are computed in light of new information from ab initio calculations of the dissociative recombination process, and our recently revised understanding of the Martian dayside ionosphere. Only about 60% of the dissociative recombinations occur in channels in which the O atoms are released with energies in excess of the escape velocity. Futhermore, we find that the computed escape fluxes for O depend greatly on the nature of the ion loss process that has been found necessary to reproduce the topside ion density profiles measured by Viking. If it is assumed that the ions are not lost from the gravitational field of the planet, as required by an analysis of nitrogen escape, the computed average O escape rate is 3 × 10 ⁶ cm ⁻² s ⁻¹ , much less than half the H escape rates inferred from measurements of the Lyman alpha dayglow, which are in the range (1 – 2) × 10 ⁸ cm ⁻² s ⁻¹ . Suggestions for restoring the relative escape rates of H and O to the stoichiometric ratio of water are explored. The final resolution of this problem may have to await a future aeronomy mission to Mars.