Evaluation of isotopic fractionation of oxygen ions escaping from terrestrial thermosphere

Abstract

Oxygen isotopic ratios in contemporary lunar soils indicate a highly mass-independent fractionation (MIF) of Δ17O≈25 per mil. This fraction is comparable to the ratio for stratospheric ozone and is thus supposed to have originated from the terrestrial upper atmosphere through transport of oxygen ions by the intense Earth wind. In this paper, we develop a one-dimensional chemical model that can solve for the ionic and neutral compositions of oxygen isotopes by including the isotopic effects in dynamical, photolytic, and chemical processes. We first confirm that the isotopic ratios of O+, O2+, O, and O2 are nearly mass dependent, Δ17O≈0 per mil, when the isotopic effects of multiple (eddy, molecular, and ambipolar) diffusion processes are taken into account. We examine which photolytic and chemical processes can produce a strong MIF for oxygen species and select four candidates: photo-dissociative ionization of O2, photo-dissociation of O2 by Lyman-α solar emissions, charge exchange between O+ and O2+, and atomic exchanges between O and O2. We estimated the oxygen isotopic ratio to be Δ17O≈1–4 per mil at the height of 100–400km on the basis of a statistical treatment of atom mechanics. These values are smaller than those in the lunar case, and we conclude that a more rigorous quantum mechanical treatment of photo-dissociation processes will be necessary before we can make an evaluation of the role of Earth-escaping oxygen isotopes.