Mass fractionation of noble gases in diffusion-limited hydrodynamic hydrogen escape.

Mass fractionation by hydrodynamic hydrogen escape is a promising mechanism for explaining the observed elemental and isotopic abundance patterns in terrestrial planet atmospheres. Previous work has considered only pure hydrogen winds. Here, the theory of mass fractionation by hydrogen escape is extended to atmospheres in which hydrogen is not the only major constituent. Analytical solutions are derived for cases in which all relevant atmospheric constituents escape; both analytical and numerical solutions are obtained for cases in which important heavy constituents are retained. In either case the fractionation patterns that result can differ significantly from those produced by pure hydrogen winds. Three applications of the theory are discussed: (1) The observed fractionation of terrestrial atmospheric neon with respect to mantle neon can be explained as a by-product of diffusion-limited hydrogen escape from a steam atmosphere toward the end of accretion. (2) The anomalously high Martian (SNC) 38Ar/36Ar ratio is attributed to hydrodynamic fractionation by a vigorously escaping, nearly pure hydrogen wind. (3) It is possible that the present high Martian D/H ratio was established during the same hydrodynamic escape phase that fractionated argon, but the predicted degree of D/H enrichment is sensitive to other, less well constrained parameters.