Mass fractionation in hydrodynamic escape

Abstract

We show that mass fractionation occurs during the course of hydrodynamic escape of gases from the atmosphere of an inner planet. Light gases escape more readily than heavy gases. The resultant fractionation as a function of mass yields a linear or concave downward plot in a graph of logarithm of remaining inventory against atomic mass. An episode of hydrodynamic escape early in the history of Mars could have resulted in the mass-dependent depletion of the noble gases observed in the Martian atmosphere, if Mars was initially hydrogen rich. Similarly, a hydrodynamic escape episode early in Earth's history could have yielded a mass-dependent fractionation of the xenon isotopes. The required hydrodynamic escape fluxes and total amounts of hydrogen lost from the planets in these episodes are large, but not impossibly so. The theory of the mass fractionation process is simple, but more work will be needed to put together an internally consistent scenario that reconciles a range of data from different planets.