Evolution of Earth's Noble Gases: Consequences of Assuming Hydrodynamic Loss Driven by Giant Impact

A simple analytic model of hydrodynamic escape is applied to atmospheric loss from Earth under conditions that are unproven but could plausibly have existed following deposition of thermal energy by a giant Moon-forming impact. Primordial xenon in the primary (pre-impact) atmosphere is readily fractionated to its contemporary nonradiogenic isotopic composition by appropriate selection of parameters in the equations governing the escape process. Subsequent mixing of the fractionated residuals of lighter primordial noble gases surviving in the post-escape atmosphere with solar-composition gases outgassed from the deep planetary interior yields close matches to the present-day abundances and isotopic compositions of atmospheric krypton and argon. Replication of present-day neon composition requires an additional later episode of hydrodynamic H 2escape, now powered by extreme-ultraviolet (EUV) solar radiation just intense enough for entrainment and loss of Ne but not of heavier species. Requirements for EUV flux intensity and planetary water inventory are substantially reduced compared to an earlier model of EUV-driven Xe loss from Earth. A noteworthy result of this approach is the close agreement of the noble gas elemental composition characterizing the pre-impact terrestrial atmosphere with that derived for Venus's primary atmosphere from a parallel evolutionary model involving only solar EUV radiation as an energy source. No claim is made that the modeling parameters used here adequately describe the complex and rapidly evolving physical nature of the post-impact terrestrial atmosphere, or that these solutions are unique. But they do suggest a basic unity in primordial noble gas distributions on the two planets, and point to separate mechanisms that could account for divergent evolution to their presently radically different compositional states.