EVOLUTION OF EARTHS NOBLE-GASES - CONSEQUENCES OF ASSUMING HYDRODYNAMIC LOSS DRIVEN BY GIANT IMPACT

A simple analytic model of hydrodynamic escape is applied to atmospher ic loss from Earth under conditions that are unproven but could plausi bly have existed following deposition of thermal energy by a giant Moo n-forming impact. Primordial xenon in the primary (pre-impact) atmosph ere is readily fractionated to its contemporary nonradiogenic isotopic composition by appropriate selection of parameters in the equations g overning the escape process, Subsequent mixing of the fractionated res iduals of lighter primordial noble gases surviving in the post-escape atmosphere viith solar-composition gases outgassed from the deep plane tary interior yields close matches to the present-day abundances and i sotopic compositions of atmospheric krypton and argon, Replication of present-day neon composition requires an additional later episode of h ydrodynamic Hz escape, now powered by extreme-ultraviolet (EUV) solar radiation just intense enough for entrainment and loss of Ne but not o f heavier species, Requirements for EUV flux Intensity and planetary w ater 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 charact erizing the pre-impact terrestrial atmosphere with that derived for Ve nus's primary atmosphere from a parallel evolutionary model involving only solar EW radiation as an energy source, No claim is made that the modeling parameters used here adequately describe the complex and rap idly evolving physical nature of the post-impact terrestrial atmospher e, or that these solutions are unique. But they do suggest a basic uni ty 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. (C) 1997 Aca demic Press.