Primordial xenon

Current results of this exercise, using the Xe component compositions listed in Table 1, are reviewed in Pepin (2000). Applied to Earth, a modeling procedure involving derivation of fractionation relationships between nonradiogenic terrestrial and meteoritic Xe compositions simultaneously identifies a composition named U-Xe as primordial Xe, and establishes the presence of an atmospheric Xe component due principally to fission of extinct as noted earlier in this chapter. Hydrodynamic escape of U-Xe leaves its fractionated residue on Earth. Evolution of atmospheric Xe from this early composition to its present isotopic state by subsequent degassing of fission and radiogenic 

In contrast to Earth, Martian Xe apparently did not evolve from a U-Xe progenitor. Modeling derivation of primordial Xe composition on Mars is based on analyses of atmospheric gases trapped in glassy phases of SNC meteorites (Swindle 2002, this volume). Present ambiguities in this data base are such that two different solar-system Xe compositions, carbonaceous chondrite (Cl)-Xe and SW-Xe, are possible candidates—but not U-Xe. Exclusion of U-Xe as the dominant primordial atmospheric inventory on Mars, despite the implication of the terrestrial modeling that it was a major component of the nebular gas phase, requires that accretion of Cl- or SW-Xe-rich materials from sources more localized in space or time has overwhelmed the isotopic signature of its presence. 

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Pepin R. O. (2000) On the isotopic composition of primordial xenon in terrestrial planet atmospheres. Space Sci. Rev. 92, 371-395. 

SNC-based geochemical model of Mars bulk composition which calls for a -40% mass fraction of volatile-rich, oxidized Cl-like material in the planet. However subsequent expansion and recalculation of Xe data from the SNC meteorites now suggest that solar-wind Xe is a viable alternative to Cl-Xe as the principal atmospheric constituent on early Mars (see Primordial Xenon section below), raising the challenging question of how Mars could have acquired a large solar Xe component while Earth apparently did not. 

Swindle T. D. and Jones J. H. (1997) The xenon isotopic composition of the primordial martian atmosphere contributions from solar and fission components. J. Geophys. Res. 102, 1671-1678. 

In the first evolutionary epoch, note that the extent of xenon fractionation from primordial to present composition is similar on both Earth and Mars despite the much smaller mass of Mars, the apparent differences (U-Xe versus SW-Xe) in their precursor xenon, the much greater overall depletion of martian noble gases, and the possibility that escape episodes were powered by... 

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