Accretion of volatile-rich planetesimals

Comet accretion models. Noble gases, as well as water, carbon, and nitrogen, could have been supplied to the inner planets by accretion of volatile-rich icy comets scattered inward from the outer solar system. Although noble gas isotopic distributions in comets are unknown, solar isotopic compositions would be expected in cometary gases acquired from the nebula. There is experimental evidence that the relative elemental abundances of heavier species (Xe, Kr, and Ar) trapped in water ice at plausible comet formation temperatures ( 30 K) approximately reflect those of the ambient gas phase, and trapped noble gas abundances per gram of water are substantial (Bar-Nun et al. 1985 Owen et al. 

The compositional characteristics of an icy planetesimal source in comet accretion models for Venus, discussed below, require occlusion of nebular noble gases with approximately unfractionated elemental ratios for Ar Kr Xe but much lower Ne. Thermodynamic modeling suggests that noble gases incorporated in clathrates do indeed have low Ne/Ar ratios, but do not reflect ambient gas-phase compositions for Xe/Ar and Kr/Ar and instead are strongly enriched in the heavier species (Lunine and Stevenson 

Clathrated gases therefore appear unlikely to be the source of atmospheric noble gases, at least for Venus, and one must appeal to physical adsorption on ice. 

At likely nebular temperatures and pressures at its radial distance. Mars is too small to have condensed a dense early atmosphere from the nebula even in the limiting case of isothermal capture (Hunten 1979 Pepin 1991). Therefore, regardless of the plausibility of gravitational capture as a noble gas source for primary atmospheres on Venus and 

some other way is needed to supply Mars. An early inward flux of icy planetesimals would have contributed to all three of the terrestrial planets, even if it dominated the noble gas budget only on Mars. Accretion of a substantial mass of volatile-rich bodies resembling the present-day carbonaceous chondrites may be another possibility. Swindle et al. (1986) pointed out that the isotopic composition of Martian atmospheric Xe calculated from their SNC meteorite measurements was compatible with that of mass-fractionated Cl-Xe, consistent with Dreibus and Wanke s (1985, 1987, 

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