Protosolar Carbon Isotopic Composition: Implications for the Origin of Meteoritic Organics

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Abstract
New ion probe isotopic measurements of carbon trapped within the 50 nm thick surface layer of lunar regolith grains strongly suggest that solar wind C is depleted in 13C by at least 10% relative to terrestrial C. In order to account for the general 13C enrichment of planetary C relative to solar C, we propose that the main carriers of C in these objects, i.e., organics, were formed in an environment that allowed a strong isotopic enrichment of 13C in the solid phase. Such an environment is most likely a dense and warm circumstellar or interstellar gas medium, which could well correspond to the nebula surrounding the proto-Sun, where isotopic fractionation could be triggered by photochemical reactions.