Prebiotic Synthesis on Meteorite Parent Bodies: Insights from Hydrogen and Carbon Isotope Models

Abstract

Several mechanisms could produce the biorelevant compounds in carbonaceous meteorites. These include radiation-driven reactions in the interstellar medium, gas-phase mineral-catalyzed reactions in the solar nebula, and aqueous chemistry in meteorite parent bodies. The ratio of heavy-to-light isotopes in a compound can constrains its formation history: a reaction's substrates, mechanisms, and physiochemical conditions impact isotope ratios. Studies of the stable isotope compositions of meteoritic organic compounds have focused on sample- and molecular-average isotope measurements and have interpreted those data via qualitative or semi-quantitative models. Here we create quantitative models (i.e., explicitly fit to measurements) for hydrogen and carbon isotope compositions of organic compounds in primitive carbonaceous meteorites and use these models to reach broader conclusions regarding the environments, substrates, and chemical processes that contributed to pre- and early-solar-system organic synthesis. The hydrogen model fits measured molecular-average deuterium concentrations in a compound class (e.g., amines, carboxylic acids) as linear combinations of hydrogens with similar chemical environments. In the chondrites studied, methyl hydrogens are amongst the most deuterium-enriched moiety and hydrogens attached to α-carbons are the least. Deuterium enrichment is inversely related to both a compound class's water solubility and a meteorite sample's degree of aqueous alteration and terrestrial weathering. These values suggest that ISM-sourced compounds reacted to form deuterium-enriched molecules on meteorites' parent bodies and the enrichments were attenuated through exchange with water during aqueous alteration on the parent body and subsequent terrestrial processing. The carbon model fits the δ13CVPDB of products from various reaction mechanisms by applying isotope effects to reactant δ13C measurements. The model with the most accurate δ13C fits of the compounds in the Murchison meteorite (62 % of previous measurements fit by model) and the lowest average residuals (5 ‰) uses the integrated aldehyde network (oxidation, reductive amination, and Strecker synthesis on aldehydes and ketones) to produce straight-chain compounds that undergo formaldehyde addition to create branched-chain compounds. Formaldehyde addition has not been previously considered in prebiotic chemical reaction networks, but the best-fit network's ability to fit compounds that span over 100 ‰ in carbon isotope abundances makes it an attractive chemistry to explore.

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