Prebiotic synthesis on meteorite parent bodies: Insights from hydrogen and carbon isotope models

Several mechanisms could produce the biorelevant organic compounds contained in carbonaceous meteorites. The ratio of heavy-to-light stable isotopes can constrain a compound's formation history because substrates, mechanisms, and physiochemical conditions impact isotope ratios of a reaction's products. Prior stable isotope studies of meteoritic organic compounds have interpreted 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 meteorite and use these models to reach broader conclusions regarding the environments, substrates, and chemical processes that contributed to their synthesis.

The hydrogen isotope model gathers previous molecular-average δD measurements, sorts them into compounds classes (e.g., amines, carboxylic acids), and fits the measurements in each class as linear combinations of the non-exchangeable hydrogen moieties, which are determined by their chemical environment (e.g., R-CH₃, R-CH(NH₂)(COOH)). In the chondrites studied, methyl hydrogens are the most deuterium-enriched moiety (up to 5000 ‰) and hydrogens attached to α‑carbons are the least. Deuterium enrichment is inversely related to a moiety's acidity and a meteorite sample's degree of aqueous alteration and terrestrial weathering, which suggests that ISM-sourced compounds reacted to form deuterium-enriched organic molecules before parent body accretion or during parent body evolution and that those D enrichments were variably attenuated through exchange with water during aqueous alteration on the parent body and subsequent terrestrial processing.

The carbon isotope model tests five potential chemical reaction networks by taking δ¹³C of reactants for each network and applying isotope effects to determine a range of product δ¹³C from minimal to complete reactant conversion. The model that fits compounds in the Murchison meteorite most accurately (70% of previous measurements fall within the values predicted by model ±5‰) with the lowest average residuals (6‰) uses an integrated aldehyde network (oxidation, reductive amination, and Strecker synthesis acting on an initial pool of aldehydes and ketones) to produce straight-chain compounds, which then react with aldehydes to create branched-chain compounds. The addition of aldehydes provides a parsimonious fit the carbon isotope contents of compounds that span over 100‰ in δ¹³C, making it an attractive working hypothesis.