Unusual isotopic enrichments are present in organic molecules found in the ISM and in meteorites. In particular, high deuterium, 13C and 15N enhancements are found. The D/H ratio of ~ 10-5, fixed by nucleosynthesis that occurred in the “Big Bang,” cannot be altered by stars. Carbon and nitrogen isotopes, on the other hand, can be influenced by stellar ejecta, in particular 12C/13C and 14N/15N ratios. Ratios as high as D/H ~ 0.4 have been observed in gas-phase interstellar molecules1. However, chemical interpretation is simplistic at best, as patterns of deuterium enrichment remain puzzling. For example, D prefers to exchange on the methyl, as opposed to the hydroxyl, group in CH3OH, and while highly deuterated methanol is common in the interstellar medium (ISM), deuterated EtOH is not. In meteoritic compounds, pronounced deuterium enrichments are found in amino, carboxylic and sulfonic acids. One of the highest ratios, D/H ~ 0.0012, has been found in 2-amino isobutyric acid2. The highest deuterium enrichments are found in the Insoluble Organic Matter (IOM), where D/H ~ 0.003. Unfortunately, typical isotopic analysis of such compounds cannot distinguish where the isotopic substitution occurs, unlike interstellar observations. In the ISM, such extreme chemical fractionation is thought to arise from differences in zero-point bond energies that become important at low interstellar temperatures in ion-molecules reactions. The isotopic enrichment is then thought to be passed on to the early solar system and then be preserved in meteorites. Is this really the case? We have been examining the link between interstellar and meteoritic isotope anomalies through observational and laboratory studies. Broadband spectral line surveys of interstellar and circumstellar sources have revealed interesting chemical trends in isotope ratios in the gas phase; new meteoritic measurements have been carried out in the solid-state. Initial findings suggest that 13C and 15N enrichments in meteorites may simply reflect local nucleosynthesis, while those for deuterium are true chemical effects. Meteoritic deuterium enrichments are likely to be interstellar in origin, but may reduced in magnitude due to partial aqueous processing. A summary of the current findings will be presented.

Extreme Isotope Ratios in Meteoritic Material: A Gas-Phase Interstellar Origin?

253rd National Meeting (2017)

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