Chiral selection, isotopic abundance shifts, and autocatalysis of meteoritic amino acids

Abstract

The discovery of amino acids in meteorites has presented two clues to the origin of their processing subsequent to their formation: a slight preference for left-handedness in some of them and isotopic anomalies in some of their constituent atoms. Numerous models have been developed to explain these phenomena. In this article we present theoretical results from the supernova neutrino amino acid processing (SNAAP) model, which uses electron antineutrinos and the magnetic fields from source objects such as supernovae or colliding neutron stars to selectively destroy one amino acid chirality, and the same antineutrinos to create isotopic anomalies. For plausible magnetic fields and electron antineutrino fluxes, nonzero, positive enantiomeric excesses, ees, defined to be the relative left/right asymmetry in an amino acid population, are reviewed for two amino acids, and conditions are suggested that would produce ee>0 for all of the α-amino acids. This is accomplished by electron antineutrinos produced by the cosmic object interacting with the N14 nuclei in the amino acids. The relatively high-energy antineutrinos that produce the ees would inevitably also produce isotopic anomalies. A nuclear reaction network was developed to describe the reactions resulting from them and the nuclides in the meteorites. At similar antineutrino fluxes, assumed recombination of the detritus from the antineutrino interactions is shown to produce appreciable isotopic anomalies in qualitative agreement with those observed for D/H1 and N15/N14. The isotopic anomalies for C13/C12 are predicted to be small, as are also observed. Autocatalysis, a chemical enrichment of a small imbalance, may be necessary for the SNAAP model, or for any model suggested so far, to produce the largest ees observed in meteorites. Thus a new autocatalysis model is developed that includes both the spins of the nuclei and the chirality of the amino acids. Autocatalysis allows the constraints of the original SNAAP model to be relaxed, providing more flexibility in its application, and increasing the probability of meteoroid survival in sites where amino acid processing is possible. The SNAAP model thus explains all of the observed phenomena related to processing of the amino acids in meteorites. These results have obvious implications for the origin of life on Earth.