# Multiple formation pathways for amino acids in the early Solar System based on carbon and nitrogen isotopes in asteroid Bennu samples

## TL;DR

Asteroid Bennu samples show amino acids formed through different processes in the early Solar System, offering clues about the origins of life.

## Contribution

The study reports first intramolecular δ13C values for glycine in extraterrestrial materials and distinct formation pathways for amino acids in Bennu.

## Key findings

- Bennu's glycine likely formed in primordial ices, unlike Murchison's glycine, which formed under aqueous conditions.
- Bennu amino acids show highly 15N-enriched values, suggesting formation in cold outer regions of the early Solar System.
- D- and L-glutamic acid in Bennu have distinct δ15N values, challenging assumptions of isotopic uniformity in chiral pairs.

## Abstract

Meteorites, asteroids, and comets can host prebiotic organic compounds produced by a range of processes occurring before, during, and after accretion of parent bodies. Stable isotopic measurements of these compounds provide insights into these processes, which link how planets and life developed. We measured stable isotope values of amino acids, aldehydes, and ketones extracted from samples of asteroid Bennu retrieved by NASA’s OSIRIS-REx mission. Carbon isotope values suggest that Bennu’s glycine formed mostly in primordial ices entrained in the early Solar System, whereas glycine in Murchison, a compositionally similar meteorite, formed under mild, aqueous conditions in a protoplanetary body. Additionally, nitrogen isotopes imply that the enantiomers (mirror-image molecules) of glutamic acid in Bennu samples experienced distinct formation or alteration conditions.

Samples collected from the carbonaceous near-Earth asteroid Bennu and delivered to Earth by NASA’s OSIRIS-REx mission contain organic molecules relevant to prebiotic chemistry. Stable isotopic measurements of extraterrestrial soluble organic matter provide critical insights into the formation pathways and alteration histories of such molecules, which hold significance for understanding the origins of life. We leverage state-of-the-art techniques for picomolar-scale isotopic analyses of amino acids in samples of Bennu and, for comparison, the carbonaceous meteorite Murchison. We report intramolecular δ13C values for glycine, which have not previously been measured in extraterrestrial materials; molecular-averaged δ13C values for amino acids, aldehydes, and ketones; and δ15N values for glycine, β-alanine, and D/L-glutamic acid. Intramolecular carbon isotope patterns of glycine in Bennu contrast with those in Murchison, suggesting distinct formation pathways. We explore several formation mechanisms and hypothesize that the observed glycine in Murchison formed dominantly by a Strecker-like synthesis under aqueous conditions, whereas the glycine currently found in Bennu may have formed mainly by modified radical–radical reactions in primordial ices at the cold, outer reaches of the early Solar System and retained its isotopic values throughout accretion and multiple episodes of aqueous alteration. This hypothesis is supported by the highly 15N-enriched δ15N values in Bennu amino acids (+170 to 277‰). Differences in the δ15N values of D- and L-glutamic acid (Δ = 87‰) in Bennu affirm published reports of enantiomeric differences in meteoritic amino acids and challenge the assumption of isotopic uniformity between amino acid chiral pairs.