Abundant extraterrestrial amino acids in the primitive CM carbonaceous chondrite Asuka 12236

Abstract

The Asuka (A)-12236 meteorite has recently been classified as a CM carbonaceous chondrite of petrologic type 3.0/2.9 and is among the most primitive CM meteorites studied to date. Here, we report the concentrations, relative distributions, and enantiomeric ratios of amino acids in water extracts of the A-12236 meteorite and another primitive CM chondrite Elephant Moraine (EET) 96029 (CM2.7) determined by ultra-high-performance liquid chromatography time-of-flight mass spectrometry. EET 96029 was highly depleted in amino acids and dominated by glycine, while a wide diversity of two-to six-carbon aliphatic primary amino acids were identified in A-12236, which had a total amino acid abundance of 360 AE 18 nmol g À1 , with most amino acids present without hydrolysis (free). The amino acid concentrations of A-12236 were double those previously measured in the CM2.7 Paris meteorite, consistent with A-12236 being a highly primitive and unheated CM chondrite. The high relative abundance of a-amino acids in A-12236 is consistent with formation by a Strecker-cyanohydrin dominated synthesis during a limited early aqueous alteration phase on the CM meteorite parent body. The presence of predominantly free glycine, a near racemic mixture of alanine (D/L~0.93-0.96), and elevated abundances of several terrestrially rare nonprotein amino acids including a-aminoisobutyric acid (a-AIB) and racemic isovaline indicate that these amino acids in A-12236 are extraterrestrial in origin. Given a lack of evidence for biological amino acid contamination in A-12236, it is possible that some of the Lenantiomeric excesses (L ee~3 4-64%) of the protein amino acids, aspartic and glutamic acids and serine, are indigenous to the meteorite; however, isotopic measurements are needed for confirmation. In contrast to more aqueously altered CMs of petrologic types ≤2.5, no Lisovaline excesses were detected in A-12236. This observation strengthens the hypothesis that extensive parent body aqueous activity is required to produce or amplify the large L-isovaline excesses that cannot be explained solely by exposure to circularly polarized radiation or other chiral symmetry breaking mechanisms prior to incorporation into the asteroid parent body.

Presolar stardust in highly pristine CM chondrites Asuka 12169 and Asuka 12236

“…">3.The higher abundances of presolar grains in A12236 and especially A12169 than seen in other CM chondrites provide further strong support to petrographic and chemical evidence (Glavin et al. 2020; Kimura et al. 2020; Noguchi et al.…”

“…More recently, Glavin et al. (2020) found a much higher abundance of amino acids in A12236 than in Paris, providing further support that this meteorite has been less affected by parent‐body alteration than any previously studied CMs.…”

“…2020) and organic chemical (Glavin et al. 2020) evidence that these samples have seen even less alteration than the Paris meteorite and that the matrix of A12169 is among the most primitive chondritic material available for study.…”

Extraterrestrial hydroxy amino acids in CM and CR carbonaceous chondrites

“…More specifically, unaltered CM and CR type 3 (petrologic type ≥2.6, see discussion below) carbonaceous chondrites show racemic isovaline, but more aqueously altered type 1 or type 2 carbonaceous chondrites (petrologic type 2.0–2.5, see discussion below) contain l ‐isovaline excesses (Glavin and Dworkin 2009; Glavin et al. 2010, 2020b; Martins et al. 2015).…”

Amino acid abundances and compositions in iron and stony‐iron meteorites

“…Although abiotic chemistry should produce racemic mixtures of chiral compounds in the absence of a chiral driving force, enantiomeric excesses of certain amino acids, including isovaline, have been observed in a variety of meteorites. The L-isovaline excesses observed in CB (up to 14%) and CH (up to 20%) chondrites are among the highest reported (Burton et al 2013;Glavin et al 2020). Rosenberg et al (2008) showed that spin-polarized secondary electrons emitted from the photolysis of an iron-nickel magnetic substrate result in chiral selective reactivity of (R)-or (S)-2-butanol.…”