Molecular distribution, ¹³C‐isotope, and enantiomeric compositions of carbonaceous chondrite monocarboxylic acids

Abstract

The water‐soluble organic compounds in carbonaceous chondrite meteorites constitute a record of the synthetic reactions occurring at the birth of the solar system and those taking place during parent body alteration and may have been important for the later origins and development of life on Earth. In this present work, we have developed a novel methodology for the simultaneous analysis of the molecular distribution, compound‐specific δ13C, and enantiomeric compositions of aliphatic monocarboxylic acids (MCA) extracted from the hot‐water extracts of 16 carbonaceous chondrites from CM, CR, CO, CV, and CK groups. We observed high concentrations of meteoritic MCAs, with total carbon weight percentages which in some cases approached those of carbonates and insoluble organic matter. Moreover, we found that the concentration of MCAs in CR chondrites is higher than in the other meteorite groups, with acetic acid exhibiting the highest concentration in all samples. The abundance of MCAs decreased with increasing molecular weight and with increasing aqueous and/or thermal alteration experienced by the meteorite sample. The δ13C isotopic values of MCAs ranged from −52 to +27‰, and aside from an inverse relationship between δ13C value and carbon straight‐chain length for C3–C6 MCAs in Murchison, the 13C‐isotopic values did not correlate with the number of carbon atoms per molecule. We also observed racemic compositions of 2‐methylbutanoic acid in CM and CR chondrites. We used this novel analytical protocol and collective data to shed new light on the prebiotic origins of chondritic MCAs.

Extraterrestrial organic compounds and cyanide in the CM2 carbonaceous chondrites Aguas Zarcas and Murchison

“…The total amounts of aliphatic carboxylic acids in Murchison are 1.7 times higher than those in Aguas Zarcas; however, this difference in concentrations is largely due to the higher concentrations of formic acid and acetic acid observed in Murchison compared to Aguas Zarcas, as the concentrations of most of the other carboxylic acids are relatively similar in both meteorites ( Table 1). The total abundances of carboxylic acids in Murchison reported here are similar to those previously reported (Aponte et al 2019); however, the abundance of acetic acid in the Murchison fragment studied here is about half that reported by Aponte et al (2019). Like the differences seen in concentrations for specific amines and carbonyl compounds, the differences in acetic acid concentration in Murchison could be related to derivatization efficiencies and/or sample heterogeneity (Simkus et al 2019a).…”

“…Carboxylic acids are the most abundant organic species in both meteorites, while amino acids, amines, carbonyl compounds, and cyanide comprised only between 8% and 11% of the total organics we investigated in this work. Carboxylic acids have been observed as one of the most abundant organic compounds in the CM2 chondrite subtype (Epstein et al 1987;Krishnamurthy et al 1992;Huang et al 2005;Aponte et al 2011Aponte et al , 2019. It is possible, however, that other aliphatic species not studied here could be even more abundant than carboxylic acids in Aguas Zarcas, Murchison, or other carbonaceous chondrites.…”

“…The PFBHA-derivatives were separated from the aqueous solution using DCM (2 9 2 mL), carried through an acid wash step (3 mL of 0.4 M HCl), and concentrated for analysis. The portions of the water extract allocated for analysis of carboxylic acids were acidified with 40 µL 2 M NaOH, dried under vacuum, and then derivatized with 2pentanol using previously described methods (Aponte et al 2019; see supporting information for further details). The dry residues were suspended in 50 lL of 6 M HCl, 20 lL of 2-pentanol, 100 lL of DCM, and heated at 100°C for 15 h in sealed PFTE-lined screw cap vials in a heating block.…”

“…Triplicate injections of derivatives were made in split mode (split flow: 5 mL min À1 , held for 1 min) in aliquots of 1 µL. The mass spectra were used to identify and quantify the meteoritic carbonyl compounds and carboxylic acids by comparison to reference standards and application of calibration curves as described elsewhere (Aponte et al 2019;Simkus et al 2019bSimkus et al , 2019c.…”

Analyses of Aliphatic Aldehydes and Ketones in Carbonaceous Chondrites

“…The higher concentrations of carbonyl compounds in low aqueously altered CR meteorites, relative to those in CI and CM, is consistent with larger concentrations of aliphatic amino acids, amines, and MCAs previously seen in this chondrite class relative to CI and CM. 15,30,[34][35][36] The higher abundance of soluble organic compounds in CR chondrites than in CI and CM chondrites may be related to the lower levels of aqueous alteration and thermal processing this chondrite class experienced, or it could be a signature of the original organic budget accreted in the CR parent body.…”