The origin of amino acids in lunar regolith samples
Jamie E. Elsila, Michael P. Callahan, Jason P. Dworkin, Daniel P. Glavin, Hannah L. McLain, Sarah K. Noble, Everett K. Gibson Jr.
Geochimica et Cosmochimica Acta
In Press, Accepted Manuscript, Available online 20 October 2015
“We analyzed the amino acid content of seven lunar regolith samples returned by the Apollo 16 and Apollo 17 missions and stored under NASA curation since collection using ultrahigh-performance liquid chromatography with fluorescence detection and time-of-flight mass spectrometry. Consistent with results from initial analyses shortly after collection in the 1970s, we observed amino acids at low concentrations in all of the curated samples, ranging from 0.2 parts-per-billion (ppb) to 42.7 ppb in hot-water extracts and 14.5 ppb to 651.1 ppb in 6M HCl acid-vapor-hydrolyzed, hot-water extracts. Amino acids identified in the Apollo soil extracts include glycine, d- and l-alanine, d- and l-aspartic acid, d- and l-glutamic acid, d- and l-serine, l-threonine, and l-valine, all of which had previously been detected in lunar samples, as well as several compounds not previously identified in lunar regoliths: α-aminoisobutyric acid (AIB), d- and l-β-amino-n-butyric acid (β-ABA), dl-α-amino-n-butyric acid, γ-amino-n-butyric acid, β-alanine, and ε-amino-n -caproic acid. We observed an excess of the l enantiomer in most of the detected proteinogenic amino acids, but racemic alanine and racemic β-ABA were present in some samples.
We also examined seven samples from Apollo 15, 16, and 17 that had been previously allocated to a non-curation laboratory, as well as two samples of terrestrial dunite from studies of lunar module engine exhaust that had been stored in the same laboratory. The amino acid content of these samples suggested that contamination had occurred during non-curatorial storage.
We measured the compound-specific carbon isotopic ratios of glycine, β-alanine, and l-alanine in Apollo regolith sample 70011 and found values of -21‰ to -33‰. These values are consistent with those seen in terrestrial biology and, together with the enantiomeric compositions of the proteinogenic amino acids, suggest that terrestrial biological contamination is a primary source of the amino acids in these samples. However, the presence of the non-proteinogenic amino acids such as AIB and β-ABA suggests the possibility of some contribution from exogenous sources.
We did not observe a correlation of amino acid content with proximity to the Apollo 17 lunar module, implying that lunar module exhaust was not a primary source of amino acid precursors. Solar-wind-implanted precursors such as HCN also appear to be at most a minor contributor, given a lack of correlation between amino acid content and soil maturity (as measured by Is/FeO ratio) and the differences between the δ13C values of the amino acids and the solar wind.”
Distinguishing between the multiple potential sources of amino acids found in low concentrations in lunar regolith samples was difficult given the technology available during early analyses of these samples in the 1970s. Our re-examination of seven lunar regoliths with modern technology has added some additional constraints to the debate. The identities and abundances of amino acids measured in the current work are in good agreement with previous measurements. Our analysis of the enantiomeric composition of these amino acids shows a strong excess of the l-enantiomer of several proteinogenic amino acids. This observation, coupled with δ13C isotopic values of -21‰ to +33‰ for glycine, β-alanine, and l-alanine in the Apollo 70011 sample, suggests that terrestrial biological contamination is a primary source of a large portion of the detected amino acids.
The presence of AIB, an amino acid that is rare in the terrestrial biosphere, and racemic β-ABA, a nonproteinogenic amino acid, suggest potential contributions from exogenous sources such as meteorites, micrometeorites, or IDPs (Thomas-Keprta et al., 2014). These compounds were not present in sufficient abundance to measure their δ13C isotopic values. Both AIB and β-ABA are found in carbonaceous chondrites, and AIB has also been identified in some micrometeorites, but this is the first report of their detection in lunar regolith samples.
Previous work suggested that amino acid precursors such as HCN could be present in the lunar regolith samples due to contamination from lunar module exhaust or implantation from solar wind. Our results do not show strong contributions from either of these sources. We saw no significant differences in total amino acid content between a sample collected under the lunar module exhaust compared to one collected 6.5 km away, nor did we observe the correlations between sample maturity and amino acid content expected from solar wind contributions.
This work highlights the fact that despite thoughtful and careful contamination control (Flory and Simoneit, 1972) and curation efforts, trace organics in extraterrestrial samples can be overwhelmed by terrestrial sources. Future missions emphasizing organic analysis must consider not only contamination control but witness samples and contamination knowledge efforts to understand the unavoidable contamination background (Dworkin et al., 2015, Sandford et al., 2010 and Summons et al., 2014). This work also highlights the lasting value of sample return missions. The techniques used in our study were not yet invented when the samples were collected; curation of the samples for future work allowed us to address the question of the origins of the amino acids detected in lunar regolith samples in ways that the original investigators were unable to resolve.”