Depleted carbon isotope compositions observed at Gale crater, Mars

Obtaining carbon isotopic information for organic carbon from Martian sediments has long been a goal of planetary science, as it has the potential to elucidate the origin of such carbon and aspects of Martian carbon cycling. Carbon isotopic values (δ C ) of the methane released during pyrolysis of 2...

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Published in:	Proceedings of the National Academy of Sciences - PNAS Vol. 119; no. 4; p. 1
Main Authors:	House, Christopher H, Wong, Gregory M, Webster, Christopher R, Flesch, Gregory J, Franz, Heather B, Stern, Jennifer C, Pavlov, Alex, Atreya, Sushil K, Eigenbrode, Jennifer L, Gilbert, Alexis, Hofmann, Amy E, Millan, Maëva, Steele, Andrew, Glavin, Daniel P, Malespin, Charles A, Mahaffy, Paul R
Format:	Journal Article
Language:	English
Published:	United States National Academy of Sciences 25-01-2022
Subjects:	Astrochemistry Carbon Carbon cycle Carbon dioxide Carbon isotopes Cosmic dust Depletion Gas analysis Isotopes Lasers Mars Mars craters Methane Molecular clouds Organic carbon Photolysis Photoreduction Physical Sciences Pyrolysis Sediments Tunable lasers methane Mars pyrolysis carbon isotopes Gale crater
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Summary:	Obtaining carbon isotopic information for organic carbon from Martian sediments has long been a goal of planetary science, as it has the potential to elucidate the origin of such carbon and aspects of Martian carbon cycling. Carbon isotopic values (δ C ) of the methane released during pyrolysis of 24 powder samples at Gale crater, Mars, show a high degree of variation (-137 ± 8‰ to +22 ± 10‰) when measured by the tunable laser spectrometer portion of the Sample Analysis at Mars instrument suite during evolved gas analysis. Included in these data are 10 measured δ C values less than -70‰ found for six different sampling locations, all potentially associated with a possible paleosurface. There are multiple plausible explanations for the anomalously depleted C observed in evolved methane, but no single explanation can be accepted without further research. Three possible explanations are the photolysis of biological methane released from the subsurface, photoreduction of atmospheric CO , and deposition of cosmic dust during passage through a galactic molecular cloud. All three of these scenarios are unconventional, unlike processes common on Earth.
Bibliography:	ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 Author contributions: C.R.W. and P.R.M. designed research; C.H.H., G.M.W., C.R.W., G.J.F., H.B.F., and J.C.S. performed research; C.H.H., G.M.W., C.R.W., G.J.F., H.B.F., J.C.S., A.P., S.K.A., J.L.E., A.G., A.E.H., M.M., A.S., D.P.G., C.A.M., and P.R.M. analyzed data; C.H.H. performed laboratory pyrolysis experiments; C.H.H. and G.M.W. wrote the paper with contributions from all authors; C.R.W. wrote method description for the tunable laser spectrometer data analyses; G.M.W. calculated sulfur isotopes and analyzed EGA profiles; C.R.W. and G.J.F. developed and implemented methods for calculating the methane isotopic composition from MSL-TLS spectra; H.B.F. calculated BSW abundances from EGA profiles and calculated sulfur isotopes; A.P. contributed to the concept of interplanetary dust as an explanation of results; S.K.A. contributed to the concept of the photochemical explanations of the results; H.B.F., J.C.S., and J.L.E. contributed to numerous discussions of results and the development of explanations for the results; A.G. and A.E.H. contributed new reagents/analytic tools; and C.A.M. and P.R.M. led discussions for instrument team for this work. Edited by Mark Thiemens, Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA; received August 26, 2021; accepted December 1, 2021
ISSN:	0027-8424 1091-6490
DOI:	10.1073/pnas.2115651119