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Depleted carbon isotope compositions observed at Gale crater, Mars

House, Christopher H. and Wong, Gregory M. and Webster, Christopher R. and Flesch, Gregory J. and Franz, Heather B. and Stern, Jennifer C. and Pavlov, Alex and Atreya, Sushil K. and Eigenbrode, Jennifer L. and Gilbert, Alexis and Hofmann, Amy E. and Millan, Maëva and Steele, Andrew and Glavin, Daniel P. and Malespin, Charles A. and Mahaffy, Paul R. (2022) Depleted carbon isotope compositions observed at Gale crater, Mars. Proceedings of the National Academy of Sciences of the United States of America, 119 (4). Art. No. e2115651119. ISSN 0027-8424. doi:10.1073/pnas.2115651119.

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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 (δ¹³CVPDB) 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 δ¹³C 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 ¹³C 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 CO₂, and deposition of cosmic dust during passage through a galactic molecular cloud. All three of these scenarios are unconventional, unlike processes common on Earth.

Item Type:Article
Related URLs:
URLURL TypeDescription Information ItemNASA Planetary Data System
House, Christopher H.0000-0002-4926-4985
Wong, Gregory M.0000-0003-0136-6373
Franz, Heather B.0000-0001-9716-5597
Pavlov, Alex0000-0001-8771-1646
Atreya, Sushil K.0000-0002-1972-1815
Eigenbrode, Jennifer L.0000-0003-3089-1986
Hofmann, Amy E.0000-0001-6869-5118
Glavin, Daniel P.0000-0001-7779-7765
Mahaffy, Paul R.0000-0003-1896-1726
Additional Information:© 2022 the Author(s). Published by PNAS. This open access article is distributed under Creative Commons Attribution License 4.0 (CC BY). 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. We thank the MSL, Mastcam, and SAM teams for their dedicated work operating the mission, crafting and sequencing scripts that are sent to the Curiosity rover and the SAM instrument to make these measurements possible, and their support in data collection, analyses, and discussions. We also acknowledge the NASA Mars Exploration Program for providing support through the MSL Participating Scientist Program (C.H.H., J.L.E., M.M.), and the NASA Earth and Space Science Fellowship and NASA Space Grant Programs for participant support (G.M.W.). Additionally, the NASA Astrobiology Center for Isotopologue Research project facilitated the collaboration involving intramolecular isotopes (A.G.). We also thank Dr. Richard Becker for useful discussions, Dr. Todd Sowers and Zhidan Zhang for assistance with laboratory experiments, and two anonymous reviewers for their comments and excellent suggestions. Data Availability: All MSL SAM data are available at the Geosciences Node of NASA’s Planetary Data System ( All study data are included in the article and/or SI Appendix. 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. The authors declare no competing interest. This article is a PNAS Direct Submission. This article contains supporting information online at
Funding AgencyGrant Number
NASA Earth and Space Science FellowshipUNSPECIFIED
Subject Keywords:Gale crater; Mars; carbon isotopes; pyrolysis; methane
Issue or Number:4
Record Number:CaltechAUTHORS:20220119-460984800
Persistent URL:
Official Citation:Depleted carbon isotope compositions observed at Gale crater, Mars. Christopher H. House, Gregory M. Wong, Christopher R. Webster, Gregory J. Flesch, Heather B. Franz, Jennifer C. Stern, Alex Pavlov, Sushil K. Atreya, Jennifer L. Eigenbrode, Alexis Gilbert, Amy E. Hofmann, Maëva Millan, Andrew Steele, Daniel P. Glavin, Charles A. Malespin, Paul R. Mahaffy. Proceedings of the National Academy of Sciences Jan 2022, 119 (4) e2115651119; DOI: 10.1073/pnas.2115651119
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:112974
Deposited By: Tony Diaz
Deposited On:19 Jan 2022 20:07
Last Modified:25 Jul 2022 23:14

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