Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere

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Abstract

Almost a decade after methane was first reported in the atmosphere of Mars1,2 there is an intensive discussion about both the reliability of the observations3,4—particularly the suggested seasonal and latitudinal variations5,6—and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited1,6, a process on or below the planet’s surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes7. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible8. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates9, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios3,4. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.

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Figure 1: Methane mixing ratios during ultraviolet irradiation of Murchison meteorite samples in a photochemical reactor.
Figure 2: Methane emission rates from ultraviolet irradiation of Murchison meteorite samples as a function of temperature and at different pressures.
Figure 3: Stable carbon and hydrogen isotope composition of CH 4 from terrestrial sources and from carbonaceous chondrites.

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Acknowledgements

F.K. was supported by the ESF (EURYI award to F.K.) and DFG (KE 884/2-1). Measurements at Utrecht University were funded by the Dutch NWO (grant number 865.07.001). A.M. was supported by the NERC (NE/F020422/1) and a Royal Society Leverhulme Trust senior research fellowship. We thank M. Greule, F. Althoff, A. Jugold, C. Tubbesing and H. F. Schöler for technical support; H. van Weelden from the Utrecht Medical Center for measuring the spectral distribution of the Xe lamp and calibrating the Waldmann ultraviolet meter; C. Brenninkmeijer, J. Lelieveld and C. Cockell for advice; and C. Jäger for providing data on ultraviolet absorption by minerals.

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F.K., I.V., A.M. and T.R. planned the study. I.V., F.K. and M.F. carried out the experiments. F.K., I.V., A.M., U.O. and T.R. worked on the scientific interpretation and wrote the manuscript.

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Correspondence to Frank Keppler.

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The authors declare no competing financial interests.

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Keppler, F., Vigano, I., McLeod, A. et al. Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere. Nature 486, 93–96 (2012). https://doi.org/10.1038/nature11203

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