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Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics

Abstract

The detection of methane on Mars1,2,3 has revived the possibility of past or extant life on this planet, despite the fact that an abiogenic origin is thought to be equally plausible4. An intriguing aspect of the recent observations of methane on Mars is that methane concentrations appear to be locally enhanced and change with the seasons3. However, methane has a photochemical lifetime of several centuries, and is therefore expected to have a spatially uniform distribution on the planet5. Here we use a global climate model of Mars with coupled chemistry6,7,8 to examine the implications of the recently observed variations of Martian methane for our understanding of the chemistry of methane. We find that photochemistry as currently understood does not produce measurable variations in methane concentrations, even in the case of a current, local and episodic methane release. In contrast, we find that the condensation–sublimation cycle of Mars’ carbon dioxide atmosphere can generate large-scale methane variations differing from those observed. In order to reproduce local methane enhancements similar to those recently reported3, we show that an atmospheric lifetime of less than 200 days is necessary, even if a local source of methane is only active around the time of the observation itself. This implies an unidentified methane loss process that is 600 times faster than predicted by standard photochemistry. The existence of such a fast loss in the Martian atmosphere is difficult to reconcile with the observed distribution of other trace gas species. In the case of a destruction mechanism only active at the surface of Mars, destruction of methane must occur with an even shorter timescale of the order of 1 hour to explain the observations. If recent observations of spatial and temporal variations of methane are confirmed, this would suggest an extraordinarily harsh environment for the survival of organics on the planet.

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Figure 1: Column-averaged methane mixing ratio calculated by the global climate–chemical model.
Figure 2: Idealized tracer experiments.
Figure 3: Maximum enhancement created by a local source of tracer in a column-averaged tracer field, as a function of tracer lifetime at the surface of Mars.

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Acknowledgements

The LMD Martian global climate model has been developed with the support of CNRS, ESA and CNES. We thank R. M. Haberle and F. Montmessin for their contributions to an early phase of this work, as well as P.-Y. Meslin and R. Wordsworth for discussions.

Author Contributions F. L. and F. F. conceived the experiments and wrote the paper. F. L. performed the experiments.

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Correspondence to Franck Lefèvre.

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Lefèvre, F., Forget, F. Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics. Nature 460, 720–723 (2009). https://doi.org/10.1038/nature08228

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