Background levels of methane in Mars’ atmosphere show strong seasonal variations

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Science  08 Jun 2018:
Vol. 360, Issue 6393, pp. 1093-1096
DOI: 10.1126/science.aaq0131

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  • Jumping to conclusions

    I worry that the key claim of this article ("The background levels of methane ... exhibit a strong, repeatable seasonal variation.") is in fact only weakly supported (< a 1.5 "sigma" claim) by the available evidence from a statistical point of view. As far as I can see there is no statistical modelling conducted herein to test the stated hypothesis against alternatives, nor a clear quantification of what specifically is considered "strong" and "seasonal" from a mathematical point of view. One way to quantify the shape of the signal perhaps seen in this dataset would be to examine the Bayesian posterior in the amplitude of a sinusoidal annual trend. Constructing this posterior myself with a regular Bayesian analysis suggests a 15% subjective probability that the amplitude is less than 0.1 ppbv, although even to choose that threshold as the boundary of "strong" variation might be too (generously) low. Bayesian bagging suggests that such a sine model is not a terrible misspecification and assigns an even higher posterior probability of 20% to the possibility that the amplitude is less than 0.1 ppbv.
    For this reason one would hope that once more data becomes available it can be compared against the trend shown here and the conclusions updated through the Science publication platform if negative.

    Competing Interests: None declared.
  • Variation of atmospheric methane concentration on Mars
    • Svatopluk Civis, researcher, J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejškova 3, CZ18223 Prague 8, Czech Republic.
    • Other Contributors:
      • Antonin Knizek, PhD student, J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejškova 3, CZ18223 Prague 8, Czech Republic.

    In the issue of Science published on 8 June 2018, a team of scientists from NASA responsible for the Curiosity rover management published an article titled: „Background levels of methane in Mars‘ atmosphere show strong seasonal variation’(1). The article presents a detailed analysis of the past 5 years worth of measurement by Curiosity and concludes that strong seasonal fluctuation of methane concentration exists on Mars. In the Supplementary Information, the authors compare the results in terms of temperature, UV irradiation, atmospheric pressure throughout the Martian year.
    The authors state that: ’Existing models including atmospheric transport and circulation are unable to reproduce the reported high concentrations of methane and its spatial and temporal variability, even when including possible clathrate release, surface/regolith adsorption/desoption, seasonally variable production from UV breakdown of surface organics, or proposed mechanisms of rapid loss.’ The authors neglected to mention a viable abiotic explanation of the seasonal methane emission that we proposed in (2). Encouragingly, the predictions we made in that paper happen to fit very well to the new data. In our scenario, methane is a product of a photocatalytic reduction of CO2, the main constituent of the Martian atmosphere. We compared the production rates of methane to other conditions on Mars, namely water atmospheric content, atmospheric pressure and UV irradiation.
    Back then, of course,...

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    Competing Interests: None declared.

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