Testing for Methane on Mars

And the three leading theories are…

The Curiosity rover landed on Mars four years ago next week, and is still investigating methane concentrations at Gale Crater.

Based on new research presented by Renyu Hu and colleagues from the California Institute of Technology, we might be getting closer to solving the mystery of methane spikes on Mars. The researchers propose three hypotheses to explain observed methane peaks of seven parts per billion measured by the Curiosity rover in Gale Crater, which is 10 times higher than background values. The neat thing is that the hypotheses are testable.

The first hypothesis suggests that the soil in Gale Crater adsorbs methane when dry and releases it to the atmosphere when the relative humidity in the Martian soil is high enough for perchlorate salts to attract water vapor from the atmosphere and dissolve in that water. The second hypothesis—the most exciting one—suggests that microbal life on Mars is the cause, and that microorganisms convert organic matter in the soil to methane when the microbes are in liquid solutions. The third hypothesis is that the bursts of methane are produced by deep subsurface aquifers.

Fortunately, the Curiosity rover should be able to find out which explanation is the most likely one. If either of the first two hypotheses are right, the methane variability is seasonal and should repeat every year. The methane abundance should peak in early winter if it’s due to inorganic adsorption; if it’s biological in origin, it should peak in late winter. And sporadic spikes would favor the third hypothesis. Of course, there could be other explanations not considered by the authors.

They also caution, justifiably, that it might be hard for Curiosity to validate any of the hypotheses because of changing wind patterns, which would move the methane around on the surface and make it more difficult to pinpoint the source. But additional insights could come from measured water vapor concentrations in the Martian soil, or the detection of any degradation products of methane and its isotopic compositions. For those answers we may have to wait for the ExoMars rover, scheduled to launch in 2020.

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