The Curiosity rover recently detected a background of 0.7 ppb and spikes of 7
ppb of methane on Mars. This in situ measurement reorients our understanding of
the Martian environment and its potential for life, as the current theories do
not entail any geological source or sink of methane that varies sub-annually.
In particular, the 10-fold elevation during the southern winter indicates
episodic sources of methane that are yet to be discovered. Here we suggest a
near-surface reservoir could explain this variability. Using the temperature
and humidity measurements from the rover, we find that perchlorate salts in the
regolith deliquesce to form liquid solutions, and deliquescence progresses to
deeper subsurface in the season of the methane spikes. We therefore formulate
the following three testable hypotheses. The first scenario is that the
regolith in Gale Crater adsorbs methane when dry and releases this methane to
the atmosphere upon deliquescence. The adsorption energy needs to be 36 kJ/mol
to explain the magnitude of the methane spikes, higher than existing laboratory
measurements. The second scenario is that microorganisms convert organic matter
in the soil to methane when they are in liquid solutions. This scenario does
not require regolith adsorption, but entails extant life on Mars. The third
scenario is that deep subsurface aquifers produce the bursts of methane.
Continued in situ measurements of methane and water, as well as laboratory
studies of adsorption and deliquescence, will test these hypotheses and inform
the existence of the near-surface reservoir and its exchange with the
atmosphere.Comment: Accepted for publication on Astrobiolog
