Methane release and the carbon cycle on Mars

It has been suggested that the present release rate of methane to the Martian atmosphere could be the result of serpentinization in the deep subsurface, followed by the conversion of H2 to CH4 in a CO2-rich fluid. Making this assumption, we show that the cryosphere could act as a buffer storing, under the form of micron-size methane clathrate particles, the methane delivered from below by hydrothermal fluids and progressively releasing it to the atmosphere at the top. From an extrapolation of the present CH4 release rate back to the past, we calculate that up to several hundred millibars (∼200–2000 mbar) of CO2, resulting from the oxidation of the released CH4, in addition to the volcanic supply (∼400 mbar), should have accumulated in the atmosphere in the absence of a CO2 sink. We reassess the capability of escape to have removed CO2 from the atmosphere by C non-thermal escape and show that it is not significant. We suggest that atmospheric carbon is recycled to the crust through an active subsurface hydrological system, and precipitates as carbonates within the crust. During episodic periods of magmatic activity, these carbonates are decomposed to CO2 dissolved in running water, and CO2 can react with H2 formed by serpentinization to build CH4. CH4 is then buffered in the subsurface cryosphere, above the water table, and finally released to the atmosphere, before being recycled to the subsurface hydrological system, and converted back to carbonates. We propose a typical evolution curve of the CO2 pressure since the late Noachian based on our hypothesis. Contrary to the steady state carbon cycle at work on Earth, a progressive damping of the carbon cycle occurs on Mars due to the absence of plate tectonics and the progressive cooling of the planet.