Constraining methane release due to serpentinization by the observed D/H ratio on Mars

It has been suggested that Mars' atmospheric CH₄ could be produced by crustal hydrothermal systems. The two most plausible mechanisms proposed so far, not exclusive from each other, are homogeneous formation by fluid–rock interaction during magmatic events and serpentinization of ultramafic rocks. The first goal of the present paper is to provide an upper limit on the release rate of serpentinization-derived CH₄. Due to the release of numerous H₂ molecules together with one CH₄ molecule, followed by thermal escape of all released H atoms to space and subsequent H isotopic fractionation, even a relatively modest serpentinization-derived CH₄ release acting over geological time scales may result in a significant enrichment of D wrt H in Mars' cryo-hydrosphere, including atmosphere, polar caps and subsurface reservoirs. By assuming that the CH₄ release rate has been proportional to the volcanic extrusion rate during the last 4billion years, we calculate the present D/H ratio resulting from the crustal oxidation due to serpentinization, including the additional effect of sulfur oxidation. We show that this rate doesn't exceed 20% (within a factor of 2) of the estimated present value of the CH₄ release rate. If not, the present D/H ratio on Mars would be larger than observed (~5 SMOW). This result suggests that, either the production of CH₄ is sporadic with a present release rate larger than the average rate, or there are other significant sources of CH₄ like homogeneous formation from mantle carbon degassing or bacterial activity. Second, assuming further that most of the H isotopic fractionation observed today is due to serpentinization, we show that a ~400m thick global equivalent layer of water may have been stored in serpentine since the late Noachian. This result doesn't depend on the chemical form of the released hydrogen (H₂ or CH₄). Such a quantity is generally considered as the amount required for explaining the formation of valley networks on Mars. Serpentinization therefore appears as a potentially efficient sink of water on Mars, much more efficient than O escape' for having removed large amounts of water from the hydrosphere.