Differentiating biotic from abiotic methane genesis in hydrothermally active planetary surfaces

Molecular hydrogen (H-2) is derived from the hydrothermal alteration of olivine-rich planetary crust. Abiotic and biotic processes consume H-2 to produce methane (CH4); however, the extent of either process is unknown. Here, we assess the temporal dependence and limit of abiotic CH4 related to the presence and formation of mineral catalysts during olivine hydrolysis (i.e., serpentinization) at 200 degrees C and 0.03 gigapascal. Results indicate that the rate of CH4 production increases to a maximum value related to magnetite catalyzation. By identifying the dynamics of CH4 production, we kinetically model how the H-2 to CH4 ratio may be used to assess the origin of CH4 in deep subsurface serpentinization systems on Earth and Mars. Based on our model and available field data, low H-2/CH4 ratios (less than approximately 40) indicate that life is likely present and active.