FIG. 1.

Schematic illustration of potential methane origins and seepage on Mars. Biotic methane from present-day microbial activity may only occur in the subsurface, even at great depths (shown near surface only for graphical reasons), and the gas may use faults and fractured rocks for exhalation to the surface. Geologic methane, not involving living organisms, includes abiotic processes and generation from ancient organic material of biotic derivation, either in sedimentary or igneous rocks (see text). CO₂ and H₂ (methane precursors in FTT reactions) can derive from C-bearing rocks, atmosphere-rock interactions, magmatic fluids, serpentinization (olivine hydration), radiolysis, and silicate cataclasis. Methane can be generated also in organic-rich source rocks and may accumulate in permeable/porous reservoir rocks. Temporary reservoirs, like clathrates, may host any type of methane. Irrespective of origin, gas generally migrates along faults or permeable layers and exhales to the surface through focused macro-seeps or diffuse microseepage. Volcanoes, as on Earth, may be very minor methane emitters. While this image highlights that gas seepage stems mainly from gas reservoirs or accumulations, in some cases gas may migrate to the surface directly from source rocks, if there are preferential pathways (faults, fractures) for degassing. Spatial scales of the surface manifestations are described in the text.