Methane on Mars and Habitability: Challenges and Responses

Recent measurements of methane (CH_4) by the Mars Science Laboratory (MSL) now confront us with robust data that demand interpretation. The baseline level of CH_4 (~1 ppbv, parts per billion by volume) requires a minimum production source of 1.7 × 10^7 mol year^(-1), while the pulses of CH_4 (~10 ppbv) require a source orders of magnitude larger (~5 × 10^9 mol year^(-1) if originating from a point source). What does this CH_4 represent in terms of interior geochemical processes, or is Martian CH_4 a biosignature? Discerning how CH_4 generation occurs on Mars may shed light on the potential habitability of Mars. There is no evidence of life on the surface of Mars today, but microbes might reside beneath the surface. In this case, the carbon flux represented by CH_4 might serve as a link between a putative subterranean biosphere on Mars and what we can measure above the surface. Alternatively, if there is no life, CH_4 records modern activity. We ask the fundamental question: how active is Mars, geochemically and/or biologically? In this report, we examine geological, geochemical, and biogeochemical processes related to our overarching question. The Martian atmosphere and surface is an overwhelmingly oxidizing environment, and life requires pairing of electron donors and electron acceptors, i.e., redox gradients, as an essential source of energy. Therefore, a fundamental and critical question regarding the possibility of life on Mars is, "Where can we find redox gradients as energy sources for life on Mars?" Hence, regardless of the pathway that generates CH_4 on Mars, the presence of CH_4, a reduced species in an oxidant-rich environment, suggests the possibility of redox gradients supporting life and habitability on Mars. Recent missions such as ExoMars Trace Gas Orbiter (TGO) may provide mapping of the global distribution of CH_4. To discriminate between abiotic and biotic sources of CH_4 on Mars, future studies should use a series of diagnostic geochemical analyses, preferably performed below the ground or at the ground/atmosphere interface, including measurements of CH_4 isotopes, methane/ethane ratios, H_2 gas concentration, and species such as acetic acid. Advances in the fields of Mars exploration and instrumentation will be driven, augmented, and supported by an improved understanding of atmospheric chemistry and dynamics, deep-subsurface biogeochemistry, astrobiology, planetary geology, and geophysics. Future Mars exploration programs will have to expand the integration of complementary areas of expertise to generate synergistic and innovative ideas to realize breakthroughs in advancing our understanding of the potential of life and habitable conditions having existed on Mars. In this spirit, we conducted a set of interdisciplinary workshops. From this series has emerged a vision of technological, theoretical, and methodological innovations to explore the Martian subsurface and to enhance spatial tracking of key volatiles, such as CH_4.