Methane on Mars and Habitability: Challenges and Responses

Abstract

Recent measurements of methane (CH4) by the Mars Science Laboratory (MSL) now confront us with robust data that demand interpretation. Thus far, the MSL data have revealed a baseline level of CH4 (∼0.4 parts per billion by volume [ppbv]), with seasonal variations, as well as greatly enhanced spikes of CH4 with peak abundances of ∼7 ppbv. What do these CH4 revelations with drastically different abundances and temporal signatures represent in terms of interior geochemical processes, or is martian CH4 a biosignature? Discerning how CH4 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 CH4 would serve as a link between a putative subterranean biosphere on Mars and what we can measure above the surface. Alternatively, CH4 records modern geochemical activity. Here we ask the fundamental question: how active is Mars, geochemically and/or biologically? In this article, we examine geological, geochemical, and biogeochemical processes related to our overarching question. The martian atmosphere and surface are an overwhelmingly oxidizing environment, and life requires pairing of electron donors and electron acceptors, that is, 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 CH4 on Mars, the presence of CH4, 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 may provide mapping of the global distribution of CH4. To discriminate between abiotic and biotic sources of CH4 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 CH4 isotopes, methane/ethane ratios, H2 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 CH4.

Advective Fluxes in the Martian Regolith as a Mechanism Driving Methane and Other Trace Gas Emissions to the Atmosphere

“…This mechanism could regulate gas emissions into the atmosphere as a function of winds and meteorological conditions and thus reconcile the methane abundance variation detected by MSL and several emission mechanisms proposed to date from subsurface reservoirs (Yung et al, 2018), which have difficulties to match the observed variation in methane abundance (Webster et al, 2018;Yung et al, 2018); that is, the observed variation could be mostly driven from local meteorology around the hypothetical reservoir(s), instead of from the mechanism emitting or producing gas in origin. Atmospheric chemistry and dynamics could also play a significant role in the resulting gas abundance in the atmosphere.…”

“…Methane production by methanogenic microorganisms has been proposed (Atreya et al, 2007;Krasnopolsky et al, 2004;Mumma et al, 2009), living in the subsurface where ionizing radiation is less damaging for organic molecules (e.g., Blanco-López et al, 2018;Dartnell et al, 2007). However, other abiotic sources are perhaps considered more plausible (Webster et al, 2015(Webster et al, , 2018Yung et al, 2018). These include serpentinization of olivine and pyroxene in the crust (Oze & Sharma, 2005) (a process that would require liquid water), and the destabilization of clathrates containing trapped abiotic or biogenic methane formed in the past in quite a different climatological environment (Chassefière, 2009;Chastain & Chevrier, 2007;Prieto-Ballesteros et al, 2006).…”

Serpentinite and the search for life beyond Earth

“…Although present deep groundwater flow has not been detected on Mars, methane detections reported by some investigations [61][62][63][64] could be the result of ongoing serpentinization reactions in the subsurface [65][66][67]. However, there remain significant questions about the detections, sources of methane, and persistence in the atmosphere [68][69][70], especially in light of the recent non-detection of methane by the ExoMars Trace Gas Orbiter [71]. NASA's InSight mission [72] may soon offer a more complete understanding of the crustal structure variations, density and heat flow of Mars to constrain where serpentinization may occur and where serpentinites could be located.…”

Future Opportunities in Scientific Ocean Drilling: Illuminating Planetary Habitability

“…To maintain essential life functions, not only geophysical constraints, such as temperature, but also the supply of water and bio-available nutrients and energy sources are crucial (Hoehler and Jørgensen, 2013;Lever et al, 2015;Jørgensen and Marshall, 2016;LaRowe et al, 2017;Ijiri et al, 2018;Tanikawa et al, 2018;Parkes et al, 2019). It may follow that this geospherebiosphere interaction must be the essential driving force not only for Earth's deep biosphere but also for any possible ecosystems on Mars and other celestial bodies (Dzaugis et al, 2018: Yung et al, 2018Stamenković et al, 2019).…”