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.

Keywords: CH4; Mars; Mars instrumentation; Subsurface redox conditions.

FIG. 1.

Summary of TLS measurements of CH₄, both (a) including and (b) excluding the high spike data from year 1 (ca. January 2014). Data from Webster et al. (2015, 2018). TLS, Tunable Laser Spectrometer.

FIG. 2.

(a) Mixing ratios of minor species in the atmosphere of Mars (Hu et al., 2012). (b) Simulation of CH₄ with a source of 75,000 t/year at the Martz crater at L_s = 80° and a lifetime of 1 month. Based on a model by Lefèvre and Forget (2009).

FIG. 3.

Global chemical trends of basalt degassing at pressures ranging from 1000 to 10⁻⁶ bar, encompassing subaerial venting conditions expected to prevail on telluric bodies. The total volatile abundances in the basalt are 600 ppm CO₂, 1000 ppm H₂O, 1000 ppm S. Taken from Gaillard and Scaillet (2014).

FIG. 4.

Free-energy profile of a hydrothermal pathway (in purple) to methane (Seewald et al., 2006) is contrasted with the reduction profiles of the acetogenic bacteria (triangles) and methanogenic archaea (squares); both biological mechanisms use the acetyl coenzyme-A pathway. We can think of the geochemical pathway as a chemical siphon while the much more rapid biochemical pathways are driven by chemiosmosis over the intermediates formate and formaldehyde (or the formyl group). Adapted from Maden (2000).

FIG. 5.

(Left) Column-integrated, zonal mean daytime water abundance for present-day Mars, modeled by the LMD MGCM. (Right) Column-integrated, zonal mean daytime water abundance from the MGS-TES instrument for MY 26. From Navarro et al. (2014b). LMD, Laboratoire de Météorologie Dynamique; MGCM, Mars general circulation model; MGS, Mars Global Surveyor; TES, Thermal Emission Spectrometer.

FIG. 6.

(Left): Limb radiances at 463 cm⁻¹ measured by MCS (top) and dust mass mixing ratio retrieved from MCS measurements (bottom), exhibiting a detached dust layer at 50–60 km altitude over Tharsis (Heavens et al., 2015). The density-scaled optical depth (d_zτ) is related to the optical depth at 0.67 μm (dτ) by d_zτ = −dτ/ρ, where ρ is atmospheric density in units of kg/m³. (Right): Mesoscale model simulation showing density-scaled dust opacity in a so-called rocket dust storm, in which dust can be lifted to altitudes 30–40 km within a few hours (Spiga et al., 2013). MCS, Mars Climate Sounder.