Methane as a biomarker in the search for extraterrestrial life: Lessons learned from Mars analog hypersaline environments
Methane released from discrete regions on Mars, together with previous reports of methane determined with ground-based telescopes, has revived the possibility of past or even extant life near the surface on Mars, since 90% of the methane on Earth has a biological origin. This intriguing possibility is supported by the abundant evidence of large bodies of liquid water, and therefore of conditions conducive to the origin of life, early in the planet's history. The detection and analysis of methane is at the core of NASA’s strategies to search for life in the solar system, and on extrasolar planets. Because methane is also produced abiotically, it is important to generate criteria to unambiguously assess biogenicity. The stable carbon and hydrogen isotopic signature of methane, as well as its ratio to other low molecular weight hydrocarbons (the methane/(ethane + propane) ratio: C1/(C2 + C3)), has been suggested to be diagnostic for biogenic methane. We report measurements of the concentrations and stable isotopic signature of methane from hypersaline environments. We focus on hypersaline environments because spectrometers orbiting Mars have detected widespread chloride bearing deposits resembling salt flats. Other evaporitic minerals, e.g., sulfates, are also abundant in several regions, including those studied by the Mars Exploration Rovers. The presence of evaporitic minerals, together with the known evolution of the Martian climate, from warmer and wetter to cold and hyper-arid, suggest that evaporitic and hypersaline environments were common in the past. Hypersaline environments examined to date include salt ponds located in Baja California, the San Francisco Bay, and the Atacama Desert. Methane was found in gas produced both in the sediments, and in gypsum- and halite-hosted (endolithic) microbial communities. Maximum methane concentrations were as high as 40% by volume. The methane carbon isotopic (δ13C) composition showed a wide range of values, from about -60 ‰ to -30 ‰, while the hydrogen isotopic composition (δ2H) ranged from about -350 to -300‰. These isotopic values are outside the range generally considered to be biogenic, however incubations of the sediments and salt crusts revealed that the methane is indeed produced there. The highest rate of methane production was 20 nmol/g/d, in a gypsum crust with endolithic microbial communities. Currently we are studying the mechanisms that control the isotopic signatures of methane in these environments. These studies are of special relevance given the projected analysis of Mars atmospheric methane by the Mars Science Laboratory in 2012, and by the ExoMars Trace Gas Orbiter in 2017.