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Applicant -- The Chemical Composition of Cave Air and its Astrobiological Significance

Wednesday, February 28, 2018
Caves are considered high priority targets for astrobiological investigation because they are isothermal, provide shielding from cosmic rays, and preserve signs of biological activity. However, the composition of cave air itself is rarely thought of in an astrobiological context. Cave air flows at a much slower rate than the atmosphere, which may enhance the potential to detect biological signatures in subterranean outgassing. Methane (CH4) is a greenhouse gas in Earth’s atmosphere, as well as its most abundant reduced trace gas. Thus, understanding its behavior in the subsurface is not only important for understanding climatic processes, but also informative in the search for extraterrestrial life because microorganisms may exploit its thermodynamic potential as a source of energy. Cave air CH­4 concentrations from the U.S., New Zealand, and Vietnam were sampled to understand its behavior in the subsurface. The carbon and hydrogen stable isotopic compositions of CH4 were also measured to pinpoint individual sources or sinks in the subsurface. All but one of the studied caves showed subatmospheric CH4 concentrations in at least one location. CH4 concentrations in the caves ranged from < 0.1 to 5 ppmv. Only three caves showed elevated CH4 concentrations relative to the atmospheric background concentration of 1.8 ppmv. CH4 concentrations generally decreased with the distance from a cave entrance and were at or below the detection level of the instruments in the least ventilated rooms. δ13CCH4 and δ2HCH4 values in the caves ranged from −27.1 to −57.2 ‰ and from +2 to −195.6 ‰ respectively. The concentrations and stable isotopic compositions of CH4 in cave air suggest that microorganisms are responsible for the subatmospheric CH4 concentrations in caves. Additionally, two minor sources of CH4 from microbial production pathways were observed entering the caves. The results show that cave air has a distinct composition compared to the atmosphere and that it is related to the action of microorganisms. Preliminary research on other reduced gases in cave air is showing that their concentrations in cave air are also influenced by microorganisms. Cave air can exchange advectively with the atmosphere at its entrances. This type of bulk flow combined with the microbial alteration of the chemical composition of cave air may allow for the remote detection of biological activity on planets like Mars. 


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