Sampling methane in hydrothermal minerals on Earth and Mars Sean McMahon 1 , John Parnell 1 and Nigel J. F. Blamey 2 1 School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK e-mail: sean.mcmahon@abdn.ac.uk 2 Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, USA Abstract: The source of Martian atmospheric methane is unknown. On Earth, hydrothermal mineral deposits contain ancient methane together with a host of chemical and geological lines of evidence for the mechanism of gas production. Such deposits are therefore potentially attractive sampling sites on Mars. In order to evaluate this potential, hydrothermal calcite veins were sampled across the Caithness region of Scotland and analysed for methane by an incremental-crushing mass spectrometry technique that may be adaptable to Mars rovers. Methane was detected in all samples. Variations in the quantity of methane released were found to relate directly to the geological history of the localities. Calcite particle size was found to affect measurements in a systematic and informative way. Oxidative weathering had no discernable effect on methane recoverability. These results suggest that the technique is sensitive and informative enough to deserve consideration for missions to Mars. Received 1 November 2011, accepted 16 January 2012, first published online 16 February 2012 Key words: Mars, methane, hydrothermal, fluid inclusion, mass spectrometry, volatile. Introduction Ground-based telescopes detected methane concentrations of the order of parts per billion by volume (ppbv) in the atmosphere of Mars in 2003 (Mumma et al. 2009), a result confirmed by the Mars Express orbiter in 2004 (Formisano et al. 2004). The methane is widely but heterogeneously distributed, with maxima of about 50 ppbv over the Elysium, Tharsis and Arabia Terrae regions in the northern hemisphere (Fonti & Marzo 2010). Methane concentrations peak during Northern Hemisphere Summer and decline thereafter, with an inferred atmospheric lifetime of only *200 Martian days (Lefèvre & Forget 2009). A major science objective of forthcoming ESA and NASA missions is to identify the processes that produce, release and destroy methane on Mars. Possible near-surface release mechanisms include dis- sociation from clathrate hydrates and desorption from mineral grains (e.g. clays and zeolites) in the regolith. Although near- surface microbial activity is the most prolific source of terrestrial atmospheric methane, its viability in the Martian near surface is limited by the presence of strong oxidants and the absence of liquid water, among other factors (Clark 1998). Potential deep subsurface methane release mechanisms include Fischer–Tropsch-type organic synthesis, thermal degradation of higher hydrocarbons, carbonate reduction, serpentinization reactions and microbial methanogenesis, which may form part of a deep biosphere (Fisk & Giovannoni 1999). All of these processes are associated with hydrothermal activity on Earth (Jones et al. 1983; Horita & Berndt 1999; McCollom et al. 1999; Lowell & Rona 2002; Svensen et al. 2004). Hydrothermal systems are thought to have mediated some of the largest fluxes of methane to the atmosphere in Earth’s history, triggering global warming across the Precambrian–Cambrian transition (Chen et al. 2009) and in the earliest Eocene (Svensen et al. 2004). Sampling of methane on Mars is desirable for isotopic and other studies that may help to determine its origin. Although atmospheric concentrations are prohibitively low for direct sampling, it may be possible to extract methane from geo- logical materials associated with extant or extinct hydrother- mal activity. Such materials include direct precipitates and mineral alteration products generated by hydrothermal fluids. On Earth, methane and other fluids are trapped both within crystals (as ‘fluid inclusions’) and at crystal grain boundaries during the crystallization (or recrystallization) of calcite, quartz and other minerals along fractures and at discharge sites. Given the likelihood that Martian methane is (or was originally) produced in or channelled through hydrothermal systems, hydrothermal mineral deposits represent attractive candidates for methane sampling sites on Mars. Such sites may also yield chemical and geological evidence of the mechanism of methane formation and associated biological activity; microfossils, microbial fabrics and microbially mediated mineral precipitates have all been recovered from hydrother- mal systems on Earth (Parnell et al. 2010). Objectives We investigated the potential for sampling and detecting methane in hydrothermally precipitated calcite veins using an incremental-crushing mass spectrometry technique previously found to detect methane in serpentinites (Parnell et al. 2010), International Journal of Astrobiology 11 (3): 163–167 (2012) doi:10.1017/S1473550412000067 © Cambridge University Press 2012 163