Evidence for methane in Martian meteorites

Abstract

The putative occurrence of methane in the Martian atmosphere has had a major influence on the exploration of Mars, especially by the implication of active biology. The occurrence has not been borne out by measurements of atmosphere by the MSL rover Curiosity but, as on Earth, methane on Mars is most likely in the subsurface of the crust. Serpentinization of olivine-bearing rocks, to yield hydrogen that may further react with carbon-bearing species, has been widely invoked as a source of methane on Mars, but this possibility has not hitherto been tested. Here we show that some Martian meteorites, representing basic igneous rocks, liberate a methane-rich volatile component on crushing. The occurrence of methane in Martian rock samples adds strong weight to models whereby any life on Mars is/was likely to be resident in a subsurface habitat, where methane could be a source of energy and carbon for microbial activity.

Figure 1. Output from crush fast scan-mass spectrometry analyses of Martian meteorites.

Output shown from during CH₄ (a) and H₂ (b) release. Output is over 80 cycles (∼20–25 s) at m/z=15 and m/z=2, representing ionized CH₃+ and H₂+ during sample crushing. The ionized volatiles produce an instantaneous current in the detector proportional to their amount. The signals from Martian meteorites are shown above quartz blanks, in units of 10⁻¹⁰ amps above background. Hydrogen generates more current than an equal volume of methane because of a difference in sensitivity factor. Doublet peaks may represent gas release from successively crushed inclusions. Peaks are displaced laterally for clarity only. The meteorite methane peak signals are 2 to 3 orders of magnitude higher than the highest signal from the blank.

Figure 2. Comparison of gas compositions in Martian meteorites with those in terrestrial samples and carbonaceous chondrites.

The terrestrial samples are from basalts and terrestrial fluids. Each point corresponds to one burst of volatiles produced by an increment of crushing. Gases of interest are normalized to CO₂. All Martian meteorites plot within the data field for terrestrial basalts; both are more methane-rich than terrestrial fluids and carbonaceous chondrites. Errors are obtained from molar volumes of individual species±one weighted s.d. (see Methods). One Tagish Lake analysis has large error bar, but in most cases error bars are smaller than data symbols. Data sources: terrestrial serpentinites, and terrestrial basalts, both crushed in same manner as the meteorites.

Figure 3. Comparison of Martian meteorite data with mixing lines between Martian and terrestrial atmospheres.

Data are from ref. . Each point corresponds to one burst of volatiles produced by an increment of crushing. Gases of interest are normalized to CO₂. Ar/O₂ plot (a) implies a primary Martian signature with limited contamination from the terrestrial atmosphere. CH₄/O₂ plot (b) implies substantial enrichment in CH₄ by a non-atmospheric source, most likely in the Martian crust. The Ar/O₂ meteorite data lie close to the composition of terrestrial soil gas, but the CH₄ enrichment excludes soil gas as a major contaminant. Together the plots show that contamination from terrestrial atmosphere was minimal, and that the gas signature is crustal rather than atmospheric. Errors are obtained from molar volumes of individual species±one weighted s.d. (see Methods).