Christopher R. Webster and others report that by analogy with Earth, methane in the Martian atmosphere is a potential signature of ongoing or past biological activity. During the last decade, Earth-based telescopic observations reported “plumes” of methane of tens of parts-per-billion by volume (ppbv), and those from Mars orbit showed localized patches, prompting speculation of sources from sub-surface bacteria or non-biological sources. From in situ measurements made by the Tunable Laser Spectrometer (TLS) on Curiosity using a distinctive spectral pattern unique to methane, we here report no detection of atmospheric methane with a measured value of 0.18 ±0.67 ppbv corresponding to an upper limit of only 1.3 ppbv (95% confidence level) that reduces the probability of current methanogenic microbial activity on Mars, and limits the recent contribution from extraplanetary and geologic sources.
Methane is the most abundant hydrocarbon in our solar system, and is found in the atmospheres of several planets and satellites. On Earth, 90-95% of atmospheric methane is biologically-produced, either from extant or fossil sources, and is easy to identify and quantify with confidence using spectroscopic methods. For Mars, three possible origins have been proposed: geologic, biotic, and exogenous. Over the last decade, there have been several reports of methane detection from Earth and from Mars orbit. Observations with the Canada-France-Hawaii Telescope (CFHT) found a global average value of 10 ±3 ppbv. The Planetary Fourier Spectrometer (PFS) on the Mars Express (MEX) spacecraft found a global average abundance of 10 ±5 ppbv, later updated to 15 ppbv, with indications of discrete localized sources, and a summer time maximum of 45 ppbv in the north polar region. A search for methane from the Infrared Telescope Facility (IRTF) and the Keck-2 telescope reported methane release in plumes from discrete sources in Terra Sabae, Nili Fossae and Syrtis Major, with the largest plume containing 19,000 tons of CH4 in March 2003; seasonal changes with a summer time maximum of ~45 ppbv near the equator were seen.
Methane abundances later retrieved from a second instrument in Mars orbit, the Thermal Emission Spectrometer (TES) of Mars Global Surveyor (MGS), reported methane abundances as intermittently present (1999-2003), ranging from 5 to 60 ppbv in locations where favorable geological conditions such as residual geothermal activity. The authors' reported upper limit of 1.3 ppbv is significantly lower than the methane abundances reported from Mars remote sensing spacecraft observations and those from Earth telescopic observations, including both the earlier high values of typically tens of ppbv and the more recently reported upper limits of 7-8 ppbv.
Although TLS samples only the very lowest part (~1 m) of the Mars atmosphere compared to the other observations that are vertical column-integrated results, the atmospheric scale height (~10 km) and mixing time (~few months) suggests that the authors' measured upper limit is representative of the global mean background level. With an expected photochemical lifetime of methane in the Martian atmosphere of hundreds of years, there currently remains no accepted explanation for the existence and distribution of the reported plumes, nor of the apparent disappearance of methane over the last few years.The authors' result sets an upper limit that is ~6 times lower than other recent measurements and greatly reduces the probability of significant methanogenic microbial activity on Mars and recent methane production by serpentinization or from exogenous sources including meteoritic, interplanetary dust and cometary infall.
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