Report

Low Upper Limit to Methane Abundance on Mars

Science  18 Oct 2013:
Vol. 342, Issue 6156, pp. 355-357
DOI: 10.1126/science.1242902

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No Methane to Be Found

On Earth, atmospheric methane is mostly produced biologically. Atmospheric methane has also been detected on Mars, but these reports have been controversial. Based on data from the Sample Analysis at Mars instrument suite on the Curiosity rover, which arrived at the surface of Mars in August 2012, Webster et al. (p. 355, published online 19 September) report no methane, with an upper limit of only 1.3 parts per billion by volume, about 6 times lower than previous measurements.

Abstract

By analogy with Earth, methane in the Martian atmosphere is a potential signature of ongoing or past biological activity. During the past 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 subsurface bacteria or nonbiological sources. From in situ measurements made with the Tunable Laser Spectrometer (TLS) on Curiosity using a distinctive spectral pattern specific to methane, we 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), which reduces the probability of current methanogenic microbial activity on Mars and limits the recent contribution from extraplanetary and geologic sources.

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