Non-Detection of Methane in the Mars Atmosphere by the Curiosity Rover

By analogy with Earth, methane in the atmosphere of Mars is a potential signature of ongoing or past biological activity on the planet. During the last decade, Earth-based telescopic and Mars orbit remote sensing instruments have reported significant abundances of methane in the Martian atmosphere ranging from several to tens of parts-per-billion by volume (ppbv). Observations from Earth showed plumes of methane with variations on timescales much faster than expected and inconsistent with localized patches seen from orbit, prompting speculation of sources from sub-surface methanogen bacteria, geological water-rock reactions or infall from comets, micro-meteorites or interplanetary dust. From measurements on NASAs Curiosity Rover that landed near Gale Crater on 5th August 2012, we here report no definitive detection of methane in the near-surface Martian atmosphere. Our in situ measurements were made using the Tunable Laser Spectrometer (TLS) in the Sample Analysis at Mars (SAM) instrument suite6 that made three separate searches on Martian sols 79, 81 and 106 after landing. The measured mean value of 0.39 plus or minus 1.4 ppbv corresponds to an upper limit for methane abundance of 2.7 ppbv at the 95 confidence level. This result is in disagreement with both the remote sensing spacecraft observations taken at lower sensitivity and the telescopic observations that relied on subtraction of a very large contribution from terrestrial methane in the intervening observation path. Since the expected lifetime of methane in the Martian atmosphere is hundreds of years, our results question earlier observations and set a low upper limit on the present day abundance, reducing the probability of significant current methanogenic microbial activity on Mars.