Low Upper Limit to Methane Abundance on Mars

Abstract

International audienceBy 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

Mars methane detection and variability at Gale crater

“…This supplementary material repeats some of that published earlier (33) that is here updated and extended to include discussion of the enrichment experiments, spectral difference plots, correlation results of the TLS methane measurements with a variety of observed quantities like relative humidity, water abundance, ground and surface air temperatures, etc., and finally to present arguments for ruling out terrestrial contamination.…”

“…Using the current data, the published value of 0.18 ± 0.67 ppbv and the upper limit of 1.3 ppbv in Webster et al (33) should be revised to 0.88 ± 0.81 ppbv and 2.2 ppbv, respectively. Although the revised mean for this 6-sol group is 0.88 ppbv, at 95% confidence level the upper limit is 0.88 +1.645*SEM = 2.2 ppbv, because in this case a one-sided Student distribution needs to be considered for upper limits.…”

“…Changes to data processing since the first Science paper (33) results: All our data have now been reprocessed with the same new algorithm that reduces any susceptibility to variations in the laser plate temperature by automatically rejecting the few 2-minute points that resulted from a laser plate temperature changing by more than 0.1 deg/minute. The need for this quality control is because the spectral lines will blur somewhat during the 2-minute average by an amount dependent on the laser plate temperature; the faster the laser plate temperature changes during the 2-minute collection, the smaller the methane spectral lines will be in depth.…”

NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance

“…Especially the data measured in (Mumma et al 2009) indicate that the lifetime should be much shorter (Holmes et al 2015), perhaps as short as 200 days. The recent observations by the Mars Science Laboratory (MSL) at Gale Crater indicate a background level of 0.2-0.8 ppb (parts per billion), showing a seasonal pattern, and localized pulses of CH 4 reaching 10 ppb (Webster et al 2013(Webster et al , 2015. These observations suggest that at least two different kinds of emission processes produce the background and the pulses of methane, either through geology or (bio)geochemistry related processes, or combined.…”

UV production of methane from surface and sedimenting IDPs on Mars in light of REMS data and with insights for TGO

“…Following initial null results (Webster et al, 2013) that limited methane values below 1 ppbv, SAM-TLS found evidence for large spikes of methane up to ~7 ppbv which persisted over relatively short timescales of tens of sols and a background concentration of methane of < 0.7 ppbv (Webster et al, 2015). This value was more than 3 times lower than predicted by the UV-CH 4 model and is less than can be explained by the discrepancy between pre-flight modelling of UV flux (Moores et al, 2007) and the observed UV flux (Smith et al, 2016).…”