Mars' missing methane

22 September 2013
Posted by Dominic Ford.

This image was taken by Mastcam: Left (MAST_LEFT) onboard NASAThe latest of NASA's rovers to explore the red planet Mars has suggested its less likely than previously thought that microbial life may be lurking beneath the planet's surface.

The Curiosity rover touched down on the Martian surface in August 2012, and since then it has been taking regular samples of the air around it, monitoring its composition. Although we've known for over a century that carbon dioxide is the principal constituent of the Martian atmosphere, there's been much debate about what other gases might be present in trace quantities.

Over the past decade, planetary scientists have been especially interested by evidence from ground-based telescopes and orbiting spacecraft, suggesting that plumes of methane could be seen rising from certain geographic structures on Mars's surface.

This was a surprise, as more than 90% of the methane in the Earth's atmosphere is produced by biological rather than geological mechanisms. For scientists hoping that the depths of the Martian soil might provide an environment where microbial life such as bacteria could thrive, this was very exciting news.

However, the result has always been somewhat in doubt. Methane molecules survive for hundreds of years in the Earth's atmosphere, and models of Martian weather suggest that plumes of the gas would spread around its whole globe within a few years. If there were regions of the Martian surface spewing out plumes of methane, the whole planet's atmosphere would be expected to be rich with the accumulated emissions of the gas over hundreds of years.

Writing in the journal Science this week, Christopher Webster from the Jet Propulsion Laboratory and his colleagues on Curiosity's Science Team report that the Curiosity rover has found no evidence for methane in the air that it has sampled over the past year.

These measurements are almost certainly more trustworthy than the previous claimed detections of methane, since Curiosity is actually able to sample Martian air directly, rather than looking at it from afar. The questions remains, though, as to what the orbiting spacecraft were seeing, if it wasn't methane gas.

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