13th July 2011.
In 2004 three research teams reported finding the spectral signature of methane in the atmosphere of Mars. Their data was collected using ground-based telescopes and an instrument flying onboard the Mars Express spacecraft called the Planetary Fourier Spectrometer (PFS). The PFS scientists also reported seasonal and even daily variations in methane abundance, with peaks spotted in late spring in the northern hemisphere. In 2009 this variation appeared to be confirmed by further analysis and observation by of one of the ground-based telescope teams.
The methane detections were very low level, only a few tens of parts per billion by volume (ppbv), but even so the results were cause for interest. The lifetime of methane in the martian atmosphere is just a few hundred years; if methane did indeed persist, there had to be a current – or very recent – source of the gas. Given that there is no atmospheric source, and based on our understanding of the martian environment, it would most likely be generated by one of two processes: a hydrological mechanism called serpentinization, or the metabolism of (hypothetical) methanogenic microbial life in the subsurface.
(Release of the gas by clathrate hydrates has also been suggested; however these crystalline solids are not a source in and of themselves, but rather a mechanism of sequestration and release. If clathrates were shown to be linked to the episodic release of methane, the point of genesis would remain to be found.)
One of the most recent advances in the debate was made in a paper which appeared in late 2010. It called into question the ground-based observations, citing confusion with spectral signatures found in Earth’s atmosphere which, the authors suggested, were recorded as the telescopes looked towards Mars.
Until these concerns are addressed, that leaves the PFS data. Terrestrial contamination is not an issue for measurements taken from martian orbit, and so the available observations appear to be consistent with a trace amount of methane in the atmosphere of Mars, somewhere in the order of 10–15 ppbv. This is very close to the detection limit of the PFS instrument, and so the scientific community is being understandably cautious.
The hypothesis that methane is currently being produced by life is understandably an enticing one. Life here on Earth began in an anoxic environment, and current thinking is that methanogenic microbes were amongst the first to evolve around 3.7 billion years ago. If on Earth, then why not on Mars?
There are two missions planned which should be able to answer the question one way or the other: the Mars Science Laboratory, recently rechristened Curiosity, due for launch later on this year; and following that in 2016 will be the ExoMars Trace Gas Orbiter, which is designed to, amongst other things, answer the methane question once and for all.
Formisano, V., S. Atreya, et al. (2004). “Detection of methane in the atmosphere of Mars.” Science 306(5702): 1758.
Zahnle, K., R. S. Freedman, et al. (2010). “Is there methane on Mars?” Icarus 212(2): 493-503.