The case of the missing methane
The layperson doesn’t often think of scientific experiments as being quite the sort of nail-biting thriller as, say, a football match. However, a recent study conducted by the Mars rover has all the makings of mystery and suspense. This study was originally reported in the journal Science (C.R. Webster et al., Sep 2013), and covered in the media by BBC in an article titled “Curiosity rover’s methane result challenges life theory”.
Much of the excitement regarding space exploration derives from the possibility that we will find new things in other planets, in particular the possibility that we will find life elsewhere. Mars has long been a candidate, partly based on tantalizing evidence of methane (CH4) and speculation about how this could have come about due to biological activity.
How do we measure methane on Mars?
During the past decade, many earth based studies, typically telescopic observations, have reported “plumes” of methane of tens of parts-per-billion-by-volume (ppbv). Such studies have included, e.g., the Canada-France-Hawaii telescope (which reported 10 ±3 ppbv), the Infrared telescope facility and the Keck-2 telescope , which reported summertime seasonal spikes as high as 45 ppbv, and even space based studies such as the Planetary Fourier Spectrometer on the Mars Express spacecraft which reported 10 ±5 ppbv. These findings, which are encouraging to spectators seeking a new discovery, are tempered by the growing realization that the dynamics of methane in the Martian atmosphere are not entirely trivial, the half life of methane appearing to be quite short and contradicting some of the estimates obtained from space-based studies. All of this made a robotic expedition to the surface necessary.
The robotic mission carried a sophisticated cargo of scientific instrumentation., including the Tunable Laser Spectrometer (TLS) of the Sample Analysis at Mars (SAM) instrument suite. This is an especially sensitive instrument, with a spectral resolution (0.0002 cm-1) that is far superior to the ground-based and orbiting spectrometers previously used. The experiment involves 1-2 hour runs involving laser scans running every second to probe the methane spectral region. The data is analysed by looking at differences between full and empty cell results, to focus attention on only the relevant feature of the martian air.
What did Curiosity find?
To date, on land, there has been no detection of methane. The measurements after appropriate statistical treatment suggest an upper limit of 1.3 ppbv, just over an eighth of the previous estimates. Indeed, estimates that took into account effects such as plume dispersion still expected to find over 6 ppbv. Of course, this may not be the entire story. Measurements such as these by the rover are but one peek into a very complex planetary scale system. Many further effects are possible that have resulted in lowered methane levels, including rapid removal by oxidants and destruction by electric fields associated with storms. Yet, based on current evidence, one must note that this lowers the probability of significant methanogenic microbial activity on Mars, in other words—life forms.
How did the media report this story?
Quite well, actually, and in a much more accessible form than the original paper. The title of the news article, “Curiosity rover’s methane result challenges life theory”, explains the significance of the result and the essential mystery that may otherwise be lost in the detail about spectroscopy and ppbv values! Moreover, the news article interviews additional experts who suggest why this journal article, and this Mars rover experiment, may not yet be the last word. For instance, perhaps the ground levels observed by the rover, at that particular site, are quite different from levels in the mid-atmosphere. Indeed, an upcoming European Space Agency study, involving an orbiter, is setting up to explore precisely this hypothesis.
Finally, this scientific mystery story says something quite promising about the role of robotic experiments in remote parts of the solar system. This very interesting negative result has only become possible due to a spectacular feat of engineering, to launch a complex robot into operation in a hostile world. With the march of this type of technology, and the possibility of increasing autonomy and agility on the part of these robots carrying sensors, one hopes to see a whole new level of experimentation and hypothesis testing that would enlighten us with richer answers, and ultimately more nuanced and new mysteries to ponder.
CR Webster, PR Mahaffy, SK Atreya, GJ Flesch, KA Farley, & the MSL Science Team (2013). Low upper limit to methane abundance on Mars. Science, in press. doi: 10.1126/science.1242902