Planetary scientists can tell a lot about a planet by studying the light reflected off its surface, and filtered through its atmosphere. One of the things that these techniques had taught them about Mars was that the planet contains a lot of methane in its atmosphere. This was very exciting, since once of the best sources of methane is the exretions of living organisms. But a few months ago, the Mars Science Laboratory (MSL), more commonly known as the Curiosity rover, poured cold water on the dream of finding life on Mars by reporting that its extremely sensitive instruments for analysing the atmosphere had found practically no methane at all.Previous data, collected with telescopes on Earth and satellites orbiting Mars, had found that methane concentrations in some regions reached as high as 45 parts per billion (ppb). This seems low, compared to the 700ppb of Earth’s atmosphere before the industrial revolution, but one of the main sources of atmospheric methane is biology. Many bacteria produce methane as a waste product, and as a result the Earth has relatively high concentrations of the gas in its atmosphere. Methane is quite persistent, but still vanishes from the atmosphere too fast for non-biological processes to keep up. For this reason, the discovery of such high concentrations in the martian atmosphere was very exciting to astrobiologists as it offered real evidence in favour of there being life on Mars.
However, Curiosity has contradicted these findings. It is equipped with an extremely sensitive instrument to analyse the air on Mars, and found no methane at all. It took six samples at dates from October 2012 to June 2013, and no methane was detected at any of these times. The instrument used, the Tunable Laser Spectrometer, is operating in a general purpose mode, which can detect methane at concentraitons as low as 1.3 ppb, so it is still possible that the gas is present at lower concentrations than this. “It would have been exciting to find methane, but we have high confidence in our measurements, and the progress in expanding knowledge is what’s really important,” said Chris Webster of NASA’s Jet Propulsion Laboratory in Pasadena, California. “We measured repeatedly from Martian spring to late summer, but with no detection of methane.”
The mystery then is why measurements from the ground differ so widely from those taken off-planet, assuming that all measurements were accurate. “There’s no known way for methane to disappear quickly from the atmosphere,” said one of the Webster’s colleagues, Sushil Atreya of the University of Michigan, Ann Arbor. “Methane is persistent. It would last for hundreds of years in the Martian atmosphere. Without a way to take it out of the atmosphere quicker, our measurements indicate there cannot be much methane being put into the atmosphere by any mechanism, whether biology, geology, or by ultraviolet degradation of organics delivered by the fall of meteorites or interplanetary dust particles.”
Thanks to atmospheric mixing due to wind, if methane exists at a few isolated locations at the concentrations detected from orbit, then it should be detectable anywhere on the planet. The fact that this is not happening is another part of the puzzle. But whatever the answer, a lack of methane doesn’t quite close the book on eventually finding life on mars. “This important result will help direct our efforts to examine the possibility of life on Mars,” said Michael Meyer, NASA’s lead scientist for Mars exploration. “It reduces the probability of current methane-producing Martian microbes, but this addresses only one type of microbial metabolism. As we know, there are many types of terrestrial microbes that don’t generate methane.”