As if the news on earth weren’t depressing enough, the latest dispatches from Mars are also gloomy. It turns out the red planet doesn’t have any atmospheric methane. Earthlings longing for inter-galactic companionship may have to set their sights elsewhere, for the gas is an important chemical signature of microbial activity. On earth, more than 90 per cent of methane is produced by living organisms. A series of tests conducted by Curiosity, NASA’s rover on Mars, indicates an insignificant amount of methane on the planet: 1.3 parts per billion by volume. This tiny amount — about six times lower than previous estimates — greatly “reduces the probability” of ongoing microbial activity and, possibly, of any microbial life in the past. The results, published in Science, come as a surprise as a series of observations made from satellites and earth-bound telescopes had found evidence of higher amounts of methane. But some of these studies were mired in controversy, and recent measurements had lowered the upper limit. Although most of the studies found seasonal abundance or sudden spikes, it has not been established that the spikes were associated with seasons on a repetitive basis. Also, given that the methane molecule has a lifetime of about 300-600 years in the atmosphere, its near absence is a setback for seekers of life on Mars.
The consolation, however, is that its absence does not automatically rule out the existence of life. Even on earth, not all organisms necessarily produce methane. Unlike the evidence collected earlier, every find of Curiosity has strengthened the possibility that Mars had been a habitable environment in the past. Two definite, separate sources have confirmed the presence of liquid water, the most essential prerequisite for habitability. The presence of carbon, hydrogen, oxygen and nitrogen has further increased the habitability quotient. Going by this, more tests at several other locations need to be undertaken before methane’s absence can be fully confirmed. Aside from further tests by Curiosity, the 2016 launch of the ExoMars Trace Gas Orbiter by the European Space Agency and Curiosity’s successor in 2020 may possibly settle the matter. The search for life on Mars has been based on our understanding of life as seen on earth. While all life forms on earth are based on carbon, science does not rule out the tantalising possibility of silicon-based organisms in our universe, though the theme has held more attraction for sci-fi writers than scientists. One thing’s for sure: such is our fascination for other forms of life that the search for extraterrestrials will never end. Let’s just call it curiosity.