After almost a year of searching, NASA's Curiosity rover has turned up no traces of the four-pronged hydrocarbon known as methane. This special molecule is regarded by many as a chemical signature of past or present life on the red planet. That means there's no life on Mars, right? Wrong.
Life on Mars?
First and foremeost: this is not the first time NASA has reported an absence of methane on Mars. The Agency made almost the exact same announcement last November, when Curiosity's initial scans of the planet's atmosphere failed to turn up any sign of the organic molecule in the Martian firmament. The newly published findings – which appear in the latest issue of Science – are essentially a confirmation of those early observations. Curiosity has had a few more months to scan for CH4 with its arsenal of highly sensitive scientific equipment (the big gun in the rover's hunt for methane being its Tunable Laser Spectrometer) and has come up empty handed.
More specifically, lead scientist Chris Webster and crew have reported that if there is methane on Mars, it exists in quantities smaller than 1.3 parts per billion by volume (ppbv). That's the lowest quantity the Tunable Laser Spectrometer can accurately detect. It's also roughly six times lower than other recent estimates.
But what does this mean for the search for life on Mars? In the words of Webster and his colleagues, the finding "greatly reduces" the probability that there are methane-producing microorganisms currently living on Mars. Important finding? Obviously. A conclusive ruling in the debate over past and present life on Mars? No. No on several fronts.
As we've explained before, methane is not the end-all-be-all of evidence for or against life on Mars. An absence of methane ≠ an absence of life, the same way that a positive trace of methane would not be conclusive evidence for Martian microbes. To quote Michael Meyer, NASA's lead scientist for Mars exploration, Webster's findings address "only one type of microbial metabolism [and] there are many types of terrestrial microbes that don't generate methane." Similarly, there are non-biological processes capable of producing and consuming methane. I could go on, but suffice to say that methane is not a robust metric for the presence (or absence) of life.
Why it Really Matters
And so what's really interesting about these results is that they appear to conflict with lot of other recently collected data on the composition of Mars's atmosphere. In 2009, researchers led by planetary scientist Michael Mumma concluded that big – albeit highly localized – plumes of methane are released sporadically from beneath the planet's surface. Since then, additional observations made from Earth and Mars orbit have supported the existence of localized methane concentrations of up to 45 parts per billion in the Martian atmosphere.
What Curiosity provides that these previous investigations don't is, quite literally, an on-the-ground assessment of Mars's atmospheric makeup. On one hand, this is great. Curiosity can stick its scientifically sophisticated nose in the air and sniff out the chemical makeup of its surroundings – not from Earth, not from orbit, but from right there in Gale Crater! This in situ approach offers researchers a degree of specificity that is unmatched by observations made with ground-based telescopes, or satellites in Mars orbit.