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.
But then, do Curiosity's findings really contradict those made in other studies? Remember: previous observations have pointed to the existence of highly localized patches of methane that are eliminated very quickly from large, vertical columns of the atmosphere. Who's to say that Gale Crater isn't just a methanic dead-zone? If it is, can Curiosity (which samples from the first meter of air above the floor of Gale Crater – what Webster's team admits is "the very lowest part of the Mars atmosphere") really speak to the composition of the planet's atmosphere as a whole?
Webster and his team say it can. Their line of reasoning is as follows: it takes several hundred years for methane to disappear via photochemical processes, but only a few months to circulate throughout the Martian atmosphere and mix evenly with any pockets of air lacking in methane. This, write the researchers, suggests that the measured upper limit of 1.3 ppbv "is representative of the global mean background level." More than that – to quote study co-athor Sushil Atreya of the University of Michigan, Ann Arbor – it suggests that "there cannot be much methane being put into the atmosphere by any mechanism, whether [biological], [geological], or by ultraviolet degradation of organics delivered by the fall of meteorites or interplanetary dust particles."
Emphasis added, because, Michael Mumma (who, you'll recall, led the 2009 study on Martian methane plumes) takes a decidedly different stance.
"These findings are actually consistent with our results," Mumma said of Webster's team's findings, in an interview with National Geographic. "We reported that the methane releases are likely to be sporadic and that the methane is quickly eliminated in the atmosphere."
Emphasis added again, because, as Mumma notes, his team's findings hinge largely on the rapid elimination of methane from the atmosphere. Which brings us to the contentious mystery at the center of this whole story.
In Mumma's paper, his team proposes a very short lifetime for methane in the Martian atmosphere – between 0.4 and 4 years. This lifetime, Webster's team criticizes, would require "powerful destruction mechanisms that have not been identified to date." And while several models for such mechanisms have been proposed, Webster and crew claim there remains no evidence for their existence at Mars, nor for their ability to reduce the lifetime of methane by the factor of 100 or more compared to its photochemical lifetime:
With an expected photochemical lifetime of methane in the Martian atmosphere of hundreds of years, there currently remains no accepted explanation for the existence and distribution of the reported plumes, nor of the apparent disappearance of methane over the last few years.
In case you somehow missed it: them's fightin' words! The true nature of Martian methane is a planetary mystery with what now appear to be at least two surprisingly divergent explanations. Determining which is closest to reality will require years of further investigation, conducted from the surface of Mars, from the planet's orbit, and even from right here on Earth. Regardless of whether Mars is – or ever was – home to microbial life, the truth of the matter is that there's some weird-ass chemistry happening on the Red Planet that we're just beginning to understand! And that's pretty damn amazing as it is.
The researchers' findings are published in the latest issue of Science.