Is there life on Mars? The answer may be blowing in the wind.
NASA’s Curiosity rover has detected fluctuating traces of methane – a possible sign of life – in the thin, cold air of the Martian atmosphere, researchers announced today at a meeting of the American Geophysical Union.
Across Mars and within Gale Crater, where Curiosity is slowly climbing a spire of sedimentary rock called Mount Sharp, the methane exists at a background concentration of slightly less than one part per billion by volume in the atmosphere (ppb). However, for reasons unknown, four times across a period of two months the rover measured much higher methane abundances, at about ten times the background level. Further in-situ studies of the methane emissions could help pin down whether Mars has life, now or in its deep past, though it is unclear when or if those studies will ever take place. The findings are published in the journal Science.
“Most of the methane on Earth is produced by biology, and the hope has been that ‘methane on Mars’ could be reduced to ‘life on Mars,’” says lead author Chris Webster, a senior research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “But we cannot yet distinguish whether the high methane levels we’re seeing are being produced geochemically or biologically.” Webster and his team believe the unexpected bursts of methane are produced relatively nearby, somewhere north of the rover, before being carried to Curiosity on prevailing winds.
The findings are a dramatic reversal from Curiosity’s earlier results released one year ago, in which it used data gathered over a third of a Martian year to all but rule out significant quantities of methane in the Martian air. That null result, it is now clear, was due to the actual background level of Martian methane lying just below the threshold of detectability for the standard operations of Curiosity’s instruments.
To sniff out the methane, the Curiosity team had to look longer and harder. For these new results, they collected data over the course of a full Martian year, and gathered “enriched” samples of Martian air that were stripped of carbon dioxide to amplify fainter traces of methane. The one-part-per-billion methane background they eventually found, Webster says, translates to about 200 metric tons of the gas flowing through the Martian atmosphere each year. The Earth, by comparison, annually has about half a billion metric tons of methane cycling through its air.
Most of Earth’s methane comes from anaerobic bacteria living in low-oxygen environments, such as stagnant water and the guts of animals, though abiotic processes such as hot water flowing through mineral-rich rock can also produce the gas. Mars’s minuscule methane background is broadly consistent with what should be produced by ultraviolet light striking the carbon-rich debris of meteorites, comets, and interplanetary dust that periodically fall to the Red Planet. But this mechanism cannot easily explain the methane spikes that Curiosity observed, as it calls for large, very recent meteoritic impacts or airbursts in the vicinity of Gale Crater that would leave clear signs which vigilant orbiting spacecraft should have spotted by now. Alternatively, the Curiosity team suggests the methane spikes may come from unseen, buried deposits of clathrates, lattices of ice that can trap gases such as methane in their crystalline structure.
Another possibility is that the methane spikes aren’t small, transient events produced near Curiosity, but that they are instead whiffs of larger methane releases occurring much farther away on the planet. For more than a decade, various teams using noise-riddled observations from Earthbound telescopes or interplanetary orbiters have claimed to see signs of massive methane releases in the Martian atmosphere, at varying concentrations of between ten to nearly sixty ppb. In 2009, the leader of one of those teams, Michael Mumma, a senior scientist at NASA’s Goddard Space flight Center in Greenbelt, Md., announced the detection of giant, globe-girdling plumes of methane periodically venting from localized regions on the Martian surface.
Other researchers, notably Kevin Zahnle of NASA’s Ames Research Center in Calif., cast doubt on the reality of the plumes. Zahnle says that the effects of Earth’s atmosphere could have contaminated Mumma’s telescopic data, and that the plumes’ purported transience required the unlikely existence of some potent planet-wide chemical catalyst to scrub the methane from the air. Curiosity’s early sniffs of seemingly methane-free Martian air were widely seen as a more definitive repudiation, since even with some unknown methane-scrubbing catalyst such huge plumes would still have left clearly detectable enhanced concentrations of the gas distributed throughout the planet’s atmosphere.
Though the huge plumes he claimed in 2009 have now fallen out of favor, Mumma still suspects something like them, venting in large amounts far away from the rover, could be the source for Curiosity’s methane spikes. Arguments for a weaker, more local release, he notes, rely on assumptions about wind patterns in and around Gale Crater that are not yet fully backed by available data.
“What the Curiosity results really confirm is that we still do not understand the release and persistence of methane on Mars,” Mumma says. “In a nutshell, this is very exciting because it very clearly shows methane has a source on the planet.”
That source, however, could also be the rover itself, which has components known to have outgassed small amounts of methane in the past. “The rover has a lot of methane in it, that is not disputed,” says Zahnle, who is authoring a forthcoming commentary in Science on the findings. “The real issue is what is the source of the methane in the samples: rover or Mars?”
The Curiosity team, Webster says, has gone to heroic lengths to test against possible confounding effects from the rover, repeatedly monitoring all its relevant components for signs of methane contamination. The team even carefully analyzed rock samples from the section of Curiosity’s traverse where it detected enhanced methane, just in case the rover’s heavy wheels happened to crush any deposits of gas-rich material under its treads. Time after time, their results suggested the most plausible conclusion was that the methane spikes Curiosity measured were genuine signs of mysterious processes occurring elsewhere, outside of the rover’s immediate vicinity.
If those processes are biological, and Martian microbes are even now belching methane from subsurface refuges, Webster believes it is within our grasp to now find out. “On Earth, bacteria are lazy, or rather, inefficient,” he says. “They like to use a lighter isotope of carbon, carbon-12. So the methane they produce is depleted in a heavier isotope, carbon-13, by up to fifteen percent.” If Curiosity is lucky enough to observe another methane spike, Webster argues that relatively minor tweaks to the process of gathering “enriched” air samples could allow the rover to measure the ratio of carbon-12 to carbon-13 well enough to distinguish between a biotic and abiotic source. All that’s needed, essentially, is a larger number of measurements and a longer “enrichment” time for Curiosity’s air samples.
Such measurements, however, face competition as steep as the climb up Mount Sharp that the rover is now attempting. The last time Curiosity sniffed the air for methane, Webster says, was five months ago.
Curiosity was meant to be a mission to study signs of habitability on ancient Mars, not signs of life on Mars in the present day, says Paul Mahaffy, a senior Curiosity team member at NASA’s Goddard Space Flight Center. An intensive search for more Martian methane could easily prevent Curiosity accomplishing those primary goals. Other, future missions, such as Europe’s ExoMars orbiter and rover launching later this decade, or NASA’s next rover, a Curiosity clone slated for launch in 2020, could potentially take the next necessary steps in unraveling the mystery of Martian methane. Webster notes that instruments now exist which are a thousand times more sensitive than Curiosity’s methane-sniffing kit, instruments that could in theory discern the gas’s potentially biotic origins with ease. But those instruments are presently not planned for flight on any upcoming mission from NASA or any other space agency.
“We’ll continue to monitor for the methane, but unfortunately these experiments are power-hungry,” Mahaffy says. “They consume a lot of [Curiosity’s] resources, and there is always, always a lot of geology to do.”
At the same meeting at which the tantalizing methane results were announced, the Curiosity team also presented yet more evidence that, early in its life, Mars was warm and wet, far more Earthlike and probably capable of sustaining life. Alas, barring a major shift in the pace and emphasis of our present explorations, the question of whether the planet sustains life now may remain unanswered for many years to come.