Methane gas is easy to detect and a sensitive indicator of biological activity. Based on previous observations, astronomers thought it was present in the Martian atmosphere. But a super-sensitive instrument aboard Curiosity found none at all.
NASA sent the beefy Curiosity rover to Mars to find out if the Red Planet is or ever has been a suitable place for life. Mission officials repeatedly stress that it's not looking for life, but even so some just-announced measurements have dealt a setback to the prospects for finding Martian lifeforms.
In yesterday's online edition of Science, mission scientists led by Christopher Webster (Jet Propulsion Laboratory) reveal that a sensitive instrument on the rover has found no trace of methane in the Martian atmosphere. Officially, the team reports an upper limit of just 0.18 part per billion (by volume). But given the uncertainty of ± 0.67 ppbv, they conclude there's been "no detection of methane."
The results involve measurements by a tunable laser spectrometer (TLS), part of the Sample Analysis at Mars (SAM) instrument housed inside the rover, that can analyze composition of the thin Martian air far more sensitively than has ever been possible before. SAM gulped in six separate samples of the atmosphere over an 8-month stretch that corresponded to late spring and summer at the rover's landing site inside Gale crater.
Methane (CH4) is the most abundant hydrocarbon in the solar system. The trace amounts in Earth's atmosphere are unstable and must be constantly replenished — almost entirely by biologic activity (decomposing organic matter, flatulent livestock, and so on).
Over the past 15 years, various ground-based observers have reported finding spectroscopic evidence for methane in the Martian atmosphere. Some have reported occasional fresh injections from localized sites.
Some researchers have questioned whether the ground-based observations are seeing Martian methane, as opposed to the whiffs present in Earth's atmosphere. And there've been debates about how the gas could come and go so quickly, as the ground-based observations suggest, even though CH4 should linger for centuries around Mars. Still, detections by two orbiters, NASA's Mars Global Surveyor and ESA's Mars Express, seemed to confirm the gas's existence at an abundance of roughly 15 to 30 ppbv.
Models suggest that any Martian methane is unlikely to arise from volcanic activity or the occasional splat of a comet, so scientific opinion has been leaning toward biological activity as the most likely source.
Understandably, Curiosity's negative finding comes as a surprise. "It would have been exciting to find methane," Webster notes in a press release, "but we have high confidence in our measurements." Michael Meyer, who oversees NASA's exploration efforts at Mars, points out that Martian life hasn't been ruled out. "As we know," he says, "there are many types of terrestrial microbes that don't generate methane."
Curiosity will keep sniffing in the months ahead, and the SAM instrument has a means to concentrate the Martian atmosphere in order to check for methane again at levels far below 1 part per billion.