| In the thin and chilly air above the surface of Mars, NASA's Curiosity rover just caught a whiff of something remarkable: methane. During a routine experiment with its Sample Analysis at Mars (SAM) instruments, Curiosity detected concentrations of this organic compound at 21 parts per billion — far above the usual low background levels. The scientists on the experiment described the detection as "incredible," and themselves as "dumbfounded." This presence of this gas — usually produced on Earth by living creatures, like microbes and belching cattle — raises the tantalizing possibility that it could be a sign of life. Scientists scrambled to cancel Curiosity's weekend plans in favor of another SAM experiment. But by the time they'd tested again, methane levels had dropped down to below 1 part per billion — barely even there. "That's the story," Paul Mahaffy, the principal investigator for the SAM instrument suite, said at an astrobiology meeting Monday. "A plume came. And a plume went." Now, scientists are wondering what it all means. Methane has been detected on Mars before. Since the mid 2000s, observations by the European Space Agency's Mars Express orbiter and Earthbound telescopes have found traces of the gas in the atmosphere, and Curiosity has several times measured spikes higher than 5 parts per billion at ground level. But the gas is elusive. The plumes usually vanish as soon as they are detected, and the ESA's Trace Gas Orbiter, which arrived at Mars in 2016 specifically to search for methane, has yet to detect even a whiff. With the current set of measurements, "we have no way of telling if the methane source is biology or geology, or even ancient or modern," Mahaffy said. As exciting as it would be if the gas came from tiny microbes living in Martian, it could also be a product of interactions between water and rock happening underground. It could reflect processes happening beneath Curiosity's wheels, or a relic of something that occurred billions of years ago. In a statement from the Jet Propulsion Laboratory, where Curiosity was built and is operated from, project scientist Ashwin Vasavada said that the rover doesn't have instruments capable of discerning the exact origin of the plumes. But by combining observations with other spacecraft, scientists could pinpoint where the gas is appearing. "The methane mystery continues," Vasavada said. | 
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