The tension as Curiosity rover  scientists began their spiel during a press teleconference was palpable. For months of weekly press conferences, reporters had been asking about Curiosity's analyses of atmospheric methane on Mars. If the rover was finding even a part per billion (ppb) or so of methane, there would be a chance that life—life on Mars, today—was producing it.
But no Martians turned up this time. Christopher Webster of NASA's Jet Propulsion Laboratory in Pasadena, California, is the instrument lead for the Tunable Laser Spectrometer (TLS), Curiosity's atmospheric analyzer. He reported that after four analyses, he could say only that, with 95% confidence, there is between 0 ppb and 5 ppb of martian methane.
That range of concentrations rules out only one possible scenario to explain a methane gush that astronomers on Earth detected in 2003 , according to atmospheric modeler Malynda Chizek of New Mexico State University, Las Cruces. (One more whiff of methane in 2006 was reported and nothing since.) Chizek has run an atmospheric model that calculates how fast processes like solar ultraviolet irradiation will destroy methane on Mars. In one run, she simulated what would happen if a gush of methane like the one observed in 2003 recurred every year—a realistic scenario if the martian spring thaw releases methane trapped in ice or produced underground by bacteria. Under those conditions, the simulation suggested, Curiosity would detect 20 ppb to 35 ppb—far above Curiosity's new upper limit of 5 ppb.
All the remaining scenarios are still in play, however. The 2003 gush could have been a once-in-a-century release involving life … or not. Or the 2003 observations were in error, and a few ppb of methane are lingering in the air from volcanic eruptions or the ultraviolet irradiation of organic-rich cosmic dust drifting into the atmosphere. Or, perhaps, methane on Mars is really down at the few-hundred-parts-per-trillion level, and neither life nor methane-belching volcanoes have anything to do with it.
Curiosity should still be able to settle everyone's questions about methane on Mars. Webster is hoping that concentrating the methane in future Curiosity samples will allow the detection of methane at concentrations as low as 100 parts per trillion. It will just take time, he said, weeks or months of time. Patience.