NASA's Curiosity rover was sent to Mars to drill into ancient rocks and look for evidence that the planet most like Earth in the solar system had the chemistry and environments to support microbial life.
But the rover, which landed the Gale Crater impact basin in August 2012, also has been scouring the thin Martian atmosphere for another potential life sign -- methane. Since the gas also can be produced geologically, any findings promised a meaty debate.
That discussion can be shelved, perhaps permanently, new findings from a team of Curiosity scientists shows. The most extensive search yet for methane in Mars' atmosphere has come up empty.
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"It's disappointing, of course. We would have liked to get there and found lots of methane and measure all the isotopes," lead researcher Christopher Webster, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., told Discovery News.
"These are the most sensitive measurements made. It will not be corrected later in time. I'm very confident our measurement is correct," he added.
Which is not to say that earlier findings of methane plumes in the planet's atmosphere, made by Mars orbiters and infrared telescopes on Earth, were wrong.
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Rather, the measurements from Curiosity, taken over an eight-month period, add a new twist into an already intriguing mystery.
"The plumes were already hard to explain before they disappeared," Webster said. "Suddenly, the whole interpretation of the earlier observations is stuck."
On Earth, methane lasts 300 years in the atmosphere before it is broken down by ultraviolet rays from the sun. Taking into account Mars' greater distance from the sun and reduced atmospheric pressure, a plume of methane measured in 2003 should have been an easy find for Curiosity's tunable laser spectrometer.
The instrument shoots infrared beams at air samples and measures how much energy is absorbed at particular wavelengths, a process that reveals concentrations of methane, carbon dioxide, water vapor, and isotopic variations of these gases.
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Planetary scientist Michael Mumma, a physicist at NASA's Goddard Space Flight Center in Greenbelt, Md., who was among the first to detect methane in Mars' atmosphere, said the new findings from the Curiosity Mars Science Laboratory means Mars has an unidentified mechanism for destroying methane, a process that occurs much faster than the 200 years or so that would be expected given the planet's photochemistry.
"We think that if Mars Science Lab lasted long enough and made sufficient measurements at a regular cadence that it should, at some point, see methane, if indeed there is another release on the planet. So far, we and other investigators have reported releases at several sites, several times much larger than the value reported by MSL," Mumma told Discovery News.
"If any of those releases are correct -- and we expect future ones to occur -- then some of that methane will make its way toward Gale Crater and MSL should in fact see an enhanced level," Mumma said.
The research appears in this week's Science.