A gas strongly associated with life on Earth has been detected again in the Martian atmosphere, opening a new chapter in a decade-old mystery about the on-again, off-again appearance of methane on Mars.
The latest discovery comes from NASA's Curiosity rover, which in addition to analyzing rocks and soil samples, is sniffing the air at its Gale Crater landing site.
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A year ago, scientists reported that Curiosity had come up empty-handed after an eight-month search for methane in the atmosphere, leaving earlier detections by ground-based telescopes and Mars-orbiting spacecraft an unexplained anomaly.
"We thought we had closed the book on methane. It was disappointing to a lot of people that there wasn't significant methane on Mars, but that's where we were," Curiosity scientist Christopher Webster with NASA's Jet Propulsion Laboratory in Pasadena, Calif., told Discovery News.
Then, on the day after Thanksgiving in 2013, methane readings shot up in Gale Crater and stayed high while Curiosity made three more measurements over the next 60 days.
"It was a complete surprise," Webster said.
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Six weeks later, the rover's Tunable Laser Spectrometer scanned again and found methane concentrations were back down to about seven parts per billion, a background level that dovetails with previous measurements by telescopes and Mars orbiters.
Methane concentrations stayed at that background level through July 9, 2014, the last measurement included in a report to be published in this week's Science.
The discovery revives two somewhat dormant investigations. The first and most difficult to answer is tracing the source of the methane and figuring out what triggers its periodic release into the atmosphere.
One possibility is that Mars hosts past or present colonies of methane-producing microbes known as methanogens. It is highly unlikely that Curiosity -- or India's newly arrived methane-hunting Mars-orbiting spacecraft -- has the sensitivity to chemically analyze isotopes in the methane molecules and identify whether the gas is produced by biological processes or geochemical ones.