NASA scientists say their original theory that an increase in methane observed in Mars' atmosphere was a seasonal change turned out not to be true, and they still haven't been able to pinpoint the source,
Scientific American reports.
The methane spike was interesting partly because it can be produced by living organisms, though it also has other causes.
The Mars rover Curiosity first recorded the unexpected rise in methane in late 2013 and early 2014. It was speculated at the time to be a seasonal event, occurring in Curiosity's first autumn on the Red Planet.
But another autumn has since passed with no similar spike in methane.
"It was an episodic release, still unexplained," NASA officials said. "However, the rover's measurements do suggest that much subtler changes in the background methane concentration — amounts much less than during the spike — may follow a seasonal pattern."
The changes could be attributable to seasonal pressure changes or changes in the amount of ultraviolent light, the officials said.
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