Methane in the atmosphere of Mars could indicate the presence of microbial life on the Red Planet, a fact that was not forgotten when a huge spike of the gas was picked up over the course of several weeks back in late 2013 and early 2014 by NASA’s Curiosity rover.

The spike saw methane levels rise from around 0.7 parts per billion to 7 parts per billion, prompting speculation that some sort of biological process could be at work.

Now though, over two years later, Curiosity has failed to pick up any other similar surges of methane, meaning that the mysterious spike doesn’t appear to be a seasonal event.

“It was an episodic release, still unexplained,” NASA wrote.

“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.”

Source: Unexplained Mysteries