NASA's Curiosity rover has just sniffed out some unusually high levels of methane in the Martian atmosphere. Now comes the question of where this mysterious spike of methane came from. Did it have a geological source or, perhaps, was it “burped” out by microbial life?

The discovery of high methane concentrations on Mars is exciting because it’s a gas that’s usually created on Earth by biological life, although it can also be created through geothermal processes. Furthermore, methane wouldn’t stick around for long here as once it reaches the atmosphere, it's broken down quickly by radiation from the Sun within just a few centuries (which in geological terms is the blink of an eye). This suggests that the methane was either produced relatively recently or has been trapped beneath the surface.

"With our current measurements, we have no way of telling if the methane source is biology or geology, or even ancient or modern," Paul Mahaffy of NASA's Goddard Spaceflight Center in Maryland, said in a statement.

It's not the first time Curiosity has detected methane on Mars since it landed on the Red Planet in 2012, however, this is the highest-ever reading of the gas at the planet’s surface. According to NASA, the rover's Sample Analysis at Mars (SAM) tunable laser spectrometer detected levels of methane at around 21 parts per billion. Back in June 2013, Curiosity detected another sudden spike of methane in the Martian atmosphere above an ice sheet east of Gale Crater. The gas appeared to linger around for a couple of months before mysteriously drifting away.

Just to make things a little more confusing, the European Space Agency's Trace Gas Orbiter has been keeping tabs on Mars for over a year and detected no methane whatsoever. 

To explain this unpredictability, it's been speculated that methane might rise and fall along with Mars’ seasons. Alternatively, the gas might get destroyed as it rises higher into the thin atmosphere. However, most of the evidence around methane on Mars and what we can glean from it is very hazy at the moment. NASA scientists hope to collect more methane observations from different probes with the aim of locating the sources of the gas and to increase their understanding of how long it lasts in the Martian atmosphere.

To date, there is no proof that Mars has ever been home to biological life, past or present. Some of the most convincing evidence of life on Mars hints it might have been possible billions of years ago during the ancient Noachian time period – a reference to Noah's flood – when Mars might have hosted liquid water, a promising sign it may have been habitable for microorganisms.

"Are there signs of life on Mars?" Michael Meyer, lead scientist for NASA's Mars Exploration Program, said last year after another confirmed methane observation. "We don't know, but these results tell us we are on the right track."

Whatever the nature of this latest methane spike, it’s sure to add more fuel to the debate.