Closer to finding aliens on Mars? NASA finds traces of gas that could be a sign of life near a crater on the Red Planet

NASA has identified a gas on Mars that is produced by living creatures on Earth, baffling scientists about what could be hiding in the Red Planet.

The Curiosity Rover detected a steady stream of methane coming from Gale Crater, appearing at different times of day and fluctuating seasonally – sometimes reaching 40 times higher than usual.

While NASA has yet to find life on the Martian world, scientists believe the source comes from deep in the ground.

The team has suggested that methane could be encased under solidified salt and only seep out when temperatures rise on Mars - or when Curiosity rolls over the crust and cracks it.

On Earth, this simple molecule, made up of one carbon atom and four hydrogen atoms, is usually a sign of life: gas passed by animals as they digest food. 

NASA's Curiosity Mars Rover detected methane coming from near Gale Crater - but not all the time. Scientists wanted to know why.

Scientists used this sample of Mars's soil to run an experiment on how it forms a crust, trapping methane under the planet's surface during the day.

READ MORE: Uranus is gassier than we thought! 

The planet is not completely packed with ice, scientists have found.

NASA's Curiosity Mars Rover has been roaming the surface of Mars since 2012, and in all that time, the most perplexing thing it found was a steady stream of methane coming from Gale Crater.

The spot in Gale Crater where methane came out was the only such spot on the planet where Curiosity has detected the gas. 

But Curiosity has not spotted any cows on Mars, nor has it found any people who just ate a large helping of cabbage.

In lab experiments that mimic the conditions of Martian soil, scientists were able to simulate what may be happening.

Over a long time, salts bubble up from deep beneath the rocky, dusty surface of the planet, a substance known as 'regolith.'

These salts are called perchlorates, and they are abundant on Mars. 

The perchlorates, which are toxic, are abundant in the ice that's trapped under the planet's surface. 

As ice will do when there's too little atmosphere, this ice evaporates gradually. And as this salty vapor filters through the regolith, it leaves a bit of itself behind.

The perchlorate salts trapped in Mars's permafrost evaporates out and gets trapped in the soil. There, it forms a crust that traps methane below the surface during the day.

When the team bubbled salty vapor up through the simulated Mars regolith, it created this crust that could trap gas.

When enough of these salts accumulate in the regolith, they form a sort of shell - like sand at the beach when it dries into a brittle crust, or like the puck of coffee grounds that's left behind after pulling a shot of espresso.

'On Mars, such a process can occur naturally over a long period of time in the shallow permafrost regions, and it may be possible for enough salt to accumulate in the top layer to form a seal,' wrote the scientists behind the new study, which was published in the journal JGR Planets.

At the same time that the salty vapor bubbles up, so does methane.

Its source remains a mystery.

It could be from some kind of living things, or it could be from geological processes beneath the planet's surface, still invisible to human scientists.

Wherever it's coming from, it ends up trapped under this salt crust. 

By pumping different concentrations of perchlorates through the simulated Mars regolith, scientists found that three to 13 days was long enough for this impermeable crust to form.

The rocky soil surface of Gale Crater traps methane beneath it, but Curiosity may be releasing it when it cracks the crust.

Curiosity is the only NASA craft that has detected methane on the planet. It has not been detected in Mars's atmosphere.

It also required a 5 to 10 percent concentration of perchlorate to create a solid salt crust.

They pumped neon gas up beneath the crust, as a substitute for methane, confirming that the layer was robust enough to trap gas beneath it. 

But then, when the planet's temperature goes up during certain times of day or certain seasons, this crust breaks, letting the methane out.

And that's when Curiosity would detect methane in the air.

It's not just temperature that can crack the crust, though.

The crust is likely about two centimeters thick, a little less than an inch. And Curiosity is plenty heavy enough to crack through it as it rolls over, the team behind the study wrote.

'To test this hypothesis, it would be beneficial to take methane measurements when the rover just arrives at a location with abundant high-salt content features (like salt veins),' they wrote. 'Another test would be to try to ingest Martian air while drilling into the salt-rich surface.'

NASA has not yet tried such an experiment.