NASA's Curiosity rover has detected "startlingly high" amounts of methane in the Mars atmosphere, according to a New York Times report, which could suggest microbes are currently living on the Red Planet.

Key points: Methane could be a sign of life on Mars, given it is often produced by biological processes on Earth

Methane could be a sign of life on Mars, given it is often produced by biological processes on Earth Scientists planned a follow-up experiment to confirm their findings, which were not published by NASA

Scientists planned a follow-up experiment to confirm their findings, which were not published by NASA Researchers are unsure of the source of the gas but a leading theory suggests it came from an underground reservoir

The exact reading, recorded on Wednesday, is unknown, but it was significant enough to prompt excitable chatter from scientists who received the data on Thursday.

NASA had not announced the discovery, however The New York Times broke the story after obtaining an email from mission project scientist Ashwin Vasavada.

"Given this surprising result, we've reorganised the weekend to run a follow-up experiment," he said in the email.

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More details about the reading are expected by Monday, but Thomas Zurbuchen from NASA's Science Missions Directorate has already advised caution.

"While increased methane levels measured by Mars Curiosity are exciting, as possible indicators for life, it's important to remember this is an early science result," Dr Zurbuchen said in a tweet.

"To maintain scientific integrity, the science team will continue to analyse the data before confirming results."

Why do we care about methane on Mars?

While methane has unpleasant connotations on Earth, its presence on Mars could confirm life exists on the Red Planet.

The gas, commonly associated with burping cows, is largely produced by anaerobic microbes called methanogens.

Methane can be a by-product of termites chewing through wood, as well as decaying animals and rotting plants in airless, underground conditions.

Non-biological sources of methane are possible, with volcanoes also producing methane on Earth.

However, in a post on the NASA website, the space agency learns towards the theory that methane on Mars is evidence of life in some form.

"A leading idea is that methane on Mars is being released from underground reservoirs created by past biology," NASA's Mike Mumma said.

Methane could be produced by biological or non-biological processes. ( Supplied: NASA/JPL-Caltech/SAM-GSFC/University of Michigan )

Reading conflicts with ESA report

Curiosity's methane reading comes just months after a report from the European Space Agency (ESA) confirmed its Roscosmos ExoMars Trace Gas Orbiter did not detect methane.

"We did not detect any methane over a range of latitudes in both hemispheres, obtaining an upper limit for methane of about 0.05 parts per billion by volume, which is 10 to 100 times lower than previously reported positive detections," scientists wrote in an article published by Nature in April.

This result was unexpected, given the high readings of methane gas picked up by Curiosity in 2013 and 2014.

In a report about the readings, scientists could not pinpoint where the methane, which was picked up during Curiosity's crossing of the Gale crater, was coming from.

Scientists wrote the readings suggested Mars was "episodically producing methane from an additional unknown source".

So when the ESA mission did not catch a whiff of methane on its most recent trip, the mystery deepened.

Methane spikes had subsided since 2014 but researchers did not expect a complete lack of gas readings.

"The new non-detection of methane by the ExoMars Orbiter could mean that Mars has some unexpected way of destroying methane, or that only some parts of Mars release methane — and possibly only at certain times," a spokesperson speculated in a NASA blog post from April.

NASA gave little away about the June reading over the weekend, leaving the issue open to debate.

"We don't yet know where methane on Mars comes from," Dr Zurbuchen said.

What about the rover fart theory?

During a seminar in 2015, NASA Ames Researcher Center scientist Kevin Zahnle floated the idea that the methane could have had Earthly origins.

"I am convinced that they really are seeing methane … but I'm thinking that it has to be coming from the rover," he said.

He pointed out that the rover had a small chamber containing methane at a concentration about 1,000 times higher than that of the whiff Curiosity picked up back in 2014.

The rover "fart" theory captured people's imagination but, as Astrobiology Magazine reported in 2015, critics of this line of thinking claimed there had been no evidence of a leakage.

A 'selfie' taken by the Curiosity rover on Mars. ( NASA )

Research scientist Chris Weber from NASA's Jet Propulsion Laboratory said comparing a methane reading to the concentration of gas in the rover chamber was misleading.

"You have to look at the amount of methane, not the concentration," he told Astrobiology Magazine.

"The concentration of methane on the rover may seem high, but the actual amount is very small because the chamber is very small.

"To produce the amount we detected in Mars' atmosphere, you'd need a gas bottle of pure methane leaking from the rover.

"And we simply don't have it."