Every year or two, NASA announces that one of its probes has discovered evidence of life on Mars.  The spectacularly successful missions of rovers and orbital laboratories have begun to pry open the secrets of Mars's past and have offered tantalizing clues of life that existed in the past and exists today.

But they are nothing more than that – clues to the possibility of life.  Other explanations, less exciting than the discovery of life on Mars, have been offered for many of these claims.

Now comes compelling evidence of microbial life on Mars.  The rover Curiosity, exploring the bottom of a crater where it is believed water was present a billion years ago, recorded huge "spikes" in methane gas – 10 times the background level of methane in the atmosphere – strongly suggesting that microbes are present near the surface.

Independent:

Mysterious spikes of methane that cannot easily be explained by geology or other theories have been found by an instrument on the robot, which landed on the planet in 2012. Scientists can’t be sure what is causing the spikes, but it is possible that it could be very small, bacteria-like living organisms.

If the gas is coming from living microbes then it would mark one of the biggest discoveries in history. On Earth, 95% of methane comes from microbial organisms, but there are many non-biological processes that can also generate the gas.

Scientists have said that the rover now has to test and re-test the possibility of life, ahead of an unmanned mission in 2020 that would look for the source of the methane.

Previous satellite observations have detected unusual plumes of methane on the planet, but none as extraordinary as the sudden "venting" measured by Curiosity at the 96-mile wide Gale Crater, where evidence suggests water once flowed billions of years ago.

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Nasa reported last year that that Gale Crater contained the remains of an ancient freshwater lake where there may have been a hospitable environment for life in the distant past.

The laboratory onboard the rover has been sniffing methane in the atmosphere a dozen times in the last 12 months. In late 2013 and early 2014, the amount of methane flared up, and then receded.

At a press conference yesterday at the American Geophysical Union's convention in San Francisco, Nasa scientists said that besides living microbes, other possible explanations for the methane include the Sun's rays degrading organic material left behind by meteors.

But that explanation, they added, still relies on the original material being deposited by an object that would have measured several metres across and left a large crater - no sign of which was visible.

The scientists said other theories, such as volcanic deposits trapped in ice called clathrates, were ruled out by the short time-scale of the spikes.

NASA scientists have been very circumspect in their claims, having been burned back in the 1990s when they let President Clinton make an announcement that evidence of life had been found in a Martian meteor that landed near the South Pole.  It turns out that the evidence was less than convincing, considering that natural mineral processes could have been responsible.

But their caution is tinged with excitement.  It's probable that no robot explorer will ever be able to prove beyond a doubt that life exists on Mars.  That job will be left for the first humans who travel to the Red Planet sometime in the next 20-25 years.