By Paul Rincon
BBC News Online science staff
New data showing that patterns of water and methane in Mars' atmosphere overlap may have important implications for the idea that the planet could harbour life.
The finding does not, by itself, suggest life
The finding comes from the Mars Express probe in orbit around the Red Planet.
If microbes are making methane seen in Mars' atmosphere, they would rely on water, so the association between the two has excited some researchers.
But other scientists have pointed out that this overlap could just as easily be explained by alternative processes.
Not all of these processes necessarily involve microbial life.
Dr Michael Mumma, a planetary scientist at the US space agency's Goddard Space Flight Center in Greenbelt, US, said one possibility was that methane molecules could be trapped in a water-ice matrix (methane hydrate) on Mars.
"If you happened to warm that beyond the liquidation temperature then you would free both methane and water together," he told BBC News Online.
"The origin of that methane could be [biological] or [non-biological] but it would clearly stem from an earlier time when water was abundant and methane hydrates could form."
The Planetary Fourier Spectrometer has detected methane
Another possibility is that geothermal activity from underground heat sources on Mars generates methane through the oxidation of iron contained in hot basaltic rocks. The process, known as serpentisation, releases hydrogen which combines with carbon to form methane.
The detections have been made by the Planetary Fourier Spectrometer (PFS), a scientific instrument carried by Mars Express. Its principal investigator is Vittorio Formisano, director of research at the Institute of Interplanetary Space Physics in Rome, Italy.
The PFS is designed to determine the composition of the Martian atmosphere from the wavelengths of sunlight absorbed by molecules in it and from the infrared radiation they emit.
The scientists then analyse the "spectra" generated by the instrument.
Dr Mumma said his own team had found the same result in their data. But, he said: "I was quite surprised when I saw [the announcement] and a little worried. There are lines of water vapour in the spectral regions that are very strong compared with methane lines.
"So even weak water lines could masquerade as a methane feature if you're unfortunate enough to have one overlapping the spectral interval in which you think you have methane."
Data from the PFS shows that at 10-15km above the surface, water vapour is well mixed and uniform in the atmosphere. However, close to the surface, it is more concentrated in three broad equatorial regions: Arabia Terra, Elysium Planum, Arcadia Memnonia.
Here, the concentration is three times higher than in other regions observed. These areas also correspond to those where the US space agency's Mars Odyssey spacecraft has detected a presumed water-ice layer a few tens of centimetres below the surface.
Methane was detected in the atmosphere of Mars in March by Earth-based telescopes and by the PFS on Mars Express.
PFS data shows that the highest concentrations of methane overlap with the areas where water vapour and underground water-ice are also concentrated. The possible implication is that water and methane have a common underground source on Mars.
Some project scientists speculate that geothermal heat beneath Mars causes ice and other material to move towards the surface: the so-called "ice table" hypothesis.
Researchers have speculated that microbial life may exist in liquid water below this ice table.
It has been suggested that the organisms are methanogens, microbes that produce methane as a waste product of their life process.
Professor Colin Pillinger, of the UK's Open University and chief scientist on the British-led Beagle 2 mission, told BBC News Online: "On Earth, places where methane is released in copious amounts are permafrost regions where you get peat bogs that are frozen for long periods of time.
"When the thaw sets in, the little microbes that live there say 'yipee' and off they jolly well go," he explained.
"We had been monitoring methane release from peat bogs as a function of season, watching it rise as the water table rose and gave the organisms something to work with. This could be an analagous situation.
"If [Formisano] has identified a correlation between water and methane it really is very interesting. But the only way to prove it of course is to get down there and measure it in situ.
By this, Professor Pillinger meant that a lander able to carry out the appropriate experiments would need to be sent to the surface of Mars to investigate.
"I've got an instrument sitting in the lab, ready to do the business," he said.