An Italian scientist working on the Mars Express probe says gases detected in the planet's atmosphere may indicate life exists on the Red Planet today.
Vittorio Formisano told a Dutch space conference methane and formaldehyde could signify biological activity.
But the cautious professor, from the Institute of Interplanetary Space Physics in Rome, said only soil analysis on the planet could prove it.
He is the principal investigator on the Mars Express Planetary Fourier Spectrometer (PFS).
The instrument is designed to determine the composition of the Martian atmosphere, and was able to confirm the presence in the Martian 'air' last March of small quantities of methane.
Scientists were surprised by the presence of such a short-lived gas because it should not be detectable unless it is being constantly replenished in some way.
The conservative view is that the source is geological - volcanic processes may be producing the gas, although active signs of this have yet to be observed. There may also be large reserves in the Martian soil of ancient methane-ice, so-called clathrates, which are slowly melting and releasing the gas into the atmosphere.
But there is also the theory that microbes are responsible. Some micro-organisms on Earth, known as methanogens, produce methane as a waste product and a number of scientists argue similar lifeforms could exist on the Red Planet, too.
Professor Formisano expressed his views on the subject at the European Space Agency's Mars Express Science Conference in Noordwijk.
He said that if the methane was considered in isolation, it appeared too small a source to be biogenic in origin.
However, he argued, if the formaldehyde detected in the atmosphere was viewed as a byproduct of the oxidation of methane, it would imply much more methane was being produced each year - and this could be explained by life more easily.
PFS data shows that the highest concentrations of methane overlap with the areas where water vapour and underground water-ice are also concentrated.
One of these areas includes the equatorial Elysium region, where Mars Express scientists think they may have seen pack ice covered by a thin 'lag' of dust and volcanic ash.
The researchers told the meeting earlier this week that a vast upwelling of water could have broken through a series of fractures known as the Cerberus Fossae, flooding an area about 800km by 900km, to a depth of about 45m.
A paper detailing the pack ice discovery will appear next month in the prestigious scientific journal Nature. A paper discussing formaldehyde was submitted to Nature by Professor Formisano but was rejected by 'one referee out of three', the Rome scientist said.
And some researchers point out that the area of the spectrum in which formaldehyde appears in the PFS data coincides with an area of significant interference from solar radiation.
Andrew Zilouf
Gases Indicate Life on Mars
Gases detected in Mars' atmosphere indicate that suggestions that the planet contains life may not just be a load of hot air.
