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.
Mars Express has been orbiting the Red Planet for a year now
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.
Confirmation came through hard data, not "fantasy", he told BBC News.
"[My observations] should not be taken as a statement that there is life on Mars today, because we need to go there, to drill the soil, take samples, and analyse them before possibly concluding that life is there," Professor Formisano said.
The researcher is 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.
The observation is fascinating because the gas is short-lived; it is broken down by sunlight, and 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.
Murray and his team have seen features that look like fractured ice floes
"If you consider only methane which is observed in the Martian atmosphere, it would be 150 tonnes a year; if you consider formaldehyde then you have 2.5 million tonnes [of methane] per year, which is much more," he said.
"And the correlation indicates the sources are in the soil, underground."
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.
And this was probably a very recent event, said Professor John Murray, from the Open University, UK.
"This was around five million years [ago]. That may sound old but in geological terms that is yesterday," he told BBC News.
"We know that these eruptions have occurred throughout Mars' geological history. The fact that they occurred just five million years ago essentially tells us we have vast reservoirs of liquid water beneath the surface today.
"Therefore, if life can develop that is where you are going to find it."
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.