Both of those phenomena, along with a third mooted methane source -- hydrothermal activity -- remain among the big unknowns about Mars, and the Paris-based agency stressed it needed to know more about the "exciting" find.

Methane measuring one part in 100 million was detected by a device called the Planetary Fourier Spectrometer, which uses the spectral fingerprints from light reflected through the planet's atmosphere to analyse the chemical composition of gases.

The methane comprises just a tiny part of the five percent of the Martian atmosphere that is not carbon dioxide (CO2).

"The measurements confirm so far that the amount of methane is very small -- about 10 parts in a thousand billion -- and so its production process is probably small," ESA said in a press statement.

"However, the exciting question 'where does this methane come from?' remains."

The methane, a carbon-based gas, would have to be continuously produced by a source or had been produced relatively recently, it said.

This is because methane can only survive a few centuries in the Martian atmosphere before being oxidised to form water and carbon dioxide.

"Based on our experience on Earth, the methane production could be linked to volcanic or hydro-thermal activity on Mars," Vittorio Formisano, the spectrometer's principal investigator, said.

Another hypothesis, also based on what happens on Earth, would be biological activity, such as fermentation, he said.

"If we have to exclude the volcanic hypothesis, we could still consider the possibility of life," said Formisano.

But, he said, "the thing to understand is how exactly the methane is distributed in the Martian atmosphere. Since the methane presence is so small, we need to take more measurements. Only then we will have enough data to make a statistical analysis and understand whether there are regions of the atmosphere where methane is more concentrated."

Mars Express was slotted into orbit around the Red Planet on December 25 and has already taken remarkably detailed stereoscopic images of the terrain and detected large reservoirs of water on the planet's South Pole.

Two US rovers, which landed on Mars in January, have revealed that the planet was once awash with a salty sea that, NASA scientists say, could have supported life.

SPACE.WIRE