Data obtained by the ESA probe Mars Express and the US spacecraft Mars Odyssey show that in some equatorial locations, low-atmosphere levels of water vapour and methane "significantly overlap," ESA said in a press statement.

The concentrations also coincide with an ice layer a few tens of centimetres (a couple of feet) below the surface.

These curiosities point to "a common underground source," such as volcanic or hydrothermal activity or, perhaps, bacterial life, which may exist in the water below the ice table, the agency said.

In the latter case, the bacteria would produce methane as a natural living process. The gas would be released to the surface and thence to the atmsophere.

"We have a new piece in a puzzle about understanding whether there is possible life, past or present, on Mars," ESA science spokesman Roberto Loverde said to AFP.

Methane comprises just a tiny part of the Martian atmophere, 95 percent of which is carbon dioxide.

The theory behind the presence of methane, a carbon-based gas, is that it would have have to be continuously produced by a source or had been produced relatively recently.

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

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