The evidence comes from infra-red imaging, which probed under dust deposited over the millions of years and found dense networks of dry valleys, whose branching bear the hallmarks of having been carved out by rain.

The research, published in the US journal Science, could prompt a rewrite of the Martian history books, for it suggests the planet had a longer "summer" than anyone thought.

The conventional theory is that Mars had a balmy climate during its infancy, a period called the Noachian era, in which vast volumes of water flowed on its surface, cutting valleys and eroding the craters left by asteroids.

Then, around 3.6 billion years ago -- coincidentally, just when the first signs of life emerged on Earth -- the planet froze, entering the so-called Hesperian epoch, which lasted around half a billion years.

What remained as water has almost always been locked up as ice, either at the poles or (so it is hoped) close to the surface, according to this theory.

The French study, led by Nicolas Mangold of the University of Paris South, contends though that the rain-carved valleys date from near the end of the Hesperian -- at a time when the temperature was, supposedly, far too cold to permit precipitation.

Their analysis is based on data sent back the thermal Emission Imaging System (THEMIS), which has been scanning the planet since October 2001 aboard a NASA orbiter, Mars Odyssey.

THEMIS' images show images of valleys with extensive branching, typical of the erosion on Earth caused by rainwater, as well as meandering curves and inner channels on the valley floors.

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