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Planetary pools
Did Nasa probe find water on Mars?
Space exploration: special report
Nasa scientists are believed to have told the White House that they have spotted evidence of a liquid form of water on Mars.
The revelation - not confirmed by Nasa officials and believed to be secret until publication in a scientific journal next week - raises once again the possibility of life on the Red Planet.
The implications of the discovery of living creatures, however primitive, would raise the almost abandoned prospect of a manned mission to Mars. It would also be a sorely needed coup for Nasa, which lost two spacecrafts last year as they ended long journeys to Mars.
Britain would be thrust into the space spotlight: the planet's next visitor will be the European Mars Express, which will set down in 2003 a British lander called Beagle 2, designed to sniff for evidence of water and life in the Martian subsoil, the layer of soil beneath the surface overlying the bedrock .
Mars is apparently arid. It is cold and its carbon-dioxide atmosphere is too thin to support a liquid form of water. But the Martian landscape is scarred by what look like drainage patterns - and even ancient shorelines - indicating water in some warmer past era.
After claims in 1996 by Nasa scientists that they could see fossil microbes in a meteorite from Mars, a series of robot space probes began heading for the mysterious landscape.
Several space websites were carrying reports yesterday that the Mars Global Surveyor, which has been orbiting the planet since 1997, had apparently detected seasonal deposits of ice in regions far from its poles.
Ice on the planet's surface would evaporate and disappear but, if it appeared again, it would indicate a permanent source of water below the surface.
The same spacecraft has apparently also seen pools of water at the bottom of Valles Marineris, a giant canyon which runs for thousands of miles near the Martian equator, one of the few spots on Mars warm enough for water to be liquid. Because of the canyon's huge depth, it could have high enough atmospheric pressures to support a liquid form.
Colin Pillinger, of the Open University, heads a group which is building Beagle 2, Britain's first Mars probe. It will drill into the Martian subsoil and try to detect traces of biochemicals that could only have been made by living things.
"The way I hear it, the guys that are involved are saying 'no comment', and everybody else is saying, 'what could they have seen?' There is a story about the possibility of streams, and the possibility of something which has changed from when they looked at it before.
"[Nasa] might even be about to say that they have seen something volcanic, but I wouldn't have thought they would call Bill Clinton to say they had seen a lava flow," he said.
The US space agency lost its $125m (£82m) Mars Climate Orbiter mission in September because of a mix up between metric and imperial measures in its command software. In December, the £109m Mars Polar Lander disappeared mysteriously just as it should have begun to brake to begin a slow descent. Two micro-probes worth about £20m were supposed to plunge deep into the Martian subsoil, but those were also lost.
Nasa's spokesman Don Savage would only confirm yesterday that there would be a press conference on June 29. But later he promised reporters a press conference later today.