By BBC News Online science editor Dr David Whitehouse
There is currently water on the surface of Mars, Nasa scientists believe.
The evidence is contained in pictures taken by the Mars Global Surveyor spacecraft, which is in orbit around the Red Planet.
The images show what appears to be brackish water seeping from beneath the Martian surface.
The discovery, if confirmed, will mark a turning point in our exploration of the Red Planet, with future missions to Mars being directed to the locations of seepage.
Deep canyons
Although there is abundant evidence that water flowed freely, billions of years ago, on the Martian surface, it was thought the planet's current atmosphere would be too thin to retain any water there today.
However, it seems this may not be the case at the bottom of deep canyons and craters.
Some of these areas are among the lowest parts of Mars, being several kilometres below the average surface level.
This would make the atmospheric pressure slightly higher and possibly high enough to support pools of salty water.
Scarred surface
This is what Nasa scientists think they may have discovered in the central part of the mighty Valles Marineris, the 6,000-km long (3,700 miles) canyon that scars the Martian surface.
The complex terrain of the canyon floor includes a great variety of landforms, including landslides, volcanic deposits, windblown material and sediments from long-gone seas. It is several kilometres below the average surface level of Mars.
In the MGS images of the area, the scientists think they can see layered terrains that they suspect were deposited by ancient shallow seas or by water deposits that were once below an ice layer.
It is here that there are features which appear to indicate brackish water is seeping from beneath the surface of the planet. The seepage may be seasonal, as it has not been seen in all images of the region.
Similar seepage may also have been detected on the walls of at least two craters in other regions of Mars.
Future missions
The depths of the canyon will now become the focus for the search for life on Mars.
Testifying before the science committee of the House of Representatives on Tuesday, Nasa's chief said that to look for life on Mars "we should follow the water".
If life ever developed on Mars in the past, and the evidence from Mars rocks blasted to the Earth is controversial, then it may be hanging on in the damp regions.
At one level, this discovery is not that surprising. Scientists know that water once flowed on Mars and that it is still probably trapped beneath the surface.
But finding water springs is an unexpected and very welcome discovery. Now, it seems that the water has been found, investigating it will become the main thrust for future Mars exploration.
Putting a lander down on the wet surface and looking for life will become a major priority.
Pictures of the water on Mars
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