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Canadian researchers have discovered that a white, salty substance churned up by the Mars Spirit rover is the first "on-the-spot" evidence of water just beneath the surface of the Red Planet.

The discovery by physicists at the University of Guelph is the first solid proof based on soil samples, and reinforces earlier evidence from satellite images suggesting water lies trapped under the barren landscape.

But how widespread that water is remains a mystery - and finding ample sources of water on the Martian surface will be critical to the success of any human mission to the planet.

Still, lead researcher and professor emeritus Iain Campbell said in an interview yesterday that the discovery is an important piece of the puzzle, and most likely represents the remnants of oceans or pools that have since largely evaporated.

"The Mars rovers looking at the geology and the geochemistry have strong proof for water in the geological past. The orbiting satellites have got very strong evidence for water in the present. Our measurement is the first ... quantitative measurement of water in a well-defined lump of material on the surface of the planet," Prof. Campbell said.

The distinctive white substance was caught in the rover's wheels as it moved across the soft red surface in the Columbia Hills region of the planet.

The Spirit rover is one of two robotic vehicles operated by NASA to accumulate evidence that liquid water once flowed on Mars's surface. A second rover, Opportunity, operates on the opposite side of the planet. The six-wheeled robotic vehicles are armed with a panoramic camera and a robotic arm that can drill into rock.

An X-ray spectrometer on the arm of Spirit captured the data and transmitted it back to Earth to be analyzed by Prof. Campbell and his team. They have been working on the project for two years.

The research, based on four samples taken from various locations in the area, concluded that the images were showing a sulphur-rich soil that contained up to 16 per cent water. The paper is under review by the Journal of Geophysical Research: Planets.

Prof. Campbell declined to speculate on the concentration of water on the planet's surface based on the samples he analyzed.

"It this going to be mine-able? I don't know. Was this just coincidence that the rover turned up four of these samples? There must be a lot more," he said.

He added, "Whether there's a little of this stuff or a lot, I can't say."

Previous probes have found evidence that water, a crucial element for life, once flowed on the now-barren landscape. Past missions have sent back images of what appears to be dried-up riverbeds, and a recent study has suggested there may be reserves of frozen water beneath the surface of the planet, likely from ancient seas that could have been home to some form of life.

Prof. Campbell said he's grateful that the four samples were dug up by the rover's wheels so scientists could detect the presence of water by looking at a substance instead of satellite images.

"I think it's a very important piece of the jigsaw that we've been able to supply," he said. "There were indicators for water. We've come along and said there's a new approach, a way to process the data you may have never thought were useful. They're telling you there's water."