Giant Martian lake traced

Mars at different elevations: Red and white is high, and blue is low

By Dr David Whitehouse BBC News Online science editor

New maps show that Ma'adim Vallis, one of the biggest valleys on Mars, formed when a large lake overflowed over a low point in its perimeter.

Ma'adim Vallis: Probably formed when a giant lake overflowed

After mapping contours that link the ancient lake's shoreline and the overspill region, researchers say the water could have cut the deep valley and flooded several impact craters downstream.

They calculate the lake would have been about 1.1 million square kilometres in area and 1,100 meters deep.

It is yet one more piece of evidence that shows Mars had a past that was wet and warm. A picture is emerging of the planet as one vast "lakeland" in which life could possibly have developed.

Long time ago on Mars

This is now how scientists think Ma'adim Vallis may have been created

Ma'Adim Vallis

A sinuous valley on the Red Planet

Situated in the southern hemisphere on the planet's highlands

Measures 861 km in length

Named in 1973 after the word for 'Mars' in Hebrew

About 3.5 billion years ago, on the northern slopes of the Martian Highlands, the surface level of a lake was slowly rising.

The lake itself had been formed several hundred million years previously, during a great bombardment of the planet by asteroids.

The lake lay in a 450 km-wide basin that was filled with water by inflowing streams along its flanks that also covered its floor with sediment.

Soon, the water level rose to the rim and still the water continued to flow into it.

Suddenly, the lip of the crater rim holding the lake in place crumbled under the weight of water, and, as it was breached, water cascaded down the slope.

True nature

Spilling northward, the water found the route of least resistance, moving from crater to crater and gouging out a deep channel.

This channel - 861 km long, 8-15 km wide and up to 2,100 m deep - is Ma'adim Vallis.

Originally, it had been thought the valley was created by a groundwater source. Now, new detailed images from the Mars Global Surveyor spacecraft that measure the elevation of land features have revealed the valley's true nature.

Researchers from the Smithsonian Institution in Washington DC and the University of Virginia at Charlottesville estimate that the lake that fed the valley had an area of a million square km.

They also estimate that the volume of water that spilled out to form the Ma'adim Vallis was about 100,000 cubic km.

The research is reported in the journal Science.