Page last updated at 16:55 GMT, Friday, 20 June 2008 17:55 UK

Mars probe makes 'ice discovery'

Tiny clumps of material in the bottom left of the trench on 15 June (left) are gone by 19 June (right)

Nasa's Phoenix spacecraft has found compelling evidence of ice at its landing site on Mars, scientists say.

Chunks of a bright material found in a trench dug by the craft have disappeared over four Martian days, suggesting they have vaporised.

While digging in another trench, the lander's arm connected with a hard surface at the same depth.

The finds lend weight to suggestions water is locked up in a permafrost layer just below the dusty surface.

Missing matter

"It must be ice," said Dr Peter Smith, Phoenix's principal investigator, who is based at the University of Arizona, Tucson.

"These little clumps completely disappearing over the course of a few days - that is perfect evidence that it's ice," he said.

"There had been some question whether the bright material was salt," he added. "Salt can't do that."

This is not ice-melt - temperatures are too low for that. Rather, very low pressures on Mars mean exposed ice cannot survive long at the surface. Over time, it will sublimate - the ice will erode straight to water vapour.

The dice-sized chunks were exposed in a trench informally known as Dodo-Goldilocks. The trench was dug and photographed on the spacecraft's 20th day on Mars.

Four days later when the trench was snapped again, some of the chunks had disappeared.

The news is a big fillip for mission scientists. Previously, an analysis of a soil sample had yielded no trace of water.

While evidence of ice on Mars has been gathered before, part of Phoenix's mission is to search out evidence to support the idea that the polar region of the planet could be habitable - in the past and even now.

'Same depth'

Further confirmation of the ice theory came from another trench, known as Snow White 2.

Digging there was halted when the scraper on the lander's robotic arm hit a hard surface just under the soil layer.

"We have dug a trench and uncovered a hard layer at the same depth as the ice layer in our other trench," said Ray Arvidson, of Washington University in St Louis, who is lead scientist for the robotic arm.

The arm also stopped three times earlier while digging in a "polygon".

This automatic reaction is a programmed response triggered when the scoop hits a hard, sub-surface region.

The polygons are soil features seen on Earth when permafrost layers in soil expand and contract as temperatures rise and fall.

Phoenix now seems to have confirmed that similar features on Mars are caused by the same processes as those on our planet.

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