Nasa's Phoenix lander has unearthed "perfect" evidence of ice on Mars.

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Digging in another trench has been halted as the lander's arm hit a hard surface - also thought to be ice.

The find lends weight to suggestions that water from Mars' ancient past is locked up in a permafrost layer buried close to the planet's surface.

Missing matter

"It must be ice," said Dr Peter Smith, Phoenix principal investigator of the University of Arizona, Tucson, in a statement.

"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."

The dice-sized chunks were unearthed in a trench informally known as "Dodo-Goldilocks", which Phoenix dug and photographed on the 20th day of its stay on Mars.

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

Earlier in the mission hopes of discovering ice were fading as soil samples scooped up earlier by Phoenix 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.

More confirmation of the ice theory could come from two other trenches. In one, known as "Snow White 2", digging was halted when the scraper on the lander's robotic arm hit a hard surface just under the soil layer.

Similarly the robotic arm automatically halted work when directed three times to dig in a "polygon" close by the lander site.

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

"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 thoses on this planet.

(BBC)

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