The bright lumps at the top of the trench are believed to be ice
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Nasa's Phoenix lander has unearthed compelling evidence of ice on Mars, the US space agency believes.
Chunks of a bright material found in a trench dug by the craft have disappeared over four Martian days, suggesting they have vapourised.
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 close to the planet's surface.
"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."
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.
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, St Louis, who is co-investigator 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.
"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 this planet.