Nasa scientists say they have discovered what they believe is ice in the polar regions of Mars.

The Phoenix Mars lander was digging a trench in the soil of the Martian arctic when it came across chunks of bright material, the mission’s chief investigator said today.

The dice-sized pieces later vanished, meaning they must have been frozen water that vaporised after being exposed, Peter Smith of the University of Arizona said.

“These little clumps completely disappearing over the course of a few days, that is perfect evidence that it’s ice,” Mr Smith said. “There had been some question whether the bright material was salt. Salt can’t do that.”

The Phoenix probe is searching for frozen water in order to determine whether the arctic region of the Red Planet, which has never been photographed or visited before, might have ever supported life.

The bright material spotted in the bottom of a trench dubbed “Dodo-Goldilocks” that Phoenix was digging on June 15. When the probe looked into the trench again early yesterday, several of the crumbs were gone, Nasa said.

However, an initial soil sample heated in a science instrument failed to yield evidence of water.

Phoenix’s robotic arm, meanwhile, encountered a hard surface while digging another trench yesterday and scientists were hopeful of uncovering an icy layer, the space agency said. That trench is called “Snow White 2.”

The arm went into a “holding position” after three attempts to dig further, which is expected when it reaches a hard surface, Nasa said.

In 2002, the orbiting Mars Odyssey detected hints of a vast store of ice below the surface of Mars’ Arctic region, a permafrost wasteland at a latitude similar to northern Canada, known as Vastitas Borealis.

The arctic terrain which Phoenix is exploring has polygon shapes in the ground similar to those found in Earth’s permafrost regions. On Earth, the patterns are caused by seasonal expansion and shrinking of underground ice.

Scientists have been using names from fairy tales and mythology to designate geological features around Phoenix and the trenches it has been digging.

Phoenix touched down near Mars’ north pole on May 25, the first successful landing in the planet's polar regions.

The $420 million mission is planned to last 90 days and hopes to learn more about the geological history of water – the key to unlocking past climate change – as well as determine whether the area might be, or have ever been, habitable.

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