Ice on Mars surface

WASHINGTON (Reuters) - Scientists working on NASA's Phoenix Mars Lander mission are reporting what they call compelling evidence that the robot craft has found ice while digging on the Martian surface.

NASA is expected to give details on the discovery during a news conference on Friday.

The small science probe landed safely last month on a frozen desert at the Martian north pole to search for water and assess conditions for sustaining life.

Small chunks of bright material described as the size of dice have disappeared from inside a trench where they were photographed by the craft earlier this week, NASA said in a statement late on Thursday.

This has convinced scientists the chunks were ice -- frozen water -- that vaporized after digging exposed it, NASA said.

"It must be ice," said mission principal investigator Peter Smith of the University of Arizona. "These little clumps completely disappearing over the course of a few days. That is perfect evidence that it's ice. There had been some question whether the bright material was salt. Salt can't do that."

The presence of water on Mars is a hot topic for scientists. They have presented strong evidence in recent years of huge deposits of frozen water at the Martian poles and point to geological features that indicate that large bodies of water have flowed on the planet's surface in the distant past.

Water is a key to the question of whether life, even in the form of mere microbes, has ever existed on Mars. On Earth, water is a necessary ingredient for life.