Los Angeles (AP): The apparent discovery of ice near Mars' north pole has scientists asking: Did the frozen water melt at some point in the planet's long history to create an environment friendly for life?

The Phoenix spacecraft exposed bright white crumbs at the bottom of a trench while digging near Mars' north pole earlier this week. The bits disappeared in new photos sent back on Thursday, convincing scientists that the magic act was evidence of ice that vapourised after being exposed to the sun.

"The fact that there's ice there doesn't tell you anything about whether it's habitable," chief scientist Peter Smith of the University of Arizona said yesterday during a teleconference from Tuscon.

To judge whether the Martian polar environment could be hospitable, scientists are using the spacecraft's instruments to study minerals in the soil and ice for hints of carbonates and sulfates, which are formed by the action of liquid water.

Preliminary results from an experiment that baked a soil sample in one of Phoenix's test ovens failed to yield evidence of water. A data glitch on the lander this week prevented scientists from getting the results right away from the last testing phase.

Water is a prerequisite for life, but it's just one piece of the equation. Scientists generally agree that organic carbon and an energy source like the sun are also considered necessary ingredients.

Mars is arid and dusty, constantly bombarded by radiation and with no apparent trace of water on its surface. But carvings of channels and gullies on the Martian surface suggest a wetter past.