The lander's robotic arm started digging trenches into Martian soil after touching down near the planet's north pole on May 25, revealing a white substance that scientists had said could be either salt or ice.

Phoenix flexed its arm again to enlarge a trench on June 15. It then took pictures of eight bright bits of material the size of dice inside the hole, which scientists dubbed "Dodo-Goldilocks."

When the lander took new photographs of the trench four days later on Thursday, the material had vanished, settling the debate about whether it was salt or ice.

Scientists at the Jet Propulsion Laboratory in Pasadena, California concluded that the material was frozen water that evaporated when exposed to the sun. Salt would not have reacted that way, scientists said.

"We found what we were looking for," Phoenix science team member Mark Lemmon said in a news conference. "We came to this site because we were expected to find water ice."

Scientists believed that a vast sheet of ice was hiding in the planet's North pole after NASA's Mars Odyssey surveyed it in 2002.

"If you had a big broom and swept this area off, we are on an icesheet," said Phoenix Principal Investigator Peter Smith of the University of Arizona, Tucson.

"We have found the proof that we have been seeking that showed that this bright material really is water ice, and not another substance," Smith said.

"Now we know for sure that we are on an icy surface and we can really meet the science goal of our mission at the highest level," he said.

Besides evidence of water, the three-month Phoenix mission is also hoping to find life-supporting organic minerals in the polar region.

The probe is equipped with oven-like instruments that can melt any ice collected by the robotic arm and analyze the water.

The trick, Smith said, is for Phoenix to move ice samples fast enough from the ground into one of the lander's eight ovens within 30 minutes before it evaporates in the atmosphere.

"Just the fact that there's ice there doesn't tell you if it's habitable," he said.

"With ice and no food it's not a habitable zone. We don't eat rocks. We have to have carbon chain materials that we ingest into our bodies to create new cells and give us energy. That's what we eat and that's what has to be there if you're going to have a habitable zone on Mars."

Water filtered down on Mars may have left its mark on surrounding minerals, and impurities in the ice could tell a great deal about the climactic history of this region of the planet.

Mars is currently too cold for liquid water but it is possible that in some distant past the polar regions were warmer, scientists posit.

Water is a main ingredient for life and the polar region at some point may have been habitable: that is a puzzle Phoenix is exploring.

Phoenix's robotic arm made contact in another trench Thursday with a hard surface scientists believe could be an icy 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 in St. Louis, co-investigator for the robotic arm.

After trying to crack further into it, the arm became immobilized, which is the expected programmed reaction for when it hits a hard surface.

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