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 vaporized 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 Friday during a teleconference from Tucson.
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 today 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. Some scientists speculate that water may have evaporated into the atmosphere and the rest trapped beneath the surface in the form of ice.
"The holy grail is to find water near the surface of Mars," said astrobiologist Mitch Sogin of the Marine Biological Lab in Woods Hole, Mass., who is not part of the mission.
Phoenix's latest discovery is not a total surprise. In 2002, the orbiting Mars Odyssey spacecraft spied evidence of a reservoir of frozen water near the planet's poles. Phoenix, which landed May 25, is the first robotic craft to reach out and touch it.
Scientists not involved in the mission said the Phoenix team makes a compelling case for the presence of ice.
"It's not unexpected, but finding it is different than predicting it," said Bruce Jakosky, an astrobiologist at the University of Colorado at Boulder. "Everybody expected the ice to be there. That's why Phoenix went there in the first place."
The bright chunks seen in the Martian soil vanished in images taken Thursday of a trench where they were seen four days earlier. Scientists had debated whether the chunks were salt or ice, but settled on frozen water since salt would not disappear.
"We have found the proof that we've been seeking," Smith said.
Smith said the ice, which appeared to be pure, was found 2 inches deep in the trench.
Digging in another trench, Phoenix hit a hard surface believed to be an icy layer, which will also be tested.
The big question is whether the ice ever melted and remained stable long enough as a liquid.
"If so, one of the requirements is satisfied for life as we know it," said Kenneth Nealson, a geobiologist at the University of Southern California, who had no role in the mission.