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SAN FRANCISCO (AP) -- New observations by a NASA spacecraft orbiting Mars show a planet rich in water, but suggests that for billions of years it has done little other than remain frozen in the soil. The finding challenges theories that Mars was once a warm, wet place hospitable to life.
Instead, the current Martian surface -- a cold, dusty and overwhelmingly dry place -- may have been the norm for much of the planet's history, scientists said Sunday during a briefing at the fall meeting of the American Geophysical Union.
``Mars may have water, but it's cold,'' said Philip Christensen, of Arizona State University, Tempe. ``It's there, but it can't do much.''
Since NASA's 2001 Mars Odyssey satellite arrived in orbit around the planet more than a year ago, it has turned up evidence that there is lots of ice mixed in its soil, buried as little as 18 inches from the surface, said William Boynton, of the University of Arizona, Tucson.
Spread across the planet, the amount of water is not huge by Earth standards -- it's the equivalent of about two Lake Michigans. In places, however, ice makes up 70 percent of the soil by volume, a significant concentration.
``It's a lot of ice,'' Boynton said.
Previously, scientists speculated that large amounts of water once moved about Mars, falling as precipitation to the surface, where it flowed in rivers and streams to pool in lakes, perhaps even oceans.
Odyssey and the Global Surveyor have been remotely prospecting for the telltale traces of minerals that might have formed in such environments.
Surveyor has found significant deposits of one such mineral, called hematite, at a location NASA may visit next year with a pair of rovers it intends to launch.
But Odyssey has yet to find other such minerals and has turned up minerals present in volcanic rocks that, on Earth at least, are quickly weathered by water. That suggests the water on Mars has largely stayed put.
Even if it did flow on the surface of Mars, carving the river channels visible to this day, it probably only did so for brief periods of time, Christensen said.
Scientists allow that life may have gained a toehold below the surface of Mars, in spots warm enough to melt the ice that peppers its soil.
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