Apparent indications of surface water, including features that look like channels and river valleys, suggest the Red Planet might once have been warm and wet enough to sustain liquid water, and therefore to allow for the possibility of Earth-type life.

But new findings reported in the current edition of the journal Science show that while there may be lots of water ice, there is nowhere near enough carbon dioxide to ever warm the planet up enough to make the water drinkable.

"There's definitely not liquid water," researcher Shane Byrne said in a telephone interview from the California Institute of Technology. "There's just a 3-kilometer (1.86 mile) thick ice sheet, like Greenland on Earth."

Astronomers have figured for decades that the martian south pole had plenty of frozen carbon dioxide, unlike Mars' north pole, which appeared to have only a trace. New thermal images made by NASA's Mars Global Surveyor and Mars Odyssey probes show that both poles are short on dry ice and long on water ice, which lurks about eight yards below the surface.

'SWISS CHEESE' ON MARS

These two robotic spacecraft sent back high-resolution pictures of what look like the holes in Swiss cheese, but are actually flat-bottomed pits in the surface dry ice at the martian poles, Byrne said.

These pits change seasonally, with the bottoms heating up in the martian summer. That indicates the bottoms of these eight-yard-deep (eight-meter-deep) pits are made of water ice, which can warm up, and not frozen carbon dioxide, which simply evaporates or sublimes into the martian atmosphere, Byrne said.

The floors of the Swiss cheese pits are the tops of the thick layer of water ice at the poles.

The water ice would never warm up enough to melt on its own, being dozens of degrees below freezing.

Andrew Ingersoll, a Caltech professor and Byrne's co-author for the Science article, said this scenario is bad news for the idea of terraforming, a visionary approach to heating up Mars enough to unlock its frozen water.

"Mars has all these flood and river channels, so one theory is that the planet was once warm and wet," Ingersoll said in a statement. He noted that a large amount of carbon dioxide in the martian atmosphere it thought to be one way to have a "greenhouse effect" to capture solar energy to melt the ice.

"If you wanted to make Mars warm and wet again, you'd need carbon dioxide, but there isn't nearly enough if the polar caps are made of water," Ingersoll said. "Of course, terraforming Mars is wild stuff and is way in the future; but even then, there's the question of whether you'd have more than a tiny fraction of the carbon dioxide you'd need."

If Mars was ever warm and wet, it must have had far more carbon dioxide that it apparently has now, the researchers said.

Images are available at http://www.gps.caltech.edu/~shane/swiss_press.html.