exposed to H it becomes...guess what?

In the long list of questions that make up this puzzle, two related queries have long stood out in the minds of scientists and the public alike: Is there liquid water on Mars? And does this seemingly barren planet harbor some kind of life?

NASA's Mars Phoenix Lander, slated to touch down in the northern polar region of Mars on Sunday, will aim to help answer these two key questions as it surveys a tiny piece of the planet.

Scientists have been keen to find evidence of water on Mars because it is essential to life as we know it, and having an "onsite" source of H2O would be crucial to any future manned missions to the planet.

"Liquid water is the holy grail on Mars. Where is it? Does it exist at all?" said Phoenix principal investigator Peter Smith of the University of Arizona.

For the first half of the 20th century, it was thought that liquid water sloshed around all over the surface of Mars, in dark patches assumed to be seas covering portions of the planet's surface (not to mention astronomer Percival Lowell's infamous canals, later shown to be optical illusions). Mariner 4's 1965 flyby, which returned the first images of the planet's surface, dashed hopes of finding any Martian seas: The surface looked as inactive and pockmarked with craters as the moon.

Mariner 9, however, found signs that liquid water had once flowed across the Martian landscape through ancient river beds, as well as evidence of water erosion. Other missions, including two current rovers, Spirit and Opportunity, have found ample evidence that water once flowed through rivers, pooled in lakes and spewed from hydrothermal vents.

But this liquid water flowed mostly in very ancient times, when conditions on Mars were much different than they are today. Now, the planet's atmospheric pressure is too low (about 1/100th of Earth's) for liquid water to last on the surface. The only place on the surface where water exists is at the poles, and there it is found only in its frozen form.

In February 2002, NASA's Mars Odyssey orbiter extended the known regions of water on Mars when it detected the signature of water ice just under the surface of the Martian arctic regions, and lots of it.

"It's not just a little bit that you might expect to get frozen into the ground from the atmosphere, but it's like 70 to 80 percent of the upper meter of the surface is ice," Smith said. "The amount of ice was a huge surprise."

Because the arctic regions of the planet haven't been explored from the surface and the underground ice has so far only been detected indirectly, this subsurface arena is "all of a sudden this mystery zone in my opinion," Smith said.

Exactly how these substantial subsurface layers of water ice formed is unknown. Some scientists say it could be a remnant of an ancient northern sea that has been theorized to have existed when Mars was much warmer. It may also have formed as water vapor froze out of the atmosphere, which is partly how the polar caps on Mars form today. But this deposition typically only creates a small amount of soil-trapped ice.

"So you wonder how you can get 70 percent water in just the pore spaces [between soil grains], it doesn't make any sense!" Smith said. "So there must be some other way that you're getting this large amount of water in that area."

Phoenix will aim to shed some light on the origin of this ice, which is expected to be so cold that it will be as hard as concrete, and to characterize it by chipping away pieces for analysis. Smith and the other Phoenix investigators also hope the lander will help determine whether or not the ice periodically melted and wet the Martian soil to create a habitable zone that could have possibly supported some form of Martian life.

"You know there's ice there now, it's probably too cold to melt — the question is, in the last million or couple million years has there been a 'wet zone' up there, if you like, where you really did get liquid water, you wet the soil, and would that be a habitable zone on Mars?" Smith told SPACE.com. 

err.... what H is that ????

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