By Andrea Thompson

updated 12:23 p.m. ET May 19, 2008

With the dozens of orbiters, landers and rovers that scientists have sent to Mars, you would think we'd have a good handle on just what makes our planetary neighbor tick. But even with all the pictures snapped of its rocky terrain, all the measurements taken from orbit and the soil samples scooped up, we've still barely begun to solve the puzzle that is the Red Planet.

The sheer amount of surface area of the planet left to investigate speaks to how difficult the project is. Mars' diameter is only half that of Earth and its mass is only a tenth of Earth's. But it has no oceans. Its surface area is equivalent to all of the continents on Earth, "so it's going to take a while to understand," said Ray Arvidson of Washington University in St. Louis, chairman of the Phoenix mission's landing site working group.

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.

Where's the water?
Scientists have been keen to find evidence of water on Mars because it is essential to life as we know it, and having an "on-site" 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.

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