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 the Earth and its mass is only 1/10th of the 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 awhile to understand," said Phoenix landing site working group chairman Ray Arvidson of Washington University in St. Louis.

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 "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.

Hunt for a habitable zone

The idea of "little green men" living on Mars has captured the human imagination in one form or another for decades. Countless science fiction books and movies, such as H.G. Wells' "The War of the Worlds," have featured Earth invasions by menacing Martian neighbors. Even Percival Lowell thought his canals were dug by an intelligent Martian civilization.

But when Viking 2 landed on the surface of the red planet in 1976 and sampled the ruddy soils, it found no signs of life: no microbes, no organic materials and certainly no "little green men" with laser guns.

The barren surface, frigid temperatures, thin atmosphere and lack of liquid water decrease the likelihood that Martian life might exist or have survived if it once existed, at least in any biological form we're familiar with. But recent discoveries of microbes living in extreme environments on Earth renewed speculation that life could be hiding away somewhere on Mars.

The soil at the surface of Mars ? the type sampled by the Viking mission ? is actually dust that gets blown all over the planet and is constantly bombarded by ultraviolet radiation from the sun, which is known to be harmful to life on Earth.

But the subsurface ice layer discovered by Odyssey is under about 2 to 4 inches (5 to 10 centimeters) of dust, so it would be protected from UV rays (though not from DNA-damaging cosmic rays), so the soil-ice layers would be more likely to have organics, Arvidson said ? if they even exist.

Smith noted that there are microbes on Earth that freeze during the polar winter, but survive and thaw out again come spring, "like nothing happened." Similarly, Mars experiences much warmer periods when the wobble of its axis sets it at a different angle with respect to the sun ? but this happens on much longer time scales than Earth seasons.

"Can that [microbe thawing] happen over 500,000-year periods? I don't know. Who knows? Life can evolve to all kinds of extreme environments and this one would be especially extreme," Smith added.

If life did thrive on Mars during more hospitable times, it could perhaps have left some telltale sign of organic compounds in the Martian soil. Phoenix is designed to analyze the soil near the subsurface ice layers for traces of organics; finding these chemicals wouldn't prove the existence of life on Mars, but it would bolster plans to probe the planet further and to find an answer to this long-standing question.

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