A soft touchdown in Mars' northern arctic plains set for Sunday is just the first step for NASA's Phoenix Mars Lander. If the dust clears, solar-power arrays deploy and all equipment checks out, Phoenix will then have some digging to do.

While its rover cousins continue to investigate the surface of the red planet (as they have since early 2004), the $462 million dollar Phoenix mission aims to see what's underneath the soil. "Our voyage is down; we dig," said Phoenix principal investigator Peter Smith of the University of Arizona.

At its landing site in the Vastitas Borealis near Mars' north pole, Phoenix is designed to scoop up samples of Martian soil, as well as the layers of rock-hard ice beneath, in the hopes of shedding light on when and how the ice formed and whether it has ever melted and moistened the surrounding soils. This information could shed light on whether this little-studied area of the planet could ever have been habitable for life, though Phoenix's mission isn't to find life itself.

"We're literally scratching the surface, and it's a stepping stone," Smith said. "If we see something that's unexpected and absolutely fascinating and interesting, I would expect NASA would want other missions, that it would go take the next step in the polar regions."

Soil and ice

The vast layers of ice underlying the Vastitas Borealis were discovered in 2002, when the Mars Odyssey orbiter detected the signature of water below the top few inches of ruddy dust that coats the planet. Phoenix will provide the first direct look at this frozen subsurface layer from its landing site at 68 degrees north latitude and 233 degrees east longitude.

"What Phoenix is trying to do is follow the water and validate what we think we discovered from orbit," said Phoenix landing site working group chairman Ray Arvidson of Washington University in St. Louis.

Phoenix's 7.7-foot (2.3-meter) robotic arm will dig down through the soil to the ice layer below, which is expected to be at about -136 degrees Fahrenheit (-93 degrees Celsius). At that temperature "the bonds [in the water] are so strong [that the ice is] as strong as a concrete sidewalk," Arvidson said.

At the end of the robotic arm is a rasp, about the size of your pinky finger, that will rotate down into the ice and kick up tiny pieces into the scoop for analysis by instruments aboard the lander.

One of the key measurements Phoenix is designed to make is the abundance of the different isotopes (which are versions of the same element with different atomic weights) of hydrogen and oxygen in the water ice. The most common form of hydrogen has no neutrons, but one of its isotopes, deuterium, has one neutron. Oxygen commonly has eight neutrons (this is called oxygen-16), but one of its stable isotopes has 10 (called oxygen-18). Phoenix's mass spectrometer will measure the ratios of the isotopes of these two elements, "and that should be a signature of the processes involved in making that ice," Arvidson said.

Here is what those details could reveal about ice on Mars: One theory is that the ice is in equilibrium with the scant amount of water vapor in Mars' atmosphere and froze out of the air and into the pore spaces between the soil grains. Because Mars' gravity is weaker than Earth's, it can only hold on to heavier elements in its atmosphere, so it has a higher ratio of deuterium and oxygen-18 to their lighter isotopes. If the mass spec examines the isotopic ratios of the water and the air "and if they're identical, it means that the water in the atmosphere is in contact, in equilibrium with the ice," Arvidson explained.

"But suppose it's a different isotopic composition ? it means that ice was inplaced in some other time, when water in the atmosphere had a different isotopic composition," Arvidson told SPACE.com. "So we're trying to get at the past history and the role of water at the high latitudes."

Signs of life

The lander also is set to scoop up samples of soil near the ice layer to look for signs of potential habitability. Because the ice has been so cold for so long, "it's been in a deep-freeze, and if there are any organics, they should be very well preserved," just as food can be preserved in your freezer, Arvidson said.

The frozen ground on Mars today probably isn't too hospitable a place for life, so mission scientists aren't expecting to get to the pole and find "little green men," or even "little green microbes" ? instead the lander will look for conditions that could support them.

Specifically, the instruments on Phoenix will analyze the soil to see if the water ice layer was once ever a liquid water layer.

"Liquid water changes soil, ice doesn't do much of anything," Smith explained. "Ice is like another form of rock. Nothing happens because ice is nearby ? it has to melt."

So if the lander's instruments find evidence of clays, salts or carbonates ? all of which are transformed by water ? in the soil, that would mean that "the soil was wet with liquid water" or was blown in from somewhere else on the planet that once had liquid water, Smith explained.

In the search for signs of life on Mars, "there's not a magical formula that we're looking for," Arvidson said, but there are a few key conditions that would increase the likelihood that Mars at least at some point harbored life.

The first is the ice itself, "because water and habitability kind of go together," Arvidson said. Phoenix will also dissolve soil samples in four teacup-sized beakers that have electrodes to measure the soil's pH (level of acidity) and oxidation potential, which can affect an organism's ability to carry out certain key biochemical reactions. It will also look for certain elements (carbon, hydrogen, oxygen, phosphorus and sulfur) that go hand-in-hand with life, on Earth at least.

Gases given off when soil samples are heated in tiny ovens aboard the spacecraft will show whether any organic compounds, which could be traces of past life, are present in the soil. But scientists have to make certain that any detected organics didn't just make the trip with the lander from Earth.

"If we get a hit like that, we are going to be totally, totally, like, probably for two or three days, making sure we haven't goofed in some way," Arvidson said.

"In fact, it's really tough. If we measure organics, the first thing we think is, 'It's terrestrial; we brought it with us.' The second thing is that it's from the asteroids and comets," Smith agreed. "It would take a considerable amount of evidence before we could talk about biology."

Martian weather

When Phoenix's three-month primary mission is completed (likely in September) at the end of the northern hemisphere summer on Mars, the lander will switch modes to become a weather station.

The weather instrumentation aboard the lander, provided by the Canadian Space Agency, includes a 4-foot (1.2-meter) mast with sensors at three heights that can monitor temperature. A wind telltale at the top of the mast shows the wind direction and speed.

A probe that can measure the moisture level of soil also is designed to measure the relative humidity of the Martian air. Such measurements characterizing the atmosphere at high latitudes have never been made before, Arvidson said.

Phoenix is also equipped with a lidar (for "light detection and ranging") tool that can measure dust and ice particles in the atmosphere. The tool sends powerful laser pulses vertically into the air, which then scatter off the particles, some returning to the instrument. This information will help scientists track changes in particle abundance and learn how clouds and dust plumes move and form in the Martian atmosphere.

Mission scientists are also hoping that as summer ends and the polar ice cap expands, Phoenix will be able to watch the process. "That would be totally cool," Arvidson says, since the ice cap formation has never been observed from the surface. Scientists don?t even know if the white coating observed from satellites is frost, snow or slabs of ice.

"If we're lucky, what we'll see is the accumulation of ice, water ice, and dust, and maybe even CO2 [carbon dioxide] ice," Arvidson said.

Eventually, as the sun sets (though it rises and descends in the sky each "sol," or Martian day, the sun remains about the horizon throughout the northern hemisphere summer above the arctic circle, just as it does on Earth) and the craft is encased in this advancing ice, it will end its mission for good.

Because no craft has ever ventured this far north on Mars (the closest was Viking 2's landing at 48 degrees latitude), scientists have little idea what to expect from any of the analyses Phoenix will perform. Whether they'll find signs of a muddy Martian past or organics is anybody's guess.

"I can't tell you what we're going to find, because this is really exploration and discovery," Arvidson said.