Phoenix to go digging on Red Planet

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.

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