For example, the trenches around Earth-based polygons in the Arctic fill with meltwater and then refreeze, forming wedges of ice.
In the drier Antarctic, however, the trenches fill with sand.
"If we find ice wedges in these cracks [on Mars], that would be a big discovery for our science team, because that would mean that there's liquid water," Smith said.
"We have not yet seen what we're going to be digging into, but it's probably going to be very similar. This is a scientist's dream right here on this landing site."
Readying for Experiments
Once the check-up phase is complete, the craft's robotic arm, capped with a digging scoop, can unfold and begin scraping up soil and ice.
"This arm has a lot of capability to dig through hard soil but not solid ice," Smith said.
To take up ice, "we have a rasp in the back [of the arm's scoop]. It's actually the same kind of tool used by ice carvers."
The main deck of Phoenix, which resembles a cluttered kitchen table, sports a range of equipment to analyze samples taken from different depths.
One of the key instruments on board is the Thermal and Evolved-Gas Analyzer, which will use eight tiny ovens that can reach 1,800 degrees Fahrenheit (982 degrees Celsius) to vaporize samples.
An electronic "nose" in the machine called a mass spectrometer can then identify what the released gases contain, including any organic compounds.
If organics turn up, burning scrapings from a ceramic blank brought from Earth will tell scientists whether the materials are truly Martian or if they are unavoidable contaminants.
"We have a limited number of ovens, so we want to make sure we know what we're doing," JPL's Blaney said.
That's because each oven was designed for one-time use.
"Once it's filled, it moves to another part of the instrument to be sealed," UA's Smith told National Geographic News.
"And when you bake something to over a thousand degrees, it tends to get real hard—you'd need a chisel to get that stuff out."
Phoenix will also use a device that includes a miniature wet chemistry lab and two microscopes to determine how the soil reacts with liquid water.
Samples will be mixed with water brought from Earth in four one-use-only beakers.
Sensors inside the beakers record the soil's acidity, what elements are present, and whether these materials form soluble salts—a sign that the soil might have been hospitable to life when the planet was wetter.
Active Water Cycle
In addition to its search for habitable conditions, Phoenix's meteorological station is now the first high-altitude weather monitor on the red planet.
"This is an area where we have all that dynamic weather going on," JPL's Blaney said of the north pole.
Data from the station could help answer questions about Mars's water cycle, which previous studies had suggested is still very active.
(Related: "Mars's Ice Patchy, Water Cycle Quite Active, Study Reveals" [May 2, 2007].)
"We'll be able to watch how the water moves in and out of the soil," Blaney said.
In addition, "it gets so cold [in winter] that [carbon dioxide] in the atmosphere is turned into dry ice on the surface."
Studying these and other exchanges can help researchers better understand how climate processes work on Mars, which could ultimately improve our understanding of Earth's more complex climate systems.
Phoenix could help test theories that Mars has cyclical ice ages similar to those recorded on Earth.
The meteorological equipment will also study how the atmosphere varies across the seasons.
A laser beam sent vertically into the atmosphere sends light reflecting off particles of dust, and a telescope on the craft can record the size, location, and abundance of Martian particulates.
In addition, a thin mast on the lander will take temperature readings from three different heights.
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