Phoenix is the first mission to target Mars' northern polar region. NASA's Mars Polar Lander (MPL), launched in 1999 toward the planet's southern pole, crashed upon landing.

The two Vikings missions, Pathfinder, Opportunity and Spirit all landed "in the dry regions of the equatorial zone," said Peter Smith, a research scientist at the University of Arizona in Tucson and principal investigator for the Phoenix mission.

"These dry regions are places where we can expect to discover the ancient history of Mars as written into the rocks.... Phoenix is doing something quite different."

Just as layers of ice and permafrost on Earth can provide clues to our planet's recent climate history, Phoenix scientists say, subsurface ice at the Phoenix landing site will yield clues to Mars' recent climate history.

The term "recent" needs to be taken with a grain of salt. These are geologists talking; recent to them is some time in the last fifty thousand years.

Mars today is frozen solid from pole to pole. But it may not always have been so. It's possible that, in "recent" times, the northern polar region of Mars has gotten warm enough for liquid water to form. And because Mars wobbles on its axis, there's a good chance that it has.

Earth's obliquity - the tilt of the axis on which it spins - is stabilized by the moon. But over the course of about 125,000 years, Mars goes through an obliquity cycle, oscillating between being more nearly upright and being tilted farther over on its side.

It's during the tilted-over times that the martian poles warm up, possibly enough for subsurface ice to melt and for liquid water to be present for months at a time. Because of this potential thawing near the poles, some scientists think that Mars' polar regions are the best places to look for signs of life.

"When that axis tilts - and we know it does - to the point where it's almost pointing at the sun, we can expect an opportunity where the ice might actually melt to liquid water, and change the properties of the soil," Smith says.

"It would leave signatures in the soil, it would change the mineralogy, it would leave salt deposits, and it might even shape the [ice] grains in a way that we can understand with our instruments.

"Our highest goal," he added, "is to see if this creates a habitable zone on Mars where we might find organic materials, we might find the presence, at least periodically, of liquid water, and we might find chemical energy sources" that could drive the biochemical reactions of life.

Phoenix will not look directly for signs of life, though. Rather it will investigate the habitability of its landing site. Phoenix has a robotic arm that will be able to scoop up soil and ice samples and instruments that will study the visual appearance and chemical makeup of those samples.

"We have not designed the instruments as life-detection instruments. We ... have a little ovens that heat the soil samples, and as we get to temperatures where, say, a carbonate would decompose, we'll see a little puff of carbon dioxide come out, and by analyzing the gases that come out and the temperatures at which we see transitions, we'll be able to characterize clays, carbonates, sulfates and other type of materials that are formed through the action of liquid water onto volcanic soils. So we're looking for the changes caused by liquid water," Smith said.

If evidence is strong for the liquid water in the recent past, the polar regions will move to the top of the list of target sites for future landers or rovers designed to look explicitly for evidence of martian life.

But first Phoenix has to land safely. And it's not easy to land a spacecraft on Mars. There's a lot of smashed-up hardware littered about the planet's surface to drive home this point.

The last three vehicles to land successfully on Mars all used airbags to cushion the blow. NASA's gotten pretty good at dropping spacecraft wrapped in airbags on the planet. Phoenix instead will use a thruster system.

A similar system was used successfully to land the two Viking spacecraft in 1976, but no other spacecraft, before or since, has ever used thrusters to land safely on the martian surface.

That point is not lost on NASA engineers. "We've been working for the last five years on testing this vehicle," said Phoenix Project Manager Barry Goldstein, who is with the Jet Propulsion Laboratory in Pasadena, Calif.

The Phoenix engineering team has systematically addressed dozens of problems that may have been responsible for the failure of MPL. (The exact cause of the failure is still unknown.)

"We feel that we have adequately tested this vehicle. That being said ... we fire 26 pyrotechnic events in the last 14 minutes of this vehicle, and each of those have to work perfectly for this mission to come off as we've planned."

But if it does, Smith said, and if Phoenix finds evidence of past liquid water near Mars' north pole, the discovery could well "change the direction of Mars exploration."

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