Phoenix is in many ways similar to the two Viking landers sent to Mars by NASA in the 1970s. Like Viking, Phoenix will stay put once it lands. And like Viking, it will search directly for organic material in the martian soil. Indeed, it will be the first spacecraft since Viking to do so. (Two previous missions designed to search for organics - NASA's Mars Polar Lander and ESA's Beagle II - were lost due to technical problems.)

But Phoenix's destination is unique. It will touch down at roughly 68.5 degrees north latitude, the martian equivalent of central Greenland or northern Alaska. During Mars's northern winter, this area is inaccessible, buried under a deep layer of carbon dioxide frost - dry ice. As spring approaches, the dry ice thaws, exposing a region of permafrost: a deep layer of water ice, frozen hard as granite, lying below a thin layer of dusty soil. It is this ice that Phoenix will explore. Scientists want to know whether this ice has ever melted, and what type of material is trapped within it - in particular whether it contains any organic substances.

"Organic" in this context means any type of hydrocarbons, not necessarily biologically produced compounds like proteins and lipids. Hydrocarbons are produced abiotically in many places in the cosmos. Both the surface and atmosphere of Saturn's moon Titan, for example, are rich in hydrocarbons like methane and ethane. Hydrocarbons are also common on comets and asteroids. Indeed, one of the mysteries of Mars is that, although comets and asteroids have been slamming into the planet for billions of years, bringing with them a host of organic compounds, when Viking searched for these compounds in the martian soil, it didn't find them.

The most common explanation for the Viking results is that Viking looked only at surface soil, and conditions on the surface of Mars work to destroy organic compounds. Mars is bombarded by intense ultraviolet radiation, which can break apart the chemical bonds in organic molecules. In addition (although the notion is yet to be proven), most scientists believe that superoxides are present at the martian surface. Similar to the household disinfectant, hydrogen peroxide, only stronger, these superoxides break down organic molecules through aggressive chemical reactions.

One advantage Phoenix will have over Viking in the quest for organics is that Phoenix is headed for the martian permafrost. It will examine not only soil at the surface, but also ice buried below the surface, where the effect of radiation is reduced and where superoxides are less likely to be present. Indeed, ice acts as a preservative for organics.

Peter Smith, of the University of Arizona's Lunar and Planetary Laboratory, is the principal investigator for Phoenix. He explains the importance of ice with a kitchen analogy. "In your kitchen, you don't leave your foodstuffs on the counter, because it quickly decomposes. You put your food in the freezer, and its organic materials are preserved. And we suspect that on Mars, if there are organic materials, the best place to find them preserved will be in the polar regions, associated with this ice."

Some researchers have also argued that even if organic material was present at the Viking sites, the landers' experiments could have missed them. What Viking did - and what Phoenix will do as well - was to heat up soil samples in tiny ovens, and then "sniff" the gases released to figure out what was in the oven. University of Arizona Professor William Boynton, the lead scientist for Phoenix's TEGA instrument, explained the procedure with another culinary analogy.

"If you're baking chocolate chip cookies in your kitchen at home, essentially anyone walking into the kitchen can immediately sniff and tell exactly what's in the oven," Boynton says.

It won't be quite that simple on Mars. Phoenix doesn't have a nose, much less a human brain to identify smells. Rather, as a sample is heated, various gases will be released at different temperatures. Each gas released will be a puzzle piece; to see the big picture, scientists will have to fit the pieces back together.

"When you start to vaporize something," says John Marshall, a Phoenix co-investigator who is with the SETI Institute, "you're looking at the disintegrated products. So it isn't as though the thing's just coming off in the pristine form of organics, you're actually decomposing that organic, and then you have to recompose it theoretically to understand what it was you cooked." Phoenix's TEGA (Thermal and Evolved Gas Analyzer) has a set of eight ovens that will bake samples of soil and ice collected by the spacecraft's robotic arm, and a mass spectrometer that will catalog the gases released. The Viking landers contained similar technology. But while Viking heated its samples to only 500 C (930 F), Phoenix will heat its samples to 1000 C (1830 F).

"There's a pretty big class of organic molecules - many are actually thought to be the most likely to be present on Mars" - that "could have been present in the Viking samples, and the samples may not have gotten hot enough in order to see them," Boynton says.

Even if Phoenix finds organics, it won't have any way to determine whether they are of biological or non-biological origin. Phoenix won't answer the question: was there ever life on Mars? But by microscopic inspection of the ice, and of any particles within it, the spacecraft will be able to shed light on a related question.

"The real question we're trying to answer here is, 'Has that ice melted?'" says Smith. Melted ice - i.e., water - in contact with soil, and shielded from harsh surface conditions, could have provided a habitable environment.

If organics are present, and if the ice, at some point in the past, melted - if the permafrost was once a habitable environment - things begin to get exciting. Microbes have been found in 100,000-year-old ice in Antarctic permafrost, and when the ice was melted, the microbes revived and multiplied readily. [http://news.bbc.co.uk/2/hi/science/nature/6935146.stm] Viable microbes have been recovered from even older ice in the Siberian permafrost. So perhaps, if life was ever present in the polar regions of Mars, it is still present, on hold, merely waiting for the next big thaw.

But that is pure speculation. Phoenix will not clarify whether the martian permafrost has a life story to tell. It was not designed to answer such questions.

"The greatest result we can find," Smith says, "is that there is a wealth of complex organics associated with this ice, and that would give us the sense that this is the place to go to search for life on Mars. Then you'd probably want wheels, and mobility, and a long-term mission. We don't have any of those things, and we are just taking the first step."

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