The Mars Lander's To-Do List

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AP

The Phoenix lander on the arctic plains of Mars

You're not very likely to find a penguin on Mars — or a seal or a puffin or a polar bear either. But that doesn't mean there aren't interesting things going on in the planet's polar regions. Life requires water, after all, and water — at least in the form of ice — is found in abundance at the poles. That's why the Mars Phoenix lander is en route to pay a call there, with a first ever touchdown in the Martian Arctic set for this Sunday at 7:53 p.m. (EDT).

The Mars Phoenix is just the latest in a small fleet of relatively inexpensive spacecraft (a few hundred million dollars apiece, which is tag-sale prices by spaceship standards) NASA has launched toward Mars in the last dozen years. The Mars Express and Mars Odyssey orbiters have done the true yeoman's work, extensively mapping the planet from high overhead. The Sojourner, Spirit and Opportunity rovers have made the headlines, toddling around in the soil of the Red Planet and sending back portfolios of pictures. But it's Phoenix that could make the most thrilling discoveries.

Mars, as scientists now know, was once a very wet planet, running with rivers and teeming with oceans and seas much like the Earth. But its low gravity and thin atmosphere allowed most of that water to vanish into space. What was left retreated into the subsoil or, significantly, contracted into the poles. Phoenix, a stationary lander in the style of the old Viking ships that touched down on the planet in 1976, will get a chance to dig into that frozen polar rind.

Unlike the rovers, which relied on almost comical but remarkably reliable airbags to bounce down on the surface, Phoenix will use the braking-rocket and foot-pad technique pioneered by the Viking and lunar Surveyor probes. Once on the surface, it will deploy a suite of scientific instruments to study the terrain around it: stereoscopic cameras, microscopic imagers, electrochemistry analyzers, meteorological sensors. Most dramatically, it will also unstow and flex a powerful, 8-ft. (2.35-m) robotic arm, equipped with a camera of its own.

The arm is fitted with a movable scoop and what NASA calls "ripper tines," sharp teeth able to chew through a concrete-like permafrost a lot tougher than the powdery soil found at lower Martian latitudes. The scoop will be able to dig about 19 in. deep (.5 m), or about the depth at which NASA scientists believe the ice meets the soil. It will then transfer what it gouges out to the spacecraft itself, where the onboard science lab will examine it for organic materials, biochemical processes and other signs of life.

As with all its Mars landers, NASA is keeping the expectations for Phoenix's lifespan low. If the ship operates for just a few months before the punishing climate of the Martian poles kills it, the mission will still be considered a success. Like the other Mars landers before it, however, this one is designed with a much longer stay in mind. The Spirit and Opportunity rovers bounced down on Mars in 2004 with a similar lowball goal of just 90 days of operational life. Yet despite arthritic wheels and joints and the need for periodic power-conserving naps, both of them roll on. Phoenix, resting on the ice on its three static pads, won't roll anywhere. But what it learns just standing in one place could push planetary science further than it's ever gone before.