Mars probe lands successfully, prepares for digging mission

PASADENA, Calif. -- After successfully landing on the planet's surface, NASA's Phoenix Mars Lander spent its first full day in the Martian arctic plains checking its instruments in preparation for an ambitious digging mission to study whether the site could have once been habitable.

Sol 1, as the days are known on Mars, was a busy time for the three-legged lander, which set down Sunday in relatively flat terrain cut by polygon-shaped fissures. The geometric cracks are likely caused by the repeated freezing and thawing of buried ice.

"We've only looked at one tiny little slit" of the landing site, principal investigator Peter Smith of the University of Arizona, Tucson said Monday.

Phoenix planned to take more views of its surroundings to help scientists zero in on a digging site and also take images of its onboard instruments, including its trench-digging robotic arm.

Early indications show the protective cover around the arm did not unwrap all the way after landing, but it should not affect the ability to unstow the arm, said Barry Goldstein, project manager at NASA's Jet Propulsion Laboratory.

The earliest engineers would move Phoenix's 8-foot-long arm will be today, but it'll be another week before the lander takes the first scoop of soil.

After the initial taste test, Phoenix will spend the rest of the mission clawing through layers of soil to reach ice that is believed to be buried inches to a foot below the surface.

Scientists were especially interested in how the polygon patterns in the ground formed at Phoenix's landing site. The fractures look similar to those found on Earth's polar regions. Arvidson said Phoenix appeared within reach of a shallow trough that could be a potential place to dig.

"I was just afraid that it'll be so flat and homogenous and that we'd be digging in soil and we wouldn't know the context" of how it formed, Arvidson said.

Launched last summer, Phoenix sailed through 422 million miles of space over a period of about 10 months. The riskiest part of the journey came seven minutes before landing, when Phoenix, operating on autopilot, had to use the atmosphere's friction, deploy its parachute and fire its dozen thrusters to slow to a 5 mph thump.

The lander executed the maneuver almost flawlessly. The only snag came when it released the parachute seven seconds later than expected. The late timing caused the spacecraft to land slightly down range from its bull's-eye mark.

Two hours after touchdown, Phoenix beamed back a flood of images revealing the first-ever peek of the polar horizon. It also sent back images of its unfolded heat shield and another of its foot planted in soil next to pebble-sized rocks.

Smith, the chief scientist, said Phoenix slid a bit after landing.

NASA on Monday released a grainy black-and-white image captured by its Mars Reconnaissance Orbiter, which had a bird's-eye view of Phoenix coming down on its parachute. The parachute appeared as a white speck connected to Phoenix, which looked like a dot.

Alfred McEwen of the University of Arizona's Lunar and Planetary Laboratory said the camera aboard Reconnaissance Orbiter has taken many unique pictures of Mars, but

"this one's really unique."

"It's the first time any camera has imaged an actual descent through an atmosphere of another planet," said McEwen, who operates the orbiter's camera. "This will be on my Top Ten list."

Phoenix joins rovers Spirit and Opportunity on the Martian surface. During its primary three-month mission, Phoenix will study whether the ice melted in the past at the landing site and probe the soil for evidence of organic compounds. It cannot directly detect fossils or living organisms.

The $420 million mission is led by University of Arizona and managed by JPL. Unlike the twin rovers, which have been operating near the Martian equator since 2004, Phoenix has a limited lifetime. Winter will set in later this year at its landing site and likely will cover the lander with frost.

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Associated Press writer Arthur H. Rotstein in Tucson, Ariz., contributed to this report.

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On the Net:

Phoenix Mars: http://phoenix.lpl.arizona.edu