PASADENA, Calif. - NASA's Phoenix Mars Lander spent its first full day on 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, chief investigator Peter Smith of the University of Arizona, Tucson, said yesterday.

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 that should not affect the ability to unstow the arm, said Barry Goldstein, project manager at NASA's Jet Propulsion Laboratory.

Ground controllers will not know how Phoenix fared on the first day until later, when an orbiting Mars probe passes over the landing site and relays the data to Earth. The soonest that engineers would move Phoenix's 8-foot-long arm will be today, but it will be one more 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 no more than a foot below the surface.

Mission co-scientist Ray Arvidson of Washington University in St. Louis is pleased with Phoenix's progress so far. "Like a union worker, it went right to work," he said.

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.

Launched last summer, Phoenix sailed through 422 million miles of space in 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 m.p.h. thump.

The lander executed the maneuver almost flawlessly. Two hours after touchdown, Phoenix beamed back a flood of images revealing the first peek of Mars' polar horizon. It also sent back images of its unfolded heat shield and another of its foot planted in soil next to pebble-size rocks.

NASA yesterday 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 had 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."

The Lexus of Space Labs

Japan is ready

to roll out the Lexus of space station labs, a whopper in size and sophistication.

The $1 billion Kibo

laboratory - which means "hope" in Japanese - is poised for a launch Saturday aboard the space shuttle Discovery.

It will be the biggest

and most elaborate room at the International Space Station, a 37-foot-long scientific workshop, as large as a school bus.

The station

will have its own hatch to the outside for experiments and a pair of robot arms. Making it even bigger will be a closet and porch.

Seven astronauts,

including one from Japan,

will deliver the lab, along with the larger of the two robot arms.

A separate

storage room loaded with Kibo equipment went up in March. The porch for outdoor experiments, along with the smaller robot arm, will fly next year.

- Associated Press