LOS ANGELES - Like a miner prospecting for gold, NASA hopes its latest robot to Mars hits pay dirt when it lands Sunday near the planet's north pole to conduct a 90-day digging mission.

The three-legged Phoenix Mars Lander fitted with a backhoe arm is zeroing in on the unexplored arctic region where a reservoir of ice is believed to lie beneath Mars' surface.

Phoenix lacks the tools to detect signs of alien life - either now or in the past. However, it will study whether the ice ever melted and look for traces of organic compounds in the permafrost to determine whether life could have emerged at the site.

Before this robotic geologist can excavate soil, it must survive a nail-biting fall through Mars' atmosphere. Despite the rousing success of NASA's twin Mars rovers, which landed in 2004, more than half of the world's attempts to land on Mars have failed.

NASA's last attempt at a soft touchdown ended in disaster. In 1999, the Mars Polar Lander was angling for the south pole when it prematurely shut off its engine and tumbled to its death.

"There's a lot of excitement, but there's also some nervousness," conceded Ed Sedivy, program manager at Lockheed Martin Corp., which built Phoenix.

Launched last summer from Cape Canaveral, Fla., Phoenix has traveled 422 million miles for the touchdown.

The spacecraft's main tool is an 8-foot aluminum-and-titanium robotic arm capable of digging trenches 2 feet deep. Once ice is exposed - believed to be anywhere from a few inches to a foot deep - the lander will use a powered drill bit at the end of the arm to break it up.

"It'll be a construction zone," said mission coleader Ray Arvidson of Washington University in St. Louis. He predicts the ice will be "as hard as a sidewalk."

The excavated soil and ice bits will be brought aboard Phoenix's science lab. It will be baked in miniature ovens and the vapors analyzed for organic compounds, the building blocks of life.

The last time NASA did tests for organics it was on a hunt for extraterrestrial life in 1976 with the twin Viking spacecraft. No conclusive signs of life were found.

On this mission, Phoenix also will probe whether the underground ice ever melted during a time when Mars was warmer and wetter. If Phoenix finds salt or sand deposits, it might be evidence of past flowing water.

Phoenix's landing target - a shallow valley in the high northern latitudes comparable to Greenland or northern Alaska on Earth - was chosen because if organic compounds existed, they're more likely to have been preserved in ice.

"Just as in your kitchen you preserve your food in the freezer, so the planet preserves organic materials and the history of life . . . inside of the ice," said principal scientist Peter Smith of the University of Arizona, Tucson.

On Sunday, Phoenix will punch through the Martian atmosphere at 12,000 m.p.h. Seconds before touchdown, it will fire its thrusters for what scientists hope will be a soft landing.

If all goes well, ground controllers expect to get a signal at 7:53 p.m. Philadelphia time.

The tasks of the Phoenix,

a Q&A and mission details: http://go.philly.com/scienceEndText