LOS ANGELES - Like a miner prospecting for gold, NASA hopes its latest robot to Mars hits pay dirt when it lands Sunday near the red 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 thought to lie beneath the Martian 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 the soil, it must first survive a nail-biting plunge through the Martian 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 the planet have failed.

"It's kind of like first-day jitters," said Ed Sedivy, program manager at Lockheed Martin Corp., which built Phoenix. "There's a lot of excitement, but there's also some nervousness."

Launched last summer from Cape Canaveral, Fla., Phoenix has traveled 422 million miles for Sunday's 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 -- thought 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 co-leader 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 then be brought aboard Phoenix's science lab. It will be baked in miniature ovens and the vapors analyzed for organic compounds, the chemical 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 will also 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 broad 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. Researchers do not expect to find water in its liquid form at the site because it's too cold.