Scientists are scrambling to find an alternative landing site for a long-armed robot set to launch this summer on a mission to dig into Mars' icy north pole to search for signs of primitive life.
The original landing spot was nixed after images beamed back by the eagle-eyed Mars Reconnaissance Orbiter unexpectedly showed scores of bus-sized boulders littered over old crater rims on flat plains.
The gigantic rocks pose a danger to NASA's Phoenix Mars lander, which unlike the rolling twin rovers, will be stationary, mission principal investigator Peter Smith of the University of Arizona said during a news teleconference Thursday.
Scientists scouring images of the Martian arctic have narrowed options down to three possible candidates for where the spacecraft can safely touch down. They have until March to choose a destination.
The three sites are clustered around the north pole, which is believed to have a huge amount of ice just below the surface. A site dubbed Green Valley is located within a shallow valley and looks the most secure, Smith said.
"This is the first mission to actually reach down and get a handful of icy soil and analyze it," Smith said.
The Phoenix lander, scheduled to launch in August and reach the Red Planet in 2008, is the first mission of the unmanned Mars Scout program, a low-cost effort to study the Red Planet. The project's cost is capped at $386 million, but difficulties in finding a landing site and other issues have led to an extra $31 million in overrun costs.
The probe, built by Lockheed Martin Corp., has a robotic arm that can drill trenches as deep as 3 feet to search for water-ice beneath the surface. The spacecraft will test the chemistry of the soil and ice. Scientists hope it will yield clues to the geologic history of water and determine whether microbes existed in the ice.
The Phoenix mission rose from the ashes of a previous failed Mars mission. The lander was built to fly as part of the 2001 Mars Surveyor program.
The program was scrapped after the mysterious disappearance of the Mars Polar Lander in 1999. An investigative board determined the Polar Lander prematurely shut off its engine during a landing attempt near the planet's south pole, causing it to tumble about 130 feet to its death.