Invasion from Earth

Friday, scientists are set to unleash a robotic "hound," dubbed Beagle 2, from its mother ship to hunt a tiny piece of Mars for the geochemical scent of past - and perhaps present - life.

Beagle 2's release from its mother ship - the Mars Express orbiter - will represent a milestone in an unprecedented international exploration of the red planet over the next month and a half.

If all goes as planned, the European Space Agency lander is set to touch down Dec. 25 to begin a six-month probe of the landscape around it. With its miniature chemistry and geology lab, Beagle 2 is the first craft since the US Viking missions in the mid-'70s to look for evidence of life.

Then in January, two US rovers the size of golf carts are scheduled to land and begin their respective three-month geological excursions to help read the history of Mars' climate.

That history is written in the chemical makeup of the planet's rocks and minerals, exposed like layers of a cake along many of its valley and crater walls. And it's key to answering a question that has captured the imagination of earthlings for nearly a century: Could Mars have hosted life, even if only in its simplest forms?

"Mars is something magic on both sides of the Atlantic," says Agustin Chicarro, the project scientist for the Mars Express/Beagle 2 mission. The lander was named after the ship that carried Darwin on his voyage of exploration.

Mars also can be an unforgiving destination. On Dec. 9, the Japanese announced that its Nozomi spacecraft, which was due to move into an orbit above Mars' equator, would be left in its current orbit around the sun.

Launched in 1998, Nozomi was supposed to conduct joint experiments with Mars Express, which will move into orbit around the Martian poles after it lets Beagle 2 loose. But Japan's Institute of Space and Astronomical Science lost contact with Nozomi after a solar flare fried some of its circuits. Efforts to restore communications failed.

ESA has selected a basin just above the Martian equator for Beagle 2's landing spot. It's something of a geological crossroads, Dr. Chicarro explains. Volcanic formations lie to the west, while geologically older terrain lies to the south. The basin itself may have been a shallow lake at one time, and therefore may contain sedimentary rocks. The object that created the basin on impact would have spread enough debris across the countryside to give scientists a veritable smorgasbord of rocks from which to select for Beagle 2's investigations.

But while geology may be the focus for planetary scientists, much of the world will be holding its breath to see if the 32-kilogram (71-pound) lander's kit of coring and grinding tools, chemistry lab, and other sensors can detect evidence of current or past life.

That prospect seemed less likely after the Viking missions. The two landers found no evidence of life in cultures they tried to nurture from soil samples from the Martian surface. But they searched in only two sites, and their samples came from the first few millimeters of soil - well within the sterilizing range of the sun's ultraviolet radiation.

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