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.
Since then, however, biologists have found an increasing number of simple life forms on Earth in places with the same cold and dry conditions found on Mars. And geological evidence from two US orbiters currently circling the planet - Mars Global Observer and Mars Odyssey - suggest that water or ice deposits may lie just below the Martian surface in some regions.
Beagle 2 will dig deeper than the Viking experiments. It will drill cores up to 1.5 meters (about 5 feet) deep, where the sterilizing radiation from the sun doesn't reach. Samples will be gasified in several small onboard ovens, and the effluent passed through a device that analyzes the gas's chemical composition. In addition, Beagle 2 will sample the atmosphere in its vicinity.
"Some have suggested that if we find methane, even in minute traces" at the surface, "this would be a telltale sign of existing life," Chicarro says. "Methane is stable at the surface for only about four days" before it combines with oxygen to form other compounds. Thus it would need to be constantly replenished to be detected over longer periods of time.
From the baked core samples, researchers hope to tease out the ratio of two forms of carbon: carbon 12 and carbon 13. Because life appears to prefer the lighter carbon 12, researchers will be looking to see if the ratio heavily favors the lighter form.
Meanwhile, the Mars Express orbiter will be circling the planet, probing to depths of up to five kilometers with a radar designed to detect water or ice deep beneath the surface. It will be taking a variety of atmospheric measurements as well.
Meanwhile, two of NASA's newest Martian SUVs are slated to land Jan. 4 and Jan. 25 in locations that, at first blush, suggest water once sat for long periods on the surface. Gusev Crater boasts a 900-kilometer long valley that appears to slice through the crater rim. This suggests to some researchers that the channel may have been cut by water that eventually filled the crater.
If so, it may host sediment layers that could bear geochemical evidence of water bound to them.
The second target is a plain called Meridiani Planum, which is covered with the mineral hematite. On Earth, hematite is often linked to water.
Thus, researchers are interested in trying to determine the source of the deposits, which cover the plain - one of the flattest places on Mars.
Perhaps as intriguing as what researchers hope to find is what they discover serendipitously when their robotic surrogates get there.
Already, images coming back from Mars Global Surveyor and Mars Odyssey are turning long-held views of Mars on their heads.
"Almost everyone is talking about the Mars we've known since Mariner 9," says Ken Edgett, a scientist at Malin Space Science Systems in San Diego. And that, he adds, is "a different planet from the one we're now discovering."