Космологија

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The ridge and hills are near the geographic center of the Atacama, 40,000 square miles of brutal wasteland in northern Chile. It is the driest spot in the driest part of the driest desert on the planet. "There are lots of places that look as dry as this," McKay says, gesturing across the barren expanse. "But the stories people there tell, 'This is a place where it never rains,' are hyperbolic. People exaggerate."

Except when it comes to this part of the Atacama. It is the only place on Earth that is, as far as anyone has been able to tell, devoid of life. The most inhospitable environments - boiling undersea thermal vents, acidic hot springs, superbriny seas, even pools of nuclear waste - all, amazingly, harbor some living thing. But not here. No one knows why. That's why McKay's team has come: This killing turf, this parched soil, is Earth's best proxy for Mars.

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McKay has spent his career at the most Mars-like places on Earth. In pursuit of the toughest organisms known, called extremophiles, he has scuba dived in the ice-crusted lakes of Antarctica's dry valleys, climbed in Mexico to the world's highest tree line, and searched the wastes of Ellesmere Island, the planet's northernmost landmass. He once spent seven days driving through roadless Mongolian desert; the chief of the village Ehin Gol welcomed the team with a banquet. The scientists spent the next few days vomiting.

But of all the places McKay has been only the Atacama matches the utter desolation of Mars. Even the soil chemistry is, so far, indistinguishable. Like many scientists, he suspects that Mars may have harbored life once. Then it fell over a climatic cliff to extinction - just as it did here, in the former mining outpost of Yungay. "We don't know what the limiting factor is," McKay explains. "Is it a lack of one or more nutrients? Or water? Or is it something in the soil that kills off anything that tries to grow there? We don't know."

What scientists do know, or suspect, about Mars is that the air is thinner than at Earth's highest mountaintops. The temperatures are colder than Antarctica, and water in liquid form is nonexistent. But signs of long-ago riverbeds and shorelines suggest that the planet may once have been more habitable, with lakes and streams and rainfall. The air must have been thicker and the climate warmer. It may have been nearly Earth-like.

So finding what's left of life, if anything, in the Atacama may help scientists to know what to look for, and how to look for it, on Mars. It costs hundreds of millions of dollars to light the candle under missions like Spirit, Opportunity, and others to come; tens of billions, probably, if human beings are sitting at the top of the rocket, as President Bush has promised they will. So the Atacama is a kind of proving ground - not for astronauts, but for the experiments Mars-bound missions will carry. Every bit of science McKay's team does has to be simple, reliable, and durable, because it's all a dress rehearsal.

And the stage is Yungay base camp, an abandoned research station left over from a desert agriculture experiment that ran out of funding. The simple cinderblock-and-wood buildings house a spartan but effective biology lab, concrete-floored rooms where some of the team beds down - others sleep in tents, or under the stars - and a bathroom with a cold-water shower. The lab is a tiny bastion of high technology, packed with centrifuges, DNA analysis machines, racks of petri dishes and chemical reagents, and a wide assortment of beakers, flasks, vials, and test tubes. Laptops are everywhere. There's also a kitchen, one of the rare luxuries of this site, with a gas stove, a couple of plastic coolers, a sink, a table, and a single light bulb hanging from the ceiling. A generator outside hums constantly.

A few years ago, McKay, who works out of NASA's Ames Research Center in Mountain View, California, set up an ultrasensitive weather station here. After 24 months, the gauge for rainfall showed an absolute flat line. "My first inclination was, 'Well, we've got a broken sensor, we've got to go out and fix it,'" he says. So on his next trip he measured a liter of water into a bottle, carried it out to the sensor, and poured it over the detector - his own little artificial rain experiment. "I looked at the data, and there it was, a liter of water recorded, all sensors working fine. The conductivity shot up, the soil humidity shot up. Then it began to sink in for me: It really did go two years here without rain."

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