A succession of visiting spacecraft have left little doubt:
Mars is a frozen ball of arid desert, its thin atmosphere made up almost entirely from unbreathable carbon dioxide. It seems to be no place for living things. But the Red Planet used to be different. Billions of years ago, water flowed on a much warmer surface. Life may have come to Mars, possibly even before it arose on the Earth. And as we know from Earth, life is tenacious. So despite the evidence so far, this apparently sterile planet might still be alive.

Until the first planetary probes of the space age relayed their observations, many astronomers were sure that life thrived on Mars. The planet's seasonal color changes were put down to the summer growth of vegetation, and there were more arguments about the nature of Martian plants than about their existence.

But the Mariner flybys in 1971 and the Viking landers in 1976 changed all that. Mariner 9 revealed Mars to be cold and arid, scorched by ultraviolet radiation—and sterile. Experiments carried out by the Vikings appeared to confirm the lifelessness of the Martian soil: The Red Planet was almost as desolate as the Moon.

But as the data was assessed, scientists came to believe that Mars was once a very different place. All over the planet were gullies and eroded channels—signs that water had once flowed. Strewn boulders on the Martian plains resembled flood debris on Earth. And the shape and structure of some of the Martian craters suggested that incoming meteorites may have struck wet ground— a Martian swamp, or perhaps even a shallow ocean.

For liquid water to have existed on Mars' surface, the planet must have been much warmer than it is today, and its atmosphere much thicker. Four billion years ago, just 500 million years after the formation of the solar system, the Martian environment could have been just right for the evolution of life. Conditions on Mars would certainly have been better than they were on Earth at the | same time: Mars is much smaller than the Earth, and almost 50 million miles farther from the Sun, so it would have cooled more quickly from its original molten state.

Early Martian life, if it existed, may have resembled the living things that emerged on Earth a few hundred million years later. In warm, shallow water, organisms akin to blue-green algae on Earth would have appeared. Gradually they would have spread across Mars' surface, finding a foothold wherever it was moist and sunlit.

Again, as on Earth, these organisms would have begun the slow process of altering the atmosphere from a blanket of carbon dioxide to the oxygen-rich mixture that would fuel more advanced life forms. But there the two planets' biological histories diverged. On Earth, the algae dominated for eons before more advanced types of life evolved: It took more than 3 billion years from the dawn of life to the arrival of the first complex animals.

On Mars, though, complexity never had the time to evolve. Faster than life could compensate, the planet began to chill. Much of its atmosphere was lost to space, and as the core cooled, geological activity came to an end. One by one, the great volcanoes whose outgassing might have replenished the thinning air sputtered into extinction. The age of liquid water came to an end.

Probably, any Martian life ended with it. But it could not all have died at once. Forever seeking warmth and wetness, Martian organisms would have retreated into the deep valleys where a little water still remained,or into hot springs around the dying volcanoes. The battle for survival would have been fought on a microscopic scale, but it was still an epic—and life may not have lost.

Against the relentless transformation of the planet's environment, life had only one weapon. But it was a powerful one: the ability to adapt. The volcanoes died slowly—some were still active less than a billion years ago—and Martian organisms may have had a chance to carve themselves a new niche. Certainly, their descendants would have been bred for toughness. Deep inside the planet, in a few favored locations, they might still be hanging on.