In the year 1900, the French Academy of Sciences offered a prize of 10,000 francs, to be given to the first person to establish communications with beings from another world.

They excluded Mars. That was felt to be too easy, even something of a sure thing. Mars has an axial tilt very similar to that of Earth, and from the beginning of the nineteenth century our telescopes had shown the clear seasonal changes of summer and winter in each hemisphere. Near the end of the century, the astronomer Percival Lowell believed that he had seen great irrigation canals on Mars, used to move water from pole to pole and clear evidence of intelligence at work.

Close examination, particularly from spacecraft in orbit around Mars, showed that the canals were a combination of Lowell's wishful thinking and the tendency of the human eye to see lines connecting patterns of dots. The atmosphere of Mars proved to be very thin, the pressure only one percent of Earth's atmosphere and mostly carbon dioxide and nitrogen. The temperature on the surface never rises above the freezing point of water, and the polar caps are believed to be mostly solid carbon dioxide.

If you had asked me twenty years ago what my opinions were regarding life on Mars, I would have said there is none now, and there never was any. I have changed my mind. Several things have combined to make me feel otherwise. First, and most recent, I spent a good part of the past weekend examining pictures taken by the Mars Global Surveyor, a U. S. spacecraft orbiting that planet and sending back wonderfully detailed images. Many of the surface patterns look so like the channels and flows created by running water, I find it hard to imagine them arising any other way. I cannot buy the suggestion that the same effects might be created by fine erosive dusts, carried on the winds of Mars. There may be no liquid water on Mars today, but there certainly was in the past.

The natural question is, when? Mars must once have been warmer, but until recently the warm period was placed billions of years ago. That would seem to rule out the possibility of life, even deeply buried life, persisting there today.

Now it seems that the cold locking Mars in its grip may not be perpetual. Support for this idea comes from here on Earth, in the form of evidence of huge climatic changes in our own past. Somewhere between 750 million and 550 million years ago, almost the whole of this planet's surface was too cold to support life. The glaciers extended periodically almost down to the equator. And of course, there have been numerous lesser Ice Ages since then, from one of which we emerged only a few tens of thousands of years ago. If Mars is subject to anything like the same fluctuations, we may today merely be observing the planet during one of its cold spells. A thousand or a million years from now, might Mars again have its lakes and rivers?

If Mars became warm enough, there would certainly be plenty of water to flood the surface. Recent estimates suggest enough to provide a planet-wide ocean three hundred feet deep. Also, the mechanism for such warming is clear. Any small increase in temperature will cause the change of some of the surface carbon dioxide from solid to gaseous form. The presence of this in the atmosphere will trigger a further increase, since carbon dioxide is a "greenhouse gas" which very effectively traps heat from the sun. This in turn leads to still more carbon dioxide being released into the atmosphere in what is sometimes called a "runaway greenhouse effect." The result is a warmer planet with a thicker atmosphere, where plant life in particular, which depends on carbon dioxide in the air, may flourish.

All of this is encouraging to life on Mars only if some life is already there, hidden away below ground, protected from temperature extremes and awaiting the return of the warm days when surface life can again thrive.

The evidence that Mars once possessed such life forms, and still may do so, again comes from here on Earth. Unlikely as it sounds, materials expelled by volcanic eruptions do make the long trip between Mars and Earth and arrive here in the form of meteorites. Three factors help. First, because the Martian atmosphere is thin the expelled material is slowed less by air resistance; second, Mars is a smaller planet with a weaker gravity field, so the velocity needed to escape into space is less than that needed to escape from Earth; and third, the trip from Mars to Earth is all "downhill," assisted by the sun's gravity field since Earth is closer to the sun than is Mars.

Such meteorites of Martian origin are rare, but they are found and identified from their composition. In August 1996, a team of NASA scientists made an astonishing announcement. A meteorite found in Antarctica not only possessed a chemical composition strongly supporting the idea that it had originated on Mars, but it also contained within it characteristic patterns typical of single-celled life. This proposal was, to put it mildly, highly controversial, and a great argument followed. It still continues. Are there other ways in which the patterns observed within the interior of the meteorite may have arisen, other than as the fossil evidence of ancient life?

I don't know, but the past thirty years have provided abundant evidence of life's tenacity and its power to flourish even in the most hostile and extreme environments. We have lichens clinging to bare rock in high mountains, surviving fierce ultraviolet radiation from the sun and drawing sustenance from the thin air and the rock itself. We have discovered life that can withstand temperatures above the boiling point of water, gathered around deep-sea thermal vents and deriving energy not from sunlight, but from chemical processes. Life thrives in the lightless depths of the deep oceans, six miles down, where the pressure is more than six tons per square inch, and the only source of food is provided by a thin rain of organic material coming from the surface.

More and more, the question seems to be, where can life not survive? My bet is that when we, or more likely our robot spacecraft, take a look below the frigid surface of Mars, we will find living one-celled organisms, hanging in there and waiting for the good old days to roll around again.

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