Reposted from:
http://acrosstheuniverse.blogs.nytimes.com/2007/03/25/is-life-elsewhere/

Is Life Elsewhere?

Of all the questions facing mankind, perhaps the most fascinating is concerned with life beyond the Earth. Does it exist — if so, where? — and what will it be like? So far we do not have the answers, and some people still maintain that we are alone in the universe. Since there are around a hundred thousand million stars in our galaxy, many of which are attended by planets, this seems unlikely, but we cannot be sure because we have no positive proof of life anywhere except on Earth. However, I have the feeling that at least one of these fundamental points may soon be cleared up.

We may be virtually certain that many planets moving around other stars are similar to Earth in size and mass, and that many of their parent stars are of a solar type; after all, our Sun is a very ordinary star. It follows that in the galaxy there are vast numbers of planets that are suitable for life of the kind we can understand — that is to say, carbon-based life. (I do not propose here to discuss truly alien life-forms; that would lead into totally uncharted waters!) The vital question, then, is this: will life automatically appear upon a world where conditions are tolerable, and will it then develop as far as its environment permits? If we could find just one world that does support life, we would be entitled to claim that life must be widespread all over the universe. And to me, it seems that the clue may be given to us by our near neighbor, the red planet Mars.



Only a few decades ago, it was still believed that the dark areas on Mars were due to vegetation; Percival Lowell's brilliant, canal-building Martian engineers had been banished to the realm of science fiction, but Mars was not dismissed as a sterile planet. We now know that there is no vegetation and probably no surface life at all, mainly because of radiation hazards, but there is still a strong possibility that primitive life-forms exist, perhaps in underground lakes. New missions have been planned, and there is every hope that in the near future we will find out whether any living organisms survive there.



My point is this. If we find any life, no matter how primitive, it will prove that life will appear wherever it can. (I suppose there is a vague chance that organisms were blasted away from Earth and deposited on Mars, but this really does seem too unlikely to be taken seriously.) If we find that Mars today is sterile, we will still not have a definite answer to our fundamental question, but Mars may well be atypical of planets in general. I have an open mind; I hope we will find some sign of Martian life, and I admit that I will be disappointed if we do not. We must wait and see.