David Grinspoon (DG): A lot of our science must be done with assumptions that are more narrow than we can justify from first principles (We recognize this tendency when we bother to think about it). We have to look for what we know how to describe. We look for life based on carbon-in-water because we have good reason to believe it should exist elsewhere, and we can guess at its properties.

But it is important to remain aware at some level that we don't really have a good reason to rule out other chemical systems. We should not convince ourselves, through consensus, that life can only be our own type. We fall back on, "Nobody has thought of another system that works as well." This argument is lousy, because we didn't think up carbon-based life, either. Much of what we know about the biology and evolution of life on Earth comes from "reverse engineering" -- taking it apart and trying to understand what makes it tick (like a crashed flying saucer at Area 51 ;^).

You can't really do a study of all possible routes to chemical complexity in every possible stable planetary environment. So for us to say, "Carbon based life works so well, and we can't think of another way, therefore it must be the only way," is like someone who doesn't know anything about building watches finding a beautiful time-piece and saying, "This is constructed so perfectly. This must be the only way a watch could be made."

I think we'll learn the truth about alternative life forms eventually, by finding one, not by thinking one up. So as we explore the solar system and the galaxy, we should always be on the lookout for strange kinds of chemistry that seem to require radical explanations.

AM: You suggest the key to creating a living world could be a high, steady level of internally driven geologic activity. Why do you think this quality, in particular, could be the greatest promoter of life on a planet?

DG: I'm interested in looking for today's antecedents to universal principles of biology. Until we have such a theory we are always, on some level, just projecting our own likeness into the heavens, which is rather parochial. In the absence of another form of life to study, where should we look for such principles? This draws me toward thermodynamics and complexity theory.

Some likely universals are that life needs an energy supply, a steady supply of nutrient materials, and some level of environmental stability. Planets that stay active for billions of years seem most likely to provide these essential qualities. This is why I think geologically-alive planets are the best candidates for also being biologically alive.

AM: The media is full of stories about the possibility of liquid water (and therefore life) on Mars, so I was surprised when you state, unequivocally, that you think Mars is a dead planet, devoid of any life. You say that life can't barely "hang on" for billions of years - that life either thoroughly infests a planet, or it isn't there at all.

DG: I do think that Mars today is dead. Mars has an atmosphere of carbon dioxide (CO2) and a few trace gasses, but no large quantities of gasses that are grossly out of equilibrium.

This is in contrast with Earth's atmosphere, which is in flagrant violation of chemical equilibrium due to the activity of Earth's global biosphere. From this, and from the striking lack of internally driven geological activity on Mars, I infer that Mars is lifeless today. This is based on some criteria for "living worlds" that I discuss further in "Lonely Planets." It may be up to us, eventually, to bring Mars to life.

But, really, "your guess is as good as mine." Any theory about the criteria for life on a planet is only an educated hunch. We need to keep exploring. That is how we'll find the answers.

Mars could be a better place than Earth to find really early fossils, since the ancient surface of Mars has been much less chewed up by geological activity in the subsequent several billion years. And by exploring Mars, even if we don't find life, or evidence of past life, we'll gain a broader knowledge of planetary evolution. That will make us smarter about what kinds of planets can sustain life.

AM: You talk a lot about the Drake equation, which gives us a general idea of how many alien civilizations could be out there. You say there were heated battles over what were appropriate numbers to plug into the equation. Has the Drake equation ever been estimated based on what we absolutely know at this moment in time? In other words, use the number of stars known to harbor planets (about 5 percent, at present), assume only 1 habitable planet in each solar system, estimate the life span of radio communication civilizations is at least 100 years, etc. That would seem to be a more pessimistic yet practical result than the estimations, for example, of saying radio civilizations last 1 million years, which is not provable at the moment.

DG: If we do the calculation the way you suggest, using only what we know, then the answer we get is "one." There is one civilization in the galaxy that lasts (in the radio sense) for at least 100 years. This is way too pessimistic, because it assumes that absence of evidence is evidence of absence.

We can't yet do a practical calculation, because we just don't know any of the numbers in the Drake equation well enough. We do know the rate of star formation, which is a start. We don't know the number of stars with planets yet. We only have limits based on current searches, which are heavily biased toward the planets that are easily found. For all we know, almost all stars might have planets, or it could be only the roughly 5 percent that reveal themselves to our current still-crude searches. An exciting development that we can count on over the next couple of decades is that the demographics of planets in our galaxy will become known to us. We will nail down this factor in the Drake equation very soon.

People often confuse the "L" in the Drake equation with our own projected longevity, but this is a mistake. It is much more likely that we are not typical. You have to remember that the "L" refers to the average lifetime. Believing in a very long average lifetime for civilizations does not require that you believe that we will last that long, or even that most fledgling civilizations last for very long. If even a small fraction achieve extremely long lifetimes, "cosmological lifetimes," then these large numbers drive up the average. So pessimism or uncertainty about our own longevity is completely compatible with optimism about the prospects for life and intelligence on a cosmic scale.

The real value of the Drake equation is in helping us explore the consequences of different assumptions, within the still-loose constraints of current knowledge. It gives us a framework for our thought experiments. From this we learn, for example, that if civilizations last, on average, for only hundreds of years, we don't have much chance of hearing from another one. But if they last, on average, for millions of years or longer, the galaxy should be noisy with signals.

AM: In 1977, Ohio State University's Big Ear project detected a strong signal from a point within the constellation Sagittarius, and the astronomer on duty wrote, "Wow!" In the margin of the readout. The "Wow!" Signal, which lasted 72 seconds, has never reappeared. You say this signal "is consistent with an alien technological source." Could you expand on that, given we've never received an alien source to compare it to?

All I meant was that it is unexplained, and there is nothing about it that gives it away as not being a real signal from beyond our solar system. The "Wow!" Signal was a very powerful radio source that seemed to be at a great distance, and not in Earth orbit. We don't know what it was. Perhaps it was leakage from some alien radio conversation.

Many searches have been conducted in the same region of the sky, but nothing ever has been picked up since that one enticing pulse. Science can't do much with it if it doesn't repeat, but that doesn't mean it wasn't real.

David H. Grinspoon is a planetary scientist at the Southwest Research Institute in Boulder Colorado. His book Lonely Planets: The Natural Philosophy of Alien Life was published in November 2003.

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