We earthlings have long fantasized, feared and hoped that we're not alone in the universe. Yet somehow, our dreams of alien life only seem to feature the UFO-flying variety of creature. In "Life Everywhere: The Maverick Science of Astrobiology," astronomer (and author of about 40 science books) David Darling contends that "life" encompasses more than E.T. and the green-skinned go-go girls of "Star Trek." Bacterial life-forms from other planets have the potential to profoundly affect our understanding of the cosmos, as well as ourselves. Darling expertly explores the accomplishments and goals of this young, controversial science and looks with great optimism to the possibility of discovering life on Mars, on the moons of Jupiter and even on planets outside our solar system.
Darling spoke to Salon from his home in Brainard, Minn.
Can you explain what exactly astrobiology is and when it emerged?
Astrobiology is an attempt to include all of "life," whatever it may consist of and wherever it may be. It's looking for more general principles of life and is a truly universal science in the same way that physics and astronomy are. Astrobiology has become the standard name for the science, though an older name, exobiology, goes back to 1960. That term was invented by Joshua Lederberg, a geneticist, and had quite a bit to do with the early discussions of extraterrestrial life. It traces back to the late 1950s when the space age started and we were thinking of putting probes on Mars. Then, the issue was contamination -- to avoid bringing bugs back to Earth or to avoid contaminating Mars. That focused scientific attention on what might be out there.
In your book, you write, "It's beginning to seem more and more as if there's nothing special about what took place here on earth." What do you mean?
That statement is based on a number of lines of evidence. There's evidence that life on Earth goes back at least 3.8 billion years, which suggests that it got started 4 billion years ago at the latest. At that time, earth was a very hostile and unfriendly place and was being heavily bombarded from space. The speed with which life arose suggests that life may be "easy" or that there's nothing peculiar about the environment needed for life to arise. Also, the basic building blocks of life as we understand them -- liquid water, some source of energy and organic material -- seem to be increasingly common in the universe as a whole. This circumstantial evidence is raising the optimism that perhaps there was nothing special about how life got started here and that it could start in other places.
This has brought the definition of "life" into question. Why is evolution seen as such a key factor in determining what is alive?
It's the one characteristic that seems to separate life as we know it, and life as we can imagine it, from nonlife. In school they give you a laundry list of things that are supposed to tell you what's alive and what isn't. The problem with those sorts of lists is that too many things fall through the cracks; for example, crystals grow and there are a lot of individuals that can't reproduce for one reason or another. You need something more basic and that basic thing appears to be evolution in the Darwinian sense: You have a population of competing individuals with different genetic makeups and the fittest survive. Of course, it's a difficult thing to detect; you can't land on a planet and detect evolution. But if you've got evolution going on, then you must have a population of self-reproducing individuals and to reproduce you must have a metabolism and so on. It leads to things you can measure.
You mentioned that life may begin more easily than we thought. How has astrobiology changed our ideas about the conditions life requires to get started?
It used to be thought -- and this is going back to Darwin's time -- that life started at the surface of the ocean or lakes in a warm, sunny environment. That made the earth seem special because that kind of environment is rare, but now it seems that life may also have got started underground or along the bottom of the ocean alongside these hydrothermal vents. Those sorts of conditions are going to be fairly typical of planets in the solar systems.
We also know that there's a cosmic connection with life now. All planets get bombarded by comets and asteroids because they're just leftover junk from when planets are made. These things carry organic material. You find amino acids -- one of the building blocks of proteins -- inside certain meteorites. They've been found floating free in interstellar clouds so we know there's basic organic matter out there, and we know there's a delivery system that brings it to newly formed worlds.
What is interplanetary exchange?
That could be important locally, within the solar system. Whether it might be important between different star systems is a more difficult question. We know that rocks have arrived here from Mars. We're got about 17 of them at the moment. Almost certainly over millions of years there have been many tons of rocks that arrived here from Mars. The same thing will have happened the other way: Rocks from Earth will have traveled to Mars and elsewhere.
We also know that bacteria and bacterial spores can survive almost indefinitely in a dormant state for millions of years. If it's shielded by a thin layer of rock -- in other words you've got some sort of a microorganism embedded within the meteorite -- it would be protected from radiation and that sort of thing.
How do the rocks travel?
They get kicked off by collisions with asteroids. A large asteroid might come at a glancing angle and splash off all kinds of material into space. Then it drifts around and eventually a bunch of it might collide with places like Earth. It's a vehicle for getting stuff from one place to another. The evidence for life in some of these Martian meteorites -- which is very controversial at the moment -- is interesting evidence and hasn't gone away over the past five years. In fact, you could say it's actually strengthened in some ways.
Why was this controversial?
The original announcement was back in 1996 and it consisted of several lines of evidence. First of all, some scientists didn't like the way that NASA announced it at a press conference before actually publishing it in a scientific journal. That was a no-no. Then the actual evidence seemed a bit shaky and still does, except for one piece. That piece of evidence consists of magnetite crystals, chains of tiny crystals. The particular shape and character of these is only known to be formed by organisms on earth. Bacteria produced them as a kind of compass and the actual characteristics in these crystals are not known to be formed any other way than by bacterial action. Most scientists are pretty convinced this magnetite actually came from Mars. We don't know of any other way that these things can be produced other than by biology.
It represents the best piece of evidence we're got for life elsewhere. The critics say we simply don't know because we haven't been looking that long. They say that maybe there are ways that these things could form chemically.
Do astrobiologists feel that these organisms on other planets would be similar to the organisms on earth, or do they believe that they could be entirely different?
Because you only have one sample of life to go on, you have to be broad-minded. Astrobiologists are quite open to the idea that life could consist of virtually anything at this stage. That's not really the issue. It's easy to imagine life of all different kinds. Of course, people have been doing that for years in science fiction and also in science too.
There are two reasons to think that this life would be similar to earth life rather than different. The first is that carbon is the basis of life on earth. You can imagine life based on a different element, say silicon, because it's quite similar to carbon in its basic chemistry. But the problems come about when you try working out the details of how biochemistry would work based on silicon -- you start running into all kinds of issues. The second reason is that if you're trying to find life elsewhere, the first kind of life you're going to look for is the life you know, simply because you can build detectors and things that will search for signs of life as you know it. If that life is based on something completely different, we don't know how to look for it, unless we stumble on it by accident, and that's not really the best way to conduct a serious investigation.
So when you say "life," you mean life at a bacterial stage only. We're not talking about green men here.
If you're talking about life in the universe, it could be the whole spectrum from the simplest to something more advanced. But certainly within the solar system, the most that we would expect to find would be microbial life. Life in general, the bulk of it, will be microbial life, simply because microbial life is the commonest kind of life around here on earth. It's been around a lot longer than we have. It's always the first thing to develop and it will survive even as more advanced life forms evolve. Then, of course, there's the controversy of how often simple life becomes advanced.
Generally, how do astrobiologists feel about the possibility of complex life existing out there?
The issue is really about the "rare earth hypothesis." A book written last year called "Rare Earth: Why Complex Life Is Uncommon in the Universe" by Peter Douglas Ward and Donald Brownlee brought together a lot of ideas that were kicking around. The basic idea is that microbial life is very common, that the steps leading to complex life are difficult and that they happen to have happened here on Earth. However, they believe that the chain of coincidences that happened here on Earth is unlikely to have happened very often elsewhere. In talking to astrobiologists, some of them expressed a certain sympathy for that idea, but the majority said we have to be more open-minded than that. There's no reason to suppose we have to have an exact duplicate of the earth's history to produce complex life.
Do religious agendas drive any of these hypotheses?
During the writing of this book, purely by accident, I came across a religious pamphlet that was given to my wife by a pastor of a church. It was basically promoting the idea that life was unique to the earth; the idea that Earth is divinely created and we're special. I noticed that one of the co-authors was Guillermo Gonzalez. He's been writing this creationist material on the one hand, and on the other hand, he's a colleague of Peter Ward and Donald Brownlee who, by the way, were not in any way affected by religious belief in their writing of "Rare Earth." However, they had been in conversation with Gonzalez for a couple of years.
I contacted Peter Ward and asked how much Gonzalez influenced him in the writing of the book. He replied, "He's been a major influence about the importance of some features of the earth that are unique to Earth and that we believe are important in the rise of complex life." I then said to him, "Did you know that Gonzalez writes extensively as a Christian apologist, defending the view of intelligent design?" And he said, "No, I had no idea of this. Are you sure?" Then he wrote to Gonzalez and asked for an explanation and Gonzalez said he wasn't making any apologies for the fact that his religious beliefs affect his science and vice versa. He has a grant from the Templeton Foundation to study intelligent design theory.
By the way, I'm not knocking him as a scientist; he's a very competent astronomer and has produced some good work. A lot of his work is not directly relevant to this whole business. And I have no problem with creationist people, but the fact is that it is influencing the public perception of where astrobiologists stand. "Rare Earth" sold over 100,000 copies, which is extraordinary for a science book. My concern is that if the public is persuaded that there isn't very much of interest out in space other than bugs, then it might eventually filter through and stifle the very means by which the question can be decided. I don't make a habit of writing about gossip and private exchanges, but on this occasion I felt that it was important and relevant.
Currently, it seems that Europa -- a moon of Jupiter -- has the best shot of having life. Why?
Europa is thought to have a major ocean underneath the surface. The search for life is first a search for liquid water because it's something we can't imagine life without. That's not to say that you couldn't have life based on another liquid or something, but we don't know yet how that would work. As soon as we find signs of liquid water somewhere else, astrobiologists get very excited because it provides the right kind of basic environment. Europa is thought to have an extensive ocean of probably salty water beneath its icy surface. Some of the other moons of Jupiter are thought to have the same thing -- like Callisto and Ganymede. The evidence seems very, very strong that it does have a liquid layer under there. If it's been there long enough, then you have to ask what the chances are that life has evolved there. It's got an internal source of heat because that's actually what's melting the ice in the first place. It will have some organic material there, which will have been delivered by comets.
What do we need technologically to send there?
The first thing we need is to prove that there is an ocean. That involves putting an orbiter around it. That will probably be 2008. The next stage will be to put something down on the surface to sample whatever it can. After that you have to get into the ocean somehow. The idea there is to develop some kind of pencil-shaped probe with a heated tip, which will melt its way down through the ice. When it gets through, it would release a minisub, which would look around beneath the ice. That's very advanced technology. Fortunately, we've got an underground lake in Antarctica that is thousands of feet deep beneath the ice, the size of Lake Huron, and is isolated from the rest of the world and has been for thousands of years. We can test instruments there. But that's not going to happen until 2020 or something like that.
Is that when we can look forward to knowing more?
In the case of Mars, it's an ongoing process. What astrobiologists are realizing is that they're not going to discover life overnight, unless, for example, you put this probe into the ocean and there are big whalelike things swimming around. There's an ongoing program of Martian exploration. A probe is there now and there's one on its way that will go into orbit in November and which will be looking for signs of water just beneath the surface. There's actually a lander going there in 2003 that could find traces of life -- that's Beagle 2 -- that will sniff the atmosphere and look beneath the surface for signs of past and present life.
What are the conditions on Mars that make people believe there's a good chance of finding life there?
For one thing, it was a lot friendlier in the past. If you go back to when Mars was young, it actually had lakes and possibly an ocean. It had liquid water then, so all of the conditions were right for life to develop. If it did develop, could it survive? Well, it couldn't survive on the surface -- we're pretty sure of that -- there's very little atmosphere and the sun's bombarding it. The question is: Does it exist in some kind of refuge, possibly deep underground where we suspect that there still may be aquifers of liquid water and, again, a source of internal heat that could sustain underground microbes? There's certainly reason to believe that there could be life there still if there was life there in the past. It's a question of where you look. Is it going to be reasonably beneath the surface where you can shovel some soil out of the way or do you have to go a mile underground? You could need a manned expedition to do that. So Mars is going to be difficult technologically.
What about the other planets in the solar system -- have they been ruled out?
Half of them are gas giants, and unless there is some kind of life that can exist in the atmosphere of these things -- which has been suggested -- then that's a long shot. There is the idea -- the deep, hot biosphere theory -- that life can exist or is going to be common deep underground on planets. If that's the case, then even a place like Venus could possibly have life deep underground. Again, that's a long shot. Possibly there could be some kind of pre-biology on comets, particularly comets that come close to the sun. Some of the ice might melt and you might even get liquid water. We know that they contain organic matter, so it's a question of how complex that chemistry can get. Then, you look beyond the solar system -- which I think is even more interesting -- the planets of other stars.
You keep making that distinction. Why is that important?
We know that there are other planets out there. The first ones were found in 1995 around sunlike stars, 60-odd of them. All of these planets are gas giants, simply because they're the ones you're going to find first, the biggest are the easiest to detect. But there's no reason to suppose that as the instruments get more sensitive that we're not going to find smaller planets. The object is to find the smaller, Earth-like planets in the right location. Again, it's going for what you know.
If we found an Earth-like planet around another star, that would be an ideal place to look for life. Then we have to be clever in analyzing the light coming from these plants and look for traces of gas in the atmosphere, which would indicate life processes on the surface, for example, molecular oxygen. You can't go looking for molecular oxygen because there are ways to produce it without life but there are combinations of gases, and things like chlorophyll. If you found these on other planets, that would be pretty good evidence. In looking at extrasolar planets, you've got many more to look at -- by 2020 we might have thousands to look at. That's why there's so much optimism that in two decades we might have pretty firm evidence of extraterrestrial life.
What would we have to do to study those extrasolar planets?
The present instruments will find big planets, especially in reasonably close orbits. What you're looking for is the effect that the planet has on the parent star. You can't actually see the planets but you can see the way that they make the parent star wobble around. To detect Earth-like planets, you need much more powerful instruments. You need arrays of telescopes; for example, four large telescopes spread out along a beam. Then you can combine their measurements. The best place to put these is in orbit over the earth's atmosphere. There is a variety of this type of project planned over the next 10 to 20 years. One of them is called a "Terrestrial Planet Finder" -- the planned launch date is 2011 -- which will be an array of four eight-meter telescopes capable of detecting Earth-like planets in Earth-like locations around stars similar to the sun within a few tens of light-years.
What do astrobiologists argue that life in space can tell us about life on Earth?
There could be a chance that we're related to the Martians, given the exchange of rocks. If we do find a second independent instance of life, then we can learn what aspects of Earth's history were one-of-a-kind accidents and probably wouldn't happen elsewhere, and which were due to the working out of general principles of life. That's one important thing -- in terms of a universal biology, it helps to fit ourselves into a broader scheme of things. If you then discover advanced and intelligent life, then it has far more implications. We'd have a second example of a culture to compare with, and that would have ramifications in terms of our philosophies and religions.
What about the future of our planet? Will astrobiology help predict anything about that?
I don't know really. It depends on what we find and where we find it. It's difficult to date a remote planet -- you could get some idea of the age of it from the age of the star and you can measure the ages of stars pretty well. Let's say we look at a range of sunlike stars at a distance of, say, 100 light-years and these are all different ages ranging from a billion years to 7 billion years. If we found life around all of them -- that would indicate that there're no time restrictions. But say you found a cutoff at the 5 billion year point or 4.6 billion year point, which is the age of the sun, then maybe you could say that there's something that restricts life in terms of how long it can occupy a planet. You'd need a lot of statistics to reach that kind of conclusion, but in the long run you probably could.