Interview with David Grinspoon

July 10th, 2005

CI: Science was in your blood, so how early was your path decided?

DG: I was exposed to a lot of science and space exploration growing up, so I had an idea by the sixth grade. I remember my first “What do you want to be when you grow up?” answer that went beyond fireman or policeman—

CI: —Those are always good answers.

DG: Yes, and they’re what every little boy wants. When I was in sixth grade, I read a biography of Louis Agassis and decided that I wanted to be an oceanographer. Somehow that morphed into space science. I was obsessed with science fiction, starting in fifth grade with Isaac Asimov’s juvenile science fiction and going right through junior high and high school. The excitement of space exploration was a formative experience. For my generation, the first Moon landings were real, new, and futuristic. All that was going on as I was forming my identity. In high school I decided that I was going to be a nuclear physicist.

CI: You must have thought that was a better way to meet girls.

DG: [Laughs] Exactly, I thought nuclear physics would be a great way to meet women. It’s probably a better way to meet women now than it was then. But no, it was because I wanted to help perfect nuclear fusion as an energy source and thereby save the world. I went to college thinking I was going to do that, and in my freshman year I was bored by my physics classes, but excited by a couple of classes in planetary science. I still remember those professors, what the classes were called, all the specifics.

It was an exciting time—I started college in the fall of 1977, when the Viking landings on Mars were brand new. I got a job my freshman year as an undergraduate research assistant working for the head of the Viking Lander Imaging Team, the camera team, which gave me a chance to work with the first images of the surface of another planet. That was cool, and I was getting paid to do it. I was really lucky, and by that point I was hooked. When I finished college it was natural to apply to graduate school and continue in planetary science.

CI: Where did you go to grad school?

DG: The University of Arizona. My advisor was John Lewis. Then I did an NRC fellowship at NASA Ames, and Jim Pollock was my advisor. He was very influential. I’ve had great mentors all the way through, including Carl Sagan, with whom I worked sometimes during the summer.

CI: Planetary science is pretty much geology and physics and atmospheres, and life is plastered onto our one planet. When did people start thinking about the complex interplay between the biospheres and geology, and the fact that it could happen to more than one planet? When did planetary science connect with astrobiology?

DG: Some people were thinking about it all along. Exobiology existed all through the space age, in fact the term “exobiology” was coined by Joshua Liederburg in a Science paper around 1960. As soon as we started sending spacecraft to other planets, people were thinking about exobiology and planetary protection and the need to explore carefully. There was a fringe of planetary science that was interested in exobiology.

CI: But it was disreputable for quite a while.

DG: That’s right. I had the good fortune of being influenced by some of those disreputable people, one of whom was Sagan. I knew him growing up as a family friend and then I worked in his lab as an undergraduate. He urged me to take organic chemistry and biochemistry courses even though they weren’t part of a standard planetary science curriculum. Likewise, Jim Pollock was interested in astrobiology, but he was more cautious than Sagan. Chris McKay was also doing astrobiology long before it was called “astrobiology.” This core of people were talking about it for a long time, but it wasn’t renamed astrobiology and made respectable and mainstream until the late 1990s. It was interesting to see that transition.

CI: I sense an iconoclast streak in you—maybe the disreputability was not a disadvantage.

DG: Part of it was being around influential people. Sagan and a few of his associates made it seem respectable, even though they received a certain amount of ridicule from the community. It wasn’t something you could do as your main line of work back then. You could get away with doing exobiology as long as you did something else as your “serious science.” If you were observing infrared spectra of Mars you could also speculate about microbes on Mars, but you couldn’t get funding for the microbe speculation alone the way you can now. You needed something else for your bread and butter. So I was influenced by some iconoclastic people. I do have an iconoclastic streak, and I would also connect it back to my path—getting into serious science through science fiction, and this part of me that was fascinated with space science because of the intriguing possibility of extraterrestrial life.

CI: And how amazing that you could do that as your day job.

DG: It seemed too good to be true. I wrote my Ph.D. thesis in 1988, definitely before astrobiology happened, but when I read through it now, it seems like a work of astrobiology. The subject of my thesis was large-impact events and atmospheric evolution on the terrestrial planets. Large impacts were this new, hot thing in the wake of the Alvarez Hypothesis in the eighties. People hadn’t considered the effect on atmospheres, so I did some modeling of impact-generated dust clouds and how they would affect early climate.

There are chapters in my thesis on the nuts-and-bolts research, but I also had an introduction and conclusion where I gave myself license to wax more eloquently about what it all meant. Those sections are all about the possibility of life elsewhere, and how important these investigations are to constrain that. I even mention the Fermi Paradox—I was able to get away with that. Reading it now, I realize that many people, myself included, were thinking about these issues for a long time. It’s just that we didn’t have the freedom to overtly frame what we were doing as “astrobiology.”

CI: What happened to let the field burst forth in the late nineties?

DG: I attribute it to four things, the most important being the discovery of apparent signs of microfossils and other hints of life in the Martian meteorite ALH 84001, which made huge headlines all around the world, led to a presidential press conference and led Dan Golden, the NASA administrator at that time, to conclude that people are really interested in this—it gets people excited about NASA’s research. It went from something we weren’t supposed to talk about—because it could lead to ridicule and negatively affect public support for our work—to something that tapped into latent public interest and could fuel excitement for our work.

CI: In another way it was a no-brainer for Golden, because he was also faced with an aging, thirty-year-old Space Shuttle and the International Space Station that was huge pork.

DG: Exactly. That was the watershed event, but it was in the background. In the same time frame, three other things were going on. One was that the Galileo spacecraft was in orbit around Jupiter. The notion that Europa might have an ocean and the other agreed-upon requirements for habitability went from exotic speculation to almost a sure thing. That was based on the images Galileo was returning, as well as on the evidence, both from the surface geology and the magnetic field signature, that there was liquid water under the ice. Europa became habitable in the scientific mind.

At the same time, extrasolar planets were discovered and that field ramped up rapidly. This one term in the Drake equation that had been taken on faith—or at least faith supported by theory but not by evidence—suddenly became concrete. Namely, that there are lots of planets out there. Most of us thought there were, but it’s different when you know there are.

The fourth thing was the discovery of terrestrial extremophiles, which was going on in the eighties but accelerated in the nineties. People started to connect the dots between that and possible extreme environments on other planets.

CI: The coupling of the second and fourth items—liquid water on Europa and a broader sense of extremophiles and their environments—rewrote the book on habitable zones, as well.

DG: Absolutely. The surprise of the Jovian system—not just Europa, but the surprising activity throughout that system—made us realize that if we actually go out and explore, rather than just relying on theory, our assumptions might be wrong. There could be a lot of activity in realms of the Solar System—and therefore the galaxy and universe at large—that previously weren’t considered remotely habitable. In general, the idea that there’s more going on out there than we’re aware of is encouraging for those of us who like to speculate about life elsewhere.

CI: If you look at the field now, after that surge over the last decade, there’s a sense that there are still amazing things on the horizon. What kind of discoveries are plausible in the next decade that would be exciting, but might not surprise you?

DG: There is a continuing trend that, when we explore new places up close, we find surprising diversity and activity. It happened in the Jovian system in the late nineties with the Galileo mission, and it’s happening now with the Saturn system and the Cassini mission. Before we actually imaged the surface of Titan and visited it with a probe, we had a whole range of possible Titans in our mind, and it turned out to be way on one end of that range—a lot of activity, a young surface, dynamic meteorology. There’s a lot going on on Titan. It’s a surprisingly dynamic place. Recently we’ve discovered, even more surprisingly, that Enceladus is an active world that seems to have anomalous heat sources and recent, probably even ongoing activity—apparently a plume of water shooting off into space. We don’t know what the energy source is, but there’s a parallel with the Jovian system.

Places where there’s lots going on—where there’s sustained activity for cosmically significant periods of time, energy sources, liquid reservoirs, and interesting geochemistry—are better candidates for life. Finding surprising activity and unknown energy sources is encouraging for astrobiology in a general sense, whether you think any particular location has life or not. I think that trend will continue; we are getting close to completing the preliminary reconnaissance of the Solar System, so there are fewer places left to completely surprise us. There’s a spacecraft heading for Pluto and all bets are off on how active that system might be. There is some tidal interaction with its moon, but we don’t have a full understanding of sources of activity even on relatively small icy worlds, so I’m not going to make any predictions about Pluto other than to bet we’ll be surprised.

On the horizon, we are going to start finding the more Earth-like extrasolar planets very soon. We are on the threshold—certainly with the Kepler mission, if not through other means—of finding lots of terrestrial planets.

CI: You almost feel sorry for these extrasolar planet hunters because now that so many have been discovered, they can’t make headlines unless they break a record.

DG: The bar keeps getting raised. Headlines appear when they find the system that’s most like our own—they found one with a near-Jupiter and a near-Saturn, next they’ll find one with a near-Jupiter, a near-Saturn, a near-Uranus, and a near-Neptune. You can expect a continuing string of headlines as they find systems more like ours in some way.

To me, the quest isn’t simply to find places that are exact mirrors of our own Solar System. Nonetheless, finding Earth-sized planets will be compelling because that can help stimulate our imaginations. We only know about two of them; surely there are many of them out there, and we are going to start to know something about their atmospheric compositions. Maybe we’ll get lucky and find some planet that seems unambiguously to have signs of life—for example, if we found an Earth-sized planet that clearly had water and some bizarre, anomalous atmospheric brew like Earth’s. That wouldn’t prove there was life, but I think it would shift the burden of proof around the other way, to come up with a way it could not be inhabited and still have such a weird atmosphere.

CI: It would certainly be a fundamental change in the nature of science—from counting planets and very simple properties like mass, to saying something about their environments. On that same near horizon, where would you place the likelihood of showing that Mars had or has life?

DG: I think it’s mostly dead. I regard that as unproven; I would love to be wrong, but my view is that if a world is mostly dead, it’s probably all dead. Looking at Earth as the only example we have, I see life as a phenomenon that thoroughly infests a planet and becomes inculcated in every pore and every realm of that planet—in a sense, transforms it.

CI: This is a paradigm shift—instead of thinking of life as painted on a surface, it forms a biosphere. Life is integrated into the geology and the atmospheric physics and so on.

DG: It’s related to the Gaia Hypothesis, which was misunderstood and caught a bad rap—people said some silly things, like “The Earth is alive.”

CI: New Agers picked it up, too.

DG: Obviously the Earth is not a living organism. It hasn’t evolved like a living organism, it can’t reproduce, it doesn’t share these basic qualities with living organisms; nonetheless, it has some interesting properties—there are important elements that cycle through the living and non-living world. Life has fundamentally altered the conditions of the planet. Certainly the planet and life have affected each other and co-evolved in a way that’s very integral to the physical functioning of the Earth, including the realms of the Earth that are not obviously “alive,” like the atmosphere itself; even the crust and plate tectonics have been modulated by life over time. In this view, life is not something that happens on an otherwise dead world—it’s something that a planet takes on, that a planet becomes.

CI: As far as the Earth goes, some of these connections and cycles have negative feedback and are self-regulating, while others have positive feedback and run rampant. It seems very hard to predict how this might play out even on a planet with similar conditions.

DG: That’s absolutely true. I think a planet cannot be barely alive. If this whole relationship is true, planets will be either flagrantly alive or dead; therefore, because Mars is not fragrantly alive, excuse me, flagrantly alive—

CI: [Laughs] It might be fragrantly alive.

DG: Yes, with its methane. But I think life on other planets will be both flagrant and fragrant—its presence in atmospheres will be obvious. There are some exceptions to this. When life started on Earth, it was probably very fragile. But it quickly reaches a state where it takes over a planet—as it has the Earth—and becomes a stable entity that can last for billions of years, through catastrophes and planetary changes like those the Earth has suffered.

However, it can’t hang on in a barely existing state, in isolated pockets on a planet that otherwise dies out—as has been postulated for Mars—because you don’t have the reinforcing structure of these global cycles. Those cycles provide a robustness for life throughout global changes of the kind that clearly happened on Mars—and probably wiped out life on Mars, if it ever existed.

CI: The corollary of that speculation is that, for those difficult experiments with extrasolar Earths, the signatures might not be that subtle. If it bifurcates into places that are essentially dead and those that are magnificently alive, maybe it’s not going to be as hard as we think.

DG: Right. Again, I regard this as a hypothesis, as any idea about life and planets has to be. You can take any qualities of Earth you like and assume they’re universal, and you could be wrong. From looking at life on Earth, this is what I’ve concluded, but I want to proceed with humility and put in the caveat that, until we have other inhabited worlds to study, this is just an idea to test. But if it’s true, then there may well be a bifurcation into planets that are either obviously alive or dead. In that case it may not be so hard once we start getting spectroscopic data. Another caveat is: what about planets with life that exists wholly underground and has no contact with the atmosphere? This has been proposed on Europa.

CI: Calculations have suggested that most of Earth’s biomass is quite deep.

DG: True, but I would argue that on Earth, even if most of the biomass is underground, the whole biosphere is in contact with the surface over long time scales, influencing and being influenced by the atmosphere. Even life at the bottom of the ocean, seemingly not at all dependent on sunlight, exists in an ocean whose entire chemistry has been altered by the oxygen-producing qualities of green plants. So there may be some subtle ways in which Earth’s whole biosphere is at least altered by, if not dependent on, the Sun. It’s hard to imagine underground life on Earth not having any signature in the atmosphere.

People speculate about underground life on Mars, and there are different ways to interpret the methane that has been observed. There could be isolated pockets of life there, we’ll see. I’m not convinced, although I think it’s the most intriguing evidence that's come along in a long time that bears on the possibility of life on Mars. We need to understand more about its sources and isotopes; there’s more observation to be done. It does demonstrate that it’s very hard to hide a biosphere: studies have deduced that this methane is biogenic—they may or may not be right, but they show that if there is underground life on a planet like Mars, its signature will be in the atmosphere. I’m not sure about biospheres that exist completely underground and don’t affect their planet’s atmosphere.

CI: Maybe one of the lessons to go with the idea that biospheres can be very emphatic in their presence is that there is no smoking gun—the data will be hard to get, because you need a lot of biomarkers. If you just had methane or a little ozone in an extrasolar Earth, it probably wouldn’t be enough. You might need a set of tracers to be sure.

DG: Maybe. The methane on Mars is ten parts per billion, and the fact that we haven’t seen it until now—even though we’ve been taking infrared spectra of Mars for decades—tells you something about the subtlety of the signal. If you were looking at Earth the same way we have been looking at Mars, you would have seen the flagrant disequilibrium in the atmosphere decades ago. If Mars was loaded, even if it had a few parts per million methane, it would require a huge flux. Robust biospheres can produce huge fluxes of chemicals that otherwise shouldn’t be there. There might be a level of methane or something else on a planet like Mars that wasn’t quite a smoking gun, but was highly indicative of some non-equilibrium process that is very hard to explain without life. We don't know enough yet to say what the magic number or the threshold amount is, but we’ll know if we’re way above it.

CI: So, if I hold your feet to the fire, is Mars dead or nearly dead?

DG: My prediction is that it’s dead.

CI: You should put money on that, you could become rich.

DG: Or I could lose it all. [Laughs] That's my prediction, because you asked me to make one. But I don’t say that with a hundred percent confidence. Mars is worth exploring; it's a good bet that we could find fossils, because clearly conditions were more pleasant in the past, but we can’t rule out the presence of extant life, either.

CI: The public may focus on whether Mars is alive now, but for astrobiologists the history of biospheres is just as interesting as their existence.

DG: Absolutely, and that opens up a lot of real estate in the Solar System for astrobiology-related exploration. It’s not just Mars, there are many other places that could have supported life in the past. Astrobiology is largely a historical science, like geology—we are interested in reconstructing the past to understand the present. Mars isn’t boring at all if it’s dead, it just creates a different set of scientific problems. And in some ways it solves problems. If we find that Mars has an extant biosphere, extant life, then it raises some ethical issues about what humans ought and ought not to do on Mars that just don't come into play if I’m right and Mars is dead. In some ways it’s going to be much more convenient for us if Mars is dead.

CI: I want to return to something you said almost in passing, but as an outsider it strikes me as very profound, a paradigm shift. You stressed that the dynamism of the system could be the thing that correlates with the likelihood and abundance of life. That's an interesting concept. Maybe you can talk a bit more about dynamic planets.

DG: Part of the reason for this line of thought is that I’m bothered by the fact that we have this whole science built upon assumptions that come down to one data point that supports them, as everything in astrobiology does to some extent. People have written some eloquent papers on why carbon is probably the best way to make life, or why water is probably the best solvent, and there are some good arguments. They may be right that only on water worlds can you have life, but I’m still bothered by the fact that everything we bring to bear has been gained by us, on a world that has been wholly shaped by “carb-aqueous” life, which is the term somebody coined to describe life based on carbon and water. Our knowledge about the potential for organic life came by reverse-engineering life on Earth. Everything that makes us so confident that carbon is the only way, I’m not sure we could have learned it if we had to start from principles and try to invent carbon-based life through physics and chemistry, rather than dissecting the example that has been given to us.

CI: There is some danger that we tell “just so” stories of how Earth got to be this specific way.

DG: That's the fallacy of the “rare Earth” hypothesis. I would like to ask if there are other criteria we can apply to habitability, rather than looking for conditions exactly like our own and a natural history that mirrors ours. That leads me to think of planetary properties as a whole. What is unusual about the Earth, other than the fact that it has this narrow range of temperatures and pressures and chemical constituents that make it friendly for our kind of life? If you compare Earth to all the other bodies in the Solar System, with the possible exception of Io, it's by far the most geologically active world. I don’t think that's a coincidence.

CI: That is one of the “rare Earth” arguments, tectonics-as-driver.

DG: Right. I want to mention complexity theory and the ideas of people like Stuart Kauffman, who talks about life in an abstract sense as a system that uses energy and builds complexity out of flows and gradients of energy and matter, resulting in something that self-replicates, so Darwinian evolution can take over. If you look at that as an abstract idea of what you need—constant flows of energy and nutrients to provide templating building blocks—then you ask, “What kind of environment provides those constant sources of energy that facilitate complexity?” A planet with continuous geologic activity: it provides not only a source of energy, but also a constant renewal of chemical materials. It provides the ultimate physical basis for cyclic geochemical behavior.

In my mind, those ideas—complexity theory, abstracting life from a thermodynamic point of view, and looking at the Earth’s uniqueness as a planetary body with the eternal evolution that facilitates constant cycling of energy and matter—mesh to create this idea of living worlds, where the geologic activity is going to be the most important criteria ultimately, maybe even more than liquid water.

CI: Another aspect of those theoretical arguments is that they’re abstract enough that you could jump right out of the box and say biology isn’t necessary for those conditions—maybe there are other ways to code information.

DG: Then you start getting into semantics: what do you mean by biology? If you’re coding information and producing something that is self-reproducing and evolving, I would argue that it is biology. Maybe it doesn’t have to be chemical, is that what you’re getting at?

CI: What I’m really asking is: given the range of extremophiles on Earth and the interesting and varied chemical environments in the Solar System, how weird could life be?

DG: I don't think we have a good handle on that from the point of view of theory. People have written some interesting papers—Steve Benner came out with a great paper in 2004. It's the best treatment I’ve seen of possible chemical bases for life in other kinds of solvents, perhaps not using carbon. He talks about the possibility of life in non-polar solvents and the possibility of using silane, which is silicon-carbon chains; I think we’re naïve about those possibilities. We’re relatively smart about what carbon can do in water because we’ve had a lot of incentive to study that. But we are fundamentally naïve about the possible range of chemistries that could form complexity in other environments.

At this point in the conversation, people always want me to propose silicon-based life, but my fundamental position is that there may be other chemistries we haven’t thought of and may not be equipped to think of during exploration. Through exploration we find examples of things we never predicted. Planets are somewhat complex entities, and that's why planetary science is only a predictive science. Life is even more complex than planets, so it’s much harder to predict.

CI: But the field is empirical now. We have to find out as much about those strange environments as possible.

DG: There’s a lot we can do even within that ignorance—we can characterize available environments, we can learn more about exotic chemistry. It’s also pragmatic to look for water-based life throughout the universe, because we know how to look for that and we believe we know what some of its signatures might be. I’m not critical of the efforts that are being made, or even of using the Terrestrial Planet Finder to look for water worlds. Clearly we should be looking for life that’s like our own, because it’s one example of life that we know the universe can produce—but I wouldn't be surprised if we find some other kind of life that defies our expectations.

CI: If we had enough funding, we should probably be spreading our bets quite widely.

DG: And since we don't, we should be looking for what is most likely a sure thing. That's why it's reasonable to focus on looking for life that is similar to our own, while making sure to avoid having a false consensus or thinking we know more than we do. We have to keep our blinders as wide open as we can.

CI: I wanted to run through a few places in the Solar System regarding their habitability, especially bearing in mind your comments about dynamism. What about Venus? That's an old favorite of yours. Do we take it off the list as a habitable place?

DG: I don't think so, for two reasons. The first is that there is a somewhat benign environment on Venus. Venus was the first planet we explored in the space age with actual spacecraft, and it was a big disappointment to know the surface was so hostile to our kind of life. The first successful experiment on any spacecraft that went to another planet was a microwave radiometer on Mariner 2 that proved Venus was hot. That was the first thing we learned from planetary exploration—that Venus was hell. It was a big disappointment because some people thought the clouds might be made of water, and perhaps it could be clement on the surface.

But we overreacted. No, there can’t be life as we know it on the surface of Venus, but there is the possibility of life in the clouds of Venus—they’re within the right temperature range for life as we know it, and they are in a continuous dynamic environment, one with a lot of interesting energy sources and a certain amount of chemical equilibrium in the atmosphere that not yet been well explained. It is an aqueous environment, albeit one that is suffused with concentrated sulfuric acid.

CI: What’s the pressure in those clouds?

DG: It ranges from a few bars to about fifty millibars. The clouds of Venus cross through standard temperature and pressure conditions; the temperature range there is pretty much the temperature range we associate with life on Earth, it goes from about zero to a hundred bars. The main problem is that it’s concentrated acid—we don't know if that would be a problem for some other kind of life. We keep finding more and more acid-loving life on Earth. We haven’t yet found something that lives in conditions as acidic as the clouds we find on Venus, but I think when we restrict our imagination to places on other planets where terrestrial extremophiles could live, we are being extremely conservative. It's a reasonable exercise, but life on another planet is not going to be a terrestrial extremophile—it’s going to be something that has adapted to conditions on its own planet. I don't rule out life in the clouds of Venus.

CI: That’s a slight echo of Sagan’s idea that Jupiter could have buoyant creatures floating in its clouds.

DG: Sagan and Salpeter talked about floaters and sinkers; they made up this whole imaginary ecology of Jovian life, and there is definitely an echo of that in this concept. We haven’t explored that realm, nor have we characterized the clouds well. Again, I wouldn't put money on life in the clouds of Venus, but it's a plausible argument—just like the arguments for life on Mars. But if you give any credence at all to the dynamism hypothesis, then Venus has the better chance because there is active volcanism, its atmosphere is much more dynamic chemically, and there is a global sulfur cycle. There are global geochemical cycles—whether they are bio-geochemical cycles is yet to be seen.

I said there were two reasons we shouldn’t discount Venus as a habitable planet. The first was that it has some environments that are more benign than we give Venus credit for. The second is much more far out—that maybe there is some kind of life that could exist on the surface of Venus. It couldn’t be organic-based life that needs water, but we simply don’t understand the chemistry going on down there. It's a fertile environment with supercritical CO₂ at the surface, it’s not really a liquid or a gas. We don't know much about the chemistry of that environment—we certainly observe a lot of weird things on the surface of Venus that don’t have good explanations. I like to reserve some small corner of my mind against completely ruling out something exotic going on there that could be called life, with a completely different chemical basis than our life.

CI: Referencing extremophiles: in terms of temperature and pressure, that's within spitting distance of what you'd find near a hot vent at the sea floor; it’s not an outrageous physical zone.

DG: That's absolutely true, except that it could not be made of polymeric carbon compounds. No matter how well something has adapted or evolved, that type of organism can’t be there. It would have to be a wholly different thing.

CI: You've speculated about Titan and an acetylene cycle—can you talk about that?

DG: It’s fun to think about life on Titan, as an exercise on what the real universal requirements might be. It helps to break out of our normal set of assumptions, which are very Earth-centric and might be wrong. If you poll astrobiologists on the basic requirements for life, there is a broad agreement. Everybody agrees you need an energy source. That's something nobody disputes, no matter what your definition of life is. Most people think you need a liquid medium and some organic chemistry, or at least some polymeric chemistry. Those are the basic three.

In light of the fact that we’ve recently gained knowledge about Titan and its environment, it’s fun to re-examine Titan and see how it holds up against those criteria. Astrobiologists have been interested in Titan for a long time because it’s so organically rich and it has some potential for teaching us about prebiotic chemistry and early Earth. People haven’t talked much about the potential for life there today, mainly because it’s so darn cold and chemistry proceeds very slowly. But in my view, what we’ve learned so far from Cassini and Huygens heightens the possibility of extant life on Titan.

Getting back to dynamism of worlds, we’ve found that Titan is an active world. It’s got a young surface without many craters. It has apparently recent and ongoing endogenic geology, producing cryo-vulcanism and flows of various kinds on the surface; it also has active meteorology, forming rivers and other fluvial forms. Apparently all these things are happening on a short timescale, because we can’t find any craters on most of these features—they’re young, presumably ongoing. Titan is a dynamic place, and as a planetary quality that's encouraging.

More specifically, there are energy sources. You can’t have cryo-vulcanism without something melting material and gushing it out on the surface; that also means there are liquids. There are reservoirs of liquid hydrocarbons, which we think we’ve found on the surface—we’ve seen a lot of evidence for them flowing on the surface, and we’ve seen the clouds. There’s clearly some methane cycle analogous to Earth’s hydrological cycle. So there are liquids—whether you can have life in liquid hydrocarbons is an interesting question.

Going from less exotic life to something that would need liquid methane: since there is cryo-vulcanism, there are reservoirs of liquid water near the surface. It’s probably liquid water-ammonia because ammonia is such good antifreeze, but there is no reason water-ammonia couldn't make a good basis for life. We’ve got hot spots—we see things that look like hot springs, we see these cryo-vulcanic features—so melting water has flowed out of the surface at some time.

If there is life there, is there anything for it to eat? Are there nutrients? Are there energy sources? That's where acetylene comes in. We know that photochemistry is going on in the upper atmosphere of Titan. Methane is being broken up and is reforming into more complex organics, many of which are more dense and would then rain down on the surface. One of these is acetylene, which is something we have looked at a little bit because it’s energy-rich and apparently there’s a lot of it on Titan. We did some simple calculations looking at the back-reaction—reacting acetylene with gaseous hydrogen back into the methane—and it's a highly exothermic reaction, it releases a lot of energy. We know that acetylene is all over the place on the surface of Titan and presumably is mixed into the subsurface, too, because of all the activity turning over the surface. There have got to be places—in hotspots underneath the surface—where this acetylene is in contact with liquid water. So this reaction of acetylene back into methane, which releases a lot of energy, could be some basis for metabolism.

CI: So you don't have to be dependent on the intensity of solar radiation—there are plenty of chemical energy sources, and you can set up chemical networks to harness that energy.

DG: That's right. We are talking in this scheme about solar-powered life on Titan, but then people say, “Wait a minute, there isn’t much sunlight on the surface of Titan.” The sunlight is being harvested in the upper atmosphere by powerful UV photons that split up the methane, and then it’s falling to the surface. So it’s indirectly solar-powered by upper atmosphere photochemistry. The neat thing about it is that it solves this problem of why the methane hasn’t gone away: methane in Titan’s atmosphere has a lifetime of about ten million years against photochemical destruction. People have come up with various other schemes where it could be reconstituting the methane in various reactions with rocks under the surface, somewhat in parallel with arguments about Mars, although there’s a lot more methane on Titan.

CI: You've made Titan sound more alive than Mars. Planetary scientists must be frustrated that it’s going to be at least a decade before we get back there.

DG: We have to get back there. Whether or not you believe in the possibility of life there, Titan has proven—with its one entry probe and the data still coming in from the repeated close passes by the Cassini orbiter—to be one of the most interesting places in the Solar System. It’s alive in the geologic sense, there are things going on there; it’s not the forensic planetology of trying to figure out why something died billions of years ago, which is the case with most of the bodies in the Solar System. We’ve only explored a small part of it and we found an incredible diversity there, so we know when we go back and explore new places, we are going to find new things. We need to go back with mobility and the ability to explore—whether with a rover or some kind of airship remains to be seen. Whatever your opinion of these theories of possible exotic life there, there will be a broad consensus in the planetary science community that it’s a high priority. I don’t know if it will be fifteen or twenty years or less; a lot of that depends on funding and politics, things that are almost as hard to predict as life itself.

CI: To wrap up the potential habitable places, would the iconoclast in you like to make a pitch for Io?

DG: Definitely. Io has a lot going against it if you’re attached to water- and carbon-based compounds, but if you just like continuous energy sources and are willing to consider other liquids, there are levels within Io with liquid reservoirs of sulfur and perhaps sulfur dioxide. Sulfur might be underrated as a basis for life. Sulfur does a lot of weird things. It has many many different phases and allotropes and a lot of strange chemistry that hasn’t been completely characterized. It makes polymers, long chain polymers in some conditions. I’m not willing to rule Io out yet. On a scale ranging from worlds that are pretty darn dead—like our Moon—to worlds like Earth that are obviously alive, I would put Io somewhere in between; where you put it depends on how much weight you give these different qualities. No one knows how much weight to give. I’m arguing for another set of criteria, so we aren’t just looking at one set.

CI: Another aspect of your subject that you’ve thought about a lot is the connection with popular culture, and people’s expectations about what we’ll find when we do astrobiology. Let me start with where you stand on part of the “rare Earth” hypothesis. A lot of scientists probably expect the universe to be littered with microbial life, but believe that advanced forms of life, true to the Drake equation with communication and so on, will be much rarer. Is that what you think?

DG: Everybody who thinks about the problem has to believe that at some level; but that aspect of the “rare Earth” hypothesis is a tautology. It’s like saying that acorns are more common than oak trees. If you regard simple life as an evolutionary step that sometimes evolves into complex life, then it’s only logical that you will have simple life in more places than you will have complex life. To me that is not a profound conclusion. Logic will lead you to believe that simple life will be more widespread than complex life.

CI: And you would have to push all the arguments extremely hard in one direction to say that there are an enormous number of acorns and only one oak tree.

DG: Exactly. There is nothing profound about saying there are more acorns than oak trees—the real question is the ratio of acorns to oak trees. How many acorns fall in the right environments, where they can sprout into something totally different and more magnificent? Nobody knows. My inclination is to believe that advanced life is probably more common than the Rare Earth authors advocate, and part of the reason for that is another timescale argument. I think it’s likely that intelligent life goes through a short period of the kind we’re in now—where it struggles to figure out how to exist as a global entity and is threatened by its own actions, and is not wise enough in a globally-coherent way to protect itself from self-destruction or destruction from external forces. But then, if it hangs around long enough, it will learn enough about the physical universe and itself to achieve wisdom, to the point where it won’t be vulnerable to easy extinction.

There is probably a phase of extreme stability we haven’t reached yet. Once a species or a global biosphere reaches this phase, it becomes immortal—or at least extremely long-lived. If you believe that, then the transition to this state does not have to be frequent, because those super-advanced societies will accummulate. Once they form, they’re stable. When I talk about this, people say, “You are really optimistic, you think humans are definitely going to survive and be around forever.” You don’t have to be optimistic to believe this. You can believe it’s a one-in-a-thousand chance for a species like ours to achieve this state, but some still do, and they accumulate.

CI: I know you’ve made a side study of belief systems. It seems like the general public might not care that there are a lot of acorns out there—which is disappointing to astronomers and astrobiologists and planetary scientists—and some of the public already think there are a lot of oak trees and that we’ve talked to them. With the emerging science of astrobiology and the way we contemplate the universe and its potential for life, where does this disconnect, this strange parallel universe of belief systems, come from?

DG: I tend to agree that if we find microbial life even elsewhere in the Solar System, on Mars or Europa or Enceladus, that scientists will think it’s the greatest discovery ever and the general public will think it’s just kind of cool—there will be some NOVA specials, but it won’t profoundly change anything. Definite evidence of some intelligent or communicative life elsewhere might really change things. It might have a global ripple effect and change the way humans see ourselves and our relationship with the world.

There is a fascination with the idea of aliens—and a certain percentage of the population take for granted that we have already made contact with them. [Laughs] Maybe nothing would change for them, because they already believe it. Nonetheless, if the scientific authorities are saying, “This is real, we’ve actually heard from them, they are out there,” then it might go from whatever it is now—fifty percent of the public believing it—to eighty-five percent of the public, but the thirty-five percent that would be swayed would be the highly educated, scientifically literate public, who are over-represented among those making decisions and running things. It wouldn’t be just the poll numbers, there’s a little more to it than that.

CI: So many astronomers, astrobiologists, and planetary scientists grew up on science fiction, but they didn’t draw those conclusions. They used that fascination to fuel a genuine scientific pursuit of answers to those questions, whereas part of the general public—who may or may not even read science fiction, but see it represented in the pop culture—have bought the whole premise, hook, line, and sinker.

DG: If you poll the scientists who grew up on science fiction and ask them, “Do you think they are out there?“ most of them will say, “Yes, we simply don’t have evidence yet.” The difference is whether there is definitive evidence or not; but, in an odd way, the astrobiology community and the SETI community are in agreement with the UFO community regarding the ultimate question of ”Are we alone?” It’s just that some of us are holding out for evidence and some of us are willing to believe—we all want to believe, it’s just a matter of how much we are swayed by those desires.

CI: It’s like Agent Mulder with his “I Want To Believe” poster.

DG: We are almost all like Agent Mulder. It’s very hard to find scientists who say there is no intelligent life out there. They’re so rare that I expect that most of them enjoy being contrarian and aren’t expressing a deeply-held, logically-derived belief.

CI: I agree with you, I think that desire is widespread. Would you care to make a “dime-store,” Freudian, psychological speculation as to why we don’t want to be alone?

DG: I can’t answer that question without venturing a little bit into the realm of the spiritual, which maybe I should be reluctant to do—“I’m a scientist, dammit, Jim, not a theologian!” [Laughs] I think it arises from a basic, deeply-held desire for connection with the wider cosmos and other sentient creatures. Some might argue that it even arises from a deeply-felt intuition that we are not alone. Maybe there’s something deep inside us that knows that. I won’t go quite that far, but certainly the desire for connection with something bigger than us, longer-lived than us, the desire for connection with the cosmos, is an innate human desire. What’s neat about astrobiology and SETI is that it’s simultaneously a scientific quest and a spiritual quest. We can use scientific methods to go about this search that is also spiritual and has implications that could change what we know about our place in the universe. Even for scientists who wouldn’t really think of themselves in these terms, I think that widespread belief and desire is a spiritual drive we all share.

CI: If it is a matter of other consciousnesses in the universe, that takes Gaia up to a cosmic scale.

DG: Indeed. On some level it’s also a quest for self-knowledge—that may be weird, that creatures that may have evolved on a planet circling some G-type star a hundred light years away could provide us with self-knowledge. But in a sense, if they are out there, they are our relatives. Those theoretical microbes on Mars that share our DNA because of communication between the two planets obviously would be our relatives, but I mean relatives in the deeper sense—that they have evolved in the same universe. They have sprung forth from the same laws of physics, which have allowed whatever processes that happened on their planet to parallel what happened on our planet, and in both their location and our location these physical processes somehow resulted in consciousness and awareness. That’s why I say it’s self-knowledge, because it contextualizes who and what we are and lets us know who our distant relatives are, if they’re out there. Everyone wants to be reunited with their distant relatives, right?

CI: That’s a nice way to think of it, and if we ever get the ability to travel large distances and meet them, that will be the first time our atoms have been in contact since the Big Bang, which is a nice arc to the story as well.

DG: Definitely.

CI: Two more questions before we finish. What kind of music do you play in your band?

DG: I’ve been in bands intermittently since junior high school. I was in a band called Liquid Earth through high school, college, grad school, post-doc, professorship, and research science. It’s always been something I did on the side. What kind of music? It varies. It’s always been rock-based, but when I was in Tucson I went through a big reggae phase. I played in a lot of reggae bands and went through an African music-phase in grad school and post-doc. Now I’m doing stuff that’s influenced by hip-hop, African, funk.

CI: Do some of your—let’s say, more rigid—colleagues look at you squint-eyed because you still do that and obviously enjoy it?

DG: I don’t know. I was more aware of that when I was a post-doc and a young professor. You go through these phases in your scientific career when you’re more insecure and worry what people think about you. I feel that most of my colleagues appreciate that side of me, as I appreciate the things they do that are beyond strictly science. It’s fun to know people who do more than one thing. Maybe I’m naïve and there are people out there who think it’s horrible and evil, but I’m not aware of that.

CI: Were you ever tempted by that life, something so different from your day job?

DG: At various times I was cursed with being a good enough musician that I knew I would have what it took to be successful if I did only that—but I also knew that without that level of dedication, I would get enjoyment but not the success. In the past, not recently, I was tempted to pursue that. I know some professional musicians quite well, including some I really admire, and I’m sometimes jealous of their ability to focus on the music and take it to the higher level that I’ll never be able to reach without that intensity of focus or time commitment. But I’m not jealous of that lifestyle; in fact, I don’t know how they do it. People who are doing well and are well-known have to be on the road all the time, and that’s not something I want to do. The travel I have to do as a research scientist is bad enough sometimes—I’d probably be traveling ten times as much if I was a professional musician. So I’m glad I don’t have to do that.

CI: One last question—what’s your next book?

DG: I’m recovering from the last one, and just starting to conceptualize a new book. I haven’t completely narrowed it down, but I’ve been making notes and brainstorming a bit. I think it’s going to be about evolution, but that’s a huge topic and I don’t want it to be a huge book, so I’m trying to decide how to narrow the focus. It will address some of the current societal debates, while simultaneously trying to put that in a more cosmic perspective.