From illusory “canals” spied through blurry 19th century telescopes, to today’s high-endurance robotic rovers, in the search for life beyond Earth, Mars is the perennial favorite target. It is, after all, the most hospitable planet we know other than our own.
But despite the Red Planet’s watery, warm ancient past and more than a century of Earthlings’ increasingly sophisticated scrutiny, no clear evidence of Martian life has ever been found. At least, that’s the mainstream scientific consensus. But according to a new book by astrobiologist Dirk Schulze-Makuch and science writer David Darling, we’ve had good evidence of microbial life on Mars since NASA’s Viking missions in the late 1970s. Now, they argue, all that’s needed to prove beyond a reasonable doubt that we are not alone is another ambitious mission to Mars—one that, like Viking, carries a life-detection experiment.
Seed editor Lee Billings spoke with Schulze-Makuch about the evidence for life on Mars, the limits of certainty, and other promising places for biology in the solar system.
Seed: Why did you write this book?
Dirk Schulze-Makuch: The evidence for life on Mars has recently been portrayed over-skeptically and negatively. As a scientist you have to be skeptical, but here we have many strong lines of evidence for microbial life, and if you put them all together you really have a very consistent picture. So my coauthor, David Darling, and I both feel very strongly that we really have to get the public very enthusiastic about this, in order for space agencies to move forward.
Seed: But scientists also make names for themselves by overturning flawed conclusions. If the evidence is so good, why aren’t more researchers lining up to say there’s life on Mars?
DS: Well, it depends on what kind of group you’re looking at. If you ask the public, they seem to think there’s life on Mars—at least microbial life. I don’t want to speak for all scientists, of course, but I think among those who are knowledgeable there’s also a tendency toward thinking life is there as well. This is especially true for those researchers who study extremophiles, Earthly microorganisms that flourish in extreme environments. Some planetary scientists are still quite skeptical.
Seed: Your argument seems to hinge, in large part, on the results from the Viking landers. Could you summarize why these results point to life?
DS: In some ways the timing was bad for Viking. A lot of progress was made after its life-detection experiments were already on or on their way to Mars: The discovery of all the ecosystems at undersea hydrothermal vents, and the extremophile research of the early 1980s really changed how we think about life and its limitations. The Viking researchers thought life on Mars would be heterotrophic, feeding off abundant organic compounds distributed everywhere all over the Martian surface. That picture was wrong, and studies of extremophiles on Earth have made us think differently about Mars. Some people say Viking tried to do too much, too early, and as a result of its ambiguous results, nothing has happened with Martian life-detection experiments ever since.
For the Viking results, the devil is in the details. There were three life-detection experiments: the Labeled Release Experiment that yielded a positive result, the Gas Exchange Experiment that gave a negative result, and the Pyrolytic Release Experiment, which was gave ambiguous, inconclusive results. Viking’s Gas Chromatograph Mass Spectrometer (GC-MS), designed to detect organic matter, was made the appeals judge so to speak—and since it did not detect any organic matter, it was concluded at that time that Viking did not detect life.
However, the results of the GC-MS were always somewhat odd. This is because we know from the Martian meteorites that there are organics on Mars. Also, it’s been shown that the same instrument could not detect organics in the Dry Valleys of Antarctica or from hydrothermal soil, places on Earth where we know that a small but significant population of microbes makes a living. So the question is, why did the GC-MS not detect the organics present on Mars? Was the concentration too low? Were they in a form that was not detectable? Or, were they all oxidized to carbon dioxide before they could be measured as organics?
We think the latter because an extra amount of carbon dioxide was exactly what the GC-MS detected. This would be nicely explained by a hypothesis I put forward along with my colleague Joop Houtkooper, which posits that Martian organisms could use a mixture of hydrogen peroxide and water as intracellular fluids rather than water alone. When heated during the GC-MS experiment, the hydrogen peroxide would have become unstable and oxidized all the organic compounds, releasing carbon dioxide.
Such a hydrogen peroxide–water mixture would be a perfect adaptation mechanism for Martian organisms, because it would also convey antifreeze properties—down to – 56°C—and hygroscopicity, which is the ability to attract water molecules directly from the atmosphere, like honey or sugar does. That would be a huge advantage for any life on a very dry desert world such as Mars. And it would also explain the results of the Gas Exchange experiment and the Pyrolytic Release experiment. They were conducted with too much water. If you are adapted to the little water that is present on Mars, too much water will overwhelm you. It’s as if an alien race were to notice that one of us is dying of thirst, and then try to help us by placing us in the middle of the Pacific Ocean. We would drown. In a sense this may be what Viking did to Martian microbes. I think it is particularly telling that one of the Pyrolytic Release experiments conducted under dry conditions showed highly significant organic synthesis rates consistent with microbial life, while another one conducted under wetted conditions showed lower synthesis rates than even the sterilized control did.
Seed: Out of all missions that have gone to Mars—orbiters, landers, rovers—would you say that Viking has been the most ambitious?
DS: There’s no question about that in my mind. Of course Viking was the most ambitious. It was too ambitious, in some ways, because at that point we didn’t know the environment of Mars well at all. But that was the kind of bold approach everyone likes to see. Everything since then has been so incremental. We now have much better technologies, and a much better understanding of the Martian environment, but we still haven’t had a life-detection experiment since Viking! The time is ripe for another mission with a strong life-detection component. The European Space Agency’s next Mars mission, ExoMars, has that, though unfortunately I’ve heard from a couple of people involved that a lot of that technology has been cut out due to budget considerations.
Seed: What would constitute bulletproof evidence for Martian life? Aren’t you worried no evidence would be good enough to settle the debate?
DS: Today, we do really have a lot of evidence pointing in the direction of life on Mars. I think it’s actually more scientifically outrageous to think that Mars is and always has been sterile.
But I’m only half-joking when I say no one will believe until we have found life on Mars until we have a Martian microbe under a microscope that is waving back to us! But even then, there are potential difficulties. One thing I’m a bit concerned about is, what if we find life on Mars and it’s shown to be related to life on Earth? That’s not an unlikely possibility. People will argue that it’s just contamination we brought with us, or from earlier space probes that crashed there. And justifiably so: After the Viking landers particularly, but even before that, the planet probably was contaminated by material from Earth. So perhaps an Earthly microorganism survived there, and was spread by the winds or somehow managed to get underground to be protected from the harsh elements.
Still, some properties would give a good argument for a Martian organism. For example, all our biological molecules have a certain “handedness,” a left- or right-handed orientation to their structures. So if the molecules in the organisms from Mars have a different handedness than the molecules from Earth life, that would be pretty good proof.
Seed: The rest of the book is about other possible sites for life in the solar system. What should be prioritized in our explorations?
DS: Mars should be top priority. But if we find life on Mars, we won’t necessarily know whether it’s from a separate origin. Mars, Earth, and Venus are all very close to each other, and we think organisms can survive meteorite impacts and be transported from one place to another. Titan is a lower priority than Mars, since it is much, much harder to get there, but for finding life that is almost certainly of independent origin, Titan should be the top priority. If we understand organic chemistry correctly and the reactions behind it, it seems reasonable to think there should be life there. Even if we don’t find life there, we can still see how far organic chemistry can evolve in its prebiotic phase. Titan is a natural laboratory for that.
After that there are lots of places, notably the subsurface oceans in Europa, Ganymede, and some of the other moons in the outer solar system. There are even chances in those places for life that could be multicellular, more complex. But we know so very little about these places; these oceans could be acidic or toxic. It could be that there’s nothing whatsoever there, and they’re just dead. There’s even a reasonable chance, I’d say, though less than 50 percent, that there could be life in the lower atmosphere of Venus. Looking for it wouldn’t even be tough to do. NASA already tried to do it in the 1980s with a Pioneer mission, but the relevant experiment jammed and no data was returned. We should go back.
Seed: What’s the biggest problem facing the search for life elsewhere?
DS: The biggest thing is that we don’t yet understand the origin of life on Earth. Rather, we understand the persistence of life in habitable environments on this planet. There are tons of potential habitable environments elsewhere in our own solar system, and we know that life originated on Earth and spread nearly everywhere. Now if you’re on another planet, or in Europa’s subsurface ocean, okay, if you once had life there, you quite likely still do, and it’s managed to survive and to spread like on Earth. But the conditions for the origins of life are probably much more constrained than the conditions for life’s persistence. This is really the big million-dollar-question that no one has yet answered, and it connects to another problem, which is how much our views of life are shaped by our experience on Earth. It’s hard to see other possibilities, other forms life can have, what other options, avenues, and paths, life could take elsewhere. I think as we discover more and more strange planets and moons, in our solar system and beyond, most scientists will realize that it’s very important to look at these other possibilities, so that we’re somewhat prepared for what else might be out there.
Front image: Roel van der Hoorn/NASA/JPL
Originally published April 20, 2010