by Marc Kaufman
Portland State University geobiologist Radu Popa, author of the 2004 book Between Necessity and Probability: Searching for the Definition and Origin of Life, said that he lost count of the proposed answers in the scientific literature after logging at least three hundred. And the definitions keep coming. Nilton Renno, a planetary and atmospheric scientist at the University of Michigan and a member of the Mars Phoenix lander science team, recently came up with this one in a paper on the likelihood that the heat from the spacecraft’s landing created liquid water that remained visible for days: Life, he wrote, is a self-replicating heat engine with a capacity for mutation.
Perhaps the most subversive challenge to the proposed definitions of life comes not from those who think the NASA definition is incorrect, but rather from those who think “life” is not a concept we can or should define. Philosophy professor Carol Cleland, from the University of Colorado, and Chris Chyba, an astronomy student of Carl Sagan’s who now teaches at Princeton University, have argued for almost a decade that current definitions of “life” are little different from medieval definitions of “water,” which was seen then as a clear liquid with certain qualities such as wetness, transparency, tastelessness, odorlessness, and the property of being a very good solvent. We can now chuckle at the misunderstanding, since muddy water is certainly not transparent, salty water has a taste, and marshy water has a smell. Medieval alchemists classified nitric acid and some mixtures of hydrochloric acid as aqua fortis (strong water) and aqua regia (royal water) because they were such good solvents.
But water, as we now know, is H2O—two hydrogen atoms bound to one oxygen atom. Those men and women trying over the centuries to define water knew nothing about the molecules and atoms that we now know make up all matter. That didn’t come until the late eighteenth century, when Antoine Lavoisier came up with the convincing theory that matter is made up of molecules. Cleland, Chyba, and others have argued that the basic knowledge needed to make a definition of “life” is simply absent, rather like how the essential molecular nature of water was unknown during the Middle Ages. Based on her iconoclastic views—grounded in philosophy and at times a challenge to scientists—Cleland was included on a University of Colorado astrobiology team that was twice funded by NASA. Her thinking became more broadly known when she addressed a 2001 meeting called “The Nature of Life,” hosted by the American Association for the Advancement of Science. She told the audience of scientists that the search for a definition of life—something many were involved in—was a waste of time and, even worse, misleading.
“The logic of my argument was impeccable, but people just blew up at me,” recalls Cleland, an expert in the philosophy of science. It was a memorable evening. “They were yelling out their own definitions, saying this is the right definition or that is the right definition. It’s as if they totally missed my point that their approach was mistaken and there is no definition available now. I was kind of shocked and remember saying to myself, ‘These people just can’t hear what I’m saying.’ I’ve learned since then how to better talk with scientists, but I still think the whole definition project is hopeless.”
Ten years later, Cleland and Chyba’s view is no longer outlandish. Addressing a NASA-NSF gathering of many of the nation’s top practitioners of “synthetic biology” (the origins-of-life side of biotechnology), the evolutionary biologist Andrew Ellington, of the University of Texas at Austin, urged NASA to bring together a blue-ribbon panel to study and then throw out the agency’s and all other definitions of life.
“It is my position that there is no such thing as life, and that the working statement in the NASA document does science a disservice by attempting to pretend the contrary,” he told the gathering of in 2008. “‘Life’ is a term better suited for poets (or perhaps philosophers) than scientists, and the continuing attempts to determine whether a given system is alive or not harken back to quite ancient philosophers, with a similar level of resolution. I assert the following existence proof: if we haven’t figured out what life is by now, there is little hope that we will figure out a definitive definition in the near term, and there is no research program that I can imagine, at any price, that will provide such a definition.”
Ellington then made clear why he felt as strongly as he did. As is so often the case in astrobiology, the purely scientific issues are surrounded by deeply felt and highly contentious social and even political issues. “I would further argue that the reason that what is nominally a rather pointless philosophical issue has become an important one for NASA is because of its near-term political ramifications,” he said. He believes defining “life” is a dangerous endeavor because the information collected will almost inevitably weigh down science. “I can imagine a day when the head of NASA would be brought before the Supreme Court in an abortion case and asked to define life,” he told me. “And I can imagine the long and uncomfortable silence that would follow.” Let the work progress on synthesizing molecules that can do what living molecules do, and on determining if some unexpected substances have lifelike qualities, he says. But leave the definitions for later.
The controversy over a definition for “life” has actually been around for some time, even inside NASA, and it became a serious problem and even embarrassment in 1976 when the agency landed two Viking spacecrafts on Mars in a self-described search for life. To the initial delight of the Viking scientists, a key biology experiment on both Viking landers gave a strong signal that “life” had been found—meeting the painstakingly crafted criteria established before the spacecraft left Earth—and the control experiments seemed to confirm the finding. Yet the principal investigator of that experiment was held back from announcing what Viking had apparently discovered. The scientific community and NASA quickly formed a consensus that life had not been detected. The problem wasn’t with the way the instruments performed or how the experiment was carried out, but rather with the definition of life that NASA itself had put together, one based on the way metabolism is known to work on Earth.
The story is best told through the life and times of Gilbert V. Levin, a pioneer of astrobiology who began his career as a sanitary engineer searching for microbes in drinking water. He first proposed a life-detection experiment for Mars in 1959 and had his idea embraced and tested time and again by NASA before the Viking launch in 1975. He got the results he had dreamed of within ten days of the first landing of Viking. It seemed like a scientific triumph of historic proportions, but it quickly slipped away and Levin has been fighting ever since to reclaim the victory. More than ever, he says, he is convinced that his Viking experiment did find something that indeed was—had to be—living. But the scientific verdict came down against him and, despite some converts, has not significantly changed.
Levin’s experiment was conceptually quite simple: It added a number of liquid nutrients that had been “labeled” with radioactive carbon 14 to a sample of Martian soil dug up by the Viking collecting arm and pulled into the spacecraft. If these nutrients were eaten by Martian bacteria or other life forms, the gases they would inevitably release as waste would also be radioactively labeled and would be detected by an installed radiation counter. It was a simple and powerful test for a cornerstone of all definitions of “life”—the ability of an organism to use the chemicals contained in food to produce the energy it needs to maintain itself, to grow, and to reproduce. If radioactive gases were released, Levin and his NASA collaborators initially agreed, then an organism had taken in and broken down the nutrient food, and was passing the waste out when it was done. Thus the experiment’s name: Labeled Release.
After the nutrient was squirted into the soil collected on Mars, the monitoring instruments registered a surging amount of radioactive carbon dioxide gas—strongly suggesting that some organism had eaten the food and then released the gas. A follow-up control experiment heated the soil to a high temperature that would presumably kill any living organisms, and then squirted in the nutrient. This time there was no release of CO2,
an apparent confirmation that the gas had been produced by the actions of an organism that had been alive during the first experiment but was killed by the heat in the second. Viking 2 landed four thousand miles away on Mars a month-and-a-half later, and the same Labeled Release experiment was conducted. Again, the radioactive gas was detected when food was delivered to the Martian soil at what amounted to room temperature, but not after samples of the same batch of soil were heated and cooked, or when it had been stored in a dark container for several months. It certainly seemed that metabolism—a process only known to occur in living organisms—was taking place. Two other Viking biology experiments got strong reactions when food was presented in gaseous form to the soil, but the controlled versions failed to support the results. Scientists quickly concluded the reactions came from chemical, and not biological, sources. Nonetheless, Levin was convinced that he had found life on Mars.
NASA was skittish about Levin’s results from the start. Officials cautioned that all the reactions could be chemical rather than biological, and that the speed of the appearance of radioactive CO2 did not appear consistent with a biological reaction (although they admitted it wasn’t consistent with a known chemical reaction, either). What they needed to make a firm scientific judgment was the data coming from another key experiment, one designed to determine whether organic compounds—the carbon-, hydrogen-, and oxygen-based molecules essential to all life on earth—were present in the soil. That experiment used a gas chromatograph mass spectrometer (GCMS) to heat the soil until chemicals turned to vapor, and then it separated, identified, and quantified the large number of different chemicals found. The device, refined and operated by prominent MIT biochemist Klaus Biemann, was designed to measure molecules present at a level of only a few parts per billion. The Viking arm twice failed to bring in soil for the GCMS, and so NASA and the many Viking watchers had to wait for days before the testing could begin.
When the samples did arrive, the results were both surprising and seemingly unequivocal: The instrument measured no indigenous organic molecules in the soil, indicating that Martian soil had even less carbon in it than the barren lunar soils brought to Earth during the Apollo program. The strongest organic concentrations it measured were minute trace chlorine-based organics written off as contaminants brought from Earth. Without organics, the scientists concluded, there could not be life, and so any experiment suggesting otherwise had to be reinterpreted. The anticipation that Viking just might delight the world by finding life on Mars quickly turned to a conviction that Mars was lifeless—without organics, without water, and seemingly with compounds all around that rapidly bound other elements to oxygen and made them inaccessible to potential life. A consensus quickly formed that the reactions in Levin’s experiment and the others had to be chemical and not biological, and that’s the way the Viking results were presented to the world and understood by the scientists—all except for Levin and a handful of others, that is.
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The fact that at both Viking sites radioactive carbon dioxide appeared in significant amounts during his experiment and didn’t appear during the controls, that the experiments met all the criteria set out before launch for a positive finding of biological activity and life, was too much for Levin to leave undefended and let go. And so for more than thirty years he has done just that—reminding one and all about the Labeled Release results, citing tests of his experiment in extreme environments around the world, and working hard to knock down all the alternate explanations offered. Others have joined the fray in recent years, and Biemann’s mass spectrometer has been found wanting in a number of reviews by respected scientists. Those men and women don’t necessarily endorse Levin and his conclusions, but their research found numerous instances where the GCMS instrument would (in theory) and did (during testing) miss the presence of certain organic compounds in extreme Earth environments, especially when their concentrations were low. A 2010 paper by two prominent astrobiologists, Rafael Navarro-González of the National Autonomous University of Mexico and Chris McKay of NASA’s Ames Research Center, went further: They concluded the GCMS actually destroyed organics by heating them. And the chlorine-based organics that Viking scientists wrote off as trace contaminants from Earth were precisely what would be left behind if Martian organic material were heated along with surrounding Martian soil.
Even Biemann, who defends his Mars work vigorously as having determined that the Viking landing sites could not and did not support life, nonetheless does not believe it represents a final word on Martian biology. He ended a recent defense by writing: “Future missions to Mars will sooner or later answer the question of organic matter at the surface or in the near subsurface of that planet. It will require carefully designed instrumentation to carry out well planned experiments and thoughtful interpretation of the resulting data.” The implication, it would certainly seem, was that Viking did not meet that grade. The next NASA mission to search for Martian organics, the Mars Science Laboratory, will launch in 2011 and has a similar if more highly evolved GCMS that can test for organics (and unofficially for signs of life) using solvents rather than heat.
Levin, born in 1924, is now an adjunct professor at Arizona State University and has long run a firm based outside Washington, D.C., that discovered and developed a low-calorie sugar called tagatose now in final clinical trial as a diabetes drug. Behind his gentlemanly demeanor, he is a scientific warrior. When the principal investigator of the 2008 Phoenix mission to Mars told a TV interviewer that the lander was the first to touch frozen water on Mars but that the planet has no liquid water, Levin had a rejoinder on the show’s website within six minutes. “What a comedy!” he wrote. “Liquid water was discovered on Mars by the Viking lander in 1976! Ice was shown in images taken by the lander. We have published several papers proving liquid water on Mars. AND we claim that our Viking Labeled Release experiment detected living microorganisms on Mars…. Paradigm shifts are difficult, but this one has taken way too long!”
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Levin had just moved into a modest town house outside Washington when we first met, and many of his various scientific trophies and memorabilia were still in boxes. Although he has three degrees, including a doctorate from Johns Hopkins University, he believes that his beginnings in the world of sanitary engineering and that his Mars research was not done at a prestigious university are held against him.
“I’ve thought long and hard about this and I think that when the Viking results came in, NASA was confronted with evidence of life and no evidence of organics. One result came from a prominent professor and another from an unknown guy from a small company. The more conservative folks were more comfortable with Biemann and his ‘no organics,’ and that was the ballgame.” Nobody seriously questioned Biemann’s instrument until years later, when it was shown by several teams that the instrument could not detect very low levels of organic material in samples from Earth known by other means to have living microbes in them. By then decades had passed, and NASA was stuck with its no-life position because overturning it would raise a new set of other controversies. Levin’s conclusion: “Nobody in charge was brave enough to say it was wrong and NASA still doesn’t want to go near the issue. After Viking and until the present day, there have not been any life-detection experiments sent to Mars, even though finding life there would be the biggest discovery in the history of science.”
Not surprisingly, many see the Viking results and subsequent scientific approaches to Mars quite differently. For instance, Michael Meyer, the lead scientist for NASA’s Mars program, and who has a longtime involvement with astrobiology, said an essential lesson of the Viking missions was that we don’t really know how to look for life yet, an embrace, of sorts, of the Cleland and Ellington position. Levin’s experiment focused on an undisputed signature of life—metabolism—but Meyer says the results were positive but ultimately not convincing. “He might have found life,” Meyer said, “or he might have found that nonbiological processes ta
ke place on Mars very differently than they do on Earth.” The release of CO2 could have been the result of a not-yet-understood chemical reaction, for instance, if compounds with a lot of free oxygen were present. In other words, what would be a clear indication of biology and metabolism on Earth could be totally nonbiological on Mars. The upshot of the Viking life-on-Mars debate has been that NASA has studiously avoided sending life-detection experiments to the planet ever since, choosing instead to concentrate on geology, mineralogy, weather, and the search for water present and past.
In the early 2000s, the United Kingdom sent Beagle 2, a small probe designed to look for life-sustaining habitats, to Mars, and Levin tried without success to get a life-detection instrument into the mix there as well. Speaking with BBC News before the planned landing, deputy mission manager Mark Adler explained that Beagle’s mission was to better understand the water environment of Mars and not to search for life as Levin urged. “What we learnt from Viking is that it is very difficult to come up with specific experiments to look for something when you don’t really know what to look for.” But it all became moot when Beagle’s mission control lost contact with the spacecraft as it entered the Martian atmosphere, and disappeared. Levin did succeed in getting a bare-bones life-detection experiment onto a Russian mission to Mars in 1996, but that effort failed before it even reached the planet.
Still Levin is seeking vindication and has (among others) his physicist son Ron Levin working with him. In 1986, the senior Levin told a Viking ten-year reunion gathering at the National Academy of Sciences that “it is more likely than not that the Viking LR detected life.” In 1997, he argued in a paper for the Proceedings of the International Society for Optical Engineering, which society has an active astrobiology program, that twenty years of additional Mars research had convinced him that his Viking experiment had definitely detected life and that NASA and the scientific consensus were wrong. Nine years after that publication, with an appreciation of Levin’s work emanating from a new generation of Mars scientists, an Argentinian scientist proposed the name Gillevinia straata as the genus and species of the bacteria-like organism ostensibly identified by Viking. But that idea did not garner much support. Levin was not invited to give a talk at the official thirtieth anniversary of the Viking mission.
