by David Toomey
Because much of Sagan’s scholarly work, like his popular writing, speculated on the nature of extraterrestrial life (indeed, his ideas of recurrent warm, wet periods on Mars and microbes taking water from rocks now seem prescient),26 anyone might have expected him to be included on Viking’s biology team. He was not. Instead he was a member of the “imaging” team, the group that would operate the cameras (two on each lander) and analyze the images returned. Viking’s designers gave it cameras so that the geology teams might study the surrounding terrain. There was some irony in Sagan’s position, as he had once attracted no small amount of ire from a conference of lunar geologists when he called the Moon “dull,” and he was not particularly interested in geology except in cases where it might be associated with life. This happened to be one. Sagan opined that the cameras might do double duty as Viking’s fourth biology experiment, one that made fewer suppositions about the nature of its subject than even Horowitz’s. It did make one supposition, though, and it was far from insignificant: that Mars might have organisms large enough to be seen without a microscope.
Most of the biologists associated with choosing and planning the life-detecting experiments aboard Viking (and Voyager before it) assumed that any Martians would be microbes. There were two justifications for this thinking. First, microbes make up most of Earth’s life, and until quite recently (on geological timescales) they made up all of it. If our planet’s history is any guide, then microbial life is the first sort to arise, and large, complex organisms appear much later if indeed they appear at all. Second, scientists’ knowledge of Mars’s surface, gained from Earthbound observations and the Mariner series of spacecraft, showed an environment far more foreboding than the interior regions of Antarctica, where what little life that survived was microscopic.
Nonetheless, Sagan confessed a real worry that while scientists had their remote-controlled noses stuck in a few ounces of Martian soil, something big (in his own coinage, a “macrobe”) would crawl, scamper, or flitter by unnoticed. It concerned him that Viking’s cameras, called single-slit scanners, worked so slowly—with a mirror moving up and down behind a narrow vertical slit in a housing that pivoted by increments—that they might miss something that was moving quickly. So, during a test in the Mars-like landscape of Colorado’s Great Sand Dunes National Monument, Sagan borrowed a garter snake, two turtles, and a lizard from a pet store and placed them before the scanner. The camera caught the snake and turtles as a blur, and the tracks of the lizard. Sagan was reassured. Even if the cameras missed the macrobes, they could detect their evidence.
In interpreting that evidence, however, Sagan acknowledged a difficulty. “Suppose,” he said, “a fourteen-foot squamous purple ovoid with thirty tentacles came floating through the air, and Viking got a picture of it. We’d identify it as alive even though we’d never seen it before and didn’t know its chemistry simply because it was so improbable.”27 In other words, a squamous purple ovoid would obviously be more than a happenstance collection of inert matter. It would be—and here Sagan borrowed a page from Lovelock—in a state of thermodynamic disequilibrium, and anything in thermodynamic disequilibrium stood a good chance of being alive.
Sagan suggested that the thermodynamic disequilibrium of organisms that were macroscopic and surface-dwelling might be obvious. They would look top-heavy. Like dandelions, cows, and dairy farmers (or examples of each when standing, anyway), they would present a greater surface area farther from the ground than they would nearer the ground. Top-heaviness was hardly a guaranteed biosignature; and as if to make that point, the leader of the imaging team, geologist Timothy Mutch, displayed in his office photographs of ventifacts, enormous rocks shaped like fluted vases, their concave surfaces abraded by windblown sand. But it was, Sagan maintained, a good place to start.
THE VIEW FROM MARS
In July 1976, Viking 1 made a successful landing on the low-lying Chryse plain in Mars’s northern hemisphere. Almost immediately its single-line scanners whirred and clicked, and images of the surrounding landscape were radioed Earthward. They showed salmon-colored sand dunes stretching to the horizon and strewn with sharp-angled rocks. It was the first time anyone had seen a landscape from another planet, and that landscape was hauntingly beautiful. It was also, to all appearances, lifeless. “There was,” Sagan said, “not a single recognizable funny-looking thing, no obvious sign of thermodynamic disequilibrium.28 The biologists were intrigued briefly by certain rocks, and for a few days Sagan wondered about a roughly spherical one, but it was clear that except for some windblown sand, nothing within view of the lander was moving.29 The images sent from Viking 2, which landed some weeks later, showed a landscape just as barren, just as still. Or, as they say in the movies, was it?
Although Oyama and his team concluded that the oxygen produced in his “gas exchange” experiment had resulted from a chemical reaction involving hydrogen peroxide in the soil, the other two biology experiments (Levin’s and Horowitz’s) gave “presumptive positive results.” But only presumptive, and difficult to have confidence in because they were contradicted by a separate experiment: a molecular analysis of the soil, in what was later called “probably the most surprising single discovery of the mission,”30 failed to detect organic compounds.
Immediately, each member of the biology team dropped his carefully planned experimental strategy and put all energies into determining whether the reactions were biological or (merely) chemical. This would have been a fair challenge with the samples sitting in a fully equipped Earthbound laboratory, but it was particularly daunting with samples more than a million miles distant in what was, after all, a rather modest facility. Still, they soldiered on, and nine months later, with the experiments put through twenty-six separate cycles, the results were in. That was the good news. The bad news was that no one agreed on what they meant.
Oyama affirmed his earlier, nonbiological take, and Horowitz now drew a similar conclusion. Horowitz had induced changes in temperature experimentally, and he reasoned that if the reaction he measured had been biological, it would have been more sensitive to those changes. Levin had held from the start that his “labeled release” experiment detected life, and after subsequent cycles he felt only more certain.
Some said the confusion would have been avoided if NASA’s first attempt to detect life on Mars had not resorted to a million-mile Hail Mary pass, but instead had proceeded in a more methodical, stepwise fashion so that scientists could make proper studies of the chemistry of the soil and atmosphere before they designed—let alone conducted—tests for living organisms. Others thought the biological tests themselves might have been better coordinated, made more like the preliminary plans for Voyager, which had proposed thirty separate life-detecting experiments to be performed in sequence, each addressing questions raised by the one that came before. Still others countered that the Viking experiments were teaching us about Martian chemistry, and giving results so unexpected that no one would have deliberately designed experiments to test for them. In the end, if the confusion proved anything, it was that, as Sagan noted, “looking for life is hard.”31
In 1978, Levin published an article claiming that images from Chryse, on further study, revealed greenish patches on some rocks, and that the patches moved. Neither his colleagues nor the imaging team saw green; most suspected that whatever Levin saw could be attributed to shadows or dust. In the years immediately following, the consensus of scientific opinion was that, Levin’s objections notwithstanding, Viking had not found life on Mars. Whether there was life on Mars, strictly speaking, remained an unanswered question, but most who studied the results were doubtful. Horowitz made a small show of appeasing the few holdouts, noting that it was impossible to prove that the reactions measured were not biological, but only in the way it was impossible to prove that the rocks surrounding each lander were not organisms that happened to look like rocks.32 Soffen was more blunt. “I began with an optimistic view of the chances of life on Mars,” he said. “I no
w believe that it is very unlikely.”33
Thirty years later, however, this unhappy consensus was upended by two discoveries occurring a million miles apart: evidence of liquid water on Mars and intraterrestrial microbes like Bacillus infernus on (or rather, in) Earth. At present, we still don’t know whether life exists on Mars—but we now know that physics and chemistry allow its possibility.
Against these shifting views, Gilbert Levin’s have changed only slightly. Now an eighty-something adjunct professor working alongside Paul Davies, Levin is redesigning his “labeled release” experiment to identify microbes in extreme environments on Earth. As to his putative Martian microbes, their time may have come at last. Astrobiologist Dirk Schulze-Makuch of Washington State University and Joop Houtkooper of Justus Liebig University (in Giessen, Germany) recently took a second look at the Viking results and, following a line of thinking begun by Horowitz, they hold that Martian microbes—or those living on the surface, at least—would have no experience or need for liquid water, where it would freeze or sublime. But they might know and love a liquid with a lower freezing point, like hydrogen peroxide. The chemical is fatal to many terrestrial bacteria (this is the reason it may be in your medicine cabinet), but when mixed with the right compounds it can be tolerated and can actually assist functions inside cells.
Schulze-Makuch and Houtkooper claim that Viking’s molecular analysis of Martian soil may have failed to detect organic compounds (that bewildering result) because the hydrogen peroxide released from dying cells oxidized those selfsame compounds. In fact, they contend, microbes that use hydrogen peroxide could have produced nearly all the results of the Viking biology experiments. In Levin’s experiment, gas was produced when the nutrient was added, but the reaction tapered off—generally speaking, not a biological result. Schulze-Makuch and Houtkooper suggest that the tapering off represented the last gasps of microbes that, exposed to the experiment’s liquid water, had after a few seconds drowned or burst.
The researchers’ hypothesis, even untested, is intriguing and perhaps unsettling. Intriguing in that we may have already found extraterrestrial organisms, and weird ones at that. Unsettling in that inadvertently, and with the best of intentions, we may have killed them.
* * *
* Traditionally, the habitable zone for any planet (or moon) is the region where water would be liquid on its surface. For a planet whose atmosphere is chemically active, the zone’s inner boundary is the distance from its sun at which it is so warm that water vapor accumulates in the upper atmosphere, where unfiltered ultraviolet radiation splits the molecules into its components: hydrogen and oxygen. The hydrogen escapes into space, and the oxygen is eventually absorbed into surface rocks. The planet dries up, a runaway greenhouse effect begins, and soon it is too warm for liquid water. The outer boundary for a planet or moon whose atmosphere is chemically active is the distance from its sun at which carbon dioxide, a greenhouse gas, freezes out of the atmosphere and the planet becomes too cold for liquid water. As it happens, Mars orbits within our Solar System’s habitable zone; liquid water once existed on its surface not because the planet was nearer the Sun (it wasn’t), but because that surface benefited from the greenhouse effect of a more substantial atmosphere, mostly of carbon dioxide. (Hart, “Habitable Zones”)
* “Follow the water” is the most recent iteration of a long tradition with a somewhat dubious beginning. Astronomer Percival Lowell observed the planet Mars for many nights over several years, and imagined that he saw crosshatched dark lines that he believed to be vegetation growing alongside a planetwide network of canals. From such figments he produced astonishingly detailed maps and hypothesized a civilization that inspired the “Martian” science fiction of H. G. Wells, Edgar Rice Burroughs, and Ray Bradbury. (Bradbury et al., Mars and the Mind of Man)
* Biological specialties include at least these nine: anatomy, taxonomy, physiology, biochemistry, molecular biology, ecology, ethology, embryology, and evolutionary biology.
* The same name would be given the twin spacecraft that in the 1980s conducted enormously successful flybys of the Solar System’s outer planets.
CHAPTER FOUR
Starting from Scratch
Fifty years after the Viking planning sessions, scientists are again asking themselves whether they could be certain to recognize life if they or their instruments saw it. Were they obliged to use the NRC report’s provisional definition of life as a “chemical system capable of Darwinian evolution,” their answer would be no. Chemical systems are common, and although evolutionary biologists can point to innumerable examples of evolution over time, they find evolution in action difficult to identify even on Earth. Identifying a system capable of evolution might be easier, but only slightly.
So, scientists have no definition or theory of life and, since biosignatures are evidence but not proof, no set of criteria by which they can be sure to recognize it. Can they say anything about life that might assist in a search for its weirder versions? Perhaps. A list of what life needs in the way of materials and conditions might allow a search strategy wider than “follow the water,” yet still selective. It would tell scientists where they might look, and just as important—considering limits on time and resources—where they needn’t bother to look.
WHAT LIFE NEEDS
If life has a fundamental, nonnegotiable, rock-bottom requirement, it is a source of energy. Familiar life takes energy from sunlight, chemical reactions, thermal energy, and natural radiation. It uses these sources in particular, biologists presume, because they are abundant and freely available. (Interestingly, there are other sources—ultraviolet radiation, thermal gradients, and gravity—that, as far as anyone knows, familiar life does not exploit, and exactly why is far from clear.)1 In addition to an energy source to drive its chemistry, life needs a medium in which that chemistry can work—that is, a medium in which molecules are suspended and through which they can move freely and interact easily. If we are in a mood to be open-minded, this requirement may be negotiable; many science fiction writers and some scientists have imagined organisms made of atomic nuclei and magnetic fields that might survive in a vacuum, and I’ll discuss these in a later chapter. For the moment, though, let’s stay conservative, follow the NRC report’s provisional definition, and assume that all life is based in chemistry and needs a medium.
There are a great many mediums, and some are rather exotic. I’ll be prudent here too, and begin with the mediums we know best—those represented by the three “classical” states of matter. They are, of course, solids, liquids, and gases. As potential staging grounds for life, solids look like a bad bet because they don’t allow molecules enough movement. Gases look like a bad bet because they don’t allow molecules enough contact. But liquids, the happy medium of mediums, allow plenty of both. Recalling life’s great affinity for water, we might suppose that for familiar life, water is the liquid medium of choice. But the fact is—this a point made by several astrobiologists—life probably had no choice. As Dirk Schulze-Makuch observes, “Life on Earth learned to work with water because it’s the only liquid that’s really abundant.”2 In fact, in extolling the virtues of water a few chapters back, I may have been giving credit where it wasn’t quite due. From its first appearance in the remnants of cooling supernova explosions somewhere in a young universe many billions of years ago, to the steady drip of your leaky kitchen faucet, water hasn’t changed. A molecule of water was an atom of oxygen covalently bonded to two atoms of hydrogen then, and a molecule of water is an atom of oxygen covalently bonded to two atoms of hydrogen now. But life, since its first appearance, has changed enormously, and no doubt some of those changes took advantage of water’s unusual characteristics—its surface tension, the wide range of temperature at which it is liquid, and so forth. In other words, life learned to love the one it’s with.*
The truth is that astrobiologists can conceive of a variety of liquids that might serve as mediums for life, each with characteristics that life might
use to its advantage. Water carries around hydrogen ions that catalyze chemical reactions crucial for cells to metabolize nutrients, but a number of other liquids—among them hydrogen fluoride, sulfuric acid, ammonia, and hydrogen peroxide—can do the same job at least as well. Ammonia, for instance, can dissolve most organic compounds as effectively as or more effectively than water can, and it can also dissolve metals like sodium and magnesium directly into solution. If the life around us used liquid ammonia or hydrogen peroxide as its medium, we’d probably be marveling at how well its characteristics were suited for life’s needs, until we realized that we had it backward—that life evolved to take advantage of those very characteristics.
In addition to an energy source to drive its chemistry and a medium in which that chemistry can work, life needs the chemicals themselves—that is, molecules that perform specialized functions that allow an organism to metabolize and reproduce. Familiar life maintains much of its metabolism with proteins—quite a lot of them, in fact. A typical cell contains 100 million proteins3—and most are specialized to perform specific functions. Some take energy from nutrients, others assemble more proteins, and still others dispose of waste and repel intruders.
Each of these functions is a complex process involving many steps. To take one example, when a cell somewhere in your body needs insulin, certain proteins inside the cell pull apart a section of the DNA molecule pairs, exposing the particular sequence of base pairs that signifies one of the many amino acids needed to make an insulin molecule. Other proteins read the sequence and make an ad hoc and temporary copy called messenger RNA. Then, still other proteins work over the messenger RNA, slicing and splicing until they’ve fashioned the amino acid needed. Finally, the molecules of protein and DNA called ribosomes (the structures that, you may recall, may set the lower size limit for a cell) pull the newly formed amino acid together with others made the same way by other proteins, and coordinate with other ribosomes, all now pulling and pushing their own amino acids, to assemble a molecule of insulin.
