The idea that Mars, like the earth, might be the home of living beings has held our imagination since the turn of the century, when Percival Lowell thought he saw hundreds of canals crisscrossing the face of the planet, and took them as proof that Mars was inhabited.
Lowell was wrong. The canals never existed, as Mariner 9 photographs finally proved five years ago. But even though his evidence was mistaken, Lowell's conclusion may yet be vindicated: the Viking landers have returned an impressive array of biochemical data which seems to show that some form of life really does exist on Mars.
The results of the Viking life-detection experiments have been more positive than most people expected. Dr. Robert Jastrow, director of NASA's Goddard Institute for Space Studies, says, "Short of seeing something wiggling on the end of a pin, the case for life on Mars is now as complete as the Viking experiments could make it."
But no one wants to make predictions about Martian life which might be proved wrong by later evidence; scientific reputations could too easily be damaged in the process. So the Viking scientists have been extremely cautious in interpreting the results of their biology experiments. And the official NASA position straddles the fence. As Viking scientist Dr. Carl Sagan of Cornell University puts it, "We have clues up to the eyebrows, but no conclusive explanations of what we're seeing."
The Viking team has good reason to maintain this ambiguous tone in public pronouncements. After all, the discovery of any kind of extraterrestrial life will have profound and far-reaching effects; it is something which no scientist can afford to be wrong about. As The New York Times indicated in an editorial, "The scientists ... are being understandably cautious—in fact, they are leaning over backward."
This extreme cautiousness means that few people get an accurate impression of how exciting and portentous the experimental evidence is. To many, in fact, the published reports suggest that the biology results have been totally negative, or at best hopelessly inconclusive. But experiments are continuing at a brisk pace, both on Mars and in Earth-based laboratories, and the ambiguities may soon be resolved. We may, in fact, be close to the momentous occasion when NASA officials call a press conference to release the mindboggling and epoch-making news that Earth people are not alone, that we have reached out and made a tentative contact with an alien form of life.
Preliminaries over, the search for Martian life began on July 28, 1976, eight days after Viking I made its touchdown on Chryse Planitia, the Plain of Gold. On that day, at 3:30 A.M., the sampler arm reached out to dig into the surface of Mars. Whirring and clicking, the arm slowly unfurled from its storage compartment.
The arm, ingeniously designed, is made up of two curved strips of stainless steel, welded together and rolled up like a steel tape measure. Despite its flimsy appearance, it is surprisingly strong. It can push straight ahead with a force of forty pounds, and pick up a fairly large rock when extended to its full length of ten feet.
The soil samples were acquired by extending the arm at a slight downward angle and then thrusting forward to force the collector head, which resembles a small shovel with teeth, into the ground to a depth of just over an inch. The sampler's jaws then snapped shut to hold the soil, and the arm retracted to the lander body, where it dumped its scoops of soil into the openings for each of the Viking's three miniature automated laboratories. This operation took about four hours.
One soil sample was dumped into the hopper for the biology experiments, where it passed through a sieve before being funneled into three separate chambers—about a teaspoonful of soil for each of the three life detection experiments. The test chambers were then sealed against the outside environment, and the incubation began.
The Viking biology labs are very sophisticated machines, containing the most advanced set of remote-controlled instruments ever assembled. The three experiments in these robot laboratories were brilliantly conceived to provide a clear indication of the presence of life processes, even though the processes might be unfamiliar.
Each of the biology tests was based on a different speculation about what Martian life might be like. The idea was that since no one knew what to expect we should look for as many different kinds of life forms as possible, hoping that one or more guesses might be right.
Two of the experiments were based on the assumption that Martian life might resemble some of the myriad forms of bacteria existing on earth. Since bacteria were among the earliest living things and are among the simplest forms of life, they would be a logical first target in a search for alien life. They are highly adaptable, inhabiting virtually every region of this planet, no matter how harsh or inhospitable, from the dry caves of Antarctica to the depths of boiling hot springs, from the uppermost reaches of the atmosphere to the deepest ocean trenches.
The metabolisms of these different kinds of bacteria are as varied as the environments they inhabit. An essential foodstuff to one may be poison to another, but hardly a substance known is not the favorite food of some bacterial strain, somewhere. For example, strains now being developed in the laboratory have a great appetite for petroleum. They may be used someday to combat oil spills. Other strains subsist quite happily on a diet of sulfuric acid, and still others are instantly poisoned by oxygen.
When Anton van Leeuwenhoek discovered bacteria in the seventeenth century, intense debates occurred among the world's scholars as to whether these animalcules were really alive; the question was not resolved for 200 years. One hopes that the case for life on Mars can be established or disproven more rapidly.
Despite the incredible variety of bacteria, some universal characteristics are now known which clearly distinguish them from nonliving matter. All of them go through some kind of metabolism—that is, they ingest certain chemical substances, break them down and rearrange them chemically, and then release byproducts, usually as a gas. For example, every marshy area contains billions of bacteria which eat decaying plant matter and release methane gas.
In order to detect such a process, the Viking team designed two experiments which feed a nutrient solution to the Martian soil and then look for changes in the test-chamber atmosphere.
In the labeled-release experiment, the nutrient solution is essentially sugar and water, but carbon atoms in the sugar have been replaced by the relatively rare isotope carbon 14, which is radioactive. A radiation counter will detect carbon 14 in the chamber atmosphere if microbes in the soil eat the sugar and release carbon atoms in gaseous form—for example, as carbon dioxide. This will only work, of course, if the Martian bugs happen to like sugar.
In the gas-exchange experiment, the nutrient solution contains a wide variety of compounds believed to be desirable to a great many different organisms. The solution includes carbohydrates, fats, proteins, and vitamins. It is such a universal food that almost any creature, including a person, could eat it and derive some nutrition from it, although it has a foul smell and would probably cause a bad case of heartburn. This rich nutritive broth was dubbed. "chicken soup" by the scientists.
After the soup is added to the test chamber, the atmosphere is monitored for changes in concentration of several gases. Thus, this test makes fewer assumptions than the previous one in terms of both the nutrients being provided and the resulting atmospheric changes that can be detected.
However, these tests have been criticized for their radical departure from known Martian conditions in two important areas: temperature and humidity. The temperatures in the test chambers of both experiments are substantially higher than the maximum temperatures ever measured on Mars, in order to keep the water used in the tests from freezing. And since both experiments use nutrients in a water solution, the amount of water in the test chamber is vastly greater than would be encountered on the dry Martian surface.
Since Martian organisms are presumably adapted to Martian conditions, these drastic changes might be expected to affect them adversely. One Viking scientist, Dr. Norman Horowitz, said before the Viking landing, "If there are any organisms on Mars, they will surely drown or burst in Oyama's pharmacy." (Oyama is the designer of the gas-exchange experiment.)
This, as it turns out, is one very plausible way of explaining the results from the labeled-release experiment. What the test showed was an immediate outpouring of large amounts of carbon dioxide. In his first report of this result, Dr. Gilbert Levin (who designed the experiment) said, "The response that we get, in amplitude and shape, is consistent with the response we are used to seeing in terrestrial soil." Another Viking spokesman said, "If life does exist on Mars, this is what it should be doing."
Within two days the carbon dioxide production had slowed down almost to a standstill, leading some scientists to doubt its biological origin. But this is just what might have been expected if Horowitz's prediction that the Martian organisms would be drowned by the unexpected abundance of water was correct. This sequence of events corresponds not only to the results of the labeled-release experiment but also to those of the gas-exchange experiment: a strong, rapid initial production of gas, in this case oxygen, declining very quickly to a standstill.
Some scientists believe that these results could be caused by some exotic soil chemistry, perhaps involving peroxides. But in the results from the second landing site, the gas-exchange test produced only one tenth of the original response, while the results of the labeled-release test showed an increase of 30 percent. If both of these reactions stemmed from the same set of compounds, as had been theorized, they should have changed in the same way. According to Dr. Jastrow, writing in Natural History magazine, "This result seems to indicate that chemical reactions involving peroxide compounds cannot be the source of the life-like signals obtained in the microbe test. With the chemical theory for the test eliminated, a biological process is the most straightforward explanation remaining."
The gas-exchange test also produced substantial amounts of carbon dioxide which increased slowly and steadily. This contrasts with the expected behavior of living bacteria, whose growth and reproduction normally cause the volume of gas released to increase exponentially The modest production of carbon dioxide makes sense if the microbes in the sample, while not actually drowning, were so uncomfortable in the heat and humidity of the test cell that growth was inhibited.
Fortunately, one biological experiment on this mission did not drown its sample: the pyrolitic-release experiment, which provided the strongest evidence of all for Martian life. The life process being sought by this test is photosynthesis, the most fundamental of all biological activities.
Photosynthesis, the basic metabolic activity of all plants, consists of taking in carbon dioxide from the air and breaking it down into its constituent atoms; carbon and oxygen. The oxygen is then released back into the air, while the carbon undergoes some very complicated chemical activity, fueled by sunlight. The carbon atoms combine with atoms of hydrogen, nitrogen, and other elements (generally extracted from the soil), producing complex organic molecules such as carbohydrates.
The device to detect this process is in some ways the best designed of all the Viking biology experiments, since it is the only one that does not depart radically from what are thought to be normal Martian conditions.
The experiment was very simple: the spoonful of soil was sealed in a small cylindrical chamber along with some Martian air, which is mostly carbon dioxide. Then a xenon lamp was switched on to simulate Martian sunlight.
The only thing added to the chamber was a small quantity of labeled carbon dioxide. (The labeling was essentially the same as in the labeled-release test: radioactive carbon 14 atoms were substituted for ordinary carbon in a large proportion of the gas molecules. This has no effect on the chemical properties of the gas, but it is a way of identifying chemical compounds which have reacted with the atmosphere—in other words, those produced by photosynthesis during the test period.) After five days of incubation, the chamber was heated to 1200° F (hot enough to vaporize any organic compounds), and the resulting vapor was driven through a gas-chromatograph tube (a device which separates organic molecules from all the rest). The separated organics were then passed through a radiation detector, which, by responding only to those molecules which included labeled carbon from the chamber atmosphere, gives a quantitative indication of how much photosynthesis has taken place.
If no photosynthesis had occurred—that is, if no living organisms were present—the result should have been a count of fifteen. Instead, the count was ninety-six—more than a 500 percent increase over the "background" level. This is a clear-cut positive response, equal to the result given by Earth soil containing about 3000 microbes.
This is the strongest and most unambiguous piece of evidence for Martian life, and a sense of intense expectation filled Viking headquarters as the results came in. But when the excitement of those first moments began to wear off, skeptics still clung to the possibility that some strange, unknown chemical reaction might have produced the results.
And yet, after months of exhaustive experimentation, no one has been able to duplicate this reaction (or, for that matter, the results of the other two biology tests) in the laboratory. Suggestions have been made, but not tested, as to reactions that might duplicate some of the results. But all of these suggestions involve peroxides, superoxides, or ozonides, none of which are known to exist on Mars. Indeed, some of the experimental results argue strongly against their existence there.
Ultraviolet radiation is the only known agent capable of producing such compounds on the Martian surface. But the biology tests were repeated using soil from under a rock, where it had been shielded from the ultraviolet, and the results were just as strong as in the original tests. Since these chemicals are extremely reactive and unstable, it is highly unlikely that they would have survived unchanged if they had been formed before being shielded by the rock.
The chemical theory was further weakened by the latest run of the pyrolitic-release experiment, in which the response was positive after first moistening, then heating the sample. This came as a big surprise, since previous runs had shown that the reaction was eliminated by just moistening the sample or just heating it. The meaning of this paradoxical result is not yet understood, but Dr. Levin says that if peroxides were responsible for the initial reactions, the response should have been eliminated in this test. Since it was not, the peroxide model, which is the cornerstone of all the non-biological theories, has received another major setback.
The task of those who hope to find a chemical explanation for the biology test results has been made even more difficult by a series of control experiments. All three of the experiments were repeated with conditions that were identical to those in the "active" run, except that the soil was first sterilized by heating it for three hours to a temperature of 340° F, which would kill any known form of life.
After sterilization, all three experiments showed that the original reactions had been completely eliminated, confirming the fact that we had killed off the creatures whose activities had been detected in the original tests.
In order to prove that the process being observed in the pyrolitic-release experiments really was photosynthesis, the experiment was repeated with the light turned off. If some other process had been involved, this might not have affected the results. But the reaction was, once again, completely eliminated, thus establishing its dependence on light.
The pyrolitic-release experiment showed conclusively that something in the Martian soil creates organic molecules in the presence of sunlight. Since the conditions of this experiment closely matched the known Martian environment, this same process must be taking place every day on the surface of Mars.
Whether this process is biological or chemical, the steady production of organics should lead to a considerable accumulation of these compounds. But the GCMS (gas-chromatograph mass-spectrometer), designed to look for large numbers of organic molecules in the soil, has failed to find any. This mystifying discovery has been the center of the controversy over the existence of life on Mars. According to Viking chief scientist Gerald Soffen, "All the signs suggest that life exists on Mars, but we can't find any bodies!"
Many of the Viking scientists have taken this discrepancy as evidence against Martian biology, and thus as support for a chemical interpretation of the data. But this doesn't help at all to explain the apparent contradiction. Even if some strange, unknown chemical process is responsible for the production of organics, they should still accumulate: if you were to set up a confetti-making machine in the middle of a field and leave it running for a long time, confetti would pile up to a great height. A confetti detector would have no trouble finding it.
How to explain the absence of organic molecules? Some process either destroys them as quickly as they are formed (a torch burns up the confetti as it comes from the machine), or isolates them so that they escape detection (the detector lands in the next field and finds only an occasional scrap blown over by the wind). The most plausible attempts to account for such processes involve the actions of Martian organisms.
For example, these valuable molecules may simply be eaten up by living creatures in the soil. On earth, areas where food is scarce often support a large population of scavengers. (Vultures abound in desert regions.) The Martian environment is very harsh, and food may be hard to find. Thus, a Martian organism would have to be adept at searching out and quickly devouring every available scrap of organic material. This kind of efficient recycling would cut down on the buildup of waste products, and perhaps leave too little for the GCMS to detect, even if the soil was teeming with a large variety of active, healthy creatures.
Another possibility is that life exists on Mars, but only in certain pockets, or "microenvironments," as Carl Sagan and Joshua Lederberg have suggested. In other words, Mars may be a vast desert with only a few isolated oases, where life can thrive under special conditions in a small area—for example, where water has accumulated because of geothermal heating (the process which causes "hot springs" on earth). In that case, a small number of seeds or spores might be transported all over the planet by the winds, but would grow and reproduce only in these few select spots. Thus, Viking would detect a few spores, but no accumulation of organics.
This conclusion has strong support from the fact that of all the Earth soils which have been tested by the pyrolitic-release experiment, the one that comes the closest to reproducing the Martian results is Antarctic soil, which includes spores transported from elsewhere by the wind, but no active life.
Another theory is that Martian organisms may have developed hard shells to protect them from the deadly ultraviolet radiation, and that these shells might prevent the organic remains from reaching the GCMS and being detected. In short, if life exists on Mars, the apparent conflict between biology results and the organic chemistry analysis, may be easier to explain. If no life exists, the contradiction is much more difficult to resolve.
Some say the strong ultraviolet radiation which constantly bombards the Martian surface explains the supposed paradox. In theory, organic molecules might be destroyed by the UV almost as fast as they were being created, and therefore would never have chance to pile up. This theory received a major setback when the GCMS ran a test on soil from underneath rock, where it had presumably been shielded from ultraviolet radiation for millions of years. Since this test, like the previous ones, failed to detect any organics, destruction by UV seems not to be the answer.
A great deal of evidence has accumulated from the Viking mission to support the conclusion that Mars is a biologically active planet. According to Dr. Gilbert Levin, of the biology team, "The accretion of evidence has been more compatible with biology than chemistry. Each new test result has made it more difficult to come up with a chemical explanation, but each new result has continued to allow for biology."
All of the life-seeking tests showed reactions, which, says Dr. Levin, "if we had seen them on earth, would unhesitatingly have described as biological. All of them have been confirmed by control experiments. All of them have been successfully repeated at both of the Viking landing sites.
We may have set our sights too low in the Viking experiments, however. We simply weren't optimistic enough about the possibility of life on Mars, and therefore concentrated on looking for the simplest and most primitive forms of life—the ones most likely to exist.
We found what we were looking for, but we may have been examining bacteria in the midst of a rich assortment of flora and fauna. We have not eliminated any possibilities about the extent or the level of intelligence of Martian life. All we've done is to raise the bottom line: at the very least, a substantial population of microorganisms almost certainly exists.
This possibility is clearly illustrated by an exhaustive investigation conducted by Dr. Sagan. He carefully examined thousands of satellite photographs of Earth which had a limit of resolution (the ability to distinguish detail), similar to those that have been taken of Mars by the Mariner 9 and the Viking orbiters. His conclusion: At that resolution, one can find no visible evidence for the existence of any kind of life on this planet, in 99 percent of the photographs.
In fact, he determined that in order to be sure of getting unambiguous evidence for intelligent life on Earth, one would need a resolution 10 times greater than we have yet achieved in photographing most of the Martian surface. Even if an advanced civilization exists on Mars, even if large cities can be found there, we simply wouldn't have been able to see them yet. We haven't looked closely enough.
But even at the present limits of resolution, some surprising formations have been seen, the most inexplicable of which are the three-sided pyramids found on the plateau of Elysium. Scientists have tried to find a natural geological process that would account for the formation of these pyramids, some of which are two miles across at the base, but as yet their origin is far from being explained. Such tantalizing mysteries may not be fully solved until astronauts are able to make direct observations on the Martian surface.
Some argue that the harshness of the Martian environment—the low temperatures, the absence of liquid water, the ubiquitous lethal ultraviolet radiation—make it extremely unlikely that advanced forms of life would ever have evolved there. However, considerable evidence now suggests that Mars used to be a much more pleasant place to live.
For example, the Mariner photographs showed a vast network of sinuous channels covering most of the planet. Scientists agree that these must almost certainly be the dried-up beds of vast rivers which once coursed across the Martian landscape.
This has enormous implications for the past climate of Mars, and therefore for the evolution of Martian life. First of all, water is a basic prerequisite for all life that we know of. The presence of vast amounts of water flowing across the face of Mars in some ancient era, therefore, greatly increases the likelihood that advanced and even quite familiar forms of life have been able to develop there. But it also implies some other differences in the Martian environment of the past. In order for water to exist in a liquid state on the surface of Mars, which is impossible today, the atmosphere must have been much denser and the temperatures much higher.
On Mars today, the air is so thin that ice turns directly to steam without ever going through a liquid state—just as "dry ice" (frozen carbon dioxide) does on Earth. However, if a large amount of water vapor accumulated in the Martian atmosphere (for example, by the melting of part of the polar ice cap), this would increase the air pressure to the point where water could exist as a liquid, and rain would begin to fall. Rivers would begin to flow. Seas would begin to fill.
At the same time, a substantial "greenhouse effect" would occur, in which the warmth of the sun would trapped by the atmosphere faster than it could dissipate again into space, and the temperature of the entire planet would be raised—according to Sagan's calculations, perhaps by 60 degrees or more. The net result of all this would be a very comfortable, Earth-like planet, capable of supporting an array of familiar creatures. Indeed, if Sagan's reconstruction is correct, during these warmer epochs of Martian climate, a man might have been able to walk around on Mars in his shirtsleeves in perfect comfort.
Many scientists, including Dr. Sagan, believe that this sequence of events has happened not only once but repeatedly throughout Martian history, in a continuing cycle—similar, perhaps, to the cycle of ice ages, on our planet.
And if some epochs were comfortable enough to foster the development of advanced forms of life, at least some of those forms then might have been able to adapt to the changing conditions, and might still be around today—perhaps in a dormant state.
The discovery of any kind of life on Mars, however primitive it may prove to be, would profoundly increase the probability of finding other living beings elsewhere in the universe.
As long as Earth was the only life-bearing planet that we knew of, the chance existed that life might have been produced by some fluke, some unique set of circumstances which had not occurred on any other planet—a rare and exotic process so unlikely that we would never find another example.
But if the process is not unique, the universe may be teeming with all manner of beings at every imaginable stage of development. And we may someday establish communication with another race of intelligent creatures.The knowledge to be gained from such a contact can scarcely be imagined.