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.