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.