The three experiments have been extensively tested using terrestrial soil samples and, as a result, it is safe to say that it is extremely unlikely that a false positive result will be obtained.
The word "positive" translates as evidence of life processes in this summary by Harold Klein, biology team leader of the Viking mission, published about the time that the first U.S. lander went to work on the surface of Mars. Yet last November, after these same life-seeking experiments aboard both the Viking landers had shown apparently positive results in tests of Martian soil, Klein and other NASA scientists seemed unsure. In a Washington press conference summarizing the Viking findings, they announced that the results made it impossible to say that there was or was not life on Mars. That has remained NASA's official position. But unofficially, a handful of scientists support the view of Physicist Robert Jastrow, director of NASA's Goddard Institute for Space Studies. Says he in a forthcoming article in Natural History magazine: "Although the Viking experiments have contradictory elements, they seem to indicate that life, or some process closely imitating life, exists on Mars today."
Complex Reaction. Jastrow and those who hold similar views base their judgment not on new evidence but on an analysis of the biology experiments conducted by the Viking landers. The gas exchange test, based on the fact that terrestrial organisms give off gases as waste products, involved dropping a pinch of Martian soil into a warm, moist test chamber. The aim was to determine whether the sample would give off carbon dioxide, as animals would, or oxygen, as plants do. Scientists were surprised when the sample began releasing oxygen far more rapidly than plants would be expected to do. But they noted that the reaction might have a purely chemical, rather than a biological explanation; compounds called peroxides could have released the oxygen if they were present in the Martian soil.
More encouraging results came from a second test, in which a sample of Martian soil that had been moistened with a nutrient broth showed a rapid release of carbon dioxide. The result might mean that some kind of microbe was metabolizing the food provided by Viking. But cautious scientists noted that certain peroxides in the soil might also have caused the reaction to occur.
A third experiment was designed to measure the conversion of atmospheric carbon dioxide or carbon monoxide into organic matter. The results suggested that this process, which is carried out on earth in living cells, might also be taking place on Mars. In Jastrow's view, the experiment undermined the argument that peroxides might have been responsible for the results of the other tests. If peroxides were involved in this activity, certain catalysts had to be present and the reaction more complex.