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labeled-release experiment, the nutrient solution is essentially sugar and water, but carbon atoms in the sugar have been replaced by the relatively rare isotope carbon 14, which is radioactive.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.