Each Viking spacecraft consisted of an orbiter and a lander. Both Viking landers touched down on the surface of the planet in the middle of 1976. Since a major factor increasing the chances of finding life was and still is the presence of water, the best places for the Vikings to look for signs of life would have been a place like Meridiani Planum, or perhaps one of the ice caps that cover the north and south poles.

However, such locations were also the most dangerous sites for the spacecraft to land and the risk was significant that both landers could have been lost, as had occurred during the Russian attempts to land on the planet and as would occur years later, in 1999, in the case of the Mars Polar Lander.

Therefore, the Vikings being NASA’s first attempts to land on Mars, the mission scientists chose safer landing sites –Viking 1 landed in Chryse Planitia and Viking 2 landed in Utopia Planitia– even though liquid water was believed to be virtually absent in these areas. However, since the planet had wind, scientists reasoned that if life existed anywhere, the wind would blow it everywhere.

Each Viking lander carried an instrument designed to detect organic compounds –the stuff of life– plus three biology experiments designed to detect metabolism –the activity of life.

Since each biology experiment was based on a different kind of metabolic reaction that Martian lifeforms might or might not posses, a positive response to any one of the three experiments would be evidence for life –even with negative results to one or both of the other two.

On the surface of Mars, one of the three experiments found that the process of REDUCTION was occuring in the Martian soil. This experiment was know as the Pyrolytic Release (PR) test and it had been developed by Norman Horowitz, a well-known biologist. Specifically, when the gasses CO₂ and carbon monoxide (CO) were exposed to samples of Martian soil, they were REDUCED, meaning that they were chemically converted into organic molecules, the stuff of life. On Earth this is something that happens in plants and in certain microorganisms during a process called photosynthesis, a process that requires light, and which produces sugars, one category of organic compounds.

To test whether the reduction occuring in the Martian soil was due to Martian photosynthetic organisms, the experiment was conducted both in the light and in the dark. If photosynthesis were taking place, it was expected that the gasses would be reduced in the light but not in the dark. However, once the test had been conducted a few times, it became clear that not only were the gasses reduced only in tiny amounts but that the reduction occurred both in the light and in the dark! Therefore, Horowitz and the other scientists attributed the PR results to a chemical effect that was interesting but not biological.

Another of the three biology experiments was called the Gas Exchange (GEx) test and this one, developed by Vance Oyama, also found no evidence of biology in the Martian soil.

Meanwhile, Gil Levin, excited by the earlier findings of the Mariner 9 mission, had developed the third of the Viking biology experiments. It was called the Labeled Release (LR) test. At both landing sites – Chryse Planitia and Utopia Planitia– some 6,500 kilometers apart, the LR results were positive.

The LR worked as follows: A sample of Martian soil was placed inside a special container and a liquid nutrient snack was dropped onto the sample. When the experiment was conducted on Mars some of the nutrients were converted chemically into one or more gasseous compounds. The gas might have been CO₂ or CO and in either case it would have meant that the nutrients had been OXIDIZED. On the other hand, the gas might have been methane (CH₄) and in this case, we would say that the nutrients had been REDUCED. But whether the gas had been CO₂ or CO produced through oxidation, or CH₄ produced through reduction, the scientists were excited.

“Could Martian microorganisms be responsible for the chemical process, whatever it is?” they asked.

Oddly, however, a second injection of nutrients failed to re-invigorate the initial release of gas, as would have been expected with soil from Earth. Instead, some of the gas evolved after the first injection was reabsorbed into the soil. Could it have been that the initial positive response was not biological but due, instead, to a simple chemical agent present in the soil? With this question in mind, a new sample was taken, only this time, before being tested, the soil was heated to 160° C for three hours to kill any microorganisms that might be present.

This time the response did not occur and when the tests were repeated various times at both Viking landing sites, the pattern repeated. Therefore, the scientists were faced with another qustion: “If the response had been due to a simple chemical rather than microorganisms, why would it have occurred only in soil samples that were not heated prior to testing?”

The simplest answer, Levin believed, was that the Martian soil did indeed contain Maritian microorganisms. Perhaps the microbes, after reacting to the first injection had died and consequently had not reacted to the second injection of nutrients. The idea did not seem too far-fetched.

But another problem soon emerged; the search for organic molecules turned up NEGATIVE. The stuff of life, at least at the two Viking landing sites, seemed nowhere to be found. It just didn’t make sense that you could have life without a trace of the molecules that make life possible. Therefore, despite the difference between the LR responses of the heated and unheated samples, some scientists hypothesized that the soil might indeed contain a simple chemical, perhaps an oxidizing agent such as H₂O₂ that if present in high enough quantities would have converted the nutrients into CO₂. This, they explained, might have also accounted for the results of the GEx, which while negative did include the release of oxygen from the soil. And perhaps at the same time H₂O₂ would have also oxidized any organic molecules present in the soil to CO₂. An oxidizing agent would thus account for three findings: The positive result of the LR experiment, the negative result of the GEx, and the negative result of the search for the stuff of life.

VIDEO?: Juan Oró will speak in support of the H₂O₂ hypothesis; he will explain how H₂O₂ could have mimicked the effects of life on the LR experiment. During his talk, the earlier animation, showing biological oxidation of organic molecules and oxidation by H₂O₂ side by side will return. Oró has actually already been taped and, since he talks specifically about formic acid as the particular LR substrate that he believes to have reacted with H₂O₂, the animations will show formic acid as the particular organic molecule being oxidized.

But the oxidant hypothesis had a major drawback: If the Martian soil contained chemical oxidizing agents, why did the PR experiment show REDUCTION was taking place? If everything that happens in the Martian soil occurs in the absence of life then the chemical process occuring may be either oxidation or reduction BUT NOT BOTH, for these two processes are opposites of one another. For years the Viking results have remained a mystery.

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