All these years later, scientists continue to glean information from the Viking data and still debate whether the results indicate life in the soils of Mars. The pendulum has swung back and forth between chemical and biological explanations for the Viking results.

The experiment

The Labeled Release (LR) experiment produced the most tantalizing results. Like most forms of life on Earth, when we consume food, some is processed and combined with oxygen to produce energy. The gas carbon dioxide is a product of that process and is present in our expired air. The clever idea with the LR experiment was that the carbon atoms in the food were radioactive and therefore could be detected in the air if a creature in the Martian soil processed the food and generated carbon dioxide (or some other gas containing carbon). In the LR experiment a radioactive broth was added to a sample of Martian soil in a closed chamber. A detector in a side chamber measured the amount of radioactivity in the air. A series of experiments was conducted to help eliminate non-biological processes. For example, some involved heating the soil to high temperature to sterilize any micro-organisms.

Viking scientists back on Earth got very excited when the first results from both Vikings were received. There was a burst of gas production when the broth was first added to the soil. This did not happen when the soil was sterilized -- as might be expected if micro-organisms had died. However, when the unheated sample was measured over several weeks the gas production levelled off, suggesting that any micro-organisms, if present, had failed to survive and grow. Also when more broth was added there was no substantial release of gas. These conflicting results, coupled with the results of other Viking experiments led most scientists to conclude that the LR results were produced by non-biological reactions due to (unknown) chemicals in the Martian soil.

Harold Klein was Biology Team Leader for the Viking Project. He has been described as the father of exobiology -- the study of life beyond the Earth. Despite his strong desire to find life on Mars he understood the rigorous evidence that would be needed to convince the scientific world that non-terrestrial life had been discovered. Since the 1970s he has expressed skepticism about further claims of evidence of life in the Viking data. Sadly, Klein died in July this year.

Gilbert Levin was involved in the design of the LR experiment and the analysis of the results. For 25 years he has labored to show that the best explanation for the results is that they were produced by Martian organisms. He has collected microbes from Alaska and Antarctica and subjected them to the LR experiments with results that match some of the Viking results. He has experimented with a range of non-biological chemicals see if they can produce the same results. He has questioned assumptions about the absence of liquid water on Mars and the chemical effects of bombardment of the soil by ultraviolet radiation. Levin has made a tremendous contribution to the study of extra-terrestrial life and he has designed new experiments for future Mars missions that could resolve the ambiguous Viking results. Unfortunately his enthusiasm for the issue has put off some scientists who regard his claims about the Viking experiments extravagant and unfounded. Levin has pushed the pendulum towards a biological explanation for the Viking results but, as Klein pointed out, this as a long way from unambiguous evidence of life on Mars.

In a 1998 paper Klein questioned whether there were, as claimed by Levin, terrestrial organisms that could produce the LR results under the conditions encountered on Mars. If organisms were present in the Martian soil they came from extremely cold, dry conditions with no atmospheric oxygen. They were heated to about 10 degrees Celsius and held at that temperature for several days before a small amount of broth was added. Radioactive gas was detected almost immediately and increased quickly before tapering off after about two hours. After the initial reaction died down a second quantity of broth was added. There was an initial drop in radioactive gas readings -- possibly the moistened soil reabsorbed it. After this no substantial release of radioactive gas was observed. These were not results that are usually associated with micro-organisms on Earth. Klein also pointed out that the process which produced the gas was highly selective in the type of carbon compounds that it consumed -- a result that was more likely with a selective chemical reaction than a biological process.

From experience on Earth, if organisms were present in the Martian soil samples they were either dormant, and waiting for the "right" conditions to start the life processes, or they were extremely hardy organisms that managed to live normally in the harsh Martian conditions. If dormant, it seems surprising that they "awoke" so quickly when the nutrient was added. Levin refers to tests of Earthly lichen that gave a vigorous initial response and then apparently died from too much water. He suggests this might have occurred with Martian organisms. However, Klein noted that only a small quantity of broth was added in the Viking experiment so that a large portion of the soil was not drenched with the broth. Why didn't micro-organisms in the unaffected soil survive to produce gas in later experiments? The other puzzle is that the surface of Mars typically has not changed for millions of years so where did the dormant spores come from?

If the organisms were adapted to the Martian conditions it is surprising that they also found the much warmer and wetter conditions in the Viking chamber comfortable. Most organisms on Earth do not respond well to being heated to more than 40 degrees Celsius above their usual temperature. This does not rule out a remarkably tough Martian organism but is another reason to be cautious about the Viking results.

The other lingering question is the composition of the gas that was released. Radioactive carbon was present. This was probably carbon dioxide but could have been methane, some other gas or a mixture of gases. Earthly organisms generally require atmospheric oxygen to produce carbon dioxide but other sources of oxygen would be needed on Mars.