The experiment on the two Viking probes to Mars called 'labeled release' produced positive results for life, detecting excess carbon dioxide when a soil sample was treated with water and nutrients. This was thought at the time to be due to metabolism of the nutrients by micro-organisms. I recall hearing news reports at the time that came close to declaring that life had been discovered on Mars. This was quickly pulled back, though, when another experiment, the gas chromatograph-mass spectrometer found no signs of organic compounds in the soil. Finding these was considered necessary for making the case for life. It is now believed that the experiment would have destroyed these compounds when it heated the soil for analysis. Perchlorates in the soil, not known of at the time, would have caused the destruction of organic compounds at the temperatures used in the experiment. So, we have the positive results from the labeled released experiment, and nothing conclusive to the contrary. The case for life on Mars seems to be looking up. Michael

I don't think there's been any proof of life, past or present, on Mars yet. Years ago, but I don't think it was as far back as 1976, there was a rock (possibly a meteorite that was a piece of Mars) that showed what looked like fossils of germs. I don't think the Vikings did much more than show the rocky deserts of Mars. Or were they just the ones that showed craters? Maybe they just orbited (that long ago).

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The two Viking missions to Mars, in 1976, were landers. They were equipped to chemically analyze the Martian soil for signs of life, which they did. The positive results of the labeled release experiments, at both landing sites, constitute strong evidence, but not proof, that life, in the form of micro-organisms, at least, exists therein. Ad Hoc, Non-biological explanations for the results of the experiments have been proposed. However, a group of scientists, including the one responsible for the Viking labeled release experiment have always maintained that life was discovered on Mars in 1976. The meteorite found in Antarctica seems to show fossils of micro organisms, though other explanations have been proposed. It has been established that this meteorite originated on Mars. Michael