Two landers, three experiments
In 1976, Viking 1 landed near the Martian equator and Viking 2 further north, each carrying a set of experiments built specifically to test Martian soil for signs of microbial life. Under a biology team led by Harold Klein at NASA Ames, three separate experiments took different routes to the same question: gas exchange, run by Vance Oyama, labeled release, run by Gilbert Levin of Biospherics, and pyrolytic release, run by Norman Horowitz of Caltech. None of them looked for life directly; each inferred it, or ruled it out, from a chemical signature that a living metabolism should leave behind, on the assumption that Martian microbial life, if present, would process nutrients or fix carbon in a broadly recognisable way.
A positive signal
Gas exchange replaced the sealed chamber’s atmosphere with inert helium, wetted a soil sample with a nutrient solution, and monitored the resulting gases for changes in oxygen and carbon dioxide. Labeled release added a nutrient solution tagged with radioactive carbon-14 to soil and watched for radioactive gas being given off, which would suggest the nutrients were being metabolised. Pyrolytic release exposed soil to a simulated Martian atmosphere containing radioactive carbon compounds, together with light and water, then baked the sample at a high temperature to check whether carbon had been incorporated into organic material. Running alongside all three, a gas chromatograph-mass spectrometer built by Klaus Biemann at MIT heated separate soil samples and searched directly for organic molecules, sensitive enough to detect them at parts-per-billion concentrations.
No organics found
The labeled-release result was the most striking: soil in contact with the radioactive nutrient mixture began releasing radioactive gas almost immediately, and that response shrank when the soil had first been heated to a moderate temperature and disappeared entirely after heating to a higher one for several hours, a pattern that looked like a heat-sensitive biological process being switched off. Pyrolytic release reported a small but measurable amount of organic matter formed under its test conditions. Those raw measurements have not been challenged as measurements; they occurred, and later laboratory work has reproduced comparable signals under controlled conditions. What has never been settled is whether either result reflects a living process or an unfamiliar piece of Martian soil chemistry.
Levin’s dissent
The strongest challenge came from the mass spectrometer running in parallel: it found no significant organic molecules in the same soil, which many scientists took as decisive, since even dead microbial matter should leave organic residue behind. For most of the biology team, that absence outweighed the positive labeled-release signal, and the working conclusion became that Viking had not found life. That conclusion has itself been complicated since: a later Mars lander, Phoenix, found perchlorate salts in Martian soil in 2008, and it turns out perchlorate destroys organic molecules at the higher temperatures the mass spectrometer used to release them, meaning the original negative organics result may not have ruled out organic material being present in the sample at all.
Perchlorates change the picture
The unresolved argument has mattered well beyond one Martian soil sample, because it exposed how hard it is to design an experiment that can tell biology apart from unfamiliar chemistry when there is no independent way to check either possibility on-site. A 2013 study showed that perchlorate exposed to radiation can react with amino acids to reproduce the labeled-release pattern without any organism involved, strengthening the chemical explanation, yet Gilbert Levin, who designed the labeled-release experiment, has continued to argue that the heat-sensitivity of the response is hard to explain by chemistry alone. The disagreement shaped how later missions, including the Curiosity rover and the ExoMars programme, have approached the search for organics on Mars, with far more attention now paid to the soil’s background chemistry before any biological claim is made.
Still open
This is worth an hour precisely because it refuses to resolve. Nearly fifty years on, the same handful of readings, a burst of radioactive gas, an absence of organic molecules, a later discovery of a salt that could explain both, are still being argued over by people who worked on the original mission and by chemists testing new explanations. It is a rare chance to watch a genuinely open scientific question age in real time, complete with a lone dissenting expert whose objections have not gone away just because most of the field moved on. Anyone expecting a verdict on life on Mars will not get one here, and that absence is the actual point of the story.