The Viking biology results that never settled
One of Viking's three biology experiments came back positive at both landing sites. The gas chromatograph found no organic molecules at all, and the argument has never closed.
Each Viking lander carried three biology experiments packed into a single box roughly a cubic foot in volume, plus a separate gas chromatograph–mass spectrometer. A robotic arm scooped martian soil into all four. This was, and remains, the only time anyone has flown a direct life-detection experiment to another planet.
The Labeled Release experiment, run by Gilbert Levin of Biospherics, dripped a dilute nutrient solution onto the soil with its carbon atoms replaced by radioactive carbon-14. If anything in the dirt metabolised the nutrients, it would exhale tagged carbon dioxide, and the detector would count it. At both landing sites, some 6,500 kilometres apart, the counter immediately climbed. Then came the control that made it serious: a duplicate sample heated to 160°C before the nutrient was added produced no response at all. Heat-sensitive, reproducible, prompt. It is exactly the curve you would draw in advance if you were asked what a positive result should look like.
The other instruments did not cooperate. Gas Exchange saw a burst of oxygen the moment the soil was humidified — too much, too fast, and with none of the follow-through biology would give. Pyrolytic Release returned a weak signal that nobody could firmly replicate. And Klaus Biemann's mass spectrometer, sensitive to parts per billion, found no organic molecules in martian soil whatsoever. That was the result that decided it. Even with no life at all, meteorites should have been salting the surface with carbon compounds for billions of years. Finding none implied something in the soil was destroying organics, and a soil that aggressive could plausibly fake the Labeled Release signal. The consensus landed on chemistry: reactive oxidants, built by ultraviolet light in a thin, unshielded atmosphere.
Three decades later the ground shifted. In 2008 NASA's Phoenix lander found perchlorate salts in the arctic soil. Heat perchlorate with organic material — which is precisely what Viking's instrument did to every sample, baking it to hundreds of degrees before measuring anything — and you destroy the organics and produce chlorinated by-products instead. Chlorinated compounds had appeared in Viking's data and been written off as contamination from Earth-side cleaning fluid. In 2018 Curiosity's SAM instrument found genuine organic molecules preserved in three-billion-year-old mudstone at Gale crater. Viking's null result no longer means what it was taken to mean in 1976.
That does not make the Labeled Release a life detection. Most researchers still favour perchlorate and peroxide chemistry, and the honest position is that no abiotic model reproduces every feature of the data and no biological one is required by it. Levin argued for life on Mars until he died on 26 July 2021, aged 97. Nobody has flown the experiment again in fifty years.