“This suggests a robust biological response. These analyses support the interpretation that the Viking LR experiment did detect extant microbial life on Mars.” -Bianciardi, Miller, Straat and Levin
On July 20, 1976, the Viking 1 lander touched down onto the Martian surface, followed just a few weeks later by Viking 2. On board both landers were a suite of three experiments designed to look for signs of life.
While the Gas Chromatograph-Mass Spectrometer and the Gas Exchange experiment both came back negative, the Labeled Release experiment — where nutrient-rich molecules tagged with radioactive carbon-14 were added to the Martian soil — gave off a positive release of radioactive CO2.