https://www.scientificamerican.com/article/looking-for-life-on-mars-viking-experiment-team-member-reflects-on-divisive-findings/

Patricia Straat served as co-experimenter on one of the most controversial experiments ever sent to Mars: the Labeled Release instrument on the Viking Mars landers. The experiment’s principal investigator, Gilbert Levin, insists to this day that the project found extraterrestrial life. Most scientists doubt this interpretation, but the issue has never been fully settled.

When Viking 1 and 2 touched down on Mars in 1976, each carried several instruments to study the planet and look for signs of life. The Labeled Release experiment mixed small samples of Martian soil with drops of water containing a nutrient solution and some radioactive carbon. The instrument then sampled the atmosphere of its internal chamber. If it detected the radioactive carbon, the thinking went, then microorganisms in the soil must have metabolized the nutrients and emitted the carbon. The air around a control version, in contrast, heated up to temperatures thought to kill microbes, should not have any radioactive carbon.