In 1976, NASA's Viking 1 and 2 landers conducted the first and only direct experiments on Mars designed to detect biological activity. One experiment, the Labelled Release test, added nutrient-rich liquid to Martian soil and observed the release of carbon gases — a signal consistent with microbial metabolism. The results were statistically strong and repeated across samples, yet NASA ultimately dismissed them, citing the lack of organic molecules found by other instruments and suggesting non-biological chemical reactions as the cause.

For decades, the official position held that Viking found no conclusive evidence of life. However, some scientists, including Harvard astrophysicist Avi Loeb, argue that the decision to stop investigating was a major missed opportunity. They contend that the scientific community’s reluctance to pursue false positives led to a potentially far costlier false negative — ignoring the first detection of extraterrestrial life.

Despite advances in astrobiology and multiple subsequent Mars missions, no follow-up experiment has replicated the Viking metabolic test. Critics say this reflects a risk-averse culture in mainstream science, one that prioritizes avoiding errors over pursuing disruptive discoveries. With renewed interest in Mars and upcoming sample return missions, the debate over how to interpret ambiguous biosignatures remains central to the search for life beyond Earth.