The Labeled Release (LR) life detection experiment on NASA’s Viking mission to Mars in 1976 yielded positive results, yet NASA has never followed up on the findings, according to the Scientific American.
While the LR returned with positive results, the Viking Molecular Analysis Experiment failed to detect organic matter and NASA subsequently concluded that the LR had found a substance that mimicked life, but was not life. All Mars landers since then have not carried a life detection instrument to follow up on the initial results from 1976.
Yet, it would take a near miracle for Mars to be completely sterile as laboratory experiments have demonstrated that some Earth microbial species could survive the Martian environment.
The Viking LR sought to detect and monitor active metabolisms, a strong indicator of living microorganisms, and while no false positive or false negative results were obtained, it strongly supports the reliability of the LR Mars data.
Subsequent missions to Mars as well as discoveries on Earth have led to further questions on the incomplete definitive nonbiological explanation of the Viking LR results.
Although NASA announced that its 2020 Mars lander would not contain a life-detection test, formal proposals to either confirm or disprove the existence of life on Mars have been submitted.