NASA Viking Mission: A Perspective of the Labeled Release Biological Experiment on Mars

The NASA Viking mission successfully landed three separate biological experiments on Mars in 1976 with a goal to detect microbial processes or materials in samples of the planet's regolith. While the Labeled Release (LR) experiment yielded robust signals, interpretation of data as a consequence of biological response or, alternately, chemical reactivity of regolith with the experiment's constituents was challenging during the mission. Laboratory experiments, conducted with the LR Test Standards Module to elucidate flight data, did not fully explain results of the planet experiments. Scientific debate between biological and chemical proponents of the LR findings has been fueled by subsequent Mars explorations that revealed that martian samples can contain (1) complex organic compounds and water, suggesting an environment that could have been amenable to microbial life and (2) oxychlorine compounds as reactive chemicals. This perspective is that of a microbiologist who supported the LR experiment during the Viking mission.