Overview of Labeled Release Experiment

The Labeled Release (LR) experiment on the Viking Lander spacecraft was designed to detect heterotrophic life by monitoring the release of radioactive gas from a sample of Martian soil that has been inoculated with a nutrient solution. In the LR experiment, a sample of Martian soil was delivered to a culture chamber and moistened with a dilute aqueous solution of selected ¹⁴C-Labeled organic substrates. The headspace (i.e. volume of test cell above the sample) was then continuously monitored for the evolution of radioactive gas as evidence for catabolism using two solid state beta detectors. The radioactive gas accumulated for the length of the experiment, nominally eight martian days. Additional nutrient could be applied and the kinetics of gas evolution followed for an additional eight martian days. The accumulated gas was purged and the soil sample was dried after the incubation period. Upon completion of a cycle (each sample analysis is referred to as a cycle) with a positive response, a control cycle was conducted in which a duplicate portion of the same soil was added to a fresh test chamber and heat-sterilized at 160°C for three hours, cooled, and similarly incubated.

Below is a diagram of the Viking Lander with the Biology Processor highlighted in yellow. The Biology Processor housed three experiments related to biology: the LR Experiment, the Gas Exchange (GEX) Experiment, and the Pyrolitic Release (PR) Experiment (Klein, 1974). he three biology experiments searched for microbial life in the soil of Mars. In addition, the Gas Chromatograph Mass Spectrometer (GCMS) Experiment, housed in the molecular analysis processor, analyzed soil for organic compounds.

Click image to enlarge (after Moore et al., 1987)

The LR experiment principal investigator (PI) was Dr. Gilbert V. Levin of Biospherics Inc. Levin worked closely with Dr. Patricia Ann Straat, co-investigator (Co-I), to design and develop the LR experiment. For Dr. Gilbert Levin's current interpretation of the LR results, visit http://www.biospherics.com/mars/index.html (Note this link will leave the PDS LR archive).