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Additional results have recently been obtained from the Labeled Release (LR) life detection experiment on Mars. On Viking Lander 2 an experiment using surface material obtained from under a rock shows a response essentially identical with those obtained from other surface samples. Further, after a second nutrient injection, there is an initial drop in the level of radioactive gas present in the test cell, followed by a slow, gradual evolution of radioactivity over the long incubation period. This gas evolution, which proceeds at a rate declining with time, attains a final level similar to that seen just prior to second injection. Following completion of this experimental cycle a fresh Martian surface sample was heat sterilized for 3 hours at 46°C prior to nutrient injection. The resultant evolution of radioactive gas is substantially reduced in agreement with results reported previously. These results are consistent with a biological response and also greatly narrow the number of possible chemical reactants. The current status of the Labeled Release experiment on Mars is summarized.

Three different types of biological experiments on samples of martian surface material ("soil") were conducted inside the Viking lander. In the carbon assimilation or pyrolytic release experiment, (14)CO(2) and (14)CO were exposed to soil in the presence of light. A small amount of gas was found to be converted into organic material. Heat treatment of a duplicate sample prevented such conversion. In the gas exchange experiment, soil was first humidified (exposed to water vapor) for 6 sols and then wet with a complex aqueous solution of metabolites. The gas above the soil was monitored by gas chromatography. A substantial amount of O(2) was detected in the first chromatogram taken 2.8 hours after humidification. Subsequent analyses revealed that significant increases in CO(2) and only small changes in N(2) had also occurred. In the labeled release experiment, soil was moistened with a solution containing several (14)C-labeled organic compounds. A substantial evolution of radioactive gas was registered but did not occur with a duplicate heat-treated sample. Alternative chemical and biological interpretations are possible for these preliminary data. The experiments are still in process, and these results so far do not allow a decision regarding the existence of life on the plonet Mars.

This report summarizes all results of the labeled release life detection experiment conducted on Mars prior to conjunction. Tests at both landing sites provide remarkably similar evolution of radioactive gas upon addition of a radioactive nutrient to the Mars sample. The "active" agent in the Mars sample is stable to 18°C, but is substantially inactivated by heat treatment for 3 hours at 50°C and completely inactivated at 160°C, as would be anticipated if the active response were caused by microorganisms. Results from test and heat-sterilized control Mars samples are compared to those obtained from terrestrial soils and from a lunar sample. Possible nonbiological explanations of the Mars data are reviewed along with plans for resolution of the Mars data. Although such explanations of the labeled release data depend on ultraviolet irradiation, the labeled release response does not appear to depend on recent direct ultraviolet activation of surface material. Available facts do not yet permit a conclusion regarding the existence of life on Mars. Plans for conclusion of the experiment are discussed.

The 1976 Viking Labeled Release (LR) experiment was positive for extant microbial life on the surface of Mars. Experiments on both Viking landers, 4000 miles apart, yielded similar, repeatable, positive responses. While the authors eventually concluded that the experiment detected martian life, this was and remains a highly controversial conclusion. Many believe that the martian environment is inimical to life and the LR responses were nonbiological, attributed to an as-yet-unidentified oxidant (or oxidants) in the martian soil. Unfortunately, no further metabolic experiments have been conducted on Mars. Instead, follow-on missions have sought to define the martian environment, mostly searching for signs of water. These missions have collected considerable data regarding Mars as a habitat, both past and present. The purpose of this article is to consider recent findings about martian water, methane, and organics that impact the case for extant life on Mars. Further, the biological explanation of the LR and recent nonbiological hypotheses are evaluated. It is concluded that extant life is a strong possibility, that abiotic interpretations of the LR data are not conclusive, and that, even setting our conclusion aside, biology should still be considered as an explanation for the LR experiment. Because of possible contamination of Mars by terrestrial microbes after Viking, we note that the LR data are the only data we will ever have on biologically pristine martian samples. Key Words: Extant life on Mars-Viking Labeled Release experiment-Astrobiology-Extraterrestrial life-Mars. Astrobiology 16, 798-810.