Did 40-year-old Viking experiment discover life on Mars?
October 21, 2016 by Lisa Zyga
The Viking 2 Lander site, showing frost on the ground. This image was taken by Viking 2 in 1979. Credit: NASA; Viking 2 Lander image P-21873
(Phys.org)—In 1976, two Viking landers became the first US spacecraft from Earth to touch down on Mars. They took the first high-resolution images of the planet, surveyed the planet's geographical features, and analyzed the geological composition of the atmosphere and surface. Perhaps most intriguingly, they also performed experiments that searched for signs of microbial life in Martian soil.
Overall, these life-detection experiments produced surprising and contradictory results. One experiment, the Labeled Release (LR) experiment, showed that the Martian soil tested positive for metabolism—a sign that, on Earth, would almost certainly suggest the presence of life. However, a related experiment found no trace of organic material, suggesting the absence of life. With no organic substances, what could be, or seem to be, metabolizing?
In the forty years since these experiments, scientists have been unable to reconcile the conflicting results, and the general consensus is that the Viking landers found no conclusive evidence of life on Mars. However, a small minority of scientists argues that the Viking results were positive for life on Mars.
One prominent proponent of this view is Gilbert Levin, Experimenter of the Viking LR experiment. At first, Levin thought that the LR results were unclear, and stated merely that the results were consistent with biology. However, in 1997, after many years of further experiments on Earth, along with new discoveries on Mars (which NASA has now declared "habitable"), and the discovery of microorganisms living under conditions on Earth as severe as those on Mars, he and his Viking Co-Experimenter, Dr. Patricia A. Straat, have argued that the Mars results are best explained by living organisms.
Recently, Levin and Straat published a perspective piece in the journal Astrobiology in which they reconsider the results of the Viking LR experiment in light of recent findings on Mars and recent proposals for inorganic substances that may mimic the observed metabolism-like processes. They argue that none of the proposed abiotic substances sufficiently explains the Viking results, and that Martian microbes should still be considered as the best explanation of the results.
How the Labeled Release experiment worked
In the LR experiment, both the Viking 1 and Viking 2 landers collected samples of Martian soil, injected them with a drop of dilute nutrient solution, and then monitored the air above the soil for signs of metabolic byproducts. Since the nutrients were tagged with radioactive carbon-14, if microorganisms in the soil metabolized the nutrients, they would be expected to produce radioactive byproducts, such as radioactive carbon dioxide or methane.