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.
The Viking 2 landing site in 1979
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.
Before launching the Viking spacecraft, the researchers tested the experimental protocol on a wide variety of terrestrial soils from harsh environments, from Death Valley to Antarctica. In each case, the experiments tested positive for life. Then as a control, the researchers heated the samples to 160 °C to kill all lifeforms, and then retested. In each case, the experiments now tested negative. To further confirm that the experimental procedure would not produce false positives, the researchers tested it on soils known to be sterile, such as those from the Moon and the Surtsey volcanic island near Iceland, which produced negative results as expected.
Once on Mars, the LR experiment was performed after the experiment searching for organic molecules came up empty-handed. So it came as a surprise when both Viking landers, located 4,000 miles apart, collected soil that tested positive for metabolism. To rule out the possibility that the strong ultraviolet radiation on Mars might be causing the positive results, the landers collected soil buried underneath a rock, which again tested positive. The control tests also worked, with the 160 °C sterilization control yielding negative results.
In addition, it seemed that whatever was doing the metabolizing was relatively fragile, since metabolic activity was significantly reduced when heating the sample to 50 °C, and completely absent when storing the soil in the dark for two months at 10 °C. Levin and Straat believe that these results provide some of the strongest evidence that the soil contained Martian life.