1976, it was reported that "on Viking 2, the gas exchange experiment is producing analogous results to those from Viking 1. Again, oxygen disappeared once the nutrient solution came into contact with the soil. Again, carbon dioxide began to appear and still continues to evolve". Labeled release The labeled release (LR) experiment (PI: Gilbert Levin, Biospherics Inc.) gave the most promise for exobiologists. In the LR experiment, a sample of Martian soil was inoculated with a drop of very dilute aqueous nutrient solution. The nutrients (7 molecules that were Miller-Urey products) were tagged with radioactive 14C. The air above the soil was monitored for the evolution of radioactive 14CO2 (or other carbon-based) gas as evidence that microorganisms in the soil had metabolized one or more of the nutrients. Such a result was to be followed with the control part of the experiment as described for the PR below. The result was quite a surprise, considering the negative results of the first two tests, with a steady stream of radioactive gases being given off by the soil immediately following the first injection. The experiment was done by both Viking probes, the first using a sample from the surface exposed to sunlight and the second probe taking the sample from underneath a rock; both initial injections came back positive. Sterilization control tests were subsequently carried out by heating various soil samples. Samples heated for 3 hours at 160 °C gave off no radioactive gas when nutrients were injected, and samples heated for 3 hours at 50 °C exhibited a substantial reduction in radioactive gas released following nutrient injection. A sample stored at 10 °C for several months was later tested showing significantly reduced radioactive gas release. A CNN article from 2000 noted that "Though most of his peers concluded otherwise, Levin still holds that the robot tests he coordinated on the 1976 Viking lander indicated the presence of living organisms on Mars." A 2006 astrobiology textbook noted that "With unsterilized Terrestrial samples, though, the addition of more nutrients after the initial incubation would then produce still more radioactive gas as the dormant bacteria sprang into action to consume the new dose of food. This was not true of the Martian soil; on Mars, the second and third nutrient injections did not produce any further release of labeled gas." The 2011 edition of the same textbook noted that "Albet Yen of the Jet