After its flawless landing, Viking 1 performed the first LR experiment on July 30, 1976. The soil tested had been taken by the sampling arm from the surface to a depth of about four cm., placed in the distribution box and then dispensed to the LR. Immediately upon injection of nutrient, 14C-labeled gas began evolving. After about three days, the volume of the accumulating gas approached a plateau, but continued to show a very slight increase. At the end of the eight-sol Cycle 1 test, a second injection of nutrient was made. A sharp decrease in headspace gas occurred until about 20 % of it was re-adsorbed by the sample, after which a slow re-evolution of gas over the eight sols of Cycle 2 restored the full amplitude of Cycle 1. The protocol called for a control in the event of a positive response. Accordingly, a duplicate soil sample was inserted into a fresh cell, heated for three hours at 160o C to sterilize it (the control procedure established for all Viking biology experiments), allowed to cool and then was tested. It produced virtually no response, thus completing the pre-mission criteria for the detection of microbial life. Those criteria did not require a positive response to a second injection. Further, the LR tests showed that, isolated in the dark sample distribution box and held at ~ 10o C, the soil lost its activity over a period of two to three months. However, the positive responses had been obtained from soil samples that, prior to nutrient injection, had been stored several days under those same conditions. All VL1 LR results, as shown in Figure 5, support, or are consistent with, the presence of living microorganisms.
Four thousand miles away, Viking 2 landed. Its LR results there were very similar to those of VL1. Based on knowledge gained from the Viking 1 LR results, more definitive controls were run to further discern the nature of the active agent. These included moving a rock to permit taking a soil sample not exposed to UV light for geological time. Its active response refuted an initially prevalent theory that the LR response was caused by UV light activation of the soil. Another test demonstrated that even modest heating of the soil significantly depressed its response. The active agent in the soil, initially responsive at 10o C, was greatly inhibited or inactivated by heating to 46o C or 51o C, as are a variety of terrestrial microorganisms when subjected to similar thermal differentiation (e.g. E. coli v other coliforms). As with VL1, months-long storage of the soil in the distribution box inactivated the agent. All LR results of VL2 are shown in Figure 6. As with VL1, all results support, or are consistent with, the presence of living microorganisms.