biological ?
inorganic oxidant ?
Viking LR Experiment
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Despite the encouraging results of the LR, the consensus of the Viking investigators was that there was not enough evidence to conclude that life had been discovered on Mars. The main reason for this was that the GCMS tests for organic molecules were negative and most of the team members did not think it possible that lifeforms could exist without leaving organic traces in the soil. It was hypothesized that the LR response had been produced by an inorganic oxidizing agent such as hydrogen peroxide present in the soil (this agent was also believed to have caused the release of oxygen and carbon dioxide in both heated and unheated samples of a third experiment called the Gas Exchange Experiment).
Those investigators who had suggested the oxidizing agent as the cause of the LR response speculated that the agent had specifically reacted with formic acid, which was one of the substrates (ingredients) used in the LR. Therefore, for simplicity, formic acid is shown on the example on the slide even though all LR substrates can react with H2O2 under the right conditions. However, it appears unlikely that the right conditions for peroxide oxidation in the LR device occur on Mars. Laboratory studies carried out on Earth using simulated Martian soil in combination with a catalyst believed to exist in the soil (gamma-Fe2O3) have not been able to demonstrate the same type of sensitivity to heat for H2O2 as was exhibited by the agent detected on Mars. Nevertheless, the possibility remains open that a variation of the simulated soil tested would allow for H2O2 catalysis to show a heat sensitivity resembling that of a biological agent. However, it has also been determined that, to produce the LR results, H2O2 would have to make up more than 2% of the Martian soil while experiments have demonstrated that the levels of UV radiation present at the Martian surface would prevent such high levels of H2O2 from accumulating. Therefore, H2O2 has, effectively, been eliminated as a plausible non-biological explanation of the LR results. Perhaps, in the long run, the Viking program will be remembered more for producing questions than answers but questions are the beginning of knowledge.
Future life detection missions may actually incorporate an improved version of the LR that would separately test individual enantiamers (left and right handed stereoisomers) of chiral LR substrates (cysteine is the LR substrate being considered for this test). A response due to an inorganic agent would be the same for both stereoisomers, whereas a biological response would favor one or the other.