Because life is the most complex phenomenon, the detection of any chemical on Mars is unlikely to be accepted as proof of life. Therefore, the demonstration of active metabolism was the basis of the LR life detection experiment. A simple diagram of the experiment is shown in Figure 1.
The nutrients were selected for the LR based on theory and experiment. All the nutrients, or substrates, were simple Miller-Urey molecular compounds believed to have formed early on primitive Earth and, therefore, likely to have been incorporated into the earliest life forms, and probably retained throughout their evolutionary process. Each candidate nutrient was uniformly tagged with 14C. Those nutrients having optical isomers were included as racemic mixtures to make either stereoisomer available to potential Martian life. The nutrients were used in minimal concentrations in pure aqueous solution to preclude possible toxicity as sometimes occurs when microorganisms are overly dosed with organic and/or inorganic matter. Table 1 presents the LR nutrients showing their concentrations and activities.
Thousands of tests were made on microbial species, covering all types available: pure cultures, mixed cultures and soils; and many field tests of soils were conducted over a wide range of environments during the twenty years of development of the LR. Examples of field tests made with the early “sticky string” version of the instrument, which ejected and reeled in a silicone-covered string to collect its sample, are shown in Figures 2 to 4. False positives were never obtained from sterilized samples. Certainty of response from living organisms, sensitivity (to as little as ~30 cells/g), and rapidity of response provided a high level of confidence in the experiment.