The Viking Labeled Release Experiment and Life on Mars. Past life on Mars

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    Selection of a particular compound was made after its metabolism was confirmed in each of a wide variety of microbial species in pure and mixed cultures and in soils. Designated VM1, the LR nutrient solution was an aqueous solution of 2.5 x 10-4 molar concentrations each of formate, glycine, D-alanine, L-alanine, D-lactate, L-lactate, and glycolate, each uniformly labeled with 14C. The LR experiment moistens a 0.5 cc soil sample with 0.115 ml of VM1 for a total of 3.91 uCi per application. Evolution of 14C gas from the moistened soil indicates microbial activity. Following such a positive response, a duplicate of the original sample is heated to impair or destroy any organisms present. After cooling, that sample is tested exactly as was the one that produced the positive result. A positive response from the second sample indicates that the response of the first was non-biological in nature, while a negative response, demonstrating that the causitive agent had been deactivated by heat, completes the evidence for life. The temperature regimen selected by the Viking Biology Team to distinguish a biological response from a non-biological one was 160o C for 3 hr.

    Thousands of soils and microbial cultures of a wide variety of species were tested. No sample with demonstrated living organisms failed to respond. The controls readily confirmed biological activity. (Only several of the hundreds of soils were sterile.) The metabolism of as few as 50 cells in lag phase was readily detected. In a final test of an LR flight instrument, 0.5 ml of a California soil was placed under the experimental conditions to prevail on Mars. After 3 days of the reduced atmospheric pressure, humidity and temperature, the LR substrate solution was injected. Radioactive gas immediately evolved and increased through the standard 8-day LR test period (see SN103, FIG. 6). A second injection of nutrient then produced an increased response. A duplicate of the test soil was heated to 160o C for 3 hours and, after cooling, was tested. Essentially no
    response from the heated sample occurred, completing the evidence that there had been living organisms in the first sample and that they had survived exposure to the martian test conditions.

    2.2 The LR on Mars

    The results from the first LR experiment at the Viking 1 Site on Mars are shown in FIG. 1. At the Viking 2 Site, some 4,000 miles away, the soil produced a similar positive response. After eight sols (one martian sol = 24.67 Earth hr) of continuing, but slowing, evolution of gas, the soil samples at both sites were injected with nutrient a second time. Each sample promptly reabsorbed approximately 20% of the gas evolved to that point. A duplicate of the active sample was heated at 160o C for 3 hr and, upon testing, gave a nil response. In additional controls, 60% less gas was evolved after heating a duplicate sample to 46.0░C. After another duplicate sample was heated to 51░C, the response became erratic and produced 90% less gas than did the active sample. Active Mars soils, enclosed in the Viking sample distribution box and held at approximately 10░C for 2 months at one site and 3 months at the other, showed no activity upon testing. In all, 9 tests were run. All of the Viking LR results6 at both sites are summarized in Table 1.

    Taken alone, the LR results were a strong positive indication of living microorganisms in the soil of Mars. The lability of the active agent in the soil to mild temperatures is difficult to explain otherwise. The difference between the responses after the heatings at 46.0░C and 51.5░C are reminiscent of laboratory microbiology analyses, such as the standard method7 for distinguishing fecal coliforms from the coliform group, which relies on small temperature differences.

    2.3 Defects in non-biological interpretations of LR results

    Updating the last review8 with more recent information, each of the above enumerated arguments against a biological interpretation of the LR Mars data will be addressed.

    1. No Organics: The Viking GCMS, designed to identify organic matter in the martian soil, failed to detect any at either Viking Lander site9. The presence of organic matter on Mars had been generally anticipated as a result of: photochemical synthesis from primitive atmospheric constituents10, synthesis from current martian atmospheric gases11,12 by UV, infall from comets and meteorites13 and from interplanetary dust particles14 (estimated15 at some 105 kg of organic carbon per year).

    The report of organic matter, including amino acids, inside the two SNC meteorites presumed from Mars, if verified, would confirm this anticipation. A report16 on EETA79001 found L-amino acid concentrations exceeding those for D-amino acids. Such chiral preference is found only in biological material. The finding17 of low molecular weight organic compounds being solar-stripped from the Hale-Bopp comet supports the widespread formation of organic matter.

    Nonetheless, the Viking GCMS negative results were widely accepted as persuasive "evidence" that the Viking samples contained some exotic chemistry, but no life18,19.

    However, the LR's finding20 of living microorganisms, as incorporated into FIG. 6, in Antarctic soil #726, in which the GCMS concurrently had detected no organic matter, provided a direct comparison of the two instruments. Moreover, a wet-chemistry analysis of Antarctic #726, performed as a control, found21 organic matter. Recently, the same soil, maintained in bonded storage by NASA, was re-examined22. The sample was subjected to prolonged digestion in concentrated HF/HCl and then pyrolyzed at 500░C (the highest pyrolysis temperature applied by the Viking GCMS).

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