Proceedings of Spie, SPIE-The International Society for Optical Engineering,
“Instruments, Methods, and Missions for the Investigation of Extraterrestrial Microorganisms.
29 July-1 August 1997, San Diego, California
Gilbert V. Levin*
The Labeled Release (LR) life detection experiment aboard NASA’s 1976 Viking Mission reported results which met the established criteria for the detection of living microorganisms in the soil of Mars. However, a variety of reasons led to the consensus of involved scientists that the positive responses at both Lander sites were caused by a chemical agent in the soil and not by microorganisms. In the years since Viking, new information from Mars and Earth has come to bear on this issue. Perhaps most spectacular are the analyses of SNC meteorites ALH84001 and EETA79001. The Viking LR experiment and each of the major chemical theories that have been proposed to explain it are reviewed in the context of these post-Viking developments, together with some Viking data hitherto unapplied to this important issue. Each of the theories attributing the LR results to chemistry is shown to have one or more key defects. It is concluded that the Viking LR experiment detected living microorganisms in the soil of Mars. Recommendations for confirming this conclusion in the near future are given.
Keywords: Labeled Release Experiment, Viking Mission to Mars, Life on Mars, Mars, Microorganisms on Mars, Extraterrestrial Life
Fresh speculation about life on Mars was aroused by reports of possible biochemical and microbiological fossil evidence of life found in meteorites ALH840011 and EETA790012, generally accepted as of martian origin. Together with other developments concerning life on Mars, a re-examination of this major scientific issue is warranted.
In 1976, the Viking Mission Labeled Release life detection experiment (LR) returned data from Mars fully satisfying the pre-mission criteria for the detection of microbial metabolism. Nonetheless, the consensus of scientists interpreting the Viking results was that the surprising activity the LR detected in the martian surface material should be attributed to chemistry and not biology.
1.1 Principal challenges to biological interpretation of LR data
1. No organic compounds were found in martian soil analyzed by the Viking Gas Chromatograph Mass Spectrometer (GCMS).
2. H2O2 , chemically formed in the upper atmosphere, was thought to descend to the soil and, directly or through forming complexes or compounds, to oxidize the LR substrates to evolve labeled gas.
3. It was assumed that there is no liquid water on Mars, and that its absence would make life impossible; and, as a corollary, the overall environment was believed too extreme to support life.
4. The amplitude and kinetics of the LR response from Mars was thought to be “too much too soon” for any putative martian biology.
5. Although not a pre-mission criterion for life, a second injection of LR nutrient onto positive samples failed to re-invigorate the evolution of gas, as generally occurs with terrestrial soils. Instead, some of the gas evolved after the first injection appeared to be reabsorbed into the soil.
6. No visual evidence for life was reported when the Viking camera images were examined.
7. UV light from the sun was thought to activate soil particles, which then disrupted the LR nutrient upon contact, releasing labeled gas. Also, UV light’s destructive effect was held to account for the reported lack of any organic matter on Mars.
8. Clays on Mars were proposed to react with the LR nutrient to release labeled gas.
9. A non-biological explanation of the Mars LR response seemed far simpler than proposing a separate origin of life on Mars. Application of Ockham’s Razor, therefore, indicated chemistry or physics over biology.
For one or more of the reasons stated above, the majority of involved scientists believed that the LR response was non-biological. However, the bases for that belief have been weakened. Over the intervening years since Viking, many new facts have emerged. Re-examination of the data and relevant scientific discoveries, including the evidence for life in the martian meteorites, requires a new evaluation. Each of the reasons enumerated above to support the non-biological rationale will be examined under new light.
NASA selected three life detection experiments for Viking based on three different assumptions about possible martian life. Therefore, it was explained, were there life on Mars, it would be likely that only one of the experiments would detect it.
2.1 The LR life detection experiment
A brief review of the Viking LR experiment is needed for the background to this discussion. The LR applied a small drop of nutrient solution to the center of a soil sample. The liquid spread outward, providing a wet-to-moist gradient through the sample. Organic substrates in the nutrient were at very low concentrations to prevent possible toxicity. For the same reason, no co-factors, growth supplements, or inorganic compounds were added (it was presumed that the soil already would have the ingredients required to sustain any life present). No buffer was used so that the sample itself would determine the pH. A slight helium overpressure and a temperature of 10░C were maintained to assure liquidity of the water during the experiment. Phase separation by gravity provided a signal free from the noise of the radioactivity still retained in the liquid. The LR experiment was the simplest and the most sensitive to microorganisms by orders of magnitude. Unlike the Pyrolytic Release (PR) experiment, it required no intervention (an optical filter) to prevent a false positive; unlike the Gas Exchange (GEx) experiment, it did not adjust the raw data (factoring in presumed coefficients of the martian soil and adjustments for probable gas sample size) prior to interpretation.
The LR experiment3,4 is based on the widespread metabolism by microorganisms of Miller-Urey compounds5, which are presumed to have been available for pre-biological evolution on primitive Earth.