Proceedings of Spie, SPIE-The International Society for Optical Engineering,
"Instruments, Methods, and Missions for the Investigation of Extraterrestrial
29 July-1 August 1997, San Diego, California
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
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,
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
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
4. The amplitude and kinetics of the LR response from
Mars was thought to be "too much too soon" for any putative martian
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.
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
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.