The Viking Missions had 5 Methods of Testing for Life:
1.Television cameras.
No images suggesting the presence of life were ever seen.
2.A gas chromatograph combined with a mass spectrometer.
This apparatus examined the martian soil for the presence of organic
molecules. Even though sensitive to concentrations in the parts per billion (ppb) range,
no organic matter was detected (except for traces of the solvents that had been used on
earth to clean the equipment). Even if organic molecules could be formed on Mars, the
intensity of the ultraviolet light at the surface would soon destroy them.
3.The Labeled-Release (LR) experiment
Metabolism is a universal property of life on earth. The LR experiment
was designed to look for evidence of catabolism by any microorganisms that might have been
present in the Martian soil. In this experiment, a soil sample was incubated with a dilute
soup of organic molecules (such as the amino acid glycine) which had been synthesized with
the radioactive isotope 14C.. Over a period of 10 days, the atmosphere above the sample
was monitored for the appearance of radioactive gases such as carbon dioxide (CO2).
The results:
A burst of gas production when the medium was first added but not
when the soil had been preheated to kill off any microoganisms it might have contained.
However, gas production did not increase as time went on (as would be expected if
living organisms were growing in the medium) and
Later additions produced no additional gas.
Thus most scientists concluded that the gas was produced by nonliving
chemistry (brought about by oxidizing agents in the soil).
4. The
Pyrolytic-Release (PR) experiment
The PR experiment was designed to look for evidence of
anabolism.; specifically whether there were any microorganisms in the martian soil that
could synthesize complex organic molecules from carbon dioxide (CO2) and carbon monoxide
(CO). In this experiment, a mixture of radioactive CO2 and CO was introduced into a vessel
containing a soil sample.
Because anabolism requires energy and the most important source of that energy here
on earth is sunlight (for photosynthesis), the incubation mixture was illuminated with a
bright arc lamp.
After 5 days, any unreacted CO2 and CO was flushed out of the system and then the
soil sample was heated to drive off any radioactive organic molecules that might have been
synthesized.
The result:
organic
matter was detected in 7 of 9 runs. However, some positive results were achieved even on
runs where the soil had first been heated to such a high temperature that any
microorganisms present would have been killed (at least here on earth).
5. The Gas-Exchange
(GEX) experiment
In this experiment, a known mixture of gases was placed in the chamber
along with the soil sample and then analyzed periodically to see if any gases (e.g. CO2)
had disappeared from or been added to the mixture.
In the first part of the experiment, nutrient broth was added to the
chamber but not to the soil. There was a rapid release of large amounts of O2 (which
would not be expected from heterotrophic breakdown of organic substrates). This soon
subsided. smaller amounts of CO2 (an expected product of catabolism). One week
later, more nutrient broth was added; this time directly to the soil. There was another,
smaller, release of CO2 but no release of O2. The conclusion: the gases were formed by
nonbiological chemistry (oxidizing agents again).
So what can we conclude from this data?
The LR, PR, and GEX experiments all produced some positive results.
However:
all of these involved puzzling ambiguities, failing to behave as
similar tests done on earthly soil samples would have. All were later shown to be
reproducible here on earth by nonbiological chemistry. So the Viking studies
probably did not reveal the presence of life on Mars. But this is not the same as saying
that life does not now nor ever did exist on Mars!
Perhaps:
The upper layers of soil are inhospitable to life. Other places on Mars need to be
sampled.