860 Encyclopedia of the Solar System
TABLE 8 A Comparison of GEx O 2 and LR^14 C ResultsaGEx O2 Oxidantb LR CO 2 Oxidantb
Sample (nmol cm−^3 ) (KO 2 →O 2 ) (nmol cm−^3 )(H 2 O 2 →O)Viking 1(surface) 770 35 ppm/m ∼ 30 1 ppm/m
Viking 2(surface) 194 10 ∼ 30 1
Viking 2(sub-rock) 70 3 ∼ 30 1
aAfter Klein 1979.bAssuming a bulk soil density of 1.5 g cm− (^3).
prescribed intervals, a sample of the gas above the sample
was removed and analyzed by a gas chromatograph.
The GEx results were startling. When the Martian soil
was merely exposed to water vapor, it released oxygen gas
at levels of 70–700 nanomoles per gram of soil, much larger
than could be explained by the release of ambient atmo-
spheric oxygen that had been absorbed onto the soil grains.
The GEx results are summarized in Table 8. It was clear
that some chemical or biological reaction was responsible
for the oxygen release. A biological explanation was deemed
unlikely since the reactivity of the soil persisted even after
it had been heat sterilized to temperatures of over 160◦C.
Furthermore, adding the nutrient solution did not change
the result that some chemical in the soil was highly reactive
with water.
The Labeled Release (LR) experiment also searched for
evidence of heterotrophic microorganisms. In the LR ex-
periment, a solution of water containing seven organic com-
pounds was added to the soil. The carbon atoms in each
organic compound were radioactive. A radiation detector
in the headspace detected the presence of radioactive CO 2
released during the experiment. Any carbon metabolism
in the soil would be detected as organisms consumed the
organics and released radioactive CO 2.
When the LR experiment was performed on Mars, there
was a steady release of radioactive CO 2 (Table 8). When the
soil sample was heat sterilized before exposure to the nutri-
ent solution, no radioactive CO 2 was detected. The results
of the LR experiment were precisely those expected if there
were microorganisms in the soil sample. Taken alone, the
LR results would have been a strong positive indication for
life on Mars.
In addition to the three biology experiments, another
instrument, a combination of a gas chromatograph and a
mass spectrometer (GCMS), gave information pivotal to
the interpretation of the biological results. This instrument
received Martian soil samples from the same sampling arm
that provided soil to the biology experiments. The sample
was then heated to release any organics. The decomposed
organics were carried through the gas chromatograph and
identified by the mass spectrometer. The only signal was
due to cleaning agents used on the spacecraft before launch.
No Martian organics were detected. However, the samples
were heated to only 500◦C, and highly refractory organics
would not have been volatilized at this temperature. In ad-
dition, it is now known that iron compounds in the soil could
have interfered with the release of organics. The limit on the
concentration of organics that would remain undetectable
by the GCMS was one part per billion. A part per billion of
organic material in a soil sample represents over a million
individual bacterium, each the size of a typicalEscherichia
coli. This may not seem to rule out a biological explanation
for the LR results. However, all life is composed of organic
material and it is constantly exuded and processed in the
biosphere. On Earth, it is difficult to imagine life without a
concomitant matrix of organic material. This apparent ab-
sence of organic material is the main argument against a
biological interpretation of the positive LR results.
The prevailing explanation for the reactivity of the
Martian soil relies on the presence of reactive chemicals in
the Martian atmosphere. In particular, hydrogen peroxide
(H 2 O 2 ) is assumed to be produced by ultraviolet light in the
atmosphere and deposited onto the soil surface. Hydrogen
peroxide itself could explain many of the LR results includ-
ing the loss of reactivity with heating, but it cannot explain
the thermally stable results of the GEX. However, perox-
ide, possibly abetted by ultraviolet radiation could somehow
result in the production of the stable reactive chemicals re-
sponsible for the release of oxygen upon humidification and
the breakdown of organics in the LR experiment. In addi-
tion, these reactive chemicals would have broken down any
naturally occurring organic material or any material car-
ried in by meteorites on the Martian surface. Table 8 also
lists the concentration of oxidant necessary to explain the
Vikingresults for typical models of the chemistry of the
oxidants.
Amplifying the apparently negative results of theViking
biology experiments, the environment of Mars appears to
be inhospitable to life. Although the atmosphere contains
many of the elements necessary for life—it is composed
of 95% CO 2 with a few percent N 2 and Argon and trace
levels of water—the mean surface pressure is less than 1%
of sea level pressure on the Earth, and the mean temper-
ature is− 60 ◦C. The mean surface pressure is close to the
triple point pressure of water, which is the minimum pres-
sure at which a liquid state of water can exist. The low