Detection of Proteins, Carbohydrates, Lipids and Nucleic Acids
The Viking Project was formally begun by NASA on
November 15, 1968 and was composed of two orbiters and two landers. The
project had been many years in planning and had as its principal goal a
direct determination of whether extant life existed in the red soil of the
planet. The Viking mission
was a spectacular technological and scientific success; data from the
mission, especially the orbiters, have been intensively analyzed ever
since by an enthusiastic science community. It is realized however, that
the specific life detection experiments carried out by the landers were
somewhat premature and that changes to the experiment are necessary in
order to ensure the further success of future missions.
Brief Description of Instruments
Image of the VCI
Labeled Release Experiment: This experiment adds a dilute solution of radioactively marked carbon compounds to a sample of Martian soil and tests for the emission of any radioactive gases containing carbon including CO2 and CO.
Pyrolytic Release Experiment: This experiment adds radioactively marked CO2 and CO to a sample of the soil and assumes that a microorganism would synthesize these radioactive isotopes and produce organic compounds. These organic compounds are then pyrolyzed and tested for in the adjoining 14C detector.
Gas Exchange Experiment: This experiment adds a thick nutrient solution to a sample of
Martian soil under dry, humid, and wet conditions. The sample chamber is attached to a gas chromatograph/mass
spectrometer (GC/MS), which tests for the presence of various gases.
The amount of the gases is compared to a control to determine which
gases might have evolved from a biological process.
Summary of Results
If life was completely absent from Mars, as the GC/MS results suggested, these should have been the results from the biology experiments:
However, in highly simplified form, these were the actual results from Mars:
These results baffled the Viking science team.
The results from the GC/MS seemed to imply that there were no organic
compounds on Mars, and yet the biological experiments suggested that there
were positive signs of metabolism. Since two of the three
experiments gave a false positive result for the heat-sterilized control
sample, it was believed that the experimental results can be explained by
invoking only purely non-biological processes. It is now know that
the surface of Mars is a highly oxidizing environment, which can explain
the results of the Viking experiments. Extensive testing with Viking
mockups on Earth show that such a hypothesis is very likely (Yen 2000).
Carr, Dr. Michael H. and Benton Clark et al. “An
Exobiology Strategy for Mars Exploration.” Online: http://cmex.arc.nasa.gov/MarsNews/mars_papers/Docs/mars_strat.html
NASA. “Viking missions.” Online: http://cmex.arc.nasa.gov/SiteCat/sitecat2/viking.htm
NASA, “Science on Mars.” Online: http://history.nasa.gov/SP-4212/ch11-5.html
Caplinger, Michael. “Life on Mars.” Online: http://barsoom.msss.com/http/ps/life/life.html
A. S. Yen, S. S. Kim, M. H. Hecht, M. S. Frant, and B. Murray.
"Evidence that the reactivity of the Martian soil is due to superoxide ions."
Science, 2000 September 15; 289: 1909-1912.
Copyright © 2000 Massachusetts Institute of Technology
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