Definition of Life

Preliminary Steps
  Geological Survey
  Sample Collection

Present Life
  Spectroscopic Analysis
  Organic Analysis
  Biology Experiments

Past Life
  Thin Section
  Isotope Analysis


Experimental Design

Present Life: Biology Experiments

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

The Viking Composite Instrument (VCI) consists of sample chambers that are equipped to perform all three Viking experiments.  It is also equipped with a GC/MS in order to identify gaseous molecules that evolve from the samples and a 14C detector that tests for the presence of radioactively charged CO2 and other carbon based compounds.  This instrument weighs a total of 15.5 kg and runs on 55W of power.


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

Had terrestrial life been tested with the Viking biology instrument, the following results would have been expected:

Experiment Response for Sample Response for Heat-Sterilized Control
GX oxygen or CO2 emitted none
LR labeled gas emitted none
PR carbon detected none

If life was completely absent from Mars, as the GC/MS results suggested, these should have been the results from the biology experiments:

Experiment Response for Sample Response for Heat-Sterilized Control
GX none none
LR none none
PR none none

However, in highly simplified form, these were the actual results from Mars:

Experiment Response for Sample Response for Heat-Sterilized Control
GX oxygen emitted oxygen emitted
LR labeled gas emitted none
PR carbon detected carbon detected

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.

mitCopyright © 2000 Massachusetts Institute of Technology
Comments and questions to mission2004-students@mit.edu Last updated: 10 December, 2000