Two Viking spacecraft were launched by NASA in the
summer of 1975 to test once and for all whether there were signs of life
in the Martian soil or not. Each craft consisted of two modules: an orbiter
and a lander. The two landers set down on almost opposite sides of the
planet and carried experiments to test Martian soil for signs of life,
as well as several atmospheric and geological tools. The atmospheric pressure
was quickly confirmed to be between 6 and 8 millibars but gradually declined
over a few weeks. This was accounted for by the approach of winter in
the southern hemisphere which meant a drop in temperature and the production
of `dry ice' (frozen carbon dioxide) removed from the atmosphere to collect
as a frost on the southern polar cap.
Geologically, the composition of Martian rocks were
determined and found to be typically very rich in iron. This result is
significant because it indicates the possibility that Mars may not have
an iron rich core, but instead that the iron remained mixed up in the
main part of the planet. This accounts in part for the relatively low
density of Mars, the colour of the rocks (red) and also for the very weak
magnetic field (about 0.004 times as strong as Earth's). Mars is spinning
almost as fast as Earth, but has little magnetic field, implying either
that Mars has no molten iron centre, or at least a relatively small core.
More exciting though were the three biological experiments designed to
observe for the effects life would have on it's environment and the presence
of materials on Mars required to support `life as we know it'. The three
biological experiments on the Viking landers were:
- The Gas Exchange Experiment was designed to detect
anything resembling respiration (breathing). A sample of soil was collected
and placed in a sealed container along with controlled amounts of gases
and nutrients. This was then monitored for any alteration in chemical
- The Labelled-release Experiment was designed to
detect processes resembling metabolism. Again a small sample of soil
was collected and this time was moistened with nutrients containing
radioactive carbon. Any organisms would eat the nutrients and then emit
gases containing the radioactive carbon.
- The Pyrolytic Release Experiment was designed to
detect photosynthesis (the process by which plants on Earth use energy
from sunlight to synthesise organic compounds from carbon dioxide).
A soil sample was again placed in a container with radioactive carbon
dioxide and was exposed to artificial sunlight. If photosynthesis occurred,
radioactive carbon would become incorporated in the micro-organisms.
After initial excitement when all experiments showed
changes, it became apparent that Martian soil in fact contains chemicals
that cause it to effervesce (fizz) when moistened. Further analysis with
a 'gas-chromatograph mass spectrometer' showed there to be absolutely
no organic compounds in the Martian soil at all: no life was present,
and according to these results, none had ever been present in the soils
analysed. Despite this, there are some Viking project scientists who are
convinced that they did find evidence of life.