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:

1. 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 composition.
2. 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.
3. 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.