The two Viking landers had an experiment on board where they attempted to find life on the planet. The initial results said yes, there was life, but later there were questions raised about the results. What is the final verdict on the matter?
Initially it looked like there was life, then that was dashed, but newer research might be indicative of life after all.
From http://csep10.phys.utk.edu/astr161/lect/mars/viking.html, back in 1976 we had:
The 4 basic experiments that the Vikings carried out to search for evidence of life were:
- Gas Metabolism: look for changes in the atmosphere induced by metabolism in the Martian soil.
- Labeled Release: Look for release of radioactive carbon dioxide by metabolism from organic material labeled by radioactive carbon.
- Pyrolytic Release: Search for radioactive compounds in soil by heating soil exposed to radioactive carbon dioxide.
- Mass Spectrometer: Search directly in Martian soil for organic compounds known to be essential to Earth life.
Of them, only the Labeled Release experiment gave positive results, and the complete absence of any organic compounds in the Martian soil according to the mass spectrometer experiment suggests that the positive results were not evidence for life, but rather evidence for a complex inorganic chemistry in the Martian soil. Thus, the Viking verdict was that there was no evidence for present or past life on Mars.
But then in 2012, this was published in the International Journal of Aeronautical and Space Sciences:
(summary taken from a Discovery.com article, for those who can't access the ijass)
The new study took a different approach. Researchers distilled the Viking Labeled Release data, provided as hard copies by the original researchers, into sets of numbers and analyzed the results for complexity. Since living systems are more complicated than non-biological processes, the idea was to look at the experiment results from a purely numerical perspective.
They found close correlations between the Viking experiment results' complexity and those of terrestrial biological data sets. They say the high degree of order is more characteristic of biological, rather than purely physical processes.
They conclude that the complexity pattern seen in active experiments strongly suggests biology while the different pattern in the control responses is more likely to be non-biological.
So, no final verdict yet - as this is relatively new research, not universally accepted, and stemming from one set of data.