In 1976 NASA’s two Viking landers touched down on the surface of Mars. The probes conducted a host of biological experiments, including collecting samples of Martian soil to test for organic compounds – the building blocks of life – and biosignatures that could indicate the presence of microorganisms.
The landers found little evidence for organics, but the onboard Labeled Release experiment found a reactive agent in the surface material of Mars that produced increased carbon dioxide. Gilbert Levin, an engineer who designed Labeled Release, concluded that this activity was triggered by living microorganisms lurking in the Martian soil. However, that interpretation has not been widely accepted by the scientific community.
More recent research has also called into question Viking’s negative results in searching for organic compounds. A study published in December 2010 in the Journal of Geophysical Research suggested that these compounds were present on Mars, but they were just destroyed by other chemicals before Viking could detect them.