July and September 1976
Nasa's Viking 1 and 2 landers touch down on the surface of Mars. Instruments on the spacecraft provide valuable information about the planet's atmosphere and surface.
Biological experiments discover strange chemical activity in the Martian soil but no clear evidence of life or any of the organic compounds found on Earth. The planet is pronounced sterile.
This is challenged two decades later when Nasa claims to have found microbial fossils inside a Mars rock that fell to Earth.