Viking lander biological experiments
The Viking lander biological experiments were a suite of four in situ tests conducted in 1976 by the Viking 1 and Viking 2 landers to detect microbial life on Mars. The experiments—Gas Chromatograph–Mass Spectrometer (GCMS), Gas Exchange (GEX), Labeled Release (LR), and Pyrolytic Release (PR)—used soil samples collected by robotic arm and analyzed them under controlled conditions. While GCMS found no significant organic molecules, LR yielded positive, reproducible signals of radioactive gas release upon nutrient addition—signals that diminished or disappeared after heat sterilization or cold storage—prompting ongoing debate over whether the results indicate biological metabolism or abiotic oxidative chemistry, especially in light of later discoveries like perchlorate.