Fifty years ago this August, the twin Viking spacecraft were launched toward Mars. They landed during the U.S. Bicentennial in 1976 at separate locations with three experiment packages designed to detect life if it existed. Two of the three yielded negative results on both landers, but one result was ambiguous. The labeled-release experiment detected unusual activity that the researchers could not explain when radioactively labeled nutrients were added to the soil. The activity gave rise to speculations that something alive in the soil was metabolizing the nutrients — speculations that, while remote, have lingered to the present day. 

Subsequent landers, beginning with Phoenix in 2008, discovered a high concentration of perchlorates in the soil. Perchlorates are chlorinated salts, often used in fireworks. These reactive salts were found to be almost ubiquitous on Mars. Now, in the journal Icarus, NASA astrobiologist Christopher McKay and two colleagues have determined that the reactions in the Viking landers can now be explained: “Perchlorate, plus abiotic oxidants, explains the Viking results and there is no requirement to postulate life on Mars.”