Fifty years ago, humanity got its first up-close look at Mars. It was a profound disappointment—at least, for anyone who harbored dreams of a sister world just like Earth. NASA's first Mars flyby, executed by Mariner 4 in 1965, revealed the (mostly!) dry, red desert we know today.
Despite this blow to our hopes for Martian life, famed scientist Carl Sagan convinced NASA to install an astrobiology mission on the Viking landers, NASA's first probes to touch down on the Red Planet. There were four astrobiology experiments. A mass spectrometer looked for organics in the Martian soil, finding only trace amounts. The second experiment involved adding helium gas to a soil sample, then applying organics, other chemicals, and water in order to spur on life. The experiment hoped to find metabolic processes through gas exchanges, of which none were found. Another experiment heated soils for organic residue, and created a control sample for the other experiments.
And then there was the labelled release experiment. A soil sample was obtained, then water and other nutrients were added. If a carbon dioxide isotope was released, it would tentatively suggest that life was present. Of the four tests, this was the only one that gave a weak positive signal. But before anybody could get too excited, the signal faded in subsequent observations. It seemed that the Vikings had found nothing on Mars but dirt.
Or had they? Decades later, Joop Houtkooper, a professor at the University of Giessen, proposed something extraordinary: Vikings 1 and 2 actually found life, then killed it. The problem, according to Houtkooper, was that NASA tried to find life that was too Earth-like. Houtkooper published his results in Astrobiology in 2007. (Arxiv reprint available here.)