The Viking lab measured a rapid increase in oxygen, carbon dioxide and some nitrogen when a soil sample was saturated with liquid nutrients that astrobiologists thought would be tasty to Mars microbes. The out-gassing from the damp soil was like an Alka Seltzer tablet bubbling away. This reaction did not happen in samples that were sterilized by heat as a control. The second Viking lander recorded similar results 4,000 miles away.
However, the findings were dismissed almost immediately because no organic compounds were detected on the Martian surface by another Viking instrument. The building blocks of life as we know it apparently weren't there. It was like hearing music, but not finding the orchestra.
The apparent false positive in the Labeled Release experiment was attributed to peculiar properties of the Martian soil. Hydrogen peroxide in combination with other chemicals in the Martian surface had been theorized to produce false life signals. But lab experiments on Earth have never precisely duplicated the Viking data.
In 2007 Joop Houtkooper of the University of Giessen proposed that the test procedure inside Viking's onboard lab actually killed Mars microbes. He hypothesizes that they had a water-hydrogen peroxide metabolism that could not tolerate exposure to the "chicken broth" mix the soil sample were soaked in to nurture metabolic reactions. Instead, soaking microbes with water, as the Viking experiments did, would destroy them! The outgassing measured by Viking was the result of cell membranes disintegrating, he suggested.
In 2008, NASA's Phoenix lander found perchlorates in the Martian soil. These salts are powerful enough to combust organics. Tiny trace chemicals found in the Viking experiment were dismissed as contaminants from Earth. But they could really have been combusted organic compounds leftover from contact with perchlorates in the Martian soil.
In 2012, a numerical analysis of the Viking data was used to conclude the reactions measured in Viking's Labeled Release experiment were too complex to be simply oddball surface chemistry, and instead were evidence of biological activity.
The Land That Time Forgot
The Viking debate aside, we've learned a lot more about Mars since 1976. With each mission the circumstantial evidence for life has ratcheted up. Today we know that Mars has the energy, water and the chemical resources for supporting life.
Mounting geologic evidence points to Mars starting out as a habitable planet. But it grew colder and drier as its water froze, much of the atmosphere was ablated away by the solar wind, and a protective magnetic field fizzled away. Darwinian evolution should have ensured that primeval life, perhaps spawned in a great polar ocean, would find innovative ways to adapt and survive on a slowly dying world.
The MSL will continue this methodical search for life. The nuclear powered, Volkswagen-sized rover will look for environments that might support life and again look for organic compounds in the soil.
However, the rover does not have a biological lab to try again to culture any Mars microorganisms in an onboard Petri dish.
It's intriguing that very small traces of methane, perhaps from Martian organisms, have been discovered in Earth-based observations of Mars' atmosphere. What's more, the European Space Agency's Mars Express Orbiter detected traces of formaldehyde in the atmosphere. This could be the byproduct of the oxidation of methane by microbes, or instead geologic processes. If the MSL detects methane it will be able to measure the isotope ratios, which would allow astrobiologists to distinguish between a geochemical versus a biological origin of the gas.