In a recent opinion piece in Scientific American, former Viking Mars investigator Gil Levin fired up anew the argument over whether that mission did in fact discover Martian life in the 1970s, and whether we can now say there’s a conclusive case for life on the Red Planet.
He bases his case primarily on the results of Viking’s Labeled Release Experiment, for which he was the principal investigator. The experiment was designed to detect carbon dioxide in the Martian soil as a result of microbial metabolism. After Viking collected the soil, it was “spiked” with a set of organic nutrients, including amino acids. The measurements observed on Mars were roughly consistent what you would expect if life were present. However, most scientists later concluded that the released carbon dioxide was more likely due to inorganic reactions of the chemically reactive Martian soil. So for many years, Gil Levin remained the sole voice advocating that Viking had in fact found life.
Recent discoveries paint a more nuanced and complex picture. We are now confident that organic compounds do exist on Mars. And the trace organics detected by Viking, previously interpreted as contaminants, are now considered by many scientists to represent indigenous organic compounds. The apparent lack of organics was an important reason for critics dismissing Levin’s claims at the time of Viking. How could you have life if you had no organics?
Also, we now know that there is plenty of water on Mars, even today, some of it in liquid form underground, and that there were lakes on the planet’s surface when it was wetter and warmer. The puzzling Viking results could be explained biologically if putative Martian microorganisms contain strong oxidants such as hydrogen peroxide inside their cells that would have reacted with organic compounds to produce carbon dioxide when heated up—as was observed during the Viking sample preparation on Mars.
Levin’s case for Martian life may be even stronger. Personally, I think Viking’s Pyrolytic Release Experiment also was intriguing, and suggestive of the presence of life, because it showed that organic synthesis was occurring after the addition of organic nutrients.
Perhaps the mixed Viking results were due to the experiment design? Mars is drier than the driest desert on Earth, so any Martian microbe would have to be adapted to those conditions. I suspect that too much water was added during the life detection experiments, possibly putting the Martians under osmotic shock and practically “drowning” them in their first encounter with humankind!
Back to Gil Levin. He requested in his Scientific American article that a panel of expert scientists review all the pertinent data from his Viking Labeled Release Experiment in light of other evidence collected on Mars since then. Further, he suggests re-running the Labeled Release Experiments, but with compounds of different “handedness” (chirality).
Is an expert panel really needed to assess whether Viking discovered life? I don’t think so. Scientists compare older data with newly obtained results—such as those from the Curiosity Rover—all the time. And the consensus remains that the bar for claiming current life on Mars has not been reached. What we need is a new mission dedicated to life detection, especially before we send humans to Mars.
But I do agree with Levin’s second request. It would be highly instructive to run the Labeled Release Experiment one more time on Mars, with amino acids separated into left (L)-handed and right (R)-handed compounds. If carbon dioxide is produced by L-amino acids only, it would indicate life as we know it, because life on Earth—with very few minor exceptions— only uses L-amino acids. If it is produced by R-amino acids only, it would indicate life as we don’t know it, and possibly a separate origin of life on Mars (more exciting!). And if both types of amino acids react at roughly the same rate, it would support the explanation that there is no life, and that the observations are only due to a chemical reaction, because chemistry does not distinguish between the different handedness of molecules.
I was hoping that new planetary protection guidelines, as recommended by the NASA Planetary Protection Independent Review Board released last week, might make a modern life detection mission easier, or at least cheaper, by easing some of the sterilization requirements for spacecraft landing on Mars. But that does not appear to be the case. Nevertheless, a life detection mission to Mars should be a priority before sending astronauts.
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