New analysis of an experiment performed by the Viking landers suggests that evidence of microbial life in the Martian soil may have been detected 36 years ago. As one of the authors of this new paper puts it: "on the basis of what we've done so far, I'd say I'm 99 percent sure there's life there." Whoa.
The experiment that the researchers looked at has been a controversial one for a very long time. It was called the Labeled Release (LR) experiment, and it was one of a set of four different tests that the Viking landers carried to try to detect life on Mars. In the LR experiment, the lander scooped up a sample of Martian soil and dumped it into a chamber which was then sealed up. A drop of a slightly radioactive nutrient solution was added, and then the air above the soil sample was monitored so see if there was anything alive in the soil metabolizing those nutrients.
To the surprise of everyone, the LR experiment detected a steady stream of radioactivity coming out of the soil after the nutrient solution was added. Something was definitely going on. However, the other three experiments didn't come up with anything at all, and the consensus back in 1976 was that the LR result was just some chemical reaction caused by rocks as opposed to any sign of microbial life.
New research published last month in the International Journal of Aeronautical and Space Sciences has taken a fresh look at the Viking LR experiment results, and the authors of this paper seem confident that the best way to explain the data is through the existence of microbial life after all.
First, the researchers do away with the geochemical reaction hypothesis since when the Viking landers ran the LR experiment a second time (three to five months later), nothing happened, and there's no reason why a chemical that was in the soil to begin with would just stop reacting over time like that.
However, the lack of evidence for a chemical reaction is not positive evidence for life, and for that, the researchers turned to "complexity analysis," looking at how the radioactive gas signal changed over time and how those changes indicate a biological process. For example, in previous research the authors showed that the signal exhibited a circadian rhythm which synced up almost exactly with the Martian day and night cycle, something that you would expect from something living but you wouldn't expect from a rock. This same basic principle can be extended to other analyses, essentially attempting to separate live processes (which are complex) from geological and chemical processes (which are less so).
In doing this, the researchers found that they could "clearly distinguish between active and control Martian LR experiments," meaning that that there was a significant difference between experiments where they were sure there wasn't life, and where there might have been life. Based on this, the researchers state that "we believe that these results provide considerable support for the conclusion that the Viking LR experiments did, indeed, detect extant microbial life on Mars."
Of course, this is not anywhere close to proof that there's currently life on Mars, and there's still plenty of work that can be done on Earth to validate some of the techniques that the researchers have used to suggest evidence for Martian life. Further analysis should be published this August, which is, coincidentally, about the same time that Curiosity will be making its landing on Mars.