In 1976 NASA's Viking program placed two spacecraft into orbit around Mars and landed two more on the planet's surface. The landers were equipped to perform experiments intended to detect organic compounds and biological activity. While the test for biological activity returned positive results, those meant to check for organic compounds were negative, leading to the reasonable and cautious conclusion that probably only inorganic chemical reactions had been observed.
But I recall reading back in the 1980s that one of the scientists in charge of the project (Gill Levin if I'm not mistaken) was skeptical of that view. In tests on Earth, he pointed out, the life-detection experiment his group had designed sometimes failed to detect signs of life, but never had it produced a false positive. This was because it included its own check for non-biological processes: One sample was heated before testing in order to kill any Martian bacteria present. If the unheated sample produced signs of biological activity and the heated one did not, that was a good indication that life had been detected. And that's exactly what happened on Mars.
Moreover, subsequent research has offered at least one plausible explanation for the initial apparent failure to detect organic compounds.
Now a newer analysis of the Viking results claims to find strong reason to think that the positive Viking findings were correct after all. Biological chemical reactions tend to be more complex than non-biological ones, and the data from the Viking experiments appear to have a level of complexity associated with life, according to the researchers. A news report at Space.com quotes one of the authors, neuropharmacologist and biologist Joseph Miller of the University of Southern California's Keck School of Medicine, as saying, "On the basis of what we've done so far, I'd say I'm 99 percent sure there's life there."
Others, of course, are less willing to go out on that limb, but it's certainly interesting news.
A couple of other recent articles on the Viking experiments:
- 88-year-old Gill Levin still thinks the positive Viking results were valid.
- Another study finds that Viking probably did detect organics after all, suggesting that rejection of the positive results may have been premature.