Previously dismissed claims for evidence of life in Martian soil samples collected more than 20 years ago now appear to have been right on the money. So says a University of Southern California biologist who recently reanalyzed the data and presented his findings last Friday at an astrobiology symposium held during the annual meeting of the International Society for Optical Engineering.

Back in the 1970s, the NASA researchers who first studied the soil, which had been gathered by the Viking Landers 1 and 2, found clear indications of gas release that they believed came from living organisms. Others countered, however, that such gases more likely resulted from chemical reactions among highly reactive inorganic compounds in the soil, and the argument for life on the Red Planet fell by the wayside.

More than two decades later, USCs Joseph Miller, who was preparing a proposal to NASA to conduct biology experiments on future Mars missions, stumbled across the old data. A figure in an old geophysical journal report caught his eye. The gas emission, Miller noted, had been highly periodic. Subsequent investigation revealed that, in fact, the gas release signal followed a circadian rhythm. "It had a precise circadian rhythm of 24.66 hours," he reports, "which is particularly significant because its the length of a Martian day." Specifically, the gas emission fluctuations seemed to be entrained to a two degrees Celsius temperature fluctuation inside the lander.

The idea that such a pattern of gas release could have resulted from reactions among inorganic compounds in the soil now seems rather implausible, Miller says. "For one thing, there has since been research that shows that superoxides exposed to an aqueous solution like the nutrient solution in the experiment will quickly be destroyed. And yet the circadian rhythms from the Martian soil persisted for nine straight weeks," he notes. Furthermore, "there is no reason for a purely chemical reaction to be so strongly synchronized to such a small temperature fluctuation. I think back in 1976 the Viking researchers had an excellent reason to believe theyd discovered life; Id say it was a good 75 percent certain," Miller remarks. "Now, with this discovery, Id say its over 90 percent. And I think there are a lot of biologists who would agree with me."