From National Geographic:
In 1976 NASA sent two space probes, Vikings 1 and 2, to Mars to determine whether life exists on the red planet. The probes carried three experiments specially designed for the task, one of which was called the Labeled Release (LR) apparatus.
The LR experiment worked by scooping up a bit of Martian soil and mixing it with a drop of water that contained nutrients and radioactive carbon atoms.
The idea was that if the soil contained microbes, the life-forms would metabolize the nutrients and release either radioactive carbon dioxide or methane gas, which could be measured by a radiation detector on the probe.
A number of control experiments were also performed....
"The minute the nutrients were mixed with the soil sample, you got something like 10,000 counts" of radioactive molecules—a huge spike from the 50 or 60 counts that constituted the natural background radiation on Mars, said study team member Joseph Miller, a neurobiologist at the University of Southern California and a former NASA space shuttle project director.
Unfortunately, the LR experiment results were not backed up by the probes' other two experiments, both of which came out negative for life, so the space agency dismissed the possibility.
Now, after running Viking's LR data through a mathematical test designed to separate biological signals from nonbiological signals, Miller's team believes that the LR experiments did indeed find signs of microbial life in Martian soil.
"It's very possible that if you have microbes, they're living a couple of inches beneath the soil, close to water ice," he said.
For the study, Miller and mathematician Giorgio Bianciardi, of Italy's University of Siena, used a technique called cluster analysis, which groups together similar-looking data sets.
"We just plugged all the [Viking experimental and control] data in and said, Let the cluster analysis sort it," Miller said. "What happened was, we found two clusters: One cluster constituted the two active experiments on Viking and the other cluster was the five control experiments."
To bolster their case, the team also compared the Viking data to measurements collected from confirmed biological sources on Earth—for example, temperature readings from a rat—and from purely physical, nonbiological sources.
"It turned out that all the biological experiments from Earth sorted with the active experiments from Viking, and all the nonbiological data series sorted with the control experiments," Miller said. "It was an extremely clear-cut phenomenon."
The team concedes, however, that this finding by itself isn't enough to prove that there's life on Mars.
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Still, the new findings are consistent with a previous study published by Miller, in which his team found signs of a Martian circadian rhythm in the Viking LR experiment results.
Circadian rhythms are internal clocks found in every known life-form—including microbes—that help control biological processes, such as waking, sleeping, and temperature regulation.
On Earth this clock is set to a 24-hour cycle, but on Mars it would be about 24.7 hours—the length of a Martian day.
In his previous work, Miller noticed that the LR experiment's radiation measurements varied with the time of day on Mars.
"If you look closely, you could see that the [radioactive-gas measurement] was going up during the day and coming down at night. ... The oscillations had a period of 24.66 hours just about on the nose," Miller said.
"That is basically a circadian rhythm, and we think circadian rhythms are a good signal for life."