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Did the Viking rover actually discover signs of life on Mars in 1976?

In 1976, NASA sent two probes, Viking 1 and Viking 2, to the Red Planet to test for signs of life. Two of the scientists now argue that their experiments' results should be reexamined in light of more recent tests. 

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    This image mosaic taken by the Mars Exploration Rover Spirit's panoramic camera in 2004 shows an area southwest of the rover's landing site. A new paper suggests that the original Mars soil experiments in 1976 may still yield clues to the possibility of life on the Red Planet.
    NASA/JPL/Arizona State University/Cornell University/Reuters
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In a study published earlier this month in the journal Astrobiology, two researchers say the scientific community should take a closer look at a study of Mars’ soil published in 1976.

Why?

Because two NASA robots may have discovered signs of life on Mars almost four decades ago, say Gilbert Levin from Arizona State University and Patricia Ann Straat from the US National Institutes of Health.

Recommended: Man and Mars through history

It all started when NASA sent two probes, named Viking 1 and Viking 2, to Mars in 1976 to test for signs of life on the Red Planet. As the first spacecraft from Earth to reach Mars, the Viking probes conducted three studies on the planet’s biology.

To conduct one of the studies, the labed release (LR) experiment, scientists took soil picked up by the Viking probes and mixed it with nutrient-rich water. If the soil had signs of life, the scientists hypothesized, then the soil’s microbes would metabolize the nutrients in the water. Scientists conducted the experiment across a variety of Earth’s soils, from Death Valley to Antarctica, to disprove the potential for false positives. 

And to the scientists’ surprise, the microbes were metabolized and radioactive molecules were released – suggesting that Mars’ soil contained life.

However, the Viking probes’ other two experiments found no signs of life. Specifically, the Viking probes found no trace of organic material, which made the scientists ask: If there were no organic materials, what could be doing the metabolizing?  

Thus, NASA concluded that some non-biological processes must be responsible for the metabolization.

But Dr. Levin and Dr. Straat say the LR experiment needs to be reexamined in light of more recent evidence, arguing that it is possible that biological processes were responsible for the LR experiment’s results, after all. 

"Many believe that the martian environment is inimical to life and the LR responses were nonbiological, attributed to an as-yet-unidentified oxidant (or oxidants) in the martian soil," the authors write in their study. They conclude "that extant life is a strong possibility, that abiotic interpretations of the LR data are not conclusive, and that, even setting our conclusion aside, biology should still be considered as an explanation for the LR experiment."

In other words, the Viking probes didn't necessarily find evidence of life on the Red Planet in 1976 – but the possibility of such a claim needs to be left on the table, they say. 

And all of NASA's finding on Mars since the 1976 experiment, such as water and methane, supports the biological hypothesis even further, argue Levin and Straat. 

"Even if one is not convinced that the Viking LR results give strong evidence for life on Mars, this paper clearly shows that the possibility must be considered," Dr. Chris McKay, a senior editor of Astrobiology and an astrobiologist with NASA, tells Science Daily. "We cannot rule out the biological explanation. This has implications for plans for sample return from Mars and for future human missions."

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