In 1976, NASA’s Viking landers conducted a series of biological experiments designed to determine whether or not there was life present in the soil on Mars’ surface, the only such tests conducted on another planet. The results of the tests, at least at the time, perplexed the scientists in charge of the Viking landers’ experiments: while two of the three tests came up negative, the third experiment initially showed definitive signs of life—right before those same signs disappeared in subsequent runs of the same test. While the researchers studying the data rejected the positive results for a variety of valid reasons, discoveries regarding the chemistry of the Red Planet’s topsoil have brought renewed interest in the successful experiment, with one scientist hypothesizing that the very experiments meant to hunt for life there may have been toxic to the Martian microbes’ alien biochemistry.

The Viking 1 lander touched down in Mars’ Chryse Planitia on July 20, 1976, followed by the Viking 2 lander arriving at Utopia Planitia on September 3. Both landers carried a series of three experiments designed to search for biological life: the pyrolytic release experiment (PR), the labeled release experiment (LR), and the gas exchange experiment (GEX).

Included with these systems was a gas chromatograph-mass spectrometer that could be used along with the GEX test to identify any organic compounds that might be present in the Martian soil.

While the results of the PR, GEX, and Spectrometer experiments came back negative, each of the landers labeled release experiments initial results came back positive: the experiment applied drops of water, combined with nutrients that contained carbon-14 as a biological marker, to samples of Martian soil; if microbes were present in the samples they would presumably consume the nutrients—along with the carbon-14—and expel the mildly radioactive material as it metabolized the nutrients. On the first run, the tests on each of the landers came back positive: something in the Martian soil appeared to be metabolizing the added nutrients.

However, subsequent runs of the labeled release experiment failed to produce the same result, hinting that the success of the experiments first run may have been due to chemical not biological processes.

Additionally, the results of the other experiments were not promising: the PR experiment, designed to test for the presence of inert biological material, found traces of the compounds being sought after, but not enough to be considered a positive result. The GEX, combined with the mass spectrometer, also failed to detect the types of gases that were expected to have been released by native microorganisms, prompting the researchers to reject the positive results of the labeled release test.

“No bodies, no life,” was how Viking project scientist Gerald Soffen is said to have summed up the results.

However, this rejection of the LR’s positive results may have been premature: while the other tests failed to confirm the presence of organic compounds in the Martian soil, the same tests were found to have failed using soil samples from Antarctica.

High levels of ultraviolet radiation bombard the Martian surface because Mars does not have a UV-shielding ozone layer like the one surrounding Earth— this combined with chemical compounds called perchlorates found to be present in Mars’ soil by the Phoenix lander in 2008, could have sterilized the samples once they were heated as part of the tests.

More recently, Dirk Schulze-Makuch, a professor for planetary habitability and astrobiology at Technical University Berlin, says that the abrupt failure of the labeled release experiment may have been due to any microbes being present in the samples being killed off by the very water that was used to nourish them in the first place. He says that while the Viking tests were designed to detect organisms that had evolved under Earth-like temperatures, Martian organisms would have had to evolve to survive the extremely cold environment that is the norm on Mars.

While studying microbes found in Chile’s extremely arid Atacama Desert, Schulze-Makuch found “microbes that live entirely within salt rocks. These hardy organisms take advantage of a process we call hygroscopicity, by which certain salts attract water directly from the relative humidity of the air,” the process that causes table salt to clump together if in a humid environment. “For that reason, the microbes living inside salt rocks in the Atacama do not need any rain at all—just a certain amount of moisture in the atmosphere.”

Indeed, the perchlorates found in Mars’ soil—the same compounds that could sterilize it of trace organic molecules—could serve this very function, and could not only serve as a way of gathering water, but also act as a form of antifreeze that could allow organisms that have evolved to incorporate it into their biochemistry to remain active under extremely cold conditions. But adding ordinary water to such an organism could hyperhyrate the creature, killing it in the process.

“It would be as if an alien spaceship were to find you wandering half-dead in the desert, and your would-be saviors decide, ‘Humans need water. Let’s put the human in the middle of the ocean to save it!'” Schulze-Makuch explained. “That wouldn’t work either.”

“Perhaps the putative Martian microbes collected for the labeled release experiments couldn’t deal with that amount of water and died off after a while,” he continued, explaining why subsequent runs of the test failed. “Most of the runs for the pyrolytic release experiment were conducted under dry conditions, contrary to the other experiments,” an idea that could explain why the other experiment initially saw traces of organic compounds, but not enough to be definitive.

“The first run was positive for life when compared to a control run conducted later, which was designed so that no biology could have been involved. Interestingly, the only run conducted under wet conditions had less of a signal than the control.”

Schulze-Makuch also hypothesizes that Martian lifeforms “might have hydrogen peroxide in their cells—an evolutionary adaptation that would allow them to draw water directly from the atmosphere,” he explains. Many Earth-bound microbes, such as Streptococcus and Lactobacillus, naturally produce the compound.

“The mixture would have other advantages, too, such as keeping water liquid at freezing Martian temperatures, preventing the formation of ice crystals that would rupture the cells.”

Since mission designers assumed there was little evidence for life on Mars, subsequent space probes have been focused on studying Mars’ geography and atmosphere, with few evidence-of-life experiments that might take up valuable space on the craft being included on most expeditions; Mars Express’ Beagle 2 lander was an exception, having included a package designed to look for past traces of Martian life; however, the probe failed when it crashed into Isidis Planitia in December 2003.

Schulze-Makuch continued to say that “we need a new mission to Mars dedicated primarily to life detection to test this hypothesis and others,” one that could explore locations favorable to potential life; such as the planet’s Southern Highlands where life “could persist in salt rocks close to the surface.”

“We might even be able to access these rocks without drilling—a huge advantage in terms of engineering complication and cost,” he continued. “I cannot wait for such a mission to get under way.”

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