Fifty years ago, humanity got its first up-close look at Mars. It was a profound disappointment—at least, for anyone who harbored dreams of a sister world just like Earth. NASA's first Mars flyby, executed by Mariner 4 in 1965, revealed the (mostly!) dry, red desert we know today.

Despite this blow to our hopes for Martian life, famed scientist Carl Sagan convinced NASA to install an astrobiology mission on the Viking landers, NASA's first probes to touch down on the Red Planet. There were four astrobiology experiments. A mass spectrometer looked for organics in the Martian soil, finding only trace amounts. The second experiment involved adding helium gas to a soil sample, then applying organics, other chemicals, and water in order to spur on life. The experiment hoped to find metabolic processes through gas exchanges, of which none were found. Another experiment heated soils for organic residue, and created a control sample for the other experiments.

And then there was the labelled release experiment. A soil sample was obtained, then water and other nutrients were added. If a carbon dioxide isotope was released, it would tentatively suggest that life was present. Of the four tests, this was the only one that gave a weak positive signal. But before anybody could get too excited, the signal faded in subsequent observations. It seemed that the Vikings had found nothing on Mars but dirt.

"The final conclusion by NASA was that Mars was dead."

Or had they? Decades later, Joop Houtkooper, a professor at the University of Giessen, proposed something extraordinary: Vikings 1 and 2 actually found life, then killed it. The problem, according to Houtkooper, was that NASA tried to find life that was too Earth-like. Houtkooper published his results in Astrobiology in 2007. (Arxiv reprint available here.)

"The final conclusion by NASA was that Mars was dead, the surface was oxidizing and therefore no life was possible," Houtkooper said in an email. "One of the Principal Investigators, Gilbert Levin, who designed the Labeled Release Experiment, remained a dissenter to this day, stating repeatedly over the years that his experiment detected life."

How to Kill a Martian

Houtkooper's hypothesis goes something like this: Mars is cold—too cold for fresh liquid water just beneath the surface. (It's not too cold for salty water, as was announced on Monday, something the original Viking results didn't account for.) But there's also something else, something very similar to water, that doesn't freeze as readily: hydrogen peroxide. It was detected in the soil.

On Earth, hydrogen peroxide is a disinfectant that breaks down cell walls. That's why it's common in first aid kits: It rips apart bacteria. Pure hydrogen peroxide freezes at a slightly colder temperature than water, 31.2 degrees F rather than 32. But, if you mix water and hydrogen peroxide in equal amounts, you get a mixture whose freezing point can dip down to -59.8 degrees F. That's close to the not-so-balmy median temperature of the soil of Mars.

The upshot of all this is that it's possible to imagine Martian microorganisms that are hardier than their Earth-bound counterparts, and subsist not on water, like life on Earth, but on a water and hydrogen peroxide solution that doesn't freeze. If you were to test for such life and give it only water, it wouldn't go well. The hypothetical bacteria would need the proper mixture to carry out metabolism and feed ... however many cells it has. But giving the Martian soil only water essentially drowned (then boiled) the bacteria, killing it.

If this were true, it would also mean that an organism on Mars would be well-adapted to the cold by internalizing an anti-freeze agent. It may also be regional in distribution. "The practical consequences are that Martian biota may thrive better at lower temperatures and should therefore be sought for at higher latitudes," Houtkooper wrote in an abstract in 2007.

The hypothesis seemed outlandish. Instead of the weak biosignatures being a false positive, it was the mark of Viking drowning Mars bacteria. But the results would be roughly consistent with the closest analogue we have on Earth: the patches of Antarctica without ice (something like 2 percent of the continent), where a small concentration of hardy bacteria survive.

Searching for answers

The only way to find out if Houtkooper is right would be to repeat the Viking experiment, this time feeding the right mixture into the soil. It would have to be performed on Mars, lest it further kill the heat-averse microorganisms.

"A Mars sample return mission might suffer from too high temperatures and, by keeping the sample in the dark for over 6 months, might preclude photosynthesis and thus organisms might not only die, but also auto-oxidize beyond recognition," Houtkooper said.

The search for life may need some tweaking, even beyond Mars. There's a hypothesis that there could be nitrogen life on Titan. It has in common with Houtkooper's hunch that there's an anti-freeze component at work in the cellular make-up of the organisms. The other places in the solar system where water exists are also frigid, meaning some cold hardiness is required to prosper. If we can replicate the Viking experiments and find biological activity this time, it could point the way toward how best to find life in the oceans of Europa, Enceladus, or Ganymede.

There are astrobiology tests coming in the next few years, both aboard ESA's ExoMars lander and NASA's Mars 2020 rover. These missions will be able to inform more about the proof of briny flows on Mars, but in a press conference regarding the water flows of Mars, NASA planetary sciences chief Jim Green said, "If I were a microbe on mars, i would not want to live on these (flows.) I would want to live further north or south under the surface where there's more water."

But along with the water, maybe they should try a hydrogen peroxide mixture to test the results, and look under the surface. While the surface may be dead, there may have been life on Mars all along. We may have already found it. But this time, we may need to just take care to think a little outside the box of familiar biology.

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