When the Viking 1 and 2 landers analyzed the Martian sands for microbial life back in the late 1970s and early 1980s, the results were … inconclusive. Some of the experiments came back positive for microorganisms surviving the harsh conditions – today, not just in the ancient past – but others did not. It was perplexing then and still is today. Most scientists have since concluded that the positive results were erroneous, because neither lander found any abundant organic material in the soil. How could there be microbes with no organics, the building blocks of life? But maybe there’s another possibility. What if there were microbes, but the soil tests on the rovers accidentally destroyed them in the process?
That theory has been around for a while. Astrobiologist Dirk Schulze-Makuch revisited this intriguing idea in a new post for Big Think on June 27, 2023.
4 experiments to look for life on Mars
The Viking landers performed four basic experiments. The landers conducted them on samples of soil that they dug from the ground. Each lander scooped samples into their onboard laboratory. Although not as sophisticated as labs back on Earth, they were considered quite capable of detecting living microbes, if they existed.
The Labeled Release Experiment looked for signs of metabolism, chemical reactions in organisms that sustain life processes. That includes the conversion of energy in food to energy available to run cellular processes; the conversion of food to building blocks for proteins, lipids, nucleic acids and some carbohydrates; and the elimination of metabolic wastes.
The Pyrolytic Release Experiment looked for evidence of organic synthesis. That is the intentional construction of organic compounds.
Those first two experiments seemed to yield positive results. But the third experiment, the Gas Exchange Experiment, did not. First, it added small amounts of water to the samples. This was an attempt to see if any dormant microbes would be revived by moisture, since Mars is now so bone dry. It also added a “chicken soup” broth containing 19 amino acids, vitamins, other organic compounds and a few inorganic salts. Would any microbes eat this delicious meal?
Where are the organics?
Scientists were excited at the initial positive results from the Labeled Release Experiment and Pyrolytic Release Experiment. There was one big problem, however. The Vikings also searched for organics in the soil with their fourth experiment, the Gas Chromatograph-Mass Spectrometer instrument. Without organics, there could be no life. All they found were tiny traces of chlorinated organics. These are organic compounds containing at least one covalently bonded atom of chlorine. Since only those and no other organics were found, the mission scientists concluded that they were most likely earthly contaminants on the landers. They were not products of life and probably didn’t even originate on Mars.
The lack of other organics was a blow to the possibility of life inhabiting the Martian sands. As Schulze-Makuch wrote:
The Viking landers also included an instrument to detect organic compounds. It saw trace amounts of chlorinated organics, which were interpreted at the time to be the result of contamination from Earth. This led Viking project scientist Gerald Soffen to utter his famous words, ‘No bodies, no life.’ In other words, there couldn’t be Martian life without organic compounds. So Soffen concluded, as did most other scientists at the time, that the Viking project was negative as to the presence of life, or at best inconclusive.
Organics on Mars after all
As it turned out decades later, there are organics on Mars. And they’re native to the planet, not contaminants that spacecraft from Earth brought there. The Curiosity and Perseverance rovers have now confirmed them beyond any doubt. Previously, the Phoenix Mars Lander was the next mission after Viking to find organics again.
Just as Viking had found, the organics are chlorinated. It’s also now known that the Martian soil contains perchlorates, which can destroy organic molecules. This might also help explain the very low abundance of organics where the Viking landers sampled. Most of the organics found more recently have been preserved in rocks.
Just add water
One of the key objectives in the Viking experiments was to add small amounts of water to the soil samples. The idea was that microbes in the soil might be dormant due to the extremely dry and freezing conditions. Adding water might rejuvenate them to become active again. Schulze-Makuch surmised, as have others, that this may have been too much of a good thing. Instead of just providing moisture for the microbes to absorb or drink, it drowned them. Any Martian microbes would be well-adapted by now to the hostile and extremely dry conditions. Schulze-Makuch wrote:
Now let’s ask what would happen if you poured water over these dry-adapted microbes. Might that overwhelm them? In technical terms, we would say that we were hyperhydrating them, but in simple terms, it would be more like drowning them. 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!’ That wouldn’t work either.
Hygroscopic life on Mars?
Perhaps Martian microbes might be like ones on Earth in the Atacama Desert in Chile that live only on salts, no liquid water, like rain. They use a process called hygroscopicity, where some salts in soil absorb water directly from the relative humidity of the air.
As Schulze-Makuch noted, the soil at the Viking landing sites was low in salts. However, it did contain perchlorates and hydrogen peroxide. Both of those are highly hygroscopic by nature. Also, the Viking orbiters saw fog in valleys from orbit. That means the relative humidity in the mornings and evenings should have been high enough for the moisture in the fog to be available to microorganisms.
Hydrogen peroxide: Another possibility for life on Mars
The Viking experiment results were indeed frustrating for the scientists. Some of the results seemed to indicate life while others didn’t. Then there was the lack of abundant organics. Most scientists now think that the results can be explained by non-biological chemical reactions. But there’s also another possibility.
Schulze-Makuch and his colleague Joop Houtkooper at the University of Giessen in Germany had previously proposed a novel biological explanation for the Viking results. What if Martian microbes had hydrogen peroxide in their cells? Hydrogen peroxide can be deadly in large amounts, but there are actually microbes on Earth that use it and produce it. These include Neisseria sicca, Haemophilus segnis, Streptococcus and Lactobacillus.
On Mars, theoretically, microbes with hydrogen peroxide in their cells could siphon tiny amounts of water directly from the atmosphere. It would also help to keep the water from freezing again inside the microbes and damaging their cells.
Schulze-Makuch makes the case that such hydrogen peroxide microorganisms could explain the puzzling Viking results. The Gas Chromatograph-Mass Spectrometer, by design, heated the soil samples before analyzing them. The scientists wanted to see if any biological compounds would be released in the process. But in hindsight, this may have actually destroyed any microbes present. If the microbes did happen to contain hydrogen peroxide, the intense heat would likely have killed them. So, in this scenario, heat would have killed off the microbes instead of water. Moreover, the hydrogen peroxide would have reacted with the organic compounds to produce carbon dioxide. And, maybe it was a coincidence, but that is exactly what the Gas Chromatograph-Mass Spectrometer detected.
Gilbert Levin
On a related note, Gilbert Levin, who was the principal investigator for the Labeled Release Experiment for both Viking landers, maintained that we did find life on Mars. He outlined his stance in an opinion piece in Scientific American on October 10, 2019. He passed away in 2021 at the age of 97.
We may never know for absolute certainty that the Viking landers did find life – or didn’t – but the perplexing results can help scientists better plan future life-detection missions. There is still much to learn, especially in hindsight, as scientists like Schulze-Makuch remind us.
Bottom line: Did the Viking landers from a few decades ago find microbial life on Mars after all? Astrobiologist Dirk Schulze-Makuch makes the case in Big Think.
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