Mars' ancient atmosphere may not have had much oxygen after all

NASA's Curiosity rover on Mars.
Curiosity's discovery of manganese oxides in Martian rocks led researchers to believe that the planet's atmosphere used to be rich in oxygen. (Image credit: NASA)

There may have been no oxygen in the atmosphere of ancient Mars after all, a new study has found — but don't despair, there still could have been living creatures crawling on the planet's surface. 

When NASA's Curiosity rover found manganese oxide in Martian rocks in 2016, many planetary scientists rejoiced, believing that the mineral's presence was a significant hint of past concentrations of oxygen in the planet's atmosphere. The odds of past existence of life on Mars suddenly seemed higher, too, as oxygen is one of the key enablers of life on Earth.

A new study based on laboratory experiments, however, has now concluded that not only were high concentrations of oxygen not necessary for the formation of the minerals, but that the expected composition of ancient Mars' atmosphere would have prevented oxygen-reliant reactions in the first place. Instead, the scientists said, copious amounts of manganese oxide could have formed on Mars simply in the presence of halogen elements, such as chlorine and bromine, which are found on the Red Planet in greater quantities than on Earth. 

Related: Curiosity rover: 15 awe-inspiring photos of Mars (gallery)

"Oxidation does not necessitate the involvement of oxygen by definition," Kaushik Mitra, now a planetary geochemist at Stony Brook University in New York who led this study as part of his graduate research work at Washington University in St. Louis, said in a statement (opens in new tab)

Oxidation is a chemical reaction in which a molecule or atom loses electrons. The reaction doesn't necessarily involve oxygen, but in many cases leads to the formation of oxides, such as manganese oxide found on Mars.

"Earlier, we proposed viable oxidants on Mars, other than oxygen or via UV [ultraviolet] photooxidation, that help explain why the Red Planet is red," he said. "In the case of manganese, we just did not have a viable alternative to oxygen that could explain manganese oxides until now."

Kaushik and his collaborators were inspired by observations of reactions occuring during chlorination of drinking water, which involves adding molecules containing chlorine into water to kill microorganisms through oxidation. The researchers decided to test whether oxidation could be occurring in the halogen-rich environment on Mars. In a laboratory, they created water samples with a composition similar to what might have been found on ancient Mars. When they submerged fragments of manganese minerals in the water, the scientists discovered that the manganese quickly dissolved, forming manganese oxide thousands to millions of times faster than in the presence of oxygen, the researchers said in the statement. 

The key to this stunning rate of oxidation, the scientists determined, was that the water contained chlorate and bromate, forms of the halogens chlorine and bromine that are common on Mars. Bromate was particularly efficient in turning manganese into manganese oxides, enabling the reaction to proceed at a speedy pace. That held true even when the water samples had high concentrations of carbon dioxide, which prohibited the formation of manganese oxides in the presence of only oxygen. 

This finding is key to disproving the theory about past abundance of oxygen in the atmosphere of Mars that emerged after Curiosity's discovery. Scientists also believe that the atmosphere of ancient Mars was rich in carbon dioxide, so because carbon dioxide blocked the reactions with oxygen, the idea that the formation of manganese oxides required high concentrations of atmospheric oxygen appeared to no longer hold water. 

"The link between manganese oxides and oxygen suffers from an array of fundamental geochemical problems," Jeffrey Catalano, a geochemist at Washington University, St, Louis, and corresponding author of the study, said in the statement. "Halogens occur on Mars in forms different from on the Earth, and in much larger amounts, and we guessed that they would be important to the fate of manganese."

The scientists, however, stress that although oxygen may not have been present in Mars' ancient atmosphere after all, the planet still could have teemed with microscopic life forms in the past.

"There are several life forms even on Earth that do not require oxygen to survive," Mitra said. "I don't think of it as a 'setback' to habitability — only that there was probably no oxygen-based lifeforms."

The study (opens in new tab) was published on Thursday (Dec. 22), in the journal Nature Geoscience.

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Tereza Pultarova
Senior Writer

Tereza is a London-based science and technology journalist, aspiring fiction writer and amateur gymnast. Originally from Prague, the Czech Republic, she spent the first seven years of her career working as a reporter, script-writer and presenter for various TV programmes of the Czech Public Service Television. She later took a career break to pursue further education and added a Master's in Science from the International Space University, France, to her Bachelor's in Journalism and Master's in Cultural Anthropology from Prague's Charles University. She worked as a reporter at the Engineering and Technology magazine, freelanced for a range of publications including Live Science, Space.com, Professional Engineering, Via Satellite and Space News and served as a maternity cover science editor at the European Space Agency.

  • rod
    "The scientists, however, stress that although oxygen may not have been present in Mars' ancient atmosphere after all, the planet still could have teemed with microscopic life forms in the past. "There are several life forms even on Earth that do not require oxygen to survive," Mitra said. "I don't think of it as a 'setback' to habitability — only that there was probably no oxygen-based lifeforms."

    Formation of manganese oxides on early Mars due to active halogen cycling, https://www.nature.com/articles/s41561-022-01094-y?utm_medium=affiliate&utm_source=commission_junction&utm_campaign=CONR_PF018_ECOM_GL_PHSS_ALWYS_DEEPLINK&utm_content=textlink&utm_term=PID100052171&CJEVENT=fb82fd55821411ed821000770a82b839
    Do not worry, hope abounds in astrobiology to prove Mars had life. Once this is done, Mars will confirm abiogenesis belief 😊 At least that seems the goal on Mars since 1970s Viking landers 😊 The astrobiology department must be held to the same standard as the heliocentric solar system astronomy was. Provide necessary demonstration from nature that the paradigm is true. So far, nothing from the abiogenesis labs or astrobiology. Waiting patiently to see that the paradigm is true in nature, not just another probability creation story :)
    Reply
  • Pogo
    Looking at Earth’s life cycle, we see that all life was anaerobic for quite a long time before something learned how to do photosynthesis. Then there was little free diatomic oxygen because the Earth’s minerals reacted with it and became oxides as fast as it was produced for another very long time. It was not until the minerals became more or less ‘saturated’ that O2 became a thing in the atmosphere. This is only conjecture, but, Perhaps I can see the same process happening on Mars, anaerobic life, photosynthesis, oxidation of the mineralogy, then Mars got too cold and not enough atmosphere to continue the process, and the last of the O2 was used up in the rocks.
    Reply