by Ancient Mars may not have had oxygen in its atmosphere after all, a new study has found – but don’t despair, life could still have crawled on the planet’s surface.
At NASA Curious Rover found manganese oxide in Martian rocks in 2016 cheered many planetary scientists, believing the mineral’s presence was a significant clue to past oxygen concentrations in the planet’s atmosphere. The odds of the past existence of life up Mars suddenly seemed higher as well, as oxygen is one of the key factors for life Earth.
However, a new study based on laboratory experiments has now concluded that not only were high concentrations of oxygen not necessary for the minerals to form, but that the expected composition was ancient atmosphere of Mars would have prevented oxygen-dependent reactions from the outset. Instead, the scientists say, large amounts of manganese oxide could have formed on Mars simply in the presence of halogen elements like chlorine and bromine, which are found in greater abundance on the Red Planet than on Earth.
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“Oxidation, by definition, does not require the involvement of oxygen,” said Kaushik Mitra, now a planetary geochemist at Stony Brook University in New York, who led this study as part of his research work at Washington University in St. Louis, in a expression (opens in new tab).
Oxidation is a chemical reaction in which a molecule or atom loses electrons. The reaction does not necessarily involve oxygen, but in many cases leads to the formation of oxides, such as manganese oxide found on Mars.
“Previously we have proposed viable oxidants on Mars other than oxygen or via UV [ultraviolet] photooxidation, which help explain why the red planet is red,” he said. “In the case of manganese, until now, we just haven’t had a viable alternative to oxygen that could explain manganese oxides.”
Kaushik and his collaborators were inspired by observations of reactions that occur during the chlorination of drinking water, in which chlorine-containing molecules are added to water to kill microorganisms through oxidation. The researchers decided to test whether oxidation could take place in the halogen-rich environment on Mars. In a lab, they prepared water samples with a composition similar to what might have been found on ancient Mars. When they immersed fragments of manganese minerals in the water, the scientists found 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 staggering rate of oxidation, the scientists determined, was that the water contained chlorate and bromate, forms of the halogens chlorine and bromine common on Mars. Bromate was particularly efficient at converting manganese to manganese oxides, allowing the reaction to proceed at a rapid rate. This was true even when the water samples had high concentrations of carbon dioxide, which prevented the formation of manganese oxides in the presence of only oxygen.
This finding holds the key to disproving the theory about past oxygen-rich conditions in the Martian atmosphere that surfaced after Curiosity’s discovery. Scientists also believe that ancient Mars’ atmosphere was rich in carbon dioxide. Since carbon dioxide blocked reactions with oxygen, the idea that the formation of manganese oxides required high concentrations of atmospheric oxygen no longer seemed tenable.
“The association between manganese oxides and oxygen suffers from a number of fundamental geochemical issues,” Jeffrey Catalano, a geochemist at Washington University, St. Louis, and corresponding author of the study, said in the statement. “Halogens occur in different forms on Mars than on Earth and in much larger amounts, and we suspected they would be important in the fate of manganese.”
However, the scientists stress that while oxygen may not have been present in Mars’ ancient atmosphere, the planet could still have been teeming with microscopic life forms in the past.
“Even on Earth, there are several forms of life that don’t need oxygen to survive,” Mitra said. “I don’t see it as a ‘setback’ to habitability – just that oxygen-based life forms probably didn’t exist.”
The study (opens in new tab) was published in the journal Nature Geoscience on Thursday (December 22).
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