Ancient Mars had less oxygen than previously thought

When NASA’s Mars rovers discovered manganite oxides in the Gale and Endeavor craters on Mars in 2014, scientists suggested that the red planet might have had more oxygen in its atmosphere billions of years ago. However, a new experimental study conducted by the Washington University in St. Louis (WUSTL) found that, under Mars-like conditions, manganese oxides can be readily form without the presence of atmospheric oxygen.

Since Mars is rich in the halogen elements chlorine and bromine, the experts conducted laboratory experiments using these two substances to oxidize manganese in water samples that they made to replicate fluids from early Mars.

“We were inspired by reactions seen during chlorination of drinking water,” said study senior author Jeffrey Catalano, a professor of Earth and Planetary Sciences at WUSTL. “Understanding other planets sometimes requires us to apply knowledge gained from seemingly unrelated fields of science and engineering.”

The experiments revealed that halogens converted manganese dissolved in water into manganese oxide minerals thousands or even millions of times faster than oxygen could do this. Moreover, under the weakly acidic conditions thought to characterize early Mars’ surface, bromate produces manganese oxide minerals faster than any other known oxidant.

“Oxidation does not necessitate the involvement of oxygen by definition,” said study lead author Kaushik Mitra, a postdoctoral fellow at Stony Brook University, who completed this research as a graduate student at WUSTL. “Earlier, we proposed viable oxidants on Mars, other than oxygen or via UV photooxidation, that help explain why the red planet is red. In the case of manganese, we just did not have a viable alternative to oxygen that could explain manganese oxides until now.”

However, although these findings suggest there was not much oxygen on ancient Mars, there is no particular reason to conclude that there was no life either. “There are several life forms even on Earth that do not require oxygen to survive. I don’t think of it as a ‘setback’ to habitability – only that there were probably no oxygen-based lifeforms,” Mitra explained.

“We need more experiments conducted in diverse geochemical conditions that are more relevant to specific planets like Mars, Venus, and ‘ocean worlds’ like Europa and Enceladus in order to have the correct and full understanding of the geochemical and geological environments on these planetary bodies. Every planet is unique in its own right, and we cannot extrapolate the observations made on one planet to exactly understand a different planet,” he concluded.

The study is published in the journal Nature Geoscience.

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By Andrei Ionescu, Earth.com Staff Writer

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