Sorry, oxygen is not required to make these minerals on Mars

When NASA’s Mars rovers found manganese oxides in the rocks of Gale and Endeavor craters on Mars in 2014, the discovery prompted some scientists to suggest that the red planet may once again have had more oxygen in its atmosphere thousands of years ago. millions of years.

The minerals likely required copious amounts of water and strongly oxidizing conditions to form, the scientists said. Using lessons learned from Earth’s geologic record, the scientists concluded that the presence of manganese oxides indicated that Mars had experienced periodic increases in atmospheric oxygen in its past, before descending to current low levels.

But a new experimental study from Washington University in St. Louis changes this view radically.

Scientists found that under conditions similar to those on Mars, manganese oxides can easily form without an atmosphere. oxygen. Using kinetic models, the scientists also showed that oxidation of manganese is not possible in the carbon dioxide-rich atmosphere expected on ancient Mars.

“The link between manganese oxides and oxygen suffers from a number of fundamental geochemical problems,” said Jeffrey Catalano, professor of Earth and planetary sciences at Arts & Sciences and corresponding author of the study published December 22 in Geosciences of nature. Catalano is a faculty member at the McDonnell Center for Space Sciences.

The study’s first author is Kaushik Mitra, now a postdoctoral research associate at Stony Brook University, who completed this work as part of his graduate research at the University of Washington.

Mars is a planet rich in the halogen elements chlorine and bromine compared to Earth. “Halogens occur on Mars in different forms than on Earth and in much larger amounts, and we assumed that they would be important to the fate of manganese,” Catalano said.

Catalano and Mitra conducted laboratory experiments using chlorate and bromate, the dominant forms of these elements on Mars, to oxidize manganese in water samples they made to replicate fluids on the surface of Mars in the ancient past.

“We were inspired by the reactions observed during the chlorination of drinking water,” Catalano said. “Understand the other planets sometimes requires us to apply knowledge gained in seemingly unrelated fields of science and engineering.”

The scientists found that the halogens converted manganese dissolved in water into manganese oxide minerals thousands or millions of times faster than oxygen. Furthermore, under the weakly acidic conditions that scientists believe were found on the surface of early Mars, bromate produces manganese oxide minerals more rapidly than any other available oxidant. Under many of these conditions, oxygen is completely incapable of forming manganese oxides.

“Oxidation does not require the participation of oxygen by definition,” Mitra said. “Previously, we proposed viable oxidants on Mars, in addition to oxygen or through UV photooxidation, that help explain because the Red planet it’s red. In the case of manganese, we simply did not have a viable alternative to oxygen that could explain manganese oxides so far”.

The new results upset fundamental interpretations of the habitability of early Mars, which is a major driver of ongoing research by NASA and the European Space Agency.

But just because there was probably no atmospheric oxygen in the past, there’s no particular reason to believe there was no life, the scientists said.

“There are various forms of life even on Earth that do not require oxygen to survive,” Mitra said. “I don’t consider it a ‘setback’ to habitability, just that there were probably no oxygen-based life forms.”

Extremophile organisms that can survive in a halogen-rich environment, such as the salt-loving single-celled organisms and bacteria that thrive in the Great Salt Lake and Dead Sea on Earth, could also thrive on Mars.

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