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Curiosity uncovers an intriguing oxygen mystery on Mars



water curiosity clay samples
NASA's Curiosity Mars rover took this selfie on May 12, 2019 (the 2,405 Martian Day, or Sun, of the mission). NASA / JPL-Caltech / MSSS

NASA's Curiosity spacecraft continues to make discoveries that challenge our understanding of the Martian environment. The latest strange scientists who tax puzzles are the variation in oxygen levels on the planet's surface, as Curiosity's portable chemistry lab, Sample Analysis at Mars (SAM) has found.

On its journey through the Gale Crater, Curiosity found that the Martian atmosphere has a surface composition of 95% by volume of carbon dioxide (CO2), 2.6% of molecular nitrogen (N2), 1.9% of argon ( Ar), 0.16% molecular oxygen (O2) and 0.06% carbon monoxide (CO). Nitrogen and argon levels follow a predictable seasonal pattern, varying with respect to the amount of carbon dioxide. Oxygen levels, however, did not meet expected standards, increasing by up to 30% in spring and summer.

Curiosity oxygen levels spoils crater seasonal gale 1
Melissa Trainer / Dan Gallagher / NASA Goddard

The varying levels of oxygen perplexed scientists. "The first time we saw it, it was impressive," Sushil Atreya, professor of climate and space science at the University of Michigan, said in a statement.

Scientists have tried various hypotheses to explain the variation in oxygen. They checked that the SAM instrument was working properly and analyzed whether carbon dioxide molecules could be breaking in the atmosphere to create oxygen, but no approach produced results.

"We are struggling to explain this," said Melissa Trainer, planetary scientist at NASA's Goddard Space Flight Center and research leader. "The fact that oxygen behavior is not perfectly repeatable every season makes us think it's not a problem that has to do with atmospheric dynamics. There has to be some chemical source and sink that we can't yet explain."

One possibility is that oxygen levels are related to another Martian puzzle: fluctuating methane levels on the planet. In addition to expected seasonal variations in methane levels, Curiosity has detected methane peaks of up to 60% at times. Scientists cannot yet explain this finding, but they may have found a link between methane and oxygen levels: it appears that the two gases float together at certain times.

"We are beginning to see this tantalizing correlation between methane and oxygen for much of the year on Mars," Atreya said. "I think there's something to do. I don't have the answers yet. No one does."

Although oxygen and methane can be produced biologically, their presence does not necessarily indicate life on the planet. They can also be produced chemically by water and rocks. Curiosity can only detect gas levels, not their origin, so the source of this mystery remains unknown for now.

The research was published in the Journal of Geophysical Research: Planets.

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