NASA has detected unexpected and unexplained changes in oxygen from Mars.
The discovery comes after the space agency used the Curiosity rover to measure seasonal changes in gas filling the air above the Gale Crater on Mars for the first time in history.
But the data sent back from the planet left the scientists who received it perplexed. They say there is no known explanation for the unusual fluctuations in oxygen on the planet.
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"The first time we saw that, it was mind boggling," said Sushil Atreya, professor of climate and space science at the University of Michigan at Ann Arbor.
This is when researchers continue to deal with the mystery of methane on the red planet. As oxygen has now also been revealed, the amount of gas rises and plunges in a way that seems random and to which scientists have no cause.
During the study, which used an instrument to analyze air on Mars over three Martian years or just under six Earth years, scientists found that gases such as nitrogen and argon behave predictably throughout the year. The proportion of gas increases and decreases relative to the amount of carbon dioxide, which makes up 95% of Martian air.
They thought that oxygen would see the same changes. But they were shocked to find that it did rise during spring and summer, with a varying amount of oxygen in the atmosphere, suggesting that it is being produced and then removed from the air.
The researchers were so shocked by the findings that their first course of action was to verify the accuracy of the instrument used to find the data, but found it was working well. Other possible explanations based on what we know about the Martian atmosphere were also considered but rejected.
"We are struggling to explain this," said Melissa Trainer, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who led the research. "The fact that oxygen behavior is not perfectly reproducible every season makes us think it is not a problem that has to do with atmospheric dynamics. It has to be a chemical source and a source that we cannot yet explain."
The similarities between the mystery of Martian methane and oxygen may be more than coincidental, scientists speculate. It may be possible that both have the same unidentified cause.
"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 has."
Scientists now hope to continue measuring the atmosphere to try to solve the mystery of oxygen.
"It's the first time we've seen this interesting behavior over the course of several years. We don't fully understand it," said Trainer.
"To me, this is an open call to all smart people who are interested in this: see what you can create."
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