Mars' oxygen is behaving in a way scientists cannot yet explain, according to NASA.
On its website this week, the space agency noted what they learned from measuring "seasonal changes in gases" over the Mars crater.
"They have noticed something puzzling: oxygen, the gas that many Earth creatures use to breathe, behaves in a way that scientists cannot yet explain through known chemical processes," the post reads.
A rover portable chemistry laboratory ios curiosity analyzes the Martian air make-up over Gale's crater over the course of six years on Earth (or three Martian years).
Scientists expected oxygen to behave predictably, but "not so" – unlike nitrogen and argon, which follows a "predictable seasonal pattern".
NASA Administrator Thomas Zurbuchen said on Twitter that the rover "has discovered seasonal variations of oxygen on Mars that cannot be explained by known chemical processes so far."
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Obviously, oxygen levels rose by 30% in spring and summer – which they termed "over-prediction" on the NASA Web site chart.
These findings were published in an academic article this week in the journal Geophysical Research: Planets.
One of the co-authors – Sushil Atrea, a scientist at the University of Michigan – called it "a fool's errand" when the researchers first saw the measurements.
Another scientist who led the research, Melissa Coach of Maryland, in a NASA post said they were "struggling to explain this".
Scientists again checked the accuracy of the measuring device they were using, but it was "okay".
They compared the oxygen mystery to the still-unsolved methane jigsaw puzzle on Mars. Methane obviously increases by 60% during the summer, for reasons we don't know.
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University of Maryland scientist Timothy Mekonohi – also co-author of the paper – told the BBC the measurements indicate a possible "reservoir" of oxygen near the surface.
Oxygen and methane may be products of biological processes, but scientists said the rover has no way of knowing whether the gas source is the result of biology or geology.
They rely heavily on "non-biological explanations", according to NASA.
"The abiotic processes look very promising, so we will have to shut them down first before contributing to the microbes," Atrea told the BBC.
So far, researchers are reporting to the world their observations and asking Mars experts in the scientific community about any and all ideas.
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