A new Martian mystery has left scientists baffled. The oxygen in the planet’s atmosphere seems to rise every spring and summer and fall during autumn and winter, and scientists have no idea why.
The startling discovery was made after the team analysed data from the Gale Crater taken by the Sample Analysis at Mars (SAM) instrument, a microwave oven-sized box that sits within NASA's Curiosity Rover. The robot cart has been exploring the Red Planet since 2012, allowing researchers to monitor how the concentration of gases fluctuate with the seasons for the first time.
SAM confirmed the chemical composition of the Martian atmosphere: it contains 95 per cent carbon dioxide, 2.6 per cent nitrogen, 1.9 per cent argon, 0.16 per cent oxygen, and 0.06 per cent carbon monoxide, by volume. But those concentrations varied over time.
"The first time we saw that, it was just mind boggling," said Sushil Atreya, co-author of the study published in the Journal of Geophysical Research: Planets and a professor of climate and space sciences at the University of Michigan.
Carbon dioxide freezes over the Martian North and South pole in winter, lowering the air pressure across the planet’s atmosphere. It evaporates in the spring and summer, however, raising the air pressure again. Scientists expected the other gases to follow a similar pattern, rising and falling according to the amount of carbon dioxide in the air.
The concentration of nitrogen and argon followed suit, but for oxygen it varied wildly. Although oxygen does wax in spring and summer and wane in autumn and winter like carbon dioxide, the amount it fluctuated by was unpredictable. Oxygen levels can shoot up by as much as 30 per cent in the warmer seasons and sink back down to below normal levels in winter, suggesting that something was producing the gas and then taking it away again.
Researchers have no idea what could be pumping and dumping oxygen on Mars; it can either be an unknown chemical reaction from biological sources like microbes or from a physical process that occurs in Martian water or rocks. The team reckon it’s more likely the latter of the two.
They considered various possibilities, ranging from carbon dioxide and water molecules breaking apart to release added oxygen or solar rays breaking apart oxygen molecules to take them away. But it would require five times the amount of available water on Mars, and it takes too long for carbon dioxide and oxygen molecules to break apart to match the observed gas levels over the seasons.
Maybe it's something to do with the soil
“We’re struggling to explain this,” said Melissa Trainer, first author of the paper and a planetary scientist at NASA’s Goddard Space Flight Center.
“The fact that the oxygen behavior isn’t perfectly repeatable every season makes us think that it’s not an issue that has to do with atmospheric dynamics. It has to be some chemical source and sink that we can’t yet account for.”
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The team also realized that methane seems to vary haphazardly too, rising up to 60 per cent in summer for some inexplicable reason.
“We’re beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year,” Atreya said. “I think there’s something to it. I just don’t have the answers yet. Nobody does.”
Martian soil also harbors oxygen so the team looked there next. They thought that the extra solar radiation cast during the warmer months might heat up the soil, releasing oxygen into the air. The process is painfully slow, however, and it’d take a million years just to accumulate enough of the gas in the soil to boost oxygen levels for a single spring.
Still, the researchers believe that it might be some other unknown process happening in the soil. “We have not been able to come up with one process yet that produces the amount of oxygen we need, but we think it has to be something in the surface soil that changes seasonally because there aren’t enough available oxygen atoms in the atmosphere to create the behavior we see,” said Timothy McConnochie, co-author of the paper and an assistant research scientist at the University of Maryland.
“This is the first time where we’re seeing this interesting behavior over multiple years. We don’t totally understand it,” Trainer added. “For me, this is an open call to all the smart people out there who are interested in this: See what you can come up with.” ®