The Sample Analysis at Mars (SAM) instrument onboard NASA’s Curiosity rover has measured the seasonal changes in the gases that fill the air directly above the surface of Gale Crater, and noticed something baffling: oxygen behaves in a way that so far planetary scientists cannot explain through any known atmospheric or surface process.
Curiosity rover took this selfie on October 11, 2019. The rover drilled twice in this location, nicknamed Glen Etive. Just left of the rover are the two drill holes, called Glen Etive 1 (right) and Glen Etive 2 (left). Image credit: NASA / JPL-Caltech / MSSS.
Over the course of nearly five years (three Mars years) the SAM instrument inhaled the air of Gale Crater and analyzed its composition.
The results confirmed the makeup of the Martian atmosphere at the surface: 95% by volume of carbon dioxide, 2.6% molecular nitrogen, 1.9% argon, 0.16% molecular oxygen, and 0.06% carbon monoxide.
They also revealed how the molecules in the Martian air mix and circulate with the changes in air pressure throughout the year.
These changes are caused when carbon dioxide freezes over the poles in the winter, thereby lowering the air pressure across the planet following redistribution of air to maintain pressure equilibrium. When carbon dioxide evaporates in the spring and summer and mixes across Mars, it raises the air pressure.
Within this environment, the researchers found that nitrogen and argon follow a predictable seasonal pattern, waxing and waning in concentration in Gale Crater throughout the year relative to how much carbon dioxide is in the air.
They expected oxygen to do the same. But it didn’t. Instead, the amount of the gas in the air rose throughout spring and summer by as much as 30%, and then dropped back to levels predicted by known chemistry in fall.
This pattern repeated each spring, though the amount of oxygen added to the atmosphere varied, implying that something was producing it and then taking it away.
“The first time we saw that, it was just mind boggling,” said University of Michigan’s Professor Sushil Atreya.
Seasonal variations in oxygen at Gale Crater in 2012-2017. Image credit: Melissa Trainer / Dan Gallagher / NASA’s Goddard Space Flight Center.
As soon as the scientists discovered the oxygen enigma, Mars experts set to work trying to explain it.
They first double- and triple-checked the accuracy of the SAM instrument they used to measure the gases: the Quadrupole Mass Spectrometer. The instrument was fine.
They considered the possibility that carbon dioxide or water molecules could have released oxygen when they broke apart in the atmosphere, leading to the short-lived rise. But it would take five times more water above Mars to produce the extra oxygen, and carbon dioxide breaks up too slowly to generate it over such a short time.
What about the oxygen decrease? Could solar radiation have broken up oxygen molecules into two atoms that blew away into space? No, the researchers concluded, since it would take at least 10 years for the oxygen to disappear through this process.
“We’re struggling to explain this. 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,” said Dr. Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center.
To planetary scientists, the oxygen story is curiously similar to that of methane.
Methane is constantly in the air inside Gale Crater in such small quantities (0.00000004% on average) that it’s barely discernable even by the most sensitive instruments on Mars. Still, it’s been measured by SAM’s Tunable Laser Spectrometer.
The instrument revealed that while methane rises and falls seasonally, it increases in abundance by about 60% in summer months for inexplicable reasons.
With the new oxygen findings in hand, the team is wondering if chemistry similar to what’s driving methane’s natural seasonal variations may also drive oxygen’s. At least occasionally, the two gases appear to fluctuate in tandem.
“We’re beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year. I think there’s something to it. I just don’t have the answers yet. Nobody does,” Professor Atreya said.
The team’s paper was published in the Journal of Geophysical Research: Planets.
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Melissa G. Trainer et al. Seasonal variations in atmospheric composition as measured in Gale Crater, Mars. Journal of Geophysical Research: Planets, published online November 12, 2019; doi: 10.1029/2019JE006175
This article is based on text provided by the National Aeronautics and Space Administration.
