Another mystery on Mars: Oxygen appears and disappears without explanation

With the mystery of Mars methane still unresolved , the space robot Curiosity brought scientists a new puzzle: Martian oxygen.

The information came by measuring the seasonal levels of all gases in the atmosphere directly above the surface of the Gale Crater, where Curiosity is located. The result is disconcerting.

On Mars, oxygen, the gas most creatures on Earth use to breathe, behaves in a way that scientists have so far failed to explain through any known chemical process.

Over the course of three Martian years (nearly six years of Earth), a Curiosity instrument called SAM ( Sample Analysis at Mars ) inhaled the air of the Gale Crater and analyzed its composition.

The results confirmed the composition of the Martian surface atmosphere: 95% by volume of carbon dioxide (CO2), 2.6% of molecular nitrogen (N2), 1.9% of argon (Ar), 0.16% of molecular oxygen. (O 2), and 0.06% carbon monoxide (CO).

Measurements also revealed how molecules in Martian air blend and circulate with changes in air pressure throughout the year. These changes are induced when CO2 gas freezes at the poles in winter, lowering air pressure across the planet after air redistribution to maintain pressure balance. When CO2 evaporates in spring and summer and mixes on Mars, the air pressure increases.

The oxygen mystery of Mars

In this environment, data show that nitrogen and argon follow a predictable seasonal pattern, with their concentration increasing and decreasing over the year relative to the amount of CO2 in the air.

Scientists expected oxygen to keep pace, but that is not the case. Instead, the amount of oxygen in the air rises throughout spring and summer by up to 30 percent, and then returns to the levels predicted by known autumn chemistry. This pattern repeats each spring, although the amount of oxygen added to the atmosphere varies, implying that something is producing oxygen and then taking it away.

"The first time we saw this, we were racking our brains," said Sushil Atreya, professor of climate and space science at the University of Michigan.

The team set out to look for possible explanations, first considering the possibility that CO2 or water (H2O) molecules could release oxygen when they separate into the atmosphere, leading to a brief rise in oxygen. But that would consume five times more water than there is in the atmosphere of Mars, and CO2 decomposes too slowly to generate it in such a short time. What about decreasing oxygen? Couldn't solar radiation break oxygen molecules into two atoms, which would then leak into space? No, the scientists concluded, as it would take at least 10 years for oxygen to disappear through this process.

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

The amount of oxygen in the air increases throughout spring and summer by up to 30 percent, and then returns to the levels predicted by known autumn chemistry

Methane and oxygen, biological and abiotic

The history of oxygen is curiously similar to the methane mystery of Mars. Methane is constantly in the air inside the Gale Crater in such small quantities (0.00000004% on average) that it almost goes unnoticed by the most sensitive instruments ever sent to Mars. Although methane increases and decreases seasonally, it increases abundantly by about 60% in the summer months for unexplained reasons - in fact, methane also fires randomly and dramatically, but no one has yet any idea why.

With the new oxygen discoveries at hand, the NASA team wonders if a chemistry similar to the one that is generating the natural seasonal variations of methane can also explain the variations in oxygen - the two gases even float together, but only occasionally.

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

Oxygen and methane can be produced biologically (from microbes, for example) and abiotically (from water and rock-related chemicals). Scientists are considering all options, although we have no convincing evidence of biological activity on Mars.

The Curiosity robot has no instruments that can definitely tell whether the source of methane or oxygen on Mars is biological or geological. With current data, nonbiological explanations are more likely.


Article: Seasonal variations in atmospheric composition as measured in Gale Crater, Mars

Authors: Melissa G. Trainer, Michael H. Wong, Timothy H. McConnochie, Heather B. Franz, Sushil K. Atreya, Pamela G. Conrad, Franck Lefèvre , Paul R. Mahaffy, Charles A. Malespin, Heidi LK Manning, Javier Martín-Torres, Germán M. Martínez, Christopher P. McKay, Rafael Navarro-González, Alvaro Vicente-Retortillo, Christopher R. Webster, Maria-Paz Zorzano

Magazine : Journal of Geophysical Research: Planets

DOI: 10.1029 / 2019JE006175  

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