Martian Mystery: Red planet's fluctuating oxygen levels stump NASA scientists
The oxygen on Mars is behaving in a way that has baffled scientists of the National Aeronautics and Space Administration (NASA). According to experts working on the Curiosity rover mission, it is behaving in a way that so far cannot be explained through any known chemical processes.
The scientists - who measured the seasonal changes in the gases that fill the air directly above the surface of Gale Crater on Mars - found that 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.
“We’re struggling to explain this. The fact that 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 cannot yet account for”, says Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who led this research.
As the experts try to unravel the mystery, they are exploring all options - both biological and geological.
“Oxygen and methane can be produced both biologically (from microbes, for instance) and abiotically (from chemistry related to water and rocks). Scientists are considering all options, although they do not have any convincing evidence of biological activity on Mars. Curiosity doesn't have instruments that can definitively say whether the source of the methane or oxygen on Mars is biological or geological. Scientists expect that non-biological explanations are more likely and are working diligently to fully understand them”, says the study.
The oxygen story is also very similar to that of methane, which remains another mystery. According to experts, methane is continuously in the air inside Gale Crater. But it is present in such small quantities that it is barely discernible even by the most sensitive instruments on Mars. Sample Analysis at Mars (SAM)’s Tunable Laser Spectrometer was able to measure it and analysis shows that while methane rises and falls seasonally, it increases by about 60% in summer months for inexplicable reasons.
The experts wonder if the chemistry that is responsible for methane’s natural seasonal variations could also drive oxygen. “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”, says co-author of the paper Sushil Atreya, professor of climate and space sciences at the University of Michigan in Ann Arbor.
The analysis
Over three Mars years, or nearly six Earth years, an instrument in SAM portable chemistry lab inside the belly of NASA’s Curiosity rover 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.
The findings also show how the molecules in the Martian air mix and circulate with the changes in air pressure throughout the year. Within this environment, scientists 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 did not, say experts.
“Oxygen varies seasonally and interannually, independently from argon and molecular nitrogen, on timescales too fast to be explained by known chemistry”, says the study published in the Journal of Geophysical Research: Planets. The data can be used to better understand how the surface and atmosphere interact as they search for signs of habitability, say experts.
“The atmosphere of Mars is made up of primarily carbon dioxide and during the martian year, the barometric pressure is known to cycle up and down substantially as this carbon dioxide freezes out and then is re-released from polar caps. The Mars Science Laboratory Curiosity rover has now acquired atmospheric composition measurements at the ground over multiple years, capturing the variations in the major gases over several seasonal cycles for the first time”, says the study.
It says, “The abundances of some of these gases were observed to vary by up to 40% throughout the year due to the seasonal cycle. Nitrogen and argon follow the pressure changes, but with a delay, indicating that transport of the atmosphere from pole to pole occurs on faster timescales than mixing of the components. Oxygen has been observed to show significant seasonal and year-to-year variability, suggesting an unknown atmospheric or surface process at work.”
Once the scientists identified this strange behavior of oxygen, they set to work, trying to explain it. They initially checked the accuracy of the SAM instrument they used to measure the gases: the Quadrupole Mass Spectrometer, and found that it was working fine.
The experts considered the possibility of 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.
The team considered if solar radiation could have “broken up oxygen molecules into two atoms that blew away into space.” The scientists concluded that this is not the case since it would take at least 10 years for the oxygen to disappear through this process.
Among the possibilities considered by the experts was that Martian soil might be a source of the extra springtime oxygen. According to the scientists, while an experiment on the Viking landers showed decades ago that heat and humidity could release oxygen from Martian soil, it took place in conditions quite different from the Martian spring environment. It also does not explain the oxygen drop, among other problems. Other possible explanations also do not add up currently, says the team.
“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”, says co-author of the paper, Timothy McConnochie, assistant research scientist at the University of Maryland in College Park.
The SAM team will continue to measure atmospheric gases so scientists can gather more detailed data throughout each season.
“This is the first time where we’re seeing this interesting behavior over multiple years. We don’t totally understand it. 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”, says Trainer.