The presence of methane in the Martian atmosphere was a fascinating mystery to planetary scientists. This is because methane is connected to life on Earth but can also be produced geologically. Some of the best data on Mars methane comes from the Curiosity Rover, which landed on Mars after a daring descent through the atmosphere in August 2012. Now Curiosity has made another fascinating discovery: oxygen at the rover's location behaves so have not been explained by a well-known atmospheric or chemical process. The gas level rises much faster in the spring and summer months than predicted, similar to the still mysterious methane. The big question, of course, is why ?
The startling peer-reviewed findings were just published in the Journal of Geophysical Research: Planets of 12 November 2019.
Sushil Atreya, professor of climate and space sciences at the University of Michigan, said:
The first time we saw this, it was just mind-blowing.
So, what happens?
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Curiosity analyzed the composition of the air at Gale Crater over three Mars years (approximately six Earth years) using its portable chemistry laboratory Sample Analysis at Mars (SAM). The results were broadly in line with expectations and had been known for years: 95% carbon dioxide (CO2), 2.6% molecular nitrogen (N2), 1.9% argon (Ar), 0.16% molecular oxygen (O2), and 0, 06% carbon monoxide (CO). (Methane is usually present in much smaller amounts, averaging about 0.00000004%.)
Nitrogen and argon typically follow a predictable pattern that increases and decreases in proportion to the amount of carbon dioxide produced each year. This is related to the change in air pressure over the year, as carbon dioxide freezes over the poles of the planet in winter as snow and ice, which lowers the air pressure. The air pressure rises again when the carbon dioxide evaporates in spring and summer.
It's going to be funny here. Scientists had expected the oxygen to follow the same pattern as nitrogen and argon, but it did not. Oxygen levels rose by as much as 30% in spring and summer, then dropped back to normal in the fall and even below. Curiosity observed this process every spring and summer on Mars.
What are the possible explanations? The researchers have considered several options, but none explains all the results.
Was there a problem with the SAM lab? The researchers checked that the instrument was OK and working properly.
Could carbon dioxide or water molecules release oxygen when they were dissolved by solar radiation in the atmosphere? Probably not, because it requires five times more water vapor than is available to produce the observed amount of oxygen. Carbon dioxide would decompose too slowly in such a short time to produce the same amount of oxygen.
Could the later observed oxygen waste have been caused by solar radiation, which breaks down oxygen molecules? No, as this would be an even slower process that would take up to 10 years.
The scientists involved also consider it unlikely that it is caused solely by atmospheric circulation patterns. According to Melissa Trainer, a planetary scientist at NASA's Goddard Space Flight Center (GFSC), who led the research:
We have trouble explaining this. The fact that the oxygen behavior is not perfectly reproducible in every season suggests that it is not an atmospheric problem. It must be a chemical source and sink that we can not yet explain.
As Timothy McConnochie, a research associate at the University of Maryland, also noted:
We could not think of any of this. Although this process produces the amount of oxygen we need, we believe that it is something in the world Surface soil that changes seasonally because there are not enough oxygen atoms in the atmosphere to produce the behavior we observe.
In the thesis itself, each of these hypotheses is discussed in more detail, and it is explained to what extent none of these hypotheses adequately explains the results so far. Nevertheless, produces some much more oxygen in the warmer months than it should. Interestingly, it has been observed that both oxygen and methane fluctuate, at least occasionally, as follows, suggesting that there may be a common source. As Atreya also noted:
We begin to observe this tantalizing correlation between methane and oxygen for much of the Martian year. I think there is something in it. I just have no answers yet. Nobody does that.
Combining oxygen and methane is considered a biosignature on Earth because they destroy each other unless they are produced continuously and flow into the atmosphere at relatively high rates. For this reason, both gases should be in a thermodynamic imbalance state.
Most of the oxygen and methane on Earth is produced and / or consumed by life. Could that really be what happened on Mars? Or is there another unknown chemistry? The Curiosity data showed that the background methane levels decreased at the same time as the oxygen levels in the last half of the year, although the oxygen increased again earlier in the year than the methane value and was more variable from year to year. However, the larger methane peak observed by Curiosity also occurred during the same period as the increase in oxygen content in the spring. What this all means is not yet clear, and further studies are needed.
If there really is a correlation between oxygen and methane on Mars, this could be a potential biosignature. A previous study by Shawn Domagal-Goldman of NASA's Goddard Space Flight Center in 2014 found that oxygen and methane can sometimes be generated by non-biological processes, such as exoplanets.
However, our research supports the argument that methane and oxygen together, or methane and ozone together, are still powerful signatures of life. We've been really trying to put false positives on life, and we've found some, but only for oxygen, ozone, or methane itself.
These strange fluctuations in oxygen levels in Gale Crater – with one possibility with methane fluctuations and spikes is a fascinating new mystery for the Mars researchers. In summary, Trainer summarizes:
This is the first time in several years that we observe this interesting behavior. We do not quite understand it. For me, this is an open appeal to all the smart people out there who are interested in: See what you can come up with.
Conclusion: The NASA Curiosity Rover has noted unusual increases and decreases in the oxygen content in the air at the Gale Crater. In a way, these are similar to, and may even be related to, the variability of methane.
Source: Seasonal variations in atmospheric composition measured at Gale Crater, Mars