The presence of methane in the atmosphere of Mars has become an interesting puzzle for planetary scientists. That's because, on Earth, methane is related to life, but it can also be produced geologically. Some of the best data about Mars methane comes from the explorer Curiosity, which landed on Mars after descending bravely in the atmosphere in August 2012. Now Curiosity has made another interesting discovery: oxygen at the exploration site behaves in a way that has not been explained by atmospheric or chemical processes that known. Gas levels rise far more in spring and summer than predicted, similar to the mysterious methane. The big question, of course, is Why?
Confusing peer-reviewed results were only published in the November 12, 2019 edition Journal of Geophysical Research: Planet.
Sushil Atreya, professor of climate and space science at the University of Michigan, said:
The first time we saw it, it was just confusing.
So what happened?
EarthSky 2020 calendar month is available! They make amazing gifts. Order now. Go fast!
Curiosity analyzed the composition of the air in the Gale Crater for three Mars years (about six Earth years), using Analysis Samples in the Martian portable chemistry laboratory (SAM). The results are almost as expected, and have been known for years: 95% carbon dioxide (CO2), molecular nitrogen 2.6% (N2), 1.9% argon (Ar), molecular oxygen 0.16% (O2), and 0.06% carbon monoxide (CO). (Methane is usually in much smaller amounts, around 0.00000004% on average).
Nitrogen and argon tend to follow a predictable pattern each year, increasing and decreasing relative to how much carbon dioxide is there. This is related to changes in air pressure during the year, because carbon dioxide freezes when snow and ice on the poles of the planet during winter, which lowers air pressure. Air pressure rises again when carbon dioxide evaporates in spring and summer.
This is where it becomes strange. Scientists had suspected that oxygen would follow the same pattern as nitrogen and argon, but it did not. Oxygen levels increase much more in spring and summer – by as much as 30% – and then drop back to normal levels, and even below, in autumn. This same process is observed by Curiosity every spring and summer of Mars.
So what are some possible explanations? Researchers have considered several possibilities, but none explain all the results.
Are there any problems with the SAM lab? The researchers checked but the instrument was fine and functioning properly.
Can carbon dioxide or water molecules release oxygen when they burst in the atmosphere due to solar radiation? Probably not, because it will require five times more water vapor than is available to produce the amount of oxygen observed. Carbon dioxide will break down too slowly to produce the same amount of oxygen over a short period of time.
As for the decrease in oxygen seen later, could it have been caused by solar radiation breaking up oxygen molecules? No, because that would be a slower process, taking up to 10 years.
The scientists involved also think it could not have been caused only by atmospheric circulation patterns. According to Melissa Trainer, a planetary scientist at NASA's Goddard Space Flight Center (GFSC) who led the research:
We try to explain this. The fact that oxygen behavior cannot be repeated perfectly every season makes us think that it is not a problem related to atmospheric dynamics. It must be a source of chemicals and resources that we cannot account for.
As Timothy McConnochie, assistant research scientist at the University of Maryland, also noted:
We have not yet been able to find a process that produces the amount of oxygen we need, but we think it has to be something on the ground that changes seasonally because there aren't enough oxygen atoms available in the atmosphere to create the behavior we see.
The paper itself goes into more detail about each of these hypotheses and how none of them explain the results so far. However something produce more oxygen during the warmer months than it should. Interestingly, both oxygen and methane have been observed to fluctuate like this together at least on several occasions, suggesting there might be a common source. As Atreya also noted:
We began to see a tantalizing correlation between methane and oxygen for much of the Mars year. I think there is something to it. I don't have the answer yet. No one knows.
On Earth, having oxygen and methane together is considered a biosignature, because they tend to destroy each other unless they continue to be produced and flowed into the atmosphere at a relatively high rate. Therefore, both gases are said to be in thermodynamic disequilibrium conditions.
So far, most of the oxygen and methane on Earth is produced by and / or consumed by life. Could that be what happened on Mars? Or are there other unknown chemicals that occur? Curiosity's data shows that the background level of methane decreases at the same time as the level of oxygen in the last half of each year, although oxygen increases again earlier this year than methane, and varies more from year to year. However, the larger "spike" in methane seen by Curiosity, also occurs during the same time period as the increase in oxygen in the spring. What this means is not yet clear, and further studies will be needed.
If there really is a correlation between oxygen and methane on Mars, that is can to be a potential biosignature. An earlier study in 2014 by Shawn Domagal-Goldman from NASA's Goddard Space Flight Center found that while oxygen and methane itself can sometimes be created by non-biological processes, for example exoplanets, but finding them together would be more biosignature assured:
However, our research reinforces the argument that methane and oxygen together, or methane and ozone together, are still strong signs of life. We try very, very difficult to make false positive signals for life, and we find some, but only for oxygen, ozone, or methane itself.
This strange fluctuation in oxygen levels in the Gale Crater – with possible connections to fluctuations and methane spikes – is an interesting new mystery to be tried and solved by Mars scientists. As summarized by the Coach:
This is the first time we have seen this interesting behavior in several years. We do not fully understand it. For me, this is an open call for all the smart people out there who are interested in this: See what you can find.
The bottom line: NASA's Curiosity explorer has detected an unusual increase and decrease in oxygen levels in the air at the Gale Crater. In some ways this is similar to fluctuations in methane, and may even be connected.
Source: Seasonal variations in atmospheric composition measured at Gale Crater, Mars