Mars’ oxygen
The Curiosity rover has been amassing data that shows an unexpected change in the Red Planet's O2
Discoveries by the Curiosity rover reveal an unexpected change on the Red Planet
Curiosity has been trundling across the surface of Mars since 2012, revealing a lot of detail about the Martian climate and the Red Planet's geology. So far the rover has proven to be invaluable in showing that Mars could have once sustained life. It has uncovered evidence of ancient persistent water flows, discovered organic carbon in rocks and found two forms of radiation that could pose potential health risks to astronauts. But its work continues to astound and confound.
In November 2019, scientists revealed the results of a study of Curiosity's data that had been gathered directly above the surface of Gale crater, the rover's landing spot. Estimated to be as old as 3.8 billion years and 154 kilometres (96 miles) in diameter, its a significant area, but on this occasion the focus has been on the detected levels of oxygen – in particular its baffling, fluctuating behaviour.
To explain, over the course of some six years, scientists have worked out that the Martian air is mainly made up of carbon dioxide, which accounts for some 95 per cent by volume. The rest consists of molecular nitrogen (2.6 per cent), argon (1.9 per cent), molecular oxygen (0.16 per cent) and carbon monoxide (0.06 per cent), with the presence of nitrogen and argon proving constant and following a predictable seasonal pattern relative to the amount of carbon dioxide in the air.
The level of oxygen, however, is showing a very different and surprising pattern. According to a study headed by Melissa Trainer, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, oxygen levels have been seen to rise by as much as 30 per cent during the Martian spring and summer before dropping back in the autumn. What's more, when the scientists looked at the data over three Martian years, each of which lasts 687 days, they found that the pattern repeated itself each time. Scientists are now trying to discover why.
“The proportion of oxygen is found to rise in the summer by as much as a third above its predicted value of about 1,300 parts per million by volume,” explains Sushil Atreya, a planetary scientist at the
University of Michigan and part of the scientific team. “On the other hand, nitrogen and argon – both of which also have a long lifetime on Mars – behave normally.” Indeed, the top reported rise of oxygen was equivalent to an extra 400 parts per million, and this was detected by the quadrupole mass spectrometer instrument on board the Sample Analysis at Mars (SAM) – a portable chemistry lab inside Curiosity.
“The presence of oxygen in the atmosphere of Mars can be attributed to the chemistry between the main gas of the atmosphere, carbon dioxide, and a smaller component, water vapour, together with energy from the Sun,” says Atreya.
“Numerical models of the past several decades had predicted that oxygen levels in the atmosphere of Mars should remain constant throughout the year because the lifetime of the gas against all known loss processes is fairly long, far exceeding the length of a year on Mars. It was a bit jarring to discover that wasn't the case.”
There is a possibility that microbial life is behind this surge, since oxygen can be produced biologically. If this was the case then the explanation would be incredibly exciting, but scientists are leaning more towards an abiotic source, which means the surge is likely down to chemistry that is related to water and rocks.
Unfortunately there is no known chemical process that easily accounts for the observations. Scientists know that the 0.16 per cent of oxygen in the atmosphere is due to ultraviolet light reacting with carbon dioxide and water vapour to break those molecules into their components. Chemical reactions then cause the oxygen to form carbon dioxide, creating a cycle. But if this was to account for the rise which scientists have observed from Curiosity's data, then the process would need to be far more rapid.
Likewise, there would need to be five-times as much water above Mars to account for the increased levels of oxygen. As such, scientists believe a more likely source of the oxygen is hydrogen peroxide or perchlorates which exist on a widespread level in the Martian soil. If it’s the former, it would point to deep, underground sources of oxygen of a sufficient amount to account for a spike, and yet the evidence is that there is not such an abundance.
As for the perchlorates, NASA’s Viking 1 and 2 spacecraft detected these toxic, stable salts composed of chlorine and oxygen when landing on Mars in 1976, and an experiment showed that heat and humidity could release oxygen. Yet the conditions under which that research was carried out were not the same as Mars in the spring, and so no firm conclusions can be made.
Even so, in 2009 the Phoenix lander discovered that perchlorates made up between 0.4 and 0.6 per cent of a collected soil sample, so there is a good chance that oxygen could be released under a set of conditions, perhaps when cosmic radiation breaks them down. Quite why the oxygen levels drop in autumn is just as mystifying, though – something seems to be removing it from the air. The gas is also soaring and plunging randomly. If cosmic radiation really was to account for the spike each year, then it would have to be happening at a faster rate than scientists believe it to be.
“The observed oxygen rise requires a source in the surface,” Atreya tell us. “The atmosphere does not produce the increase. At least three potential
“Oxygen is found to rise in the summer by as much as a third above its predicted value”
Sushil Atreya
non-biological reservoirs exist in the surface: perchlorates, hydrogen peroxide and the rocks oxidised by water or brine in the past when Mars was presumably wetter and warmer, or even today. It appears pretty challenging to dislodge large amounts of oxygen from rocks, however.
“On the other hand, the other two sources look promising for the excess oxygen. The devil is going to be in the details, as it’s not just the increase in oxygen in summer, but also its decline in late summer and autumn back down to the levels predicted by atmospheric chemistry models which must be understood.” But that's not the end of it. As if the situation wasn't mysterious enough, the extent of the rise has been shown to vary year after year. “The amount of the summertime increase in oxygen varies somewhat from one Mars year to the next,” says Atreya. “Is that surprising? What’s surprising is that oxygen increased at all. The yearto-year fluctuations seem to reflect fluctuations in the mechanism behind the increase.”
And yet oxygen is not alone in showing such seasonal fluctuation. Methane inside Gale crater also rises in the summer and falls in the autumn, and it does so by as much as 60 per cent. This was
discovered by SAM's tunable laser spectrometer, which has done a fine job in picking methane up, given that the quantity is a tiny 0.00000004 per cent on average. Scientists wonder if there is some sort of correlation between the behaviour of oxygen and methane, with the general hunch being that it will indeed be the case.
“Both gases increase in summer, which makes me think the same mechanism that triggers the release of oxygen also causes methane to be released to the atmosphere from its subsurface reservoir, discounting the occasional spikes seen in methane,” says Atreya. “Oxygen and methane show a similar trend over part of the year, but some gaps remain in the data.“
As such, it's not proving to be a straightforward case to solve, but some potential problems can at least be ironed out. Tests on Curiosity's instruments have shown that they are not malfunctioning. “Multiple tests of the instrument and testbed experiments in the lab showed that SAM is working properly,” says Atreya. But, in terms of methane at least, only the rover is detecting methane. Go higher in the atmosphere, well away from the ground, and methane is not being picked up.
Indeed, the European Space Agency's Mars Express passed over Gale crater five hours after Curiosity observed a surge in methane of 21 parts per billion by volume and found none. And even Curiosity has found moments when the level is less than one part per billion, making the amount of methane practically bereft.
Given that the ExoMars Trace Gas Orbiter has also struggled to measure methane and yet Curiosity points to it being persistently present, it's going to take a while yet to figure this one out, let alone work out whether or not there is a connection.
“I don’t rule out biology on
Mars either, but that should be considered as the very last resort, only if all else fails”
Sushil Atreya
Going back to the oxygen, however, there is a suggestion that there is a source in the near-surface that interchanges with the atmosphere.
Does it rule out life? “Either biology or geology can generate oxygen and methane,” says Atreya. “On Earth, biology is by far the dominant process, and I don’t rule out biology on Mars either, but that should be considered as the very last resort, only if all else fails. It’s absolutely essential to rule out the abiotic processes first.”
To do that, it's clear that more studies will need to be made, and there will be missions in the future that should help scientists make headway. “Continued measurements with SAM at the surface of Mars will also fill in some gaps in the data, while laboratory studies and electrochemical modelling would be important in untangling the exciting new results,” Atreya says. “Work is ongoing in all areas, and future missions should pay close attention to processes that might be happening beneath the surface of Mars.”
For a better set of results, oxygen and methane readings could be taken more regularly. The scientists were working on the basis of 19 data points over the equivalent of six Earth years, which makes it hard to see what is happening day by day.
“As with any new finding, especially an unexpected one, you want to repeat the measurement to be certain,” says Atreya. “Except that on Mars, one has to wait two years for the same season to roll around again.”
Even so, no one can say for certain whether the phenomena being seen at Gate crater will apply across the whole of the Red Planet. It could only apply to specific areas, and that's something the ExoMars Trace Gas Orbiter hints at, given it can't detect methane.
Atreya is wary of jumping to conclusions, though, and he says this current study should be a warning for those searching for alien life. “The recent
Mars findings further reinforce the importance of continued exploration of the Solar System using state-of-the-art technology,” he says. “They are also a reminder to avoid the temptation of declaring the presence of life if and when oxygen and methane are detected together on an Earth-sized extrasolar planet with surface liquid water in the habitable zone of star. Be ultra-cautious!”