Mysterious changes: The atmosphere of Mars gives researchers – once again – puzzles. For the oxygen content of the gas envelope varies surprisingly strong and abrupt, as measured data from the Mars rover Curiosity show. In the spring, the values increase by 30 percent – that is far more than can explain common processes. Where this oxygen comes from and where it disappears in the fall is therefore still completely puzzling.
At first glance, the atmosphere of the Red Planet is not very exciting: it consists of 95 percent carbon dioxide, with small amounts of nitrogen, carbon monoxide, argon, carbon monoxide and oxygen. Typical for these gases are slight variations over the seasons: In winter, when a lot of CO2 crystallizes out as dry ice at the poles, the gas pressure and thus also the density of the trace gases decreases. In summer, the gas density increases again.
But there are strange deviations from this pattern, as measured by NASA's Mars rover Curiosity in recent years – especially methane. This trace gas changes its concentration surprisingly erratic and alternates between undetectable and up to 21 parts per billion (ppb). Due to the seasonal fluctuations alone, these strong, abrupt changes are unexplainable – and the origin of this gas is still unclear.
Surprise with martian oxygen
Now another gas provides some guesswork: the Martian oxygen. Because the Curiosity data from the last three years of Mars now show inexplicable fluctuations even with this gas, as reported by Melissa Trainer from NASA's Goddard Space Flight Center and her colleagues. In the last spring and summer, the oxygen value rose surprisingly strong and abruptly from 1,700 to 2,200 ppb – significantly more than expected on the basis of seasonal CO2 fluctuations alone.
"The first time we saw that, we were really confused," says co-author Sushil Atreya of the University of Michigan. To explain this increase, the atmosphere above the Gale crater would have to get around ten trillion O2 molecules per cubic centimeter in the spring. In addition, this seasonal O2 increase turns out differently every year.
"This is surprising because the chemical life of oxygen in the Martian atmosphere is around ten Earth years," the researchers said. The O2 content would therefore have to be far more uniform than measured by curiosity.
Where does the oxygen come from?
How can this be explained? "At the moment we are having a hard time finding an explanation," says coach. Because so far, the researchers know no process that can produce these amounts of oxygen in such a short time. "In the Martian atmosphere, there are not enough available oxygen atoms to produce the observed behavior," says co-author Timothy McConnochie of the University of Maryland.
Although there are oxygen-containing minerals in the Martian underground such as perchlorate (ClO4) and hydrogen peroxide (H2O2). These could theoretically decay under the influence of sunlight and release oxygen. But with perchlorate, the release rate would be far too low to cause such an abrupt O2 surge, as the coaches and their team explain. The peroxide, on the other hand, could release a lot of oxygen. But then you would have to simultaneously detect chlorine and hydrogen – and that was not the case.
Cause remains puzzling
Currently, researchers can only speculate about where the oxygen thrust comes from in the spring of Mars and where the gas disappears in the fall. "There must be a chemical source and a sink somewhere that we do not know yet," says coach. In theory, oxygen could be produced by both geochemical and biological processes, similar to methane. But curiosity can not distinguish with its measuring instruments, which origin of the Martian oxygen.
However, the researchers consider a geochemical origin far more likely – even if they do not yet know which could be. "So far we do not understand this," says coach. "As with all the surprising results, we now hope that further on-site measurements and theoretical considerations will shed more light on this exciting observation." (Journal of Geophysical Research: Planets, 2019; doi: 10.1029 / 2019JE006175)
Source: NASA / Goddard Space Flight Center