Mars has a very tenuous atmosphere, roughly 1 percent of Earth’s own, which is overwhelmingly made of carbon dioxide. Tiny fractions of other gases have been measured and studied over time, leading to several puzzling findings. The recently reported spike in methane is certainly among them and now we can add another one: an unusual increase in oxygen.

As reported in the Journal of Geophysical Research: Planets, Curiosity has collected years' worth of data of the amount of oxygen relative to carbon dioxide present above Gale Crater, where the industrious rover is operating. Oxygen has been seen to increase up to 30 percent during the summer months and researchers don’t know why. By the fall, the oxygen level goes back down to the expected values.

Carbon dioxide accounts for 95 percent of the Red Planet’s air. In fall and winter, it freezes over the poles and it sublimates in the hotter months. Nitrogen and Argon, which make up together 4.5 percent of the Martian atmosphere, wax and wane in relation to CO₂. Oxygen doesn’t behave like that at all.

"We're struggling to explain this," lead author Melissa Trainer, a planetary scientist at NASA's Goddard Space Flight Center, said in a statement. "The fact that the 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 can't yet account for."

The findings have reminded researchers of the other unruly and unexplained gas detection: methane. Methane was also seen to spike in the warmer months, increasing up to 60 percent over the expected values. The team thinks that oxygen and methane spikes are related but still don't know the cause of the spikes in the two gases when the Red Planet gets warmer.

"We're beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year," explained Sushil Atreya, professor of climate and space sciences at the University of Michigan in Ann Arbor. "I think there's something to it. I just don't have the answers yet. Nobody does."

Methane and oxygen can be produced by geological chemistry and also from biological systems such as microbes. Currently, there is no evidence that there’s life of Mars and this finding shouldn’t be taken as evidence of it. That said the team is still looking for a likely explanation that can explain the data, and they suspect that whatever is happening must be taking place in the top layer of soil. Hopefully, more data will offer up some clues into these curious spikes.