For three Martian years ending in 2017, the Sample Analysis at Mars (SAM) instrument aboard the Curiosity Rover analyzed the composition of the Red Planet’s air. Most chemicals in the air behaved in a predictable way, increasing and decreasing in relation to the amount of carbon dioxide at any given point in the year.

But not oxygen. It unexpectedly increased by upwards of 30 percent in the spring and summer before dropping back down to expected levels in the fall — and scientists are struggling to figure out why, according to a paper published in the Journal of Geophysical Research: Planets on Tuesday.

“The first time we saw that, it was just mind boggling,” co-author Sushil Atreya, a climate and space sciences professor University of Michigan, said in a press release.

To try to explain the phenomenon, the researchers first double- and triple-checked the accuracy of SAM. Then they considered the possibility that CO2 or water molecules were splitting to produce the extra oxygen. Finally, they calculated whether solar radiation could cause oxygen to split and leave Mars’ atmosphere.

But no matter how many explanations they considered, the scientists couldn’t find one that made sense.

“We’re struggling to explain this,” research lead Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center, said in the press release. “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 team does have one lead it still hasn’t exhausted, though: methane.

For more than 15 years, scientists struggled to explain where Mars’ atmospheric methane was coming from before finally pinpointing an ice sheet on the planet’s surface as a source in April.

While they still haven’t figured out why Mars’ atmospheric methane levels spike in the summer, they’re now wondering if that existing mystery is tied to this new one.

“We’re beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year,” Atreya said. “I think there’s something to it. I just don’t have the answers yet. Nobody does.”

READ MORE: With Mars Methane Mystery Unsolved, Curiosity Serves Scientists a New One: Oxygen [NASA]

More on Mars: Astronomers Found Mars’ Missing Methane