Curiosity’s atmospheric measurements showed exactly that pattern for nitrogen and argon, two other trace gases in the Martian atmosphere. But, for oxygen, the concentrations shot up by a third during spring.

“This was a very unexpected result, an unexpected phenomenon,” Dr. Trainer said. “There’s a lot we don’t know about the oxygen cycle on Mars. That’s become apparent.”

Adding to the mystery, the cycle was not the same each year, and the scientists could not find an obvious explanation — like temperature, dust storms or ultraviolet radiation — for what changed from year to year.

On Earth, most oxygen is generated by the photosynthesis of plants. But so far, for the Mars scientists, that is far down on the list of explanations.

“You’ve got to rule out all of the other processes first before you go there,” Dr. Atreya said.

More likely sources are chemicals like hydrogen peroxide and perchlorates known to exist in the Martian dirt. “It’s pretty clear you need a flux from the surface,” Dr. Atreya said. “Nothing in the atmosphere is going to create this kind of rise.”

But how these chemicals might release and absorb enough oxygen to explain the seasonal rise and fall is difficult to figure out, especially as there are only 19 oxygen measurements over five and a half years.

An intriguing possibility is that the oxygen mystery might be tied to another trace gas, methane, that is also acting strangely in the Martian atmosphere.