WASHINGTON, Nov. 12 -- The changes of oxygen concentrations in the Martian air baffled scientists as the gas Earth creatures use to breathe behaves in a way that cannot be explained by any known chemical processes.

The study published on Tuesday in the Journal of Geophysical Research: Planets showed that the amount of oxygen on Mars rose throughout spring and summer by as much as 30 percent, and then dropped back to normal levels in autumn in a Martian year, implying that something was producing it and then taking it away.

NASA's Curiosity rover measured, for the first time in history, the seasonal changes in the gases that fill the air directly above the surface of Gale Crater on Mars.

It revealed the makeup of the Martian atmosphere at the surface: 95 percent by volume of carbon dioxide (CO2), 2.6 percent molecular nitrogen, 1.9 percent argon, 0.16 percent molecular oxygen and 0.06 percent carbon monoxide.

Those molecules mix and circulate with the changes in air pressure throughout the Martian year, caused when CO2 freezes over the poles in the winter, thereby lowering the air pressure across the planet and redistributing air to maintain a pressure equilibrium, according to the study.

However, oxygen didn't follow a predictable seasonal pattern of waxing and waning in concentration relative to how much CO2 is in the air like nitrogen and argon.

Scientists considered the possibility that CO2 or water molecules could have released oxygen when they broke apart in the atmosphere, leading to the short-lived rise. But it would take five times more water above Mars to produce the extra oxygen, and CO2 breaks up too slowly to generate it over such a short time.

Also, the solar radiation, which can break up oxygen molecules into two atoms that blew away into space, cannot explain the oxygen decrease since it would take at least 10 years for the oxygen to disappear through this process.

The similar pattern has been observed in that of methane in the air inside Gale Crater in quite a small quantities (0.00000004 percent on average), according to the researchers.

"We're beginning to see this tantalizing correlation between methane and oxygen for a good part of the Mars year," said the paper's co-author Sushil Atreya, professor of climate and space sciences at the University of Michigan in Ann Arbor.

Oxygen and methane can be produced both biologically and abiotically. Scientists are considering all options, although they don't have any convincing evidence of biological activity on Mars.

Currently, scientists expect that non-biological explanations are more likely with Martian soil a potential source of the extra springtime oxygen.