Mars Rover Discovers Oxygen on Mars

Curiosity, NASA's SUV-sized roaming laboratory working on Mars, has been at the center of a number of Martian mysteries during its seven-year stint on the red planet. In June, its Sample Analysis at Mars instrument detected a surprising spike in methane gas that puzzled NASA scientists. The mystery of Mars methane has been partly resolved, but now Curiosity has uncovered another baffling Martian puzzle: Oxygen is acting strangely and scientists don't really understand why.

The SAM instrument is able to analyze what molecules make up the Martian atmosphere. The atmosphere on the red planet is mostly made up of carbon dioxide (CO2), but during winter, CO2 freezes over the poles, which lowers air pressure across the planet. During the thaw, in summer and spring, the CO2 raises air pressure. Researchers see molecules like nitrogen and argon change in concentration according to the season, too. But oxygen acted differently. It only makes up a tiny amount of the Martian atmosphere (around 0.16%), but it doesn't follow the expected rises and falls. During spring and summer, the concentration of oxygen was much higher than expected. In fall, it dropped back to predicted levels. Then in winter, it fell below predicted levels.

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https://www.cnet.com/news/nasas-curiosity-rover-makes-a-baffling-oxygen-discovery-on-mars/