There are a lot of rocks on Mars, and most of them wouldn’t raise an eyebrow. But one in particular has revealed new insights about the ancient Martian atmosphere.

In 2013, Mars rover Curiosity identified large amounts of the element manganese in a piece of rock – which, by all accounts, shouldn’t have been there. Now, analysts say the discovery could be proof of a once-oxygenated Martian atmosphere.

Most planetary crusts are composed of basalt, a type of rock that forms when lava is cooled near a planet’s surface. Mars is no exception, so researchers expected that Curiosity would find plenty of basalt on the red planet. But on the formation dubbed “Caribou,” the rover found something unusual: manganese.

This element can be found in basalt, but only in trace amounts. The manganese would have to be concentrated significantly to reach the levels found on Caribou. Researchers say that condensed manganese could only form on Mars if, somehow, basalt rock was dissolved in oxygenated water.

It is widely accepted that Mars was once abundant in surface water. But until recently, few would have guessed that the planet was ever oxygen-rich.

“If we could peer onto Mars millions of years ago, we’d see a very wet world,” wrote Nina Lanza, a researcher at Los Alamos National Laboratory who was involved in the analysis. “Yet we didn’t think Mars ever had enough oxygen to concentrate manganese – and that’s why we thought the data from Caribou must have been an error.”

After the original discovery, researchers sent Curiosity on a hunt for more manganese. The rover used ChemCam, a chemical analysis tool that works by vaporizing chunks of rock with a laser, to take more samples from the Gale crater. Sure enough, it found more manganese.

There are a few ways oxygen could have ended up in the atmosphere of Mars. Dr. Lanza suggests that ionizing radiation from the sun could have split water molecules, leaving oxygen and hydrogen. Mars has no magnetic field and low gravity, so hydrogen atoms, which are extremely light, could have simply floated away. The remaining oxygen would have absorbed into rock, turning it red through oxidation.

The process went a little differently on Earth. About 3 billion years ago, photosynthesizing microbes evolved from other microbial life. These organisms used sunlight to produce energy, depositing oxygen waste into an atmosphere that, until that point, was low in oxygen. This shift led to changes in Earth’s atmosphere and geology, and paved the way for a new kind of life on the planet.

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In the hunt for life-harboring exoplanets, oxygen-rich atmospheres are considered a biosignature – proof that life could exist there. But correlation doesn’t always equal causation, and Lanza notes that oxygen and life aren’t mutually inclusive. Further mineral analysis, she says, is the clear path to new insights.

“This tells us that Mars has evolved very differently than we thought it did,” Lanza told The Christian Science Monitor in a previous report. “We need to start looking for different types of minerals and other evidence about Mars’s past.”