Mars is often called the “Red Planet” due to the red hue of the rocks and dust covering the planet’s surface. How it got this red color has been a bit of a mystery, but new research may have identified the cause.

Mars is a very dry and desolate planet. It has no water on its surface. For that reason, scientists had long theorized the planet’s red hue came from the presence of hematite, an oxidized iron that can form without water. This would make sense on a very dry planet.

However, researchers here on Earth got a different answer when they replicated (created a copy of) Martian dust in a laboratory. The results indicate a different mineral, ferrihydrite, is the culprit. Ferrihydrite is also an iron oxide, but it is produced when iron reacts with oxygen and water. The presence of ferrihydrite on Mars would mean the planet was not always so dry.

“Our understanding of why Mars is red has been transformed,” Adomas Valantinas, a scientist at Brown University who led the research, told CNN.

If this new discovery holds true, it helps paint a picture of the Red Planet’s distant past. It means that at one point—billions of years ago—Mars was a water-rich planet with oxygen. When the iron in Martian rocks reacted with the water, it created rusty-red ferrihydrite. Then slowly, as the years passed, the ferrihydrite broke down into dust, explained Valantinas.

“As Martian winds spread this dust everywhere, it created the planet’s iconic red appearance,” he said.

But Valantinas and his research team will need to wait before they can confirm that there is ferrihydrite on Mars. Right now a Mars rover, called Perseverance, is collecting mineral samples on the planet. One day a future mission will pick up the rover’s samples and bring them back to Earth. Only then can scientists analyze the samples to see what minerals are there, said Jack Mustard, another researcher who took part in the study.

“When we get those back, we can actually check and see if this is right,” said Mustard.