NASA’s Curiosity rover has discovered a wealth of never-before-seen crystals on the Red Planet after accidentally hitting a rock.

On May 30, the rover accidentally hit and split a small rock while exploring Hades Vallis, a channel carved into the steep slopes of Mount Sharp in the heart of Gale Crater. When the rover’s cameras focused on the robot’s tripped-over object, scientists noticed unusual yellow crystals glowing in the newly exposed interior of the rock.

The crystals in the cracked rock were too small and delicate for the rover to properly use, but when the robot drilled into another nearby rock, it discovered the crystals were made of pure sulfur.

Sulfur has been detected on Mars before, but only in combination with other elements in compounds known as sulfates. Until now, pure sulfur, also known as elemental sulfur, had not been found on the Red Planet. Scientists suspected that elemental sulfur might exist somewhere on Mars, but were surprised to find it in surface rocks.

“It’s not supposed to be there, so we need to explain it now,” said Ashwin Vasavada, Curiosity rover project scientist at NASA’s Jet Propulsion Laboratory. “Discovering the strange and unexpected is what makes planetary exploration so exciting,” he added.

The rover did not confirm the presence of yellow crystals in other rocks, but scientists are confident that nearby rocks also contain sulfur, making this area interesting for future research.

“Finding an area of ​​pure sulfur rocks is like finding an oasis in the desert,” Vasavada said.

Curiosity, which landed in the region in 2012, has found many more interesting rocks in and around the Gediz Valley. In February, the rover took photos of “ripples” carved into the bottom of an ancient lake by Martian water millions of years ago. And in May, a rover found rocks containing manganese oxide—the best evidence yet that the Red Planet once had an oxygen-rich atmosphere similar to Earth’s.