NASA scientists say pure sulfur has been found on Mars for the first time after the Curiosity rover inadvertently uncovered a cluster of yellow crystals when it drove over a rock. And it looks like the area is filled with it. It’s an unexpected discovery — while minerals containing sulfur have been observed on the Red Planet, elemental sulfur on its own has never been seen there before. “It forms in only a narrow range of conditions that scientists haven’t associated with the history of this location,” according to .

Curiosity cracked open the rock on May 30 while driving in a region known as the Gediz Vallis channel, where similar rocks were seen all around. The channel is thought to have been carved by water and debris flows long ago. “Finding a field of stones made of pure sulfur is like finding an oasis in the desert,” said Ashwin Vasavada, Curiosity’s project scientist. “It shouldn’t be there, so now we have to explain it. Discovering strange and unexpected things is what makes planetary exploration so exciting.”

After spotting the yellow crystals, the team later used a camera on Curiosity’s robotic arm to take a closer look. The rover then took a sample from a different rock nearby, as the pieces of the rock it had smashed were too brittle for drilling. Curiosity is equipped with instruments that allow it to analyze the composition of rocks and soil, and NASA says its Alpha Particle X-Ray Spectrometer (APXS) confirmed it had found elemental sulfur.