The Curiosity rover has made its most unusual discovery on Mars: rocks made of pure sulfur. This happened when the rover accidentally drove over and cracked open a rock, revealing yellowish-green crystals never seen before on the planet. Ashwin Vasavada, Curiosity project scientist, described it as the strangest and most unexpected find of the mission, noting that luck played a significant role.

Curiosity’s team had been eager to investigate the Gediz Vallis channel, a feature believed to be created 3 billion years ago by water and debris, carved into Mount Sharp. During this exploration, the rover captured a close-up image of a rock with elemental sulfur. Previously, the rover had found sulfates like gypsum, but pure sulfur was unexpected. The discovery led scientists to investigate what this means for Mars’ history.

Read more: New Study Indicates Mars had Cold, Icy Past

Since landing on Mars in 2012, Curiosity has been ascending Mount Sharp, studying its layers to understand Mars’ wet and dry periods. In its recent investigations, the rover found pure sulfur rocks and a variety of other minerals in the Gediz Vallis channel, suggesting different environmental conditions over time. Despite challenges, Curiosity continues to explore and provide valuable insights into Mars’ geologic history.