Mars' Mysterious 'Blueberries': How Did These Iron-Rich Spheres Form?

In 2004, NASA's Opportunity rover discovered thousands of gray, iron-rich spheres on Mars, nicknamed "blueberries." These 4-mm-wide balls littered rocks near the landing site, puzzling scientists until the rover found a depression—dubbed the "Berry Bowl"—packed with the spheres, captured on the 48th Martian day of the mission.