NASA's Curiosity Mars rover took this close-up view of a rock nicknamed "Terra Firme" that looks like the open pages of a book, on April 15, 2023, the 3,800th Martian day, or sol, of the mission, using the Mars Hand Lens Imager (MAHLI) on the end of its robotic arm. The rock is about an inch across (2.5 centimeters). Rocks with unusual shapes are common on Mars, and often were formed by water seeping through cracks in a rock in the ancient past, bringing harder minerals along with them. After eons of being sand-blasted by the wind, softer rock is carved away and the harder materials are all that's left. https://photojournal.jpl.nasa.gov/catalog/PIA25828