A study led by Aditya Khuller of the University of Washington, using data from the OMEGA and CRISM instruments and the Phoenix lander, finds the cap is about 3% dust by mass, not the roughly 25% assumed before. A dusty frost forms each winter and hides older, cleaner ice that shows again in summer. Our take: cleaner ice keeps a sharper record of Mars’s past climate, and would be easier water for future explorers.