This mosaic of images from NASA's MESSENGER spacecraft shows the impact crater Hokusai, located on Mercury at a latitude of 58°N. The crater has an impressive system of rays, which extend as much as a thousand kilometers (more than 600 miles) across the planet and are the longest that have yet been identified on Mercury.
Such rays are formed when an impact excavates material from beneath the surface and throws that material outward from the crater. These bright rays, consisting of both ejecta and secondary craters that form when the ejected material re-impacts the surface, slowly begin to fade as they are exposed to the harsh space environment. Mercury and other airless planetary bodies are being constantly bombarded with micrometeoroids and energetic ions, producing an effect known as space weathering. Craters with bright rays are thought to be relatively young because the rays are still visible, indicating that they have had less exposure to such weathering processes than craters that lack rays.