The planet most covered with craters is Mercury. Its entire surface is densely cratered, due to a combination of factors. First, Mercury lacks an atmosphere, which means there is no weather or erosion to wear away the craters over time. Second, Mercury's proximity to the Sun means it is frequently hit by debris and meteors from the inner solar system. Finally, Mercury's slow rotation means that each crater remains in direct sunlight for a long time, which prevents ice or other volatile materials from accumulating and protecting the surface from impacts.