Mercury

Mercury is the smallest and innermost planet of our Solar System, a small, rocky, "terrestrial" world with a heavily cratered surface similar to Earth's Moon and no moon of its own. It has a huge, iron-rich metallic core, the second-highest density of any planet, and an extremely thin atmosphere called an exosphere. Lacking an atmosphere to regulate its temperature, Mercury experiences extreme temperature fluctuations, and water ice can be found in permanently shadowed craters at its poles.