Mercury is a rocky planet with a huge iron core which makes up a large part of its interior. The core takes up nearly 3/4 of the planets diameter, and it is about the size of the moon. Iron makes up about 70% of Mercurys total weight, making Mercury the most iron-rich planet in the Solar System. Mercurys core makes up a larger portion of the planet than others in the solar system, hinting at a chaotic beginning. On top of this core lies an outer, rocky shell which is about 350 miles (~550 km) thick. Mercurys surface is rich in sulfur, about 20 times richer than the surfaces of Earth, the Moon, and Mars. Messenger also found low surface abundances of titanium and iron. Mercury seems to have formed in conditions much more reducing—i.e., those in which oxygen was scarce—than other terrestrial planets. Mercury has little atmosphere, but what it does have is made up mostly of oxygen, sodium, hydrogen, helium, and potassium.