What Mercury made of? 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 planet's diameter. Mercury's iron core is about the size of the moon. Iron makes up about 70% of Mercury's total weight making Mercury the most iron-rich planet in the solar system. On top of this core, lies an outer, rocky shell which is about 350 miles (~550 km) thick.