CARLOS CLARIVAN / SCIENCE PHOTO LIBRARY CARLOS CLARIVAN / SCIENCE PHOTO LIBRARY
Cutaway illustration showing the internal structure of Mercury, the smallest planet of the Solar System. Mercury has a radius of 2439 kilometres (km). Data compiled by the Messenger probe that orbited the planet between 2011 and 2015 revealed its composition. It has a rocky crust about 40 km thick, that covers a 400 km thick silicate mantle. Below this is an iron sulfide (FeS) layer that is 50 km thick and a huge metallic core with a radius of 1950 km. The outer core (1000 km) is composed of molten iron while the inner core (950 km) is solid iron.
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