The inner core of the Earth is solid despite its extremely high temperature because of the intense pressure from the rest of the planet that keeps the iron atoms together by force. The pressure and density are simply too great for the iron atoms to move into a liquid state. The inner core is mostly made up of iron and has a radius of about 1,220 kilometers (758 miles) and a temperature of about 5,200° Celsius (9,392° Fahrenheit) . The pressure in the inner core is nearly 3.6 million atmospheres (atm) . The primary contributors to the inner cores heat are the decay of radioactive elements such as uranium, thorium, and potassium in Earths crust and mantle, residual heat from planetary formation, and heat emitted by the solidification of the outer core.