Why is the Earth's core still hot after billions of years?

Heat from planet formation, radioactive decay of some elements, and friction between layers keep the Earth's core at extremely high temperatures after 4.5 billion years.

At the center of the Earth, temperatures reach 5,200 degrees Celsius , nearly as hot as the surface of the Sun. What keeps these hellish temperatures going are forces that have been at work for billions of years, according to IFL Science . The Earth's core is divided into two distinct regions: an outer core, made mostly of molten iron and nickel, and an inner core, essentially a solid sphere the size of the Moon, made of two metals.

Simulation of the Earth's core. (Photo: Physic World).

Although the temperature inside the core is extreme, we cannot feel the heat at the surface except through volcanoes and geothermal springs. The outer core begins 2,889 km below the Earth's crust. Between the crust and the outer core, there is a lot of material, the mantle contains rocks that absorb and disperse heat. In fact, there is no way to measure heat directly . Because it is impossible to send probes or people deep underground, scientists calculate the temperature by studying how iron and iron-rich compounds melt under high pressure. By determining the melting temperature, they can infer the temperature at the core.

Some of this heat is a relic from the past , tied to the birth of the planet. The surface cools rapidly, while the core is the last to cool. Earth was born about 4.5 billion years ago when gravity condensed matter from the cloud of hot gas and particles surrounding the young Sun. As the molten sphere cooled, the outer layer hardened like candle wax to form the crust. However, the mantle retains some of its heat and continues to cool.

A 2011 study estimated that heat left over from the planet's formation accounts for nearly half of the heat in Earth's interior. The rest comes from the radioactive decay of uranium-238 and thorium-232 in the planet's core, accounting for about 54%. Finally, there is heat from friction caused by the movement of solid and liquid layers under enormous pressure.

The heat inside the Earth is important to the surface because it is the main force behind the tectonic plates, causing them to move around and create or destroy continents. Scientists predict that the Earth's core will cool and solidify 91 billion years after the Sun dies.