Earth’s core does not have one exact temperature, because we cannot lower a thermometer 6,000 kilometers into the planet. A simple answer is that the core reaches more than 5,000 degrees Celsius. Scientific American gives a wider estimate for the core of about 4,000 to over 7,000 kelvins, depending on which part of the core and which laboratory measurements are used.

That uncertainty is not a failure. It is just a hard measurement problem. The deepest human-made holes reach only a tiny fraction of the way down, so scientists infer core temperature from seismic waves, the density of Earth’s layers, and experiments on iron at extreme pressure. Since the core is mostly iron, the melting behavior of iron under deep-Earth pressure is one of the main clues.

The core has two main parts. The outer core is molten, mostly iron alloy. The inner core is solid even though it is extremely hot, because the pressure is enormous. USGS explains the basic layout: inner core, outer core, mantle, and crust, with a molten outer core and a solid inner core under extreme pressure.

So why has it not cooled after billions of years? First, Earth started very hot. During formation, space rocks collided and stuck together, and dense iron-rich material sank toward the center. USGS says the heat from Earth’s formation melted the entire planet, and Scientific American lists formation heat and sinking dense core material as major sources of deep-Earth heat.

Second, Earth is huge and loses heat slowly. Heat has to travel from the deep interior through the mantle and crust before it can escape into space. Scientific American compares Earth’s plates to a kind of blanket and notes that even convection in the solid mantle is not a very efficient way to dump heat quickly. A baked potato stays hot inside for the same reason, just on a much smaller scale.

Third, Earth is still making some heat. Radioactive elements such as uranium, thorium, and potassium decay over time and release energy. The Earth Observatory of Singapore describes radioactive decay as a major source of heat inside Earth. But there is an important detail: Scientific American notes that the mantle is rich in those radioactive elements, while the core is probably poor in radioactive isotopes. So radioactive heating helps keep Earth’s interior active, but the core’s heat is not simply a giant nuclear furnace.

The cooling that does happen is still important. Nature describes Earth as a heat engine driven by radiogenic decay and slow cooling, with the liquid iron core powering the magnetic field as heat is released by cooling and freezing while the solid inner core grows. USGS similarly explains that outer-core convection, helped by radioactive heating and chemical differentiation, drives the geodynamo that makes Earth’s magnetic field.

The short version is: Earth’s core is roughly thousands of degrees hot, commonly described as above 5,000 degrees Celsius, and it stays hot because Earth began hot, is enormous, loses heat slowly, and still gets internal heat from radioactive decay. It is cooling, just not fast enough for billions of years to make the deep planet cold.

References

1. Why is the earth’s core so hot? And how do scientists measure its temperature? – Scientific American
2. Why is the interior of the Earth hot – Earth Observatory of Singapore
3. What do we know about the interior of the Earth? – U.S. Geological Survey
4. How does the Earth’s core generate a magnetic field? – U.S. Geological Survey
5. Thermal and electrical conductivity of iron at Earth’s core conditions – Nature
6. Earth’s Missing Ingredient – Scientific American

Explore More

• How do scientists study Earth’s core without drilling into it?
• Why is Earth’s inner core solid if it is so hot?
• How does the outer core create Earth’s magnetic field?
• Will Earth’s core ever cool completely?
• Why does radioactive decay heat the inside of planets?