How do we know Earth’s internal temperature?

In fact, below the depths to which we have drilled, scientists do not know Earth’s precise internal temperature. Nevertheless, it is possible to make very good estimates.

The key first step in these estimates comes from figuring out the temperature of the liquid outer core. As we’ve discussed, we know from seismic studies and general understanding of the solar system that the outer core is molten iron and nickel. Therefore, the temperature in the outer core must be above the melting temperature of these metals.

You might think we would know that melting temperature, since factories melt iron and nickel all the time. However, melting temperature depends on pressure, and the pressure deep inside the Earth is millions of times as high as the pressure on the surface. (Scientists can calculate how Earth’s interior pressure must rise with depth based on the weight of overlying material at any depth.) These pressures are so high that they are very difficult to recreate in the laboratory, which makes it correspondingly difficult to measure melting temperatures for iron and nickel at those pressures. Nevertheless, laboratory experiments to date indicate that the melting temperature at core pressures must be close to 5,700°C at the bottom of the outer core.

Once scientists have this estimate of the outer core temperature, they can then use seismic wave analysis and knowledge of materials at different depths to estimate how the temperature must vary throughout the rest of our planet’s interior. The overall results are often displayed on a graph below showing what is called the “geothermal gradient” – that is, how the temperature changes with depth (Figure 1).

To summarize, scientists have very good estimates of Earth’s internal temperature based on a combination of measurements of temperature change with depth near the surface, laboratory-based estimates of the temperature required for the outer core to be molten, and applications of the laws of physics.