Scientists determined that the Earth's outer core is liquid primarily through the study of seismic waves generated by earthquakes. There are two key types of seismic waves involved:
- S-waves (secondary waves) cannot travel through liquids. Scientists observed that S-waves do not transmit through the outer core—there is an S-wave shadow zone where these waves disappear at the outer core boundary. This indicated that the outer core must be liquid since S-waves can only travel through solid materials.
- P-waves (primary waves) can travel through both solids and liquids but slow down when passing through the outer core, suggesting a change in physical state.
Additional supporting evidence comes from:
- The Earth's magnetic field, which is explained by the movement of molten (liquid) iron in the outer core.
- Laboratory experiments simulating high pressure and temperature conditions show that iron would be liquid in the outer core.
- The density and pressure within the Earth also align with the outer core being a liquid layer.
In summary, the key evidence that the outer core is liquid is the disappearance of S-waves at the outer core and the magnetic field generated by the fluid motion of molten iron there.
