What evidence suggests that the outer core of the Earth is liquid?

The assertion that S waves cannot pass through the outer core is a key piece of evidence indicating that this layer is liquid. S waves, or shear waves, are a type of seismic wave that require a solid medium to propagate. Since they do not travel through liquids, the fact that S waves are unable to be detected on the opposite side of the Earth from where an earthquake occurs implies that a liquid layer exists within the inner structure of the planet. This behavior of seismic waves is fundamental to understanding the physical properties of different Earth layers, including distinguishing between solid and liquid states.

In contrast, while P waves can travel through both solids and liquids, their speed is indeed affected by the material they are passing through. However, it is the inability of S waves to propagate through the outer core that directly indicates its liquid nature. The other statements regarding the composition and density of the outer core do not directly provide evidence of its state (solid or liquid) in the same conclusive manner as the behavior of S waves.