How we know: seismic waves

No drill has reached the mantle, yet we know the outer core is liquid because one kind of earthquake wave never arrives on the far side of the planet.

Two kinds of waves

When a fault slips, it sends waves through the whole planet. The fastest are P waves, short for primary. They push and pull the rock in the direction they travel, like a pulse passed along a stretched spring, and they can pass through solids, liquids and gases. Next come S waves, secondary waves. They shake the rock from side to side, at right angles to the direction of travel. A liquid cannot hold that kind of sideways shear, so S waves cannot travel through liquid at all.

Both waves speed up in deeper, stiffer rock. A wave that goes down at a slant therefore bends, the way light bends in water, and curves back up to the surface. Thousands of seismometers around the world time these arrivals. From the delays at different distances, seismologists work out how fast waves travel at each depth, and sudden changes in speed mark the boundaries between layers. The British Geological Survey compares it to light bending through a glass prism.

The shadow zones

Measure the angle around the Earth from the earthquake, so the point directly opposite is 180°. Direct S waves are recorded out to about 104° and then disappear completely. Something between the mantle and the centre stops them, and since S waves stop only in liquid, that layer must be liquid. P waves do something different. They also vanish beyond about 104°, then reappear from about 140° onwards. USGS explains that the liquid core bends them. P waves slow down sharply when they enter the outer core, because liquid iron carries them more slowly than the stiff rock above, so they bend steeply towards the centre and come out far away. The ring of the surface between 104° and 140° gets no direct P waves.

You can see this in the model. Watch the clock: P waves reach the opposite side of the Earth after about 20 minutes, and S waves never do. Move the Quake slider and the shadow zones move with the earthquake, which tells you they come from the inside of the Earth and not from anything special about one place on the surface. In this simplified model the last P ray that just misses the core comes up near 97°; real waves also bend around the edge of the core, which carries weak P waves a little farther.