Science Puzzle
How Do We Know Earth’s Outer Core Is Liquid?
The deepest hole ever drilled, in Russia, reached about 12 kilometres down. Earth's outer core doesn't begin until about 2,900 kilometres down, and nobody has ever seen it or taken a sample of it.
Yet geologists are confident that the outer core is liquid: a vast ocean of molten iron. How can they possibly know?
The Answer
Earthquakes act as a giant scan of Earth's insides, and one kind of earthquake wave can't get through the outer core.
A big earthquake sends two main kinds of wave through the planet. P waves push and pull, squeezing the rock in the direction they travel, like sound. They can pass through solids and liquids. S waves shake the rock from side to side. They need something rigid, which springs back when it's bent sideways, so they can travel through solid rock but not through a liquid. You can't send a side-to-side shake through water: it just flows.
Seismometers around the world record these waves after every large earthquake. On the far side of Earth from the quake there's a huge region where no S waves arrive at all, a shadow cast by something they can't cross. Working out the shape of that shadow showed that the outer core, starting about 2,900 km down, must be liquid. P waves bend sharply as they enter it, leaving a shadow zone of their own.
Scientists pieced this together in the early 1900s. In 1936 the Danish seismologist Inge Lehmann spotted faint P waves arriving where none should have been and realised there was a solid inner core inside the liquid one. The swirling liquid iron of the outer core also generates Earth's magnetic field. Volcanoes, by contrast, erupt magma that melted within a few hundred kilometres of the surface, nowhere near the core.
The principle: Seismic waves. Shear (S) waves cannot travel through liquids, so the shadow they leave on the far side of Earth reveals a liquid layer deep inside.