Internal structure of Earth

• 1. Introduction

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  The internal structure of Earth is something no human has ever seen, yet scientists describe it down to layers thousands of kilometres deep. The deepest point measured here is about 5,150 kilometres beneath the surface, where one layer ends and another begins. We have never drilled anywhere close. So how does anyone know what is down there? The answer involves earthquakes, volcanoes, diamonds, and lasers that turn metal into plasma. This documentary asks how a planet reveals its hidden interior when we cannot reach it. It asks why the rock can be both solid and able to flow. It asks what keeps a molten ball of iron from freezing, and why that matters for every living thing on the surface above it.

• 2. Reading The Hidden Depths

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  Earthquakes send waves through the entire planet, and those waves are the closest thing we have to a flashlight aimed at Earth's core. The layering of Earth has been inferred indirectly from the travel time of refracted and reflected seismic waves created by earthquakes. When a wave crosses from one layer into another, its speed changes, and that change bends the wave the way a prism bends light. This bending follows Snell's law, the same rule that governs refraction in optics.

  Reflections work like a mirror rather than a prism. A large increase in seismic velocity sends part of a wave bouncing back toward the surface, much as light reflects off glass. By timing these echoes, geophysicists map boundaries they can never touch. The behaviour of one wave type carries a decisive clue. The core does not allow shear waves, the transverse seismic waves, to pass through it.

  Seismology is only one tool in a larger kit. Scientists also study topography and bathymetry, examine rock in outcrop, and analyse samples carried up from depth by volcanoes. They measure Earth's gravitational and magnetic fields. They run experiments on crystalline solids squeezed and heated to the pressures and temperatures of the deep interior, recreating conditions that exist far below any mine shaft.

• 3. Weighing The Whole Planet

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  Earth's average density is 5.515, a figure that immediately tells scientists the surface rocks cannot be the whole story. Measurements of the force exerted by Earth's gravity allow its mass to be calculated, a mass of about 6. Astronomers reach the same number a second way, by watching the motion of satellites orbiting the planet. Average density has historically been pinned down through gravimetric experiments that relied on swinging pendulums.

  The planet's composition echoes objects from far beyond it. The makeup of Earth bears strong similarities to certain chondrite meteorites, and even to some elements in the outer portion of the Sun. Beginning as early as 1940, scientists including Francis Birch built geophysics on the premise that Earth resembles ordinary chondrites, the most common meteorite type seen striking the planet. That premise sets aside the rarer enstatite chondrites. Those formed under extremely limited oxygen, which pushed certain normally oxygen-loving elements into the metallic portion corresponding to Earth's core.

• 4. Two Ways To Slice

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  The same planet can be divided two completely different ways, and geologists keep the two systems separate on purpose. Compositionally, Earth's layers are sorted by chemistry into the crust, an outer layer chiefly of silicate minerals, the mantle, and the core, an iron-nickel central layer. The upper mantle is high in mafic rock, while the lower mantle is high in bridgmanite and ferropericlase.

  Mechanically, the planet is sorted instead by how its material behaves under stress. This approach gives the lithosphere, a rigid, brittle outer shell that includes the crust and the very top of the upper mantle. Below it sits the asthenosphere, a ductile, semi-fluid layer of the upper mantle where tectonic plates float and move. Deeper still is the mesosphere, a layer of the lower mantle where high pressures solidify the rock, and which should not be confused with the atmospheric mesosphere.

  These two schemes align only partially, so their names are usually not intermingled. A liquid outer core and a solid inner core complete the mechanical picture, while the chemical scheme stops at a single core. Choosing which set of names to use depends entirely on the scientific question being asked.

• 5. Crust And The Ancient Grains

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  Earth's crust ranges from 5 to 70 kilometres in depth, making it the thinnest layer by far. The thin parts are the oceanic crust beneath the ocean basins, between 5 and 10 kilometres thick and rich in dense iron-magnesium minerals. The thicker continental crust is less dense and rich in the felsic elements that form feldspar and quartz. Crustal rocks fall into two categories, sial, an aluminium silicate, and sima, a magnesium silicate. Sima is estimated to start about 11 kilometres below the Conrad discontinuity, a boundary that is not always distinct and can be absent in some continental regions.

  The boundary between crust and mantle shows up as two separate phenomena. The Mohorovičić discontinuity is a sharp change in seismic wave velocity caused by a change in rock density. Just above the Moho, primary seismic waves travel at speeds consistent with basalt, between 6.7 and 7.2 kilometres per second. Just below, they match peridotite or dunite, between 7.6 and 8.6 kilometres per second. In oceanic crust there is also a chemical discontinuity between ultramafic cumulates and tectonized harzburgite, seen where deep oceanic crust has been thrust onto continents and preserved as ophiolite sequences.

  Many rocks of the crust formed less than 100 million years ago, but the crust itself is ancient. The oldest known mineral grains are about 4.4 billion years old. That single figure indicates Earth has carried a solid crust for at least 4.4 billion years.

• 6. The Mantle That Flows

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  Earth's mantle reaches a depth of 2,890 kilometres, the planet's thickest layer. That depth is 45 percent of the 6,371 kilometre radius and accounts for 83.7 percent of the planet's volume, while the crust holds just 0.6 percent. The mantle is split into upper and lower portions separated by a transition zone, and its lowest part, against the core-mantle boundary, is called the D-double-prime layer. At the very bottom the pressure reaches roughly 140 gigapascals.

  Solid rock here behaves in a way that defies everyday intuition. Although the mantle is solid, its extremely hot silicate material can flow over very long timescales. This slow churning, called convection, propels the tectonic plates riding in the crust above. The heat driving it comes from the decay of radioactive isotopes in the crust and mantle, combined with leftover heat from the planet's formation.

  The deeper the rock, the more reluctantly it moves. The viscosity of the mantle ranges between 10 to the 21 and 10 to the 24 pascal-seconds, increasing with depth, partly because of rising pressure and partly because of chemical changes within. To grasp that stiffness, compare it to water at 300 kelvin, which has a viscosity of 0.89 millipascal-seconds. Even pitch, famous for barely flowing at all, sits far below the mantle's resistance to motion.

• 7. The Iron Heart

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  Earth's inner core is a solid ball with a radius of about 1,220 kilometres, roughly 19 percent of Earth's radius and about 70 percent of the radius of the Moon. It was discovered in 1936 by Inge Lehmann, who recognised it because the layer can transmit shear waves, the transverse seismic waves that prove a material is solid. Above it lies the fluid outer core, about 2,260 kilometres in height, composed mostly of iron and nickel. The transition between the two cores sits approximately 5,150 kilometres beneath the surface.

  The core's iron-rich makeup traces back to the planet's birth. In Earth's early formation, about 4.6 billion years ago, melting let denser substances sink toward the centre in a process called planetary differentiation, while lighter materials rose toward the crust. The core is thought to be largely iron, about 80 percent, with nickel and one or more light elements. Heavy elements such as lead and uranium are either too rare to matter or bind to lighter elements and stay near the surface.

  Exactly what the inner core is made of remains contested. Some researchers have argued the inner core may be a single iron crystal. In one experiment, an iron-nickel alloy was gripped between two diamond tips in a diamond anvil cell, squeezed to core-like pressure, then heated to about 4,000 kelvin. Observed with x-rays, the sample supported the idea of giant crystals running north to south. Yet a discrepancy persists. Static diamond anvil studies yield melting temperatures roughly 2,000 kelvin below those from dynamic shock laser studies, which create plasma, leaving open whether the inner core is solid or a plasma with the density of a solid.

• 8. The Shield Above Us

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  The average magnetic field in Earth's outer core is estimated at 2.5 millitesla, fifty times stronger than the field measured at the surface. That field is not a fixed magnet but a product of motion. Dynamo theory holds that convection in the outer core, combined with the Coriolis effect, generates Earth's magnetic field. The solid inner core is too hot to hold a permanent field of its own, a limit set by the Curie temperature, but it probably helps stabilise the field the liquid outer core produces.

  This invisible field reaches far beyond the planet to do essential work. The magnetic field generated by core flow protects life from interplanetary radiation and stops the atmosphere from being stripped away by the solar wind. Should the core ever freeze solid, that protection would weaken. The rate of cooling by conduction and convection is uncertain, but one estimate holds the core would not freeze for roughly 91 billion years. That number stretches well past the day the Sun is expected to expand, sterilise the surface, and burn out, meaning the dynamo will outlast the very star it shields us from.