Planetary Differentiation

Why planetary differentiation matters

A planet is not a scaled-up version of the dust and pebbles it accreted from. The material that assembled Earth — chondritic dust rich in iron, magnesium silicates, and volatiles — was, at first, roughly uniform throughout the growing body. Differentiation is the event that broke that uniformity, and almost everything we recognize about a terrestrial planet follows from it.

• Layered interiors. Cores, mantles, and crusts exist because dense and light material physically separated. Earth's iron core is ~3480 km in radius; its silicate mantle is ~2890 km thick; its crust is 5–70 km.
• Magnetic dynamos. Concentrating conductive iron into a convecting liquid core is the prerequisite for a global magnetic field, which shields atmospheres and surfaces from the solar wind.
• Chemistry of the crust. Differentiation strips iron-loving (siderophile) elements — gold, platinum, nickel, tungsten — down into the core, leaving the crust depleted in them relative to primitive meteorites.
• Volcanism and plate tectonics. A hot, chemically layered mantle convects, drives volcanism, and (on Earth) sustains plate tectonics — the engine of the carbon cycle and long-term climate.
• A cosmic archive. Iron meteorites and achondrites are the debris of differentiated worlds that were destroyed. They let us sample the cores and crusts of planets we can never visit.

How differentiation works, step by step

1. Accretion assembles a homogeneous body. Dust and planetesimals collide and stick, building a body of roughly chondritic (undifferentiated) composition. Each impact deposits kinetic energy as heat, and larger bodies bury and retain more of it.
2. Radiogenic heating raises the temperature. Live aluminium-26, incorporated when the body formed, decays to magnesium-26 with a half-life of 717,000 years, dumping ~3 MeV per decay. A body that formed within ~2 million years of the first solids carried enough ²⁶Al to melt completely.
3. Iron begins to melt and pool. The Fe–FeS eutectic melts near ~1200–1300 K, below the silicate solidus, so metal liquefies first while much of the rock stays solid. Molten metal collects into droplets and channels.
4. The iron catastrophe: runaway core formation. Dense liquid iron sinks. Converting its gravitational potential energy to heat warms the surroundings, melts more material, and lets still more iron descend — a runaway that rapidly separates metal from rock.
5. A metallic core forms; silicates float up. The iron settles into a central core; buoyant silicate melt rises to form the mantle and, at the top, a low-density crust. Incompatible and volatile elements concentrate in the outermost layers.
6. The body cools and freezes from the outside in. A crust solidifies first. Over time the core may partially freeze; on Earth an inner solid core grows while the outer core stays liquid and convects, sustaining the dynamo.

Layered interiors: a key-numbers comparison

Density is the fingerprint of differentiation. A body's mean density, combined with its moment of inertia (measured from how it responds to gravity and rotation), reveals whether mass is concentrated toward the center — the signature of a dense core.

Body	Mean density (g/cm³)	Core radius fraction	Differentiated?
Mercury	5.43	~0.83	Yes — huge iron core
Earth	5.51	~0.55	Yes — active dynamo
Moon	3.34	~0.20	Yes — small core
Mars	3.93	~0.50	Yes — dynamo now dead
Vesta (asteroid)	3.46	~0.40	Yes — melted early
Ceres (dwarf planet)	2.16	—	Partial — rock/ice, no metal core
Typical C-type asteroid	~1.3–2.0	—	No — primitive chondrite

Mercury is the extreme case: at 5.43 g/cm³ it is nearly as dense as Earth despite being a third the size, because a metallic core occupies roughly 83% of its radius. Ceres, by contrast, is largely a rock-and-ice body that separated ice from rock but never built a metal core.

A worked example: dating Earth's core with hafnium–tungsten

How do we know Earth's core formed within ~30 million years? The answer is an isotopic stopwatch. The radioactive isotope hafnium-182 decays to tungsten-182 with a half-life of ~8.9 million years. Crucially, hafnium is lithophile (it stays with silicate rock) while tungsten is siderophile (it follows iron into a core).

If a body differentiates while ¹⁸²Hf is still live, the silicate mantle keeps the hafnium, and the ¹⁸²W produced by later decay accumulates in the mantle — giving it an excess of ¹⁸²W relative to undifferentiated meteorites. If differentiation happens after ¹⁸²Hf has fully decayed (after ~50 million years), no such excess develops. Earth's mantle shows a ¹⁸²W excess of about +2 parts per 10,000 relative to chondrites, implying that metal–silicate separation — core formation — was substantially complete within roughly the first 30 million years of solar system history. The Moon-forming giant impact reset part of this clock, and modern models put the bulk of Earth's core formation at ~30–40 Myr after the first solids.

The key equation: buoyancy and the settling of iron

Whether a blob of molten iron sinks — and how fast — is governed by Stokes' law for a dense sphere falling through a viscous silicate melt:

v = (2/9) · (Δρ · g · a²) / η

• v — settling velocity of the iron droplet (m/s)
• Δρ — density contrast between iron metal and silicate melt (~4000 kg/m³; iron ≈ 7000, silicate melt ≈ 3000)
• g — local gravitational acceleration (m/s²; ~9.8 for Earth, far less in a small planetesimal)
• a — radius of the iron droplet or blob (m)
• η — dynamic viscosity of the surrounding silicate melt (Pa·s; ~1–100 for a magma ocean)

The strong dependence on droplet radius (v ∝ a²) explains why differentiation is efficient: once iron pools into larger blobs, or descends as diapirs and molten channels rather than fine droplets, it plunges quickly. A related energy relation quantifies the heat released: as a core of mass M and radius R forms, the gravitational potential energy liberated is of order ΔE ≈ (3/5)(GM²/R) × f, where G is Newton's constant and f is a factor accounting for the redistribution of mass. For Earth this amounts to enough energy to raise the whole planet's temperature by roughly 2000 K — a self-reinforcing contribution that helped sustain the iron catastrophe.

Common misconceptions

• "The core is iron ore or magma like the crust." No — it is a metal alloy, mostly iron and nickel with a few percent of light elements (sulfur, oxygen, silicon). The outer core is liquid metal, not molten rock.
• "Planets differentiate because gravity slowly pulls heavy things down over billions of years." No — it requires melting. In a solid planet, iron can't migrate. Differentiation is fast, geologically speaking, and happens while the interior is hot and partly molten.
• "All asteroids are leftover planet cores or crusts." Most asteroids are primitive, undifferentiated chondritic bodies that never melted. Only bodies that formed early and large enough — like Vesta — differentiated.
• "A magnetic field just needs an iron core." It needs a core that is at least partly liquid and actively convecting. Mars has an iron core but its dynamo shut down ~4 billion years ago, leaving only fossilized crustal magnetism.
• "Differentiation happened once and stopped." Some differentiation is ongoing: Earth's inner core is still slowly freezing, expelling light elements that power the dynamo, and Saturn is thought to be raining helium deep in its interior right now.