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Planetary differentiation is the gravitational separation of materials by density, where dense metals (iron, nickel) sink to form a core and lighter silicates rise to form mantle and crust. This process releases gravitational potential energy that heats the planet and is recorded in meteorite compositions.
Compare meteorite types (iron, stony-iron, stony) with planetary layering models. Discuss why smaller bodies (asteroids, small moons) show incomplete differentiation while large planets fully differentiate.
Imagine dropping a handful of sand and marbles into a jar of honey and watching — over time — the marbles sink and the sand floats upward. Planetary differentiation is exactly this process, played out inside a molten young planet. When a rocky body grows large enough and gets hot enough that its interior melts, gravity takes over: dense materials (iron and nickel) sink toward the center while lighter silicates (the minerals that make up rock) rise toward the surface. The result is the layered structure we observe in Earth and other planets — metallic core, rocky mantle, thin silicate crust.
The critical question is: where did the heat come from? Two sources dominate. First, as the planet grew through accretion — collisions between smaller planetesimals — the kinetic energy of those impacts converted to heat. For a body the size of Earth, this is enormous. Second, the early solar system was laced with short-lived radioactive isotopes, particularly aluminum-26 (Al-26, half-life ~700,000 years), which released intense heat as they decayed. Bodies that formed early enough — while Al-26 was still abundant — received a powerful internal heat source. Add the heat released as dense iron sank (gravitational potential energy converted to thermal energy), and you have a self-reinforcing process: melting allows sinking, and sinking generates more heat.
The evidence for differentiation is literally in our hands. Different types of meteorites correspond to different layers of ancient differentiated bodies that were later shattered by collisions: iron meteorites are the remnants of metallic cores, stony-iron meteorites come from the core-mantle boundary, and stony (chondritic) meteorites represent undifferentiated primitive material that never melted. Comparing these to seismic models of Earth's interior reveals a striking match — Earth's layers are the fully differentiated version of what meteorites sample in fragments.
Not all bodies differentiate equally. Size matters enormously. A large planet retains heat (small surface-area-to-volume ratio), stays molten for millions of years, and differentiates completely. A small asteroid cools rapidly, freezes before separation is complete, and may remain partially or entirely undifferentiated. This is why bodies like Vesta (radius ~260 km) show partial differentiation while tiny asteroids generally do not. Core composition also varies: Mars likely has a sulfur-rich iron core, Earth's is iron-nickel with lighter elements, and Mercury's is disproportionately large — reflecting the different starting compositions and impact histories of each body.