Accretion & Planetary Differentiation: Physical Mechanisms

Early solids grow from grains to aggregates, planetesimals, planetary embryos, and protoplanets. Large bodies contain heat from impacts, gravitational compression, short-lived radioactive isotopes, and later long-lived radioactive decay. Once internal materials can move, dense metals tend to sink toward a core while lower-density silicates form mantle and crust.

As a growing body becomes more massive, gravitational focusing increases the effective collision cross-section and accelerates accretion. High-energy impacts convert kinetic energy into heat, and compression plus radioactive decay add internal energy. If rock and metal melt or deform, gravity drives denser material downward and lighter material upward until the body becomes compositionally layered.

Planet densities, seismic measurements, gravity fields, magnetic fields, meteorites, and returned samples reveal differentiated interiors. Iron meteorites are interpreted as fragments of metallic cores from disrupted parent bodies, while stony meteorites sample silicate material. Planetary bulk density often exceeds crustal rock density, implying denser material at depth.

As material accumulates, gravity increases collision speeds and promotes runaway or oligarchic growth. If melting occurs, dense metal can sink and release additional gravitational energy, while silicates rise relative to metal. Volatile elements may escape, redistribute, or remain in outer layers depending on temperature and pressure.