Accretion & Planetary Differentiation: Structures & Vocabulary

Accretion is the growth of planetary bodies through collisions and gravitational capture of smaller material, while differentiation is the internal separation of materials by density after a body becomes hot enough for substantial melting or flow. Together these processes transform primitive disk material into layered planets.

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.

Differentiated terrestrial bodies commonly develop metallic cores, silicate mantles, and lower-density crusts. Heat sources include impact energy, gravitational compression, short- and long-lived radioactive isotopes, and core formation itself. Small asteroids may remain primitive while larger bodies such as Vesta show evidence of internal separation.