Planetary core: composition, structure, and role within planets

Composition and internal layers

Many terrestrial planets and large moons have cores rich in iron and nickel, often alloyed with lighter elements such as sulfur or oxygen; for example, laboratory and geochemical evidence points to iron as a dominant component in rocky bodies, referenced here simply as iron. Cores may be differentiated into layers: a solid inner core surrounded by a liquid outer core, or a single homogeneous region that is wholly solid or liquid depending on temperature and pressure. Earth's interior provides a clear example: a solid inner core coexists with a convecting liquid outer core, a configuration inferred from seismic observations and consistent with a compositional gradient.

Formation and thermal evolution

Cores form during planetary accretion and early differentiation, when denser materials sink toward the center under gravity and release gravitational energy as heat. Over time, cores cool and may partially solidify; the balance of heat sources (accretional heating, radioactive decay, latent heat from crystallization) and heat loss to space determines present-day state. In large planets, extreme pressures can stabilize phases of matter that do not occur at surface conditions.

Magnetic fields and the dynamo

The presence and nature of a global magnetic field are tightly linked to a core's state and dynamics. A convecting, electrically conducting fluid layer can sustain a magnetic dynamo; Earth's magnetic field is maintained by motion in its liquid outer core and related compositional convection. The existence or absence of a sustained magnetic field has important consequences for atmospheric retention and surface radiation environment; many observations of planetary magnetism rely on spacecraft magnetometers and theoretical dynamo models (Earth's core being the best-studied case). Lack of an observable global magnetic field does not necessarily prove a completely solid core—other factors such as slow rotation or insufficient convection can suppress a dynamo.

Examples and variations across bodies

• Mercury: a large iron-rich core occupying a surprisingly high fraction of the planet's radius; estimates suggest the core radius is about 75% of the whole Mercury radius.
• Earth: a layered core with a liquid outer region and solid inner core; its dynamo produces the planet's global magnetic field.
• Moon: a much smaller core relative to size; lunar studies indicate a core roughly 20% of the Moon's Moon radius, and its dynamo history differs from Earth's.
• Mars and Venus: both have iron-rich cores, but neither has a strong present-day global dynamo. Mars may have had an early dynamo and its current core state is constrained by geophysical data; Venus likely has a core composition similar to Earth's but lacks a robust dynamo, possibly due to slow rotation or different convective behavior.
• Gas and ice giants: these large planets often possess dense central regions that can be called "cores," composed of rock, metal and volatiles under extreme pressure. In giants the core is small in radius relative to the whole planet but can be many times more massive than Earth's core; models propose a dense central region beneath massive envelopes of hydrogen, helium or ices.

How we study cores and why they matter

Direct sampling of cores is impossible, so scientists infer their properties using seismic waves (where available), gravity field measurements, magnetic field observations, laboratory experiments on materials at high pressure, and computer models. Knowledge of cores illuminates how planets cool, generate magnetic fields, interact with their atmospheres, and evolve over billions of years. It also guides interpretation of exoplanet measurements, where mass and radius together constrain possible core sizes and compositions.

Notable distinctions

Key distinctions among planetary cores include composition (metallic vs. rock/ice), physical state (solid, partially molten, fully liquid), layering (single vs. inner/outer division), and dynamo activity. These differences help explain why planets with superficially similar sizes can have very different magnetic, tectonic and atmospheric histories.

Further reading and mission data are available through planetary science summaries and mission pages (general resources, comparative planetology). For more technical treatments consult seismology and high-pressure mineral physics sources (material properties, solid phases, liquid behavior, Earth-focused studies, magnetohydrodynamics, lunar research, radius scaling, Mercury data).