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Planetary interiors are driven by convection, density-dependent settling, and internal heat from planetary formation and radioactive decay. Temperature, pressure, and composition vary with depth, creating distinct layers and driving long-term planetary evolution and outgassing.
Start with Earth's interior structure, then apply concepts to other terrestrial planets (Mars, Venus, Mercury) using comparative data on size, composition, and thermal state. Use seismic constraints and heat-flow measurements.
Every planet is a heat engine. From the moment of formation, planetary bodies accumulate heat and slowly release it over billions of years — and the dynamics of that heat flow shape everything from surface geology to magnetic fields to the possibility of habitability. Understanding planetary interior dynamics means tracing where the heat comes from, how it moves, and what it does along the way.
Two processes supply most of a planet's internal heat. The first is accretional heat: during planetary formation, countless smaller bodies collided and merged, converting kinetic energy into thermal energy. For large planets, gravitational compression of the growing body added more heat. This was enough to melt entire planetary interiors early in solar system history, allowing denser iron and nickel to sink to the center (forming a metallic core) while lighter silicates rose (forming the mantle and crust) — a process called differentiation. The second source is radiogenic heat: long-lived radioactive isotopes — primarily uranium-238, thorium-232, and potassium-40 — decay continuously within the rocky interior, releasing heat that sustains interior temperatures over geological timescales.
This internal heat escapes the interior primarily through convection in the mantle. Even though mantle rock is solid on human timescales, over millions of years it behaves like a very viscous fluid: hot rock at depth rises slowly, cools near the surface, and sinks again, transferring heat outward. On Earth, this mantle convection is the engine behind plate tectonics — the moving plates are essentially the surface expression of underlying convective cells. On planets that have cooled more (smaller planets like Mars or Mercury lose heat faster because of their higher surface-area-to-volume ratio), convection has slowed or stopped, leaving the lithosphere rigid and geologically inactive.
Planetary size is thus a first-order predictor of interior activity. A larger planet retains heat longer, sustains convection longer, and remains geologically active longer. This is why Earth still has active plate tectonics and a convecting liquid outer core — which generates our protective magnetic field — while Mars, despite similar rocky composition, has a thick, immobile lithosphere and a much weaker magnetic field. Mercury's oversized core relative to its small mantle is likely the result of a giant impact early in its history that stripped away much of its original silicate mantle.
A key misconception to correct: internal heat is not negligible for surface processes. On Earth, volcanic eruptions, mountain building, ocean floor spreading, and the magnetic field are all direct consequences of the interior heat engine. Even the delivery of volatiles (water, CO₂, nitrogen) to the early surface through outgassing — which enabled the atmosphere and oceans — was powered by interior heat. Planets are not inert balls of rock; they are dynamic systems shaped from the inside out.