The terrestrial planets—Mercury, Venus, Earth, and Mars—are the rocky worlds of the inner Solar System. Their composition reflects the high-temperature conditions close to the young Sun, as they formed from the same primordial cloud of gas and dust. All four planets share a fundamental layered structure, having undergone internal sorting that separated materials by density. This process, called differentiation, created a dense metallic core, a rocky mantle, and a thin outer crust.
Bulk Elemental Composition
The composition of terrestrial planets is dominated by refractory materials—elements that condense into solids at high temperatures. These planets formed in the hot, inner solar nebula, where volatile compounds like water and methane remained gaseous, preventing their significant incorporation. The four most abundant elements are Iron (Fe), Oxygen (O), Silicon (Si), and Magnesium (Mg).
Iron is the densest major element, typically accounting for about one-third of the total mass of Earth, Venus, and Mars. The remaining mass is primarily silicate rock, composed of Silicon, Oxygen, and Magnesium. The proportions of these rock-forming elements are similar to those found in primitive meteorites, suggesting a shared origin for the inner planets’ building blocks. However, Mercury exhibits a significantly higher iron-to-silicate ratio, suggesting a unique formation process that removed a large portion of its original silicate material.
The Metallic Core
The innermost layer of every terrestrial planet is a metallic core, formed early in their history through a process called planetary differentiation. During this time, the planets were largely molten due to the heat generated by impacts and radioactive decay, allowing the densest materials to sink toward the center. This core is predominantly an alloy of Iron and Nickel (Ni).
For Earth, the core is estimated to be composed of 83–85 weight percent Iron, with Nickel making up approximately 5–7 weight percent. The remaining portion consists of lighter elements, such as Sulfur, Silicon, Oxygen, and Carbon, which explain the core’s observed density being lower than that of pure Iron-Nickel alloy. Earth’s core is divided into a solid inner core and a liquid outer core of molten iron and nickel. The presence of this convecting liquid metal layer generates Earth’s global magnetic field through the dynamo effect.
The metallic cores of the other terrestrial planets display variations in their state and size. Mercury has an unusually large core, comprising nearly 60% of its total mass, and maintains a global magnetic field, suggesting a liquid outer core is present. Mars has a smaller core enriched in lighter elements, possibly Sulfur, contributing to its weak or non-existent global magnetic field. Venus lacks an intrinsic magnetic field, likely due to its extremely slow rotation rate hindering the dynamo action in its metallic core.
Silicate Mantle and Crust
Surrounding the metallic core is the mantle, which constitutes the largest volume of the planet and is primarily composed of silicate rock. The mantle material is rich in dense, magnesium-iron silicates, such as the mineral olivine, giving the rock a density greater than the crustal rock above it.
The mantle, despite being solid, is hot enough to behave as a highly viscous fluid over geological timescales. This allows for slow convection currents that redistribute heat from the interior. The uppermost layer is the crust, which is chemically distinct from the mantle and is the least dense and thinnest of the three major layers.
The crust is composed of silicates, but it is made of lighter, more buoyant minerals like feldspars and quartz, which are richer in elements such as Aluminum, Calcium, Sodium, and Potassium. On Earth, the crust is divided into the denser, basalt-rich oceanic crust and the less dense, granite-rich continental crust.
Crust formation involves processes like partial melting, where lighter silicate components separate from the mantle rock and rise to the surface. This compositional layering, with the lightest silicates on the surface and the densest metals in the core, is a direct result of differentiation acting on a planetary body that was once largely molten.