Planetary differentiation

• 1. Introduction

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  Planetary differentiation is the reason you are standing on solid ground right now rather than sinking through a uniform mass of mixed rock and metal. Deep beneath your feet, iron settled toward the center of the Earth billions of years ago, drawing with it a suite of companion elements, while lighter silicates rose to form the crust you walk on. The density of that crust is roughly 2700 kilograms per cubic meter. The mantle just below reaches about 3400 kilograms per cubic meter. The planet as a whole averages 5515 kilograms per cubic meter. Those three numbers alone tell the story of a world that sorted itself out. But how does a planet do that? And what heats the rock enough to let gravity do its work? The answers lie in radioactive decay, violent collisions, and a cast of elements that followed their chemical nature rather than their mass. This documentary explores how worlds pull themselves apart and put themselves back together in a new order.

• 2. Gravity Sorts the Heap

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  Iron is the most common element likely to form a very dense molten metal phase inside a planetary body. When temperatures climb high enough to melt rock, iron drains toward the interior, pulling along siderophile elements, which are materials that readily alloy with iron, including nickel and cobalt. Yet density alone does not decide every element's fate. Some heavy elements are chalcophilic: they bind into low-density silicate and oxide compounds and ride those lighter materials upward rather than sinking with the iron. Uranium is a striking example. Pure uranium is an extremely dense element, yet it is chemically more at home as a trace element in the Earth's silicate-rich crust than in the dense metallic core. Chemistry, in that case, overrules weight. Lighter materials rise through denser ones in a process that can produce dramatic structures. A mineral called plagioclase, being light, tends to float upward and can take on dome-shaped forms called diapirs. On Earth, salt diapirs have pushed upward through surrounding rock to form salt domes in the crust. Diapirs of molten low-density silicate rocks such as granite are abundant in the upper crust as well. Even serpentinite, a hydrated low-density rock formed by alteration of mantle material at subduction zones, can rise to the surface the same way. Mud volcanoes are a near-surface, low-temperature example of the same principle.

• 3. The Heat That Started It All

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  When the Sun ignited inside the solar nebula, it drove hydrogen, helium, and other volatile materials away from the region around it. The solar wind and radiation pressure pushed those low-density gases outward, leaving behind rocks and the elements locked within them, stripped of their early atmospheres. Those rocks accumulated into protoplanets. Early protoplanets carried higher concentrations of radioactive elements than planets do today, because radioactive decay has steadily reduced the inventory over time. The short-lived radioactive isotope 26Al was probably the main source of heat in the earliest stages. The hafnium-tungsten system, which tracks the decay of two unstable isotopes, offers a potential timeline for how quickly accretion and differentiation unfolded. Beyond radioactivity, every collision during accretion released energy as local heat. In a body large enough, gravitational pressure itself generated temperatures and pressures sufficient to melt rock. Once melted zones existed, denser materials could sink and lighter ones could rise. The compositions of certain meteorites known as achondrites show that differentiation was not limited to full-sized planets; it took place in some asteroids as well, with 4 Vesta being the named example. Tidal heating is a further external source that can sustain or restart the process in bodies that orbit massive neighbors.

• 4. Reading Magma's Memory

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  Magma on Earth originates through partial melting of source rock, ultimately drawn from the mantle. As that melt forms, it preferentially extracts incompatible elements, which are elements not stable in the major minerals of the source. When magma rises above a certain depth, dissolved minerals begin crystallizing at particular pressures and temperatures. Each batch of crystals removes specific elements from the remaining liquid, gradually depleting the melt of those components. Geochemists read this record in trace elements found in igneous rocks, using the chemical fingerprint to reconstruct how much of a source melted and which minerals dropped out along the way. Thermal effects add another layer to the story. When material is unevenly heated, lighter components migrate toward hotter zones while heavier ones drift toward cooler areas, a process known as thermophoresis, thermomigration, or the Soret effect. Research on Hawaiian lava lakes provided a window into this mechanism: drilling into the lakes revealed crystals that had formed within magma fronts, and the concentration of large crystals called phenocrysts within certain magma zones demonstrated differentiation driven by the chemical melting of those crystals. The Moon preserves its own chemical record in a distinctive basaltic material known by the abbreviation KREEP. Rich in potassium, rare earth elements, phosphorus, uranium, and thorium, KREEP is thought to represent a chemical differentiate that was squeezed out between the lunar crust and mantle as the primeval magma ocean crystallized. It surfaces occasionally through eruptions, carrying that ancient chemical signature to where researchers can study it.

• 5. How Cores Form

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  Getting metal from the surface of a growing planet to its center is not a single mechanism but a collection of competing pathways. Percolation drives molten metal downward through the density contrast between metal and silicate. Diking allows metal to travel through fluid cracks that open in cold, brittle minerals, provided enough pressure builds to overcome the fracture toughness of the surrounding rock. The size of the metal body intruding and the viscosity of the surrounding material together set the pace of that sinking process. Diapirism works here too, but in reverse: large masses of dense liquid iron are sufficiently heavier than continental crust material that they force their way down through the crust toward the mantle. Direct delivery through impacts adds a fourth route: when an impactor of comparable size strikes a target body, the two objects exchange cores, mixing metallic material during the collision itself. The timing of core formation is constrained by studies of short-lived radionuclides, which indicate the process occurred during an early stage of the Solar System. Terrestrial bodies and iron meteorites share Fe-Ni alloys as their primary metallic component, and the Earth's core is dominated by the same composition. Accretion itself set the stage: terrestrial planetary bodies entered neighboring orbits, collisions followed, and bodies grew or shrank depending on the outcome. Most scenarios require multiple collisions among similarly sized objects to produce significant growth, and characteristics such as feeding zones and hit-and-run events are among the results those collisions can leave behind.

• 6. Moons and the Outer Reaches

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  Earth's Moon almost certainly formed from material splashed into orbit by the impact of a large body striking the early Earth. By the time that collision happened, differentiation on Earth had already moved many lighter silicate materials toward the surface. The impact therefore removed a disproportionate share of silicate material from the Earth, leaving most of the dense metal behind. That imbalance explains why the Moon's density is substantially less than Earth's: it never accumulated a large iron core. In the outer Solar System, differentiation follows similar physics but with entirely different materials. Instead of iron and silicate, the relevant players may be hydrocarbons such as methane, water in liquid or ice form, and frozen carbon dioxide. Lighter among these materials rise through denser surroundings just as granite diapirs rise through terrestrial crust, sorting the deep interiors of icy moons and outer-planet bodies by the same gravitational logic that structured the rocky inner planets. Dwarf planets have undergone differentiation as well, indicating that even bodies far from the Sun and far below planetary mass can accumulate enough internal heat to let gravity impose its order.