Introduction
The Moon you see tonight is the direct result of a planetary collision so violent it partially vaporised Earth — and the metal now at Earth's core sank there in a geological instant, setting the stage for the magnetic field that makes our planet liveable.
When a planetary embryo first forms from accreting planetesimals, it is broadly homogeneous — a jumble of rock, metal, and ice with no internal structure. But accretion releases enormous energy: every infalling chunk of rock converts its gravitational potential energy into heat on impact, and short-lived radioactive isotopes — particularly ²⁶Al (half-life 0.72 Myr) and ⁶⁰Fe (half-life 2.6 Myr) — provide additional heat from within. Given enough mass and the right timing of formation, this heat is sufficient to melt rock and iron, allowing gravity to sort materials by density in a process called planetary differentiation: dense iron-nickel metal sinks to the centre to form a core, while lighter silicate minerals rise to form a mantle and crust. The result is the layered interior structure we observe in every rocky world in the Solar System, from Mercury to the Moon.
Differentiation happened fast — on timescales of just tens of millions of years. The hafnium-tungsten (Hf-W) chronometer, which exploits the radioactive decay of ¹⁸²Hf (half-life 9 Myr) into ¹⁸²W (which is siderophile, tending to concentrate in iron), tells us that Earth's metallic core had largely separated from its silicate mantle within ~30 million years of t₀ (4.5673 Ga). This is a geologically brief instant — less than 1% of Earth's age — and it sets the stage for everything that followed: the geomagnetic field generated by convection in the liquid outer core, the plate tectonic cycle driven partly by mantle convection, and the protection of Earth's atmosphere from solar wind stripping.
The story of the early inner Solar System is also one of catastrophic impacts. The final assembly of the terrestrial planets from a swarm of lunar-to-Mars-mass embryos involved collisions of titanic violence. The most consequential was the formation of the Moon: a Mars-sized body called Theia struck proto-Earth approximately 50–100 Myr after t₀, ejecting enough material to form our lunar companion. Then, hundreds of millions of years later, a second pulse of heavy bombardment — the Late Heavy Bombardment (LHB) — rained projectiles onto the inner Solar System surfaces, resetting impact crater ages preserved in the Apollo lunar samples. Understanding these events reshapes our picture of when and how habitable conditions first arose on Earth.
Key Terms
The separation of an originally homogeneous planet into compositionally distinct layers (core, mantle, crust) driven by density differences and gravitational settling. Differentiation requires the planet to at least partially melt so that materials can flow. Once iron-nickel metal melts, its high density (~7,900 kg/m³) relative to silicate rock (~3,000 kg/m³) causes it to sink through the molten or partially-molten silicate mantle toward the centre. Simultaneously, lighter, lower-melting-point materials are buoyed upward to form the crust. Differentiation releases additional gravitational potential energy as dense material descends, providing further heating in a runaway process. The result is recorded in every major rocky Solar System body: Earth, Mars, the Moon, Vesta, and others all show geophysical evidence of metallic cores and silicate mantles.
A global or regional layer of molten silicate rock covering a planet's surface and extending to depth in the earliest stages of planetary formation and after giant impacts. Magma oceans are generated when accretional heating, giant impacts, or short-lived radionuclide decay raise temperatures above silicate melting points (~1,300–2,000 K). In a magma ocean, minerals crystallise from the melt in order of their melting points (fractional crystallisation), and dense minerals (e.g., bridgmanite/perovskite) sink while buoyant minerals (e.g., anorthosite) float. The lunar highlands, composed of ancient anorthositic crust, are the preserved floating crust of the Moon's primordial magma ocean. Earth almost certainly had one or more magma ocean episodes early in its history, though the evidence has been obliterated by subsequent geological activity.
The leading scientific explanation for the origin of the Moon, proposing that a Mars-sized protoplanet (named Theia) collided with proto-Earth at a glancing angle approximately 50–100 Myr after Solar System formation (t₀ = 4.5673 Ga). The oblique collision at ~4–8 km/s generated a disc of vaporised and molten material in Earth orbit; this disc cooled and accreted into the Moon within a few thousand years. Key evidence: Earth and Moon have near-identical oxygen, titanium, silicon, and chromium isotope ratios (unlike most meteorite classes); the Moon has a small iron core (~2% of its mass, vs ~32% for Earth) consistent with most iron remaining bound to Earth after the impact; the Earth-Moon system's total angular momentum matches simulations of high-energy oblique impacts; the Moon is strongly depleted in volatile elements, consistent with the extreme temperatures (~4,000–6,000 K) of the impact plume.
A hypothesised intense episode of meteorite and asteroid impacts on the inner Solar System bodies (Moon, Earth, Mars, Mercury, Venus) concentrated between approximately 4.1 and 3.8 Ga — roughly 400–700 Myr after Solar System formation. Evidence comes primarily from Apollo lunar samples: radiometric ages of impact melt rocks from different lunar landing sites cluster in the 3.8–4.1 Ga range, suggesting a distinct impact pulse rather than the tail end of normal accretion. The leading dynamical explanation is the **Nice model**, in which gravitational resonance crossing between Jupiter and Saturn destabilises the outer Solar System and scatters a large reservoir of planetesimals inward. The existence of a sharp LHB spike (versus a more gradual decline from accretion) remains debated, with some researchers arguing for a statistical bias from a few large impact basins dominating the datable sample.
A radioactive isotope system using the decay of ¹⁸²Hf (hafnium, lithophile — stays in silicate) to ¹⁸²W (tungsten, siderophile — prefers iron metal), with a half-life of 8.9 Myr. Because Hf remains in the silicate mantle and W partitions into metal, core formation separates the two elements at a precise moment in time. After that moment, the silicate mantle accumulates excess ¹⁸²W (from continued ¹⁸²Hf decay) relative to the metallic core. By measuring the ¹⁸²W/¹⁸⁴W ratio in mantle rocks and comparing to the solar (chondritic) baseline, geochemists can calculate when core-mantle separation occurred. For Earth, this system indicates core formation was essentially complete within ~30 Myr of t₀. The Hf-W system was also critical in demonstrating that the Moon-forming Giant Impact occurred ~50–100 Myr after t₀.