From planetesimals to proto-Earth and differentiation into core, mantle, and crust
1. How a rocky planet forms from dust
More than 4.5 billion years ago, the forming proto-Sun was surrounded by a protoplanetary disk – remnants of a gas and dust cloud left after the collapse of the nebula from which the Solar System formed. In this disk, countless planetesimals (rocky/icy bodies tens of kilometers in scale) collided and merged, gradually forming the terrestrial (rocky) planets in the inner Solar System. The path Earth took – from dispersed solid particles to a layered, dynamic world – was far from calm, disturbed by massive impacts and intense internal heating.
The layered structure of our planet – an iron-rich core, a silicate mantle, and a thin, rigid crust – reflects the differentiation process, when Earth's materials separated by density during partial or complete melting. Each layer formed through a long chain of cosmic impacts, magmatic segregation, and chemical partitioning. Understanding Earth's early evolution provides important insights into the overall formation of rocky planets and how fundamental factors such as the magnetic field, plate tectonics, or volatile reservoirs arise.
2. Fundamental building blocks: planetesimals and embryos
2.1 Planetesimal formation
Planetesimals are the “fundamental building blocks” of rocky planets according to the core accretion model. Initially, microscopic dust inside the disk stuck together into mm–cm sized grains. However, the “meter-size barrier” (radial drift, fragmentation) hindered slow growth. Current proposed solutions, such as streaming instability, suggest dust can concentrate in local overdensities and rapidly collapse under gravity, forming kilometer-sized or larger planetesimals [1], [2].
2.2 Early collisions and protoplanets
As planetesimals grew, gravitational runaway growth created larger bodies – protoplanets, typically tens to hundreds of kilometers in scale. In the inner Solar System, these were mostly rocky/metallic alloys, as higher temperatures allowed little ice. Over a few million years, these protoplanets merged or scattered each other, eventually coalescing into one or several large planetary embryos. It is believed Earth's embryonic mass arose from many protoplanets, each with a unique isotopic signature and elemental composition.
2.3 Chemical clues from meteorites
Meteorites, especially chondrites, are preserved fragments of planetesimals. Their chemistry and isotopic nature show the early elemental distribution of the solar nebula. Non-chondritic meteorites from differentiated asteroids or protoplanets show partial melting and metal-silicate segregation, similar to what Earth likely experienced on a larger scale [3]. By comparing Earth's bulk composition (inferred from mantle rocks and average crustal material) with meteorites, scientists infer which primary materials formed our planet.
3. Accretion duration and early heating
3.1 Earth's formation rate
The accretion process onto Earth took tens of millions of years, from the initial planetesimal collisions to the final giant impact (~30–100 million years after the Sun's formation). Hf–W isotopic chronometry shows that Earth's core formed roughly within the first ~30 million years after the start of the Solar System, indicating significant early internal heating that allowed iron to separate into the central core [4], [5]. This pace matches the formation of other terrestrial planets, each having its own collision history.
3.2 Heat sources
Several factors caused Earth's internal temperature to rise enough for melting:
- Impact kinetic energy: High-velocity collisions convert gravitational energy into heat.
- Radioactive decay: Short-lived radionuclides (e.g., 26Al, 60Fe) provided intense but brief heating, while longer-lived ones (40K, 235,238U, 232Th) continue to heat for billions of years.
- Core formation: Iron migration to the center released gravitational energy, further raising temperature and creating the “magma ocean” phase.
During these melting phases inside the Earth, denser metal separated from silicates – a fundamental step in differentiation.
4. Giant impact and late accretion
4.1 Moon-forming impact
The giant impact hypothesis states that a Mars-sized protoplanet (Theia) collided with the proto-Earth in a later accretion stage (~30–50 million years after the first solid particles). This impact ejected molten and vaporized Earth's mantle material, creating a disk of particles around Earth. Over time, the disk material coalesced into the Moon. This is supported by:
- Identical oxygen isotopes: Moon rocks are very similar to Earth's mantle isotopic signature, unlike many chondritic meteorites.
- High angular momentum: The Earth–Moon system has a large combined spin, consistent with an energetic oblique impact.
- Moon's volatile element depletion: The impact could have vaporized lighter compounds, leaving the Moon with certain chemical differences [6], [7].
4.2 Late veneer and volatile delivery
After the Moon's formation, the Earth was likely still reached by a small amount of material from leftover planetesimals – the late veneer. This may have supplemented the mantle with certain siderophile (metal-loving) elements and precious metals. Also, part of Earth's water could have arrived through such post-impact collisions, although a significant portion of water likely remained or was delivered earlier.
5. Differentiation: core, mantle, and crust
5.1 Separation of metal and silicate
During melting phases, often called "magma ocean" periods, iron alloys (with nickel and other metals) sank to the Earth's center by gravity, forming the core. Meanwhile, lighter silicates remained on top. Key points:
- Core formation: May have occurred in stages, with each major impact promoting metal segregation.
- Chemical equilibration: Metal and silicate interaction at high pressure determined element distribution (e.g., siderophile elements migrated to the core).
- Time: Isotopic systems (Hf–W etc.) indicate the core finished forming within ~30 million years from the system's start.
5.2 Mantle
The thick mantle, composed of silicate minerals (olivine, pyroxenes, deeper garnets), is the largest Earth layer by volume. After the core formed, it likely partially crystallized from a global or regional magma ocean. Over time, convection formed some compositional layers (e.g., possible two-layer mantle differentiation in the early period), but eventually mixed due to plate tectonics and hot plume circulation.
5.3 Crust formation
When the external magma ocean cooled, the early Earth crust formed:
- Primary crust: Probably basaltic in composition, formed directly from the crystallization of the magma ocean. It may have been recycled many times by impacts or early tectonics.
- Hadean and Archean crust: Only small fragments remain from that time (~4.0 billion years), e.g., the Acasta gneiss (~4.0 billion years) or Jack Hills zircons (~4.4 billion years), providing clues about early plutonic conditions.
- Continental vs. oceanic crust: Later, stable continental crust formed on Earth (more “felsic,” lighter), which thickened over time – this is very important for further plate tectonics. Meanwhile, oceanic crust, formed at mid-ocean ridges, has “mafic” chemical properties and is rapidly recycled by subduction processes.
During the Hadean eon, Earth's surface was still active – a barrage of impacts, volcanism, the formation of the first oceans – but from this chaos a solid layered geology already emerged.
6. Importance for plate tectonics and the magnetic field
6.1 Plate tectonics
Iron separation and silicate uplift along with significant heat energy after impacts sustained mantle convection. Over several billion years, Earth's crust split into tectonic plates that slide over the mantle. These are:
- Recycles the crust into the mantle, regulating atmospheric gases (via volcanism and weathering).
- Forms continents through orogenic processes and partial mantle melting.
- Creates a unique Earth "climate thermostat" through the carbonate-silicate cycle.
No other planet in the Solar System shows such plate tectonics, so it is clear that Earth's mass, water content, and internal heat are particularly significant here.
6.2 Formation of the Magnetic Field
When the iron-rich core formed, its outer liquid iron layer began to rotate and a dynamo action was established, creating a global magnetic field. This geodynamo system protects Earth's surface from cosmic and solar wind particles, preventing the atmosphere from being stripped away. Without early metal and silicate differentiation, Earth probably would not have had a stable magnetosphere and might have lost water and other volatile substances – this again emphasizes the importance of such initial separation for Earth's habitability.
7. Clues from the Oldest Rocks and Zircons
7.1 Hadean Epoch
Direct Hadean crustal rocks (4.56–4.0 billion years) are extremely rare – most have been destroyed by subduction or early impacts. However, zircon minerals in young sediment layers show U-Pb ages up to ~4.4 billion years, indicating that continental crust, a fairly cool surface, and most likely liquid water existed then. Their oxygen isotopes show traces of water activity, meaning a hydrosphere existed early on.
7.2 Archean Terranes
Around ~3.5–4.0 billion years ago begins the Archean eon – the better-preserved green shales and cratons (3.6–3.0 billion years). These regions show that although some early “plate-like” activity may have already operated, stable lithospheric blocks existed, allowing the development of another phase of Earth's mantle and crust evolution after the main accretion.
8. Comparisons with Other Planetary Bodies
8.1 Venus and Mars
Venus probably underwent similar early steps (core formation, basaltic crust), but different environmental conditions (runaway greenhouse, no large Moon, little water) led to a completely different fate. Meanwhile, Mars may have formed earlier or from different materials during accretion, becoming smaller and less able to sustain geological and magnetic activity. These contrasts with Earth's layering help understand how slight changes in mass, chemical composition, or external influences from giant planets determine planetary fate.
8.2 Moon Formation – A Source of Answers
The Moon's composition (small iron core, isotopic similarity to Earth's mantle) confirms the giant impact scenario as the final step in Earth's assembly. We do not observe a directly analogous history for other inner bodies, although Mars' small “captured” moons or the Pluto–Charon system offer other interesting parallels.
8.3 Exoplanet Perspective
Directly observing the layering processes of exoplanets is currently impossible, but it is believed that similar laws apply there as well. By observing the densities of super-Earths or the composition of atmospheres, assumptions can be made about their differentiation state. The emergence of some planets with a high iron content may indicate stronger impacts or a different nebula composition, while others that remain undifferentiated may indicate lower mass or less heating.
9. Disagreements and future directions
9.1 Timing and mechanisms
A more precise timing of Earth's accretion process – especially the moment of the giant impact – and how much partial melting occurred at each stage remains a topic of discussion. Hf–W chronometry outlines general limits, but refining them using newer isotopic techniques or better metal-silicate redistribution models is important.
9.2 Volatiles and water
Did Earth's water come mainly from local, water-bearing planetesimals, or from later cometary/asteroidal sources? The ratio of local implantation vs. late delivery influences the formation of primordial oceans. Isotopic studies (e.g., HDO/H2O ratios in comets, Earth's mantle (e.g., xenon isotopes)) help increasingly narrow down possible scenarios.
9.3 Magma ocean depth and duration
There is still debate about the extent and duration of Earth's initial magma ocean phases. Some models suggest repeated melting during large impacts. The final giant impact may have created a global magma ocean, followed by a vapor layer forming in the steam atmosphere. Observing exoplanet “lava worlds” with next-generation IR telescopes may help confirm or refute these hypotheses elsewhere.
10. Conclusion
Earth's accretion and differentiation – i.e., the path from dust and planetesimal accumulation to a layered, dynamic planet – is a fundamental process that shaped all subsequent Earth evolution: from the Moon's formation to plate tectonics, the global magnetic field, and a stable surface environment for life. Through geochemical analysis of rocks, isotopes, meteorites, and astrophysical models, we reconstruct how numerous collisions, melting episodes, and chemical segregation formed Earth's layered interior. Each of these intense birth stages left the planet suitable for persistent oceans, stable climate control, and ultimately, thriving ecosystems.
Looking ahead, new data from sample return missions (e.g., OSIRIS-REx from Bennu, or possible future far side of the Moon studies) and improved isotopic chronometry will further refine the early Earth history timeline. Combined with advanced HPC simulations, finer details will emerge: how iron droplets sank to form the core, how the giant impact created the Moon, and how and when water and other volatiles appeared before life flourished. As exoplanet observations expand, Earth's “assembly” history becomes a crucial template for understanding the fate of other similar rocky worlds across the Universe.