# 5. The Core The core is Earth's dense metallic center, making up about 15% of Earth's volume but nearly a third of its mass. It is divided into two regions with sharply different physical states despite similar composition. ## 5.1 Outer Core - Liquid iron–nickel alloy, with roughly 5–10% lighter elements (sulfur, oxygen, silicon, carbon) that lower its melting point and density relative to pure iron. - Extends from ~2,890 km to ~5,150 km depth; roughly 2,260 km thick. - Temperatures range from about 4,000°C near the CMB to nearly 5,700°C near the inner core boundary. - Convects vigorously, driven by both heat loss to the mantle and the release of light elements as the inner core freezes — this motion, combined with the Coriolis effect from Earth's rotation, generates the geomagnetic field (Section 7). ## 5.2 Inner Core - Solid iron–nickel alloy despite temperatures comparable to the surface of the Sun, because immense pressure (over 3.5 million atmospheres) raises the melting point of iron above the local temperature. - Radius of about 1,221 km — roughly 70% the size of the Moon. - Growing slowly over geological time as the outer core cools and freezes onto it from the outside in, releasing latent heat and light elements that help power outer-core convection. - Seismic evidence indicates the inner core is not perfectly uniform: it appears to have its own faint internal structure and rotates slightly differently from the mantle and crust above it ("super-rotation"), though the exact rate remains an active research question. ## 5.3 Why the Core Matters to Everything Above It The core is often treated as a geological curiosity, but it is functionally the power source for nearly every dynamic process in the crust and mantle. It is the dominant source of Earth's internal heat budget (alongside radioactive decay in the mantle and crust), and it is the sole source of the planet's global magnetic field. Sections 6 through 9 detail these interactions directly. ## 5.4 Core Formation: Timing and Evidence The mechanism of core formation — dense iron sinking under gravity — was introduced in Section 9.1. A separate question is *when* this happened and how confidently that timing is known. - **Isotopic dating:** the hafnium-tungsten (Hf-W) system is the primary tool. Hafnium is "lithophile" (it stays in silicate rock) while tungsten is "siderophile" (it partitions preferentially into metal). Because radioactive ¹⁸²Hf decays into ¹⁸²W with a short half-life (about 9 million years), the tungsten-isotope composition of mantle rock today effectively records how long core-forming metal continued separating from the silicate mantle while that radioactive clock was still running. - **Current estimate:** this evidence places core formation as largely complete within the first 30–100 million years of Solar System history — a small fraction of Earth's total 4.5-billion-year age, overlapping with the era of giant planetary impacts, including the Moon-forming impact. - **Why a magma ocean made this fast:** early Earth's surface, and likely much of its mantle, was molten from accretional heat, short-lived radioactive isotopes, and the heat released by core formation itself (Section 9.7). Dense iron could segregate and sink as droplets through this magma ocean far more efficiently than it could migrate through solid rock, which is why core formation could complete in tens of millions of years rather than the much longer timescale solid-state percolation would require. - **The ****"****late veneer****"****:** mantle rock today contains a small excess of certain metal-loving ("highly siderophile") elements beyond what core formation should have left behind — interpreted as a late addition of meteoritic material delivered after core formation had already effectively finished, adding a final, thin layer of unmixed material to the mantle rather than being incorporated into the core itself. ## 5.5 The Light Element Puzzle: What's Actually Down There The "light elements" mentioned throughout this document (Section 2, Section 5.1) are not a settled, precisely known ingredient list — their identity is one of the most actively studied open questions in deep-Earth science. - **Leading candidates:** sulfur, silicon, oxygen, carbon, and hydrogen each have independent cosmochemical or experimental support, and no single-element explanation is considered adequate — most current models favor some combination rather than one dominant light element. - **How candidates are tested:** high-pressure, high-temperature experiments (diamond anvil cells, shock-compression experiments) measure the density and sound velocity of candidate iron alloys under core-like conditions, and the results are compared against the density and velocity profile seismology actually observes for the outer core (Section 5.6) — a direct, quantitative matching exercise rather than guesswork. - **Why the identity matters beyond curiosity:** different candidate elements imply different conditions during core formation (oxygen points toward a more oxidized, high-pressure magma ocean equilibration; silicon points toward more reducing conditions), and the specific identity and abundance of light elements also affects exactly how buoyant the material released during inner-core freezing is — directly feeding into how vigorously that buoyancy can drive the compositional convection that powers the geodynamo (Section 7.2). ## 5.6 The Core Density Deficit: The Evidence Behind the Light Elements The light-element discussion above is not speculative color — it exists specifically to explain a real, quantitative mismatch between what seismology observes and what pure iron would predict. - Combining PREM (Section 8.4) with high-pressure equation-of-state data for pure iron-nickel alloy shows that the **outer core is roughly 6–10% less dense** than pure liquid iron-nickel would be at the relevant core pressures and temperatures. - The inner core shows a smaller, but still real, density deficit of roughly 2–3% relative to pure solid iron-nickel — smaller than the outer core's deficit, consistent with the inner core containing a lower concentration of light elements than the outer core, as would be expected if light elements are preferentially excluded during freezing (Section 5.1). - This density deficit is the actual empirical anchor for the entire light-element discussion: it is the specific, measured reason geophysicists are confident *something* other than pure iron-nickel must be present, even though which specific elements and in what proportion remains the open question in Section 5.5. ## 5.7 The "New Core Paradox": Thermal Conductivity and Inner Core Age A genuinely significant controversy in core science follows from a single, focused experimental result and its wide-reaching implications. - Older geodynamo models assumed a relatively low thermal conductivity for core iron, implying the core cools slowly — consistent with an ancient magnetic field (3.5+ billion years old, per the paleomagnetic evidence in Section 7.6) sustained partly by thermal convection even before much of an inner core existed to provide compositional buoyancy. - Around 2012–2013, new experimental and computational studies revised iron's thermal conductivity under core conditions sharply upward — by roughly a factor of two to three relative to earlier assumptions. - Higher conductivity means more heat is carried directly down the core's temperature gradient by conduction alone, leaving less heat available to actually drive convective motion, and implying the core has been cooling faster over its history than older models assumed. - **The paradox:** faster historical cooling pushes estimates of the inner core's age significantly younger — some estimates as low as 500 million to 1.5 billion years old, versus older estimates of up to 2–4 billion years — which creates real tension with the paleomagnetic record showing a magnetic field existing for 3.5+ billion years (Section 7.8). If the inner core is that young, something other than inner-core-driven compositional convection must have powered the dynamo for the majority of Earth's history. This is genuinely still debated in the literature — proposed resolutions include higher radiogenic heating from potassium-40 in the core, revised light-element concentrations altering the compositional convection budget, or reconsidering the conductivity measurements themselves — and it remains one of the more actively contested questions in core geophysics today. ## 5.8 The Innermost Inner Core: A Layer Within a Layer A genuinely recent finding — proposed as recently as 2021–2023 — suggests the inner core itself is not a uniform sphere, but contains its own internal boundary. - Seismologists studying waves that travel back and forth through the Earth's center multiple times (very long, near-antipodal wave paths that sample the deepest possible territory) identified a change in seismic anisotropy — the direction in which waves travel fastest — at a boundary roughly 650 km in radius, about half the total radius of the inner core. - This "innermost inner core" appears to have a distinctly different pattern of iron crystal alignment than the outer part of the inner core surrounding it, suggesting it solidified under different conditions or represents an earlier, different phase of inner-core growth history than the material that has frozen on since. - One interpretation is that this boundary is a fossilized record of some past change in core conditions or geodynamo behavior, frozen into the iron crystal structure at the time — but this remains an active, still-developing finding rather than settled consensus, and is flagged here as an area to watch rather than an established fact on the level of the rest of Section 5. ## 5.9 Comparative Planetology: Why Earth's Core Looks the Way It Does Comparing Earth's core to those of its planetary neighbors clarifies which of the specific numbers given throughout this document are universal physics and which are particular to Earth's specific combination of size, composition, and history. - **Mars** has a proportionally smaller core than Earth's, and current evidence indicates its dynamo shut down roughly 4 billion years ago — Mars still shows strong remnant magnetization frozen into its ancient crust, but no active, present-day global field. This is the direct comparison referenced in Section 7.9's discussion of atmospheric loss: without an active dynamo shielding it, Mars is thought to have lost much of its early atmosphere to the solar wind over subsequent billions of years. - **Mercury**, by contrast, has an unusually large core relative to its total size — proportionally much larger than Earth's — likely because a major impact (or preferential loss of lighter surface material during formation, given Mercury's proximity to the Sun) stripped away a disproportionate share of its original silicate mantle. Despite its small overall size, which might be expected to cool quickly and shut down a dynamo the way Mars did, Mercury still maintains an active, if comparatively weak, magnetic field today. - **The takeaway:** an active dynamo is not guaranteed simply by having a core, nor automatically prevented by being small — it depends on the specific combination of core size, light-element budget, cooling rate, and (per Section 5.7) how much of that cooling is currently being spent on inner-core growth versus pure thermal loss. Every specific number given for Earth's core elsewhere in this document (Section 2, Section 5.1–5.2) represents one particular, evidently favorable combination of these factors, not the only way a rocky planet's interior can turn out.

Key CharacteristicsMajor Functions and Features
•
•
•
• 		•
•
•
Header Header
Data 1 Data 2

Back