Earth’s metallic core is slowly losing heat, and that gradual cooling is what keeps our planet magnetically shielded and geologically alive. The inner core, a solid ball of iron and nickel roughly the size of the Moon, has been growing for something like the last billion and a half years as the liquid outer core freezes onto it from the outside in. That process releases energy that stirs the molten iron above it, generating the magnetic field that deflects solar radiation and makes surface life possible. Core decay, in the planetary sense, is not a catastrophe unfolding. It is the engine behind one of Earth’s most important features. But the process is finite, and what happens when it winds down can be seen on other worlds that have already passed that point.
Why Earth’s Core Is Still Hot
Two main heat sources keep the deep interior at thousands of degrees. The first is leftover heat from the planet’s formation, when colliding material dumped enormous kinetic energy into the growing Earth. The second is radioactive decay of elements trapped inside the planet. Uranium, thorium, and potassium are the key players. Most of the radioactive heating happens in the crust and mantle, where those elements are concentrated, but experiments have shown that potassium can dissolve into iron-sulfide melts under extreme pressure and temperature. That means potassium-40 acts as a heat source inside the core itself, not just in the rocks above it.1PubMed. Experimental evidence that potassium is a substantial radioactive heat source in planetary cores
The ratio between these two contributions matters. Radioactive heating slows over time as isotopes decay into stable products, and the primordial heat steadily leaks outward. The core cools at a rate governed by how efficiently the overlying mantle can carry that heat toward the surface. Plate tectonics plays a surprisingly direct role here: subducting slabs of cold oceanic crust sink to the base of the mantle, pulling heat away from the core-mantle boundary. Without plate tectonics, heat extraction from the core slows dramatically, with consequences visible on our nearest planetary neighbors.
The Dynamo and What Drives It
Earth’s magnetic field is generated in the liquid outer core, where convection of electrically conducting molten iron creates a self-sustaining dynamo. The fluid flow is shaped by two competing forces. The planet’s rotation produces a strong Coriolis effect that would ordinarily constrain the flow into narrow columns. At the same time, the magnetic field itself exerts a force on the moving iron. Individually, each force tends to resist large-scale flow, but together they reach an unusual balance: the magnetic field relaxes the rotational constraint and allows the broad convective motions that regenerate the field.2Annual Review of Fluid Mechanics. Rapidly Rotating Magnetohydrodynamics and the Geodynamo
The energy feeding this convection comes from two sources. Thermal convection arises because the core is hotter at depth than at the top, so hot iron rises and cooler iron sinks. Compositional convection kicks in as the inner core solidifies: lighter elements like sulfur, silicon, and oxygen get rejected from the freezing iron and rise buoyantly through the liquid. Both processes stir the outer core, but compositional convection is thought to be the more efficient driver of the dynamo today. This distinction matters because a purely thermal dynamo can shut down even while the core is still liquid, if the temperature gradient becomes too gentle to drive vigorous flow.
The pattern of heat leaving the core through the mantle also shapes the field’s behavior. Variations in the heat flux at the core-mantle boundary, driven by the structure of mantle convection above, can destabilize the dynamo and push it toward weaker or more complex field geometries.3Earth and Planetary Science Letters. Effect of realistic heat flux patterns on geodynamo simulations This sensitivity to mantle conditions is one reason the field’s strength fluctuates over geologic time and occasionally reverses polarity entirely.
When the Field Flips
Geomagnetic reversals, where the north and south magnetic poles swap, have occurred hundreds of times over the past few hundred million years. They are not signs that the core is failing. Instead, they appear to be an intrinsic feature of the dynamo’s turbulent behavior. Numerical simulations show that patches of reversed-polarity magnetic flux can form inside the core during normal convective variability. When enough of these patches grow and coalesce, they can weaken the dominant dipole field to the point of collapse, sometimes triggering a full reversal and sometimes just a temporary excursion before the original polarity recovers.4Physics of the Earth and Planetary Interiors. Dipole collapse and reversal precursors in a numerical dynamo
The current field has been weakening over the past few centuries, which occasionally makes headlines. But the field has been much weaker in the geologic past and recovered without incident. A reversal would reduce the shielding effect temporarily, potentially increasing radiation exposure at the surface and disrupting navigation systems that rely on the field, but it would not mean the dynamo had permanently wound down. The core would still be convecting. The field would rebuild in a new orientation.
How Fast the Inner Core Grows
The inner core has not been around for the entire history of the Earth. Early on, the entire core was liquid. It was only when the center cooled below the melting point of the iron alloy that crystallization began. Modeling of this process suggests the inner core cannot have reached its present size in more than about 1.7 billion years.5Physics of the Earth and Planetary Interiors. On cooling of the Earth’s core Some more recent estimates push the onset even later, to around a billion years ago, which would mean the dynamo ran on thermal convection alone for most of Earth’s history before the inner core gave it a compositional boost.
The inner core continues to grow today, at a rate of roughly a fraction of a millimeter per year. As it grows, it releases latent heat and expels light elements, both of which power the dynamo. In a sense, the slow death of the liquid core is what keeps the magnetic field alive. The paradox is real: the field exists because the core is freezing, and the field will eventually die when the freezing process either completes or slows below the threshold needed to drive convection.
Mars and the Dynamo That Stopped
Mars once had a global magnetic field. Ancient crustal rocks in the southern highlands are strongly magnetized, recording a field that existed early in the planet’s history. But Mars today has no global field, and understanding why it shut down is one of the central questions in planetary science.
The answer depends on the composition of Mars’s core, particularly how much sulfur it contains. If the sulfur content is below a certain threshold, the core may have solidified nearly completely, leaving no liquid to convect. If the sulfur content is high enough to keep the core liquid, the lack of a field might instead reflect a failure of the dynamo mechanism itself, either because thermal convection stalled or because the conditions for magnetic field generation were never met in a fully liquid core without an inner solid seed.6Journal of Geophysical Research: Solid Earth. Thermal history of Mars and the sulfur content of its core
A separate line of research points to giant impacts as a possible trigger. The massive collisions that formed Mars’s largest impact basins delivered huge amounts of heat to the interior. Simulations show that a sufficiently large impact can heat the outermost layers of the core, creating a thermally stratified blanket that prevents cooling of the deeper interior and shuts down core convection. Even after the hottest layers dissipate, the dynamo does not restart for roughly 100 million years, and the core does not fully recover its convective state for about a billion years.7Journal of Geophysical Research: Planets. Impact heating and coupled core cooling and mantle dynamics on Mars For a small planet already struggling to maintain enough heat flow, that interruption may have been permanent.
The loss of Mars’s magnetic field had real consequences for its atmosphere. Without a global field to deflect the solar wind, the upper atmosphere became vulnerable to stripping. Over billions of years, Mars lost much of its air and water, transforming from a planet with rivers and possibly oceans into the cold, dry world we see today.
Venus and the Mantle Problem
Venus is almost the same size as Earth and presumably has a similar iron core, yet it has no detectable magnetic field. The explanation most widely discussed is not that Venus’s core has cooled too much, but that it has not cooled enough. Generating a dynamo requires extracting heat from the core into the mantle. On Earth, plate tectonics keeps the mantle relatively cool, maintaining a steep temperature gradient across the core-mantle boundary. Venus appears to lack plate tectonics, and its mantle temperature may actually be increasing over time. If mantle heating reduces the heat flux out of the core to zero, core convection stops and the field dies.8Geology. Why does Venus lack a magnetic field?
This picture is not settled, though. Some researchers have pushed back on the standard model by pointing out that Venus’s surface features suggest steadier heat loss over time rather than the episodic catastrophic resurfacing traditionally assumed. If Venus has been losing heat more gradually, other explanations for the missing field become necessary. One possibility is that the core’s thermal conductivity is high enough that heat moves out by conduction rather than convection, suppressing the dynamo even with adequate cooling. Another is that Venus’s core has either completely solidified or retained a primordial chemical stratification from its formation that prevents mixing.9Earth and Planetary Science Letters. Prospects for an ancient dynamo and modern crustal remanent magnetism on Venus
Venus illustrates that core decay is not a one-size-fits-all story. The same-sized core in a different thermal environment can follow a completely different evolutionary path. Earth’s plate tectonics, often discussed as important for surface habitability, turns out to be equally important for what happens thousands of kilometers below.
The Moon’s Fading Field
The Moon, too, once generated a magnetic field from a small iron core. Paleomagnetic measurements of Apollo samples show that a lunar dynamo operated between at least 4.25 and 3.56 billion years ago, with surface field strengths reaching around 70 microtesla, comparable to Earth’s present field. But by 3.19 billion years ago, the surface field had dropped to less than roughly 4 microtesla, a precipitous decline.10Earth and Planetary Science Letters. Decline of the lunar core dynamo
The Moon’s small size worked against it. A smaller body has less internal heat to begin with and loses it faster relative to its volume. Thermal modeling suggests that convection driven by inner core growth can account for the longevity of the lunar dynamo without needing exotic mechanisms to keep it running or to switch it off.11Icarus. Magnetic field generation in the lunar core: The role of inner core growth The inner core simply grew until there was not enough liquid left, or not enough energy being released, to sustain convection. The field faded and never came back.
The lunar case is useful because it gives a concrete example of core decay running to completion on a geologically short timescale. What took the Moon a couple of billion years will take Earth much longer, but the trajectory is broadly similar.
What This Means for Exoplanets
The question of whether a rocky planet can sustain a magnetic field has become central to discussions of habitability beyond our solar system. A magnetic field shields an atmosphere from being stripped by stellar winds, particularly around M-dwarf stars, which are more magnetically active than the Sun and host many of the rocky planets discovered so far. But modeling shows that a planet’s magnetic strength does not depend on rotation rate alone. Its formation history, thermal state, age, composition, and even the geometry of the field all play roles.12PubMed. Magnetic fields in Earth-like exoplanets and implications for habitability around M-dwarfs
A planet that formed with a hotter core might generate a stronger early field but burn through its thermal budget faster. A planet with a different core composition might solidify earlier or later, changing when the compositional convection boost kicks in. Two planets with the same mass and orbit could have very different magnetic histories depending on how they assembled and what their insides are made of. This makes predicting surface habitability from mass and distance alone unreliable. Core decay is a hidden variable that telescopes cannot directly measure.
The Nuclear Engineering Meaning of Core Decay
Outside planetary science, “core decay” commonly refers to the heat produced by a nuclear reactor after it has been shut down. When control rods are inserted and the fission chain reaction stops, the fuel does not immediately go cold. Radioactive fission products trapped in the fuel continue to decay, releasing heat. This decay heat is a major factor in reactor safety, because failing to remove it can lead to fuel damage even in a shutdown reactor.13ScienceDirect. Evaluation of decay heating in shutdown reactors
Decay heat starts at a few percent of the reactor’s full operating power and drops rapidly in the first hours, then more slowly over days and weeks. But it never truly goes to zero on human timescales. Spent nuclear fuel continues generating heat for decades, which is why spent fuel pools require active cooling and why geological disposal of nuclear waste has to account for thermal output over very long periods. Repository designs must ensure that the heat from spent fuel does not damage the surrounding rock or engineered barriers.14Nuclear Engineering and Technology. High-efficiency deep geological repository system for spent nuclear fuel in Korea with optimized decay heat in a disposal canister and increased thermal limit of bentonite
The connection between the two meanings is not just a coincidence of language. In both cases, radioactive decay converts stored nuclear energy into heat, and managing that heat over time is the central engineering or geological challenge.
Other Things That Decay From the Inside Out
The phrase “core decay” also surfaces in entirely different fields, each with its own flavor of the same basic idea: deterioration starting at the structural center of something.
In forestry and wood science, heartwood decay (sometimes called heart-rot) is the breakdown of the dead inner wood of a living tree by fungi. The heartwood is no longer biologically active, making it vulnerable to colonization. Certain fungi, including some that are endophytic in living trees, specifically target heartwood and break it down from within. A tree with extensive heart-rot can look healthy on the outside while being structurally hollow inside.15PLoS ONE. Molecular fungal community and its decomposition activity in sapwood and heartwood of 13 temperate European tree species
In dentistry, internal root resorption is a condition where the mineralized interior of a tooth is eaten away by the body’s own cells. Inflammation in the dental pulp triggers clastic cells that dissolve the dentine from inside the root canal. The process can take two forms: a purely destructive version and one accompanied by repair, where the body deposits replacement tissue alongside the resorption sites.16PubMed. Internal root resorption: a review Left untreated, the tooth can be weakened to the point of fracture or perforation. The condition is usually detected incidentally on dental X-rays, since it tends to be painless until advanced.17PubMed Central. Management of internal root resorption on permanent teeth
In civil engineering, concrete exposed to flowing soft water over long periods can undergo calcium leaching, where the mineral that gives concrete its strength is slowly dissolved and carried away. This increases the porosity of the material and opens pathways for further chemical attack, including corrosion of reinforcing steel.18PubMed Central. Effects of Leaching Behavior of Calcium Ions on Compression and Durability of Cement-Based Materials with Mineral Admixtures The degradation tends to be localized to areas of highest water flow rather than uniformly distributed, as documented in a study of a hydroelectric powerhouse exposed to river water for four decades.19Construction and Building Materials. Concrete leaching of a hydroelectric powerhouse due to 40 years of exposure to river water
Stellar Cores and the End of Burning
At the largest scale, core decay describes what happens when a star exhausts its nuclear fuel. A white dwarf, the remnant of a Sun-like star, is essentially a bare stellar core slowly radiating away its residual heat. The crystallization of a white dwarf’s carbon-oxygen interior is governed by a phase diagram similar in principle to the freezing of Earth’s core: as the star cools, solid crystals form and release latent heat, temporarily slowing the cooling process. Accurate modeling of this phase transition is important for using white dwarfs as cosmic clocks, since their temperature reflects their age.
Recent work on the carbon-oxygen phase diagram has improved precision considerably, but a puzzle remains. The observed pile-up of white dwarfs at the crystallization stage is larger than models predict from latent heat and oxygen sedimentation alone. Additional mechanisms, possibly involving the separation of neon-22 into distinct phases, appear to be needed to explain the observations.20Astronomy & Astrophysics. Toward precision cosmochronology: A new C/O phase diagram for white dwarfs The crystallization of a cooling stellar core is, in some ways, the cosmic-scale version of Earth’s inner core growth: a freezing process that releases energy and alters the remaining liquid, with consequences that outlast the transition itself.