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The Earth's inner core started forming when molten iron cooled below the melting point. However, the nucleation mechanism, which is a necessary step of crystallization, has not been well understood. Recent studies have found that it requires an unrealistic degree of undercooling to nucleate the stable, hexagonal, close-packed (hcp) phase of iron that is unlikely to be reached under core conditions and age. This contradiction is referred to as the inner core nucleation paradox. Using a persistent embryo method and molecular dynamics simulations, we demonstrate that the metastable, body-centered, cubic (bcc) phase of iron has a much higher nucleation rate than does the hcp phase under inner core conditions. Thus, the bcc nucleation is likely to be the first step of inner core formation, instead of direct nucleation of the hcp phase. This mechanism reduces the required undercooling of iron nucleation, which provides a key factor in solving the inner core nucleation paradox. The two-step nucleation scenario of the inner core also opens an avenue for understanding the structure and anisotropy of the present inner core.

Analysis of a database of Precambrian palaeomagnetic intensity measurements reveals a clear transition in the Earth’s magnetic field that is probably the signature of the inner core first forming, suggesting a modest value of core thermal conductivity and supporting a simple thermal evolution model for the Earth. Tracking Earth's thermal evolution The record of the intensity and orientation of Earth's magnetic field provides a constraint on the thermal evolution of Earth through its influence on the geodynamo. Here Andrew Biggin et al . analyse a database of palaeomagnetic intensity measurements and confirm that the time-averaged Precambrian magnetic field was — as is commonly assumed — dominantly dipolar. They also find evidence for long-term variations in geomagnetic field strength, with an increase in both average field strength and variability observed to occur between 1 and 1.5 billion years ago. They conclude that this increase could be explained by nucleation of the inner core during this interval, the timing of which would support a simple thermal evolution model for the Earth. The Earth’s inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies 1 , 2 , 3 , 4 , 5 have presented radically differing estimates of the thermal conductivity of the Earth’s core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime 6 and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements 7 selected using a new set of reliability criteria 8 . Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval 9 ; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.

Yang & Song (2022, https://doi.org/10.1029/2022GL098393) first claimed existence of Earth's inner core differential rotation based on the waveform similarity of two neighboring stations AAK and KZA across an earthquake doublet and then postulated a local velocity gradient at the top of the inner core based on the difference of PKiKP‐PKIKP differential times between the stations and inferred inner core differential rotation rate. In this comment, we collectively analyze the seismic data in the region and add the data of another nearby station HORS into analysis. HORS and KZA, located in an opposite direction away from AAK, consistently exhibit high waveform similarity. Collective analysis of seismic data demonstrates the invalidity of both their logic of claiming existence of inner core differential rotation and their postulation of “a local inner core gradient” to infer differential rotation. Localized and episodic inner core surface change provides a physically consistent explanation to the seismic data. Plain Language Summary Whether Earth's inner core differentially rotates with respect to the rest of Earth or it experiences localized episodic change of its surface has been vigorously debated. While both mechanisms are derived based on temporal changes of compressional seismic waves that touch the inner core, the hypothesis of inner core differential rotation is criticized for its lack of direct supporting seismic evidence and existence of many inconsistencies in explaining the seismic data. Yang & Song (2022, https://doi.org/10.1029/2022GL098393) claimed existence of inner core differential rotation based on waveform similarity of two neighboring stations AAK and KZA across an earthquake doublet. They further postulated a local inner core velocity gradient and inferred inner core differential rotation rate. Here, we collectively analyze the seismic data in the region and add the seismic data of another nearby station HORS into analysis. Collective analysis of seismic data demonstrates the invalidity of both their logic of claiming existence of inner core differential rotation and their postulation of “a local inner core gradient” to infer inner core differential rotation. Seismic evidence is contradictory to the hypothesis of inner core differential rotation. Instead, a localized and episodic inner core surface change provides a physically consistent explanation to the seismic data. Key Points Yang and Song (2022)'s logic of claiming existence of inner core differential rotation is invalid Yang and Song (2022)'s postulation of “a local inner core gradient” and inference of inner core differential rotation are invalid Localized and episodic inner core surface change provides a physically consistent explanation to the seismic data

Earth's core is structured in a solid inner core, mainly composed of iron, and a liquid outer core. The temperature at the inner core boundary is expected to be close to the melting point of iron at 330 gigapascal (GPa). Despite intensive experimental and theoretical efforts, there is little consensus on the melting behavior of iron at these extreme pressures and temperatures. We present static laser-heated diamond anvil cell experiments up to 200 GPa using synchrotron-based fast x-ray diffraction as a primary melting diagnostic. When extrapolating to higher pressures, we conclude that the melting temperature of iron at the inner core boundary is 6230 ± 500 kelvin. This estimation favors a high heat flux at the core-mantle boundary with a possible partial melting of the mantle.

Seismic observations show the Earth's inner core has significant and unexplained variation in seismic attenuation with position, depth and direction. Interpreting these observations is difficult without knowledge of the visco‐ or anelastic dissipation processes active in iron under inner core conditions. Here, a previously unconsidered attenuation mechanism is observed in zinc, a low pressure analog of hcp‐iron, during small strain sinusoidal deformation experiments. The experiments were performed in a deformation‐DIA combined with X‐radiography, at seismic frequencies (∼0.003–0.1 Hz), high pressure and temperatures up to ∼80% of melting temperature. Significant dissipation (0.077 ≤ Q−1(ω) ≤ 0.488) is observed along with frequency dependent softening of zinc's Young's modulus and an extremely small activation energy for creep (⩽7 kJ mol−1). In addition, during sinusoidal deformation the original microstructure is replaced by one with a reduced dislocation density and small, uniform, grain size. This combination of behavior collectively reflects a mode of deformation called “internal stress superplasticity”; this deformation mechanism is unique to anisotropic materials and activated by cyclic loading generating large internal stresses. Here we observe a new form of internal stress superplasticity, which we name as “elastic strain mismatch superplasticity.” In it the large stresses are caused by the compressional anisotropy. If this mechanism is also active in hcp‐iron and the Earth's inner‐core it will be a contributor to inner‐core observed seismic attenuation and constrain the maximum inner‐core grain‐size to ≲10 km. Plain Language Summary The Earth's inner‐core is the most remote and inaccessible part of our planet. Knowledge of the inner‐core's structure comes from interpretation of the information held in seismic waves that have passed through the inner‐core. These waves show measurable variation in wave speed and damping with depth. To investigate the wave damping in the inner‐core we performed experiments that mimicked the passage of seismic waves through zinc. Zinc was used as a low‐pressure analog because it has the same crystallographic structure as the iron in the inner‐core. In these experiments, we observed new behavior in the zinc samples that can only be explained by the behavior of different directions within the zinc crystal lattice. These we named “elastic strain mismatch superplasticity” and if the same phenomena occurs in the Earth's inner‐core it could explain the seismic observations. Key Points Zinc, a low pressure analog for hcp‐iron, deforms by internal stress superplasticity during small amplitude sinusoidal‐strain deformation Internal stress superplasticity due to mechanical oscillations has not been previously reported Internal stress superplasticity is another attenuation mechanism that could be active in the Earth's inner‐core

The Earth's inner core shows distinct features, including low shear velocities and a high Poisson's ratio, often linked to hexagonal close‐packed iron (HCP‐Fe) through high‐pressure experiments and calculations. Premelting is proposed as a key mechanism behind the observed shear softening in HCP‐Fe. This study uses magnesium, an HCP‐structured analog, to explore the effects of premelting on ultrasonic velocities. Premelting begins at ∼0.97 times the melting temperature (Tm), leading to sharp nonlinear drops in shear (VS) and compressional (VP) velocities by 14.5% and 4%, respectively, with a ∼13.3% increase in Poisson's ratio. These findings, together with seismic and mineralogical data, suggest that premelting could account for the reduced VP and VS and increased Poisson's ratio observed in the Earth's inner core.

This study investigates whether the initiation time and intensification rate (INTRATE) of intensifying tropical cyclones (TCs) vary diurnally and how they are related to deep convection. TC intensifying events are identified and classified into slowly intensifying (SI) and rapidly intensifying (RI) events. RI events last ∼42 hr on average, much longer than SI events. More importantly, the onset of the TC intensification, especially RI, markedly peaks at 00–06 local time. However, the INTRATEs of both RI and SI events show very weak diurnal variability. The INTRATE continues to increase after RI initiation and slightly peaks in the late afternoon (at 90% significance level). Inner‐core convection of all intensifying events maximizes in the early morning, in phase with the peak initiation time. In short, our results suggest that the nocturnally enhanced inner‐core convection may play a role in triggering TC intensification (e.g., RI), but not maximizing the INTRATE. Plain Language Summary The outbreak of intense convection in the inner core of tropical cyclones (TCs) may promote the rapid intensification of TCs. It is well known that TC inner‐core convection varies diurnally. Recent studies found that the TC intensification rate exhibits a diurnal cycle that is in phase with the inner‐core deep convection. These studies further proposed that the nocturnally enhanced inner‐core convection may simultaneously promote the TC intensification rate. However, it is unknown whether and how the diurnal cycle of the intensification rate varies among different TC intensifying periods. Also, whether the initial time of the TC intensification exhibits an evident diurnal signal? Based on the analysis of 30‐year TC track data, this study found that TC intensification including the RI prefers to initiate in the early morning, well in phase with the inner‐core deep convection. In contrast, the intensification rate of RI TCs continues to increase after RI initiation therefore does not peak immediately with the deep convection. These results suggest that the inner‐core convective outbreak may be important in triggering the TC intensification especially RI, while the intensification rate is impacted or dominated by multiple factors that are not on the diurnal scale. Key Points The rapid intensification (RI) of tropical cyclone (TC) is more likely to initiate in the early morning and lasts ∼42 hr on average TC intensification rates of RI events slightly peak in the late afternoon but with a marginal diurnal amplitude Inner‐core deep convection likely plays a role in the RI initiation as they are diurnally in phase

Earth’s core exhibits similar elastic properties to rubber. Experiments show that a high-pressure phase of iron carbide modifies iron’s elastic properties under inner-core conditions, suggesting that carbon is the light element in the core. Geochemical, cosmochemical, geophysical, and mineral physics data suggest that iron (or iron–nickel alloy) is the main component of the Earth’s core 1 , 2 , 3 . The inconsistency between the density of pure iron at pressure and temperature conditions of the Earth’s core and seismological observations can be explained by the presence of light elements 1 , 4 . However, the low shear wave velocity and high Poisson’s ratio of the Earth’s core remain enigmatic 2 . Here we experimentally investigate the effect of carbon on the elastic properties of iron at high pressures and temperatures and report a high-pressure orthorhombic phase of iron carbide, Fe 7 C 3 . We determined the crystal structure of the material at ambient conditions and investigated its stability and behaviour at pressures up to 205 GPa and temperatures above 3,700 K using single-crystal and powder X-ray diffraction, Mössbauer spectroscopy, and nuclear inelastic scattering. Estimated shear wave and compressional wave velocities show that Fe 7 C 3 exhibits a lower shear wave velocity than pure iron and a Poisson’s ratio similar to that of the Earth’s inner core. We suggest that carbon alloying significantly modifies the properties of iron at extreme conditions to approach the elastic behaviour of rubber. Thus, the presence of carbon may explain the anomalous elastic properties of the Earth’s core.

Melting-induced core anomalies Several observed properties of Earth's solid inner core and liquid outer core remain enigmatic, including an east–west seismic asymmetry and the presence of a roughly 250-kilometre-thick layer of reduced seismic velocity at the base of the outer core. Alboussière et al . now suggest a model that can produce both of these features. Based on experiment and numerical modelling, they propose that simultaneous crystallization and melting at opposite sides of Earth's inner-core surface can result in a translational mode of thermal convection within the inner core, which also introduces an asymmetry between hemispheres. These authors show that simultaneous crystallization and melting at the surface of the Earth's inner core can result in a translational mode of thermal convection within the inner core, producing the observed stratified layer of reduced seismic velocity at the base of the outer core. The dynamical model they propose also introduces an asymmetry between hemispheres that may explain the enigmatic East–West asymmetry in seismic properties of the inner core. In addition to its global North–South anisotropy 1 , there are two other enigmatic seismological observations related to the Earth’s inner core: asymmetry between its eastern and western hemispheres 2 , 3 , 4 , 5 , 6 and the presence of a layer of reduced seismic velocity at the base of the outer core 6 , 7 , 8 , 9 , 10 , 11 , 12 . This 250-km-thick layer has been interpreted as a stably stratified region of reduced composition in light elements 13 . Here we show that this layer can be generated by simultaneous crystallization and melting at the surface of the inner core, and that a translational mode of thermal convection in the inner core can produce enough melting and crystallization on each hemisphere respectively for the dense layer to develop. The dynamical model we propose introduces a clear asymmetry between a melting and a crystallizing hemisphere which forms a basis for also explaining the East–West asymmetry. The present translation rate is found to be typically 100 million years for the inner core to be entirely renewed, which is one to two orders of magnitude faster than the growth rate of the inner core’s radius. The resulting strong asymmetry of buoyancy flux caused by light elements is anticipated to have an impact on the dynamics of the outer core and on the geodynamo.

The heat extracted from the core by the overlying mantle across the core‐mantle boundary controls the thermal evolution of the core. This in turn leads to the solidification of the inner core in association with the exsolution of light alloying elements into the liquid outer core. Although the temperature (T) at the inner core boundary (ICB) would be adjusted to account for the effects of the light elements, the melting T of Fe places an upper bound at the ICB and it is a vital point in the thermal profile of the core. Here, we determine the melting T of Fe in the multi‐anvil press by characterizing the interface of Fe‐W interaction. Our data place a tighter constraint on the melting curve of Fe between 8 and 21 GPa, that is directly applicable to small planetary bodies and serves as an anchor for melting curve of Fe at higher pressure. Plain Language Summary The melting temperature (T) of Fe is a fundamental parameter in constraining the thermal structure and evolution of the cores of the rocky planets and their satellites. Here, we precisely determine the melting T of Fe by characterizing the inter‐metallic diffusive interaction between Fe‐W at the melting transition using a new method “inter‐metallic fast diffusion” in multi‐anvil press. We measured the melting T of Fe at various fixed pressures between 8 and 21 GPa. We determined the melting T within an uncertainty of about 30 K, which is higher in precision than the reported errors in previous studies. Our data set provides a tighter constraint on the melting curve of Fe measured in the large‐volume press. In addition, we used the data set to critically evaluate the melting curve of Fe up to 80 GPa which has a large discrepancy in the existing melting data produced in laser‐heated diamond anvil cell. Key Points The melting curve of Fe was precisely measured up to 21 GPa in multi‐anvil press by Characterizing the interface of Fe‐W interaction We used our data set to critically evaluate the melting curve of Fe up to about 80 GPa which provided an independent check on diamond anvil cell datasets