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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.

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

How the differential rotation of the Earth's inner core (IC) has changed over time provides insights into the dynamics of the Earth's interior. Analyses of repeating earthquakes (doublets) have yielded different models. Here we present an event‐based investigation using individual events from long‐term earthquake sequences, which improves temporal coverage over doublet‐based approaches and provides spatial resolution for inferring the rotation rate. Results from two pathways, South Sandwich Islands to Alaska (1982–2024) and Kuril Islands to Argentina (1994–2024), reveal a consistent pattern that the IC successively rotated faster than the mantle by about 0.10°/yr $0.10{}^{\\circ}/\\text{yr}$ from the early 1980s and decelerated to a near‐zero rate around 2000, perhaps slower than the mantle after about 2010. We also provide a new way to calibrate spatial structure and constrain the average rotation rate with improved accuracy. The results are consistent with the multidecadal IC oscillation model, but do not support shorter‐term oscillations or bursts.

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

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

Here, using self-consistent ab initio lattice dynamical calculations that go beyond the quasiharmonic approximation, we show that the high-pressure high-temperature bcc-Fe phase is dynamically stable. In this treatment the temperature-dependent phonon spectra are derived by exciting all the lattice vibrations, in which the phonon-phonon interactions are considered. The high-pressure and high-temperature bcc-Fe phase shows standard bcc-type phonon dispersion curves except for the transverse branch, which is overdamped along the high symmetry direction Γ-N, at temperatures below 4,500 K. When lowering the temperature down to a critical value TC, the lattice instability of the bcc structure is reached. The pressure dependence of this critical temperature is studied at conditions relevant for the Earth's core.

The tendency in tropical cyclone (TC) rainfall is of great concern due to its remarkable contribution to global precipitation and extreme rainfall events. This study finds a decreasing trend in TC inner‐core rain rate over the western North Pacific (WNP) from 1998 to 2019. This basinwide trend is mainly induced by the decreasing TC inner‐core rain rates over the region west of 150°E, while it is seldom linked to the changes in the distribution of TC occurrence. The maximum decreases in TC inner‐core rain rate are observed over the offshore areas along the coastlines of East Asia. Further analysis reveals that the change in atmospheric stability, referred to as a dominant environmental contributor to basinwide TC inner‐core rain rate decreases shown in previous studies, only has a primary impact over the northern South China Sea. By comparison, there is a positive correlation between the variations of the aerosol optical depth and TC inner‐core rain rate over the mid‐latitude regions extending from the East China Sea to Japan. Our result highlights the linkage of the recent decreasing trends in aerosol optical depth and TC inner‐core rain rate over the WNP. The decreasing trend in inner‐core rain rate of tropical cyclones over the western North Pacific.

Compressional seismic wave reflected off the Earth’s inner core boundary (ICB) from earthquakes occurring in the Banda Sea and recorded at the Hi-net stations in Japan exhibits significant variations in travel time (from -2 to 2.5 s) and amplitude (with a factor of more than 4) across the seismic array. Such variations indicate that Earth’s ICB is irregular, with a combination of at least two scales of topography: a height variation of 14 km changing within a lateral distance of no more than 6 km, and a height variation of 4–8 km with a lateral length scale of 2–4 km. The characteristics of the ICB topography indicate that small-scale variations of temperature and/or core composition exist near the ICB, and/or the ICB topographic surface is being deformed by small-scale forces out of its thermocompositional equilibrium position and is metastable.

Reactions involving carbon in the deep Earth have limited manifestations on Earth's surface, yet they have played a critical role in the evolution of our planet. The metal-silicate partitioning reaction promoted carbon capture during Earth's accretion and may have sequestered substantial carbon in Earth's core. The freezing reaction involving iron-carbon liquid could have contributed to the growth of Earth's inner core and the geodynamo. The redox melting/freezing reaction largely controls the movement of carbon in the modern mantle, and reactions between carbonates and silicates in the deep mantle also promote carbon mobility. The 10-year activity of the Deep Carbon Observatory has made important contributions to our knowledge of how these reactions are involved in the cycling of carbon throughout our planet, both past and present, and has helped to identify gaps in our understanding that motivate and give direction to future studies.

The Earth’s core contains, in addition to iron, 5–10% nickel and many light elements such as C, O, Si and S, among others. For this work, we consider binary Fe–Ni alloys with 10% Ni as host material, doped with light elements as impurity atoms. The phases considered for the Fe–Ni host alloy are face-centered cubic (fcc) and body-centred cubic (bcc). At different concentrations of 2.5–50%, the impurity atoms C, O, Si and S were placed in the host Fe–Ni alloy to create ternary alloys. The resulting ternary systems are exposed to a pressure equivalent to that existing at the Earth’s core. As shown by their formation energies, these alloys are stable and advantageous. The calculation of the resistivity of the impurities was performed with the help of the Kubo–Greenwood formula. Compared to fcc, in the case of bcc, electrical resistivities begin to saturate at about 30% of the atomic concentration of the impurities. Thermal conductivity was also determined from electrical resistivities calculated for varying concentrations and pressures according to Wiedemann–Franz law. In the case of compression, we observe a rise in thermal conductivity of about 1.5% of the core’s internal pressure. The reported thermal conductivities support the notion of maintaining a convection-induced geodyanmo. Highlights Study the internal structure of the Earth. The electrical and electronic properties for the binary and ternary compounds exist at the Earth's core. The electrical resistivity at high pressure of ternary alloys. The thermal conductivity at high pressure and temperature of ternary alloys.