Our understanding of the iron cores in Earth and other bodies is progressing rapidly thanks to cross-fertilization between a number of observational, theoretical and experimental disciplines.
Improved seismic observations continue to provide better images and prompt refinements in structural and geodynamic models. Mineral physics provides constraints for dynamic, structural, and thermodynamic models. Improved constraints on the core heat budget, paleomagnetic observations of long-term magnetic field variations, and high-resolution numerical simulations promote the exploration of new dynamo mechanisms. Geomagnetic observations from both ground and satellite, along with magneto-hydrodynamic experiments, provide additional insight to our ever expanding view of planetary cores.
This session welcomes contributions from all disciplines, as well as interdisciplinary efforts, on attempts to proceed towards an integrated, self-consistent picture of planetary core structure, dynamics and history, and to understand their overwhelming complexity.
Dear all, we intend to use the EGU plattform for the 2 min presentations at least. Then depending on the server load we may continue with their breakout rooms system.
In case the server breakdown, we have setup our own Zoom session with breakout rooms, it will be open 15 mins before the session.
In case the server breakdown, we have setup our own Zoom session with breakout rooms, it will be open 15 mins before the session.
vPICO presentations: Mon, 26 Apr
Thermal conductivity of Earth materials under relevant high pressure-temperature conditions is crucial to determine the temperature profile in Earth’s interior, which further influences its thermo-chemical evolution and structures as well as geodynamics. In Earth’s core, iron (Fe) is the major constituent along with some candidate light elements, for instance, silicon (Si), carbon (C), sulfur (S), etc. Core’s thermal conductivity plays a key role in affecting its thermal evolution history and the magnitude of thermal and compositional sources required to operate a geodynamo. Precise and direct measurements of the thermal conductivity of Earth’s core materials under extreme conditions, however, have been very challenging due to the difficulty of experimental methods. Recently we have combined time-resolved optical techniques with high-pressure diamond cells to precisely measure the thermal conductivity of core materials, including pure Fe and Fe-Si and Fe-C alloys, etc. We found that the alloying effect by these candidate light elements results in a relatively low thermal conductivity compared to the pure Fe. Combined with thermal evolution models, our new data suggest a low minimum heat flow across the core-mantle boundary than previously expected, and therefore less thermal energy needed to run the geodynamo. In addition, the age of the inner core is constrained to be older than about two billion years.
How to cite: Hsieh, W.-P.: Low thermal conductivity of Earth’s core with implications for the geodynamo and the age of inner core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1462, https://doi.org/10.5194/egusphere-egu21-1462, 2021.
It is still controversial for an emergence of a stable region at the top of Earth’s core in theoretical modeling because both thermal conductivity of Earth’s core and heat flow across the core-mantle boundary (CMB) have not been clearly constrained from mineral physics and geophysical observations, ranging 20 to 220 W/m/K for the thermal conductivity (denoted as ) and 5 to 20 TW for the present-day CMB heat flow (denoted as QPCMB). In this study, in order to resolve these uncertainties, we try to constrain the values of thermal conductivity of Earth’s core and the present-day CMB heat flow by requiring continuous generation of geomagnetic field in addition to existence of a stable region at the top of present Earth’s core using a one-dimensional thermal and compositional evolution model.
Numerical experiments for various values of and QPCMB show that the solutions satisfying both long-term magnetic field generation and emergence of a stable region is possible only when is larger than 40 W/m/K and QPCMB is less than 18.5 TW. The specific required value of depends on QPCMB. If the expected CMB heat flow would be as large value as 17.5 TW, which is suggested by the recent studies on the core evolution theory (e.g., Labrosse, 2015), should be a high value such as about 212 W/m/K to satisfy our requirements. The thickness of an expected stable region would be about 30 km in this case. In contrast, when QPCMB is as small as that derived from numerical mantle convection models (e.g., 10 TW; Nakagawa and Tackley, 2010), the required value of decreases to 110 W/m/K. In this case, a stable region extends about 75 km thickness below CMB.
If the requirements assumed in this study is confirmed by certain geophysical observations and/or QPCMB can be restricted more precisely with some methods, our assessment scheme would be useful for evaluations of the radial convective structure of Earth’s core and for further constraint of the value of thermal conductivity of Earth’s core.
How to cite: Nakagawa, T., Takehiro, S., and Sasaki, Y.: A constraint to thermal conductivity of Earth’s core and CMB heat flow by assessment on a stable region of Earth’s core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10510, https://doi.org/10.5194/egusphere-egu21-10510, 2021.
There are many examples which show how the anisotropic diffusive coefficients crucially influence geophysical and astrophysical flows and in particular flows in the Earth’s outer core. Thus, many models concerning rotating magnetoconvection with anisotropy in the viscosity, thermal and magnetic diffusivities have been developed.
Different models correspond to different cases of anisotropic diffusivities. For example, we consider several anisotropic models: one with anisotropy in all diffusivities and other models with various combinations of anisotropic and isotropic diffusivities.
Firstly, all kind of anisotropies are reminded and described. Then, a thorough comparison of these anisotropies, especially of the physical differences among them is done. All physical systems with the above mentioned anisotropies are prone to the occurrence of convection and other instabilities. We show how different types of anisotropy cause a different convection and a different balance among the main forces in the Earth’s Outer Core (Magnetic, Archimedean, Coriolis).
As usually, to study instabilities in such systems, we use analysis in term of normal modes and search for preferred modes. In all our models, only marginal modes with zero growth rate have so far been studied. Now, we present the bravest modes, i.e. the ones with maximum growth rate. The comparison of the modes dependent on basic input parameters - Prandtl numbers, anisotropic parameter, Ekman and Elsasser numbers - is made mainly for values corresponding to the Earth’s outer core. In all our models the anisotropic diffusive coefficients are represented as diagonal tensors with two equal components different from the third one giving the chance to define simply the anisotropic parameter.
We stress how magnetoconvection problems with the anisotropy included, became more and more important among the geodynamo problems in the last years; indeed, the origin of flows necessary for dynamo action, as studied in magnetoconvection with resulting instabilities, is important, as well as the problem of the origin of magnetic fields.
How to cite: Filippi, E. and Brestenský, J.: Anisotropic diffusivities' effects in rotating magnetoconvection and geodynamo problems, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7896, https://doi.org/10.5194/egusphere-egu21-7896, 2021.
Precession driven flows are relevant for geo- and astrophysical fluid dynamics as well as industrial applications. In the context of planetary core dynamics, they are attributed to the generation of magnetic fields and/or anomalous dissipation. While precession driven flows have been frequently studied in a cylindrical, spherical or spheroidal container shape, the geometry of a triaxial ellipsoid, representing the geophysical case of core mantle boundary deformation in a tidally locked planet, has received less attention.
Here, we present results from an experimental study in a triaxial ellipsoid. The main focus of our study is on the base flow of uniform vorticity and we report a good agreement between experimental data and existing semi-analytical models. The amplitude of the time averaged uniform vorticity displays a hysteresis loop as a function of the precession forcing and we demonstrate that this observation depends on the ellipticity of the container. Our study also comprises experiments where the boundary layer is expected to be in a turbulent state. Therefore, we discuss the applicability of an effective damping coefficient in the semi-analytical models to account for the dissipation in a turbulent boundary layer.
How to cite: Burmann, F. and Noir, J.: Experimental investigation of precession driven flows in a triaxial ellipsoid, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4705, https://doi.org/10.5194/egusphere-egu21-4705, 2021.
We revisit the generation of mean zonal flows in fluid planetary interiors subjected to precession.
The main effect of precession on a (nearly) spherical fluid envelope is to make the fluid rotate along an axis tilted with respect to the rotation axis of the solid mantle. This is the so-called "spin-over" response of the fluid.
also shows that a steady shear flow develops on top of the spin-over mode due to non-linear effects in the boundary layer equation.
This mean zonal shear flow has been studied theoretically and numerically by .
With faster computers and more efficient codes, we compute this flow down to very low viscosity and compare with the inviscid theory of Busse (1968).
In addition we investigate the width and the intensity of the detached shear layer, which is controlled by viscosity and therefore not present in the theory.
We also use this problem as a benchmark to assess the benefits of using a semi-lagrangian numerical scheme, where solid-body rotation is treated exactly.
How to cite: Schaeffer, N. and Cébron, D.: Mean zonal flow driven by precession in planetary cores: numerical simulations with a semi-lagrangian scheme, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13699, https://doi.org/10.5194/egusphere-egu21-13699, 2021.
Fast changes of Earth's magnetic field could be explained by inviscid and diffusion-less quasi-geostrophic (QG) Magneto-Coriolis modes. We present a hybrid QG model with columnar flows and three-dimensional magnetic fields and find modes with periods of a few years at parameters relevant to Earth's core. These fast Magneto-Coriolis modes show strong focusing of their kinetic and magnetic energy in the equatorial region, while maintaining a relatively large spatial structure along the azimuthal direction. Their properties agree with some of the observations and inferred core flows. We find additionally, in contrast to what has been assumed previously, that these modes are not affected significantly by magnetic diffusion. The model opens a new way of inverting geomagnetic observations to the flow and magnetic field deep within the Earth's outer core.
How to cite: Gerick, F., Jault, D., and Noir, J.: Fast (non-)diffusive Quasi-Geostrophic Magneto-Coriolis Modes in the Earth's core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2176, https://doi.org/10.5194/egusphere-egu21-2176, 2021.
Magnetic fields inside planetary objects can influence their rotation. This is true, in particular, of terrestrial objects with a metallic liquid core and a self-sustained dynamo such as the Earth, Mercury, Ganymede, etc. and also, to a lesser extent, of objects that don’t have a dynamo but are embedded in the magnetic field of their parent body like Jupiter’s moon, Io.
In these objects, angular momentum is transfered through the electromagnetic torques at the Core-Mantle Boundary (CMB) . In the Earth, these have the potential to produce a strong modulation in the length of day at the decadal and interannual timescales . They also affect the periods and amplitudes of nutation  and polar motion .
The intensity of these torques depends primarily on the value of the electric conductivity at the base of the mantle, a close study and detailed modelling of their role in planetary rotation can thus teach us a lot about the physical processes taking place near the CMB.
In the study of the Earth’s length of day variations, the interplay between rotation and the internal magnetic field arrises from the excitation of torsional oscillations inside the Earth’s core . These oscillations are traditionally modelled based on a series of assumptions such as that of Quasi-Geostrophicity (QG) of the flow inside the core . On the other hand, the effect of the magnetic field on nutations and polar motion is traditionally treated as an additional coupling at the CMB . In such model, the core flow is assumed to have a uniform vorticity and its pattern is kept unaffected by the magnetic field.
In the present work, we follow a different approach based on the study of magneto-inertial waves. When coupled to gravity through the effect of density stratification, these waves are known to play a crucial role in the oscillations of stars known as magneto-gravito-inertial modes . The same kind of coupling inside the Earth’s core gives rise to the so-called MAC waves which are directly and conceptually related to the aforementioned torsional oscillations .
We present our preliminary results on the computation of magneto-inertial waves in a freely rotating planetary model with a partially conducting mantle. We show how these waves can alter the frequencies of the free rotational modes identified as the Free Core Nutation (FCN) and Chandler Wobble (CW). We analyse how these results compare to those based on the QG hypothesis and how these are modified when viscosity and density stratification are taken into account.
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How to cite: Rekier, J., Triana, S., and Dehant, V.: Magneto-inertial waves and planetary rotation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11936, https://doi.org/10.5194/egusphere-egu21-11936, 2021.
It is known that the columnar structures in rapidly rotating convection are affected by the magnetic field in ways that enhance their helicity. This may explain the dominance of the axial dipole in rotating dynamos. Dynamo simulations starting from a small seed magnetic field have shown that the growth of the field is accompanied by the excitation of convection in the energy-containing length scales. Here, this process is studied by examining axial wave motions in the growth phase of the dynamo for a wide range of thermal forcing. In the early stages of evolution where the field is weak, fast inertial waves weakly modified by the magnetic field are abundantly present. As the field strength(measured by the ratio of the Alfven wave to the inertial wave frequency) exceeds a threshold value, slow magnetostrophic waves are spontaneously generated. The excitation of the slow waves coincides with the generation of helicity through columnar motion, and is followed by the formation of the axial dipole from a chaotic, multipolar state. In strongly driven convection, the slow wave frequency is attenuated, causing weakening of the axial dipole intensity. Kinematic dynamo simulations at the same parameters, where only fast inertial waves are present, fail to produce the axial dipole field. The dipole field in planetary dynamos may thus be supported by the helicity from slow magnetostrophic waves.
How to cite: Varma, A. and Sreenivasan, B.: The role of slow magnetostrophic waves in dipole formation in rapidly rotating dynamos, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15480, https://doi.org/10.5194/egusphere-egu21-15480, 2021.
The cause for the damping of the Earth's Free Core Nutation (FCN) and the Free Inner Core Nutation (FICN) eigenmodes has been a matter of debate since the earliest reliable estimations from nutation observations were made available. Numerical studies are difficult given the extreme values of some of the parameters associated with the Earth's fluid outer core, where important dissipation processes can take place. We present a linear numerical model for the FCN that includes viscous dissipation and Ohmic heating. We find an asymptotic regime, appropriate for Earth's parameters, where viscous and Ohmic processes contribute equally to the total damping, with the dissipation taking place almost exclusively in the boundary layers. By matching the observed nutational damping we infer an enhanced effective viscosity matching and validating methods from previous studies. We suggest that turbulence caused by the Earth's precession can be a source for the FCN's damping.
How to cite: Triana, S., Rekier, J., Trinh, A., Dehant, V., and Zhu, P.: Ohmic and viscous damping of the Earth's Free Core Nutation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12492, https://doi.org/10.5194/egusphere-egu21-12492, 2021.
Spherical Slepian functions (or ‘Slepian functions’) are mathematical functions which can be used to decompose potential fields, as represented by spherical harmonics, into smaller regions covering part of a spherical surface. This allows a spatio-spectral trade-off between aliasing of the signal at the boundary edges while constraining it within a region of interest. While Slepian functions have previously been applied to geodetic and crustal magnetic data, this work further applies Slepian functions to flows on the core-mantle boundary. There are two main reasons for restricting flow models to certain parts of the core surface. Firstly, we have reason to believe that different dynamics operate in different parts of the core (such as under LLSVPs) while, secondly, the modelled flow is ambiguous over certain parts of the surface (when applying flow assumptions). Spherical Slepian functions retain many of the advantages of our usual flow description, concerning for example the boundary conditions it must satisfy, and allowing easy calculation of the power spectrum, although greater initial computational effort is required.
In this work, we apply Slepian functions to core flow models by directly inverting from satellite virtual observatory magnetic data into regions of interest. We successfully demonstrate the technique and current short comings by showing whole core surface flow models, flow within a chosen region, and its corresponding complement. Unwanted spatial leakage is generated at the region edges in the separated flows but to less of an extent than when using spherical Slepian functions on existing flow models. The limited spectral content we can infer for core flows is responsible for most, if not all, of this leakage. Therefore, we present ongoing investigations into the cause of this leakage, and to highlight considerations when applying Slepian functions to core surface flow modelling.
How to cite: Rogers, H., Beggan, C., and Whaler, K.: Application of spherical Slepian functions to the inversion of virtual observatory satellite magnetic data into localised regions of flow on the core-mantle boundary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2857, https://doi.org/10.5194/egusphere-egu21-2857, 2021.
Temporal changes of the inner core over several years have been well observed by different studies, especially those using high-quality repeating earthquakes (i.e., doublets). The phenomenon has commonly been interpreted as the differential rotation of the inner core shifting its interior heterogeneities. However, an alternative interpretation, the rapid growing or shrinking at the inner core boundary (ICB), is favored by some studies. On the other hand, estimates of the inner-core rotation rate vary by an order of magnitude.
In this study, we used high-quality doublets from our previous systematic global search and analyzed the temporal changes (in terms of arrival times and waveforms) of inner core waves (both the refractive PKIKP and the reflective PKiKP) at the distance range between 128° and 142°. Using SKP (or PP) phase as a reference to eliminate possible clock errors, we found that the temporal changes are mostly from the PKIKP arrivals and always start before the onset of the late-arriving PKiKP. The observation is consistent with the proposal of differential rotation and rules out the ICB as the sole source of the temporal changes.
On the other hand, we discovered compelling evidence of the differential rotation. Stations AAK and KZA in Kyrgyzstan are virtually the same distance to the doublets along the South Sandwich Islands (SSI) and hence are referred to as twin stations by us. The fortuitous geometry captures the underlying local structures, which have complex lateral velocity gradients. The yearly temporal change from different doublets also varies a lot, but surprisingly, it strongly correlates with the underlying velocity gradient, providing unequivocal evidence for the rotation of the inner core. The rotation rate could be accurately determined as 0.127° ± 0.006° per year at 95% confidence level in 1991-2010. In other words, when the lapse of a doublet is about 6.3 years, the inner core structure sampled by the earlier event to AAK is captured by its later repeater to KZA, which agrees very well with the real data.
We believe that the above results largely resolve the debates on the origin of the temporal changes of the inner core and provide the most precise estimation of the differential rotation rate for the 1991-2010 time period.
How to cite: Yang, Y. and Song, X.: Nature of inner-core temporal changes and a precise estimate of differential inner-core rotation rate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9152, https://doi.org/10.5194/egusphere-egu21-9152, 2021.
Scattered waves composing the coda of the PKiKP wave, reflected by the boundary of Earth's inner core at pre-critical range, reveal the existence of small-scale heterogeneity in the uppermost inner core. Since the shape this coda envelope is relatively insensitive to intrinsic viscoelastic attenuation, seismograms synthesized using the Axisem code (Nissen-Meyer, 2014) are exploited to determine whether heterogeneity spectra consistent with the coda envelope of PKiKP can contribute to the attenuation observed in long range PKIKP waves transmitted through the deeper inner core. Peng et al. (2008) have shown that a range of possible parameters describing an exponential autocorrelation of small-scale heterogeneity can fit observed PKiKP coda envelopes, with the rms P velocity fluctuation trading off against the corner scale length parameter "a" of the heterogeneity spectrum. Testing the effects of a series of "a's" and velocity fluctuations that fit PKIKP coda envelopes we determined upper bounds to “a” and the rms P velocity fluctuation below 300 km depth in the inner core. Parameter combinations of “a” > 2 km and rms dVp/Vp > 2% can be eliminated from consideration because they produce too strong a coda following PKIKP. In the antipodal range (178o to 180o) we found that there exists a strong focusing of multiply scattered waves affecting the pulse width and coda of PKIKP. The parameter combination "a"= 2 km and rms=1.2% produces a strong PKIKP coda, which is not observed in antipodal data. This, coupled with the fact that Axisem ignores out-of-plane scattering, suggests that the attenuation of PKIKP observed beyond 160o is dominated by viscoelastic rather than scattering attenuation and that the rms P velocity fluctuation must decrease by at least a factor of 2 below 300 km to be consistent with the coda of antipodal PKIKP waves.
Peng, Z., Koper, K.D., Leyton, J.E., Shearer, P., J. Geophys. Res., 113(B9), B09312, doi:10.1029/2007JB/005412, 2008.
Nissen-Meyer, T., van Driel, M., Stähler, S. C., Hosseini, K., Hempel, S., Auer, L., Colombi, A., and Fournier, A. Solid Earth, 5, 425-446, https://doi.org/10.5194/se-5-425-2024, 2014.
How to cite: Cormier, V. and Wickramathilake, R.: Constraints on small-scale heterogeneity in Earth's inner core determined from transmitted P waves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13037, https://doi.org/10.5194/egusphere-egu21-13037, 2021.
The inner core contains strong seismic heterogeneity, both laterally and from the surface to the centre. Accurately resolving the seismic structure of the inner core is key to unravelling the evolution of the core. Seismic models of inner core structure are often limited by their parameterization, which means it is difficult to interpret which features of the inner core are real (e.g. hemispheres or the inner most inner core). To overcome this we conduct seismic tomography using transdimensional inversion on a high quality data set of 5296 differential and 2344 absolute P-wave travel times. By taking a transdimensional approach we allow the data to define how the model space is parameterized and this provides us with both the mean structure of the inner core but also the probability distributions of each model parameter. This allows us to identify which regions of the model space are well constrained and likewise which regions are poorly constrained. We compare results from a static MCMC model and a transdimensional MCMC model, this provides confidence in our results as both models show clear similarities in structure. From no prior assumptions on inner core structure we recover many first order observations: such as anisotropic hemispheres and an isotropic outer inner core (OIC) along with potential observations of an inner most inner core. With higher resolution than previous inner core tomography we can provide more detailed interpretation of inner core structure and draw conclusions with greater confidence. We also conduct transdimensional inversions on a subset of our data which does not contain South Sandwich Islands (SSI) events which are considered by many to be unreliable or contaminated with mantle structure. The overall inner core structure remains largely the same however, showing that the SSI data does not significantly alter our final interpretations.
How to cite: Brett, H., Hawkins, R., Lythgoe, K., Waszek, L., and Deuss, A.: 3D Transdimensional Seismic Tomography of the Inner Core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4159, https://doi.org/10.5194/egusphere-egu21-4159, 2021.
The Slichter mode (1S1) is the longest-period mode of the free oscillations of the Earth. The period of the Slichter mode directly depends on density jump between the outer liquid and the inner solid core which makes the detection of this oscillation very important for gaining a more detailed insight into the structure of the Earth’s interior. Reliable empirical data on the detection of Slichter mode are absent, which is associated with the rather low amplitude of this mode on the surface.
In this work, for the first time, an attempt is made to detect the Slichter mode using the strain data from the largest 2010 Chilean earthquake recorded by the Baksan laser interferometer–strainmeter (Sternberg Astronomical Institute of the Moscow State University (SAI MSU)) with a measuring arm length of 75 m in the Elbrus region, the Northern Caucasus.
A new asymptotically optimal algorithm for data analysis is developed. The algorithm uses the maximum likelihood method and takes into account the features of the detected signal and the properties of seismic noise. The algorithm is based on the fundamental principles of the theory of optimal signal reception against the background of non-Gaussian noise, which provides the most effective signal detection in accordance with the Neumann-Pearson optimality criterion. Simultaneously with the detection, the degenerate frequency of the mode and splitting parameter b are estimated. Applying the developed algorithm to the strain data of the Chilean earthquake yielded two sets of the most probable candidates for the Slichter mode parameters: T1 = 5.905 h at b1= 0.1038 and T2 = 6.581 h at b2= 0.1046. The obtained sets of the Slichter mode parameters have a false-alarm probability of 0.012 and 0.005, respectively.
The comparison of our results with the theoretical models and the previous results of experimental determinations of the period of the Slichter mode shows a close correspondence of the period T1 = 5.905 h to the period in the CORE11 model (Widmer et al., 1988); the difference is below 1.5%. In the case of the PREM models (Rosat et al., 2006), the obtained periods correspond to the density jumps between the inner and outer cores of Δρ1 = 0.456 g/cm3 for T1= 5.905 h and Δρ2 = 0.360 g/cm3 for T2 = 6.581 h.
This work is supported by the MSU Interdisciplinary Scientific and Educational School of Moscow University "Fundamental and Applied Space Research" and the Russian Foundation for Basic Research under Grant No Grant No 19-05-00341.
How to cite: Milyukov, V., Vinogradov, M., Mironov, A., and Myasnikov, A.: Detection and estimation of the Slichter mode based on strain observation of the 2010 Chilean earthquake, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5022, https://doi.org/10.5194/egusphere-egu21-5022, 2021.
Growth of the solid inner core is generally considered to power the Earth's present geodynamo. Cristallisation of a solid central inner core has also been proposed to drive the lunar dynamo and to generate a magnetic field in smaller bodies. In a previous work, we estimated the compaction of planetary cores for different scenarios of growth (with or without supercooling) and different sizes of the inner core. Our main results indicated that small inner cores are unlikely to compact efficiently the liquid trapped during the first steps of the growth.
This is especially true for small bodies for which the typical size of the core is similar to the compaction length. The light elements are thus trapped during the cristallisation, reducing the release of latent heat and of light elements. We present here a model to include the effect of an inefficient compaction in the energy budget of a planetary core and investigate the implications for the dynamo evolution in small bodies. We apply this model for the evolution of the core of the Moon.
How to cite: Lasbleis, M.: Inefficient compaction in small planetary cores -- application to the Moon, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14959, https://doi.org/10.5194/egusphere-egu21-14959, 2021.
Recent theoretical studies have tried to constrain Mercury’s internal structure and composition using thermal evolution models. The presence of a thermally stratified layer of Fe-S at the top of an Fe-Si core has been suggested, which implies a sub-adiabatic heat flow on the core side of the CMB. In this work, the adiabatic heat flow at the top of the core was estimated using the electronic component of thermal conductivity (kel), a lower bound for thermal conductivity. Direct measurements of electrical resistivity (ρ) of Fe-8.5wt%Si at core conditions can be related to kel using the Wiedemann-Franz law. Measurements were carried out in a 3000 ton multi-anvil press using a 4-wire method. The integrity of the samples at high pressures and temperatures was confirmed with electron-microprobe analysis of quenched samples at various conditions. Unexpected behaviour at low temperatures between 6-8 GPa may indicate an undocumented phase transition. Measurements of ρ at melting seem to remain constant at 127 µΩ·cm from 10-24 GPa, on both the solid and liquid side of the melting boundary. The adiabatic heat flow at the core side of Mercury’s core-mantle boundary is estimated between 21.8-29.5 mWm-2, considerably higher than most models of an Fe-S or Fe-Si core yet similar to models of an Fe core. Comparing these results with thermal evolution models suggests that Mercury’s dynamo remained thermally driven up to 0.08-0.22 Gyr, at which point the core became sub-adiabatic and stimulated a change from dominant thermal convection to dominant chemical convection arising from the growth of an inner core. Simply considering the internal structure of Mercury, these results support the capture of Mercury into a 3:2 resonance orbit during the thermally driven era of the dynamo.
How to cite: Berrada, M., Secco, R., and Yong, W.: Adiabatic Heat Flow in Mercury with a Fe-8.5wt%Si Core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1417, https://doi.org/10.5194/egusphere-egu21-1417, 2021.
The Earth’s inner core is primarily composed of solid iron and is exposed to pressures of ~330-360 GPa and to temperatures corresponding to that of the surface of the sun. Its transport and rheological properties determine the rotational dynamics and deformation of the inner core. However, the rheology of the inner core is poorly understood. In a recently published paper in Scientific Reports (1Ritterbex & Tsuchiya 2020), we propose a theoretical mineral physics approach based on the density functional theory to constrain the viscosity of hexagonal close packed (hpc) iron, the most likely phase of iron stable in the inner core. Since plastic deformation is rate-limited by atomic diffusion at the extreme pressure and temperature conditions of Earth’s center, we quantify self-diffusion in hcp iron non-empirically. Results are used to model the rate-limiting creep behavior of hcp iron, suggesting dislocation creep to be a potential mechanism driving inner core deformation which might contribute to the observed seismic anisotropy of the inner core. The associated viscosity agrees well with geodetic estimates supporting that the inner core is significantly less viscous than Earth’s mantle. We demonstrate that the predicted low viscosity of hcp iron is consistent with a strong gravitational coupling between the inner core and mantle compatible with seismic observations of small fluctuations in the inner core rotation rate. We will discuss why the inner core is too weak to undergo translational motion, one of the hypotheses to explain the hemispherical patterns of seismic anisotropy in the inner core. Instead, our results provide evidence that mechanical stresses of tens of pascals are sufficient to deform hcp iron by dislocation creep at extremely low geological strain rates, comparable to the candidate forces able to drive inner core convection.
1S. Ritterbex and T. Tsuchiya (2020). Viscosity of hcp iron at Earth's inner core conditions from density functional theory. Scientific Reports 10, 6311. [doi:10.1038/s41598-020-63166-6]
How to cite: Ritterbex, S. and Tsuchiya, T.: Viscous strength of hcp iron at conditions of Earth’s inner core, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1920, https://doi.org/10.5194/egusphere-egu21-1920, 2021.
Earth’s core is a Fe-rich alloy with a significant contribution from cosmochemically abundant light elements such as sulfur. Understanding the phase stability and structural properties of iron-rich sulfides at core conditions is critical for assessing the core’s composition and dynamics. In the current study, we examined the high-pressure polymorphism of Fe2S coexisting with Fe to outer-core pressures and high temperatures by combining in-situ powder and single-crystal X-ray diffraction techniques. We further conducted single-crystal X-ray diffraction experiments on Co2P as a low-pressure analog of Fe2S. Analyses of the powder X-ray diffraction patterns indicate an orthorhombic Fe2S phase coexisting with Fe between 25 and 170 GPa at moderate temperatures. Above 85 GPa, the orthorhombic Fe2S phase transitions to a hexagonal lattice that is stable on the liquidus to 140 GPa. Using single-crystal diffraction techniques, the orthorhombic structure of Fe2S was solved and refined to the C23 structure (Co2P type, Pnma, Z=4) at 90 GPa and quenched from 2380 K. While upon quenching at 100 GPa from 2650 K, a hexagonal lattice was identified and indexed to a unit cell compatible with a C22 Fe2S phase (Fe2P type, P-62m, Z=3), confirming the phase relations inferred in our powder diffraction experiments. The C23 Fe2S unit-cell parameters fit between 25 and 170 GPa reveal a highly compressible a axis, where the a axis is about 3 times more compressible than the b and c axes. To 48 GPa, C23 Co2P shows analogous anisotropic compression behavior to that observed at higher pressures in C23 Fe2S. Structural analysis of Co2P demonstrates that the anisotropic compression of these C23 phases is attributable to bond angle distortion and bond length compression parallel to the a direction and that the Co2P-type structure is compressing towards a Co2Si-type structure. These results display the mechanism for anisotropic compression observed in C23 Fe2S and support previous observations of a C37-like Fe2S phase above 190 GPa. Through this work, we determined that Fe2S is the relevant Fe-rich sulfide to at least outer core pressures and high temperatures and assessment of the phase transition and compression behavior of the Fe2S and Co2P analogs provides insight into the material properties and dynamics of Earth’s complex core.
How to cite: Zurkowski, C., Lavina, B., Chariton, S., Greenberg, E., Tkachev, S., Prakapenka, V., and Campbell, A.: Phase stability and structural properties of Fe2S and its analog Co2P at high pressures and temperatures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1862, https://doi.org/10.5194/egusphere-egu21-1862, 2021.
Iron is a key constituent of planetary core and an important technological material. Here, we combine in situ ultrafast X-ray diffraction at free electron lasers with optical-laser-induced shock compression experiments on polycrystalline Fe to study the plasticity of hexagonal close-packed (hcp)-Fe under extreme loading states. We identifiy the deformation mechanisms that controls the Fe microstructures and observe a significant time-evolution of stress over the few nanoseconds of the experiments. These observations illustrate how ultrafast plasticity studies can reveal distinctive materials behavior under extreme loading states and will help constraining the pressure, temperature, and strain rate dependence of materials behavior in planetary cores.
How to cite: Merkel, S., Hok, S., Bolme, C., Mao, W., and Gleason, A.: Crystal plasticity in shock-compressed hcp-iron, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13568, https://doi.org/10.5194/egusphere-egu21-13568, 2021.
Seismic waves traveling through the outer core have been used for a long time to study heterogeneity at the core mantle boundary (CMB) and in lower mantle. Earth's velocity structure opens a window for waves that are scattered at 3D structures in the lower mantle to arrive at the Earth's surface prior to the waves that would propagate in a 1D spherically symmetric model. These precursors are particularly well observed as they are not hidden in the coda waves of earlier phases. At epicentral distances below 140° PKPab and PKPbc waves scattered close to the CMB can arrive as precursors to PKPdf that travels through the inner core (IC). These waves have been studied extensively and provided important information about the structure of the mantle close to the CMB. However, theory predicts that PKP waves can also be scattered to distances above 155°. These waves have not been well observed before, partly because they arrive at the surface only after the inner core PKPdf phase that has far larger amplitudes at lower frequencies. Here we report on the observation of an emergent arrival of seismic energy at distances above 155° that is consistent with the onset times of scattered PKPbc energy. The key to observe this scattered phase is the use of signals from large deep earthquakes which are strong high frequency sources. As basis for the observation we used records of the Japanese Hi-Net stations that allowed to observe the scattered waves in the distance range between 135o and 165o when combining records of two events in Peru and Argentina. The Brazilian seismic network provided observations of a deep Bonin Islands event in the distance range from 145o to 175o. Using frequencies around 6Hz we show (A) energy in this frequency band propagates to epicentral distances beyond 170°, (B) attenuation in the IC completely removes the energy of the PKPdf phase, (C) energy scattered close to the CMB arrives prior to PKPab wave forming a precursor that we call PKPab precursor. This observation extends the frequency range and opens a new time-distance window for investigations of deep Earth heterogeneity.
How to cite: Sens-Schönfelder, C., Bataille, K., and Bianchi, M.: Observation and modelling of the seismc high frequency PKPab precursor at distances larger than 155o, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16259, https://doi.org/10.5194/egusphere-egu21-16259, 2021.
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