A Magma Ocean at the base of Earth's mantle (BMO), if stirred by sufficiently vigorous convection, may have a sufficient electrical conductivity to sustain a magnetic field.
However, this possibility rests on several results that are based on knowledge obtained mostly from numerical simulations of Earth-core dynamos, which arguably operate in a different regime -- both in terms of geometry (a thick spherical shell) and dynamical balance.
With the help of dedicated numerical simulations in a thin spherical shell geometry, we study how such magnetic fields would look like (intensity, geometry) and what are the required conditions for an Earth-like magnetic field to be produced by a BMO.

How to cite: Schaeffer, N., Aurnou, J., and Labrosse, S.: Magnetic field generation by a Basal Magma Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14023, https://doi.org/10.5194/egusphere-egu25-14023, 2025.

While the Earth's magnetic field has existed for 4Gyr or more, its inner core is much younger - recent estimates for the age of the inner core go back no further than 1.5Gyr. Consequently, Earth’s dynamo has been running in a full sphere for much of its life, in contrast to the present day dynamo operating in a spherical shell. However, despite their geophysical relevance, full shpere dynamos remain rare in literature.

Here, we present results from a first parameter study on rapidly rotating dynamos in a full sphere geometry, representative of the Earth's dynamo before the nucleation of the inner core. Since we cannot rely on the buoyancy release of the inner core, our dynamos are driven by internal heat sources and fixed flux boundary conditions take account of the secular cooling of the planet. We show that - depending on the input paramters (Ekman, Rayleigh and magnetic Prandtl number) - such dynamos can produce a variety of different solutions, including dipolar and multipolar dynamos as well as stable and chaotically-reversing dynamos.

How to cite: Burmann, F., Luo, J., Marti, P., and Jackson, A.: Early-Earth dynamos in a full-sphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1349, https://doi.org/10.5194/egusphere-egu25-1349, 2025.

The geomagnetic field is generated and self-sustained by dynamo action in the Earth's liquid outer core. The dynamo is driven by thermo-chemical convection that derives energy from the secular cooling and inner core growth. In addition, the geodynamo is controlled by thermally inhomogeneous core-mantle boundary (CMB). The CMB controls the heat transfer from the core to the mantle. Such heterogeneous CMB heat flux affects the flow and magnetic field patterns generated by the dynamo. The present study investigates the back reaction of magnetic fields on the onset of convective instability inside the inner core tangent cylinder by incorporating various laterally varying thermal structures at the top plate of a plane layer convection model. Different orientations of imposed magnetic fields of various strengths have been implemented at various rotation rates. Consequently, localised convective flow clusters have been developed in the regions of heat flow higher than the mean heat flux as a consequence of imposed laterally heterogeneous thermal structures. Additionally, convective clusters have developed with both odd and even orders of thermal heterogeneity, resulting in laterally asymmetric and symmetric structures respectively. As a result of rapid rotation, small-scale columnar rolls are formed in a weak magnetic field, regardless of the magnetic field orientation. However, under a strong magnetic field with a horizontally imposed magnetic field, large-scale convection rolls are developed.

How to cite: Barman, T., Haldar, T., and Sahoo, S.: Back reaction of magnetic field on rotating convection in presence of thermal heterogeneity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16896, https://doi.org/10.5194/egusphere-egu25-16896, 2025.

In the present study the combined convection in a rapidly rotating plane layer under the conditions that are characteristic of the near-polar regions in the planetary interiors is investigated. The combined thermal and compositional convection in a slowly rotating plane layer was previously considered for oceans, where convection is supported by thermal effects and is suppressed by compositional effects. The present work analyses the occurrence of convection by both of these effects with a predominant compositional effect in the Earth’s outer core and with various effects in the deep interiors of the known planets and moons. The self-consistent estimates of typical physical quantities give similarity coefficients for the small ratio dissipation/convection generation (s coincides with inverse Rayleigh number) and the ratio thermal convection/compositional convection (r). The third small coefficient (δ linked to the Ekman number) is the ratio of the characteristic size normal to the axis of rotation to the layer thickness. The effect of the important parameters δ and s on the stability of the combined thermal and compositional convection in a rapidly rotating plane layer is proposed in the literature by Starchenko (2017). To investigate the linear stability of this problem here, the normal mode method is employed. The critical values of  s and A (the critical wave number) observed to be depend on r for different values of δ and both Prandtl numbers that could imitate Solar System’s planets and moons at different ages. The obtained results coincide with those obtained by pioneers in the literature. The weakly nonlinear behaviour near to the primary instability threshold has been investigated using the spatiotemporal Landau-Ginzburg (LG) equation with cubic nonlinearity. Using the multiple scale analysis, the LG equation obtained and it is similar to those in the literature having different relaxation time, nonlinear coefficient, and coherence lengths. The heat transfer rate is studied using these coefficients. This equation is used to determine the domain for Eckhaus and zigzag as secondary instabilities.

How to cite: Rani, H., Nayak, K., Starchenko, S. V., and Rameshwar, Y.: Linear and Weakly Nonlinear Stability of Combined Convection in a Rapidly Rotating Plane Layer in Planetary Convection Models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1151, https://doi.org/10.5194/egusphere-egu25-1151, 2025.

It is widely accepted that the convection of the liquid metallic outer core is the driver of the dynamo-produced magnetic field in terrestrial bodies, yet the core composition and the processes which occur within the core are difficult to study due to the extreme temperatures (T) and pressures (P). By examining the phase changes that occur with varying P, T, and composition (X), phase diagrams may be constructed for expected core mimetic compositions. The constructed phase diagrams of Fe-Si alloys along with known or modeled P,T conditions of solid/liquid phases within the cores of interest can then be used to determine likely compositions of these cores.

Experiments were conducted in a 1000-ton cubic anvil press at P in the range 2-5 GPa and T into the liquid state. A central 5-hole BN cylinder held 5 different Fe-Si sample compositions simultaneously with a thermocouple located at the base of the BN cylinder, and was surrounded by a graphite furnace within  a pyrophyllite cubic pressure cell. Fe-Si samples were prepared from pure Fe up to 33 wt% Si using mixtures of powders with known compositions. Following quenching of each experiment, the samples underwent electron microprobe analysis and along with textural analyses, these are used to map the T-X phase diagram at constant P. These phase diagrams will then be applied to the cores of small terrestrial bodies, such as the Moon, Mercury, and Vesta, to identify potential core compositions that are consistent with observational data and models that employ prescribed interior compositions and phases.

How to cite: Kalman, B., Yong, W., and Secco, R.: Phase Transitions of Solid and Liquid Fe-Si Alloys with Applications to Planetary Core Composition and Dynamo Processes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2585, https://doi.org/10.5194/egusphere-egu25-2585, 2025.

The Earth’s inner core presents interesting properties such as seismic velocity anisotropy and a complex internal structure that is still under investigation. Establishing the phase diagram of the relevant iron alloys and, first, of pure iron itself is necessary to improve our understanding of planetary cores.

The iron phase diagram at high pressure and temperature is still discussed despite numerous experimental and simulation studies. Indeed, discrepancies still exist on the melting curve and the existence of a high pressure and high temperature cubic phase is debated. New techniques must be developed to address those issues.

The European X-ray Free Electron Laser (EuXFEL) offers a high brilliance pulsed X-ray beam. The pulses duration is below 50 fs and can be synchronized with the DiPOLE 100-X laser, enabling X-ray diffraction experiments during dynamic compression. This type of experiments was first tested at the EuXFEL in 2023 by an international consortium that was followed by a second experiment in 2024.

Those experiments allow the collection of high-quality in situ X-ray diffraction data and visar measurements of shocked and off-Hugoniot Iron. After establishing procedures for the processing of such data, comparison between results from the two experiments will ensure repeatability. In this presentation, we will show first results including solid phases and melt diffraction patterns collected during those experiments.

This work is the result of experiments performed under the EuXFEL 2740 and 6659 community proposals led by M. McMahon, K. Appel, J. Eggert and G. Morard.

How to cite: Ginestet, H. and Merkel, S. and the EuXFEL 2740 and 6659 community proposals: Laser driven shock compression of Iron at the EuXFEL, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9786, https://doi.org/10.5194/egusphere-egu25-9786, 2025.

The growth of Earth's solid inner core powers the geodynamo in the liquid outer core, creating a global magnetic field that helps to shield the planet from harmful solar radiation. However, the origins of the inner core are still not fully understood. Traditional models of core evolution overlook the necessity for liquids to be supercooled below their melting point before freezing. Recent estimates of the required supercooling for the inner core's homogeneous nucleation are unrealistically high and conflict with the expected current thermal structure of the core. Through molecular dynamics simulations, we show that nucleation from an Fe_1-xC_x liquid, with x=0.1-0.15, reduces the supercooling requirement to 250-400 K, broadly compatible with expected current thermal profiles of the core. Though these compositions are not a complete description of core chemistry, which requires at least ternary systems, they are consistent with a number of constraints derived from seismology, mineral physics, and geochemistry. Crucially, our results demonstrate that whilst some potential compositions of the core cannot explain the presence of the inner core, others can. The nucleation process of the inner core can therefore provide a new and strong constraint on core composition.

How to cite: Wilson, A., Davies, C., Andrew, W., and Alfè, D.: Constraining the Composition of Earth’s Core: Insights from Nucleation in FeC Liquids, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11647, https://doi.org/10.5194/egusphere-egu25-11647, 2025.

Dynamo action in liquid Fe planetary cores varies according to alloyed light elements such as S and Si. This study experimentally constrains the thermal conductivity of Fe-S-Si alloys at planetary core conditions, which may be used in combination with thermal evolution models to find the total thermal convective force in the core. A sample of Fe alloy with 16wt%S and 2wt%Si was chosen as a predicted composition of the core of Asteroid 4 Vesta, based on studies of HED meteorites [1-2]. This is near the miscibility limit of S and Si in liquid Fe [3].

Experiments were performed at 2-5 GPa in a 1000-ton cubic anvil press and at up to 9 GPa in a 3000-ton multi-anvil press. Temperatures as high as 2100 K into the melt of Fe-S-Si. The electrical resistivity of the liquid Fe-S-Si alloy was measured in situ; to find the electronic component of the thermal conductivity, the Wiedemann-Franz Law was used. To confirm the sample composition and homogenization, electron microprobe analysis was performed on samples recovered from various stages of melting, yielding compositional maps of Fe, S, and Si across each sample.

The individual effects of S and Si on the electrical resistivity of liquid Fe are seen in the results for the conditions of small planetary cores. Fe-16wt%S-2wt%Si has an electrical resistivity of 300-450 µΩ·cm at the complete melt in the pressure range of 2-7 GPa. Pure Fe at the same pressures is at most half this value [4], meaning that a moderate amount of S greatly decreases thermal conductivity in the liquid core. These results may be used to find the adiabatic heat flux at the top of the core of a given planetary body, with direct application to the formation of a magnetic dynamo in the liquid cores of objects such as Vesta, Ganymede, and Mars.

References:

[1] Steenstra, E.S., Dankers, D., Berndt, J., Klemme, S., Matveev, S., van Westrenen, W., 2019. Icarus, v. 317, p. 669-681.

[2] Pringle, E.A., Savage, P.S., Badro, J., Barrat, J.-A., Moynier, F., 2013. Earth Planet. Sci. Lett., v. 373, p. 75-82.

[3] Chabot, N.L., Wollack, E.A., Klima, R.L., Minitti, M.E., 2014. Earth Planet. Sci. Lett., v. 390, p. 199-208.

[4] Yong, W., Secco, R.A., Littleton, J.A.H., Silber, R.E., 2019. Geophys. Res. Lett., v. 46, p. 11065-11070.

How to cite: Lenhart, E., Yong, W., and Secco, R.: The heat flux through the cores of small terrestrial planetary bodies from electrical resistivity measurements of liquid Fe-S-Si at high pressures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2878, https://doi.org/10.5194/egusphere-egu25-2878, 2025.

The instabilities occurring in a horizontal layer of ferromagnetic fluid which is heated from below and kept rotating about vertical axis in the presence of vertical magnetic fluid is analysed using weakly nonlinear analysis. It is observed that either stationary convection or oscillatory convection occurred as the first instability depending on the control parameters. The amplitude equations are derived in the vicinity of the onset of stationary convection and oscillatory convection by assuming the fluid as anisotropic. At the onset of stationary convection the conditions for generalized Eckhuas instability and near the Liptz point the conditions for zig zag instability are obtained. Localized convection is studied from cubic-quintic amplitude equation. Heat transfer rate in terms of the Nusselt number is computed from the cubic amplitude equation. From the cubic-quintic amplitude equation, the subcritical fluid behavior near the onset of oscillatory convection is analysed.

How to cite: Rameshwar, Y., Jozef, B., Anil Kumar, O., and Rani, H. P.: Weakly Nonlinear Analysis of Rotating Anisotropic Ferromagnetic Rayleigh-Bénard convection, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14312, https://doi.org/10.5194/egusphere-egu25-14312, 2025.

The Fe-Ni alloy is believed to be the main component of Earth's core. Yet, Ni’s effects on the inner core’s structure and formation process are often disregarded due to its similarity to Fe. Using ab initio simulations, we find that Ni can stabilize bcc structures and accelerate Fe’s crystallization at high temperatures and inner core pressures. We computed the Gibbs free energy and phase diagram for liquid and solid solutions of Fe-Ni alloys under inner core conditions, providing new insights into the possible structure of the inner core. The results offer new constraints for the study of the core’s composition and formation.

How to cite: Sun, Y., Wei, L., Ho, K.-M., and Wentzcovitch, R.: The effect of Ni on the formation and structure of Earth’s inner core, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20372, https://doi.org/10.5194/egusphere-egu25-20372, 2025.

An upper bound of heat transfer has been published recently [1] for the first time in the case of compressible convection. This concerns only the anelastic liquid approximation, but the best place where such an approximation is valid might well be the core of terrestrial planets. In this work, I will apply this result to the specific case and geometry of the outer core. A big difference is also the fact that not only viscous dissipation but also Joule heating are sources of entropy production.

For a given forcing (Rayleigh number), we will see that there is a trade-off between a maximal heat flux and a maximal Joule dissipation. An upper bound can be obtained for both quantities, but they cannot both reach that bound.

We shall also consider the case of terrestrial planets of larger radii than the Earth. A number of exoplanets are suspected to be in that case. We will investigate the consequences of larger compressibility on their internal structure [2] and obtain upper bounds of heat flux and Joule dissipation.

[1] T Alboussière, Y Ricard, S Labrosse, "Upper bound of heat flux in an anelastic model for Rayleigh–Bénard convection", JFM 999, 2024

[2] Y Ricard, T Alboussière, "Compressible convection in super-Earths", PEPI 341, 2023

How to cite: Alboussiere, T.: Bounds on heat transfer and dissipation in the core, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13013, https://doi.org/10.5194/egusphere-egu25-13013, 2025.

The geomagnetic fields are generated by dynamo action driven by thermo-chemical convection in the deep interior. The convective instability occurs due to the density gradient of fluid, which depends on the temperature (thermal) and light element concentration (chemical composition), and they diffuse at different rates. We investigate rotating double-diffusive convection (RDDC) in the two-dimensional plane layer. We focus on classical convection, a diffusive regime with unstable thermal and chemical composition gradients. A systematic investigation on the impact of various thermal and compositional boundary conditions, such as fixed temperature, fixed composition, fixed heat flux, and fixed compositional flux, and their combinations, on the onset of convection by fixing mechanical boundary condition as no-slip is carried out in the present study. In particular, we choose a compositionally dominated regime by fixing the Rayleigh ratio (ratio of thermal to compositional Rayleigh number) equal to 0.5 for both non-rotating and rotating cases. With varying compositional Rayleigh numbers, the critical thermo-chemical Rayleigh number is estimated at the onset. The onset Rayleigh number, with fixed temperature and compositional boundary condition at both the upper and bottom boundary, is higher than fixed flux conditions for both the non-rotating and rotating cases, and this trend persists with increase (decrease) in compositional (thermal) Prandtl number at the low diffusivity ratio regime. However, at the high diffusivity ratio regime, the trends substantially change with changing diffusivity.

How to cite: Singh, S., Barman, T., and Sahoo, S.: Impact of boundary condition on the onset of thermo-chemical convection at the Earth’s core, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9161, https://doi.org/10.5194/egusphere-egu25-9161, 2025.

The Earth's liquid outer core is mainly composed of iron and nickel. The secular cooling of the inner core releases latent heat and light elements, driving convection in the liquid outer core and promoting the upward transport of these lighter elements, thereby forming thermal and compositional driven convection. However, due to the uncertainty of the temperature distribution within the liquid outer core, two types of convection may occur: top-heavy and salt-fingers double-diffusive convection, the latter characterized by a thermal stable stratification where the thermal gradient is stabilizing. Most dynamo models, however, do not account for such complex driving mechanisms. Instead, they simplify the system by assuming no distinction between thermal and compositional convection, which is termed the co-density model. In our study, we compared the top-heavy double-diffusive model with the co-density model within the strong field regime, where the Lorentz force plays a significant role. Our results suggest that, under strong fields and similar magnetic Reynolds numbers, different types of buoyancy do not show significant differences in driving the dynamo process. Furthermore, we investigate the effects of varying the strength of thermal stratification on the dynamo. Our analysis indicates that when the thermal stratification becomes sufficiently strong, it can suppress convection entirely, ultimately halting the dynamo process.

How to cite: Fan, W. and Lin, Y.: Double-diffusive convection driven dynamos in the strong-field regime, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8115, https://doi.org/10.5194/egusphere-egu25-8115, 2025.