Melting-induced stratification above the Earth's inner core due to convective translation

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(2-6) and the presence of a layer of reduced seismic velocity at the base of the outer core(6-12). 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

Thermal convection in Earth's inner core with phase change at its boundary

Inner core translation, with solidification on one hemisphere and melting on the other, provides a promising basis for understanding the hemispherical dichotomy of the inner core, as well as the anomalous stable layer observed at the base of the outer core - the F-layer - which might be sustained by continuous melting of inner core material. In this paper, we study in details the dynamics of inner core thermal convection when dynamically induced melting and freezing of the inner core boundary (ICB) are taken into account. If the inner core is unstably stratified, linear stability analysis and numerical simulations consistently show that the translation mode dominates only if the viscosity $\eta$ is large enough, with a critical viscosity value, of order $3 10^{18}$ Pas, depending on the ability of outer core convection to supply or remove the latent heat of melting or solidification. If $\eta$ is smaller, the dynamical effect of melting and freezing is small. Convection takes a more classical form, with a one-cell axisymmetric mode at the onset and chaotic plume convection at large Rayleigh number. [...] Thermal convection requires that a superadiabatic temperature profile is maintained in the inner core, which depends on a competition between extraction of the inner core internal heat by conduction and cooling at the ICB. Inner core thermal convection appears very likely with the low thermal conductivity value proposed by Stacey & Davis (2007), but nearly impossible with the much higher thermal conductivity recently put forward. We argue however that the formation of an iron-rich layer above the ICB may have a positive feedback on inner core convection: it implies that the inner core crystallized from an increasingly iron-rich liquid, resulting in an unstable compositional stratification which could drive inner core convection, perhaps even if the inner core is subadiabatic.Comment: 25 pages, 12 figure

Experiments on the fragmentation of a buoyant liquid volume in another liquid

We present experiments on the instability and fragmentation of volumes of heavier liquid released into lighter immiscible liquids. We focus on the regime defined by small Ohnesorge numbers, density ratios of order one, and variable Weber numbers. The observed stages in the fragmentation process include deformation of the released fluid by either Rayleigh-Taylor instability or vortex ring roll-up and destabilization, formation of filamentary structures, capillary instability, and drop formation. At low and intermediate Weber numbers, a wide variety of fragmentation regimes is identified. Those regimes depend on early deformations, which mainly result from a competition between the growth of Rayleigh-Taylor instabilities and the roll-up of a vortex ring. At high Weber numbers, turbulent vortex ring formation is observed. We have adapted the standard theory of turbulent entrainment to buoyant vortex rings with initial momentum. We find consistency between this theory and our experiments, indicating that the concept of turbulent entrainment is valid for non-dispersed immiscible fluids at large Weber and Reynolds numbers

Core crystallization and chemical evolution of iron meteorites parent bodies

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Dynamique de la cristallisation de la graine : expériences et modèles.

Seismic observations of the Earth's inner core reveal unexpected structural complexity, its most striking property being its elastic anisotropy. The inner core has been slowly crystallizing from the liquid outer core, and its structure and dynamic may be in part related to the process of solidification. A stability analysis of the solidification front suggests that the inner core boundary is morphologicaly unstable, and that a dendritic layer should have developped. This dendritic layer may extend deep in depth, but thermosolutal convection associated with the solidification and compaction of the solid matrix are expected to efficiently expell the residual liquid. We present an experiment of solidification under a large gravity, which allows to increase the strength of the convective phenomenon associated with solidification. It is shown that the enhancement of solutal and thermal transport have important consequences on the structure of the dendritic layer, and increases significantly its solid fraction. A last part is devoted to the mechanisms from which the seismis anisotropy may originates. It is shown that the internal dynamic of the inner core depends critically on the thermal and solutal evolution of the outer core. If the inner core is relatively old, its is likely to be stably stratified, and vertical motions are inhibited. Heterogeneous growth of the inner core results in an shallow shear layer. If the inner core has grown rapidly, a thermaly driven convective episode may have occured in its early history.La graine terrestre telle que vue par la sismologie présente une anisotropie de ces propriétés élastiques et une structure étonnament complexe. La graine cristallise lentement à partir du noyau liquide, et il est possible que sa structure et sa dynamique soit liée à sa cristallisation. Une analyse de stabilité du front de solidification suggère que celui-ci est instable vis-à-vis d'une instabilité morphologique qui se manifeste par la formation d'une zone biphasique où des dendrites solides coexistent avec un liquide enrichi en soluté. Du point de vue thermodynamique, cette couche biphasique pourrait s'étendre jusqu'au centre de la graine, mais il est probable que la convection thermo-solutale associée et la compaction de la matrice solide y réduisent rapidement la fraction liquide. Une seconde partie présente un dispositif expérimental de cristallisation sous forte gravité, permettant d'intensifier les phénomènes de convection liés à la cristallisation. Il y est montré que la convection thermo-solutale rétroagit fortement sur la structure de la zone dendritique, et y augmente significativement la fraction solide. Une dernière partie porte sur les mécanismes susceptibles d'être à l'origine de l'anisotropie. L'importance de l'évolution thermochimique du noyau est mis en avant. Si la graine est relativement âgée, celle-ci est stratifiée de manière stable et les mouvement verticaux y sont interdits. La déformation induite par une croissance hétérogène est focalisée dans une couche cisaillante à la surface de la graine. Si la croissance de la graine est rapide, elle a pu convecter au début de son histoire et être progressivement stabilisée par la stratification chimique

Asymmetric dynamics of the inner core and impact on the outer core

International audienceThe history and present state of knowledge of the dynamics of the inner core are outlined in this paper. The observations that motivated ideas on the dynamical processes are introduced, but the main objective is really to concentrate on the diverse dynamical models that have been and are currently proposed for the formation and evolution of the inner core. A deliberate choice has been made of reproducing key figures from the literature in a didactic attempt to provide clear and quick identification for these models. This review looses impartiality concerning recent models, notably those aiming at explaining the hemispherical asymmetry. A preference for an intrinsic dynamic mode of the inner core is expressed, as opposed to the distant influence of the dynamics of the mantle through heat-flux heterogeneities. Meanwhile, the opinion is conveyed that the dynamics of the inner core is largely not understood yet and that every model must be considered with a critical eye

Thermochemical convection in Earth's inner core

International audienceThe dynamics of Earth's inner core depends critically on whether it is stably stratified or unstably stratified. We propose here a general analysis of the thermal evolution of the inner core. Whether the geotherm in the inner core is superadiabatic or not depends on the inner core solidification rate, on the thermal diffusivity of iron at inner core conditions, and on the ratio of the Clapeyron slope to the adiabatic gradient in the inner core. The temperature field within the inner core can be destabilizing--and could drive convection--if the growth of the inner core is fast enough. The effect of radiogenic heating is probably small, and, perhaps surprisingly, can even stabilize the inner core against convection. The uncertainties are such that it is not possible at present to conclude about the likelihood of thermal convection in the inner core, but recent estimates of the Core-Mantle Boundary (CMB) heat flux and inner core conductivity favour convection. Thermal convection is more likely early in the inner core history, a consequence of the secular decrease in cooling rate of the core. In addition, solidification-induced partitioning of the light elements may induce a stable density stratification within the inner core. We develop a numerical model of thermochemical convection in a growing inner core, which couples the evolution and dynamics of the inner core with the thermal and compositional evolution of the outer core. Melting and crystallization associated with deformation of the Inner Core Boundary (ICB) would be of importance for the style of convection if the viscosity is large, but we focus here on the case of low viscosity for which phase change associated with dynamic topography at the ICB is expected to play a secondary role. In this regime, convection is typical of high Rayleigh number internally heated convection, with cold plumes falling from the ICB. Several possible scenarios can lead to a layered inner core, either because of cessation of thermal convection due to the decrease in cooling rate of the core, or because of a compositional stratification which can confine convection in the deep inner core, or stabilize the whole inner core. For each of these scenarios, it is possible to find plausible sets of parameters (inner core age, viscosity, magnitude of the compositional stratification) for which the radius at which convection stops corresponds to the radius of the seismically inferred innermost inner core