Renaud Deguen

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

In addition to its global North–South anisotropy, there are two other enigmatic seismological observations related to the Earth’s inner core: asymmetry between its eastern and western hemispheres and the presence of a layer of reduced seismic velocity at the base of the outer core. This 250-km-thick layer has been interpreted as a stably stratified region of reduced composition in light elements. 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.

Turbulent metal–silicate mixing, fragmentation, and equilibration in magma oceans

Abstract Much of the Earth was built by high-energy impacts of planetesimals and embryos, many of these impactors already differentiated, with metallic cores of their own. Geochemical data provide critical information on the timing of accretion and the prevailing physical conditions, but their interpretation depends critically on the degree of metal–silicate chemical equilibration during core–mantle differentiation, which is poorly constrained. Efficient equilibration requires that the large volumes of iron derived from impactor cores mix with molten silicates down to scales small enough to allow fast metal–silicate mass transfer. Here we use fluid dynamics experiments to show that large metal blobs falling in a magma ocean mix with the molten silicate through turbulent entrainment, with fragmentation into droplets eventually resulting from the entrainment process. In our experiments, fragmentation of the dense fluid occurs after falling a distance equal to 3–4 times its initial diameter, at which point a sizable volume of ambient fluid has already been entrained and mixed with the dense falling fluid. Contrary to previous assumptions, we demonstrate that fragmentation of the metallic phase into droplets may not be required for efficient equilibration: turbulent mixing, by drastically increasing the metal–silicate interfacial area, may result in fast equilibration even before fragmentation. Efficient re-equilibration is predicted for impactors of size small compared to the magma ocean depth. In contrast, much less re-equilibration is predicted for large impacts in situations where the impactor core diameter approaches the magma ocean thickness.

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

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

\eta

Rayleigh–Bénard convection in a creeping solid with melting and freezing at either or both its horizontal boundaries

is large enough, with a critical viscosity value, of order

Structure and dynamics of Earth's inner core

3 10^{18}

Controls on geomagnetic reversals and core evolution by mantle convection in the Phanerozoic

Pas, depending on the ability of outer core convection to supply or remove the latent heat of melting or solidification. If

Experiments on turbulent metal-silicate mixing in a magma ocean

\eta

Eccentricity of the geomagnetic dipole caused by lopsided inner core growth

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.

Thermochemical convection in Earth’s inner core

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.