On crystallization at the inner core boundary

It is widely believed that a mushy layer develops at the ICB while the outer core material freezes onto the inner core. This view is obtained in reference to metallurgical or analog experiments on solidification carried out in the laboratory where morphological instability almost always follows constitutional supercooling. However, morphological instability is not an automatic consequence of supercooling: supercooling and stable solid-liquid interface may coexist under certain conditions such as might perhaps occur at the ICB. If these conditions are realized, there could be both solidification on the ICB and crystallization of small solid particles in the supercooled layer (slurry layer). We made a linear stability analysis of a unidirectionally advancing solid-liquid interface under the ICB conditions. Several values of liquidus slope against concentration of light material (mc) are used for the stability analysis. It was confirmed that supercooling and interstability may coexist if |mc| is low although this value of |mc| is believed to be too small as the liquidus slope at the ICB. The estimated amount of supercooling in the layer is of order 0.1K, and this provides an estimate of the size and population of solid particles in the slurry layer. However, even though the value of |mc| is small, to sustain such slurry layer at the ICB is unlikely due to (i) the difficulty of continuous supply of enough number of nuclei into the layer and (ii) the Rayleigh-Taylor instability by the compositional buoyancy. It is most probable that mushy layer exists at the ICB