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Using ab initio simulations on Fe-Ni-S-C-O-Si liquids, we constrain the origin and composition of the low-velocity layer E at the top of Earth's outer core. We find that increasing the concentration of any light element always increases velocity and so a low-velocity and low-density layer (for stability) cannot be made by simply increasing light element concentration. This rules out barodiffusion or simple sedimentation of a light phase for its origin. However, exchanging elements candepending on the elements exchangedproduce such a layer. We evaluate three possibilities. First, crystalization of a phase from a core may make such a layer, but only if the core contains more than one light element and only if crystalizing phase is very Fe rich. Second, the E layer may result from incomplete mixing of an early Earth core with a late impactor, depending on the light element compositions of the impactor and Earth's core. Third, using thermodynamic models for metal-silicate partitioning, we show that a reaction between the core and an FeO-rich basal magma ocean can result in a light and slow layer.

Plain Language Summary The Earth's outer core is mostly made of liquid iron at extremely high temperatures (up to similar to 6000 K) together with a small amount of other elements such as oxygen or silicon. The high temperatures generate vigorous convection and so the core is generally considered to be well mixed. Nevertheless, evidence from seismology as far back as the 1980s show that there is a layer at the top of the Earth's core of a hundred kilometers or so thick. We call this the E layer. There are a number of ideas of what this layer is made of and how it formed, but the data on the properties of iron and its light elements has not been available to test these ideas. In this paper we use recent ab initio data to rule out many ideas for its formation. We also show that the properties of the E layer can be explained as (a) reaction between the liquid iron core and the rocky mantle above it, (b) the incomplete mixing of an early Earth core with a late impactor, or (c) the residue from crystallisation of a very Fe-rich phase at the top of the core.