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Core formation efficiency for rocky super-Earths

For rocky planets it is typically assumed that they are able to differentiate into a silicate mantle and a metal core, due to the fact that metals are denser than silicates and should sink towards the gravitational center, i.e. the core of the planet.

However, more massive planets experience a higher pressure and compressibility in their interior, which can strongly impact the differentiation of the planet, potentially leading to inefficient core formation and even to coreless planets (Elkins-Tanton and Seager, 2008; Lichtenberg, 2021). By using a mantle convection code, we show that even over geological timescales, and depending on the size and distribution of iron droplets forming during the magma ocean crystallisation or later on due to phase transition disproportionation in the silicate mantle, the iron may indeed never be able to sink to the centre of the planet to form a metal core. Since the ability to form a (large) core should decrease with increasing planetary mass, this study suggests that the mantle of super-Earths may be more iron-rich and therefore more reducing than for Earth, which would be reflected in their atmospheric composition and could potentially be confirmed by future observations of exoplanetary atmospheres.

Elkins-Tanton and Seager, 2008, Coreless Terrestrial Exoplanets, ApJ 688, 628-635
Lichtenberg, 2021, Redox Hysteresis of Super-Earth Exoplanets from Magma Ocean Circulation, ApJL 914:L4


Lena Noack1
1Freie Universität Berlin, Germany
GeoBerlin 2023