Differentiation of silicates and iron during formation of Mercury and high-density exoplanets

Recent MESSENGER observations confirmed that Mercury is dominated by a disproportionally large iron-rich core, and also showed a surprisingly high content of a moderately volatile element K on the planet's surface. This latter observation challenges several popular models for the iron-rich composition of the planet that invoke extreme heating of the Mercury's surface. Here we examine mechanisms by which such high-density planets can form in the context of the Tidal Downsizing (TD) model, in which planet formation takes place inside massive gaseous fragments born by the self-gravitational instability in the outer cold disc. The fragments migrate inward and are tidally disrupted in the inner ∼1 au disc region. Grains settle inside the fragments into dense planetary-mass cores before the disruption of the fragments. The disruption leaves behind terrestrial-like proto-planets, which are all that remains after the gaseous component of the fragment is dispersed and accreted by the parent star. In our model for the proto-Mercury in particular, iron grains are assumed to have higher mechanical strength than silicate grains, and the strength of the latter varies depending on the temperature of the surrounding gas according to experimental data. We present two exploratory calculations that contrast grain sedimentation inside a relatively low temperature gas clump, T ≈ 700 K, where silicate grains are amorphous and thus ‘weak’, to that in a higher temperature clump, T ≈ 1200 K, where the silicates are sintered and ‘strong’. In the weak silicate case, iron grains are found to sediment faster than the silicate grains due to their higher mechanical strength. An iron-dominant low-mass proto-planet is left behind in this scenario when the host fragment is destroyed by tides and the silicates are accreted (together with the gas) by the Sun. Since Mercury formation in this model occurs before Solar Nebula dispersal, volatile elements such as K could then be accreted from the Nebula on the surface of the planet, potentially explaining MESSENGER's observations. In the hotter strong silicate fragment, however, both silicates and iron sediment into the proto-cores. This leaves behind more massive (≲1 M⊕) silicate-dominated planets. Comments on what this model implies for the massive cores of Uranus/Neptune and high-density exoplanets such as CoRot-7b, Kepler-10b and Kepler-36b are made.