From bulk compositions to interior structure models: Why does it matter where the iron ends up?

Abstract:

When the first models of the interior state of rocky exoplanets where developed, typically not more information than the observed mass and radius (if at all) of the planets were employed, leading to studies of Earth-like planets with variations in for example planet size or core-to-planet ratio. Meanwhile, a robust theoretical and observational basic framework was developed connecting the stellar spectra to the likely compositions of planetary building blocks considering condensation temperatures of different species and minerals, devolatilization trends, accretion models -- allowing for a fair estimate of the possible compositional range of specific exoplanets. But one crucial information is still typically not obtained by these models - the state of differentiation of the planet, in another words if a full or partial differentiation into a silicate mantle and iron core occurred, or if the mantle is strongly enriched in iron.

However, the differentiation efficiency plays an important role for the later evolution of a rocky planet - in the core, the mantle, and at the surface of the planet. The pressures and temperatures in the core (especially close to the core-mantle boundary) strongly vary with differentiation efficiency, if a core forms at all, and therefore influences if the core can be (partly) liquid and lead to dynamo activity. Mantle properties depend on the exact mineralogy: more iron in the mantle makes the mantle rocks thermally less conductive (but with higher electrical conductivity), denser, less viscous, and more easy to melt. We will discuss the implications of iron differentiation for the long-term evolution of core and mantle.