Bulk terrestrial exoplanet compositions, and their effect on mantle evolution

Abstract:

Recent developments have driven the geophysical and astrophysical communities closer together than ever before, in a joint effort to understand terrestrial exoplanets. We aim to contribute to this effort by studying terrestrial exoplanets in light of a property which could vary widely among planetary systems: bulk planet composition. The bulk composition of a terrestrial planet affects many interior properties, including core size, mantle physical properties, and mantle melting behaviour. This may in turn affect the interaction between interior and atmosphere, which makes it potentially important for habitability, and potentially observable once atmospheric characterisation becomes available.

Here, we aim to study the geodynamical behaviour of terrestrial planets with a variety of bulk compositions. We base compositions on our previous work, where we constrain the bulk terrestrial exoplanet compositional range based on stellar abundances from the Hypatia catalogue [Hinkel+, 2014]. We considered a simplified model of condensation and rock-metal differentiation to constrain bulk planet and bulk silicate compositions of exoplanets in the habitable zones of stars in the Solar neighbourhood [Spaargaren et al., in prep.]. A number of end-member compositions that span this compositional is translated to mantle mineralogical profiles, using Perple_X [Connoly, 2005]. Associated physical properties of mantle materials are prescribed in a geodynamical model of mantle evolution. For this end, we use StagYY [Tackley, 2008], to model mantle convection in a 2D spherical annulus geometry. We present mantle evolution for the full life-time of a terrestrial planet (10 Gyr), for all of our end-member compositions. We thus hope to shed some light on how bulk composition of a terrestrial exoplanet affects the interior thermal and compositional evolution. We pay special attention to dynamical behaviour of the lithosphere (i.e., whether it develops plate tectonics-like behaviour, or whether it remains in a stagnant lid regime), since this behaviour has great consequences for interior thermal evolution, and interaction between the interior and the atmosphere. Our future work will focus on coupling this model with volatile exchange between mantle, lithosphere, and atmosphere.