Exploring the effects of terrestrial exoplanet bulk composition on long-term planetary evolution

New terrestrial exoplanets are being discovered at an ever faster pace, and each discovery leads to a widening of our understanding of planetary diversity. A key aspect in the quest to better quantify terrestrial planet diversity is to gain information on plausible bulk compositions, as this physical-chemical quantity determines the planet's structure, which in turn controls physical properties of the its layers (core, mantle, crust, atmosphere). Recent insights in the expected range of bulk planet compositions allow us to investigate how this fundamental parameter affects the evolution of the planetary interior and surface, and consequently to guide next-generation ground- and space-based telescopic observations of exoplanet properties, such as atmospheric composition.

Here, we first simulate mantle mineralogies for exoplanets with various bulk compositions, using a Gibbs energy minimization algorithm, Perple_X. Using mineralogy and resulting physical properties, we employ a 2D global-scale model of thermochemical mantle convection to investigate the variations between Earth-sized exoplanets of different compositions in terms of interior evolution. We include the effects of composition on planet structure, mantle physical properties, and mantle melting. We investigate how composition affects thermal evolution, and whether it has an effect on the propensity of a planet towards plate tectonics-like behaviour.

In general, Earth tends to have an average composition for most elements, except for iron, which it is relatively rich in, and therefore it has an above average core size. Our preliminary results show that core size (and thus iron abundance) affects convective vigor, and thus thermal evolution of the interior. We further find major differences for planets with different ratios of Mg-silicates, as these minerals control mantle viscosity, and thereby thermal evolution. Planets with lower Mg/Si than Earth will have a significantly stronger mantle, impeding cooling on planetary lifetimes, while planets with much higher Mg/Si have weaker upper mantles, impacting surface mobility. Stellar Mg/Si is a good indicator of the relative abundances of these minerals, and can be an important source of information. Therefore, the host stellar abundances seem to be an indicator of rocky planet properties, and can be used in the target selection for future missions.