Hakim, Kaustubh

PhD thesis supervised by Dominik, Carsten; van Westrenen, Wim (University of Amsterdam)

Abstract: Approximately one-third of about four thousand known exoplanets are possibly rocky in nature. The chemical and physical diversity of rocky exoplanets is well beyond that of the rocky planets of our solar system; Mercury, Venus, Earth, and Mars. In this thesis, we investigate the mineralogy, structure, evolution, and habitability of rocky exoplanets by implementing laboratory and computational tools from geosciences. We derive a new ab initio equation of state of iron using density functional theory valid at and beyond the core pressures of super-Earths. We show that extrapolations of equations of state to pressures beyond their validity range result in significant errors in estimating the interior properties of planets. We perform high-pressure high-temperature experiments on chemical mixtures representative of the bulk composition of carbon-rich rocky exoplanets and find that graphite (and diamond, depending on pressure) is the dominant carbon-bearing mineral. Such exoplanets likely contain an iron-rich core, a silicate mantle, and a graphite outer shell. We also show that silicon carbide, another carbon-bearing mineral thought to be an important constituent of these exoplanets, is stable only if all oxidized iron in the planetary interior is reduced to its metallic state. Using a parameterized mantle convection model, we compute the thermal evolution of rocky exoplanets with a graphite outer shell and find that the graphite shell exhibits a thermal shielding effect resulting in a slower cooling of the planet. A priori these graphite-shell exoplanets are likely uninhabitable because of a lack of life-building elements at their surface other than carbon, however thin graphite shells might still allow habitability.

ISBN: 9789463234191
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