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Core formation sets the thickness, composition and surface evolution of rocky planet crust

Presentation #0705 in the session “Planetary Interiors”.

Published onMar 17, 2021
Core formation sets the thickness, composition and surface evolution of rocky planet crust

The search for habitable worlds outside of our solar system requires constraining the geological characteristics of rocky planets, such as their bulk chemistry and degree of internal differentiation. Evidence from our solar system indicates that the amount of iron that enters a planet’s core directly affects the bulk composition of the surrounding silicate mantle, which in turn defines the petrophysical properties of derivative crusts. To explore how the extent of core formation influences the thickness and chemical make-up of a planet’s crust, we used a Gibbs free energy minimisation procedure that simulated adiabatic mantle decompression melting and crust production in chondritic planets with core mass fractions between 0.34 and 0.16. We found that the core mass fraction that develops during planetary accretion and differentiation exerts a first-order control on the thickness and mineralogy of juvenile oceanic crust, which affects the planet’s surface geodynamic behaviour. Planets with large core mass fractions (≥0.32) and iron-poor mantles (about 0-4 wt. % FeO; cf. Mercury) generate a thin, feldspar-rich crust that can carry relatively little water during burial or subduction into the planet’s interior. By contrast, rocky planets with smaller core mass fractions (≤0.24) and iron-rich mantles (up to 25 wt. % FeO; cf. Mars) develop olivine- and pyroxene-rich crusts that are thick and vertically stratified. These minerals readily weather at planetary surfaces to incorporate volatiles and stabilise hydrous minerals during metamorphism that can effectively transport water from the hydrosphere to mantle depths. Additionally, the thicker crust and deeper underlying melt-depleted mantle generated on these planets would result in a more buoyant lithosphere that is more difficult to subduct, and hence cannot undergo the densification necessary to sustain plate tectonics. As a planet ages and cools, the influence of core mass fraction on both crust thickness and hydration capacity become more pronounced, underscoring the sustained role of core formation in setting the surface evolution of rocky planets. Future exploration for terrestrial exoplanets should therefore consider core mass fraction and bulk-mantle chemistry as a primary control on potential habitability.

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