The initial condition for the long-term evolution of terrestrial planets

Theme: Solid Earth Dynamics

Primary Supervisor:

Maxim Ballmer

Earth Sciences, UCL

Maxim Ballmer's Profile Picture

Project Description:

Early in their history, terrestrial planets evolve through stages of large-scale melting, or magma oceans, due to the energy release during accretion and differentiation. Any magma ocean is thought to become progressively enriched in FeO and incompatible elements during fractional crystallization. The resulting upwards enrichment of the cumulate (=crystal) package(s) drives gravitational over-turn(s) of the incipiently solid mantle, and ultimately stabilizes a FeO-enriched molten layer at the core-mantle boundary (CMB), or “basal magma ocean” (BMO). The BMO itself will freeze by fractional crystallization, ultimately stabilizing a thick FeO-enriched layer at the CMB. Such a layer, however, would be too dense to be entrained by mantle convection, a scenario that is ruled out by geophysical observations, at least for Earth.

In this project, we will investigate the consequences of a previously neglected mechanism, BMO reactive crystallization, on long-term planetary evolution. Reaction is driven by chemical disequilibrium between the mantle and BMO. The related BMO reactive cumulates should range from Mg-enriched bridgmanite (MgSiO3) to FeO-enriched pyrolite, but the detailed compositions will be calculated using available thermodynamic models. The long-term thermochemical evolution of the mantle (e.g., fate of the cumulate package) will be addressed by geodynamic modeling. The predicted thermochemical mantle structures will be compared to lower-mantle seismic signature of Earth, using available constraints for physical properties of mantle materials at high pressure-temperature conditions. Finally, results will be applied to terrestrial planets in general. Such an effort is expected to yield systematic relationships between planet mass/composition, deep-mantle structure and long-term thermal evolution.

Policy Impact of Research:

On Earth, long-term material cycles through the mantle set up life-sustainable conditions at the surface. This cycling is controlled by the initial condition after planetary accretion and differentiation. Indeed, studying mantle evolution is key to understand the conditions for habitability, and gauge the potential for extraterrestrial life in our galaxy.

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