Accretion and differentiation of the terrestrial planets with implications for the compositions of early-formed Solar System bodies and accretion of water

David Rubie, Seth Jacobson, Alessandro Morbidelli, David O’Brien, Edward Young, Jellie de Vries, Francis Nimmo, Herbert Palme, Daniel Frost
Icarus 248, 89–108 (2015)

The formation of Earth, Venus and Mars and core-mantle differentiation of these planets are modelled by combining six N-body accretion simulations with the multi-stage core formation model of Rubie et al. (2011, Earth and Planetary Science Letters 301, 31-42). The planets grow through impacts with smaller bodies (Mars-size embryos and smaller planetesimals) that cause deep melting, magma ocean formation and episodes of core formation. Modelling the chemical consequences of core formation is based on experimental studies of how elements partition between liquid metal and silicate at high pressure combined with chemical mass balance. The aim is to produce a model Earth with a mantle that has the chemical composition of the Earth’s primitive mantle. This approach requires that the chemical compositions of starting bodies in the proto-planetary disk are defined in terms of oxygen and water contents. Results show that bodies that formed close to the Sun were highly reduced (low oxygen contents with almost all Fe present as metal) whereas with increasing distance from the Sun, compositions were increasingly oxidized. Beyond 6-7 astronomical units (AU), bodies contain 20 wt% water ice (see figure). By performing least-squares fits, model Earths are produced with realistic primitive mantle compositions.

The results show that metal-silicate segregation occurred at pressures around 70% of the core-mantle boundary pressure at the time of each accretional impact. Water was added to the Earth’s interior after around 60-80% of the planet had formed. The results predict that Earth’s iron-rich core contains 8–9 wt% silicon, 2–4 wt% oxygen and 10–60 ppm hydrogen.

This contribution is the subject of an article by Jeff Taylor at the University of Hawaii:

Compositions of starting bodies in the proto-planetary disk as a function of distance from the Sun. Close to the Sun, almost all iron and about 20% of available silicon are present as metal. With increasing distance, iron becomes increasingly oxidized and beyond 6-7 AU, bodies are fully oxidized, contain 20 wt% water and no metal. The red arrows show parameter values that were adjusted during least-squares fitting.

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