Formation of a solid inner core during the accretion of Earth

• Published in: Journal of geophysical research. Solid earth 2017-05, Vol.122 (5), p.3248-3285
• Main Author: Arkani‐Hamed, Jafar
• Format: Article
• Language: English
• Subjects: Accretion; accretion of Earth from Moon to Mars‐sized bodies; Adiabatic flow; Compressibility; Cores; Deposition; Earth core; Earth gravitation; Earth models; Embryos; Evolution; Geophysics; inner core solidification during Earth's accretion; Mars; Mars satellites; Mathematical models; Melting; Moon; Physical properties; Planetary cores; Pressure; Solidification; Symmetry; Temperature; Temperature distribution; Temperature effects; Temperature rise; Thermal evolution; Velocity
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1002/2016JB013601

The formation of an inner core during the accretion of Earth is investigated by using self‐gravitating and compressible Earth models formed by accreting a total of 25 or 50 Moon to Mars‐sized planetary embryos. The impact of an embryo heats the proto‐Earth's interior differentially, more below the impact site than elsewhere. The rotating core dynamically overturns and stratifies shortly after each impact, creating a spherically symmetric and radially increasing temperature distribution relative to an adiabatic profile. Merging of an embryo to the proto‐Earth increases the lithostatic pressure that results in compressional temperature increase while further enhances the melting temperature of the core causing solidification. A total of 36 thermal evolution models of the growing proto‐Earth's core are calculated to investigate effects of major physical parameters. No solidification is considered in the first 21 models where modified two‐body escape velocities are used as the impact velocities of the embryos. At the end of accretion, temperatures in the upper part of the core are significantly different among these models, whereas temperatures in the deeper parts are similar. The core solidification considered in the remaining 15 models, where impact velocities higher than the modified two‐body escape velocities are adopted, drastically changes the temperature distribution in the deeper parts of the core. All of the models produce partially solidified stiff inner cores, 1000–2100 km in radius, at the end of accretion, where the solid fraction is larger than 50%. The innermost of the stiff inner cores is completely solidified to radii 250–1500 km. Plain Language Summary This article shows that if Earth is formed by the accretion of large Moon to Mars‐sized bodies, then it is highly likely that a solid inner core was created during the accretion. Key Points Thermal evolution of Earth's core during accretion Inner core formation during the accretion of the Earth Innermost part of the core of Earth is formed during accretion