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Abstract The accretion of Earth and the formation of a metallic core released a large amount of primordial heat and may have enabled its evolution into a habitable world. Metal‐silicate segregation likely occurs in super‐Earth exoplanets as well, but its influence on their initial thermal states has not been fully examined. Here we calculated the energy released during core‐mantle differentiation of super‐Earths for a range of planet radii and core mass fractions. We found that the energy of differentiation increases with planet mass for rocky planets with Earth‐like composition, and it peaks at 55% core by mass in Earth‐sized rocky planets. Using the latest mineral physics constraints on the equations‐of‐state and melting curve of relevant phases, we modeled the initial thermal profiles and assessed the extent of melting in initial iron cores for plausible heat retention efficiencies. Our results suggest that following accretion and metal‐silicate differentiation, the cores of most super‐Earths are expected to be at least partially molten, a necessary condition for the generation of a magnetic field. Based on the largely molten state of Earth's core at the present day, we place a lower bound of 7% retention of accretional energy as primordial heat in rocky planets.
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Vestiges of impact-driven three-phase mixing in the chemistry and structure of Earth’s mantle
Highly siderophile elements (HSEs; namely Ru, Rh, Pd, Re, Os, Ir, Pt, and Au) in Earth’s mantle require the addition of metals after the formation of Earth’s core. Early, large collisions have the potential to deliver metals, but the details of their mixing with Earth’s mantle remain unresolved. As a large projectile disrupts and penetrates Earth’s mantle, a fraction of its metallic core may directly merge with Earth’s core. Ensuing gravitational instabilities remove the remaining projectile’s core stranded in Earth’s mantle, leaving the latter deprived of HSEs. Here, we propose a framework that can efficiently retain the metallic components during large impacts. The mechanism is based on the ubiquitous presence of a partially molten region in the mantle beneath an impact-generated magma ocean, and it involves rapid three-phase flow with solid silicate, molten silicate, and liquid metal as well as long-term mixing by mantle convection. In addition, large low-shear-velocity provinces in the lower mantle may originate from compositional heterogeneities resulting from the proposed three-phase flow during high-energy collisions.
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- PAR ID:
- 10509974
- Publisher / Repository:
- PNAS
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 120
- Issue:
- 43
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.2309181120
