Birth and decline of magma oceans in planetesimals. Part 2: Structure and thermal history of early accreted small planetary bodies

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Cyril Sturtz, Angela Limare, Stephen Tait, Édouard Kaminski

JGR Planets, First Published: 30 November 2022

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“Key Points

  • We model the thermal and structural evolution of radiogenic heated early accreted planetesimals that experienced an internal magma ocean
  • The efficiency of crystal segregation can vary according to model parameters leading to both well-mixed and layered planetary mantles as possible outcomes
  • Crustal thickening by crystal flotation increases the lifetime of the magma ocean and favors crystal segregation”

“This is the second of two companion papers that present a theoretical and experimental study of the thermal history of planetesimals in which heating by short-lived radioactive isotopes generates an internal magma ocean and the subsequent cooling and crystallization thereof. We study the conditions required to form and preserve basal cumulates and flotation crusts, and the implications for the thermal evolution of planetary bodies. Our model predicts that planetesimals larger than 30km can reach 1300oC and a melt fraction of 40 vol%, producing a solid-like to liquid-like rheological transition that triggers an internal magma ocean. In the magma ocean regime core-mantle differentiation occurs very quickly and the mantle convects under a relic of chondritic material whose thickness is controlled by the temperature of rheological transition. We show that the magma ocean episode is associated with time-dependent crystal segregation and no re-entrainment. Segregation of crystals is essentially constrained by their size and by their density difference with respect to the melt, the latter being fully determined by the planetesimal’s initial composition. Olivine cumulates are likely to form at the core-mantle boundary. Under certain particular conditions, a flotation crust can also form, which reduces the efficiency of heat evacuation by convection, thereby enhancing the magma ocean’s lifetime and the efficiency of crystal segregation. Two types of large-scale mantle structure are possible outcomes: a well-mixed upper mantle above an olivine cumulate, or a more finely layered ”onion-shell” structure.”

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