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Rheological Structure of the Mantle of Super-Earths : Insights from Mineral Physics

Explore the dynamics of mantle convection and thermal evolution in super-Earths, and investigate the possibility of plate tectonics and dynamo operation. Understand the role of tidal heating and orbital evolution in these planets, and study the migration of exoplanets since their formation. Analyze the rheological properties and internal structure of super-Earths' mantle, including the B1 to B2 transition and the role of MgO. Investigate the viscosity-mass relationship and the effects of temperature and pressure.

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Rheological Structure of the Mantle of Super-Earths : Insights from Mineral Physics

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  1. Rheological Structure of the Mantle of Super-Earths: Insights from Mineral Physics Shun-ichiro Karato Yale University Department of Geology and Geophysics New Haven, CT, USA

  2. Dynamics of a super-Earth mantle convection, thermal evolution Does plate tectonic operate on super-Earths? Does dynamo operate in super-Earths? tidal heating orbital evolution How much have exo-planets migrated since their formation? Rheological properties

  3. Internal structure of a super-Earth P to ~ 1 TPa (1000 GPa) T to ~5000 K

  4. Viscosity of planetary materials depends strongly on T and P. P-effect is potentially very large!

  5. Viscosity-mass relationship Viscosity of solids increases with P at low P. Is this valid at higher P in super-Earths?

  6. conventional models new model

  7. Internal structure of a super-Earth (B1 B2 transition) (dissociation of post-perovsktie (?), Metallization (?)) MgO is the softest phase in a super-Earth’s mantle.

  8. MgO (V* decreases with depth (pressure))

  9. Viscosity changes also with crystal structure. normalized temperature MgO (B1 or B2) is the softest mineral in the deep mantle. normalize viscosity B1

  10. Materials with B1 structure are the softest among various oxides. Materials with B2 structure are even softer than those with B1 structure. B2 B1

  11. Viscosity changes when mechanisms of atomic motion change. V*vacancy >0 V*interstitial <0 vacancy mechanism interstitial mechanism (from (Ito and Toriumi, 2007)) (from Karato (1978))

  12. B1

  13. Conclusions • Viscosity of the deep mantle of a super-Earth might decrease with pressure. • the Rayleigh number increaseswith planetary mass • reduces plate thickness, increases convective stress with planetary mass making plate tectonics possible in large planets, which would otherwise be difficult. • high tidal energy dissipation

  14. (low viscosity  higher heating rate, faster orbital evolution)

  15. Could plate tectonics operate on a super-Earth? How does resistance and driving force for plate tectonics change with planetary mass? resistance: plate thickness  Rayleigh number driving force: convective stress  Rayleigh number A large Rayleigh number  high stress, thin planet  promote plate tectonics How does the Rayleigh number change with planetary mass? P-effect on viscosity is often ignored. Is it justifiable? (Valencia et al., 2007)

  16. P to ~1 TPa (1000 GPa) T to 5000 K

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