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Lowermost Outer Core and the ICB

Lowermost Outer Core and the ICB. August 7, 2008. Bin Chen, Vernon Cormier, Shan Dou, Garrett Euler, Lili Gao, David Gubbins, Kuang He, Svetlana Kharlamova, Jie Li, Hongfeng Yang, …. (sorted alphabetically by last names). PKP-Cdiff Phase. Seismic Observations.

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Lowermost Outer Core and the ICB

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  1. Lowermost Outer Core and the ICB August 7, 2008 Bin Chen, Vernon Cormier, Shan Dou, Garrett Euler, Lili Gao, David Gubbins, Kuang He, Svetlana Kharlamova, Jie Li, Hongfeng Yang, … (sorted alphabetically by last names)

  2. PKP-Cdiff Phase

  3. Seismic Observations • Flattened velocity gradient at base of outer core from AK135 travel times and PKP-Cdiff travel times • Variable  at ICB from PKiKP/PcP amplitude ratios (mosaic ICB) and unexplained high amplitude of PKiKP at distances > 50o • Higher attenuation of PKP-Cdiff with distance than can be explained by AK135 type velocity gradients

  4. Flattened Velocity Gradient in the Lowermost Outer Core(F Layer) Zou et al., 2008 (JGR)

  5. Variable  at ICB

  6. High Attenuation of PKP-Cdiff • Volumetric scattering in F layer • Glassy F layer • Bumpy ICB • Viscoelasticity in F layer

  7. Glassy F Layer PREM2 PREM2 with glassy F layer PKP-AB PKP-AB PKIKP PKIIKP PKP-Cdiff PKIKP PKIIKP PKP-Cdiff Note: differences in PKP-Cdiff decay and PKIIKP amplitude

  8. Snowing ICB – Solid vs. Liquid

  9. Time (Depth)‏ Snowing ICB – Solid vs. Liquid

  10. Snowing ICB – Solid vs. Liquid Solid liquid Solid

  11. Mercury's Snowing Core? l s l s l s Liquid Composition Double Snow State Ganymede-like State (Chen et al., 2008, GRL)‏ Why Does it Snow? Solid Composition

  12. Assumptions • Solid particles form at 150 km above ICB, and sinks down • These solid particles contain mainly Fe, and light elements • Adding light elements to Fe decreases the density • Solid particles partially dissolve into the liquid OC • The remaining solid particles contain less light element • The released material from solid particles is denser than the surrounding liquid. • Density of liquid increases with depth

  13. Model Input Density Profile Bulk Modulus Solid Fraction Ideal Solution Theory Reuss Averaging Voigt Averaging ~ PREM value of OC ~ PREM value of IC

  14. Model Output (G = 0)‏

  15. Conclusions and Discussion Conclusion Snow model is possible to explain the Vp anomaly 150 km above ICB, for certain density and bulk modulus profiles Future Perspective Thinking hard ... More accurate data from mineral physics More accurate model Thermodynamic constraints Geodynamic Constraints?

  16. Geodynamic Model of the Lowermost Outer Core COMPOSITIONALLY STABLE τ ~ 100 gy depth X0 X0+ΔX ρ0 ρ0+ΔρX THERMALLY UNSTABLE depth τ ~ 100 my T0 ρ0 ρ0+ΔρT T0+ΔT

  17. Double Diffusive ConvectionExamples from Oceans K (thermal diffusivity) >> D (molecular diffusivity)‏

  18. Range of DDC Behavior From Turner 1973

  19. DDC Behavior in the Lowermost Outer Core V RaT Q Le = 1.43x10-3 Pr = 4.3x10-2 XW: RaT~ -0.04RaX+680 XZ: RaT~ -700RaX+660 RaT~1025 RaT/RaX~0.2 T~ 100 my Unstable Oscillations UNSTABLE W X RaX STABLE Fingers P Z

  20. Effect of Prandtl Number W & V RaT Q Le = 1.43x10-3 Pr = ∞ XW: RaT~ RaX+660 XZ: RaT~ -700RaX+660 RaT~10? RaT/RaX~0.2 UNSTABLE X RaX Infinite Prandtl Number = no inertia = no overstability = stable Finite Prandtl number DDC modeling - oceanographic codes? STABLE Fingers

  21. Formation of Layering with DDC - Theory is poor • - Layers form from lateral variations - Layering is stable - A mechanism for stronger attenuation in the lowermost outer core?

  22. Summary and Outlook • Lowermost Outer Core is anomalous • low Vp gradient • high attenuation • Various ways to model this • Glassy layer, chemical layer, … • Outlook • Geographical variations of density jump and low velocity gradient • Bumpy ICB • Scatterers in lowermost outer core • Thermodynamic calculations • Conservation of energy and mass • Refine density and velocity calculations from mineral physics • Finite Pr modeling

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