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

PKP-Cdiff Phase

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 > 50 o Higher attenuation of PKP-Cdiff with distance than can be explained by AK135 type velocity gradients

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

Variable  at ICB ReferencesPhasesdistance ( o )density jump (g/cm 3 )Vs contrast (km/s) Koper and Pyle, JGR, 2004PKiKP / PcP0 ~ 500.3(0.2)2.0(0.5) Koper et al., EPSL, 2005PKiKP / P50 ~ (0.24)2.82(0.32) Souriau et al., GJI, 1989PKiKP / PcP0 ~ ~ 1.66 Shear and Masters, GJI, 1990 PKiKP / PcPPcP: 20 ~ 70Body Waves: < 1.0Body Wave: > 2.5 PKiKP / PP: 70 ~ 90Normal Modes: 0.55Normal Modes: 3.45 Normal Modes Cao and Romanowicz, EPSL, 2004PKiKP / PcP10 ~ ~ 0.92 ~ 3 PREM (Dziewonski and Anderson, PEPI, 1981) Normal Modes 0 – AK135 (Kennett et al., GJI, 1995) PKP, PKKPPKP: PKiKP: Masters and Gubbins, PEPI, 2003Normal Modes 0.82(0.18)

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

PKP-AB PREM2PREM2 with glassy F layer Note: differences in PKP-Cdiff decay and PKIIKP amplitude Glassy F Layer PKIKPPKP-CdiffPKIIKPPKIKPPKP-CdiffPKIIKP PKP-AB

Snowing ICB – Solid vs. Liquid

Time (Depth)‏

Snowing ICB – Solid vs. Liquid Solid liquid

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

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

Model Input Density ProfileBulk Modulus Solid Fraction Reuss Averaging Voigt Averaging Ideal Solution Theory ~ PREM value of OC ~ PREM value of IC

Model Output (G = 0)‏

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 More accurate data from mineral physics More accurate model Thermodynamic constraints Geodynamic Constraints? Thinking hard...

Geodynamic Model of the Lowermost Outer Core depth T0T0 T0+ΔTT0+ΔT ρ0ρ0 ρ 0 +Δρ T ρ 0 +Δρ X ρ0ρ0 THERMALLY UNSTABLE COMPOSITIONALLY STABLE X0X0 X0+ΔXX0+ΔX τ ~ 100 gy τ ~ 100 my

Double Diffusive Convection Examples from Oceans K (thermal diffusivity) >> D (molecular diffusivity)‏

Range of DDC Behavior From Turner 1973

DDC Behavior in the Lowermost Outer Core Ra T Ra X STABLE Fingers Unstable Oscillations UNSTABLE X Z W V P Q Le = 1.43x10 -3 Pr = 4.3x10 -2 XW: Ra T ~ -0.04Ra X +680 XZ: Ra T ~ -700Ra X +660 Ra T ~10 25 Ra T /Ra X ~0.2 T~ 100 my

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

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?

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