Lowermost Outer Core and the ICB

Lowermost Outer Core and the ICBBin 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 > 50o

• 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

References Phases distance (o) density jump (g/cm3) Vs contrast (km/s)

Koper and Pyle, JGR, 2004 PKiKP / PcP 0 ~ 50 0.3(0.2) 2.0(0.5)

Koper et al., EPSL, 2005 PKiKP / P 50 ~ 90 0.52(0.24) 2.82(0.32)

Souriau et al., GJI, 1989 PKiKP / PcP 0 ~ 45 1.35 ~ 1.66

Shear and Masters, GJI, 1990

PKiKP / PcP PcP: 20 ~ 70 Body Waves: < 1.0 Body Wave: > 2.5

PKiKP / P P: 70 ~ 90 Normal Modes: 0.55 Normal Modes: 3.45

Normal Modes

Cao and Romanowicz, EPSL, 2004 PKiKP / PcP 10 ~ 70 0.6 ~ 0.9 2 ~ 3

PREM (Dziewonski and Anderson, PEPI, 1981)

Normal Modes 0 – 180 0.60 3.5

AK135 (Kennett et al., GJI, 1995)PKP, PKKP PKP: 100-180 0.565 3.5

PKiKP: 80-120

Masters and Gubbins, PEPI, 2003 Normal 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

PREM2 PREM2 with glassy F layer

Note: differences in PKP-Cdiff decay and PKIIKP amplitude

Glassy F Layer

PKIKP PKP-CdiffPKIIKP PKIKP PKP-CdiffPKIIKP

PKP-AB

Snowing ICB – Solid vs. Liquid

Snowing ICB – Solid vs. Liquid T

ime (D

epth

)

Snowing ICB – Solid vs. Liquid

Solid

Solid

liquid

Why Does it Snow?

Mercury's Snowing Core?

Double Snow State

Ganymede-like State

(Chen et al., 2008, GRL)

l s

sl

sl

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 Profile Bulk ModulusSolid Fraction

Reuss Averaging

Voigt Averaging

Ideal Solution Theory

~ PREM value of OC

~ PREM value of IC

Model Output

(G = 0)

Conclusions and Discussion

ConclusionSnow model is possible to explain the Vp anomaly 150 km above ICB, for certain density and bulk modulus profiles

Future PerspectiveMore accurate data frommineral physics More accurate modelThermodynamic constraints

Geodynamic Constraints?

Thinkinghard ...

Geodynamic Model of the Lowermost Outer Core

depth

T0 T0+ΔT

depth

ρ0 ρ0+ΔρT

ρ0+ΔρXρ0

THERMALLY UNSTABLE

COMPOSITIONALLY STABLE

X0 X0+ΔX

τ ~ 100 gy

τ ~ 100 my

Double Diffusive ConvectionExamples from Oceans

K (thermal diffusivity) >> D (molecular diffusivity)

Range of DDC Behavior

From Turner 1973

DDC Behavior in the Lowermost Outer CoreRaT

RaX

STABLE

Fingers

UnstableOscillations

UNSTABLE

X

Z

W

V

P

QLe = 1.43x10-3

Pr = 4.3x10-2

XW: RaT~ -0.04RaX+680XZ: RaT~ -700RaX+660

RaT~1025

RaT/RaX~0.2

T~ 100 my

Effect of Prandtl Number

Infinite Prandtl Number = no inertia = no overstability = stable

Finite Prandtl number DDC modeling - oceanographic codes?

RaT

RaX

STABLE

Fingers

UNSTABLE

X

W & V

QLe = 1.43x10-3

Pr = ∞

XW: RaT~ RaX+660XZ: RaT~ -700RaX+660

RaT~10?

RaT/RaX~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