core beliefs - isterre · core beliefs: progress and puzzles on earth’s core and its dynamo les...

Peter OlsonJohns Hopkins University

University of New Mexico

Core Beliefs:Progress and Puzzles on Earth’s Core and its Dynamo

Les HouchesOctober 2017

Planetary Cores across the Solar System

Some Fundamental Questions about Earth’s Core

1. What is the composition of the core, esp. its light element composition?

2. How fast is the core cooling?

3. What is the age of the solid inner core? Why is it anisotropic?

4. Can we quantify critical transport properties in the core:

electrical & thermal conductivity, viscosity, etc.

5. How is the geodynamo maintained, at present and in the deep past?

Note: you might think that most of these have been answered!

Hirose et al. Ann Rev EPS 2013

Light elements in the outer core: 65 years without consensus

Light element composition is vital for:

(1) dynamo energetics

(2) core formation processes

Cumulative number of supporting articles

References: 1978: [Loper, 1978]; 1979: [Gubbins et al., 1979]; 1983: [Stacey and Loper, 1983]; 1984: [Stacey and Loper, 1984]; 1992: [Buffett et al., 1992]; 1995: [Lister and Buffett, 1995]; 1996: [Buffett et al., 1996]; 2002a: [Anderson, 2002]; 2002b: [Labrosse, 2002]; 2003: [Buffett, 2003]; 2004: [Gubbins et al., 2004]; 2005: [Hernlund et al., 2005]; 2006a: [Zhong, 2006]; 2006b: [Lay et al., 2006]; 2007a: [van der Hilst et al., 2007]; 2007b: [Stacey and Loper, 2007]; 2008: [Korenaga, 2008]; 2009: [Tateno et al., 2009]; 2010: [Nakagawa and Tackley, 2010]; 2011: [Wu et al., 2011]; 2012: [Pozzo et al., 2012]; 2013: [Gomi et al., 2013]; 2014: 2015a: [Gubbins et al., 2015]; 2015b: [Olson et al., 2015]; 2015c: [Nimmo, TGP2, 2015]..

References: 1978: [Loper, 1978]; 1979: [Gubbins et al., 1979]; 1983: [Stacey and Loper, 1983]; 1984: [Stacey and Loper, 1984]; 1992: [Buffett et al., 1992]; 1995: [Lister and Buffett, 1995]; 1996: [Buffett et al., 1996]; 2002a: [Anderson, 2002]; 2002b: [Labrosse, 2002]; 2003: [Buffett, 2003]; 2004: [Gubbins et al., 2004]; 2005: [Hernlund et al., 2005]; 2006a: [Zhong, 2006]; 2006b: [Lay et al., 2006]; 2007a: [van der Hilst et al., 2007]; 2007b: [Stacey and Loper, 2007]; 2008: [Korenaga, 2008]; 2009: [Tateno et al., 2009]; 2010: [Nakagawa and Tackley, 2010]; 2011: [Wu et al., 2011]; 2012: [Pozzo et al., 2012]; 2013: [Gomi et al., 2013]; 2014: 2015a: [Gubbins et al., 2015]; 2015b: [Olson et al., 2015]; 2015c: [Nimmo, TGP2, 2015]; 2015d: [Biggin et al., 2015].

References: Bullard, 1949; Keeler, 1971; Stacey, 1977a; Mattasov, 1977b; Stacey & Anderson, 2001; Bi et al., 2002; Stacey & Loper, 2007; Gomi et al., 2012a; de Koker et al., 2012b; Seagle et al., 2013; Pozzo et al., 2014; Zhang et al. 2015

Outer Core Thermal Conductivity Estimates

Nature, June 2016:Ohta et al.

Konôpková et al.

20

16

• Energetics: Heat loss to the mantle (8-16 TW) + chemical differentiation via inner core solidification (~2x106 kg/s) drive thermo-chemical convection in the molten outer core.

• Dynamics:: Convection in the outer core induceselectric currents that sustain the geomagnetic field.

• Timescales:

Outer Core: magnetic diffusion td ~ 400 kyr;convective turnover time tc~ 200 yr; Rm= td/tc ~ 2000 ( Rmcritical ~ 40)

Mantle: Wilson cycle ~ 200 - 400 Myr.

Inner Core: Growth time O(1000) Myr

• Implications:

1) Core cooling drives the geodynamo today

2) Alternative energy sources in the deep past

The Geodynamo Process

CMB

Thermo-chemical Evolution of the Core: the “Standard Model”

Olson, Science 2013; Labrosse, PEPI 2015

Predicted Core Evolution with Qcmb = 13 TW

Landeau et al., EPSL 2017

Inner Core Nucleation (ICN) Age Predictions in Ma

Dynamo status:

Adiabatic core heat flow:

Mantle GCMs

Olson G^3 2016

Olson, Phys. Today (2013)

Core Evolution Stages

Q: Can we find this transition in the paleomagnetic record?

Pre-ICN Post-ICN

Driscoll, GRL 43, 2016; Biggin et al. Nature 526, 2015

Before ICN: Precipitation-driven convection in the core?

Iron rain/Iron snow Mg, Si precipitation

Mercury:Dumberry & Rivoldini, Icarus 248 (2015)

Ganymede:Christensen, Icarus 247 (2015)

Early Earth:O’Rourke & Stevenson, Nature 529 (2016)Nimmo et al., Nature (2016)etc, etc.

Where is the Frontier?

Juno Psyche

More urgent (my humble opinion): a discovery mission to Earth’s core!

Kepler-62e

A Mission to Earth’s Core:

THE Best Natural Laboratory

for

Understanding Planetary Cores & Dynamos

CMB

Earth’s Core has it all:

• Convecting liquid outer core

• Multi-element composition

• Heterogeneous solid inner core

• Active dynamo

• Control by mantle dynamics

• Long paleomagnetic history

Why a Core Mission? Progress by small working teams has not provided

definitive answers to long-standing questions about Earth’s core or the cores

of other planets.

Mission Structure: A comprehensive approach by an international team led by a

Principal Investigator. Scientific objectives, participants, and instrumentation are

competitively selected.

Mission Objectives:

1. Determine structure, state, and composition of Earth’s core

2. Determine how a planetary dynamo works, how it started and evolved

3. Understand interactions with Earth’s mantle, crust, ocean, and atmosphere

4. Drive new technology

The Challenge of Inner Core Anisotropy

BCC FCC

DVp~3%

HPC

+ =

Lincot et al., GRL (2015; 2016)

40K ton closed die press

Shultz Steel

South Gate, CA

80K ton press

Deyang, China

High Pressure – Large Volume Technology

Possible Core Array Locations (for m>6.0 & z>100 km eqks)

N America E Asia

Brazil N Africa

Dynamo Equations

(w/ Thermo-chemical convection)

Navier-Stokes:

Magnetic Induction:

Continuity: Co-density Transport:

Co-density (temperature + light elements):

Self-sustaining dynamo: starting with a weak seed magnetic field,

maintain a strong (reversing) B indefinitely

Dynamo Parameters

----- Inputs -------

Earth’s Core Dynamo Models

(Numerical, Laboratory)

E (Ekman #) rotational

constraint

10-9 (turbulent)

10-13 (laminar)

10-3 - 10-7

> 10-5

Ra (Rayleigh #)

convective forcing

1025 - 1030 104 - 1010

mechanical forcing

Pr (Prandtl #) viscous/

thermal diffusion

0.1 - 1 ~ 1

~ 0.1

Pm (magnetic Prandtl #)

viscous/magnetic diffusion

----- Outputs ------------

10-5 - 10-6 0.05 - 20

~ 10-5

Rm (magnetic Reynolds #)

fluid velocity

500 - 2000 40(critical) - 1500

< 100

(Elsasser #)

magnetic energy density

0.1 - 1 0.1 - 1

0.1 - 1

Rol (local Rossby #)

turbulence

~0.1 0.01 - 0.1

1 - 100

Numerical Dynamo Resolution vs. Simulated Time

Rudolph & Zhong, G^3, 2014Zhong & Rudolph, G^3, 2015.

Mantle Global Circulation Models (mGCMs) with Plate Motion Constraints

Mantle Structure + Plate Velocity Boundary Conditions

Deep Mantle Heterogeneity

& Geoid

MantleMelt & Volatile

FluxesLi et al.

G^3, 2016

Geodynamo ReversalsOlson et al., PEPI, 2013Inner Core Age, Olson G^3 2016

Core Mantle Boundary Heat Flux History from Mantle GCMs

Zhang & Zhong (2011}; Rudolph & Zhong (2014); Zhong & Rudolph (2015)