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