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The interior structure and evolution of Mercury

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The interior structure and evolution of Mercury

COME-IN meeting Amsterdam

10 February Marie-Hélène Deproost

(2)

Introduction Observables

Mercury: observables and interior structure

Density and magnetic field

Density: very high

! large core

Magnetic field:

more than 2 orders of magnitude weaker than on Earth

strong equatorial asymmetry

! partly liquid core + solid inner core

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 2 / 34

2440 km

~2000 km

Observables

(3)

Introduction Observables

Mercury: observables and interior structure

Rotation and tides

Rotation:

3:2 spin-orbit resonance librations:

observation = 455 ± 19 m > solid planet ' 190 m

! liquid core

with grav. field: mantle polar moment of inertia if solid inner core: correction

obliquity + grav. field: planet polar moment of inertia Moment of inertia: constraints on mass distribution

! densities, liquid-solid interface Tides:

mass distribution modification ) grav. field modification (Love number k2)

! density, elasticity

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 3 / 34 Introduction Observables

Mercury: observables and interior structure

Rotation and tides

Rotation:

3:2 spin-orbit resonance librations:

observation > solid planet! liquid core

with grav. field: mantle polar moment of inertia if solid inner core: correction

obliquity + grav. field: planet polar moment of inertia Moment of inertia: constraints on mass distribution

! densities, liquid-solid interface Tides:

mass distribution modification ) grav. field modification (Love number k2)

! density, elasticity

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 3 / 31

Observables

(4)

Introduction Observables

Mercury: observables and interior structure

Surface observations

Surface composition:

surface content in iron and sulfur

abundance of radioactive elements (K, Th, U)

! core and mantle composition Surface features: secular cooling

no prolonged resurfacing and near-surface alteration lobate scarps: contraction ⇠ 7 km

! solid inner core, core composition, crust formation

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 4 / 31

Observables

(5)

Introduction Observables

Mercury: observables and interior structure

Surface observations

Surface composition:

surface content in iron and sulfur

abundance of radioactive elements (K, Th, U)

! core and mantle composition Surface features: secular cooling

no prolonged resurfacing and near-surface alteration lobate scarps: contraction ⇠ 7 km

! solid inner core, core composition, crust formation

)

observables alone not enough: theoretical model needed...

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 4 / 31

Observables

(6)

PhD presentation Core

Core heat and entropy budget

Objectives:

inner core growth: heat generated at the inner core boundary

! heat budget core dynamo:

liquid metal core rotating and cooling

energy conversion in the core ) not in heat budget entropy generated

! entropy budget

) energy and entropy balances in the core for different compositions:

Fe-FeS, Fe-FeSi and Fe-S-Si

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 12 / 34

Thermal evolution:

energy and entropy budgets

(7)

PhD presentation Core

Core heat and entropy budget

Heat budget

Qtot(rcmb) = Qcmb = Qs + QL + Qg + QT + Qh

CMB heat flow Secular cooling Latent heat (QL)

Gravitational energy (Qg) Tidal heating (QT )

Radioactive heating

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 13 / 34

Thermal evolution:

energy and entropy budgets

(8)

Thermal evolution:

energy and entropy budgets

PhD presentation Future work

Future work

Core model: heat budget

Qtot(rcmb) = Qcmb = Qs + QL + Qg + QT + Qexsol + Qsnow solid inner core: QL, Qg , QT

sulfur exsolution: Qexsol

iron snow: Qsnow = QL,snow + Qg ,snow

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 27 / 31 PhD presentation Future work

Future work

Core model: heat budget

Qtot(rcmb) = Qcmb = Qs + QL + Qg + QT + Qexsol + Qsnow solid inner core: QL, Qg , QT

sulfur exsolution: Qexsol

iron snow: Qsnow = QL,snow + Qg ,snow

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 27 / 31 PhD presentation Future work

Future work

Core model: heat budget

Qtot(rcmb) = Qcmb = Qs + QL + Qg + QT + Qexsol + Qsnow solid inner core: QL, Qg , QT

sulfur exsolution: Qexsol

iron snow: Qsnow = QL,snow + Qg ,snow

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 27 / 31 PhD presentation Core

Core heat and entropy budget

Heat budget

Qtot(rcmb) = Qcmb = Qs + QL + Qg + QT + Qh

CMB heat flow Secular cooling Latent heat (QL)

Gravitational energy (Qg) Tidal heating (QT )

Radioactive heating

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 13 / 34

(9)

PhD presentation Core

Core heat and entropy budget

Entropy budget

EJ = Es + EL + Eg + ET + Eh (Ea + Edi↵ )

Entropy available for the dynamo

Sources:

Es: secular cooling EL: latent heat

Eg: gravitational energy ET : tidal heating

Eh: radioactive heating

Sinks:

Ea: Entropy of conduction

Edi↵ : Entropy of molecular diffusion

Marginal dynamo: EJ = 0

M-H Deproost (KU Leuven-ROB) PhD first oral December 7, 2016 16 / 34

Thermal evolution:

energy and entropy budgets

(10)

Core model

1950 - 2050 km

Fe-S

- small core:

~ 2-3 wt%

- large core:

~ 5 wt%

Fe

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initial temperature: 1900 - 2200 K

(11)

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Small core: solid iron core + iron snow Large core: iron snow

Marginal dynamo: Core melting

Solid: small core (1950 km) Dashed: large core (2050 km)

solid core

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(12)

Marginal dynamo: radioactivity

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abundance:

Uranium: 90 ppb Thorium: 155 ppb

Potassium: 1288 ppm Core: Uranium + Thorium

mantle

(13)

Marginal dynamo: core-mantle boundary temperature

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solidus

Solid: small core (1950 km) Dashed: large core (2050 km)

Lower mantle not melted

(14)

Mantle model

Mantle

2439 km Crust

Stagnant lid

Regolith 2440 km

2400 km

1950-2050 km 2310-2370 km

Upper thermal boundary layer

Lower thermal boundary layer

Convective part

60% olivine 40% pyroxene

temperature: 2100 K

(15)

Global model: heat fluxes

Solid: small core (1950 km) Dashed: large core (2050 km)

Small core:

inner core dynamo Large core:

iron snow no dynamo

mantle convection stops

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(16)

Global model: budget contributions

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Solid: small core (1950 km) Dashed: large core (2050 km)

Main contribution: latent heat

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(17)

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Global model: stratification

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small core large core

(18)

Global model: temperature

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Lower mantle not melted

Solid: small core (1950 km) Dashed: large core (2050 km)

(19)

Conclusions

(20)

Back-up

(21)

Modèle de core + mantle (conductivité thermique) composition, densités, …

sulfur concentration (wt) Tcmb = 1900 K

rcmb = 1980 km 0.032776

Tcmb = 1900 K

rcmb = 2050 km 0.0570619

Tcmb = 2000 K

rcmb = 1950 km 0.0187252

Tcmb = 2000 K

rcmb = 2050 km 0.0534317

Tcmb = 2100 K

rcmb = 1950 km 0.01561

Tcmb = 2100 K

rcmb = 2080 km 0.0498513

Tcmb = 2200 K

rcmb = 1950 km 0.0151242

Tcmb = 2200 K

rcmb = 2050 km 0.0463206

(22)

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