Magnetic Field Evolution of Magnetic Field Evolution of Terrestrial PlanetsTerrestrial Planets

Doris BreuerDoris BreuerInstitute of Institute of PlanetaryPlanetary ResearchResearchDLR BerlinDLR Berlin

ContentContent Part IPart I

IntroductionIntroductionMotivationMotivationObservationsObservations

MechanismsMechanisms forfor magneticmagnetic fieldfield generationgenerationin in thethe corecore

Thermal Thermal convectionconvectionCompositionalCompositional convectionconvection

ContentContent Part IIPart II

ExamplesExamplesMarsMarsEarthEarthMercuryMercuryGalileanGalilean SatellitesSatellites

ConclusionsConclusions

WhyWhy itit isis InterestingInteresting to to StudyStudy thetheMagneticMagnetic FieldField

General General understandingunderstanding of of dynamodynamo actionaction

MagneticMagnetic fieldfield evolutionevolution and and magnetizedmagnetizedcrustcrust helpshelps to to constrainconstrain

Thermal Thermal evolutionevolutionInteriorInterior structurestructureGeologicalGeological and and tectonictectonic processesprocessesEvolution of Evolution of thethe atmosphereatmosphere

SelfSelf--GeneratedGenerated MagneticMagnetic FieldFieldOf Of thethe terrestrialterrestrial planetsplanetsand and majormajor satellitessatellites, , Earth, Mercury, and Earth, Mercury, and GanymedeGanymede areare knownknown to to havehave selfself--generatedgeneratedmagneticmagnetic fieldsfields

Mars, Venus, Moon, Mars, Venus, Moon, IoIo, , Europa, and Europa, and CallistoCallisto lack lack selfself--generatedgenerated magneticmagneticfieldsfields

PlanetaryPlanetary DataDataMercury Venus Earth Mars Ganymede

Radius 0.38 0.95 1.0 0.54 0.41

Mass 0.055 0.815 1.0 0.107 0.018

Density[kg/m3]

5430. 5250. 5515. 3940. 1940.

ρ0 [kg/m3] 5300. 4000. 4100. 3800. 1800.

Moment ofInertia factor

0.34 ? 0.3355 0.3662 0.3105

Rc/Rp 0.8 0.55 0.546 0.5 0.3

DipoleMoment[1013 T m³]

0.43 - 1577.

WhatWhat AboutAboutMagneticMagnetic FieldField Generation in Generation in thethe

PastPast??

Remanent Remanent CrustCrust MagnetizationMagnetization

Magnetized crust provides information Magnetized crust provides information aboutabout

History of the magnetic fieldHistory of the magnetic fieldGeological and tectonic processesGeological and tectonic processes

Acuna et al., 1999

OriginOrigin of Remanent of Remanent MagnetizationMagnetizationThermal remanent Thermal remanent magnetizationmagnetization (TRM)(TRM)

If a magnetic mineral is cooled in an ambient If a magnetic mineral is cooled in an ambient magnetic field through a temperature characteristic magnetic field through a temperature characteristic of the material, of the material, Curie temperatureCurie temperature, it will begin to , it will begin to acquire a very large acquire a very large remanentremanent magnetization. As magnetization. As the material cools through the Curie temperature, the material cools through the Curie temperature, domains begin to form, in alignment with the domains begin to form, in alignment with the ambient field. The magnetic field is then frozen into ambient field. The magnetic field is then frozen into the rock and is extremely stable.the rock and is extremely stable.

MagnetiteMagnetite ~ 853 K~ 853 KHematiteHematite ~ 953~ 953Iron ~ 1043 KIron ~ 1043 K

Remanent Remanent CrustCrust MagnetizationMagnetizationConstraintConstraint on on EarlyEarly Dynamo ActionDynamo Action

EarthEarthAge of magnetized crust between 3.5 Age of magnetized crust between 3.5 GaGa and and todaytoday

MoonMoonAge of magnetized crust between 3.9 and 3 Age of magnetized crust between 3.9 and 3 GaGa

MarsMarsAge of magnetized crust between 4.5 and 4 Age of magnetized crust between 4.5 and 4 GaGa??

Venus, MercuryVenus, MercuryNo data availableNo data available

WhatWhat do do wewe KnowKnow AboutAbout thetheOriginOrigin of of thethe MagneticMagnetic FieldField??

‘‘Bar MagnetBar Magnet‘‘ in in thethe PlanetsPlanets??

No! No! VariationsVariations withwith time (time (e.ge.g. polar . polar wanderwander, , reversalsreversals) ) cancan bebe observedobserved

No! No! MagneticMagnetic material in material in thethe interiorinterior isis well well aboveabove thetheCurie Curie temperaturetemperature

DecayingDecaying Old Old MagneticMagnetic FieldField??

EquationEquation of of magneticmagnetic inductioninduction

( )∂∂

t

Bu B B= ∇ × × + ∇Dma

2

1ma

c

Dμσ

=

uu velocityvelocity fieldfieldBB magneticmagnetic fieldfieldt timet time

MagneticMagnetic diffusiondiffusion coefficientcoefficient

σσcc electricalelectrical conductivityconductivityμ μ magneticmagnetic permeabilitpermeabilitäätt

DecayingDecaying Old Old MagneticMagnetic FieldField??

In In thethe casecase of of uu = 0= 0CharacteristicCharacteristic diffusiondiffusion time:time:

L L characteristiccharacteristic lengthlength scalescale ((planetaryplanetary radiusradius))

Fast Fast decaydecay of of magneticmagnetic fieldfield isis inconsistentinconsistent withwithobservationsobservations

2410 adiff

ma

LD

τ = ≈

MagneticMagnetic FieldField GenerationGeneration

NecessaryNecessary condictionscondictionsforfor existenceexistence

A A conductingconducting fluidfluid

Motion in Motion in thatthat fluidfluid

CowlingCowling‘‘ss Theorem Theorem requiresrequires somesomehelicityhelicity in in thethe fluidfluidmotionmotion

CoresCores

TheThe magneticmagnetic fieldsfields of of terrestrialterrestrial planetsplanets and and satellitessatellites areareproducedproduced in in theirtheircorescoresThereThere isis littlelittle doubtdoubtthatthat thethe planetsplanets and and mostmost of of thethe majormajorsatellitessatellites havehave ironiron--richrich corescores

DynamosDynamosHydromagneticHydromagnetic dynamosdynamos

Thermal Thermal dynamosdynamos

Chemical Chemical dynamosdynamos

G. Glatzmeier‘s Dynamo model for Earth

Thermal DynamoThermal DynamoThermal Dynamo

stabilystratified

noconvectioninthecore

efficient heat transferfromthe lower mantle

thermal convectioninthecore

inefficient heat transferfromthe lower mantle

Fluid motion in the liquid iron core due to thermal buoyancy(=> cooling from above)

‘Critical‘ heat flow out of the core

‘‘CriticalCritical‘‘ HeatHeat FlowFlow::HeatHeat FlowFlow AlongAlong thethe CoreCore AdiabateAdiabate

c

p

thermal conductivity of the coreT temperaturez depth

thermal expansivityg acceleration due to gravityC specific heat capacity

cc c c

ad p

c

TgdTq k kdz C

k

α

α

⎛ ⎞= =⎜ ⎟⎝ ⎠

‘‘CriticalCritical‘‘ HeatHeat FlowFlow

Mars, Mercury Mars, Mercury 5 5 -- 20 mW/m20 mW/m22

Earth, Venus Earth, Venus 15 15 –– 40 mW/m40 mW/m22

GalileanGalilean SatellitesSatellites, Moon < 7 mW/m, Moon < 7 mW/m22

Large Large uncertainitiesuncertainities duedue to to poorlypoorly knownknownparametersparameters

VigourVigour of of CoreCore ConvectionConvection

A A sufficientlysufficiently large large ΔΔT T betweenbetween thethe corecore and and thethe mantlemantle isis requiredrequired in in order to order to drivedrive thermal thermal convectionconvection in in thethe corecore

IfIf ΔΔT T isis tootoo smallsmall thanthanthethe corecore will will bebe coolingcoolingbyby conductionconduction

Sohl and Spohn, 97

Chemical DynamoChemical Dynamo

CompositionalCompositionalbouyancybouyancy releasedreleased bybyinner inner corecore growthgrowth

DifficultDifficult to to stopstopoperatingoperating

Mercury Model by Conzelmann and Spohn

Chemical DynamoChemical DynamoChemical Dynamo

ExistenceExistence of light of light alloyingalloying elementselements in in thethecorecore likelike S, O, SiS, O, Si

CoreCore temperaturetemperaturebetweenbetween solidussolidus and and liquidusliquidus

Fe

FeFe

Fe-FeSFe-FeS Fe-FeS

CoreCore CompositionComposition: Earth: EarthIndirectIndirect informationinformation fromfromseismologyseismology

SeimicSeimic velocitiesvelocities of of thethe corecore constrainconstraindensitydensity and and thereforetherefore compositioncomposition

Inner solid Inner solid corecore22--3% 3% lessless densedense thanthan pure iron pure iron ~ 8% S / Si ?~ 8% S / Si ?

OuterOuter fluidfluid corecore55--10 % 10 % lessless densedense thanthan pure iron pure iron ~ 8% S / Si and 8% O ?~ 8% S / Si and 8% O ?

25

Galile

anSa

tellite

s

Mercu

ry

Mars

26

Galile

anSa

tellite

s

Mercu

ry

Mars

27

Melting Melting TemperaturesTemperatures in in thethePlanetaryPlanetary CoresCores

0 10 20 30 40 50Pressure (GPa)

1000

1200

1400

1600

1800

2000

2200

2400

2600

Tem

pera

ture

(K)

Fe

Fe-FeS eute

ctic

Galilean Satellites

Core liquidus depending on S content

Low S content

High S content

‘‘ClassicalClassical‘‘ Earth Earth Chemical DynamoChemical Dynamo

Fe

FeFe

Fe-FeSFe-FeS Fe-FeS

P

T

Mantle Core

Tprofile core liquiduscore liquidus

Solid

inne

r core

Solid Fe-richinner core

Low Low PressurePressureChemical DynamoChemical Dynamo

P

T

Mantle Core

Tprofile

core liquidus

Fe snowMantle

Iron snow may form a solid inner coreor remelt upon sinking (chemical iron gradient)

Liquid Fe- FeS core

Solid (bouyant) FeS layer

P

T

Mantle Core

Tprofile

core liquidus

rising FeS-solid

Mantle

Low Low PressurePressureChemical DynamoChemical Dynamo

Power Power RequirementsRequirementsDynamo Dynamo convertsconverts thermal and thermal and gravitationalgravitational energyenergy intointo magneticmagnetic energyenergy

Power Power neededneeded to to sustainsustain geomagneticgeomagneticfieldfield isis setset byby thethe ohmicohmic losseslosses ((dissipationdissipationduedue to to electricalelectrical resistanceresistance))

EstimatesEstimates of of ohmicohmic lossloss forfor thethe Earth Earth covercover a a widewide rangerange (0.1 to 3.5 TW)(0.1 to 3.5 TW)

L

i

th

c

gravitational energy

E latent heat of soldification

dm rate of inner core growthdt

dE rate of change of heat content of the coredt

A heat conducted along adiabatCarnot effic

i i thdiss g g t L c c

g

c

dm dm dEE E A qdt dt dt

E

q

χ χ

χ

⎛ ⎞Φ = + − −⎜ ⎟⎝ ⎠

iency factor

OhmicOhmic DissipationDissipation

Dynamo Dynamo EfficiencyEfficiency

Thermal Thermal dynamodynamo efficiencyefficiency isis restrictedrestrictedbyby CarnotCarnot efficiencyefficiency χχtt to to bebe a a fewfewpercentpercent

Chemical Chemical dynamodynamo isis notnot restrictedrestricted; ; χχg g = 1= 1

SomeSome First First ConclusionsConclusionsThermal Thermal convectionconvection ((fluidfluid corecore withoutwithout inner inner corecoregrowth): growth): InefficientInefficient of of dynamodynamo generationgeneration

CompositionalCompositional convectionconvection (inner (inner corecore growth): growth): EfficientEfficient of of dynamodynamo generationgeneration; ; difficultdifficult to to stopstop

TheThe mantlemantle determinesdetermines whetherwhether a a terrestrialterrestrialplanet has planet has corecore convectionconvection and and whetherwhether itit cancanhavehave a a dynamodynamo

ComparisonComparison plateplate tectonicstectonics and and stagnantstagnant lidlid convectionconvection

0 750 1500 2250 3000 3750 4500Time (Ma)

16001700180019002000210022002300

Cor

e te

mpe

ratu

re (k

)

0 750 1500 2250 3000 3750 4500Time (Ma)

-10

0

10

20

30

40

50

Cor

e-m

antle

hea

t flo

w (m

Wm

2 )

ComparisonComparison plateplate tectonicstectonics and and stagnantstagnant lidlid convectionconvection

SOME EXAMPLESSOME EXAMPLESMarsMarsEarth / VenusEarth / VenusMercuryMercuryGanymedeGanymede and and EuropaEuropa

MARSMARS

MarsMarsMars

Magnetic Field HistoryMagneticMagnetic FieldField HistoryHistory

No No presentpresent--dayday dynamodynamo

MagnetisationMagnetisation of of oldestoldest partspartsof of thethe MartianMartian crustcrust

No No magnetisationmagnetisation of large of large impactimpact basinsbasins

⇒⇒ Dynamo Dynamo actionaction beforebefore thethelarge large impactsimpacts ~4 Ga ~4 Ga

oror⇒⇒ Dynamo Dynamo actionaction afterafter large large

impactsimpacts

`The Great Nothing`

Dynamo Action Dynamo Action BeforeBefore Large Large ImpactsImpacts

Pro Pro ‘‘EasiestEasiest‘‘ explanationexplanation: (: (oldold surfacesurface –– magnetizedmagnetized, , youngyoung surfacesurface –– nonnon--magnetizedmagnetized))MagnetizationMagnetization of of oldold SNC SNC meteoritemeteorite (age 4.4 Ga)(age 4.4 Ga)

ContraContraThermal Thermal dynamodynamo notnot veryvery efficientefficientDifficultDifficult to to explainexplain thethe non non magnetizedmagnetized areasareas in in thethesouthernsouthern hemispherehemisphereNorthern Northern hemispherehemisphere has has oldold crustcrust belowbelow youngyoungsurfacesurface butbut almostalmost no no magnetizationmagnetization

Dynamo Action After Large Dynamo Action After Large ImpactsImpacts

Pro Pro Inner Inner corecore growth growth moremore efficientefficientNon Non magnetizedmagnetized areaarea in in thethe southernsouthern hemispherehemisphere

ContraContraChemical Dynamo Chemical Dynamo difficultdifficult to to stopstopLateLate strongstrong crustcrust productionproduction ((e.ge.g. . plutonismplutonism) ) necessarynecessary butbut notnot observedobserved on on thethe surfacesurfaceEarlyEarly HesperianHesperian volcanicvolcanic plainsplains ((aboutabout 3.7 3.7 –– 3.2 3.2 Ga) Ga) showshow no no magnetizationmagnetization

Melting Melting TemperaturesTemperatures in in thetheMartianMartian CoreCore

0 10 20 30 40 50Pressure (GPa)

1000

1200

1400

1600

1800

2000

2200

2400

2600

Tem

pera

ture

(K)

Martian coreMartian mantle

Fe

Fe-FeS eute

cticFe-Fe

S (14% S)

Usselman

n

core adiabate

core adiabate

Fe-FeS (14% S) Fei et al.

Thermal Evolution ModelsThermal Evolution Models

WhatWhat cancan wewe learnlearn aboutabout Mars Mars fromfrom thetheconstraintsconstraints on on thethe magneticmagnetic fieldfield historyhistory??

HeatHeat transfertransfer mechanismmechanism((plateplate tectonicstectonics versusversus stagnantstagnant lidlidconvectionconvection))

CompositionComposition((drydry versusversus wetwet MartianMartian mantlemantle))

Early Plate Tectonic Regime Versus

Stagnant Lid Convection

EarlyEarly PlatePlate TectonicTectonic Regime Regime VersusVersus

StagnantStagnant Lid Lid ConvectionConvection

CoreCore TemperaturesTemperatures

0 500 1000 1500 2000 2500 3000 3500 4000 4500time (Ma)

1850

1950

2050

2150

2250

2350

core

tem

pera

ture

(K)

1020 Pa s

1022 Pa s

1021 Pa s

One-Plate Planet 1021 Pa s

Breuer and Spohn, 2003

EarlyEarly MartianMartian (thermal) (thermal) dynamodynamo possiblepossiblewithwith a a superheatedsuperheated corecore

DryDry VersusVersus WetWet MartianMartian MantleMantle

1700 1800 1900 2000 2100Initial mantle temperature (K)

1600

1700

1800

1900

2000

2100

2200

2300

Cor

e-m

antle

tem

pera

ture

(K)

mol

ten

core

inne

r cor

e gr

owth

η = 1022 Pas

core liquidus

η = 1021 Pas

η = 1020 Pas

η = 1019 Pas

Models Models withwith a a weak/wetweak/wet viscosityviscosity showshow presentpresent--daydayinner inner corecore growth growth forfor UsselmanUsselman datadata; ; inconsistentinconsistentwithwith thethe lack of a lack of a presentpresent dynamodynamo..

ConclusionsConclusions Mars IMars I

EarlyEarly plateplate tectonicstectonics consistentconsistent withwith earlyearlystrongstrong magneticmagnetic fieldfield

StagnantStagnant lidlid convectionconvection consistentconsistent withwithearlyearly magneticmagnetic fieldfield ifif thethe corecore isissuperheatedsuperheated byby moremore thanthan 100 K.100 K.

ConclusionsConclusions Mars IIMars II

In In casecase of of UsselmanUsselman datadata and 14 wt.% S, and 14 wt.% S, a a drydry, , stiffstiff mantlemantle isis moremore likelylikely to to explainexplainearlyearly magneticmagnetic fieldfield

In In casecase of Fei of Fei datadata ((strongstrong decreasedecrease of S of S withwith pressurepressure in in eutecticeutectic compositioncomposition) and ) and 14 wt.% S, Mars 14 wt.% S, Mars indirectlyindirectly ‘‘provesproves‘‘ thatthat a a thermal thermal dynamodynamo cancan existexist

EarthEarth

QuestionsQuestions

WhenWhen diddid thethe inner inner corecore growth?growth?

ThemalThemal dynamodynamo activeactive beforebefore inner inner corecoregrowth?growth?

RadioactiveRadioactive elementselements in in thethe corecore??

ConstraintsConstraints on Thermal on Thermal Evolution Models Evolution Models forfor thethe EarthEarth

ObservedObserved magneticmagnetic fieldfield evolutionevolution

SurfaceSurface heatheat flowflow

Inner Inner corecore radiusradius

Stevenson et al., 1983

Evolution of the Earth‘s Core-Mantle Heat Flow

Evolution of Evolution of thethe EarthEarth‘‘ss CoreCore--MantleMantle HeatHeat FlowFlow

Stevenson et al., 1983

Evolution of the Earth‘sMagnetic Field

Evolution of Evolution of thethe EarthEarth‘‘ssMagneticMagnetic FieldField

Thermal Chemical

Difficult to stopoperating

ConclusionsConclusions EarthEarthCurrentCurrent thermal thermal evolutionevolution modelsmodels showshowgrowth of inner growth of inner corecore betweenbetween 1.5 and 3 Ga1.5 and 3 Ga

MagneticMagnetic fieldfield mustmust bebe poweredpowered byby thermal thermal dynamodynamo in in thethe earlyearly evolutionevolutionPotassiumPotassium in in thethe corecore isis requiredrequired dependingdependingmainlymainly on on ohmicohmic dissipationdissipation and and thethe corecore adiabateadiabate

OnsetOnset of inner of inner corecore dependsdepends ononSolidusSolidus and and adiabateadiabate of of thethe corecoreContentContent of of radioactiveradioactive elementselements e.ge.g. K (. K (thethe higherhigherthethe potassiumpotassium contentcontent thethe youngeryounger thethe inner inner corecoregrowth)growth)TwoTwo--layeredlayered versusversus oneone--layeredlayered convectionconvection

MercuryMercury

MercuryMercury’’s Magnetic Fields Magnetic Field

Mariner 10 discovered Mercury's planetary Mariner 10 discovered Mercury's planetary magnetic field and magnetospheremagnetic field and magnetosphereThe planetary magnetic field is sufficient to The planetary magnetic field is sufficient to stand off the solar wind (at least most of stand off the solar wind (at least most of the time)the time)

22:10 22:20 22:30 22:40 22:50 23:00

16 MARCH 1975

BS - BOW SHOCKMP - MAGNETOPAUSECA - CLOSEST APPROACH

MA

GN

ETI

C F

IELD

MA

GN

ITU

DE

(nT)

BS MP (5) BSCA MP

UPSTREAMWAVES

UW

22:10 22:20 22:30 22:40 22:50 23:00

16 MARCH 1975

BS - BOW SHOCKMP - MAGNETOPAUSECA - CLOSEST APPROACH

MA

GN

ETI

C F

IELD

MA

GN

ITU

DE

(nT)

BS MP (5) BSCA MP

UPSTREAMWAVES

UW

Thermal HistoryThermal History

New thermal history New thermal history calculations using full calculations using full convection codesconvection codes

Planet cools mostly Planet cools mostly by thickening its by thickening its lithospherelithosphere

Dynamo Dynamo DrivenDriven bybyCompositionalCompositional ConvectionConvection

CompositionalCompositionalbouyancybouyancy releasedreleased bybyinner inner corecore growth growth afterafter a 100 a 100 –– 1000 Ma1000 MaSmall Small amountamount of S of S requiredrequired; ; consistentconsistentwithwith geochemicalgeochemicalmodelsmodels

Europa andEuropa andGanymedeGanymede

63

Ice or Ice + thin ocean

Ganymede has a magnetic field while Ganymede has a magnetic field while EuropaEuropa has not!has not!

Ganymede Ganymede –– EuropaEuropa DichotomyDichotomy

DeepDeep interiorsinteriors areare similarsimilarGanymedeGanymede‘‘ss MoIMoI of of 0.3105 suggests e.g.0.3105 suggests e.g.–– core radius: 800 kmcore radius: 800 km–– silicate shell: 900 kmsilicate shell: 900 km–– ice shell: 900 kmice shell: 900 km

EuropaEuropa‘‘ss MoIMoI of 0.346 of 0.346 suggests e.g.suggests e.g.–– core radius: 600 km core radius: 600 km –– silicate shell: 800 kmsilicate shell: 800 km–– Ice shell: 150 kmIce shell: 150 km

Inner Inner CoreCore GrowthGrowth

Present-day inner core growthInner core growth early in the evolution, remanent magnetic field possible

Temperature Profiles of Ganymede

GanymedeGanymede, Europa, Europa

IfIf GanymedeGanymede‘‘ss corecore formedformedlatelate

SlowSlow differentiationdifferentiation isis lesslessfavourablefavourable forfor a a sufficientsufficient ΔΔT to T to drivedrive a thermal a thermal dynamodynamo. .

IfIf corecore formedformed earlyearlyDynamo Dynamo actionaction possiblepossible withwithchemicalchemical dynamodynamo ifif lowlow S S contentcontent oror weakweak rheologyrheology

TidalTidal heatingheating in Europa in Europa maymay frustratefrustrate presentpresentdynamodynamo actionaction

Ganym

ede

Europa

ThermoThermo--chemicalchemical dynamodynamo

IfIf an inner an inner corecore growthsgrowths: a : a longstandinglongstandingmagneticmagnetic fieldfield generationgeneration isis likelylikely

ExistenceExistence of a of a growinggrowing inner inner corecore dependsdependson:on:–– ContentContent of light of light elementselements–– HeatHeat transporttransport mechanismmechanism of of thethe mantlemantle

Fluid core

Efficiency of mantle coolingStagnant lid Plate tectonics

Ligh

t ele

men

tsin

the

core

PresentPresent magneticmagnetic fieldfield dependingdepending on on compositioncomposition and and heatheat transporttransport

t1

Inner core growthMagnetic field generation

Inner core growthMagnetic field generation

Fluid core

Large inner coreWeak magnetic field?

Efficiency of mantle coolingStagnant lid Plate tectonics

Ligh

t ele

men

tsin

the

core

Mars

Earth

Mercury

Venus

t2

t2

PresentPresent magneticmagnetic fieldfield dependingdepending on on compositioncomposition and and heatheat transporttransport

Ganymede

Inner core growthMagnetic field generation

Fluid core

Solid coreNo magnetic field

Stagnant lid Plate tectonics

Ligh

t ele

men

tsin

the

core

Mars

Earth

Mercury

Venus

t3

t3

t3

Large inner coreWeak magnetic field?

LaterLater in time in time ……..

Efficiency of mantle cooling

Ganymede

General General ConclusionsConclusionsEarlyEarly dynamosdynamos forfor oneone--plateplate planetsplanets areare likelylikely ififtherethere isis a a sufficientlysufficiently large large ΔΔT as a T as a consequenceconsequenceof of corecore formationformationThermal Thermal dynamosdynamos forfor oneone--plateplate planetsplanets will last will last aboutabout 100 100 –– 500 Ma500 MaExtendedExtended dynamodynamo actionaction requiresrequires efficientefficient corecorecoolingcooling and an IC and an IC freezefreeze--outout (Earth, Mercury, (Earth, Mercury, and and GanymedeGanymede))

Earth: Earth: plateplate tectonicstectonicsMercury: Large Mercury: Large corecore and and lowlow S S contentcontentGanymedeGanymede: : weakweak rheologyrheology ((plateplate tectonicstectonics?) ?) duedueto to waterwater

Magnetic Field Evolution of Terrestrial PlanetsContent Part IContent Part IIWhy it is Interesting to Study the Magnetic FieldSelf-Generated Magnetic FieldPlanetary DataWhat About �Magnetic Field Generation in the Past?Remanent Crust MagnetizationOrigin of Remanent MagnetizationRemanent Crust Magnetization�Constraint on Early Dynamo ActionWhat do we Know About the Origin of the Magnetic Field?Decaying Old Magnetic Field?Decaying Old Magnetic Field?Magnetic Field GenerationCoresDynamosThermal Dynamo‘Critical‘ Heat Flow:�Heat Flow Along the Core Adiabate‘Critical‘ Heat FlowVigour of Core Convection Chemical DynamoChemical DynamoCore Composition: EarthMelting Temperatures in the Planetary Cores‘Classical‘ Earth Chemical DynamoLow Pressure �Chemical DynamoPower RequirementsDynamo EfficiencySome First ConclusionsComparison plate tectonics and stagnant lid convectionSOME EXAMPLESMARSMars Magnetic Field HistoryDynamo Action Before Large ImpactsDynamo Action After Large ImpactsMelting Temperatures in the Martian CoreThermal Evolution ModelsEarly Plate Tectonic Regime �Versus �Stagnant Lid ConvectionCore TemperaturesDry Versus Wet Martian MantleConclusions Mars IConclusionsMars IIEarthQuestionsConstraints on Thermal Evolution Models for the EarthEvolution of the Earth‘s Magnetic FieldConclusions EarthMercuryDynamo Driven by Compositional ConvectionEuropa and�GanymedeGanymede – Europa DichotomyDeep interiors are similarInner Core GrowthGanymede, EuropaThermo-chemical dynamoPresent magnetic field depending on composition and heat transport Present magnetic field depending on composition and heat transport Later in time ….General Conclusions