Magnetic Field Evolution of Magnetic Field Evolution of Terrestrial Planets Terrestrial Planets Doris Breuer Doris Breuer Institute of Institute of Planetary Planetary Research Research DLR Berlin DLR Berlin
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