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Does Ganymede Have a Does Ganymede Have a Dynamo?Dynamo?
Jennifer PalgutaJennifer Palguta
December 2, 2004December 2, 2004
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OverviewOverviewPossible sources for Ganymede’s intrinsic Possible sources for Ganymede’s intrinsic field:field:– MagnetoconvectionMagnetoconvection– Remanent magnetization due to Jupiter’s Remanent magnetization due to Jupiter’s
magnetic fieldmagnetic field– Internal active dynamo Internal active dynamo – Remanent magnetization due to an internal Remanent magnetization due to an internal
dynamo which is no longer activedynamo which is no longer active
A present vs. past dynamo.A present vs. past dynamo.
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Discovery of Ganymede’s Magnetic FieldDiscovery of Ganymede’s Magnetic Field
Discovered after the 1996 Discovered after the 1996 flybysflybys
Evidence for an internal field is Evidence for an internal field is unambiguous unambiguous
Equatorial field magnitude ~ Equatorial field magnitude ~ 720 nT720 nT
While observations require an While observations require an internal field they do not internal field they do not indicate its sourceindicate its source
Sarson et al. 1997
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Internal structure IInternal structure IGanymede’s internal structure and thermal Ganymede’s internal structure and thermal state determines which source for the state determines which source for the magnetic field is most likely.magnetic field is most likely.
2 0
sin)sin()cos(1),,(n
n
mnmnmnm
n
PmSmCr
R
r
GMrV
The only non-zero gravity parameters of The only non-zero gravity parameters of importance are the monopole GM and the 2 importance are the monopole GM and the 2 quadrupole coefficients (not entirely correct).quadrupole coefficients (not entirely correct).
GM = 9887.83 kmGM = 9887.83 km33 s s-2-2
Mean density = 1942 kg mMean density = 1942 kg m-3-3
C/(MRC/(MR22) = .311) = .311
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Internal Structure IIInternal Structure II
The low value for the The low value for the MOI indicates MOI indicates Ganymede is strongly Ganymede is strongly differentiated.differentiated.
The density requires The density requires that Ganymede have a that Ganymede have a large water-ice large water-ice component.component.
A three-layer model A three-layer model appears most consistent appears most consistent with observations.with observations.
http://www.solarviews.com
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Three-layer ModelThree-layer ModelConsists of a metallic core overlaid by 2 Consists of a metallic core overlaid by 2 spherical shells.spherical shells.2 options for the metallic core:2 options for the metallic core:– FeFe– Fe-FeSFe-FeS
Requires heating to at least the Fe-FeS Requires heating to at least the Fe-FeS melting temperature.melting temperature.Sources:Sources:– AccretionalAccretional– RadiogenicRadiogenic– Tidal Tidal
Implications?Implications?
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MagnetoconvectionMagnetoconvection
Produces a field due to an externally Produces a field due to an externally imposed magnetic field. imposed magnetic field.
Requires an electrically conducting fluid to Requires an electrically conducting fluid to be affected by the imposed field.be affected by the imposed field.
2 possible regions2 possible regions– Ocean of salty waterOcean of salty water– Metallic inner coreMetallic inner core
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Salty Ocean ISalty Ocean IPlotting the melting Plotting the melting temperature vs. depth with temperature vs. depth with the temperature profile the temperature profile suggests an ocean.suggests an ocean.Temporal variations arise in Temporal variations arise in the jovian magnetosphere the jovian magnetosphere because Jupiter’s tilted because Jupiter’s tilted dipole moment changes its dipole moment changes its orientation as the planet orientation as the planet rotates.rotates.In principle, the time varying In principle, the time varying field drives inductive field drives inductive currents within the ocean. currents within the ocean. These induced currents These induced currents produce a time-varying produce a time-varying magnetic moment.magnetic moment.
Kivelson et al. 2002
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Kivelson et al. 2002
1010
Salty Ocean IISalty Ocean IIKivelson et al. used G1, G2, G28 to determine whether Kivelson et al. used G1, G2, G28 to determine whether an inductive response is present.an inductive response is present.The external field for G1 and G2 was directed radially The external field for G1 and G2 was directed radially outward.outward.The external field for G28 was directed radially inward.The external field for G28 was directed radially inward.Were there an induced magnetic moment it would be Were there an induced magnetic moment it would be antiparallel to the radial component of the external field antiparallel to the radial component of the external field and its orientation for G28 would differ from that for G1 and its orientation for G28 would differ from that for G1 and G2.and G2.The time-varying field at Ganymede has an amplitude of The time-varying field at Ganymede has an amplitude of ~100 nT. ~100 nT. The induced dipole moment will be at most ~ 6% of the The induced dipole moment will be at most ~ 6% of the permanent magnetic moment.permanent magnetic moment.
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Inner Metallic CoreInner Metallic CoreThe induced magnetic field is of the The induced magnetic field is of the same order of magnitude or smaller same order of magnitude or smaller than the imposed field. A stronger than the imposed field. A stronger driving force is required.driving force is required.Including an induced field does little to Including an induced field does little to the resultant solution (essentially it the resultant solution (essentially it operates by a “pure” dynamo process operates by a “pure” dynamo process and the induced magnetic field is and the induced magnetic field is effectively negligible).effectively negligible).Dynamo action rather than Dynamo action rather than magnetoconvection is a more likely magnetoconvection is a more likely explanation because Ganymede’s explanation because Ganymede’s magnetic field is so much larger (by a magnetic field is so much larger (by a factor of six) than the ambient jovian factor of six) than the ambient jovian magnetic field.magnetic field.
Sarson et al. 1997
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Magnetoconvection ConclusionsMagnetoconvection Conclusions
For Ganymede, it is difficult to see how For Ganymede, it is difficult to see how magnetoconvection could produce an intrinsic magnetoconvection could produce an intrinsic field so much stronger than the local ambient field so much stronger than the local ambient field.field.
It seems unlikely that Ganymede’s intrinsic field It seems unlikely that Ganymede’s intrinsic field is generated solely by magnetoconvection.is generated solely by magnetoconvection.
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Remanent MagnetizationRemanent Magnetization
Magnetization would occur in a layer of rock Magnetization would occur in a layer of rock which was once heated above the Curie which was once heated above the Curie temperature but has since cooled below it.temperature but has since cooled below it.Muller and McKinnon used meteoritic Muller and McKinnon used meteoritic compositions and equilibrium condensation compositions and equilibrium condensation values to estimate the mineralogy of values to estimate the mineralogy of Ganymede’s interior:Ganymede’s interior:– Ganymede is rich in magnetiteGanymede is rich in magnetite
Potential source of strong remanent magnetism.Potential source of strong remanent magnetism.SourcesSources– Jovian fieldJovian field– PaleodynamoPaleodynamo
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Remanent Magnetization from JupiterRemanent Magnetization from Jupiter
Requires no differentiation between rock and Requires no differentiation between rock and iron.iron.
Lack of differentiation provides an upper limit on Lack of differentiation provides an upper limit on magnetization from the jovian field.magnetization from the jovian field.– Magnetite content not reduced (7.5-16.5%)Magnetite content not reduced (7.5-16.5%)– Higher effective susceptibility, c (ratio of the remanent Higher effective susceptibility, c (ratio of the remanent
magnetization to the local magnetic field)magnetization to the local magnetic field)
Model: series of concentric shells cooling Model: series of concentric shells cooling through the Curie temperature one after the through the Curie temperature one after the other.other.
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Remanent Magnetization from Jupiter:Remanent Magnetization from Jupiter:Results IResults I
The resulting remanent field The resulting remanent field saturates when its strength at the saturates when its strength at the top of the ferromagnetic layer is ~ top of the ferromagnetic layer is ~ to the external field.to the external field.
The magnetic field interior to the The magnetic field interior to the shell is reduced and subsequent shell is reduced and subsequent shells are magnetized by shells are magnetized by progressively weaker net fields.progressively weaker net fields.
Therefore, remanent Therefore, remanent magnetization by the jovian field magnetization by the jovian field cannot produce a dipole field cannot produce a dipole field stronger than the background stronger than the background field.field. Crary and Bagenal 1998
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Remanent Magnetization from Jupiter:Remanent Magnetization from Jupiter:Results IIResults II
As the jovian field decreases As the jovian field decreases the remanent dipole increases the remanent dipole increases toward the new background toward the new background field strength, overshoots, field strength, overshoots, then decreases towards the then decreases towards the background field strength.background field strength.Even for unrealistically high Even for unrealistically high values of c the current values of c the current strength at the top of the strength at the top of the ferromagnetic layer would be ferromagnetic layer would be < 150 nT.< 150 nT.This corresponds to an This corresponds to an equatorial strength < 6% of equatorial strength < 6% of the observed value.the observed value. Crary and Bagenal 1998
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Remanent Magnetization from Jupiter:Remanent Magnetization from Jupiter:ConclusionsConclusions
To achieve the requisite level of To achieve the requisite level of magnetization in such a layer to account magnetization in such a layer to account for Ganymede’s intrinsic magnetic dipole for Ganymede’s intrinsic magnetic dipole moment requires a sufficiently large moment requires a sufficiently large concentration of magnetite and an external concentration of magnetite and an external magnetizing field larger than the present magnetizing field larger than the present jovian magnetic field at Ganymede’s orbit.jovian magnetic field at Ganymede’s orbit.The necessary requirements fall outside The necessary requirements fall outside reasonable parameter values.reasonable parameter values.
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Active Internal DynamoActive Internal DynamoRequires differentiation into a rocky layer Requires differentiation into a rocky layer and an iron/iron sulfide core.and an iron/iron sulfide core.
Heating would occur early in the history Heating would occur early in the history and sustaining core convection in the and sustaining core convection in the small body until today is difficult to explain. small body until today is difficult to explain.
The cooling rate required for the onset of The cooling rate required for the onset of convection in the core ~ 300-400 K/Ga.convection in the core ~ 300-400 K/Ga.
This exceeds values expected from the This exceeds values expected from the decline of the radioisotope heat budget.decline of the radioisotope heat budget.
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Active Internal Dynamo: Active Internal Dynamo: ConclusionsConclusions
Hard to explain how an active dynamo Hard to explain how an active dynamo could exist without invoking additional could exist without invoking additional energy sources.energy sources.PossibilitiesPossibilities– Recent capture into resonanceRecent capture into resonance– Large amounts of sulfurLarge amounts of sulfur– Decay of a lot Decay of a lot 4040KK
There might once have been an active There might once have been an active dynamo. dynamo.
2020
Remanent Magnetization from PaleodynamoRemanent Magnetization from PaleodynamoDoes not require ongoing Does not require ongoing convection.convection.The remanent field is ~1-6.8% of The remanent field is ~1-6.8% of the maximum dynamo field.the maximum dynamo field.Requires a maximum dipole field Requires a maximum dipole field of over 11 of over 11 μμT.T.Estimates of lunar Estimates of lunar paleomagnetism suggest that the paleomagnetism suggest that the Moon once had a dynamo-driven Moon once had a dynamo-driven magnetic field of 10-100 magnetic field of 10-100 μμT.T.Ganymede has a greater metal Ganymede has a greater metal content, larger conduction core, content, larger conduction core, faster rotation rate, possible faster rotation rate, possible enhancement by jovian field.enhancement by jovian field.One problem: It’s possible One problem: It’s possible Ganymede’s field reversed itself. Ganymede’s field reversed itself. Increases the necessary Increases the necessary maximum dynamo field to 462 maximum dynamo field to 462 μμT.T.
Crary and Bagenal 1998
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ConclusionsConclusions
A dynamo is required to explain the A dynamo is required to explain the observed field (though other methods observed field (though other methods might contribute to the field). might contribute to the field).
An active dynamo is hard to explain.An active dynamo is hard to explain.
Dynamo action in the past is more likely Dynamo action in the past is more likely and is capable of causing significant and is capable of causing significant magnetization.magnetization.
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BibliographyBibliography
Anderson, J. D. et al. Anderson, J. D. et al. Gravitational constraints on the internal structure of GanymedeGravitational constraints on the internal structure of Ganymede . . Nature, Nature, 384384, 541-543 (1996)., 541-543 (1996).
Crary, F. J. and F. Bagenal. Crary, F. J. and F. Bagenal. Remanent ferromagnetism and the interior structure of Remanent ferromagnetism and the interior structure of GanymedeGanymede. JGR . JGR 103103, 25,757-25,773 (1998)., 25,757-25,773 (1998).
Kivelson, M. G., et al. Kivelson, M. G., et al. The Permanent and Inductive Magnetic Moments of The Permanent and Inductive Magnetic Moments of GanymedeGanymede. Icarus, . Icarus, 157157, 507-522 (2002)., 507-522 (2002).
Kivelson, M. G., et al. Kivelson, M. G., et al. Discovery of Ganymede’s magnetic field by the Galileo Discovery of Ganymede’s magnetic field by the Galileo spacecraftspacecraft. Nature, . Nature, 384384, 537-541 (1996)., 537-541 (1996).
Sarson, G. R., et al. Sarson, G. R., et al. Magnetoconvection Dynamos and the Magnetic Fields of Io and Magnetoconvection Dynamos and the Magnetic Fields of Io and GanymedeGanymede. Science, . Science, 276276, 1106-1108 (1997)., 1106-1108 (1997).
Schubert, G. et al. Schubert, G. et al. The magnetic field and internal structure of GanymedeThe magnetic field and internal structure of Ganymede. Nature, . Nature, 384384, 544-545 (1996)., 544-545 (1996).
Showman, A. P. and R. Malhotra. Showman, A. P. and R. Malhotra. The Galilean SatellitesThe Galilean Satellites. Science, . Science, 286286, 77-84 , 77-84 (1999).(1999).
Stevenson, D. J. Stevenson, D. J. Planetary magnetic fieldsPlanetary magnetic fields. EPSL. . EPSL. 208208, 1-11, (2003)., 1-11, (2003).