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Does Io have a dynamo?Does Io have a dynamo?

Yasong GeYasong Ge

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OutlineOutline

Overview of IoOverview of Io Io’s interior structureIo’s interior structure Io’s interaction with Jupiter’s magnetosphereIo’s interaction with Jupiter’s magnetosphere Controversy on Io’s internal fieldControversy on Io’s internal field

Galileo’s first flyby of IoGalileo’s first flyby of Io Galileo’s encounters with Io in 1999 and 2000Galileo’s encounters with Io in 1999 and 2000

Io’s condition for a dynamoIo’s condition for a dynamo ConclusionConclusion

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Overview of IoOverview of Io

The Jupiter’s The Jupiter’s innermost innermost satellitesatellite

The most The most volcanic body volcanic body knownknown

Within an Within an intense intense radiation beltradiation belt

JI EG

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Io’s interior structureIo’s interior structure Io is in hydrostatic Io is in hydrostatic

equilibrium. equilibrium. Io almost certainly has Io almost certainly has

a metallic core with a a metallic core with a radius between 550 radius between 550 and 900 km for an Fe-and 900 km for an Fe-FeS core or between FeS core or between 350 and 650 km for an 350 and 650 km for an Fe core (Anderson, Fe core (Anderson, 2001).2001).

Silicates3500 kg m-3

Fe-FeS5150 kg m-3

From Nimmo’s lecture

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Io’s interaction with Jupiter’s fieldIo’s interaction with Jupiter’s field

Alfven WingAlfven Wing

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Magnetic signature of Io by Magnetic signature of Io by Galileo’s first flybyGalileo’s first flyby

The Voyage 1 detected The Voyage 1 detected magnetic perturbation magnetic perturbation of ~5% of the ambient of ~5% of the ambient jovian magnetic field jovian magnetic field about 11Rabout 11RIo Io (R(RIo Io

=1821km) below Io, =1821km) below Io, which comfirmed the which comfirmed the presence of a field-presence of a field-aligned current.aligned current.

A decrease of nearly A decrease of nearly 40% of the background 40% of the background jovian field was jovian field was recorded by Galileo at recorded by Galileo at closest approach closest approach (898km) of Io, that is (898km) of Io, that is 695 nT decrease in a 695 nT decrease in a background of 1835 nT background of 1835 nT (Kivelson, 1996).(Kivelson, 1996).

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Kivelson’s explanationKivelson’s explanation Plasma sources alone Plasma sources alone

appear incapable of appear incapable of generating generating perturbations as large perturbations as large as those observed, as those observed, which was obtained which was obtained with the plasma with the plasma parameters of Voyage.parameters of Voyage.

An intrinsic An intrinsic magnetic field magnetic field of of amplitude consistent amplitude consistent with dynamo action at with dynamo action at Io would explain the Io would explain the observations observations (Kivelson, 1996).(Kivelson, 1996).

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Modeling on Io’s interactionModeling on Io’s interaction Modeling (Krishan, Modeling (Krishan,

1997)1997) With assumptions of With assumptions of

“Long Wake” and “Long Wake” and Voyage 1 plasma data.Voyage 1 plasma data.

The plasma effects The plasma effects can account for only a can account for only a fractrion (<30%) of the fractrion (<30%) of the observed depression observed depression and the principle cause and the principle cause of the reduction in the of the reduction in the field strength is a field strength is a source source inside of Io.inside of Io.

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Plasma observations of Galileo’s Plasma observations of Galileo’s first flyby of Iofirst flyby of Io

A plausible thick and dense ionosphere (Frank, A plausible thick and dense ionosphere (Frank, 1996)1996) The measurement of torus mass densities are about The measurement of torus mass densities are about

two times greater than those inferred from Voyage two times greater than those inferred from Voyage flyby.flyby.

The magnetic perturbation due to the plasma The magnetic perturbation due to the plasma interaction is doubled to ~500nT.interaction is doubled to ~500nT.

The gradients in the plasma pressure give rise to The gradients in the plasma pressure give rise to currents which cause magnetic perturbation between currents which cause magnetic perturbation between 100 to 200 nT.100 to 200 nT.

No need for a magnetized Io interior.No need for a magnetized Io interior.

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SimulationsSimulations Saur,1999: 3-D, two-Saur,1999: 3-D, two-

fluid simulationfluid simulation The constant The constant

homogeneous Jovian homogeneous Jovian background field as the background field as the only magnetic fieldonly magnetic field

Gives a good fit to the Gives a good fit to the plasma dataplasma data

Electric current of 10 Electric current of 10 million A gives observed million A gives observed magnetic field magnetic field perturbation reported by perturbation reported by Kivelson.Kivelson.

No need for an No need for an internal fieldinternal field

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Galileo’s encounters with Galileo’s encounters with Io in 1999 and 2000Io in 1999 and 2000

Data from the encounter on November Data from the encounter on November 26,1999, with closest approach beneath 26,1999, with closest approach beneath Io’s south polar regions, were lost Io’s south polar regions, were lost (Kivelson,2001).(Kivelson,2001).

The trajectories of the passes that are The trajectories of the passes that are available to study are all at relatively low available to study are all at relatively low latitude (Kivelson,2001).latitude (Kivelson,2001).

Data from I27 pass rule out a strongly Data from I27 pass rule out a strongly magnetized Io but do not rule out a weakly magnetized Io but do not rule out a weakly magnetized Io (surface equatorial field of magnetized Io (surface equatorial field of the order of Ganymede’s but smaller than the order of Ganymede’s but smaller than the background field at Io) (Kivelson,2001)the background field at Io) (Kivelson,2001)

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Galileo’s encounters Galileo’s encounters with Io in 1999 and with Io in 1999 and

2000 cont’d2000 cont’d

Models suggest that if Models suggest that if Io is magnetized, its Io is magnetized, its magnetic moment is magnetic moment is not strictly antialigned not strictly antialigned with the rotation axis.with the rotation axis.

Cannot concludeCannot conclude if Io if Io does or does not have an does or does not have an internal magnetic internal magnetic moment, nor can we moment, nor can we exclude a timing varying exclude a timing varying induced magnetic induced magnetic moment.moment.

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Galileo’s encounters Galileo’s encounters with Io in 1999 and with Io in 1999 and

2000 cont’d2000 cont’d Saur’s simulation Saur’s simulation

(Saur, 2002):(Saur, 2002): 3-D, two-fluid3-D, two-fluid Only Jovian Only Jovian

background fieldbackground field

Good fit for the first Good fit for the first pass.pass.

Explanation of Explanation of Magnetometer data Magnetometer data without Io’s intrinsic without Io’s intrinsic field.field.

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Private communication with Private communication with Galileo’s teamGalileo’s team

The flybys after I27 still didn’t give the data The flybys after I27 still didn’t give the data that can definitely decide if Io has an that can definitely decide if Io has an intrinsic magnetic field (private intrinsic magnetic field (private communication with Dr. Russell). communication with Dr. Russell).

There is no further work on the after There is no further work on the after passes.passes.

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Self-sustained intrinsic field?Self-sustained intrinsic field? Even though Io could have an Even though Io could have an

internal field, the field may not be a internal field, the field may not be a self-sustained intrinsic field in the self-sustained intrinsic field in the absence of the ambient jovian field absence of the ambient jovian field (Sarson,1997).(Sarson,1997).

Pre-Galileo modeling on Io’s Pre-Galileo modeling on Io’s dynamo:dynamo: If the surface heat flow and the If the surface heat flow and the

tidal heating rate are in tidal heating rate are in disequilibrium, periods of disequilibrium, periods of dynamo action alternating with dynamo action alternating with periods of no magnetic field periods of no magnetic field generation are likely generation are likely (Wienbruch, 1995).(Wienbruch, 1995).

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Io’s thermal stateIo’s thermal state Tidal dissipation is Tidal dissipation is

brought to the surface brought to the surface by rapid ascent of by rapid ascent of magma, rather than by magma, rather than by convection (Moore, convection (Moore, 2001)2001)

Moore’s model also Moore’s model also shows that either Io is shows that either Io is currently out of currently out of thermal equilibrium, or thermal equilibrium, or other heat transport other heat transport mechanism such as mechanism such as melt segregation melt segregation determines Io’s determines Io’s thermal state (Moore, thermal state (Moore, 2003).2003).

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Other options if Io has intrinsic field Other options if Io has intrinsic field

Ferromagnetism? Ferromagnetism? (Cheng, 1996)(Cheng, 1996)

Tidal instability Tidal instability (Kerswell, 1998)(Kerswell, 1998) Lab realization for Lab realization for

extreme ‘stirring’ and extreme ‘stirring’ and dissipationdissipation

The energy to sustain The energy to sustain Io’s magnetic field Io’s magnetic field would come ultimately would come ultimately from Jupiter’s rotational from Jupiter’s rotational energy.energy.

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ConclusionConclusion

It is difficult to decide if Io has an intrinsic It is difficult to decide if Io has an intrinsic magnetic field by Galileo’s flyby magnetic field by Galileo’s flyby observations.observations.

Modeling indicates that the thermal state Modeling indicates that the thermal state of Io seems not favorable for convection in of Io seems not favorable for convection in the mantle, thus maybe not for a dynamo the mantle, thus maybe not for a dynamo either.either.

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ReferencesReferences Anderson, J. D., et al., Io’s gravity and interior structure, Anderson, J. D., et al., Io’s gravity and interior structure, J. Geophysics. Res.,J. Geophysics. Res., 106106(E12), 32,963, 2001(E12), 32,963, 2001 Beatty, J. K., et al, The new solar systemBeatty, J. K., et al, The new solar system Cheng, A. F., and C. Paranicas, Implications of Io’s magnetic signature: Ferromagnetism?, Cheng, A. F., and C. Paranicas, Implications of Io’s magnetic signature: Ferromagnetism?, Geophys. Res. Geophys. Res.

Lett.,Lett., 23, 2879,1996 23, 2879,1996 Frank, L. A., et al., Plasma observations at Io with the Galileo spacecraft, Frank, L. A., et al., Plasma observations at Io with the Galileo spacecraft, ScienceScience, 274, 394, 1996, 274, 394, 1996 Kerswell, R. R., and Willem V. R. Malkus, Tidal instability as the source for Io’s magnetic signature, Kerswell, R. R., and Willem V. R. Malkus, Tidal instability as the source for Io’s magnetic signature,

Geophys. Res. Lett.,Geophys. Res. Lett., 25, 603,1998 25, 603,1998 Khurana, K. K., et al., Interaction of Io with its torus: Does Io have an internal magnetic field?, Khurana, K. K., et al., Interaction of Io with its torus: Does Io have an internal magnetic field?, Geophys. Res. Geophys. Res.

Lett.,Lett., 24, 2391,1997 24, 2391,1997 Kivelson, M. G., et al., A magnetic Signature at Io: Initial Report from the Galileo Magnetometer, Kivelson, M. G., et al., A magnetic Signature at Io: Initial Report from the Galileo Magnetometer, ScienceScience, ,

Vol. 273, 337,1996Vol. 273, 337,1996 Kivelson, M. G., et al., Magnetized or unmagnetized: Ambiguity persists following Galileo’s encounters with Kivelson, M. G., et al., Magnetized or unmagnetized: Ambiguity persists following Galileo’s encounters with

Io in 1999 and 2000, Io in 1999 and 2000, J. Geophysics. Res.,J. Geophysics. Res., 106106(A11), 26,121, 2001 (A11), 26,121, 2001 Moore, W. B., The thermal state of Io, Icarus Moore, W. B., The thermal state of Io, Icarus 154154, 548, 2001, 548, 2001 Moore, W. B., Tidal heating and convection in Io, Moore, W. B., Tidal heating and convection in Io, J. Geophysics. Res.,J. Geophysics. Res., 108108(E8), 5096, 2003(E8), 5096, 2003 Sarson, G. R., et al., Magnetoconvection Dynamos and the Magnetic Fields of Io and Ganymede, Science, Sarson, G. R., et al., Magnetoconvection Dynamos and the Magnetic Fields of Io and Ganymede, Science,

276,1997276,1997 Saur, J., et al., Three-dimensional plasma simulation of Io’s interaction with the Io plasma torus: Asymmetric Saur, J., et al., Three-dimensional plasma simulation of Io’s interaction with the Io plasma torus: Asymmetric

plasma flow, plasma flow, J. Geophysics. Res.,J. Geophysics. Res., 104104(A11), 25,105, 1999(A11), 25,105, 1999 Saur, J., et al., Interpretation of Galileo’s Io plasma and field observations: I0, I24, and I27 flybys and close Saur, J., et al., Interpretation of Galileo’s Io plasma and field observations: I0, I24, and I27 flybys and close

polar passes, polar passes, J. Geophysics. Res.,J. Geophysics. Res., 107107(A12), 1422, 2002(A12), 1422, 2002 Wienbruch, U and T. Spohn, A self sustained magnetic field on Io?, Wienbruch, U and T. Spohn, A self sustained magnetic field on Io?, Planet. Space. Sci.,Planet. Space. Sci., Vol. 43, No. 9., Vol. 43, No. 9.,

1045,19951045,1995

2020

Thank you!Thank you!