1 Does Io have a dynamo? Yasong Ge

2 Outline Overview of Io Overview of Io Io’s interior structure Io’s interior structure Io’s interaction with Jupiter’s magnetosphere Io’s interaction with Jupiter’s magnetosphere Controversy on Io’s internal field Controversy on Io’s internal field Galileo’s first flyby of Io Galileo’s first flyby of Io Galileo’s encounters with Io in 1999 and 2000 Galileo’s encounters with Io in 1999 and 2000 Io’s condition for a dynamo Io’s condition for a dynamo Conclusion Conclusion

3 Overview of Io The Jupiter’s innermost satellite The Jupiter’s innermost satellite The most volcanic body known The most volcanic body known Within an intense radiation belt Within an intense radiation belt J IE G

4 Io’s interior structure Io is in hydrostatic equilibrium. Io is in hydrostatic equilibrium. Io almost certainly has a metallic core with a radius between 550 and 900 km for an Fe- FeS core or between 350 and 650 km for an Fe core (Anderson, 2001). Io almost certainly has a metallic core with a radius between 550 and 900 km for an Fe- FeS core or between 350 and 650 km for an Fe core (Anderson, 2001). Silicates 3500 kg m-3 Fe-FeS 5150 kg m -3 From Nimmo’s lecture

5 Io’s interaction with Jupiter’s field Alfven Wing Alfven Wing

6 Magnetic signature of Io by Galileo’s first flyby The Voyage 1 detected magnetic perturbation of ~5% of the ambient jovian magnetic field about 11R Io (R Io =1821km) below Io, which comfirmed the presence of a field- aligned current. The Voyage 1 detected magnetic perturbation of ~5% of the ambient jovian magnetic field about 11R Io (R Io =1821km) below Io, which comfirmed the presence of a field- aligned current. A decrease of nearly 40% of the background jovian field was recorded by Galileo at closest approach (898km) of Io, that is 695 nT decrease in a background of 1835 nT (Kivelson, 1996). A decrease of nearly 40% of the background jovian field was recorded by Galileo at closest approach (898km) of Io, that is 695 nT decrease in a background of 1835 nT (Kivelson, 1996).

7 Kivelson’s explanation Plasma sources alone appear incapable of generating perturbations as large as those observed, which was obtained with the plasma parameters of Voyage. Plasma sources alone appear incapable of generating perturbations as large as those observed, which was obtained with the plasma parameters of Voyage. An intrinsic magnetic field of amplitude consistent with dynamo action at Io would explain the observations (Kivelson, 1996). An intrinsic magnetic field of amplitude consistent with dynamo action at Io would explain the observations (Kivelson, 1996).

8 Modeling on Io’s interaction Modeling (Krishan, 1997) Modeling (Krishan, 1997) With assumptions of “Long Wake” and Voyage 1 plasma data. With assumptions of “Long Wake” and Voyage 1 plasma data. The plasma effects can account for only a fractrion (<30%) of the observed depression and the principle cause of the reduction in the field strength is a source inside of Io. The plasma effects can account for only a fractrion (<30%) of the observed depression and the principle cause of the reduction in the field strength is a source inside of Io.

9 Plasma observations of Galileo’s first flyby of Io A plausible thick and dense ionosphere (Frank, 1996) A plausible thick and dense ionosphere (Frank, 1996) The measurement of torus mass densities are about two times greater than those inferred from Voyage flyby. The measurement of torus mass densities are about two times greater than those inferred from Voyage flyby. The magnetic perturbation due to the plasma interaction is doubled to ~500nT. The magnetic perturbation due to the plasma interaction is doubled to ~500nT. The gradients in the plasma pressure give rise to currents which cause magnetic perturbation between 100 to 200 nT. The gradients in the plasma pressure give rise to currents which cause magnetic perturbation between 100 to 200 nT. No need for a magnetized Io interior. No need for a magnetized Io interior.

10 Simulations Saur,1999: 3-D, two- fluid simulation Saur,1999: 3-D, two- fluid simulation The constant homogeneous Jovian background field as the only magnetic field The constant homogeneous Jovian background field as the only magnetic field Gives a good fit to the plasma data Gives a good fit to the plasma data Electric current of 10 million A gives observed magnetic field perturbation reported by Kivelson. Electric current of 10 million A gives observed magnetic field perturbation reported by Kivelson. No need for an internal field No need for an internal field

11 Galileo’s encounters with Io in 1999 and 2000 Data from the encounter on November 26,1999, with closest approach beneath Io’s south polar regions, were lost (Kivelson,2001). Data from the encounter on November 26,1999, with closest approach beneath Io’s south polar regions, were lost (Kivelson,2001). The trajectories of the passes that are available to study are all at relatively low latitude (Kivelson,2001). The trajectories of the passes that are available to study are all at relatively low latitude (Kivelson,2001). Data from I27 pass rule out a strongly magnetized Io but do not rule out a weakly magnetized Io (surface equatorial field of the order of Ganymede’s but smaller than the background field at Io) (Kivelson,2001) Data from I27 pass rule out a strongly magnetized Io but do not rule out a weakly magnetized Io (surface equatorial field of the order of Ganymede’s but smaller than the background field at Io) (Kivelson,2001)

12 Galileo’s encounters with Io in 1999 and 2000 cont’d Models suggest that if Io is magnetized, its magnetic moment is not strictly antialigned with the rotation axis. Models suggest that if Io is magnetized, its magnetic moment is not strictly antialigned with the rotation axis. Cannot conclude if Io does or does not have an internal magnetic moment, nor can we exclude a timing varying induced magnetic moment. Cannot conclude if Io does or does not have an internal magnetic moment, nor can we exclude a timing varying induced magnetic moment.

13 Galileo’s encounters with Io in 1999 and 2000 cont’d Saur’s simulation (Saur, 2002): Saur’s simulation (Saur, 2002): 3-D, two-fluid 3-D, two-fluid Only Jovian background field Only Jovian background field Good fit for the first pass. Good fit for the first pass. Explanation of Magnetometer data without Io’s intrinsic field. Explanation of Magnetometer data without Io’s intrinsic field.

14 Private communication with Galileo’s team The flybys after I27 still didn’t give the data that can definitely decide if Io has an intrinsic magnetic field (private communication with Dr. Russell). The flybys after I27 still didn’t give the data that can definitely decide if Io has an intrinsic magnetic field (private communication with Dr. Russell). There is no further work on the after passes. There is no further work on the after passes.

15 Self-sustained intrinsic field? Even though Io could have an internal field, the field may not be a self-sustained intrinsic field in the absence of the ambient jovian field (Sarson,1997). Even though Io could have an internal field, the field may not be a self-sustained intrinsic field in the absence of the ambient jovian field (Sarson,1997). Pre-Galileo modeling on Io’s dynamo: Pre-Galileo modeling on Io’s dynamo: If the surface heat flow and the tidal heating rate are in disequilibrium, periods of dynamo action alternating with periods of no magnetic field generation are likely (Wienbruch, 1995). If the surface heat flow and the tidal heating rate are in disequilibrium, periods of dynamo action alternating with periods of no magnetic field generation are likely (Wienbruch, 1995).

16 Io’s thermal state Tidal dissipation is brought to the surface by rapid ascent of magma, rather than by convection (Moore, 2001) Tidal dissipation is brought to the surface by rapid ascent of magma, rather than by convection (Moore, 2001) Moore’s model also shows that either Io is currently out of thermal equilibrium, or other heat transport mechanism such as melt segregation determines Io’s thermal state (Moore, 2003). Moore’s model also shows that either Io is currently out of thermal equilibrium, or other heat transport mechanism such as melt segregation determines Io’s thermal state (Moore, 2003).

17 Other options if Io has intrinsic field Ferromagnetism? (Cheng, 1996) Ferromagnetism? (Cheng, 1996) Tidal instability (Kerswell, 1998) Tidal instability (Kerswell, 1998) Lab realization for extreme ‘stirring’ and dissipation Lab realization for extreme ‘stirring’ and dissipation The energy to sustain Io’s magnetic field would come ultimately from Jupiter’s rotational energy. The energy to sustain Io’s magnetic field would come ultimately from Jupiter’s rotational energy.

18 Conclusion It is difficult to decide if Io has an intrinsic magnetic field by Galileo’s flyby observations. It is difficult to decide if Io has an intrinsic magnetic field by Galileo’s flyby observations. Modeling indicates that the thermal state of Io seems not favorable for convection in the mantle, thus maybe not for a dynamo either. Modeling indicates that the thermal state of Io seems not favorable for convection in the mantle, thus maybe not for a dynamo either.

19 References Anderson, J. D., et al., Io’s gravity and interior structure, J. Geophysics. Res., 106(E12), 32,963, 2001 Anderson, J. D., et al., Io’s gravity and interior structure, J. Geophysics. Res., 106(E12), 32,963, 2001 Beatty, J. K., et al, The new solar system Beatty, J. K., et al, The new solar system Cheng, A. F., and C. Paranicas, Implications of Io’s magnetic signature: Ferromagnetism?, Geophys. Res. Lett., 23, 2879,1996 Cheng, A. F., and C. Paranicas, Implications of Io’s magnetic signature: Ferromagnetism?, Geophys. Res. Lett., 23, 2879,1996 Frank, L. A., et al., Plasma observations at Io with the Galileo spacecraft, Science, 274, 394, 1996 Frank, L. A., et al., Plasma observations at Io with the Galileo spacecraft, Science, 274, 394, 1996 Kerswell, R. R., and Willem V. R. Malkus, Tidal instability as the source for Io’s magnetic signature, Geophys. Res. Lett., 25, 603,1998 Kerswell, R. R., and Willem V. R. Malkus, Tidal instability as the source for Io’s magnetic signature, Geophys. Res. Lett., 25, 603,1998 Khurana, K. K., et al., Interaction of Io with its torus: Does Io have an internal magnetic field?, Geophys. Res. Lett., 24, 2391,1997 Khurana, K. K., et al., Interaction of Io with its torus: Does Io have an internal magnetic field?, Geophys. Res. Lett., 24, 2391,1997 Kivelson, M. G., et al., A magnetic Signature at Io: Initial Report from the Galileo Magnetometer, Science, Vol. 273, 337,1996 Kivelson, M. G., et al., A magnetic Signature at Io: Initial Report from the Galileo Magnetometer, Science, Vol. 273, 337,1996 Kivelson, M. G., et al., Magnetized or unmagnetized: Ambiguity persists following Galileo’s encounters with Io in 1999 and 2000, J. Geophysics. Res., 106(A11), 26,121, 2001 Kivelson, M. G., et al., Magnetized or unmagnetized: Ambiguity persists following Galileo’s encounters with Io in 1999 and 2000, J. Geophysics. Res., 106(A11), 26,121, 2001 Moore, W. B., The thermal state of Io, Icarus 154, 548, 2001 Moore, W. B., The thermal state of Io, Icarus 154, 548, 2001 Moore, W. B., Tidal heating and convection in Io, J. Geophysics. Res., 108(E8), 5096, 2003 Moore, W. B., Tidal heating and convection in Io, J. Geophysics. Res., 108(E8), 5096, 2003 Sarson, G. R., et al., Magnetoconvection Dynamos and the Magnetic Fields of Io and Ganymede, Science, 276,1997 Sarson, G. R., et al., Magnetoconvection Dynamos and the Magnetic Fields of Io and Ganymede, Science, 276,1997 Saur, J., et al., Three-dimensional plasma simulation of Io’s interaction with the Io plasma torus: Asymmetric plasma flow, J. Geophysics. Res., 104(A11), 25,105, 1999 Saur, J., et al., Three-dimensional plasma simulation of Io’s interaction with the Io plasma torus: Asymmetric plasma flow, J. Geophysics. Res., 104(A11), 25,105, 1999 Saur, J., et al., Interpretation of Galileo’s Io plasma and field observations: I0, I24, and I27 flybys and close polar passes, J. Geophysics. Res., 107(A12), 1422, 2002 Saur, J., et al., Interpretation of Galileo’s Io plasma and field observations: I0, I24, and I27 flybys and close polar passes, J. Geophysics. Res., 107(A12), 1422, 2002 Wienbruch, U and T. Spohn, A self sustained magnetic field on Io?, Planet. Space. Sci., Vol. 43, No. 9., 1045,1995 Wienbruch, U and T. Spohn, A self sustained magnetic field on Io?, Planet. Space. Sci., Vol. 43, No. 9., 1045,1995

20 Thank you!