Does Ganymede Have a Dynamo?

1. Does Ganymede Have a Dynamo? Jennifer Palguta December 2, 2004

2. Overview • Possible sources for Ganymede’s intrinsic field: • Magnetoconvection • Remanent magnetization due to Jupiter’s magnetic field • Internal active dynamo • Remanent magnetization due to an internal dynamo which is no longer active • A present vs. past dynamo.

3. Discovery of Ganymede’s Magnetic Field • Discovered after the 1996 flybys • Evidence for an internal field is unambiguous • Equatorial field magnitude ~ 720 nT • While observations require an internal field they do not indicate its source Sarson et al. 1997

4. Internal structure I • Ganymede’s internal structure and thermal state determines which source for the magnetic field is most likely. • The only non-zero gravity parameters of importance are the monopole GM and the 2 quadrupole coefficients (not entirely correct). • GM = 9887.83 km3 s-2 • Mean density = 1942 kg m-3 • C/(MR2) = .311

5. Internal Structure II • The low value for the MOI indicates Ganymede is strongly differentiated. • The density requires that Ganymede have a large water-ice component. • A three-layer model appears most consistent with observations. http://www.solarviews.com

6. Three-layer Model • Consists of a metallic core overlaid by 2 spherical shells. • 2 options for the metallic core: • Fe • Fe-FeS • Requires heating to at least the Fe-FeS melting temperature. • Sources: • Accretional • Radiogenic • Tidal • Implications?

7. Magnetoconvection • Produces a field due to an externally imposed magnetic field. • Requires an electrically conducting fluid to be affected by the imposed field. • 2 possible regions • Ocean of salty water • Metallic inner core

8. Salty Ocean I • Plotting the melting temperature vs. depth with the temperature profile suggests an ocean. • Temporal variations arise in the jovian magnetosphere because Jupiter’s tilted dipole moment changes its orientation as the planet rotates. • In principle, the time varying field drives inductive currents within the ocean. These induced currents produce a time-varying magnetic moment. Kivelson et al. 2002

9. Kivelson et al. 2002

10. Salty Ocean II • Kivelson et al. used G1, G2, G28 to determine whether an inductive response is present. • The external field for G1 and G2 was directed radially outward. • The external field for G28 was directed radially inward. • Were there an induced magnetic moment it would be antiparallel to the radial component of the external field and its orientation for G28 would differ from that for G1 and G2. • The time-varying field at Ganymede has an amplitude of ~100 nT. • The induced dipole moment will be at most ~ 6% of the permanent magnetic moment.

11. Inner Metallic Core • The induced magnetic field is of the same order of magnitude or smaller than the imposed field. A stronger driving force is required. • Including an induced field does little to the resultant solution (essentially it operates by a “pure” dynamo process and the induced magnetic field is effectively negligible). • Dynamo action rather than magnetoconvection is a more likely explanation because Ganymede’s magnetic field is so much larger (by a factor of six) than the ambient jovian magnetic field. Sarson et al. 1997

12. Magnetoconvection Conclusions • For Ganymede, it is difficult to see how magnetoconvection could produce an intrinsic field so much stronger than the local ambient field. • It seems unlikely that Ganymede’s intrinsic field is generated solely by magnetoconvection.

13. Remanent Magnetization • Magnetization would occur in a layer of rock which was once heated above the Curie temperature but has since cooled below it. • Muller and McKinnon used meteoritic compositions and equilibrium condensation values to estimate the mineralogy of Ganymede’s interior: • Ganymede is rich in magnetite • Potential source of strong remanent magnetism. • Sources • Jovian field • Paleodynamo

14. Remanent Magnetization from Jupiter • Requires no differentiation between rock and iron. • Lack of differentiation provides an upper limit on magnetization from the jovian field. • Magnetite content not reduced (7.5-16.5%) • Higher effective susceptibility, c (ratio of the remanent magnetization to the local magnetic field) • Model: series of concentric shells cooling through the Curie temperature one after the other.

15. Remanent Magnetization from Jupiter:Results I • The resulting remanent field saturates when its strength at the top of the ferromagnetic layer is ~ to the external field. • The magnetic field interior to the shell is reduced and subsequent shells are magnetized by progressively weaker net fields. • Therefore, remanent magnetization by the jovian field cannot produce a dipole field stronger than the background field. Crary and Bagenal 1998

16. Remanent Magnetization from Jupiter:Results II • As the jovian field decreases the remanent dipole increases toward the new background field strength, overshoots, then decreases towards the background field strength. • Even for unrealistically high values of c the current strength at the top of the ferromagnetic layer would be < 150 nT. • This corresponds to an equatorial strength < 6% of the observed value. Crary and Bagenal 1998

17. Remanent Magnetization from Jupiter:Conclusions • To achieve the requisite level of magnetization in such a layer to account for Ganymede’s intrinsic magnetic dipole moment requires a sufficiently large concentration of magnetite and an external magnetizing field larger than the present jovian magnetic field at Ganymede’s orbit. • The necessary requirements fall outside reasonable parameter values.

18. Active Internal Dynamo • Requires differentiation into a rocky layer and an iron/iron sulfide core. • Heating would occur early in the history and sustaining core convection in the small body until today is difficult to explain. • The cooling rate required for the onset of convection in the core ~ 300-400 K/Ga. • This exceeds values expected from the decline of the radioisotope heat budget.

19. Active Internal Dynamo: Conclusions • Hard to explain how an active dynamo could exist without invoking additional energy sources. • Possibilities • Recent capture into resonance • Large amounts of sulfur • Decay of a lot 40K • There might once have been an active dynamo.

20. Remanent Magnetization from Paleodynamo • Does not require ongoing convection. • The remanent field is ~1-6.8% of the maximum dynamo field. • Requires a maximum dipole field of over 11 μT. • Estimates of lunar paleomagnetism suggest that the Moon once had a dynamo-driven magnetic field of 10-100 μT. • Ganymede has a greater metal content, larger conduction core, faster rotation rate, possible enhancement by jovian field. • One problem: It’s possible Ganymede’s field reversed itself. Increases the necessary maximum dynamo field to 462 μT. Crary and Bagenal 1998

21. Conclusions • A dynamo is required to explain the observed field (though other methods might contribute to the field). • An active dynamo is hard to explain. • Dynamo action in the past is more likely and is capable of causing significant magnetization.

22. Bibliography • Anderson, J. D. et al. Gravitational constraints on the internal structure of Ganymede. Nature, 384, 541-543 (1996). • Crary, F. J. and F. Bagenal. Remanent ferromagnetism and the interior structure of Ganymede. JGR 103, 25,757-25,773 (1998). • Kivelson, M. G., et al. The Permanent and Inductive Magnetic Moments of Ganymede. Icarus, 157, 507-522 (2002). • Kivelson, M. G., et al. Discovery of Ganymede’s magnetic field by the Galileo spacecraft. Nature, 384, 537-541 (1996). • Sarson, G. R., et al. Magnetoconvection Dynamos and the Magnetic Fields of Io and Ganymede. Science, 276, 1106-1108 (1997). • Schubert, G. et al. The magnetic field and internal structure of Ganymede. Nature, 384, 544-545 (1996). • Showman, A. P. and R. Malhotra. The Galilean Satellites. Science, 286, 77-84 (1999). • Stevenson, D. J. Planetary magnetic fields. EPSL. 208, 1-11, (2003).