Exploring the interior of icy satellites using magnetic induction Krishan K. Khurana Institute of Geophysics and Planetary Physics University of California at Los Angeles

2 Giant planets and their major icy satellites

3 Europa Ganymede Callisto 1560 km 2634 km 2400 km radius 2990 kg m kg m kg m -3 density C/MR hr hr hr period Meet the icy moons of Jupiter, worlds of rocks and ice. The moons are phase locked with Jupiter.

4 Record of impact craters reveals the surface ages of solar system bodies

Arbela Sulcus Regional View Ganymede Europa Ganymede Europa

6 Multiple passes showed that Ganymede is surrounded by a mini- magnetosphere that rocks with the ~10 hour period of Jupiter’s rotation.

B Induced (t) The principle behind electromagnetic induction The total field The primary and secondary fields shown separately B Primary (t) –Eddy currents generate a secondary or induced field which reduces the primary field inside the conductor. –The induced field can be detected with a sensor. Eddy currents

Jupiter provides the primary field The Galilean satellites are located in the inner and middle magnetosphere of Jupiter. Because the dipole and rotation axes of Jupiter are not aligned, the moons experience a varying field in their frame.

Galileo Observations at Europa E4 and E14 passes showed signatures consistent with induced dipolar fields from currents flowing near the surface. The direction of the dipole moment was directed towards Europa in both cases (as expected). A subsequent pass (E26) confirmed that the dipole moment flipped in response to the different orientation of Jupiter’s field as expected from theory. E14

10 Galileo Observations at Callisto During the C3 flyby, the magnetic field of Jupiter was directed radially outward. During the C9 flyby, the magnetic field of Jupiter was directed radially inwards. The observed induction signature also showed opposite polarities. This confirms that electromagnetic induction and not a permanent dipole is the source of the observed signature.

11 And what about Callisto’s surface? Callisto appears inactive on every length scale.

The inductive response for Ganymede Induced moment in a metallic sphere M yo = 49 nT 100% response 82% response Kivelson et al. Icarus, 2002

The Likely Material Table 1. Conductivities of common materials and their skin depths for a ten-hour wave.

Detailed modeling shows that: “Near” surface conductors are required to fit Europa, Ganymede & Callisto measurements Source of field cannot be far below the surface because the field strength falls like (r/R surf ) 3 and signature would become too weak to detect. We know that Europa’s H 2 O layer is ~ 150 km thick, Ganymede and Callisto’s > 400 km. Global sub-surface oceans of at least a few km thicknesses and located at a depth of a few to tens of km for Europa and ~ 150 km for Ganymede and Callisto are required to explain the observed signatures.

Moving beyond Galileo’s capabilities... Galileo’s passes yielded short time series, so only the dominant frequency of the primary signal (the inverse of the synodic rotation period of Jupiter  11 hours) was considered. –Measurements at this frequency did not establish separately the conductivity and the thickness of the ocean, but rather a combination. The source signal at Europa actually contains other predictable frequencies including that arising from its 85 hours orbital period. –The depth to which a signal penetrates into a conductor increases with the period of the signal.

B(f) anticipated at a Europa Orbiter period arises from ellipticity & small tilt of orbit. The spectra contain many peaks. Greatest power occurs at the inverse of Jupiter’s rotation period. Power at Europa’s orbital

What could a Europa Orbiter measure? The Jovian field predicted at the nominal orbit of a Europa orbiter (polar orbit, altitude ~ 200 km) Three periodicities appear: – the 2 hour period of the spacecraft orbit, – the 11.1 hr rotation period of Jupiter – the 85 hour orbital period of Europa. Curves plotted are for: – no ocean (black) – a 3 km thick ocean (  = 2.75 S/m, red) – a 100 km thick ocean (  = 2.75 S/m, cyan).

Sounding Europa at the two main frequencies The blue curves show response from a wave at Europa’s rotation period and the red curves show response at Europa’s orbital period. When the amplitude curves are parallel to each other, conductivity and shell thickness cannot be separately identified. There is a parameter space domain where the induction amplitude curves at the two frequencies intersect each other. In this domain one can obtain the values of the ocean thickness and its conductivity uniquely from measurements made at the two frequencies. Khurana et al. 2002, Astrobiology

19 Some thoughts about other icy satellites The Uranian and Neptunian icy satellites also experience strong varying fields at the diurnal period because of the strong tilted magnetic dipoles of these planets. We should be able to use these strong inducting fields to probe the interiors of their icy satellites. Titan and the other icy satellites of Saturn on the other hand experience almost no time varying field because the dipole and rotation axes of Saturn are aligned. A mysterious periodicity at the rotation period of Saturn was reported by Stephane Espinosa in the radial and azimuthal components with an amplitude of ~ 4 nT (JGR, 2003, 1085). Prof. Fritz Neubauer has suggested that the changes in the ionospheric conductivity of Titan would create a signal at the orbital period of Titan. The interior of Titan could be experiencing a measurable inducing AC field from the ionospheric electric current modulations.

20 Temporal signal in the magnetosphere of Saturn

Conclusions Electromagnetic induction probing is now a proven technique for locating liquid water in the interiors of the icy satellites of the solar system. So far the limited nature of the data from the Galilean satellites has meant that we can only make qualitative statements about the thicknesses and the conductivities of the oceans. Future data from multiple points and multiple frequencies should allow us to uniquely determine these parameters. The technique should work well for the icy satellites of Uranus and Neptune as well. More work is required to adapt the technique for probing the interiors of the Kronian satellites.

22 End/ Reserve slides follow

Assume a uniform oscillating primary field B primary (t) = B o (t). When  (i.e. conductivity becomes infinite), the external solution is: i.e., a uniform + dipole field. The surface strength (polar) of the dipole field is B o and the dipole moment is directed opposite to the primary field. Induction from an infinitely conducting shell B Primary (t) B Induced (t)

Sounding at two magnetic frequencies Longer period waves penetrate deeper thus providing more information on structure.

Induction from a finite-conductivity shell

Induced field from a plane half-space The electrodynamic equation describes the convection and diffusion of the magnetic field in a conductor: In the absence of convection in the conductor, the equation reduces to the well known diffusion equation: The solution for a conducting half-space plane(z >0) in the presence of a uniform oscillating field is: which shows that the field decays in the conductor by an e folding within a skin depth S. The diffusion time for the field is:

Induction from a finite-conductivity shell

Induction from a finite-conductivity shell (cont)

Why might a melted layer be buried as deep as ~170 km inside of Ganymede? a possible temperature profile that could produce a melted layer The melting temperature of ice varies with pressure and therefore with depth. Temperature increases with depth from  180º C= ~ 100ºK at the surface. Minimum melting temperature occurs about 170 km depth. Ocean might be in this zone.

30 Pull-Apart Bands Analogous to terrestrial crustal spreading regions at mid-oceanic ridges. (24X16 km 2 )

Chaos (Conamara Region) Evidence that the crust is being destroyed by tectonics. (35X50 km 2 )

Ganymede: A Moon with Magnetism (with thanks to Torrence Johnson)

Galileo Measurements of Induction from Europa Induction dipole moment from several Galileo passes plotted against predictions from a highly conducing shell model.

For Ganymede, the most probable values (i.e., 82% response) given by our analysis would be expected if: an ocean of thickness greater than a few km is buried about 170 km beneath the surface. This depth of burial is reasonably consistent with the pressure dependence of the melting temperature of ice. And there is now more compelling evidence of subsurface water at some past time in images.