The Galileo spacecraft has been orbiting Jupiter since 7 December 1995, and encounters one of the four galilean satellites-Io, Europa, Ganymede and Callisto-on each orbit Initial results from the spacecraft's magnetometer have indicated that neither Europa nor Callisto have an appreciable internal magnetic field, in contrast to Ganymede and possibly Io Here we report perturbations of the external magnetic fields (associated with Jupiter's inner magnetosphere) in the vicinity of both Europa and Callisto We interpret these perturbations as arising from induced magnetic fields, generated by the moons in response to the periodically varying plasma environment Electromagnetic induction requires eddy currents to flow within the moons, and our calculations show that the most probable explanation is that there are layers of significant electrical conductivity just beneath the surfaces of both moons We argue that these conducting layers may best be explained by the presence of salty liquid-water oceans, for which there is already indirect geological evidence in the case of Europa

Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto

On 3 January 2000, the Galileo spacecraft passed close to Europa when it was located far south of Jupiter's magnetic equator in a region where the radial component of the magnetospheric magnetic field points inward toward Jupiter. This pass with a previously unexamined orientation of the external forcing field distinguished between an induced and a permanent magnetic dipole moment model of Europa's internal field. The Galileo magnetometer measured changes in the magnetic field predicted if a current-carrying outer shell, such as a planet-scale liquid ocean, is present beneath the icy surface. The evidence that Europa's field varies temporally strengthens the argument that a liquid ocean exists beneath the present-day surface.

/pdf/measurements-a-stronger-case-for-a-subsurface-ocean-at-2nhatlahxt.pdf

Measurements: A Stronger Case for a Subsurface Ocean at Europa

Superionic conductors are compounds that exhibit exceptionally high values of ionic conductivity within the solid state. Indeed, their conductivities often reach values of the order of 1 Ω−1 cm−1, which are comparable to those observed in the molten state. Following Faraday's first observation of high ionic conductivity within the solids β-PbF2 and Ag2S in 1836, a fundamental understanding of the nature of the superionic state has provided one of the major challenges in the field of condensed matter science. However, experimental and theoretical approaches to their study are often made difficult by the extensive dynamic structural disorder which characterizes superionic conduction and the inapplicability of many of the commonly used approximations in solid state physics. Nevertheless, a clearer picture of the nature of the superionic state at the ionic level has emerged within the past few decades. Many different techniques have contributed to these advances, but the most significant insights have been provided by neutron scattering experiments and molecular dynamics simulations. This review will summarize the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors, beginning with a comparison of the behaviour of two of the most widely studied binary compounds, AgI and β-PbF2. Each can be considered a parent of two larger families of highly conducting compounds which are related by either chemical or structural means. These include perovskite-structured oxides and Li+ containing spinel-structured compounds, which have important commercial applications in fuel cells and lightweight batteries, respectively. In parallel with these discussions, the relative importance of factors such as bonding character and the properties of the mobile and immobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorder which characterizes superionic behaviour will be assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.

/pdf/superionics-crystal-structures-and-conduction-processes-1q9bpg4zfv.pdf

Superionics: crystal structures and conduction processes

http://www.igpp.ucla.edu/people/mkivelson/Publications/234-ICRUS147329.pdf

Subsurface Oceans on Europa and Callisto: Constraints from Galileo Magnetometer Observations

Summary
During bays and similar magnetic variations the vectors representing changes in the geomagnetic field tend to lie on or close to a plane. The orientation of this plane varies from one observatory to another. At coastal observatories it almost invariably tilts upward towards the nearest deep ocean.

/pdf/the-influence-of-continents-and-oceans-on-geomagnetic-1yr8htx2ti.pdf

The Influence of Continents and Oceans on Geomagnetic Variations

Summary
The vectors representing geomagnetic changes over intervals less than one hour tend to be confined to a plane at most temperate latitude stations. In many cases this plane is almost horizontal, but sometimes it is steeply inclined. This appears to be caused by currents induced asymmetrically in the Earth, possibly due to the highly conducting oceans.

/pdf/directions-of-rapid-geomagnetic-fluctuations-4yv4p94lyg.pdf

Directions of Rapid Geomagnetic Fluctuations

Introduction to geomagnetism

Near most coastlines the vertical component of the geomagnetic variation field is abnormally large and correlates positively with the inland horizontal component. This phenomenon is known as the geomagnetic coast effect. The few coastal locations at which it is absent are tectonically anomalous. The ratio of vertical to horizontal components decreases inland at a rate which depends on the geological nature of the continent. The effect varies only slightly with period, reaching a broad maximum between periods of 30 and 90 min. A number of models have been investigated by either calculation or analogue modeling, but the complete problem dealing with finitely conducting irregular oceans overlying a less conducting lithosphere has not yet been solved. Eddy currents flowing entirely in the seawater would be induced only by the vertical component of the magnetic field. Observations, combined with analogue model experiments, suggest that induction by horizontal components is also important. This requires flow of current in the lithosphere beneath the oceans.

https://www.leif.org/EOS/RG017i008p01999.pdf

The geomagnetic coast effect

The Tamar conductivity anomaly

Summary

Extensive 2-D forward modelling has been used to study conductive structures beneath the Cordillera, the Rocky Mountains and the Rocky Mountain Trench of western Canada. Both large-scale geology and magnetovariation anomalies indicate generally 2-D structure. Eleven stations close to a profile transverse to strike were chosen from two magnetovariation arrays. The response parameter modelled at these stations is the real part of Z/H for a linearly polarized uniform horizontal primary field, along the profile and transverse to the structures, calculated from single-station transfer functions at periods of 600 and 1500 s. Induction arrows indicate that the quadrature phase response is related mainly to currents induced in sedimentary rocks in another conductor. Very large real Z/H, at stations in the Main Ranges, and small values 50 km SW of these, in the Rocky Mountain Trench, can be modelled by currents induced in highly conductive rocks, rising to within a few kilometres of the surface, beneath the Main Ranges. This conductive ridge is well constrained in position and depth. It is joined to the Canadian Cordilleran Regional conductive layer, and is essentially a thickening of the CCR conductor along its edge. The best purely inductive model places the top of the CCR conductor within 5 km of the surface, but it can be modelled at greater depth if the entire conductive structure carries a small amount of channelled current in addition to the induced currents. Available magnetotelluric soundings indicate a midcrustal depth to the CCR conductor SW of Revelstoke, but depths of 5 km or less under the Rocky Mountains and Trench, in good agreement with our models fitted to long-period magnetovariation fields. The nature of the CCR conductor has been considered elsewhere: its high conductivity may be due to silicate partial melt in interconnected pores at upper mantle depths, with saline hot water in interconnected pores and fractures at crustal depths.

W. D. Parkinson

Papers

Directions of Rapid Geomagnetic Fluctuations

Introduction to geomagnetism

The geomagnetic coast effect

The Tamar conductivity anomaly

Models of conductive structure under the Canadian Cordillera