Fig. 6.26. A cutaway view of the possible internal structure of Europa, showing a metallic iron, nickel core, surrounded by rock or silicate, with a thin outer layer of water in ice or liquid form, or perhaps a thin ice layer overlying a liquid water ocean. (Credit: NASA/JPL/Caltech)

are approximately those expected for moons responding as perfectly conducting spheres. Such a response requires a globally distributed highly conducting medium located close to the surface of the moon. This result has been interpreted as support for the presence of a salty sub-surface ocean. A subsurface ocean with the salinity typical of Earth's oceans and a few kilometers thickness can easily produce the observed induction response. Perhaps the most spectacular result of these observations is that the magnetic evidence indicates the existence of an ocean on Europa at this present epoch, not just during the recent past [405]!

The ice shell itself has been estimated by Shoemaker and others to be ~10 km thick. The ability of large craters to support central peaks, and the unusual morphologic transitions compared to craters on Ganymede and Callisto both suggest that the shell is greater than 5 km and could be as thick as 20 km according to Nimmo et al., [406]. Though the thickness of the ice remains controversial, it is clear that it has been disrupted extensively from below [407]. E

The discovery by the Galileo magnetometer team of what appeared to be an £

induced magnetic field with certain properties at Europa suggests that a large c ¡X

fraction of the water layer is a briny, liquid ocean ~100 km deep. If the ocean o lacked appreciable solutes, or if it was thinner than a few tens of kilometers or *j> O

frozen completely, it would not conduct current to explain the observed magnetic signature. Thus, the evidence for a subsurface ocean is strong [408]. The water

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