Introduction

The possibility that, at the pressures encountered in the planets, materials ordinarily non-metallic at low pressures might transform into metals has been discussed for more than forty years. Two main ideas have been considered: one, that metal silicates, such as olivine, might become metallic at pressures developed in the core of the Earth, and the other, that hydrogen, helium and other light elements might transform to metals at pressures encountered in the major planets. Sufficient is now known about changes of density in metallic transformations under high pressure to be sure that the jump of density between the mantle and the core of the Earth is too great to be explained by such a transformation and in the preceding chapters on the terrestrial planets it has been assumed that the difference between the core and mantle of the Earth is one of composition (see also Anderson, 1977). It is otherwise with Jupiter and Saturn. The mean densities of those planets are too low for them to be composed of anything but hydrogen, helium and other materials of low atomic number, and the likelihood of a metallic transformation of hydrogen in particular is crucial to a discussion of their internal structures. One of the first studies of the metallic transformation in hydrogen (Kronig, de Boer and Korringa, 1946) was prompted by the idea of Kuhn and Rittman (1941) that the inability of the core of the Earth to support shear waves might be because it was of solar composition, that is, mainly of hydrogen, and by the subsequent suggestion of van der Waals that at core pressures the hydrogen might be metallic.