While Jupiter and Saturn have many similarities to each other, they are both very different from Uranus and Neptune, which form a separate pair of planets. The mean internal structures of Uranus and Neptune are shown in Figure 2.10 (after Guillot, 1999b). The small radii of Uranus and Neptune tell us immediately that, although hydrogen and helium account for the bulk of the molecules in the observable atmosphere, the planets cannot be predominantly composed of hydrogen and helium since the density would then be far too low. Instead, their radius is close to the characteristic radius of icy materials and thus these planets are thought to be composed predominantly of ice, with only a thin outer envelope of hydrogen and helium. It would be possible to match the planets' mean density by having a hydrogen atmosphere over a small rocky core, but this would have a moment of inertia which is very
Figure 2.10. Interior models of Uranus and Neptune. From Guillot (1999b). Reprinted with permission of the American Association for the Advancement
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