We cannot directly sample the interiors of these outer planets. However, we can use physical laws to construct computer models of the interior. We can then see which models produce results that agree with observations. In Fig. 25.13 we show the internal structures of Jupiter, Saturn, Uranus and Neptune. To allow comparison, they are all scaled relative to the size of the planet. In that way, we can compare what fraction of the interior is made up of the various sections.

For Jupiter, the outermost layer is a hydrogen-helium envelope. With that, extending from a radius 10 000 to 54 000 km, is a liquid region. This liquid is hydrogen. The pressure in this region rises to 40 million times the atmospheric pressure on the Earth's surface. Under these conditions, the hydrogen forms into a metal. This metallic hydrogen contains 73% of the planet's mass. Within the liquid region is the core. The core may be made of rock and ice materials, though it has been suggested that the hydrogen


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