continues right to the center. The pressure is 80 million bar; the temperature is about 25 000 K. The core contains only 4% of Jupiter's mass.

The transition from the gaseous region to the liquid region is probably a gradual one. The transition from the normal liquid hydrogen to the metallic hydrogen probably takes place over a small change of radius. We think that the interior has excess energy stored from the time of the collapse of the planet. The energy is so large that it has not all escaped yet. This is probably the source of the excess energy that Jupiter gives off.

The general structure of Saturn is probably very similar to that of Jupiter, as shown in Fig. 25.13. There are some differences, however. Saturn has a larger core, containing about 26% of the mass. The central pressure is 50 million bar, and the central temperature is about 20 000 K.

There is a smaller metallic hydrogen zone. The range of radii is from 16 000 to 28 000 km. The temperature in this zone ranges from 9000 to 12 000 K. This zone contains about 17% of Saturn's mass.

Our understanding of Saturn's excess heat is not as good as that for Jupiter's. Some other explanation is needed. It has been suggested that some of the energy comes from helium condensing and sinking through the less dense material towards the core.

Uranus and Neptune have higher densities than Jupiter and Saturn. This suggests a different composition. The cores are rock. The rock is mostly silicon and iron. Over the core is a mantle. This mantle probably contains liquid water, ammonia and methane. Over the mantle is a crust of hydrogen and helium. It may be in the form of high density gas. The central pressure is about the same for both planets, about 20 million bar. The central temperature is about 7000 K.

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