Direct measurements of the conditions deep inside Jupiter are of course impossible. Calculations of these conditions are, however, constrained in a number of ways. The low mean density, only 1.3 times that of water, implies that the composition must be very different from the terrestrial planets with their typical mean density of live times that, of water. Jupiter must be composed almost entirely of hydrogen and helium, since only these two elements can have such a low density at planetary temperatures and pressures. By laboratory standards, although not by astronomical standards, the temperatures and pressures inside Jupiter are very high and it is necessary to predict the properties of the materials making up the planet. It is less difficult to do this for hydrogen and helium than for other elements. Any theoretical model of the interior of Jupiter must explain the origin of the large internal heat source and the way in which the energy is transported to the sur¬face. Because of the relative simplicity of the properties of the material in Jupiter, and this need to include the internal heat source, it is likely that the understanding of Jupiter’s internal structure will soon improve to such an extent that it will be quite accurately known. The interior of Jupiter may then be better understood than any other planet – except possibly the Earth!

One theory for the origin of Jupiter is that it formed in a similar manner to a star. In the nebula from which the Sun condensed, there was a second region where the density was sufficiently greater than average for the material to collapse under its own gravitation. Because of the very low mass, the internal temperature never became hot enough to start nuclear fusion. For a condensation of the mass of Jupiter, the temperature would soon reach a maximum of about 40 000 K after which the body would cool rapidly. It seems that it would reach a state with the present, radius and energy production of Jupiter after about 4 x 109 years, which is just the age of the Solar System.

An alternative theory assumes that there is a high density core with a composition similar to that of the terrestrial planets. This core must have formed first and then built up a body the size of Jupiter by gravitational capture of interplanetary gas. This gas would be part of the nebula from which the Sun formed and as such would presumably have the same composition as the Sun, i.e. almost entirely hydrogen and helium.

In the liquid and solid states, hydrogen and helium can only mix together in certain proportions. Because of this there may be separate layers of hydrogen-rich material and helium-rich material inside Jupiter; the details depend on the central temperature. One theoretical model, which has only hydrogen-rich layers, except for a possible dense core, is shown in figure 11.11. Another possibility is a helium-rich core surrounded by a layer rich in metallic hydrogen above which is a molecular-hydrogen-rich atmosphere. Again there may be a dense heavy-element core. In each layer, energy is carried outwards by convection, whereas across the boundaries the energy flow is by conduction.

The most probable source of the internal energy is gravitation.If Jupiter contracted by just one millimetre per year, then, with a central temperature of 10 000 K, the observed excess luminosity would result. Alternatively there may be gravitational separation of the hydrogen and helium; this can give the present luminosity with a lower temperature. It is very unlikely that simple cooling, – by- radiating the heat acquired during formation, is a major contributor to the energy production. Table 11.2 shows the results of one calculation of the pressure and density in the interior of Jupiter. Like all the theories described here, these results are uncertain and should be treated with some caution, but they do give the probable general trend of the conditions inside Jupiter.

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