Jupiter

Like its atmosphere, Jupiter's interior is mostly made up of hydrogen and helium. The planet also has a rocky core that is comparatively small, but nevertheless is 10 times more massive than the Earth. The hydrogen, accounting for 71% of Jupiter's mass, is in molecular form throughout a shell that extends from the base of the atmosphere down to about 75% of the planet's radius. Such a shell is a huge ocean: the hydrogen is kept in the liquid phase by a very high pressure, due to the weight of the atmosphere above it.

Below the liquid molecular hydrogen (H₂) ocean there lies a shell of liquid metallic hydrogen (H). This is quite an exotic state of matter whose existence is inferred by measurements of Jupiter's magnetic field. In the liquid molecular state, hydrogen atoms are bound in a diatomic molecule. As such molecules slide by each other at a short distance they keep their separate identities. The electrons of each molecule move around the two nuclei of hydrogen, but remain bound to them.

At high enough temperatures and pressures, the electrons become able to move freely around the liquid and the molecules lose their individuality. This freedom of electron movement means that the liquid is essentially a metal, made of free hydrogen nuclei and electrons.

As we were mentioning, Jupiter as a whole emits almost double the energy it receives from the Sun. Computer models show how such energy may be produced. Jupiter has not reached a complete hydrostatic equilibrium. The planet formed from the contraction of gas and rocks due to their mutual gravitational attraction, until an almost complete equilibrium was reached between gravity and the pressure of the planet's constituents. Such a process is almost over, but Jupiter is still contracting a little further and very slowly. The contraction is a source of energy. It transforms gravitational potential energy into kinetic energy of the atoms, and ultimately the latter transforms into heat. The heat then finds its way out of the planet by processes of convection in the liquid interior as well as in the gaseous atmosphere.

Another process is also probably at work inside Jupiter, namely the differentiation between hydrogen and helium. Helium atoms are heavier than hydrogen atoms and molecules. Within any fluid, the heavier atoms and/or molecules always tend to sink, leaving a higher percentage of the lighter ones in the upper layers. The only obstacle to a complete separation between hydrogen and helium is the thermal agitation of the atoms in the fluid. If the temperature is really high, the differentiation is practically negligible.

In Jupiter's interior the temperature is high. Helium and hydrogen are therefore well mixed, but some differentiation is occurring. Helium atoms are slowly finding their way down through Jupiter's interior, leaving the outer regions of the planet with a slightly higher ratio of hydrogen to helium abundance. The differentiation is also a source of heat, gravitational in nature. This process is the main source of heat for Saturn, since its interior is cooler than Jupiter's.

A large fraction of Jupiter's interior is liquid and metallic; therefore it is a conductor. Magnetic fields ensue when electric currents move within conductors, and that is precisely what happens in Jupiter's metallic hydrogen shell. Electric currents on a planetary scale run through it, creating the strong magnetic field that we measure from Jupiter's orbit. Jupiter is also a strong radio emitter, at radio wavelengths of a few decimeters. Individual electrons in the outer atmosphere of Jupiter spiral along the magnetic field lines, thereby emitting radio waves. Technically, this kind of emission is called 'synchrotron' radiation.