What we know of Jupiter's interior, we must infer from data recieved from spacecraft. Although we have sent the galileo probe into the atmosphere, we have had no "lander" to tell us of what lies below the atmosphere. Jupiter's low density and atmospheric composition lead us to believe that Jupiter's composition further down is not very different from that above. We also know that Jupiter radiates more energy into space than it recieves from the Sun, therefore, the planet must be hot inside. The heat is probably left over heat form Jupiter's formation.

     Beneath the atmosphere, density, temperature and pressure increase. Hydrogen can no longer exist in a gaseous state, and so it becomes liquid hydrogen. Deeper into Jupiter's interior, eventually the pressure reaches 2 million atmospheres. At this incredible pressure, hydrogen molecules are stripped of protons and electrons which move about freely and are able to conduct electricity. This is known as liquid mettalic hydrogen.

     Within 14,000 km of Jupiter's center, the temperature has reached 40,000 K. If Jupiter has any kind of solid core, it would be here, and would likely be composed of heavy elements.