The Interior of

Most of the interior of Jupiter is liquid (primarily hydrogen and about 10% helium). The central temperatures are thought to lie in the 13,000-35,000 degree Celsius range, and the central pressure is about 100 million Earth atmospheres. We infer indirectly that the small core (perhaps a few tens of Earth masses) is rocky.

Liquid Metallic Hydrogen

The inner layers of highly compressed hydrogen on Jupiter are in a state that can only be produced on the Earth in the most extreme laboratory conditions--in fact under the extreme conditions of powerful laser laboratories, of which the most powerful in the world is currently here at the UR (the Laboratory for Laser Energetics). (These lasers converge on a target producing an extremely large pressure.) Normally, hydrogen does not conduct heat or electricity very well, which are defining characteristics for a metal. Thus, under normal conditions hydrogen is not a metal. Under the extreme pressure found deep inside Jupiter, theory suggests that the electrons are released from the hydrogen molecules and are free to move about the interior. This causes hydrogen to behave as a metal: it becomes conducting for both heat and electricity.

The intense magnetic field of Jupiter is thought to result from electrical currents in this region of metallic hydrogen that is spinning rapidly and thought to compose 75% of the planet's mass. We are limited in our understanding of Jupiter's interior by the fact that we have little laboratory knowledge of the behavior of hydrogen under such conditions and are forced to rely on theory, but this will change as the field of laser laboratory astrophysics emerges. Conversely, the properties of Jupiter provide a stringent test of our understanding of matter under such extreme conditions.

Internal Source of Energy

Jupiter radiates 1.6 times a much energy as falls on it from the Sun. Thus, Jupiter has an internal heat source. It is thought that much of this heat is residual heat left over from the original collapse of the primordial nebula to form the Solar System, but some may come from slow contraction (liquids are highly incompressible, so Jupiter cannot be contracting very much.) This internal heat source is presumably responsible for driving the complex weather pattern in its atmosphere, unlike the Earth where the primary heat source driving the weather is the Sun.

A Large Planet or a Failed Star?

Calculations suggest that Jupiter lacks the mass to initiate hydrogen fusion reactions in its core and become a star by about a factor of 100. Since factors of 100 are not so large on astrophysical mass scales, it is legitimate at some level to view Jupiter as either a very large planet, or a failed star.