Jupiter is highly oblate (flattened). Plus, Jupiter has a very high rotation rate (once every 9.8 hours). These two facts combine to tell us that Jupiter has a very small solid core.

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Jupiter's interior:

Jupiter's interior consists mostly of hydrogen and helium. These elements are gaseous at the top of Jupiter's atmosphere down to several thousand kilometers. At this point, the pressures and temperatures reduce the gases into a liquid state. The liquid hydrogen, in molecular form at these levels (H2), continues to be compressed further reaching a metallic state. This occurs in a transition zone located 20,000 km below the atmosphere. Notice that at no time is there any real ``surface'' as one drops into Jupiter's interior. At the very center of Jupiter is a small (15 Earth masses) rocky core, leftover from the icy dust particles that originally collected in the early solar nebula.

A planet absorbs energy from the Sun in the form of light and converts the energy into heat. The heat is then reradiated back into space (mostly from the nightside of the planet). Based on how much energy Jupiter absorbs from the Sun, then its mean temperature should be 105 K (about -280 F). However, IR and radio measurements of Jupiter show that it has a mean temperature of 125 K, or 20 degrees too warm. In other words, Jupiter radiates about twice as much energy as it receives. Conservation of energy requires that this heat come from someplace and the only reservoir is the core of Jupiter. Thus, this extra heat is leftover energy from the time of Jupiter's formation.

Many textbooks refer to Jupiter as a ``failed star''. This is due to the fact that if Jupiter were slight more massive the temperatures in its core would have reached the ignition point for thermonuclear fusion. This is the process where stars due hydrogen into helium and release energy (i.e. the star shines). If Jupiter were 4 to 5 times more massive our Solar System would have had two stars.

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Jupiter's radiation output:

IR and radio measurements revealed two components to Jupiter's radiation output.

A thermal component, associated with the leftover heat of formation (see above) and a non-thermal component. The non-thermal component is associated with radiation that does not follow a Planck curve but follows what is know as a power-law spectrum. A spectrum that associated with synchrotron radiation.

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Jupiter's magnetic field:

The magnetic field of Jupiter is 19,000 times stronger than the Earth's magnetic field. Even with a large rocky core and high rotation rate, the magnetic field is too strong. The origin of Jupiter (and other Jovian planets) strong magnetic field is the metallic hydrogen shell that surrounds Jupiter's rocky core. Metal is an excellent conductor of electric current and supplies the energy for the generation of an intense and large magnetic field.

A strong magnetic field can capture charged particles from the solar wind (i.e. high speed protons and electrons) and particles ejected from the inner moon, Io. These particles are trapped in the inner magnetic belts and are reflected back and forth between the north and south magnetic poles.

A visible result of this interaction is aurora or northern lights on Jupiter.

The interaction of Jupiter's strong magnetic field and nearby space produces a region known as Jupiter's magnetosphere. The magnetosphere has several features:

North and South magnetic poles

current sheet along the magnetic equator

plasma torus associated with Io's orbit

magnetopause - the boundary where the magnetosphere encounters the solar wind

Magnetosphere movie

The rapid rotation of Jupiter spews charged particles into a current sheet around the magnetic equator of Jupiter. Inside this current sheet orbits the moon Io. The current sheet sweeps up ejected ions from Io's geysers to make a plasma torus. The region around this plasma torus and the inner moon system is intensely radioactive with levels around 1000 times the radioactive levels of the Earth's surface. This region of space is inhabitable by man or machine without heavy shielding.

The magnetosphere of Jupiter encounters the solar wind at about a million kilometers from the planet. The bow shock from this boundary reaches beyond the orbit of Saturn.

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