Interiors of the Jovian Planets

Jupiter and Saturn are primarily hydrogen and helium (as is the Sun). Their outer layers are molecular hydrogen (H₂), their middle layers are liquid metallic hydrogen (more on this in a second), and their inner regions are composed of rocky, icy type material (the seed from the planet formation process).

The liquid metallic hydrogen is an odd state of hydrogen. Such a state was first predicted for the high pressures one finds in the interiors of Jupiter and Saturn many years ago (the core has a pressure of 12 million bars ~ 12 million times as large as sea level pressure on the Earth!!). Such a state for hydrogen was not reproduced in Terrestrial labs until fairly recently. The fact that the hydrogen is liquid means that if you poured it into a cup, it would assume the shape of the cup, but would not spread out throughout the entire volume. The fact that the hydrogen is metallic means that it will conduct electricity. Note that the fact that this layer can flow and can conduct electricity means that Jupiter and Saturn can support large internal electrical currents and should thus show large magnetic fields.

How do we know these general properties?

density ---> made of primarily light stuff ---> hydrogen and helium
seismology ---> no
magnetic fields ---> yes; all jovians have strong magnetic fields ---> molten electrically conducting interiors
nonspherical shapes (flattening due to rapid rotation) ---> interior structure

An interesting sidelight is that Jupiter and Saturn radiate more energy than they receive from the Sun.

Jupiter radiates 1.5-2 times as much energy as it receives from the Sun
Sature radiates 2-3 times as much energy as it receives from the Sun

Are Jupiter and Saturn therefore considered to be stars? Where does this excess heat come from?

Answers: No and we now consider the issue. Well, for Jupiter the answer is easy. We know that planets form hot and then cool as they age. Given Jupiter's large mass and size, it takes Jupiter a long time to cool. This scenario explains Jupiter's excess heat. Saturn, however, is another matter. Saturn is less massive and smaller than Jupiter and yet radiates more heat (in a relative sense) ---> the scenario does not work. What's the scoop?

Helium Rain

Saturn cools faster than Jupiter and so, after ~2.6 billion years, atmosphere gets cool enough for helium to condense and rain out.

As the helium rains through the planet it gains energy because gravity pulls it inward. As it moves through the liquid H₂, friction slows the drops down and heats up the hydrogen. This extra heat is then radiated by Saturn.

A natural prediction of this model is that the atmosphere of Saturn should contain less helium than does the atmosphere of Jupiter. Is this true? Yes, the atmosphere of Jupiter is more than 10 % helium while the atmosphere of Saturn is 6 % helium. Amazing!

A prediction is that in the future, Jupiter should show the same behavior.