(chapters 9-12 in Beatty and Chaikin; chapters 10 and 11 in Christiansen and Hamblin)
Saturn is similar in size (121,000 km diameter) to Jupiter, (144,000), but much less massive, 95 times the mass of the Earth, versus, 318 for Jupiter. This implies a bulk density of 0.7 g/cc for Saturn, versus 1.3 g/cc for Jupiter. At first glance one might suspect a difference in the makeup of the two planets, and in fact Saturn contains a slightly higher proportion of hydrogen than does Jupiter, but this difference is not enough to explain the large discrepancy in bulk density.
To understand what is going on, we have to think about what supports the planets against gravity. Recall that the sun is held up against its self gravity by gas pressure gradients, while the terrestrial planets are held up by electron degeneracy pressure, the usual chemical forces that we think about as providing solid or liquid bodies with their ability to resist compression. This incompressability is captured in the rubric that "no two bodies can occupy the same space at the same time". One upshot of incompressability is that adding material to a body will increase the volume of the body. Jupiter and Saturn have large enough masses that the material in their interiors is degenerate, but their envelopes are not degenerate. Instead, the envelops are supported by pressure gradients like those in the sun. If we were to add hydrogen to Saturn, material near the core that is currently nondegenerate would be compressed and become degenerate; the mass of the planet would increase but its size would remain almost constant.
Both Jupiter and Saturn are stratified bodies. The outer layers of both consist primarily of molecular hydrogen (H2); the bottom of this layer has a pressure of about 4 million bar. At higher pressures, the molecular bonds break and the hydrogen acts like a conductor; this state is refered to as metallic hydrogen. On Jupiter this occurs at a depth of about 15,000-20,000km; on Saturn it occurs at a depth of about 30,000km. Both Jupiter and Saturn may have silcate and/or water cores of about 10 Earth masses, but the evidence for this is weak. If they exist, the cores have radii of about 10,000km. Like Jupiter, Saturn emits more energy than it receives from the sun. In the case of Jupiter, this energy is believed to be primordial heat trapped in the planet during its accretion. Since Saturn is so much less massive than Jupiter, it should have cooled faster, and hence should be emitting less heat than Jupiter. One suggestion for the source of heat is the separation of helium from hydrogen, which will release gravitational binding energy as the denser helium drops toward the center of the planet.
Satellites and Rings
The Saturn system contains a large number of moons and the most extensive set of rings in the solar system.
Uranus has a large obliquity (angle between its spin axis and its orbital axis) of 82 degrees. Currently the spin axis is pointed nearly at the sun. It is also set off from the other giant planets in that it puts out no more heat than it receives from the sun. The atmosphere shows little band structure, consistent with the lack of an internal heat source. The magnetic field of the planet is unusual as well, being inclined to the spin axis by about 60 degrees, and not centered on the planet. The rather high bulk density of 1.28 g/cm3, nearly twice that of Saturn, suggest the presence of a substantial amount of water and silicates in the interior of Uranus.
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