Key Ideas

Composition and Structure: Jupiter and Saturn are both much larger than Earth. Each is composed of 71% hydrogen, 24% helium, and 5% all other elements by mass. Both planets have a higher percentage of heavy elements than does the Sun.

• Jupiter probably has a rocky core several times more massive than Earth. The core is surrounded by a layer of liquid “ices” (water, ammonia, methane, and associated compounds). On top of this ice layer is a layer of helium and liquid metallic hydrogen and an outermost layer composed primarily of ordinary hydrogen and helium. All of Jupiter’s visible features are near the top of this outermost layer.
• Saturn’s internal structure is similar to that of Jupiter, but its core makes up a larger fraction of its volume and its liquid metallic hydrogen mantle is shallower than that of Jupiter.
• Jupiter and Saturn both rotate so rapidly that these planets are noticeably flattened, or oblate
• Both Jupiter and Saturn emit more energy than they receive from the Sun. Presumably both planets are still cooling.

Atmospheres: The visible “surfaces” of Jupiter and Saturn are actually the tops of their clouds. The rapid rotation of the planets twists the clouds into dark belts and light zones that run parallel to the equator. Strong zonal winds run along the belts and zones.

• The outer layers of both planets’ atmospheres show differential rotation: The equatorial regions rotate slightly faster than the polar regions.
• The colored ovals visible in the Jovian atmosphere represent gigantic storms. Some, such as the Great Red Spot, are quite stable and persist for many years. Storms in Saturn’s atmosphere seem to be shorter-lived.
• There are presumed to be three cloud layers in the atmospheres of Jupiter and Saturn. The reasons for the distinctive colors of these different layers are not yet known. The cloud layers in Saturn’s atmosphere are spread out over a greater range of altitude than those of Jupiter, giving Saturn a more washed-out appearance.
• Saturn’s atmosphere contains less helium than Jupiter’s atmosphere. This lower abundance may be the result of helium raining downward into the planet.

Magnetic Fields and Magnetospheres: Jupiter has a strong magnetic field created by currents in a metallic hydrogen layer. With less liquid metallic hydrogen, Saturn’s magnetic field is much weaker than Jupiter’s.

Rings: Saturn is circled by a system of thin, broad rings lying in the plane of the planet’s equator. This system is tilted away from the plane of Saturn’s orbit, which causes the rings to be seen at various angles by an Earth-based observer over the course of a Saturnian year.

Structure of the Rings: Saturn has three major, broad rings (A, B, and C) that can be seen from Earth. Other, fainter rings were found by the Voyager spacecraft.

• The principal rings of Saturn are composed of numerous particles of ice and ice-coated rock ranging in size from a few micrometers to about 10 m. Most of the rings exist inside the Roche limit of Saturn, where disruptive tidal forces are stronger than the gravitational forces attracting the ring particles to each other.
• Each of Saturn’s major rings is composed of a great many narrow ringlets. The faint F ring, which is just outside the A ring, is kept narrow by the gravitational pull of shepherd satellites.
• Jupiter’s faint rings are composed of a relatively small amount of small, dark, rocky particles that reflect very little light.