Key Ideas and Terms

7-1 Dynamic atmospheres of Jupiter and Saturn change rapidly

• Jupiter and Saturn are both much larger than Earth. Each is composed of 71% hydrogen, 24% helium, and 5% all other elements by mass.
• 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 are a layer of helium and liquid metallic hydrogen and an outermost layer composed primarily of ordinary hydrogen and helium.
• 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.
• 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. 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.
• The outer layers of both Jupiter and Saturn’s atmospheres show differential rotation as the equatorial regions rotate slightly faster than the polar regions.

7-2 Uranus and Neptune have seemingly quiet atmospheres

• Both Uranus and Neptune have atmospheres composed primarily of hydrogen, helium, and a few percent of methane.
• Methane absorbs red light, giving Uranus and Neptune their greenish-blue color.
• Much more cloud activity is seen on Neptune than on Uranus. This is because Uranus lacks a substantial internal heat source.

7-3 Saturn’s moon Titan and Neptune’s moon Triton exhibit unexpected atmospheres

• Titan, the largest natural satellite of Saturn, is covered with a thick, hazy carbon dioxide atmosphere that may have sufficient energy for methane-based weather and lakes to occur near the surface.
• Triton has a tenuous nitrogen atmosphere and a young, icy surface indicative of tectonic activity.

7-4 All Jovian planet atmospheres are encircled by complex ring systems

• 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.
• From Earth, Saturn is observed to have three major, broad rings composed of unconnected ring particles, ice and ice-coated rock ranging in size from a few micrometers to about 30 feet (10 m) that are kept separate by a tidal force. The A ring and the B ring are separated by the Cassini division.
• 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.
• Jupiter’s faint rings are composed of a relatively small amount of small, dark, rocky particles that reflect very little light.
• Uranus and Neptune are both surrounded by systems of thin, dark rings. The low reflectivity of the ring particles may be due to radiation-darkened methane ice.