Saturn's Atmosphere
Like Jupiter, Saturn has colorful banding in its atmosphere, as shown in Figure 23. Saturn's atmosphere is the largest of any planet in the solar system. Saturn also has the lowest average density. In fact, if there were a bathtub big enough, Saturn would float in the water. Saturn's atmosphere is made mostly of hydrogen and helium, with small amounts of other elements. Scientists think that the helium in Saturn's atmosphere condenses, creating helium “raindrops” that fall into the middle of the planet. As the helium drops fall, their potential energy is converted into heat. This keeps the planet warm, and explains why Saturn releases more heat than Jupiter.
Saturn's Moons
Astronomers have found at least 62 moons orbiting Saturn. Most of these moons are very small. However, one moon, Titan, is larger than Mercury. Titan is the only planetary moon with a thick atmosphere and active streams and lakes. In 2004, the space probe Cassini began a series of flybys of Titan, and detected what appear to be lakes of liquid methane on the surface.
Uranus
Because Uranus is so far from the sun, it is very cold. Uranus's atmosphere is mostly hydrogen and helium. It also has a large amount of methane, which gives the planet a distinct blue-green appearance.
Structure of Uranus
Like Jupiter and Saturn, Uranus has a dense core. This core is thought to be surrounded by a thick mantle layer of liquid water and dissolved ammonia, somewhat similar to a bottle of window-cleaning solution. A layer of liquid hydrogen and helium surrounds the mantle.
Uranus has rings, although they are not as visible as Saturn's. The particles that make up these rings are very dark, about as reflective as lumps of coal. Uranus has at least 27 moons, though most are quite small.
A Tilted Planet
The most unusual characteristic of Uranus is that it lies nearly on its side, as shown in Figure 24. 
The axis of Uranus's rotation is tilted more than 90º. Uranus rotates in a direction opposite to the direction of its revolution around the sun. Uranus's rings and moons revolve about the same tilted axis. Because of the tilt of Uranus's axis, its northern and southern hemispheres alternate being exposed to the sun for many years. This causes immense storms that disturb the planet's otherwise smooth appearance. No one knows for sure why Uranus's axis is so tilted. Scientists hypothesize that the tilt may have been caused by a collision with another large planetary body early in its history.
What do scientists hypothesize about the cause of Uranus's tilt?
Figure 24 Uranus is unusual in that its axis is tilted so that the planet rotates nearly on its side. Uranus's rings and moons revolve around its greatly tilted axis of rotation. Inferring In what way must the seasons on Uranus be unusual?
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