Page 6 - Curriculum Visions Dynamic Book

For condensation to have been an important process, we have to imagine that the giant planets formed as space gradually became colder. As a result, some elements stopped drifting around as gases and became first liquids and then solids.
In regions very distant from the warming radiation of the Sun space has become cold enough even for elements with low boiling points, such as ammonia and methane, to become liquids. That is the difference that separates the gas planets from the rocky planets. On the rocky planets ammonia and methane are present only as gases and readily escape from their atmospheres.
There is a big difference in size between the largest pair of gas giants—Jupiter and Saturn—and the smallest pair— Uranus and Neptune. This also points to some differences in composition and properties.
Uranus and Neptune are the smaller of the gas giants. They have cores of rocky materials and contain simple metallic compounds. But in addition, they have solid water, ammonia, and methane enveloping their rocky cores. They also have methane, helium, and hydrogen in their atmospheres. The biggest of the giants—Jupiter and Saturn—have hydrogen and helium in their cores. That makes them unique, since it takes an immensely powerful gravity to compress these light gases into liquids.
Rings and moons
All the Jovian planets have rings that contain a mixture of ice and dust, and they have many satellites: small captive bodies we more commonly call moons. These moons move around their planets just as the planets move around the Sun. Each moon behaves as though it were a planet, while the dust behaves as though it were an asteroid belt.
 The solar system’s largest moon, Ganymede (see pages 30–32), orbiting the gaseous world of Jupiter.
Ganymede, which, in contrast to Jupiter, is a rocky body, is larger than the planets Mercury and Pluto and Saturn’s largest moon, Titan.
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