Beyond Mars, the Solar System is a realm of ice and frozen gas. In this frigid zone, far from the Sun, where solar heat is only a vestige of what we receive on Earth, the giant planets formed. The low temperature--typically about 150 Kelvin (over 100 degrees below zero on the Fahrenheit scale) allowed bodies condensing there, within the solar nebula, to capture hydrogen and hydrogen-rich gases, such as methane, ammonia, and water. Because these gases were far more abundant in the young Solar System than the silicate- and iron-rich material from which the terrestrial planets condensed, planets that formed in this cold environment had more material available for their growth. As a result, these cold planets became vastly larger than those near the Sun, and they developed very different structure and composition. The four largest planetsJupiter, Saturn, Uranus, and Neptune are composed mainly of gaseous and liquid hydrogen and its compounds. Although these giant bodies may have cores of molten rocky matter, they lack solid surfaces and, consequently, have no surface features, such as mountains and valleys. Rather, it is their atmospheric features that give them such different appearances. Pluto, however, is an exception to these rules. By far the smallest planet in the Solar System, Pluto has little in common with the giant planets and more closely resembles their larger moons, which are composed of ice and rock.

The moons of the outer Solar System range in size from very small bodies to those as large as Mercury. As they orbit their parent planets, they form families rather like miniature Solar Systems. Some of the larger moons have brightly colored surfaces, others have numerous craters, and a few have surface features unlike anything seen elsewhere in the Solar System. A few moons even have active volcanoes. In fact, astronomers consider these diverse bodies, virtually unknown before the space age, to be some of the most interesting members of the Sun's family.