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 planets: Jupiter,
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. |