The Solar System

CHAPTER 20 | EARTH: THE STANDARD OF COMPARATIVE PLANETOLOGY 427

and fl ooded the deeper impact basins. Later, as the environment

cooled, water fell as rain and fl ooded the basins to form the fi rst

oceans. Note that on Earth, basin fl ooding was fi rst by lava and

later by water.

Th e fourth stage, slow surface evolution, has continued for at

least the past 3.5 billion years. Earth’s surface is constantly chang-

ing as sections of crust slide over and against each other, push up

mountains, and shift continents. In addition, moving air and

water erode the surface and wear away geological features.

Almost all traces of the fi rst billion years of Earth’s history have

been destroyed by the active crust and erosion.

Terrestrial planets pass through these four stages, but diff er-

ences in mass, temperature, and composition among the planets

size and temperature are important. Th e second question is more
complex. Where did those atmospheres come from? To answer
that question in later chapters, you will have to study the geologi-
cal history of these worlds.

SCIENTIFIC ARGUMENT

Why do you expect the inner planets to be high-density worlds?

In Chapter 19, you saw how the inner planets formed from hot

inner parts of the solar nebula. No ice solidifi ed there, so the inner

planets could grow only from particles of rock and metal able to

condense from hot gas. So, you expect the inner planets to be

made mostly of rock and metal, which are dense materials.

As you visit the Terrestrial planets, you will fi nd craters almost

everywhere. What made all of those craters?

Earth as a Planet

Like all the terrestrial planets, Earth formed from the
inner solar nebula about 4.6 billion years ago. Even as it took
form, it began to change.

Four Stages of Planetary Development

Th ere is evidence that Earth and the other Terrestrial planets,
plus Earth’s moon, passed through four developmental stages
(■ Figure 20-2).
Th e fi rst stage of planetary evolution is diff erentiation, the
separation of material according to density. As you have already
learned, Earth is diff erentiated, meaning, separated into layers of
diff erent density. Th at diff erentiation is understood to have
occurred due to melting of Earth’s interior caused by heat from a
combination of radioactive decay plus energy released by infall-
ing matter during the planet’s formation. Once the interior of
Earth melted, the densest materials were able to sink to the
core.
Th e second stage, cratering, could not begin until a solid
surface formed. Th e heavy bombardment of the early solar sys-
tem made craters on Earth just as it did on the moon and other
planets. As the debris in the young solar system cleared away, the
rate of cratering impacts fell gradually to its present low rate. You
will learn in the next chapter that evidence provided by lunar
crater counts and rock samples indicates there was a temporary
large increase in the impact cratering rate near the end of the
heavy bombardment, and this violent event likely would have
aff ected all the planets.
Th e third stage, fl ooding, began as radioactive decay contin-
ued to heat Earth’s interior and caused rock to melt in the upper
mantle, where the pressure was lower than in the deep interior.
Some of that molten rock welled up through cracks in the crust

20-2

■ Figure 20-2

The four stages of Terrestrial planetary development are illustrated for

Earth.

Four Stages of Planetary Development

Differentiation

produces a dense

core, thick mantle,

and low-density crust.

The young Earth was

heavily bombarded in

the debris-filled early

solar system.

Flooding by molten

rock and later by

water can fill

lowlands.

Slow surface evolution

continues due to

geological processes,

including erosion.