The Solar System

CHAPTER 20 | EARTH: THE STANDARD OF COMPARATIVE PLANETOLOGY 425

A Travel Guide to the

Terrestrial Planets

If you visit the city of Granada in Spain, you will probably

consult a travel guide. If it is a good guide, it will do more than

tell you where to fi nd museums and restrooms. It will give you a

preview of what to expect. You are beginning a journey to the

Earth-like worlds, so you should consult a travel guide and see

what is in store.

Five Worlds

You are about to visit Mercury, Venus, Earth, Earth’s moon, and

Mars. It may surprise you that the moon is included in your

itinerary. It is, after all, just a natural satellite orbiting Earth and

isn’t one of the planets. But the moon is a fascinating world of its

own, it makes a striking comparison with the other worlds on

your list, and its history gives you important information about

the history of Earth and the other planets.

■ Figure 20-1 compares the fi ve worlds. Th e fi rst feature you

might notice is diameter. Th e moon is small, and Mercury is not

much bigger. Earth and Venus are large and quite similar in size,

Nature evolves. Th e world was diff erent 20-1

yesterday.

— PRESTON CLOUD, COSMOS, EARTH AND MAN

P

lanets, like people, are more alike than they are
diff erent. Th ey are described by the same basic princi-
ples, and their diff erences arise mostly because of small
diff erences in background. To understand the planets, you can
compare and contrast them to identify those principles, an
approach called comparative planetology.
Earth is the ideal starting point for your study because it is
the planet you know best. It is also a complex planet. Earth’s
interior is molten and generates a magnetic fi eld. Its crust is
active, with moving continents, earthquakes, volcanoes, and
mountain building. Earth’s oxygen-rich atmosphere is unique in
the solar system. Th e properties of Earth will give you perspective
on the other planets in our solar system.

Mercury is slightly more than a third

the diameter of Earth, has no

atmosphere, and is heavily cratered.

Radio-wavelength radiation can

penetrate the clouds, and radar

maps of the surface of Venus

reveal impact craters, volcanoes,

and solidified lava flows.

Venus, 95 percent the

diameter of Earth, has a

thick cloudy atmosphere

that hides its surface from

view. Seen through an

Earth-based telescope, it is

a featureless white ball.

Planet Earth, the basis for comparative planetology of

the Terrestrial planets, is a water world. It is widely covered

by liquid water, has polar caps of solid water, and has an

atmosphere rich in water vapor and water-droplet clouds.

Volcanoes

Earth’s moon is only

one-fourth Earth’s

diameter. It is airless

and heavily cratered.

Polar cap of frozen water

and carbon dioxide

Mars, slightly over

half Earth’s diameter,

has a thin

atmosphere and a

rocky, cratered crust

marked by volcanoes

and old lava flows.

■ Figure 20-1

Planets in comparison. Earth and Venus are similar in size,
but their atmospheres and surfaces are very different. The
moon and Mercury are much smaller, and Mars is intermediate
in size. (Moon: UCO/Lick Observatory: All planets: NASA)