The Solar System

CHAPTER 20 | EARTH: THE STANDARD OF COMPARATIVE PLANETOLOGY 439

■ Figure 20-11

(a) Satellite observations of ozone concentrations over Antarctica are shown here as red for highest concentration and violet for lowest. Since the
1970s, a hole in the ozone layer has developed over the South Pole. (b) Although ozone depletion is most dramatic above the South Pole, ozone
concentrations have declined at all latitudes. (NASA/GSFC/TOMS and Glenn Carver)

Average ozone each

October above Halley

Bay Station, Antarctica

350

300

250

200

150

100

50

0

Ozone (D

U)

1950 1960 1970 1980 1990 2000

a b earY

Average data from

4 years in the 1970s

show no ozone hole.

Average data from 4 years

in the 1990s show the rapid

development of an ozone hole.

What Are We? Scientifi c Imagineers

One of the most fascinating aspects of

science is its power to reveal the unseen.

That is, it reveals regions you can never

visit. You saw this in earlier chapters when

you studied the inside of the sun and stars,

the surface of neutron stars, the event

horizon around black holes, the cores of

active galaxies, and more. In this chapter,

you have “seen” Earth’s core.

An engineer is a person who builds

things, so you can call a person who

imagines things an imagineer. Most

creatures on Earth cannot imagine situa-

tions that do not exist, but humans have

evolved the ability to say, “What if?” Our

ancient ancestors could imagine what

would happen if a tiger was hiding in the

grass, and we can imagine the inside of

Earth.

A poet can imagine the heart of Earth,

and a great writer can imagine a journey to

the center of Earth. In contrast, scientists

use their imagination in a carefully

controlled way. Guided by evidence and

theory, they can imagine the molten core of

our planet. As you read this chapter you saw

the yellow-orange glow and felt the heat of

the liquid iron, and you were a scientifi c

imagineer.

Human imagination makes science

possible and provides one of the great

thrills of science—exploring beyond the

limits of normal human experience.

SCIENTIFIC ARGUMENT

Why does Earth’s atmosphere contain little carbon dioxide and

lots of oxygen?

Sometimes as you build a scientifi c argument, you must contradict

what seems, at fi rst glance, a simple truth. In this case, because

volcanic outgassing releases mostly CO 2 , N 2 , and water vapor, you

might expect Earth’s atmosphere to be very rich in CO 2. Luckily for

the human race, CO 2 is highly soluble in water, and Earth’s sur-

face temperature allows it to be covered with liquid water. The CO 2

dissolves in the oceans and combines with minerals in seawater

to form deposits of silicon dioxide, limestone, and other mineral

deposits. In this way, the CO 2 is removed from the atmosphere

and buried in Earth’s crust. Oxygen, in contrast, is highly reac-

tive and forms oxides so easily you might expect it to be rare in

the atmosphere. Happily for us animals, it is continually replen-

ished as green plants release oxygen into Earth’s atmosphere faster

than chemical reactions can remove it. Were it not for liquid-water

oceans and plant life, Earth would have a thick CO 2 atmosphere

with no free oxygen.

Now follow up on your argument. Why would an excess of CO 2

and a defi ciency of free oxygen be harmful to all life on Earth

in ways that go beyond mere respiration?