The Solar System

430 PART 4^ |^ THE SOLAR SYSTEM

Pa

ndS

waves

An earthquake

sends seismic

waves through

Earth’s interior.

In the S-wave

shadow, only

P waves can

be detected.

oN

P

or

Sw

av

es

wP

eva

s

well. Th ese changes cause sound waves to be refracted as they

travel through Earth’s interior, meaning that, instead of following

straight lines, seismic waves curve away from the denser, hotter

central regions. Geoscientists can use the arrival times of refl ected

and refracted seismic waves from distant earthquakes to con-

struct a model of Earth’s interior.

Such studies confi rm that the interior consists of three parts:

a central core, a thick mantle, and a thin crust. S waves provide

an important clue to the nature of the core. When an earthquake

occurs, no direct S waves pass through the core to register on

seismographs on the opposite side of Earth, as if the core were

casting a shadow (■ Figure 20-5). Th e absence of S waves shows

that the core is mostly liquid, and the size of the S-wave shadow

fi xes the size of the core at about 55 percent of Earth’s radius.

Mathematical models predict that the core is also hot (about

6000 K), dense (about 14 g/cm^3 ), and composed of iron and

nickel.

Earth’s core is as hot as the surface of the sun, but it is under

such tremendous pressure that the material cannot vaporize.

Because of its high temperature, most of the core is a liquid.

Nearer the center the material is under even higher pressure,

How can studying what can’t be seen save

your life? Science tells us how nature works,

and the basis for scientifi c knowledge is evi-

dence gathered through observation. But much

of the natural world can’t be observed directly

because it is too small, or far away, or deep

underground. Yet geologists describe molten

rock deep inside Earth, and biologists discuss

the structure of genetic molecules. So how can

these scientists know about things they can’t

observe directly?

A virus, which can be as common as a cold

or as deadly as Ebola, contains a tiny bit of

genetic information in the form of a DNA mol-

ecule. You have surely had a virus, but you’ve

never seen one. Even under the best electron

microscopes, a virus can be seen only as a

hazy pattern of shadows. Nevertheless scien-

tists know enough about them to devise inge-

nious ways to protect us from viral disease.

A virus is DNA hidden inside a protective

coat of protein molecules, which is a rigid

molecular lattice almost like a mineral. In fact,

a culture of viruses can be crystallized, and

the shapes of the crystals reveal the shape and

structure of the virus. Unlike a crystal of cal-

cite, however, a crystal of viruses also contains

genetic information.

Scientists can make a vaccine to protect

against a certain virus if they can identify a

unique molecular pattern on the protein coat.

The vaccine is harmless but contains that

same pattern and trains your body’s immune

system to recognize the pattern and attack

it. Vaccines signifi cantly reduce the danger

of common illness such as chicken pox and

infl uenza, and they have virtually wiped out

devastating diseases like polio and smallpox in

the developed world. Researchers are currently

working on a vaccine for HIV/AIDS that would

potentially save millions of lives.

Even though viruses are too small to see,

scientists can use chains of inference and the

interaction of theory and evidence to deduce

the structure of a virus. Whether it is a virus

or the roots of a volcano, science takes us into

realms beyond human experience and allows us

to see the unseen.

The electron microscope allows biologists to

deduce the elegant structure of the virus that

causes the common infection called pink eye.

(From Virus Ultrastructure: Electron Micrograph Images.

Copyright 1995 by Linda M. Stannard. Reprinted with

permission from Linda M. Stannard. Found at: http://www.web.

uct.ac.za/depts/mmi/stannard/adeno.html)

20-2

Studying an Unseen World

■ Figure 20-5

P and S waves give you clues to the structure of Earth’s interior. No direct

S waves from an earthquake reach the side of Earth opposite their source,

indicating that Earth’s core is liquid. The size of the S wave “shadow” tells

you the size of the liquid outer part of the core.