The Solar System

426 PART 4^ |^ THE SOLAR SYSTEM

Another important way you can study a planet is by follow-

ing the energy fl ow. In the preceding chapter you learned that the

heat in the interior of a planet may be partly from radioactive

decay and partly left over from the planet’s formation, but in any

case it must fl ow outward toward the cooler surface where it is

radiated into space. In the process of fl owing outward, the heat

can cause convection currents, magnetic fi elds, plate motions,

quakes, faults, volcanism, mountain building, and more. Heat

fl owing outward through the cooler crust makes a large world

like Earth geologically active (How Do We Know? 20-1).

In contrast, the moon and Mercury, both small worlds, cooled

quickly inside, so they have little heat fl owing outward now and

are relatively inactive.

Atmospheres

When you look at Mercury and the moon in Figure 20-1, you

can see their craters, plains, and mountains clearly; they each

have little or no atmosphere to obscure your view. In compari-

son, the surface of Venus is completely hidden by a cloudy atmo-

sphere even thicker than Earth’s. Mars, the medium-sized planet,

has a relatively thin atmosphere.

You might ponder two questions. First, why do some worlds

have atmospheres while some do not? You will discover that both

but Mars is a medium-sized world. You will discover that size is
a critical factor in determining a world’s personality. Small worlds
tend to be geologically inactive, while larger worlds tend to be
active.

Core, Mantle, and Crust

Th e Terrestrial worlds are made up of rock and metal. Th ey are
all diff erentiated, which means they are each separated into layers
of diff erent density, with a dense metallic core surrounded by a
less-dense rocky mantle, and a low-density crust on the
outside.
As you learned in Chapter 19, when the planets formed,
their surfaces were subjected to heavy bombardment by left-
over planetesimals and debris in the young solar system. You
will see lots of craters on these worlds, especially on Mercury
and the moon, many of them dating back to the heavy bom-
bardment era. Notice that cratered surfaces are old. For
example, if a lava fl ow covered up some cratered landscape
after the end of the heavy bombardment, few craters could be
formed later on that surface because most of the debris in the
solar system was gone. When you see a smooth plain on a
planet, you can guess that surface is younger than the heavily
cratered areas.

The So-Called Scientifi c Method

What causes change? One of the best ways
to think about a scientifi c problem is to follow
the energy. According to the principle of cause
and effect, every effect must have a cause,
and every cause must involve energy. Energy
moves from regions of high concentration to
regions of low concentration and, in doing so,
produces changes. For example, coal burns to
make steam in a power plant, and the steam
passes through a turbine and then escapes
into the air. In fl owing from the burning coal
to the atmosphere, the heat spins the turbine
and makes electricity.
Scientists commonly use energy as a key
to understanding nature. A biologist might
ask where certain birds get the energy to fl y
thousands of miles, and a geologist might
ask where the energy comes from to power
a volcano. Energy is everywhere, and when
it moves, whether it is in birds or molten
magma, it causes change. Energy is the
“cause” in “cause and effect.”
In earlier chapters, the fl ow of energy from
the inside of a star to its surface helped you

understand how stars, including the sun, work.

You saw that the outward fl ow of energy sup-

ports the star against its own weight, drives

convection currents that produce magnetic

fi elds, and causes surface activity such as

spots, prominences, and fl ares. You were

able to understand stars because you could

follow the fl ow of energy outward from their

interiors.

You can also think of a planet by follow-

ing the energy. The heat in the interior of

a planet may be left over from the forma-

tion of the planet, or it may be heat gener-

ated by radioactive decay, but it must fl ow

outward toward the cooler surface, where it

is radiated into space. In fl owing outward,

the heat can cause convection currents in

the mantle, magnetic fi elds, plate motions,

quakes, faults, volcanism, mountain building,

and more.

When you think about any world, be it

a small asteroid or a giant planet, think of

it as a source of heat that fl ows outward

through the planet’s surface into space. If

Heat fl owing out of Earth’s interior generates

geological activity such as that at Yellowstone

National Park. (M. Seeds)

you can follow that energy fl ow, you can

understand a great deal about the world.

A planetary astronomer once said, “The most

interesting thing about any planet is how its

heat gets out.”

20-1

Understanding Planets: Follow the Energy