The unique Earth

Earth is a unique planet—the only world known

to support any life. It has liquid water on

its surface and lots of oxygen. The thick

atmosphere protects the surface from

radiation and meteorites and the strong

magnetic field shields us from harmful

particles streaming out from the Sun.

EARTH

INSIDE EARTH

Earth has the highest density of any planet in the solar system

because its core is mainly made of iron. The very high pressures at

the center mean that the inner core remains solid, even at 11,000°F

(6,000°C). The outer core is made of molten metal and the

surrounding mantle is a thick layer of partly molten rock. Floating

on top of this is a thin, rocky skin called the crust.

The habitable zone

Earth is at just the right distance from the

Sun for liquid water to exist. Any closer

and the oceans would boil away: any

farther away and the planet would freeze.

The availability of liquid water is very

important. Life on Earth can exist

wherever there is water—without

it life would die. The part of the

solar system where conditions

are suitable for life is

known as the habitable

zone. Earth is the only

planet found here.

Earth has a strong magnetic field, which creates a magnetic

bubble around the planet. Shaped like a tadpole, it extends

about 40,000 miles (64,000 km) from Earth’s surface in the

sunward direction and more in other directions. The magnetic

field usually protects satellites and astronauts that are inside the

bubble from blasts of particles from the Sun. However, massive

solar explosions can weaken it and severe space weather can

cause widespread power cuts and communication blackouts.

The red and green curtains of light that

appear in the night sky at the North and

South poles are known as the northern

lights (aurora borealis) and southern

lights (aurora australis). The auroras are

caused when high-energy particles from

the Sun pour through weak spots in Earth’s

magnetic field, colliding with atoms in the

upper atmosphere and giving off light.

THE PERFECT PLANET

We live on the most amazing rock in the universe. Despite all our efforts to find new, habitable worlds, ours is the only planet so far that has the right conditions for life. Situated at just the right  distance from our Sun, it is not too hot nor too cold. The key to life is liquid water, which Earth  has in abundance. It drives our weather and makes plants grow, forming the basis of the food chain for  animals. Earth is also the only planet we know of that has enough oxygen to keep us alive.

Earth’s seasons

We live our lives according to Earth’s timetable. With a few exceptions, we get up and work in the day and go to sleep at night. The Sun shining on Earth  produces day and night. It also plays a role in creating the seasons—spring, summer, fall, and winter. 

EARTH AND MOON

An alien flying past would see the Earth and Moon appearing to change shape. Sometimes the alien would see Earth fully lit, as a bright blue and green disk, sometimes half-illuminated, and sometimes  fully in shadow—with various stages in between.  

The different shapes are called phases. We can see the Moon’s phases from Earth.

Earth’s seasons

We live our lives according to Earth’s timetable. With a few exceptions, we get

up and work in the day and go to sleep at night. The Sun shining on Earth

produces day and night. It also plays a role in creating the seasons—spring,

summer, fall, and winter.

EARTH AND MOON

An alien flying past would see the Earth and Moon

appearing to change shape. Sometimes the alien

would see Earth fully lit, as a bright blue and green

disk, sometimes half-illuminated, and sometimes

fully in shadow—with various stages in between.

The different shapes are called phases. We can see

the Moon’s phases from Earth.

u AN ALIEN’S

VIEW Earth and

the Moon appear

here in first-quarter

phase—half in

daylight, half in night.

EARTH

Day and night

Because the Earth is tilted as it spins, the period of daylight

changes throughout the year, unless you live on the equator.

The polar regions experience this to the extreme, with very long days in summer and very long nights in winters. North of the Arctic Circle and south of the Antarctic Circle, the Sun does not rise in midwinter or set in midsummer. Because of this, areas such as northern Norway and Alaska are known as the “land of the midnight Sun.”

SEASONS

Unless you live near the equator or the poles, you will experience four seasons:

spring, summer, fall, and winter. At the equator, the period of daylight hardly

changes and the Sun is high in the sky, so it is always warm. Our spinning

Earth is tilted at 23.5 degrees to

the plane of its orbit. When the

North Pole is tilted toward

the Sun, it is summer in

the northern hemisphere

and winter in the

southern hemisphere.

When the North Pole is

tilted away from the Sun,

it is winter in the northern

hemisphere and summer in

the southern hemisphere.

. IN HOT WATER

This map shows how sunlight

affects sea temperatures around

the world, with warm waters in

red around the equator, cooling

through orange, yellow, and green.

Cold waters are shown in blue.

Earth

The seasonal Sun

temperature is influenced by the length of the day and by

the seasons. In the summer, the Sun is above the horizon for longer

and higher in the sky. Less heat is absorbed by the atmosphere and

more heat is absorbed by the ground. In the winter, the Sun is above

the horizon for a shorter length of time. During the long nights, more

heat escapes to space than is provided by the Sun during the day.

VEGETATION patterns (green) change according  

to how much light is received in each season.

On the surface

Earth’s surface is constantly changing. Although covered by a rocky crust, it is far from stiff and static. The crust is divided into huge slabs, called plates, which move very slowly around Earth. The surface is also changed by rivers, glaciers, wind, and rain, which help shape the world around us.

Earthquakes and volcanoes

The edges of plates are dangerous

places to live. Major earthquakes

occur where plates collide and cities

such as San Francisco or Tokyo, which

lie near active plate boundaries, suffer

from frequent, large earthquakes.

Many volcanoes occur at plate boundaries, where one  plate slides under  another, allowing  molten rock  to escape to  the surface.

EARTH’S PLATES

The rocky plates that make up the crust float on Earth’s dense mantle.

They move between 1 and 6 inches (3 and 15 cm) a year, changing the

positions of the continents over time. Some plates move apart, others

slide toward or past each other. Their movements build mountain

ranges and cause earthquakes, tidal waves, and volcanic eruptions.

Mountain ranges

Most continents have mountain ranges. These occur where two plates collide, pushing the crust up to form high peaks. Standing at 29,029 ft (8,848 m), Mount Everest is the highest mountain in the world. It is part of the Himalayan mountain range that formed when the Indian plate crashed into the Eurasian plate. There are also volcanic mountains that rise from the seabed.  The tallest of these is Mauna Kea, an inactive volcano in Hawaii. Measured from the ocean floor, Mauna Kea is even taller than Everest.

Waterworld

As streams and rivers flow down from high ground, they pick up sediment and small rock fragments.  

These abrasive particles grind away at the landscape. Over time, this process wears away  mountainsides and carves out deep canyons. Rivers  can also build up and create new landscape features  by depositing mud and silt as they approach the sea.  

The sea itself is a massive force of change—the waves grind away at cliffs and shorelines, changing coastlines and forming spectacular shapes in the rocks.

Windswept 

In dry places with little water or plant life, wind is the major

source of erosion. The wind blasts rocks at high speed, carrying away loose

particles of rock and grinding these against existing landscape features.

Over years, this wears down rocks and can produce some amazing shapes—arches, towers, and strange,  wind-blown sculptures.

Rivers of ice

Glaciers are large, moving sheets of ice that occur at the poles and high in mountain ranges. Some barely move, while others surge forward, traveling as fast  as 65–100 ft (20–30 m) a day. These rivers of ice dramatically alter the landscape, eroding rock, sculpting mountains, and carving out deep glacial valleys. Glaciers pick up rocks and debris, dragging them along and leaving holes or depressions in the valley floor. As the glaciers melt, they produce lakes and leave boulders strewn across the landscape.

Up in the air

Life could not survive on Earth without

the thick blanket of gases known as the

atmosphere. The atmosphere protects us

from harmful radiation and small incoming

meteorites. It also provides us with our

weather and helps keep Earth warm.

OZONE HOLE

The atmosphere contains a form of oxygen known as

ozone. Ozone is important because it helps block harmful

ultraviolet radiation coming from the Sun. In 1985, a hole

in the ozone layer was found over Antarctica and a

smaller hole was found over the Arctic a few years later.

These holes were caused by the

release of man-made chemicals

called chlorofluorocarbons

(CFCs). These chemicals are

now banned, but the ozone

holes are likely to remain for

many years and are closely

watched by satellites in space.

ATMOSPHERIC ZONES IT’S ALL A GAS

The atmosphere extends about 600 miles

(1,000 km) into space. It is thickest near the

ground and quickly becomes thinner as you

move upward. The most common gases in

the atmosphere are nitrogen (78 percent)

and oxygen (21 percent). Other gases

include argon, carbon dioxide, and

water vapor.

WATER CYCLE

The water cycle is a continuous movement of water between  Earth’s surface and its atmosphere.

 It is powered by heat from the sun and provides us with  a constant source of  freshwater.

Storm forces

Hurricanes are the most powerful storms on Earth. Storms over  tropical waters become hurricanes  when wind speeds reach more  than 75 mph (120 km/h).  Hurricanes in the southern  hemisphere spin in a clockwise  direction, while those in the  northern hemisphere spin counterclockwise.

Life on Earth

Earth is the only place we know where life

exists. Life is found almost everywhere on

the planet—from the highest mountains to

the deepest ocean trenches. It is even found

in boiling hot springs and inside solid rock.

Life begins

The first life-forms were simple,

single cells that probably lived in the

oceans and hot springs. Over billions

of years, single-celled organisms

became a lot more

complex and

multicellular

life evolved.

THE ORIGINS OF LIFE

The first simple life-forms probably appeared

on Earth about 3.8 billion years ago. No one

knows how life began but scientists think

it may have started in the oceans, since the

land was very hot and the atmosphere was

poisonous. Others think comets or meteors

brought complex chemicals from outer space.

However it began, simple molecules formed

and began to copy themselves, eventually

growing into cells, and then colonies.

Over time, these evolved into more

complicated organisms that began

to colonize the land.