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 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.
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.
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
daylight, half in night.
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.”
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
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.
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.
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.
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.
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.
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.
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
(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
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.
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.
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
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.