Knowledge-Encyclopedia

ENCYCLOPEDIA



smithsonian

ENCYCLOPEDIA

LONDON, NEW YORK, MELBOURNE, CONTENTS
MUNICH, AND DELHI
SPACE
US Senior Editor Rebecca Warren
US Editor Kate Johnsen THE UNIVERSE 10
The Big Bang 12
Senior Editors Shaila Brown, Daniel Mills, Ben Morgan Galaxies 14
Senior Art Editors Vicky Short, Smiljka Surla Star birth 16
Star death 18
Editors Lizzie Munsey, Sam Priddy, Alison Sturgeon The Sun 20
Designers Daniela Boraschi, Tannishtha Chakraborty, Richard Horsford, The Solar System 22
Inner planets 24
Hedi Hunter, Fiona Macdonald Outer planets 26
Visualizer Peter Laws The Moon 28

Illustrators Peter Bull, Rob Cook, FOREAL™, Mike Garland, Mark Garlick, SPACE EXPLORATION 30
Gary Hanna, Jason Harding, Arran Lewis, Maltings Partnership, Medi-Mation, Astronomy 32
Peter Minister, Gerson Mora and Anna Luiza Aragão/Maná e.d.i., Moonrunner Mission to the Moon 34
Exploring the planets 36
Design, Ian Naylor, Alex Pang, Dean Wright and Agatha Gomes
DK Picture Library Emma Shepherd, Rob Nunn
Jacket Designer Laura Brim
Producer, pre-production Francesca Wardell
Producer Alice Sykes
Managing Editors Julie Ferris, Paula Regan
Managing Art Editor Owen Peyton Jones
Publisher Sarah Larter
Art Director Phil Ormerod
Associate Publishing Director Liz Wheeler
Publishing Director Jonathan Metcalf

Contributors Kim Bryan, Robert Dinwiddie, Jolyon Goddard, Ian Graham,
Reg G. Grant, Jacqueline Mitton, Darren Naish, Douglas Palmer,
Philip Parker, Penny Preston, Sally Regan, David Rothery,
Carole Stott, Paul Sutherland, Chris Woodford, John Woodward

First American Edition, 2013
Published in the United States by DK Publishing, 345 Hudson Street

New York, New York 10014
13 14 15 16 17 10 9 8 7 6 5 4 3 2 1

001—187527—Oct/13
Copyright © 2013 Dorling Kindersley Limited

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in any form or by any means, electronic, mechanical, photocopying,
recording, or otherwise, without the prior written permission of both the
copyright owner and the above publisher of this book. Published in Great

Britain by Dorling Kindersley Limited.
A catalog record for this book is available from the Library of Congress.

ISBN: 978-1-4654-1417-5
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THE SMITHSONIAN
Established in 1846, the Smithsonian—the world’s largest museum and
research complex—includes 19 museums and galleries and the National
Zoological Park. The total number of artifacts, works of art, and specimens
in the Smithsonian’s collection is estimated at 137 million. The Smithsonian
is a renowned research center, dedicated to public education, national

service, and scholarship in the arts, sciences, and history.

EARTH NATURE

PLANET EARTH 40
Inside the Earth 42
Earth’s climate 44

TECTONIC EARTH 46
Plate tectonics 48
Volcanoes 50
Earthquakes 52

EARTH’S RESOURCES 54
Rocks and minerals 56

WEATHER 58
Hurricanes 60
The water cycle 62

SHAPING THE LAND 64
Caves
Glaciers 66 HOW LIFE BEGAN 76
68 Timeline of life 78
EARTH’S OCEANS 80
The ocean floor 70 The dinosaurs 82
72 Tyrannosaurus rex 84
How fossils form
86
THE LIVING WORLD 88
Plant life 90
Green energy
92
INVERTEBRATES 94
Insects 96
Butterfly life cycle
98
VERTEBRATES 100
Fish 102
Great white shark 104
Amphibians 106
Frog life cycle 108
Reptiles 110
Crocodile 112
Birds 114
How birds fly 116
Mammals 118
African elephant
120
SURVIVAL SECRETS 122
Habitats 124
American desert 126
Amazon rainforest 128
African savanna 130
Coral reef 132
Animal architects 134
Predators and prey

HUMAN BODY 138 SCIENCE 190
140 192
BODY BASICS 142 MATTER 194
Building blocks 144 Atoms and molecules 196
The skeleton 146 Atom smasher 198
Muscle power Solids, liquids, and gases 200
The skin 148 The elements 202
150 Chemical reactions
FUELING THE BODY 152 Material world 204
From mouth to stomach 154 206
The intestines 156 FORCES 208
In the blood 158 Laws of motion 210
The heart 160 Engines 212
Fighting germs 162 Simple machines 214
Cleaning the blood Flotation 216
Air supply 164 Magnetism 218
166 Gravity
IN CONTROL 168 Flight 220
Nervous system 170 222
Brainpower 172 ENERGY 224
How vision works 174 Electromagnetic spectrum 226
Inside the ear 176 Signals from space 228
Taste and smell Light 230
Control chemicals 178 Telescopes 232
180 Sound 234
LIFE CYCLE 182 Heat 236
A new life 184 Electricity 238
Life in the womb 186 Power network
Growing up Radioactivity 240
Genes and DNA 242
ELECTRONICS 244
Digital world
Robotics

HISTORY 248 Imperial China 294
250 Rulers of India 296
THE ANCIENT WORLD 252
The first humans 254 THE MODERN WORLD 298
The first towns 256 The slave trade 300
Early empires 258 The Enlightenment 302
Ancient Egypt 260 The American Revolutionary War 304
The pharaohs 262 The French Revolution 306
Ancient Greece 264 The Industrial Revolution 308
Ancient Athens 266 The Age of Steam 310
The Roman Empire 312
Roman society The Civil War 314
268 World War I 316
THE MEDIEVAL WORLD 270 Trench warfare 318
Viking raiders 272 World War II 320
Fortresses 274 Modern warfare 322
Wars of faith 276 The Cold War 324
World religions 278 The 1960s 326
The Ottoman Empire 280 The 21st century
The Silk Road 282 328
Samurai warriors 346
284 REFERENCE 350
THE AGE OF DISCOVERY 286 GLOSSARY 359
Voyage to the Americas 288 INDEX
Ancient Americas 290 ACKNOWLEDGMENTS
The Renaissance 292
Shakespeare’s theater



SPACE

When you look into the blackness of the night sky, you are
peering into the fathomless depths of the Universe. Stars,
planets, and galaxies stretch into space, not just farther than
you can see, but farther than you can imagine.

10

THE CELESTIAL BODIES Asteroid
UNIVERSE Rocky lumps left over from
The Universe is at least 99.999999999999 the formation of the Solar
The Universe is the whole of existence—all of space, percent empty space. Floating in this vast, System are called asteroids.
matter, energy, and time. The Universe is so vast that it dark void are all sorts of different objects, They range in size from
seems unimaginable, but we do know that it has been which astronomers call celestial bodies. boulders to bodies close to
steadily expanding following its beginning 13.8 billion They range from grains of dust to planets, the size of a dwarf planet.
years ago in an explosive event called the Big Bang. stars, and galaxies. Our Solar System
includes a star (the Sun) and a large family
of planets and moons that formed from
the same cloud of gas that gave birth to
the Sun. In recent years, planets have
been seen around hundreds of other
stars, showing that our Solar System
may be one of billions in our galaxy.

UNDERSTANDING Closed Looking back in time
THE UNIVERSE A dense Universe would bend itself into a
closed shape. Traveling in a straight line Because light takes time to travel,
People used to think of the would bring you back to your starting point. when we look into space we are looking
Universe as a giant sphere, but back in time. The most distant objects
we now know that things are visible are galaxies photographed by
not so simple. The Universe the Hubble Telescope. We see them as
probably has no center or outer they were 13 billion years ago. The
edge. Only a fraction of it—the Universe extends far beyond these,
observable Universe—is visible but it’s impossible to see objects
to us. The whole Universe may much further because their light
be vastly bigger than this, hasn’t had time to reach us.
perhaps infinitely so.
Furthest objects
The light from the faintest galaxies in this
photo from the Hubble Space Telescope
took 13 billion years to reach Earth.

The shape of space Open What’s the matter? 27% 23%
If the Universe isn’t dense enough, it might dark matter helium
The three dimensions of space stretch into an open shape, making it The elements hydrogen and
are bent by the force of gravity infinite in size with no outer edge. helium make up 98 percent of 68% 75%
from matter in the Universe into the matter we can see in the dark energy hydrogen
a fourth dimension that we can’t Flat Universe. But there doesn’t
see. This is hard to visualize, so Just the right amount of matter would give seem to be enough matter to 5% matter
scientists use the metaphor of a the Universe a flat shape. This would also account for the way stars and
two-dimensional rubber sheet to be infinite in size with no outer edge. galaxies are pulled by gravity. 2% other
explain the idea. The mass of the As a result, astronomers think elements
Universe could bend this rubber galaxies contain dark matter,
sheet in one of three ways, which we cannot see. There is MAKEUP OF THE UNIVERSE
depending on how densely also an unknown force making
packed with matter the Universe the Universe expand, known
is. Most scientists now think the as dark energy.
shape of the Universe is flat.

THE SCALE Earth and Moon Solar System Stellar neighborhood
OF SPACE Earth is 7,926 miles (12,756 km) wide. The Sun’s family of eight planets occupy The nearest star to the Sun is Proxima
Our nearest neighbor in space—the Moon— a region of space 5.6 billion miles (9 billion Centauri, which is just over four light years
The Universe is so vast that orbits Earth at a distance of 238,855 miles km) wide. If Earth were a soccer ball, it away. There are around 2,000 stars within
we cannot appreciate its size (384,400 km). If Earth were the size of a would take five days to walk across this 50 light years of the Sun. These make up
without making leaps of scale. soccer ball, the Moon would be the size of part of the Solar System. The nearest star our stellar neighborhood, which is a tiny
In this series of pictures, each a cantaloupe about 69 ft (21 meters) away. would be a 58-year walk away. fraction of the Milky Way galaxy.
stage represents a microscopic
speck of the image to its right.
When dealing with the vast
distances in space, miles
aren’t big enough. Instead,
astronomers use the speed of
light as a yardstick. Light is so
fast it can travel around the
Earth 7.5 times in a second.
One light year is the distance
light travels in a year: nearly
6 trillion miles (10 trillion km).

11

Comet Moon Dwarf planet Planet Stars Nebula
Comets are chunks of ice Also called a natural Dwarf planets are larger A planet is a large and These luminous balls of A glowing cloud of gas and
from the outer reaches of satellite, a moon is a body than asteroids but smaller nearly spherical object that gas, such as the Sun, shine dust in space is known as a
the Solar System. Some that orbits a planet. Earth than planets. Like planets, orbits a star and has swept by generating their own nebula. Some nebulae are
grow long tails of gas and has only one moon but they are round in shape. its orbital path clear of nuclear power. Stars come clouds of wreckage created
dust as they approach the the planet Jupiter has Pluto (above) is the best debris. The Solar System in a wide range of types, by dying stars. Others give
Sun and are warmed by it. 67, including Io (above). known dwarf planet. has eight planets. temperatures, and sizes. birth to new stars.

IS THERE ANYBODY Arecibo message Numbers 1 to 10 Pioneer plaque
OUT THERE? in binary, reading
In 1974, scientists used the left to right The robotic spacecraft Pioneer 10
One of the biggest questions in giant Arecibo radio telescope and Pioneer 11 visited the planets
science is whether life is unique in Puerto Rico to broadcast a Chemical formula of Jupiter and Saturn in 1973–74 and
to Earth or has arisen on other radio message toward the star DNA (the molecule then flew off into deep space. If
worlds. And if life has appeared cluster M13. The message that carries the aliens ever discover either craft
elsewhere, could intelligent contains symbols (right) that blueprint of life) drifting through interstellar space,
beings have evolved? represent human beings, our they will find a gold-plated plaque
Scientists have set up projects base-10 counting system, the Shape of DNA engraved with a message from Earth.
to watch and listen for signals DNA molecule, and the Solar molecule
from extraterrestrials, and System. More a publicity stunt Hydrogen Man and
messages have been sent to than a serious attempt to atom woman in front
the stars to inform any aliens contact aliens, the broadcast of Pioneer craft
out there of our existence. will take 25,000 years to
reach M13, and a reply will
take 25,000 years to return.

SETI Human figure and
population of Earth
Astronomers involved in the in 1974
SETI (search for extraterrestrial
intelligence) project use powerful Earth’s position
radio telescopes to scan the skies in Solar System
in search of artificial radio signals
broadcast by alien civilizations. RADIO Arecibo Telescope,
The SETI project has been running TELESCOPE which sent message
since 1960, but it has so far found
no conclusive evidence of alien Position of Plan of Solar
signals, despite some false alarms. Sun in Milky System, with
Way galaxy Pioneer’s flight path

Milky Way galaxy Local Group of galaxies Supercluster Universe
The Milky Way is a vast cloud of 200 billion The Milky Way is just one of perhaps seven Clusters of galaxies exist in even larger Superclusters are thought to form a vast
stars. Its shape resembles a pair of fried trillion galaxies in the observable Universe. groupings called superclusters. We live web of filaments riddled with enormous
eggs held back to back, with a central bulge Galaxies exist in groups called clusters, held in the Virgo Supercluster, which is one voids containing no galaxies. The true size
surrounded by a flat disk. It measures together by gravity. The Milky Way is part of millions of superclusters in the known of the Universe is a mystery, and only a
100,000 light years across the disk and of a cluster known as the Local Group, Universe. Between these are immense fraction of it is visible to us. The Universe
2,000 light years deep through the bulge. which is about 10 million light years wide. empty areas called cosmic voids. may even be infinite in size.

12 space THE UNIVERSE 570 thousand million million million miles (1 million million million
million kilometers)—the diameter of the observable Universe.

The Big Bang The expanding Universe Rate of
expansion
About 14 billion years ago, the Universe materialized The illustration below does not show the increases
out of nothing for unknown reasons. Infinitely smaller shape of the Universe, which is unknown.
than an atom to begin with, the Universe expanded to Instead, it is a timeline that shows how the
billions of miles across in under a second—an event Universe has expanded and changed since the
called the Big Bang. Big Bang. We know the Universe is expanding
because the most distant galaxies are rushing
Time came into existence when the Universe began, so the apart at rapid speeds. By running the clock
question “What happened before?” has no meaning. Space also backward, astronomers figured out that the
came into existence. The Big Bang was not an explosion of expansion began 13.8 billion years ago
matter through space—it was an expansion of space itself. at a single point: the Big Bang.

At first the Universe consisted of pure energy, but within First galaxies form
a trillionth of a second some of this energy turned into matter,
forming a vast soup of subatomic particles (particles smaller
than atoms). It took nearly 400,000 years for the particles to
cool down enough to form atoms, and then another 300 million
years before the atoms formed planets, stars, and galaxies.
The expansion that began in the Big Bang continues to this
day, and most scientists think it will carry on forever.

Stars form

The Universe began 9

as something called a singularity: a Atoms form
point of zero size but infinite density.

Protons and
neutrons form

Energy turns
into particles

The Universe begins 8

7
6
5

4

2 Within a tiny fraction of a
second, the Universe balloons in
size from trillions of times smaller
than an atom to the size of a city.
The rate of expansion then slows.

3 3 The intense energy of the
2 newborn Universe creates matter.
At first, the matter is a soup of
1 particles and antiparticles. These
crash into and cancel each other
1 The Universe appears out of out, turning back into energy. But
nowhere. At the start, the Universe some of the matter is left over—
consists purely of energy and is this will eventually turn into atoms
infinitely dense and unimaginably and later stars and galaxies.
hot—18 billion trillion trillion °F
(10 billion trillion trillion °C).

13

11

Discovery of the Big Bang

The first scientific evidence for the Big Bang was found
in 1929, when astronomers discovered that light from
distant galaxies is reddened. This color change happens
when objects are moving away from us, making
lightwaves stretch out and change color. The more
distant the galaxies are, the faster they are rushing
away. This shows that the whole Universe is expanding.

STATIC UNIVERSE

No change
in starlight
10
EXPANDING UNIVERSE

Light waves
stretched

Big Bang afterglow

More evidence of the Big Bang came in the 1960s,
when astronomers detected faint microwave radiation
coming from every point in the sky. This mysterious
energy is the faded remains of the intense burst of
energy released in the Big Bang.

MICROWAVE MAP
OF WHOLE SKY

Solar System forms

7 At 300 million years, stars Changing elements 24% helium
appear. Stars form when great 76% hydrogen
clouds of gas are pulled into tight For hundreds of millions
4 The Universe is now about 1 knots by gravity. The pressure and of years, the Universe EARLY UNIVERSE 75% hydrogen
microsecond old and 60 billion miles heat become so intense in the consisted almost entirely UNIVERSE TODAY 23% helium
(100 billion km) wide. The leftover dense pockets of gas that nuclear of hydrogen and helium 1% oxygen
particles begin to form protons and reactions begin, igniting the star. —the very simplest 0.4% carbon
neutrons—the particles that today chemical elements. 0.4% neon
make up the nuclei of atoms. But the 8 At 500 million years, the first After stars appeared, 0.1% iron
Universe is too hot for atoms to galaxies are forming. Galaxies are new elements began to 0.1% nitrogen
form yet. Light cannot pass through enormous clouds of stars, held be made in the cores + traces of other
the sea of particles, so the young together by gravity. of dying stars. All the
Universe resembles a dense fog. complex elements in our
9 Now 5 billion years old, the bodies were forged in
5 After 379,000 years, the Universe consists of vast clusters dying stars this way.
Universe cools enough for atoms of galaxies arranged in threads,
to form. The Universe is now an with gigantic voids between them. Big Bounce theory
enormous cloud of hydrogen and The voids get ever bigger as space
helium. Light can pass through continues to expand. At 8 billion What caused the Big Bang? We may never know for sure,
space more easily, and the Universe years, the expansion of the but some scientists have suggested that there may have
becomes transparent. Universe begins to accelerate. been lots of big bangs, with the Universe expanding after
each one and then shrinking again. This theory is called
6 Half a million years after the Big 10 Our Solar System forms at 9 the Big Bounce because the process repeats itself.
Bang, matter is spread out almost billion years. When the Universe is
evenly in the Universe, but tiny 20 billion years old, the Sun will Universe expands Big Bang Universe shrinks
ripples exist. Working on these expand in size and destroy Earth.
denser patches, gravity begins TIME
pulling the matter into clumps. 11 The Universe will carry on
expanding forever, becoming
cold and dark everywhere.

14 space THE UNIVERSE 200  billion—the approximate number 23 —the number of times our Solar
of stars in the Milky Way galaxy. System has orbited the Milky Way.

Galaxies The Milky Way Orion Arm
Our Solar System lies in
Our Sun belongs to a giant whirlpool of stars If you could look down on the Milky Way galaxy from this small arm. Many of the
called the Milky Way. Huge collections of above, the view would be like flying over a glittering stars we see in the night
stars are called galaxies, and like all galaxies city at night. Most of the galaxy’s 200 billion stars are sky are in the Orion Arm.
the Milky Way is unimaginably vast. in the central bulge. Curving around this are two vast
spiral arms and several smaller arms. The Milky Way
Galaxies come in many shapes and sizes. Some are is thought to be a barred spiral (see panel), but we
spirals like our own galaxy, but others are fuzzy can’t see its shape clearly from Earth since we view
balls or shapeless clouds. The smallest have just it from the inside. In the night sky, the Milky Way
a few million stars. The largest contain trillions. appears only as a milky band of light.

Although they look packed with stars, galaxies MILKY WAY OVERHEAD VIEW
are mostly empty space. If you made a scale model
of the Milky Way with a grain of sand for each star, SCUTUM-CENTAURUS ARM
the nearest star to the Sun would be 4 miles (6 km)
away. The furthest would be 80,000 miles (130,000
km) away. The stars in a galaxy are held together
by gravity and travel slowly around the galactic
heart. In many galaxies, including ours, a
supermassive black hole lies hidden in the
center. Stars and other material are sucked
into this cosmic plughole by gravity
and disappear forever.

10,000 light-years

1 Galactic center 20,000 light-years
This photo from an
infrared (heat-sensitive) NORMA ARM
telescope shows stars
and gas clouds packing 30,000 light-years
the center of the Milky
Way. A supermassive 40,000 light years
black hole lies hidden
somewhere in this area. MILKY WAY SIDE VIEW

2 Solar System Main disc containing arms
Our Solar System
is in a minor spiral arm
called the Orion Arm.
We orbit the center of
the galaxy once every
200 million years,
traveling at about 120
miles (200 km) a second.

3 Crab Nebula
Clouds of gas and dust
occur throughout
the Milky Way, especially
in the spiral arms. The
Crab Nebula is a cloud of
wreckage left behind by a
dying star that exploded.

4 Globular cluster
Not all the Milky Way’s
stars are in the main disc
of the galaxy. Many are in
globular clusters—tightly
packed balls of ancient
stars floating above and
below the galaxy in a
spherical region called
the halo.

7 trillion—the approximate number of 6,000 years—the length of time it would take to count 15
galaxies in the observable Universe. the Milky Way’s stars at a rate of one a second.

Perseus Arm Galaxy shapes
This is one of the Milky Way’s two
major spiral arms. We see stars in Astronomers classify galaxies into just
this arm when we look toward the a few main types, depending on the
constellation Perseus. shape we observe from Earth.

3 Circles show
2 distance from the
center of the galaxy.

Spiral Barred spiral

A central hub of stars A straight bar runs

YOU ARE HERE is surrounded by spiral across the center,

SAGITTAR arms curving out. connecting spiral arms.

IUS ARM CROSS SECTION THROUGH

MAIN DISC OF GALAXY Elliptical Irregular

Bulge formed More than half of all Galaxies with no clear

by spiral arm galaxies are simple shape are classified

Central bar ball shapes. as irregular.
1

PERSEUS ARM How spiral arms form

The stars in a galaxy orbit the center, taking
millions of years to make one circuit. Spiral
arms appear where stars pass in and out of
crowded areas, like cars passing temporarily
through a traffic jam. One theory is that
these traffic jams happen because the orbits
of different stars don’t line up neatly.

If stars all had neat,
parallel orbits, the
galaxy would have
no spiral arms.

Gas cloud If stars’ orbits don’t
line up neatly,
crowded zones

form, giving the
galaxy spiral arms.

Dark lane formed Colliding galaxies
by dust
Sometimes galaxies crash and tear each
other apart. Individual stars don’t collide, but
gas clouds do, and gravity pulls the colliding
galaxies into new shapes.

Scutum-Centaurus Arm
This is the second of the two
main spiral arms. The area
where it joins the central bar
is rich in star-forming clouds.

Central bulge

4 Globular cluster

End of the Milky Way
In 4 billion years our galaxy will collide with the
Andromeda galaxy. This artist’s impression shows
what the sky might look like as they merge.

16 space THE UNIVERSE The brightest stars emit 6 million 13  billion years—the age of
times more light than the Sun. the oldest known star.

Star birth The largest stars

Stars have been forming throughout the Universe in the night sky are big enough to
for most of its life. They take shape in vast clouds
where thousands of stars are born at a time. swallow our Sun a billion times.

The clouds that give birth to the stars are cold and dense How new stars form
and consist mainly of hydrogen gas. The newly formed
stars are huge spinning globes of hot, glowing gas—mainly The star-forming process begins when the cloud becomes
hydrogen, with helium and small amounts of other unstable and breaks up into fragments. Gravity pulls the
elements. Much of this material is packed tightly into the material in a fragment into an ever-tighter clump, and the
stars’ cores, and it is here that nuclear reactions release clump slowly forms a sphere as it shrinks. Now a protostar,
energy in the form of heat and light. this star-to-be keeps on shrinking, its core getting denser and
hotter. Eventually the pressure and temperature are so high
that nuclear reactions begin, and the star starts to shine.

Interstellar Cloud breaks up Protostar Rotating disc
gas cloud into fragments

1 Interstellar cloud 2 Fragments form 3 Protostar
Stars are born within Now unstable, the cloud breaks A protostar forms. Gravity
enormous, cold, dense clouds of up into fragments of different size pulls material into its core,
gas and dust. The process of star and mass. The most massive and where the density, pressure,
formation may be triggered if dense of these fragments are and temperature build up. The
something disturbs the cloud, gradually pulled by their own more matter the original cloud
such as a collision with another gravity into tighter clumps. These fragment contained, the greater
cloud or a shockwave from a shrinking fragments will eventually the temperature and pressure
supernova explosion. turn into protostars. rise as the protostar develops.

Starbirth nebulas

Clouds of gas and dust in space
are called nebulas. Much of the gas
and dust in a nebula is debris from
old stars that exploded when they
ran out of fuel. Over millions of
years, this material is recycled to
make new stars. Starbirth nebulas
are among the most beautiful
objects in space, their colorful
clouds illuminated from within by
the blue light of newborn stars.

Orion Nebula
The Orion Nebula is one of the closest
star-forming regions to Earth. In the
night sky it looks like a fuzzy star in
the sword of Orion. In reality, it is a
vast cloud of gas and dust thousands
of times bigger than the Solar System.

17

Types of star Brighter Supergiant Classifying stars
MAIN SEQUENCE STARS The Hertzsprung–Russell
A star begins to shine when LUMINOSITY Blue diagram is a famous
nuclear reactions in its core supergiants graph that astronomers
convert hydrogen into helium use to classify stars. The
and release energy. It is then White dwarfs Red giants graph plots brightness
called a main sequence star. Not Sun against temperature
all main sequence stars are the Dimmer and reveals that there
same—they differ in size, Cooler are distinct groupings of
temperature, color, brightness, Hotter TEMPERATURE stars, such as red giants
and the amount of matter they Star begins to shine (dying stars) and main
contain. When stars begin to run sequence stars (ordinary
out of fuel and near the end of stars). Astronomers also
their lives, they stop being main classify stars by color,
sequence stars and may swell which is linked to their
up and turn into red giants or temperature: hot stars
shrink to become white dwarfs. are blue; cooler stars
are orange or red.
Jets of gas
Leftover material
may form planets

4 Spinning disc 5 A star is born 6 Planets form
The growing mass at the Squeezed by the force of gravity, Not all the material from the
center creates a gravitational the protostar’s core becomes so hot gas cloud has been used to make
pull, drawing ever more gas and dense that nuclear reactions the star. The leftovers form a
and dust inward. A little like occur, and the star begins to shine. spinning disc of gas and dust
water going down a drain, the The glowing core produces an outward around the star. This debris may
material being pulled in starts pressure that balances the inward pull be lost into space, or it may clump
to spin around. Powerful winds of gravity, making the star stable. It is together to form planets, moons,
develop, blowing jets of gas now a main sequence star. comets, and asteroids.
out from the center.

Star clusters

Stars are not formed singly—they
are born in clusters from the same
cloud of material at roughly the
same time. Eventually, the stars of
a cluster will drift apart and exist
alone in space, or with a close
companion or two. Our Sun, like
about half of the stars nearest to
us, is alone. About a third of the
stars in the night sky are in pairs,
bound together by gravity.

Pleiades cluster
A handful of the 5,000 or so stars
that make up the Pleiades cluster
can be seen with the naked eye. In
about 250 million years time, the
stars will have dispersed and the
cluster will no longer exist.

18 space THE UNIVERSE Supergiant stars can grow to 1 billion
times the volume of our Sun.
Four ways to die
Small stars Star begins
Stars can die in four different ways, all of which are Stars with less than half the mass of the Sun fade away to shrink
shown on these pages. Our Sun, a typical star, will very slowly. Once the hydrogen in the core is used up,
follow the central path, but not yet—it has enough the star begins to feed off hydrogen in its atmosphere.
fuel to keep shining for 5 billion years. When larger But it doesn’t generate enough gravity to use other
stars die, they turn hydrogen into heavier chemical elements as fuel, so it slowly shrinks to become a
elements such as carbon and oxygen, which are black dwarf. This will take far longer than the
later recycled to form new stars and planets. All age of the Universe—up to a trillion years.
the atoms in your body were created this way.

5 billion tons—the weight of one
teaspoonful of material from
the core of a neutron star.

Stable star
Every young star goes
through a stable phase in
which it shines steadily.

Star death Medium stars Star expands
When a Sunlike star has used up the hydrogen
All stars eventually run out in its core, nuclear fusion spreads outside the core,
of fuel and die. Most fade away making the star expand into a red giant. The core
quietly, but the most massive stars collapses until it is hot and dense enough to fuse
self-destruct in a huge explosion helium, but eventually it runs out of helium too.
that can outshine an entire galaxy. Finally, it becomes a white dwarf, and its outer
layers spread into space as a cloud of debris.
Like Earth, stars generate the force of
gravity, which squeezes their hot cores. The Massive stars Star expands
more matter a star has, the greater the force Stars over eight times more massive than
of gravity and the hotter and denser the core our Sun end their lives in strange and
becomes. The way a star dies depends on how violent ways. The heat and pressure inside
much matter it contains (its mass) and how the core become so great that nuclear fusion
powerfully its core is squeezed by gravity. can not only fuse hydrogen atoms together to
form helium but can fuse helium and larger
Stars make heat and light by the process of atoms to create elements such as carbon or
nuclear fusion: hydrogen atoms in the core crash oxygen. As this takes place, the star swells
together to form helium, releasing energy. In small into the largest star of all: a supergiant.
stars, when hydrogen in the core runs out, the star’s
light slowly fades. But in more massive stars, the core
is so hot and dense that fusion can spread beyond it,
changing the star’s appearance. The most massive stars
are eventually overwhelmed by their own gravity, which
crushes them so violently that they collapse into a
pinprick to create a black hole.

1  teaspoonful of material from a red 1,600 light years—the distance from 19
giant weighs less than a grain of salt. Earth to the nearest black hole.

Light intensity fades Star continues to Light becomes Black dwarf
as fuel runs out shrink and fade increasingly dim Finally, its fuel used up and

its light extinguished, the
star becomes a black dwarf—

an Earth-sized cinder.

Red giant Planetary nebula White dwarf
Nuclear fusion spreads to The star’s outer layers disperse into All that remains is the dying
the layer around the core, space as a glowing cloud of wreckage
heating it up and making —a planetary nebula. The material in core—a white dwarf. This
the star expand. Nearby this cloud will eventually be recycled Earth-sized star will slowly
planets may be swallowed to form new stars.
up by the growing giant. fade and become a cold,
dead black dwarf.

Outer layer Core

Neutron star
Up to three times heavier than the
Sun, yet just a few miles wide,
neutron stars are unimaginably
dense, fast-spinning stars.

Red supergiant Supernova Black hole
The star has grown into a supergiant. The star self-destructs in an explosion brighter The force of gravity close to a black hole
Nuclear fusion carries on inside the core, than a billion suns. Its outer layers are blasted is so intense that nothing can escape
forcing atoms together to form heavier and into space, but its massive core continues to from it—not even light. Anything falling
heavier elements, until the star’s core turns collapse in on itself. What happens next inside is torn apart by gravity and then
into iron. When this happens, the core no depends on how massive the core is. A smaller crushed into a point of infinite density.
longer generates enough outward pressure to core becomes a neutron star, but a massive
resist the crushing force of gravity, and the core never stops collapsing. It shrinks until it’s
whole star suddenly collapses, causing a billions of times smaller than an atom and
catastrophic explosion—a supernova. becomes a black hole.

20 space THE UNIVERSE

Loop prominence
Gigantic loops of glowing gas
extend high above the Sun’s
surface, anchored to the star’s
tangled magnetic field. Called
loop prominences, these gas
eruptions can last for months.

Spikes of gas
Jets of hot gas rise all the
time from the Sun’s surface,
forming towering spikes
that last just a few minutes
before collapsing. Called
spicules, these formations
can reach thousands of
miles in height. Seen from
above (right), they form
shimmering, hairlike
patterns around a sunspot.

The Sun Core
Inside the hot, dense core,
The Sun is a nearly perfect sphere of the process of nuclear fusion
hot, glowing gas. Its source of power lies releases energy. Every second,
buried deep in the central core, where a 683 million tons of hydrogen are
nuclear furnace rages nonstop, turning fused into helium in the core.
matter into pure heat and light.
Radiative zone
Slightly bigger than a typical star, the Sun is Outside the core is the
large enough by volume to swallow 1.3 million radiative zone, which is
Earths. It contains 99.8 percent of all the matter
in the Solar System, and the force of gravity not dense enough for
generated by this enormous mass keeps the nuclear fusion to take
planets trapped in orbit around it. Seen from place. Energy from the
Earth, the Sun is a life-sustaining source of light core seeps very slowly
and warmth that shines steadily on us. Closer out through this layer.
views, however, reveal a world of astonishing
violence, its seething surface bursting with vast Convective zone
eruptions that hurl fiery gases into space. In the convective zone, vast
bubbles of hot gas rise to the

surface, cool, and then fall,
transferring energy from the

Sun’s heart to the exterior.

Inside the Sun Photosphere
The Sun’s apparent surface
Scientists divide the Sun’s interior
into three distinct layers: the core, the is called the photosphere.
radiative zone, and the convective zone. Energy escapes into space
All three are made solely of gas, but the
gas gets hotter and denser toward the from here as light.
center. In the core, the temperature soars
to 27 million °F (15 million °C) and the
gas is 150 times more dense than water.

385 million billion gigawatts—the amount of 21
energy output from the Sun each second.

SIZE OF EARTH Sun statistics
COMPARED
TO THE SUN Diameter .......... 865,374 miles (1,393,684 km)
Distance from Earth ................. 93 million miles
Solar flare
A sudden burst of energy (150 million km)
from the Sun’s surface is Mass (Earth = 1) ........................................ 333,000
called a solar flare. Flares are Surface temperature ....... 10,000 °F (5,500 °C)
often followed by a coronal Core temperature.. 27 million °F (15 million °C)
mass ejection (see panel).
Energy release
Sunspots
Cooler, darker patches on the It takes only eight minutes for light from the
Sun are called sunspots. The Sun to reach Earth, but it can take 100,000
number of sunspots rises and years for energy released in the Sun’s core
falls over an 11-year cycle. to travel to the surface and emerge as light.
The journey is slow because the energy is
absorbed and reemitted by trillions of atoms
as it passes through the dense radiative zone.

100,000 years

8 minutes

EARTH

SUN

Rotation

Like all objects in space, the Sun rotates.
Unlike the Earth, which rotates as a solid
object, the Sun is a ball of gas and turns at
different speeds in different places. The
equator takes 25 Earth days to rotate once,
but the polar regions take 34 days.

34 days

25 days

24 hours 34 days

EARTH SUN

Mass ejections

Vast bubbles of superhot gas (plasma), each
with a mass of around 1.1 billion tons, erupt
from the Sun up to three times a day. Called
coronal mass ejections, these bubbles grow
millions of miles wide in a few hours and
then burst, sending a blast of charged
particles hurtling across the Solar System.
The blast waves sometimes collide with the
Earth, lighting up the polar skies with
unusually brilliant auroras.

Grainy surface 3:23 PM 6:09 PM 6:25 PM
The bubbles of hot gas that
rise up inside the Sun make its
surface look grainy. There are
some 4 million granules on the
Sun’s face, each about 600 miles
(1,000 km) wide and lasting for
around eight minutes.

22 space THE UNIVERSE If the Sun were the size of a basketball, If the Sun were the size of a basketball, Neptune
Earth would be a pea 270 ft (83 m) away. would be a strawberry 170 miles (270 km) away.
Asteroid
Asteroids are giant space rocks that
drift around the inner Solar System.
Most lie in a belt between Mars and
Jupiter, but some occasionally come
dangerously close to the Earth. The
smallest are the size of houses, while
the largest are big enough to be
classified as dwarf planets. Scientists
think asteroids are leftovers from the
material that formed the planets. All
of them together amount to less than
a twentieth of the Moon’s mass.

Sun Saturn
The Sun is like a vast nuclear The second biggest planet
power station that produces is striking for the dazzling
energy by converting hydrogen system of bright rings that
into helium. It is the only star encircle it. It has 62 moons
we can study close up. and dozens more moonlets.

Earth
Our home planet is the
only place known to
support life, thanks to the
liquid water on its surface.

Mercury
The closest planet to the Sun,
Mercury is also the smallest
planet. Its surface is scarred
by ancient craters.

Mars
Mars is a bitterly
cold, desert world. Like
Earth, it has mountains,
canyons, and icy poles.

Venus
Though similar in size to
Earth, Venus is a hellish
world where any visiting
astronaut would be
crushed and boiled alive.

Orbital distance

The scale bar below shows the relative
distances of the planets from the Sun. The
distance between one planet and the next
increases greatly as we move out through
the Solar System.
Sun
Mercury
Venus Jupiter Saturn
Earth
Mars

500 million miles 1,000 million miles
(805 million km) (1,609 million km)

26 The number of asteroids known to be
more than 124 miles (200 km) wide.

Comet Neptune
These small, icy bodies can The most distant planet, Neptune
develop spectacular tails of
gas and dust when they is a blue giant with 13 known
approach the Sun. moons. It takes Neptune nearly
164 years to orbit the Sun once.
Uranus
Blue giant Uranus orbits Kuiper Belt
the Sun tipped over on its Thousands, if not millions, of
side, perhaps because of a
crash with a smaller planet. small icy bodies occupy the
region beyond the planets.
It has 27 moons. The Kuiper Belt is home to
dwarf planet Pluto and is a

likely source of comets.

The Solar System

Jupiter The force of gravity generated by the Sun’s vast mass
The largest planet in the Solar System, keeps a family of planets and other bodies trapped in
Jupiter is more massive than all the orbit around it. Together, the Sun and all these bodies
other planets put together. It has its make up our Solar System.
own family too, with at least 67
moons, some as big as planets. Our Sun formed from a great cloud of dust and gas around 4.6
billion years ago. Vast amounts of matter were drawn in by the
The Solar System developing star, but not all of it was fully absorbed. A tiny
fraction of leftover material—a mere 0.14 percent of the Solar
The Solar System’s planets form two groups. System’s mass—formed a disc of gas and dust encircling the
There are four small, inner planets made of newborn star. Over millions of years, the grains of dust in this
rock and metal, and four giant, outer planets disc clumped together, growing into ever larger bodies until
made of gas and liquid. Between the two is they grew to the size of planets, pulled into spheres by their
a belt of rocky bodies called asteroids, and own gravity. In the inner Solar System, where the Sun’s heat
beyond the planets is a zone of icy bodies was too intense for gases to condense, planets formed from
including dwarf planets and comets. Even rock and metal. In the outer Solar System, gases condensed
farther out is a vast, spherical cloud of more to form much bigger planets.
comets—the Oort Cloud. The Solar System has
no certain outer boundary. Today the Solar System has eight planets, more than
100 moons, an unknown number of dwarf planets, and
countless millions of comets and asteroids.

Orbits A slow cannonball A very fast cannonball Dwarf planets
falls to the ground. escapes Earth’s gravity.
Every major body in the Solar System orbits Dwarf planets are round in shape
the Sun counterclockwise. The planets are on At just the right but smaller than true planets, and
near-circular orbits in the same plane as the speed, a cannonball their gravity is not strong enough
disc of gas and dust from which they formed. keeps falling but to sweep their region of space
Many smaller objects, such as dwarf planets never lands. clear of smaller debris. The most
Pluto and Eris, have stretched orbits tilted to famous dwarf planet is Pluto,
this plane. Comets arrive from all directions. How orbits work which was classified as a true
planet until 2006.
English scientist Isaac Newton was the first person to
Uranus Neptune realize why moons and planets travel in orbits: because Earth Ceres
Pluto they are trapped by gravity. To explain his theory, he Haumea
Saturn drew a giant cannon firing cannonballs off Earth. If a Makemake
Sun Eris cannonball moved fast enough, the curve of its path as
it fell back would be gentler than the curve of Earth’s Pluto
shape, and it would never land—it would stay in orbit.
Eris

SIZE COMPARED TO EARTH

Uranus Neptune

1,500 million miles 2,000 million miles 2,500 million miles
(2,414 million km) (3,219 million km) (4,023 million km)

24 space THE UNIVERSE Mercury’s iron core accounts for
two-thirds of the planet’s mass.

Inner planets This view of Venus’s Maat Mons is
surface was made using Venus’s largest
Mercury, Venus, Earth, and Mars are the Solar radar to see through the
System’s inner planets. On the face of it, they volcano
are worlds apart—but underneath the surface, planet’s thick clouds
it is a different story.

The inner planets all formed from the same material
about 4.6 billion years ago. All are a mix of rock and
metal, with interiors that are roughly divided
into layers. The heavier metals are concentrated
toward the center, while the lighter rock is on top.

Each of these planets was bombarded by
asteroids and comets early in the Solar System’s
history, and each has been affected by volcanic
activity too. Mercury’s heavily cratered face still
bears the scars of the early bombardment, but
the surfaces of the other three worlds have
changed over time.

Smooth plains
formed from lava
flows in distant past

Brightest craters
are the youngest

MERCURY VENUS

In the Sun’s glare Mercury profile Lava land Venus profile

Mercury is the smallest of the Diameter .... 3,032 miles (4,879 km) Venus is sometimes described as Diameter ..7,521 miles (12,104 km)
Solar System’s planets and lies Average surface Earth’s twin because it’s almost the Average surface
closest to the Sun. It is a lifeless temperature.................333°F (167°C) same size as our planet and has a temperature.................867°F (464°C)
world that has hardly changed One spin on axis .... 58.6 Earth days similar internal structure. But the One spin on axis ..... 243 Earth days
in 3 billion years. The planet’s One orbit of Sun......... 88 Earth days two worlds are very different. One orbit of Sun...224.7 Earth days
entire surface is pitted with Number of moons ..............................0 Number of moons ..............................0
craters formed when asteroids Any astronaut who tried to
crashed into it while Mercury Hot spots walk on Venus would be killed in Greenhouse effect
was young. The craters range The colors on this heat map seconds. The surface is as hot as Venus is hot because of a process called
from small, bowl-shaped ones of Mercury show the planet’s surface the inside of a pizza oven, and the the greenhouse effect. The Sun’s heat
to the huge Caloris Basin, which temperature. The red region, which is crushing air pressure is 90 times passes through the atmosphere and
is nearly one-third the width on the equator, faces the Sun and is greater than that on Earth. warms the ground, which then reemits
of the planet. hottest. Next warmest are yellow warmth. The reemitted warmth is
areas, then green. The planet’s polar Venus’s deadly surface is hidden trapped by the atmosphere, much as
Mercury orbits the Sun more regions (blue) are coolest. from our view by thick cloud glass traps heat in a greenhouse.
quickly than any other planet, cover, but orbiting spacecraft have
but it rotates slowly: for every used radar to see through the Sunlight warms Heat from ground
two orbits, it spins around just gloom, and landers have touched the ground is trapped by
three times. So a “day” on down to take photos. Venus is a atmosphere
Mercury (sunrise to sunrise) world of volcanoes, many thought
takes 176 Earth days. Such long to be active, and its surface is Atmosphere is
days and nights, coupled with littered with broken rock from about 50 miles
a very thin atmosphere, give solidified lava. It is permanently (80 km) deep
Mercury the greatest surface overcast, with a sickly yellowish
temperature range of all the light filtering through the cloud.
planets. In the daytime, the
surface is hot enough to melt Venus spins more slowly
lead, but at night it’s cold than any other planet. It also
enough to liquefy air. spins in the opposite direction
(clockwise) to every planet
apart from Uranus.

2,500 miles (4,000 km)—the length of Mars’s moons Phobos and 14 miles (22 km)—the height of the 25
Mars’s Valles Marineris canyon. Deimos were once asteroids. Olympus Mons volcano on Mars.

Liquid water Ice cap at Like Earth, Mars
covers most of north pole has a crust made
Earth’s surface of solid rock

Like all the inner
planets, Mars has
a core made of
red-hot iron

The reddish color
comes from iron oxide

(rust) in the soil

EARTH MARS

Living world Earth profile The red planet Mars profile

Third out from the Sun, Earth is Diameter ..7,926 miles (12,756 km) The second smallest planet in the Diameter .... 4,220 miles (6,792 km)
the largest of the inner planets. It’s Average surface Solar System, Mars is half the size Average surface
the only planet with liquid water temperature......................59°F (15°C) of Earth. It’s sometimes called the temperature..................–81°F (–63°C)
flowing freely on the surface, and One spin on axis .............. 23.9 hours red planet because of its rusty One spin on axis .............. 24.6 hours
it’s the only planet in the Universe One orbit of Sun...............365.3 days coloring. A vast canyon called One orbit of Sun...... 687 Earth days
known to sustain life. Number of moons ..............................1 Valles Marineris stretches a Number of moons ..............................2
quarter of the way around this
Earth’s surface consists of vast NORTH POLE frozen desert world. It formed long Rocky floodplain
oceans (71 percent), continents of ago when the crust of the young Mars hasn’t always been a desert. Dry
land, and two polar ice caps—all 23°.5 Axis planet split open. Elsewhere are river beds show that water flowed
supported by a thin, rocky crust. dusty plains strewn with boulders here long ago. Floods swept rocks
The crust is broken into seven Northern and giant, extinct volcanoes, across the land and dumped them on
huge segments and many smaller hemisphere including Olympus Mons—the floodplains like the one below. Mars
ones. Called tectonic plates, these Solar System’s largest volcano. may even have been warm and wet
giant slabs of rock creep slowly enough for life to flourish.
over Earth’s surface, pushed by
churning movements in the softer, Equator
hot rock that fills most of Earth’s
interior. As tectonic plates move, Southern
they bump into each other and hemisphere
grind past one another, generating
immense forces that thrust up SOUTH POLE
mountain ranges, unleash volcanic
eruptions, and trigger earthquakes. Tilted planet
These powerful forces continually Earth spins around once a day, but
change Earth’s appearance, as do it isn’t perfectly upright. Its axis—
the actions of wind and water— the imaginary line from pole to pole
and the planet’s 7 billion around which it spins—is tilted by
human inhabitants. 23.5°. So as Earth travels around the
Sun, one hemisphere and then the
other is tilted toward the Sun. This
is what causes the seasons.

26 space THE UNIVERSE 1,300 The number of times Earth could fit 1665 The year Jupiter’s Great Red
inside Jupiter’s volume. Spot was first seen.
Size of Earth
relative to Gas atmosphere
planets Jupiter’s swirling outer atmosphere
is 620 miles (1,000 km) thick and
consists mainly of hydrogen gas.

Liquid layer
The outer atmosphere merges
gradually into a deep layer of
liquid hydrogen and helium.

Blue color caused
by methane in
atmosphere

Great Red Spot JUPITER Solid core Liquid metal layer NEPTUNE
This giant storm is bigger Jupiter’s rocky core Deep inside Jupiter, intense pressure
than Earth and has been turns hydrogen into a liquid metal.
raging for over 300 years. is hotter than the
surface of the Sun.

King of the planets Jupiter profile Blue planet Neptune profile

Mighty Jupiter is the fifth planet Diameter ......................... 88,846 miles Neptune, the eighth and furthest Diameter ........................ 30,775 miles
from the Sun and the largest in (142,984 km)
the Solar System—so big, in fact, of the planets from the Sun, was (49,528 km)
that it’s 2.5 times more massive Average surface
than all the other planets put temperature.............–186°F (–121°C) discovered in 1846. Astronomers Average surface
together. Its strong gravitational One spin on axis ................. 9.9 hours
pull greatly affects the orbits of One orbit of Sun......11.9 Earth years had noticed Uranus wasn’t temperature.............–330°F (–201°C)
other bodies in the Solar System. Number of moons ........................... 67
following its expected path—there One spin on axis ...............16.1 hours
Jupiter’s fast rate of spin has The Jupiter system
stretched its surface clouds into Like a king surrounded by his courtiers, seemed to be an unseen body, One orbit of Sun 163.7 Earth years
bands, with spots (storms) and Jupiter is circled by a great number
ripples where neighboring of moons. The inner moons, including perhaps an undiscovered planet, Number of moons ........................... 13
bands swirl together. the four largest, are shown below.
Ganymede, the largest, is bigger than pulling on it. Two mathematicians—
Several craft have visited the planet Mercury. Most of Jupiter’s
Jupiter, including Galileo, which other moons are probably asteroids John Couch Adams in England and
orbited from 1995 to 2003. captured by the planet’s gravity.
Urbain Le Verrier in France—

calculated where in the sky the

undiscovered planet must be.

Within days, Neptune was spotted

from an observatory in Germany.

Neptune is slightly smaller

than Uranus and looks bluer

because its atmosphere contains

more methane. It has a deep, fluid

Metis Thebe mantle that is hot and dense and
Io contains water, ammonia, and
methane. Neptune also has a Fastest known winds
Adrastea Europa Ganymede barely visible system of rings. Its When Voyager 2 flew past Neptune in
Amalthea Callisto biggest moon, Triton, resembles 1989, it photographed white clouds
Pluto and was likely captured by blown into streaks by winds of up to
Neptune’s gravity in an encounter 1,300 mph (2,100 kph)—the fastest
billions of years ago. sustained winds in the Solar System.
This violent weather is thought to be
powered by heat from inside Neptune

since the planet is too far from the

Sun to absorb much of its warmth.

43,000 °F (24,000 °C)—the estimated 150 The number of moons and moonlets 27
temperature of Jupiter’s core. observed orbiting Saturn.

Outer atmosphere Cloud bands Outer planets
Although it looks calm, Saturn’s clouds form
fierce winds whip through bands around the planet, Four gigantic planets dominate the outer Solar System.
Saturn’s atmosphere at like those on Jupiter. Very different from the rocky, inner planets, these
1,800 kph (1,120 mph). strange worlds are huge globes of gas and liquid,
with no solid surface and hundreds of moons.

After the Sun first formed, its heat drove gases out of the inner
Solar System, leaving behind heavier compounds such as rock and
metal. The rock and metal formed the solid inner planets, while

the gases formed the outer planets. Astronomers
call the outer planets gas giants, though they
consist mostly of liquid and they have
solid cores. These four worlds
have much in common. All have
numerous moons, a deep, stormy
atmosphere, and a set of rings made
of flecks of rock or ice.

On a roll
Uranus leans so far to
one side that it appears
to roll along as it travels

around the Sun.

Gaps SATURN Main rings Rings around Uranus URANUS
Gaps in the rings are areas The rings consist of billions of sparkling Thirteen rings are known

swept clear of ice by the fragments of ice, varying in size from to circle the planet.
gravity of Saturn’s moons. microscopic to as big as a house.

Lord of the rings Saturn profile Topsy-turvy world Uranus profile

The second-largest planet and the Diameter ........................ 74,898 miles Uranus, the seventh planet from Diameter ................... 36,763 miles

sixth farthest from the Sun, Saturn (120,536 km) the Sun, was unknown to ancient (51,118 km)

shines like a bright yellow star. Even Average surface astronomers, even though it is Average surface

a small telescope will reveal its temperature........... –292°F (–180°C) just visible with the naked eye temperature........–315°F (–193°C)

most famous feature: a magnificent One spin on axis ...............10.7 hours in perfectly clear and dark skies. One spin on axis ..........17.2 hours

ring system. Despite Saturn’s size, One orbit of Sun.... 29.5 Earth years It was discovered by musician One orbit of Sun...84 Earth years

it is only half as dense as Jupiter. Number of moons ........................... 62 William Herschel from his back Number of moons ...................... 27

Its clouds form less obvious bands garden in Bath, England, in 1781. Outer atmosphere,
Uranus is similar to Neptune but the cloud layer
than Jupiter’s, but fierce storms Atmosphere
has a paler blue, almost featureless (hydrogen, helium,
blow up every 30 years or so, Ring system face. It is the coldest of all the and methane gases)
creating giant white spots. Saturn’s main rings are 220,000 planets and generates very little
miles (360,000 km) wide, yet they
Saturn’s largest moon, Titan, are only 30 ft (10 m) thick. A scale heat from within. It orbits on its Core of
has a dense atmosphere and a model of the rings made with a side—perhaps because it was silicate rock
rocky surface with seas of liquid sheet of paper would be 2 miles knocked over by a collision
methane. The Cassini spacecraft (3 km) wide. Beyond the main rings
has been orbiting Saturn since are hazy outer rings, photographed with another planet early in its Hot, liquid
2004. It released a probe, Huygens, by Cassini while the Sun was history. Its extreme tilt gives it mantle
that landed on Titan in 2005. behind Saturn (below).
very long seasons.

Uranus has a faint set of rings,

which were discovered in 1977. Ice giant
The planet’s moons are all named Uranus’s pale blue color is due to the
after characters in works by methane in its atmosphere. Water and
William Shakespeare or the ammonia have also been detected in
English poet Alexander Pope. the clouds. The planet contains less
hydrogen and helium than Jupiter and
Saturn but has more rock and water.

28 space THE UNIVERSE 18 days—the time it would take to fly to
the Moon at the speed of a jumbo jet.

Moon profile The Moon

Diameter ........................ 2,159 miles (3,474 km) The Moon is Earth’s closest neighbor in space
Average surface temperature .....–63°F (–53°C) and looms larger than any other object in the
Length of lunar day ...................... 27 Earth days night sky. Its cratered surface may be cold and
Time to orbit Earth ....................... 27 Earth days lifeless, but deep inside the Moon is a gigantic
Gravity (Earth = 1) ...........................................0.17 ball of white-hot iron.

How the Moon formed Earth and Moon have existed together in space
ever since the Moon formed as the result of a
Scientists think the Moon formed as a result cosmic collision. It orbits around our planet, keeping
of a collision between Earth and a planet the same face toward us at all times. As we gaze
4.5 billion years ago. The debris was pulled on its sunlit surface, we look at a landscape that has
together by gravity and became the Moon. barely changed since 3.5 billion years ago. Back then,
the young Moon was bombarded by asteroids.
For millions of years they blasted out surface SEA OF SHOWERS
material and formed craters. The largest of these (MARE IMBRIUM)
were then flooded with volcanic lava, creating
Impact Moon formation dark, flat plains that look like seas.
A planet smashes into A disc of debris forms.
Earth and blasts molten The particles slowly Lunar maria
rock into space. join to form a Moon. 1 Dark, flat areas known as

maria, or seas, are huge
plains of solidified lava.

Phases of the Moon Gray surface
The Moon’s surface is covered
As the Moon orbits the Earth, a changing
amount of its face is bathed in sunlight. in a layer of fine gray dust
The different shapes we see are the Moon’s an inch or so deep.
phases. One cycle of phases lasts 29.5 days.
Lunar layers
NEW MOON WAXING CRESCENT FIRST QUARTER Lunar craters
Craters exist all over the Moon. Like Earth, the Moon is made of different
WAXING GIBBOUS FULL MOON WANING GIBBOUS They range from small bowl-shaped layers that separated out long ago, when
hollows a few miles wide to the vast its whole interior was molten. Lightweight
LAST QUARTER WANING CRESCENT NEW MOON South Pole–Aitken Basin, which is minerals rose to the top, and heavier
1,600 miles (2,500 km) in diameter. metals sank to the center. The outermost
The far side Many craters, like Eratosthenes layer is a thin crust of rock like the rock
(above), have central hills that formed on Earth. Under this is the mantle—a deep
The Moon keeps the same face toward the as the ground rebounded after the layer of rock that gets hotter toward the
Earth all the time. The face we never see—the asteroid struck. center. The bottom part of the mantle is
far side—can only be viewed by spacecraft. partly molten. In the Moon’s center is an
Its crust is thicker and more heavily cratered Man on the Moon iron core heated to about 2,600°F (1,400°C)
than the near side. Elevation (height) maps Astronauts landed on the Moon six times by energy from radioactive elements.
reveal high and low areas of land. during NASA’s Apollo program. They found Scientists think the outer core is molten
a world of gray, dusty plains and rolling hills but the inner core is squeezed solid by
under an inky black sky. Below, an astronaut the pressure of the rock around it.
heads back to his rover vehicle parked near
Highlands Camelot Crater (left), where he had been
collecting samples. The large boulders were
flung out of the crater when it formed.

Height of surface

High 2

Low
South Pole–Aitken Basin

Every year, the Moon drifts 1.48 in 12 people have walked 29
(3.78 cm) further away from Earth. on the Moon.

Highlands 3
All the Moon’s hills and
mountains are the rims
of craters or central
peaks in craters.

2

SEA OF SERENITY
(MARE SERENITATIS)

3 SEA OF TRANQUILITY Hadley Rille
1 (MARE TRANQUILLITATIS) A deep gorge named Hadley Rille cuts
through flat plains at the edge of the
The mantle gets Moon’s Sea of Showers, winding for
hotter toward more than 60 miles (100 km). How it
the center. formed is a mystery, but it might be
an ancient lava channel. In July 1971,
Apollo astronauts drove their rover to
the edge of Hadley Rille to take
photographs and study it.

Inner core
A hot, 300-mile- (480-km-)
wide ball of solid iron
forms the inner core.

Outer core Inner mantle
The outer part of the Heat from radioactive
elements has partially
Moon’s iron core is melted the inner mantle.
molten or partially
Mantle
molten iron. The mantle is mainly
solid rock and is rich in
silicate minerals, which
are common on Earth.

Crust
Made of granitelike
rock, the Moon’s crust
is about 30 miles (48
km) thick on the near
side and 46 miles
(74 km) thick on
the far side.

Far side
The Moon’s far side,
which is not visible
from Earth, is covered
in craters and has no
large maria (seas).

Craters in craters
On many parts of the Moon
the craters overlap each
other, the newer ones lying
on top of older craters.

30

SPACE EXPLORING Robot explorers
THE PLANETS
EXPLORATION Robotic spacecraft can visit places too
The planets are too far for far or dangerous for human beings.
Stars and planets have fascinated people since manned missions, so robotic Launched into space by rocket, they
ancient times, but it wasn’t until the 20th century spacecraft are sent instead. The travel vast distances across space and
that exploring space became possible. In recent first to visit another planet was may take years to reach their target.
decades we have sent astronauts to the Moon, Mariner 2, a US craft that flew There are various types of spacecraft,
robotic spacecraft to the outer reaches of the Solar past Venus in 1962. Since then, each suited to a particular mission.
System, and used huge telescopes to peer across and despite a number of early
the vastness of the Universe. failures, hundreds of spacecraft FLYBY SPACECRAFT
have visited the Solar System’s Some spacecraft observe a target as they
planets, moons, asteroids, and fly past. NASA’s famous Voyager 1 and
comets. Most spacecraft either Voyager 2 flew past several planets.
fly past or orbit their target,
but some also release landers ORBITER
that touch down on the surface. An orbiter flies around a planet repeatedly,
giving it plenty of time to study its target.
Orbiters have visited the Moon and all the
planets except Uranus and Neptune.

OBSERVING THE SKIES Capturing light LAUNCH VEHICLES

For centuries, astronomers have observed the heavens Telescopes come in many Space is only 60 miles (100 km) above
with their eyes alone or used simple telescopes that different styles and designs, but the Earth’s surface and takes less than 10
magnify the view. But the visible light we see is just one basically all do the same thing: minutes to reach in a rocket. Although the
part of a much bigger spectrum of electromagnetic rays collect electromagnetic radiation journey is short, it takes tremendous power
that reaches Earth from space. Stars and other objects from space and focus it to create to escape the pull of Earth’s gravity. Launch
also emit invisible radio waves, X-rays, infrared, and an image. Earth’s atmosphere can vehicles are built to make the journey only
ultraviolet rays. Modern telescopes can see all of these, block or blur the image, so some one time, and most of their weight is fuel.
and each type of radiation reveals something different. telescopes are located on high
mountaintops or even launched World’s largest rockets
into space.
Saturn V, which sent astronauts
RADIO INFRARED VISIBLE ULTRAVIOLET X-RAY to the Moon, was the largest rocket
ever built. Its Soviet rival, the N1,
was launched four times but each
attempt ended in disaster.

Radio telescopes Infrared telescopes Optical telescopes Ultraviolet telescopes X-ray telescopes ARIANE 4 (EUROPE)—193 ft (59 m) tall
Huge, curved dishes These instruments, Using large lenses Astronomers These telescopes LONG MARCH 2F (CHINA)—203 ft (62 m) tall
are used to focus some of which are or mirrors, optical use ultraviolet capture high energy DELTA IV HEAVY (US)—236 ft (72 m) tall
radio waves given sent into space, detect telescopes gather telescopes to rays from extremely N1 (USSR)—344 ft (105 m) tall
out by sources such the heat from objects faint visible light and examine radiation hot objects. X-ray SATURN V (US)—364 ft (111 m) tall
as galaxies, pulsars, such as clouds of can see much further from the Sun, stars, telescopes only
and black holes. gas and dust. than the human eye. and galaxies. work in space. Launch sites

Mapping the stars Celestial Declination lines Many countries have spaceflight launch sites.
north pole These split the sky into Sites closer to the equator can launch heavier
Because Earth is surrounded This is the north–south segments. cargo, because rockets there are given a boost
by space, when we look point directly by the speed of Earth’s spin.
at the night sky it seems above Earth’s Right ascension lines
as though all the stars are North Pole. These divide the sky Cape Canaveral, Baikonur, Kazakhstan
pinned to the inside of a giant into east–west FL
sphere. Astronomers call this Celestial segments. Xichang,
the celestial sphere and use it south pole China
to map the positions of stars This is the Celestial equator
and planets. Vertical and point directly This imaginary line
horizontal lines are used to above Earth’s over Earth’s equator
divide the celestial sphere South Pole. divides the sky into
into a grid, just like the grid north and south
of longitude and latitude lines hemispheres.
used to map Earth’s surface.

MAJOR LAUNCH SITES

31

Magnetometer ATMOSPHERIC PROBE ROVER Solar System missions
This type of craft enters A rover is a robotic lander
Ultraviolet a planet’s atmosphere. with wheels that can drive In little more than 50 years, around 200 spacecraft have left
sensor The Galileo probe dove around. Rovers sent to Mars Earth’s orbit and headed off to explore the Solar System. More
into Jupiter’s stormy have studied its rocks for than half the missions have been to Earth’s nearest neighbors
atmosphere in 2005. signs of ancient life. in space: the Moon and the planets Mars and Venus.

Infrared sensor LANDER PENETRATOR 80
Some craft can touch down A penetrator is designed to 70
Communication on the surface of another hit its target at high speed 60 SUN
dish world. In 1976, Viking 1 and bury itself. In 2005, 50 MERCURY
became the first craft to Deep Impact penetrated 40
Asteroid detector successfully land on Mars. the surface of a comet. 30 VENUS
20 MOON
Generator 12 years and 43 days – Most visited 10
the time it took the This chart shows MARS
Pioneer spacecraft spacecraft Voyager 2 the number of 0 ASTEROIDS
Pioneers 10 and 11 made flybys of to reach Neptune from Earth. missions to the JUPITER
Jupiter and Saturn. They are now heading major bodies in SATURN
out of the Solar System into deep space. the Solar System. URANUS
NEPTUNE
PLUTO

SATELLITES LIVING IN SPACE Brain and balance
Without gravity, the inner
About 1,000 operational satellites orbit the Earth, Astronauts must adapt to a
carrying out tasks such as beaming TV signals around zero-gravity environment when ear’s balance system no
the world, gathering data for weather forecasters, and living in space. Although floating longer works, which can
spying for the military. Many thousands more pieces of weightlessly can be fun, it can make astronauts feel sick.
space junk—old satellites, discarded rocket parts, and also cause medical problems.
debris from collisions—also circle our planet. The growing Space stations are cramped places Heart
cloud of space debris is a hazard to spacecraft. with few luxuries. Astronauts eat Blood flows more
ready-made meals that are either easily, so the heart
Over 500,000 objects, including satellites and space junk, orbit Earth. freeze-dried or served in
pouches. All water is recycled, doesn’t need to
Satellite orbits including the water vapor work as hard.
from human breath. Astronauts
Some satellites are a few hundred miles above Earth’s surface, clean themselves with special Muscles
but others are much further. Some of the highest ones, such as shampoos and soaps that don’t Movement is easy when
weather, TV, and phone satellites, have geostationary orbits, need water, and they use space
which means they stay over a fixed point on Earth. Satellites toilets that suck away waste you’re weightless, and
with lower orbits change position all the time. rather than flushing with water. muscles waste away if
not used. Workouts in
Effects on the body the onboard gym help

When the human body spends a long to slow this process.
time in space, it changes. Without Bones
gravity pulling on the spine, the
body gets about 2 in (5 cm) taller. Bones weaken and
Body fluids that flow downward on become less dense.
Earth build up in the head. This gives Regular exercise is
astronauts swollen faces and blocked
noses, making food seem tasteless. essential to keep
When astronauts come back to Earth, them strong.
the return of full gravity can make
them feel extremely weak.

ORBITAL PERIOD Geostationary orbit Space stations
20 HOURS (communications
15 HOURS satellites) A space station is a crewed satellite—
a kind of orbiting laboratory in which
10 HOURS astronauts and scientists live and SKYLAB (US) SALYUT 1 (USSR)
5 HOURS work. The USSR launched the first
Medium-Earth orbit station, Salyut 1, in 1971. The US MIR (USSR)
(GPS satellites) soon followed with Skylab, in 1973.
Russia’s Mir, in use from 1986 to
2001, was the most successful station TIANGONG-1
until the US, Russia, and more than (CHINA)
10 other countries joined forces to
Low-Earth orbit build the International Space Station,
(International in orbit since 1998. China’s own
Space Station) space station prototype, Tiangong-1,
was launched in 2011.
INTERNATIONAL
SPACE STATION

32 space SPACE EXPLORATION 25 trillion miles (40 trillion km)—the distance
to the nearest star, Proxima Centauri.

3000 bce A brief history of astronomy Astronomy

Many ancient cultures followed the People have been looking up at the night sky and marveling at
Sun and stars in order to keep track of its beauty and mystery for thousands of years. Today, a whole
the time of year, and by Ancient Greek branch of science—astronomy—is devoted to studying stars.
times, astronomers had already worked
out that Earth is round. Today, powerful Professional astronomers investigate not only stars but everything to do
telescopes allow us to peer so far into with space—from the meteors that burn up spectacularly as shooting stars in
space that we can look back in time Earth’s atmosphere and the planets of the Solar System to distant galaxies
almost to the birth of the Universe. billions of light years away. Astronomy makes a rewarding hobby too, and
many amateur stargazers enjoy observing the night sky with backyard
Astronomical telescopes or binoculars. Whenever astronomers observe the sky, they are
calendars looking back in time. This is because light takes such a long time to reach us
Many monuments built from distant objects in space. We see the Moon as it was one and a quarter
by ancient peoples, such seconds ago and the stars as they were hundreds of years ago.
as Stonehenge in the UK,
align with the Sun. These
monuments may have
been used as calendars
so that farmers knew
when to sow crops.

150 ce Ptolemy The sky at night LATITUDE
The Greek astronomer 60°N
Ptolemy cataloged Ancient stargazers saw patterns 40°N
1,022 stars in 48 in the stars and named groups of 20°N
constellations. He stars after mythical beings and
believed that Earth was animals. These star patterns, Colored circles on the star Horizon 20ºN
the center of the Solar called constellations, look little chart show the night sky
System and Universe, like the objects they are meant you can see from the Horizon
orbited by the Sun, to represent, but we still use the locations highlighted
Moon, planets, and stars. old names. Today, astronomers in color above. ANTLIA SEXTANS
divide the whole sky into 88
Earth segments, each one named after HorizonHYDRA Alphard Regulus
the constellation within it. Star
1543 Copernicus charts like the one here show PYXIS
which constellations are visible VELA
Polish astronomer at a particular time and place.
This chart shows the stars you
Nicolaus Copernicus can see at midnight in January
from the northern hemisphere.
proposed that the Sun,

not Earth, is the center B
or GEMINI
of the Solar System. It PUPPIS
PhiolvleuxClusCtAerNCECRAMININSOR Pro M
was a shocking idea

since it meant Earth CARINA MACJAONRIS

must be flying through cOyNoOnCERO2S244eteAlAglnelniutOiMsalRaek4mIO2NAMldBienebtRlalaaikrgtaarenilx

space, spinning around.

1610 Galileo Galilei Canopus Adhara Sirius M36
Italian scientist Galileo PICTOR COLUMBA LEPUS Arneb Elnath
Galilei built a telescope H
and used it to study the SOUTH RETICULUM DORADO M38
night sky. He saw spots
on the Sun, mountains Betelgeuse ERIDANUS
on the Moon, and four CAELUM
moons orbiting the Rigel HOROLOGIUM HyadTAeUs RUPSlAelicaydoense
planet Jupiter.
1687 ARIES
Isaac Newton Hamal
English scientist Isaac FORNAX Mira Ecliptic
Newton worked out the PHOENIX CETUS PISCES
laws of gravity—the Orion the hunter
1990 force that makes objects One of the best-known and brightest Measuring brightness SDCeUnLePbTKOaRitos
fall to the ground. He constellations is Orion the hunter, The size of the white spots
discovered that gravity which is visible the world over. on the chart show how bright
keeps the Moon in orbit Orion includes the red giant star the stars are. Astronomers call this
around Earth and keeps Betelgeuse and the blue-white magnitude and measure it on a scale
the planets in orbit supergiant Rigel—two of the that runs backward—the smaller the
around the Sun. brightest stars in the night sky. number, the brighter the star.

Modern astronomy Star magnitudes -1 0 1 2 34 5
Today, space telescopes
such as Hubble, which
was launched in 1990,
give us breathtaking
views of distant
objects in space,
including the furthest
galaxies ever seen.

15 million trillion miles (24 million trillion km)—the distance to the 2,000 The number of stars 33
Andromeda galaxy, the furthest object visible to the naked eye. visible to the naked eye.

How telescopes work Lens Eyepiece

The invention of the telescope Eyepiece lens Light from star Focal point Light from star
revolutionized astronomy. A magnifies Light rays Smaller mirror
telescope collects more light image converge
from an object than a human
eye can. It uses this light to Refracting telescope Main mirror Reflecting telescope
form a magnified image. A convex lens bends light A concave (inward-curving)
There are two basic types entering the telescope main mirror reflects light on
of telescopes: refracting and to focus it, forming an to a smaller, flat mirror. The
reflecting. The refracting image. At the other end resulting image is magnified
telescope has a large convex of the telescope, a smaller by an eyepiece lens.
(outward-curving) lens that lens called the eyepiece
gathers and focuses the light. magnifies the image.
The reflecting telescope uses
a curved mirror instead.

Focal point

EAST The dotted white line running The Great Bear Seeing the invisible
down the chart is called the Ursa Major is named after a bear.
The seven bright stars running Professional astronomers don’t just use visible
ecliptic and shows the path of from its tail form a famous group light to see the night sky. Their telescopes can
the Sun. The planets always of stars known as the Big Dipper or also create images from wavelengths of light
stay close to the ecliptic. Plow. The last two of these point to that our eyes cannot see, such as X-rays, radio
the North Star, which is always due waves, and infrared rays. The images below
MLIENOOR BCEOREMNAICES north. In ancient times, sailors used all show Kepler’s Supernova—the wreckage
Algieba this star to find the way. left by a giant star that exploded in 1604.
LEO
VECNAANTIESCI aroli BOÖTES Pointers North X-ray image
Denebola BOREALIS star This image of Kepler’s
Alcor MizarCor C CORONA Supernova is from the
MUARJSOARMerak Be orbiting Chandra X-ray
Dubhe Plough Observatory. It shows
AURIGA URSA DRACO a cloud of incredibly
apella MINOR hot gas that emits
NX S fak LY Horizon 40ºN high-energy X-rays.
Cas HorizKoonTc2hh0uaºbbNan
LYREApVsielgoan Lyrae HERCULES Visible light image
Haedi C ASSIOCPAEIMAELOPARDALIS Horizon NORTH Very little of the object
8 Epsilon Polaris can be seen in visible
AlderaDelmtian Cephei CEPHEUS light, even in this image
Aurigae from the Hubble Space
Telescope. The bright
Algol Mir C areas are clumps of gas.
PERASlEmUach
Caph CYGNUS Infrared image
Albireo Taken by the Spitzer
TRIAMNGirUaLchUM Deneb Space Telescope, this
LACERTA infrared image shows
AANlDpRheOrMatEzDA Cygnus Rift dust clouds that were
heated by a shock wave
PEGASUS from the exploding star.

Markab Combined image
Combining all three sources produces a complete
WEST On a clear, dark night you Cassiopeia image: a shell of supernova debris expanding
can see the Milky Way— This constellation is named after a into space at 1,240 miles (2,000 km) per second.
the galaxy to which our vain queen in Greek mythology. It’s
Solar System belongs. very easy to spot in northern skies
as it looks like a funky letter W.

GALAXY GLOBULAR CLUSTER OPEN CLUSTER Star clusters are
clouds of stars.

34 space SPACE EXPLORATION 1 million people traveled to Florida to watch The Saturn V rocket was taller than
the launch of Apollo 11 on July 16 1969. the Statue of Liberty in New York City.

Escape rocket Fuel tanks
(for emergencies Tanks within the Service
Module supplied fuel to
during launch) the main engine.

Command Module Astronauts
Astronauts stayed in The crew of three stayed in

here during launch. the Command Module for
most of their journey to
Service Module and from the Moon.
This module powered
the Apollo spacecraft. Engine nozzle
Nozzle for the
Lunar Module main engine, which
The Lunar Module propelled the Apollo
was housed in an craft through space.

aluminum cone. Service Module
This module provided life-
Instrument unit support systems and power for

Third stage the crew, and housed the
This stage reached Apollo craft’s main engine.
low-Earth orbit and
then put Apollo on Mission Thrusters
course for the Moon. to the Moon Small thrusters made fine
adjustments to the Apollo
Single third-stage Humans have set foot on only one world spacecraft’s movements.
engine beyond Earth: the Moon. Just 27 daredevil
astronauts have traveled there, of whom Command Module
Interstage adapter 12 walked on its cratered, lifeless surface. The Command Module was the only part
Covering the third- of the Apollo craft to return to Earth. Its
Eight space missions visited the Moon between
stage engine, this 1968 and 1972 as part of NASA’s Apollo program. conical shape helped it withstand the
section linked the Each mission carried three American astronauts heat of reentry into Earth’s atmosphere.
inside an Apollo spacecraft, which was launched
rocket’s second by a Saturn V rocket. Apollo 8 tested the craft as Apollo spacecraft
and third stages. it orbited the Moon. Then, in a dress rehearsal prior
to landing, Apollo 10 flew close to the lunar surface. The Apollo spacecraft had three parts: the
Second stage The first of the six missions that successfully landed Command, Service, and Lunar Modules. These
The second stage on the Moon was Apollo 11 in 1969. Astronauts Neil were all linked together for the 250,000-mile
Armstrong and Buzz Aldrin touched down on the (400,000-km) trip to the Moon. Once there,
held a tank of surface in July of that year. As Armstrong took the the Lunar Module took two astronauts down
liquid hydrogen first historic step, he said, “That’s one small step for to the Moon’s surface, while the third crew
fuel and a tank man, one giant leap for mankind.” member remained in lunar orbit in the
of liquid oxygen. combined Command and Service Module (CSM).
Saturn V rocket The top half of the Lunar Module, known as the
Second-stage ascent stage, later returned the two astronauts
engines The Apollo astronauts were blasted into space to the CSM for the journey back to Earth.
inside the nose cone of the largest rocket ever
Interstage adapter built: Saturn V. Standing nearly 364 ft (111 m) tall, 21hours—the length of time
This section linked the Saturn V was as tall as a 30-story building. Apollo 11 astronauts Neil
the rocket’s first two This giant launch vehicle consisted of three Armstrong and Buzz Aldrin
rockets in one. The first two parts, or stages, lifted spent on the Moon. Apollo 17 astronauts
stages and also the Apollo craft into space, and the third stage set spent three days on the lunar surface.
covered the second- the spacecraft on course for the Moon.

stage engines.

First stage fuel
The first stage had a
tank of kerosene fuel
and a tank of liquid

oxygen to burn it.
The five engines

burned 16 tons of
fuel per second

during the launch.

Five first-stage
engines

300 The total number of hours that Beef, potatoes, and grape juice—the first meal 842  35lb (382 kg) of lunar rock and soil were
astronauts have spent on the Moon. eaten by the Apollo 11 astronauts in space. brought back by the Apollo astronauts.

There and back 11 10 The trip from the Earth Landing sites
to the Moon took The Apollo landing sites
Each of the six Apollo missions 1 about three days. were on the side of the
that landed men on the Moon 12 Moon that faces Earth.
took the same route, taking off 5
from Florida, and ending with 2
the astronauts splashing down 9
in the Pacific Ocean. 3 7
4 after which it is discarded.
1 Saturn V rocket carrying 9 CSM adjusts its course and 8
Apollo craft blasts off and Lunar Module. Third rocket heads back to Earth. 6
positions craft in Earth’s orbit. stage is now discarded. 10 Service Module is jettisoned.
2 The rocket’s third stage and 5 Apollo craft adjusts its 11 Command Module enters
Apollo craft leave Earth’s orbit course to go into lunar orbit. Earth’s atmosphere.
and head toward the Moon. 6 Lunar Module transports two 12 Command Module makes a
3 Combined Command and astronauts to lunar surface. parachute landing in the sea.
Service Module (CSM) separates 7 Third crew member continues
from the rocket. to orbit the Moon in CSM.
4 CSM turns and docks with 8 Ascent stage of Lunar Module
takes astronauts back to CSM,

Docking tunnel Ascent stage Legs and pads
Astronauts used this tunnel to The ascent stage of the Lunar Flexible legs with wide pads
move between the Command Module was the astronauts’ home on the bottom cushioned the
and Lunar Modules. while they explored the Moon. Module’s landing and kept it
stable on the surface.

Descent stage
This bottom half of the Lunar
Module acted as the launch
platform when the top half blasted
off back into space. The descent
stage stayed on the Moon.

Hatch
The astronauts
climbed through a
hatch to go outside.

Gas tanks Descent engine
The larger tank This engine was used to slow
contained helium; down the Lunar Module’s
oxygen was held in the descent during landing.
adjacent smaller tank.
Fuel tank Man on the Moon
Leg ladder This tank contained fuel The Lunar Module was the only part
Astronauts used the for the Lunar Module’s of the Apollo craft to reach the Moon’s
ladder to climb down descent engine. surface. Preprogrammed controls
to the lunar surface. maneuvered it into position above the
landing site, then an astronaut steered
the craft to touchdown. Scientific
equipment, a TV camera, and tools and
storage boxes for rock collecting were
all stored in the bottom half.

Sensing probe
Probes on the legs touched
the ground first during
landing and sent signals
to shut down the engine.

36 space SPACE EXPLORATION 4 spacecraft have visited
the planet Saturn.

Path to the planets Cassini-Huygens spacecraft

The paths of spacecraft are often carefully planned to Cassini-Huygens is the largest spacecraft
take them close to one or more planets on the way to to visit another planet. It was launched in
their final destination. Using the pull of gravity of each 1997 and arrived at Saturn in 2004. It had
planet boosts their speed and saves fuel. Cassini-Huygens two parts: the Cassini orbiter, designed to
flew past Venus, Earth, and Jupiter on its way to Saturn. orbit Saturn until 2017, and a probe called
Huygens, which touched down on Saturn’s
2. First Venus fly-by 6. Arrival at large moon Titan. The main aim of the mission Boom
Saturn was to discover more about Titan—the only A 36-foot (11-
world in the Solar System other than Earth meter) boom carries
that has a dense, nitrogen-rich atmosphere. instruments to measure
Saturn’s magnetic field.
3. Second Venus fly-by The long boom reduces
interference from other
Jupiter’s orbit instruments on Cassini.

CASSINI 5. Jupiter fly-by Large radio dish
1. Launch The large radio dish
4. Earth fly-by communicates with Earth.
It is also used to map
Landmark missions Titan, which it does by
bouncing radio waves
Since the first spacecraft to visit a planet was launched in off the surface.
1962, about 200 craft have explored the Solar System.
Some of the most famous missions are shown here.

Descent capsule Solar panel

Venera 7 Lunokhod 1
The first craft to touch down Russian-built Lunokhod 1 was
on another planet, Venera 7 the first lunar rover. It landed
landed on Venus in 1970. It on the Moon in 1970 and spent
lasted 23 minutes before the 322 days exploring, traveling
searing heat destroyed it. a total of 6.5 miles (10.5 km).

Radio dish

Spectrometer

Voyager 1 Sojourner rover
Launched in 1977 and still The first rover to explore
operational, Voyager 1 is another planet was Sojourner.
the furthest manmade object It reached Mars in 1997 and
from Earth. It visited Jupiter spent 12 weeks studying the
in 1979 and Saturn in 1980. soil and taking photos.

Rovers Small radio antenna
This is one of two small
A rover is a robotic radio dishes that serve as
vehicle built to explore backups in case the main
the surface of a planet or dish breaks down.
moon. Four rovers have
landed successfully on CASSINI ORBITER (GREY)
Mars. They receive radio
commands from Earth HUYGENS
but find their way LANDER
around and carry out
tasks independently. (RED)

Curiosity lands
The Curiosity rover was
lowered on to Mars in 2012
by a rocket-powered craft.

98,346  mph (158,273 kph)—the top 53  spacecraft have attempted 27  missions to Mars 37
recorded speed of Cassini-Huygens. to reach the planet Mars. have ended in failure.

Radio antennas Thruster Exploring
The three long antennas of Cassini has four the planets
the radio and plasma wave thrusters. These small
science (RPWS) instrument engines adjust the craft’s
detect radio waves generated flight path precisely.
by Saturn’s outer atmosphere.
While manned spacecraft have ventured
Plutonium power no further than the Earth’s Moon, robotic
supply (one of three) craft have visited all the planets in the Solar
System—and more than 100 moons.
Main engine
Robotic spacecraft can visit places that would prove
lethal to astronauts, such as the scalding surface of
Venus or the deadly radiation belts around Jupiter.
Packed with scientific instruments, telescopes, and
cameras, they carry out dozens of experiments
during their missions and capture thousands of
images, which are sent back to Earth by radio.

Helium tank Triple parachute
Helium gas from this tank Packed under Huygens’s back cover were
pushes fuel from the fuel three parachutes that opened in turn to slow
tanks to the engine. the lander’s descent onto Titan. Huygens
discovered a world of freezing, orange-brown
Fuel tanks plains littered with pebbles of ice.
The fuel tanks carry two
different liquids that burst
into flame when mixed.

Cosmic dust analyzer
This device measures the
size and speed of cosmic
dust particles in space.

Front case of
Huygens probe

Fuel for
thrusters

Back cover of
Huygens probe

Inner body case of
Huygens probe

Experiment platform

Huygens carried a range

of scientific instruments

to study conditions on

Plasma spectrometer Saturn’s moon Titan.

This device measures

charged particles

trapped by Saturn’s Heat shield
powerful magnetic field. Without a heat shield, the Huygens

probe would have burned up like a

shooting star when it entered the

atmosphere of Saturn’s moon Titan.

The shield was made of silica fiber

tiles able to withstand temperatures

up to 2,700°F (1,500°C).



EARTH

Oceans of water, an oxygen-rich atmosphere, and the
existence of life make Earth a unique planet. Its surface
is continually changing as plates slowly shift and the
relentless force of erosion reshapes the land.

40

PLANET THE CHANGING EARTH

EARTH Look at a map and it will show you the position of the continents, but
in fact our world is always changing. Earth’s surface is split up into
Earth formed about 4.5 billion years ago, but it was a large slabs called tectonic plates. The plates steadily shift around,
very different place then. Its surface was a hot inferno carrying continents and oceans with them. When they collide, new
of mostly molten rock, with little or no liquid water mountain ranges are pushed up. Afterward, over millions of years,
and no oxygen in the atmosphere. Since then Earth wind, water, and ice gradually wear the mountains down.
has developed oceans, continents, an oxygen-rich
atmosphere—and life. 250 MILLION YEARS AGO,

EARTH’S CONTINENTS ALL

JOINED TOGETHER, FORMING

AN ENORMOUS SUPERCONTINENT

KNOWN AS PANGAEA.

UNIQUE PLANET Inside our planet CRUST OUTER CORE
Different types of crust The only liquid layer,
Earth is the only place in the Earth’s interior has layers. make up Earth’s surface the outer core is mainly
Universe known to support life. Scientists discovered this and its ocean floor. The iron but also contains
It is thought that life developed by studying the paths by crust under the surface some nickel and small
after water began to collect on which earthquake waves is thicker and contains amounts of other
the Earth’s surface. Eventually, pass through the planet. more rock types. substances.
tiny life forms evolved that
could survive on water, sunlight, Thickness MANTLE INNER CORE
and chemicals in the water. This rocky layer is This is solid, and is
These microbes added oxygen 3.7–56 miles (6–90 km) denser than the crust. mostly iron with some
to the atmosphere—an essential It is mostly solid, nickel. Its temperature
step for the development of 1,790 miles (2,880 km) although it can very is very hot—about
plants and animals. slowly deform and flow. 9,900°F (5,400°C).
1,400 miles (2,255 km)

755 miles (1,215 km)

Earth’s atmosphere What’s in a layer? Key CONTINENTAL CRUST
OCEANIC CRUST
The atmosphere of Earth Earth’s crust and mantle are mostly Silicon dioxide MANTLE
is made up of several made of minerals called silicates, Aluminum oxide CORE
different gases. which are a combination of silicon Iron and iron oxides
dioxide and metal oxides. The mantle Calcium oxide
0.9% ARGON 0.1% OTHER is rich in magnesium-containing Magnesium oxide
silicates, while the two different Nickel
21% types of crust have less magnesium Other
OXYGEN and more aluminum and calcium. The
core is dominated by metallic iron.
78% No part of it has ever been brought
NITROGEN to the surface, but its composition
has been worked out by scientific
methods such as studying
earthquake waves.

NITROGEN – 78% The oceans 25%
A gas that can be fixed in the soil as LAND
well as loose in the atmosphere. Plants Earth’s surface and atmosphere
need nitrogen from the soil to survive. contain the equivalent of 333 million 75%
miles³ (1.39 billion km³) of water. WATER
OXYGEN – 21% There are regions of deep ocean
Essential for animals to breathe, as well as shallow seas that cover Water world
oxygen was absent until microbes areas around the edges of the Almost three-quarters
evolved that could use sunlight to continents—these are called of Earth’s surface is
turn carbon dioxide and water into continental shelves. Earth’s surface water. Over 97 percent
carbohydrates, releasing oxygen. has not always been as dominated of Earth’s water is found
by liquid water. In the past, during in the oceans.
ARGON – 0.9% ice ages when the polar ice caps
An inert gas (one that doesn’t were much thicker and more
react with other substances). extensive, so much water became
locked up in them that sea level was
OTHER – 0.1% at least 400 ft (120 m) lower than it
These include carbon dioxide is today, exposing the continental
(C0₂), which was once abundant, but shelves as dry land.
is now mostly incorporated into
materials such as limestone rock.

41

Continental drift into each other, creating new land masses Plate movement
and moving the oceans in a process called
Over millions of years, tectonic plates have “continental drift.” The continents get rearranged because
moved, shifting around the continents on Earth’s they are carried along as parts of
surface. Chunks of continents split away and push moving plates. This process has been
going on for billions of years, and is
North America, India moves Australia is joined South America Australia moves into thought to be caused by slow, heat-
Europe, and parts of north to Antarctica separates from Africa the Pacific Ocean driven movements in Earth’s mantle.
Asia are one landmass

012

200 MILLION YEARS AGO 130 MILLION YEARS AGO 70 MILLION YEARS AGO TODAY Yearly shift
The supercontinent India has escaped from the South America has split Australia has separated Plates typically move at a rate of about
Pangaea has just begun southern landmass, and is from Africa, while in the from Antarctica, and India 1 in (2.5 cm) in a year. That’s about as fast
to break into two main slowing moving north, north, North America is has collided with Asia, as your fingernails grow. Some move
landmasses. toward Asia. splitting from Europe. forming the Himalayas. faster—up to 4 in (10 cm) a year.

LOOKING AT EARTH A spinning planet Magnetic Earth Geographic Magnetic
North Pole north pole
Our planet is far from smooth— Earth’s gravity would pull it into Because Earth’s outer core is liquid,
its continents and ocean floors the shape of a sphere, but its the planet’s rotation stirs it into Geographic
are scarred and pitted with rotation makes it bulge slightly. motion. This motion causes electric South Pole
marks caused by movement This means its diameter at the currents to develop in the liquid
of plates. Earth’s place in space equator is 25 miles (41 km) more iron itself. Any pattern of electric
also affects its shape, as than the distance between its poles. currents creates a magnetic field,
constantly spinning makes it and in Earth’s case, the field is
bulge out around the middle Bulge at similar to what would be produced
so it is not a perfect sphere. middle by a large bar magnet inside the
Spinning also creates a magnetic planet. The field protects Earth
safety field around the planet. Direction from damage by harmful, energetic
of rotation particles that come from the Sun.
MOUNTAINS
Not quite round The magnetic field Magnetic Magnetic
MAKE UP ABOUT At the moment, scientists think The magnetic poles do not coincide south pole force
that Earth’s equatorial bulge is exactly with the geographic
ONE-FIFTH growing at a rate of 0.3 in (7 mm) (rotational) poles, and they gradually
every 10 years. change position over time.
OF THE EARTH’S

LANDSCAPE.

Earth’s surface

The solid surface of the Earth ranges
from about 35,750 ft (10,900 m) below
sea level in the Challenger Deep (part of
the Pacific’s Mariana trench) to 29,029 ft
(8,848 m) above sea level at the summit
of Everest, which may be rising at about
0.16 in (4 mm) per year. The surface of
most land areas is less than 1,650 ft
(500 m) above sea level.

Elevation Mountains and trenches
Over 13,125 ft (4,000 m) Earth’s solid surface is far from flat. This map shows its elevations and
6,500–13,125 ft (2,000–4,000 m) depths—from the highest mountain peaks to the deepest ocean trenches.
3,300–6,500 ft (1,000–2,000 m)
1,600–3,300 ft (500–1,000 m)
800–1,600 ft (250–500 m)
300–800 ft (100–250 m)
0-300 ft (0–100 m)

Sea depth
0–800 ft (0–250 m)
800–6,500 ft (250–2,000 m)
6,500–13,000 ft (2,000–4,000 m)
Below 13,000 ft (4,000 m)

42 earth PLANET EARTH 4.5 billion years—the
approximate age of Earth.

Inside the Earth Continental crust
Thicker and less
We can’t explore much of the Earth—our deepest mines dense than the
only travel about a mile into the crust. However, there oceanic crust.
are scientific ways to find out what it is like inside.

Geologists are able to study rocks from all depths within the
Earth’s crust, because collisions between continents push up
rock that used to be below the surface, forming mountains.
In some areas, collisions have even unearthed vast swathes
of the mantle. Volcanoes also sometimes erupt lumps of
rock from the mantle. Under the mantle is the core, which
has never been seen at the surface. However, scientists
have used the waves from earthquakes to work out
that the core is split into two layers—a liquid outer
core and a solid inner core.

Volcanoes in Hawaii
The Hawaiian islands in the mid-Pacific have
been built by volcanic eruptions. The rock that
formed them was pushed to the Earth’s surface
by hot rock moving upward in the mantle.

Rifting
Two tectonic plates pull away

from each other, and new
land is created between them.

Oceanic crust
Thinner and

denser than the
continental crust.

9,900°F

(5,400°C) – the approximate
temperature of Earth’s inner core.

Layered Earth Subduction
When two plates
Earth is made up of many rocky layers. The meet, one can be
top layer is the crust. Below that, uniform and pushed underneath
slightly denser rock forms the mantle. The crust
and the top of the mantle form a single rigid the other.
layer together, which is called the lithosphere.
This is broken into sections called tectonic plates. Pacific Ocean
Below the lithosphere is the asthenosphere. Only
tiny parts of the asthenosphere are liquid, but it
is soft enough to move, pushing around the
plates above. Under the mantle lies the core.
The outer core is a liquid mix of iron and sulfur,
while the inner core is solid iron and nickel.

There are four main layers inside the Earth. From the outside 43
in, they are the crust, mantle, outer core, and inner core.

Lithosphere The atmosphere
The rigid outer shell,
made of the crust and the Earth’s atmosphere is made up of gases, which are
top layer of the mantle. held in place by gravity. There is no clear boundary to
the outer edge of the atmosphere—it just fades into
Mantle space. Outer space is generally thought to begin about
A solid layer that 62 miles (100 km) above the surface.
is Earth’s thickest.
Exosphere 80 MILES
Outer core This is the outer zone. 130 KM
Molten iron and Gas molecules can escape
sulfur. Currents in into space from here.
this liquid generate
Earth’s magnetic field. Satellite

Inner core Thermosphere
A ball of iron and nickel, which In this zone, temperature
is very hot but solid because
of the immense pressure. increases with height.

Mantle plume
An upwelling within
the mantle sends
magma to volcanoes.

Hot spot
A volcanic site above
a mantle plume.

Aurora

Mesosphere
A zone where

temperature
decreases with

height.

Stratosphere 50 MILES
Absorption of 80 KM
ultraviolet sunlight Meteors
adds energy to the
stratosphere, so 30 MILES
50 KM
temperature
increases with 10 MILES
16 KM
height here.
Airplane
Weather
balloon

Troposphere
All weather
occurs in
this layer.

South American Clouds
coast

Crust
Earth’s outer layer, which is
distinct from the mantle,
though both are made of rock.

44 earth PLANET EARTH

Tundra Coniferous forest
Lichen, herbs, and Coniferous trees are cold
resistant, and have narrow,
grasses grow on needle-shaped leaves.
the thin soils in
Mediterranean
these regions, Tough evergreen trees and
which are too cold
for trees to live in. shrubs grow where the
summers are hot and dry and
TROPIC OF CANCER
winters are warm and wet.
What makes a climate?
Desert
A climate is an average weather pattern that occurs in Little or no vegetation
a set area over many years. The climate experienced at grows in this dry region
a certain location is influenced by its distance from the
equator, elevation above sea level, the amount of sunlight with low rainfall.
it gets, and how nearby circulation patterns in the ocean
and atmosphere affect it. Tropical rainforest
Warm, wet climates
Temperature Rainfall support more plant and
Places near the Earth’s equator Atmospheric circulation creates animal species than any
generally have higher average zones of high and low rainfall other land environment.
temperatures and more around the Earth—some areas
diverse ecosystems. get more rain than others. The Amazon
rainforest
The seasons
Earth’s biomes
There are four seasons in a year: spring, summer, fall, and
winter. Each has its own climate conditions and hours of Communities of plants and animals
daylight. The seasons differ in the northern and southern are different as a result of the
hemispheres and are more distinct further away from the climate they develop in. There are
equator, because of the tilt of the Earth’s axis of rotation. five main groups of communities,
or biomes—water, deserts, forests,
Northern Spring Winter grasslands, and tundra. Most of
hemisphere tilts Tilted axis these biomes can be broken down
toward the Sun further: for example, there are
in summer coniferous forests, deciduous
forests, and tropical rainforests.
Sun The size and location of Earth’s
biomes has changed very slowly
Antarctica over geological time. The most
recent changes have mainly been
caused by human activity.

1.5million species of
plants and animals
live in tropical
rainforests.

Summer Fall

0.7 in (2 cm)—average annual rainfall in the Sahara Desert, 57°F (14°C)—the average 45
compared with 70 in (180 cm) a year in a tropical rainforest. temperature on Earth’s surface.

Key

Ice cap

Tundra

Coniferous forest

Deciduous forest

Deciduous forest Tropical rainforest
With year-round rain, Temperate grassland
broadleaf trees shed Tropical grassland
their leaves during cool Mediterranean
winters and regrow them
in warmer summers.

Desert

The Himalayas

EQUATOR Mountains

Tropical grassland
Grasses dominate where
summer temperatures
are high and winter
temperatures are low.

TROPIC OF CAPRICORN

Earth’s climate Ice cap
An area of ice
Earth is the only planet in the Solar System that supports and snow that can
life. It can do this because it has a unique atmosphere, water, cover mountains.
and weather that living organisms can survive in.

Almost every part of our planet is occupied by some form of life—from the
deepest oceans to the highest mountains. However, the different climates
across the world affect how many and what type of species live in each
place. Tropical regions are teeming with life while only extreme survivors
can live in polar wastes and deserts.

46

SHIFTING PLATES TECTONIC
EARTH
Earth’s surface appears to be still, but it is actually a collection of
plates that is always moving. These plates move around due to
currents deep inside the Earth. Plates that are under oceans are
much thinner and less dense than those under continents—where
they push into each other the oceanic plate gets forced down
underneath the continental plate.

Jigsaw planet The outside layer of the Earth is broken up into giant
pieces called tectonic plates. Over millions of years
Tectonic plates fit together to make up Earth’s surface. They move constantly, these plates move, bump together, overlap, and
and can change our planet’s features, depending on how they meet. Where slide past each other, in the process making new
they push together, mountains and volcanoes form. Where they pull apart, areas of ocean floor, building mountains, causing
new ocean floor is created. earthquakes, and creating volcanoes.

NORTH ASIA EARTHQUAKES
AMERICA
Most earthquakes happen where tectonic plates rub against each
AFRICA AUSTRALIA other, in places called faults. Some faults move with a steady, very
gradual creep. In other places, a length of fault can remain locked
SOUTH for years, decades, or centuries, before giving way in a few seconds.
AMERICA Earthquakes are caused by the shaking of the ground after a rupture
like this. The shallower the depth where the rupture begins, the
more severe the shaking is at the surface.

ANTARCTICA Measuring quakes

Tectonic plate boundaries Key Transform The magnitude of an earthquake is a measure of the energy it releases. It
Plate boundaries are classified depending on Convergent Uncertain can be measured using the Richter scale, where a difference of one point
whether plates are moving together (converging), Divergent corresponds to a 30-fold difference in energy. The intensity of an earthquake
apart (diverging), or past each other (transform). can be measured using the Mercalli Intensity scale (below), which grades
earthquakes from I to XII, according to their effects.

THE LARGEST TECTONIC PLATE I-II Hardly felt by VII-VIII General alarm,
III-IV people, but can IX-XI cracks appear in
IS THE PACIFIC PLATE. IT IS V-VI be measured by XII buildings, tree
instruments. branches break.
THE ONLY LARGE PLATE THAT
DOESN’T CARRY A CONTINENT. Felt indoors as Most buildings
a quick vibration. destroyed,
How plates move Hanging objects underground pipes
swing slightly. torn apart.
No one knows exactly why tectonic plates move, but scientists think
it is likely that they shift around on top of currents in the mantle layer Rocking motion Almost all
underneath. These currents are thought to move in steady loops—rising felt by most buildings
when an area is heated by processes at the center of Earth, then sinking people, buildings destroyed, rivers
down again when they are cooled by moving nearer to the surface. tremble. change course.

Plate is pushed Tectonic Upper Currents in Living with 10
into the mantle plate mantle the mantle earthquakes
9
Earthquakes can occur
anywhere in the world, but 8
the most damaging ones
usually happen near plate 7
boundaries. Earthquakes can
be very dangerous—buildings 6
can fall down and huge cracks
open up in the ground. In 0
earthquake-prone areas, Biggest quakes
buildings can be designed There have been many powerful earthquakes
to sway when there is an in the last 100 years. The largest, in Chile, was
earthquake, so that they strong enough to move rivers.
Plate are not shaken apart. Their MAGNITUDE (RICHTER SCALE)
movement foundations must be built in Chile
solid rock rather than on
sandy or wet ground. May 22, 1960
Prince William Sound,
Alaska March 28, 1964

Northern Sumatra
December 26, 2004

Honshu, Japan
March 11, 2011

Kamchatka
November 4, 1952

Core

Moving mantle
Though mostly solid, the mantle moves. As currents rise,
cool, and sink, they drag the plates around on top of them.

47

VOLCANOES Eruption types Huge ash plume Rain of ash Volcanic bomb

A volcano occurs where molten Volcanoes can erupt with Loud explosions Magma Medium-height
rock called magma erupts up from massive force, a small explosion, from vent ash plume
under Earth’s surface. An exploding or even just a steady dribble.
volcano is one of the most The way each volcano erupts Plinian Vulcanian
incredible and dangerous sights on depends on how thick its magma The most explosive and violent A violent eruption that begins
our planet. Volcanoes often form is and how much gas is in the type of eruption. A steady, with a cannonlike explosion.
near the boundaries between magma. In a gas-rich magma, powerful stream of gas and Lumps of rock called bombs
tectonic plates, but they can also violent expansion of bubbles can magma is blasted into the air. are thrown out.
form elsewhere, at hotspots shatter the magma and project
where hot rock moves upward volcanic ash into the sky with
from deep inside the Earth. There huge force, creating an ash
are about 550 active volcanoes cloud. More gentle eruptions
on land, and more under the sea. feed lava flows that ooze slowly
down the side of the volcano.
Lava
Small or no Lava lake Crack
Hot, molten rock that flows across ash cloud in crater
the ground is called lava. It keeps Lava fountain Solidified lava Flowing
this name even after it has cooled Stream of lava
down and solidified into rock. Shower of runny lava
Lava comes in different forms, lava bombs
depending on what it is made
of, how stiff or runny it is, and Strombolian Hawaiian Fissure or Icelandic
how fast it flows. Short and explosive eruption Usually mild eruptions that These are quiet, without loud
that creates showers of cinders create fountains and streams explosions. They happen along
and lava bombs. of runny lava. long cracks in the ground.

A’a lava Ash columns HEIGHT OF COLUMN31 MILES
This is a thick lava flow made of basalt. Its NONEXPLOSIVE50 KM
surface is made up of loose, broken, and When volcanoes erupt, GENTLE
sharp chunks of lava that can tumble down they can create tall columns EXPLOSIVE28 MILES
the front of the lava flow as it moves. of ash. The height of these SEVERE45 KM
columns varies, depending CATACLYSMIC
Pahoehoe lava on the amount of energy PAROXYSMAL25 MILES
This is a basalt lava with an unbroken in the eruption, and how COLOSSAL40 KM
surface. As it flows, its surface skin is much magma is thrown SUPERCOLOSSAL
gradually stretched. The end result can end out. The most energetic MEGACOLOSSAL22 MILES
up looking either smooth or ropelike. phase of a major volcanic 35 KM
eruption can last for
Blocky lava many hours. 18.5 MILES
These chunks of lava form when stiff- 30 KM
flowing lava breaks up into angular blocks. How high?
These lava blocks have smoother faces Column height is one way of 15.5 MILES
than a’a lava. estimating how explosive a 25 KM
volcanic eruption is. Eruption
types have different names 12.5 MILES
and correspond to different 20 KM
grades of the Volcanic
Explosivity Index. 9 MILES
15 KM

6 MILES
10 KM

3 MILES
5 KM

0

1 2 34 56 7 8 9
VOLCANIC EXPLOSIVITY INDEX

Volcanic bombs Cinders Ash cloud Volcanic fallout

If there is wind, a volcanic ash cloud will be
blown to one side of the volcano, so that ash
from the cloud falls to the ground in a belt that
can extend hundreds of miles away from the
volcano. Although it is cold by the time it
reaches the ground, ash fallout can strip the
leaves from plants and is dangerous to inhale.
Aircraft must avoid flying through airborne
ash, because it can clog up their engines.

What falls where?
The larger fragments created in an eruption, such
as bombs and cinders, fall closest to the volcano.
Fine ash travels the furthest.

48 earth TECTONIC EARTH Tectonic plates are made of Earth’s crust together with
the top layer of the mantle, called the lithosphere.

Plate tectonics

Earth’s surface may seem fixed but in fact is made up 3
of lots of huge slabs called tectonic plates. These plates
move slowly but constantly, and movements between
them create earthquakes and volcanoes.

Most tectonic plates carry both oceans and continents, 2

though a few are almost entirely oceanic. Where two

plates pull apart under an ocean, new ocean floor is

formed. Where plates are pushed together, dramatic

changes to the landscape can occur. If both edges

are continental, a huge mountain range will form in

the collision zone. If one plate is oceanic and the

other continental, the oceanic edge will usually

be pushed under its neighbor. Fiery volcanoes

occur along the edges of these boundaries, 15
35
which are called subduction zones. 1

16
20

4

Earth’s plates 9
21
The top layer of Earth is like a jigsaw, with
seven or eight large plates and dozens of 8
smaller, more fragmented plates. These plates
float around, moving on top of the hotter
layers below. Their slow, steady movement
can change the size of the oceans, and carry
continents around the globe.

Key 20 North Andes 33
21 Altiplano
1 Pacific 22 Anatolian Mid-Atlantic Ridge 18 6
2 North American 23 Banda Most of this ridge lies 34
3 Eurasian 24 Burma about 8,200 ft (2,500 m)
4 African (Nubian) 25 Okinawa underwater—Iceland is
5 African (Somalian) 26 Woodlark
6 Antarctic 27 Mariana the only part of it
7 Australian 28 New Hebrides above sea level.
8 South American 29 Aegean
9 Nazca 30 Timor Plate boundaries Volcanoes Trench
10 Indian 31 Bird’s Head Plate movement
11 Sunda 32 North Bismarck The edges of tectonic plates Continental
12 Philippine 33 South Sandwich meet up in different ways. The crust
13 Arabian 34 South Shetland plates move apart, toward
14 Okhotsk 35 Panama each other, or past each other. Magma Oceanic crust
15 Caribbean 36 South Bismarck Earthquakes can occur in any Earthquakes
16 Cocos 37 Maoke of these circumstances, and
17 Yangtze 38 Solomon studying earthquakes can help Convergent
18 Scotia us work out where plate Where a plate with oceanic crust moves toward a plate with
19 Caroline boundaries lie. Sometimes thicker, continental crust, it will be pushed down, or subducted.
there are so many cracks that The oceanic plate then melts, and can create volcanoes as magma
it is impossible to tell exactly bubbles up to the surface. If two continents collide, they will push
where one plate ends and the up against each other, creating mountain ranges.
next one begins.