The Earth, man's home, is a planet. The Earth has special characteristics, and
these are important to man. It is the only planet known to have the right
temperature and the right atmosphere to support the kind of environments and
natural resources in which plants and man and other animals can survive. This
fact is so important to man that he has developed a special science called
ecology, which deals with the dependence of all living things will continue to
survive on the planet. Many millions of kinds of plants and animals have
developed on Earth. They range in size from microscopic plant and animals to
giant trees and mammoth whales. Distinct types of plants or animals may be
common in many parts of the world or may be limited to a small area. Some kinds
thrive under conditions that are deadly for others. So some persons suggest that
forms of life quite different from those known on Earth might possibly survive
on planets with conditions that are far different from conditions on Earth. Many
persons believe that the Earth is the only planet in the solar system that can
support any kind of life. Scientists have theorized that some primitive forms of
life may exist on the surface of Mars, but evidence gathered in 1976 by unmanned
probes sent to the Martian surface seems to indicate that this is unlikely.
Scientist at one time also believed that Venus might support life. Clouds always
hide the surface of Venus, so it was thought possible that the temperature and
atmosphere on the planet's surface might be suitable for living things. But it
is now known that the surface of Venus is too hot--an average of 800 F (425
C)--for liquid water to exist there. The life forms man is familiar with could
not possibly live on Venus. The Earth has excellent conditions for life. The
temperature is cool enough so that liquid water can remain on Earth's surface.
In fact, oceans cover more than two thirds of the surface. But the temperature
is also warm enough so that a small fraction of this water is permanently
frozen--near the North and South Poles and on some mountain tops. The Earth's
atmosphere is dense enough for animals to breathe easily and for plants to take
up the carbon dioxide they need for growth. But the atmosphere is not so dense
that it blocks out sunlight. Although clouds often appear in the sky, on the
average enough sunlight reaches the surface of the Earth so that plants
flourish. Growing plants convert the energy of sunlight into the chemical energy
of their own bodies. This interaction between plants and the sun is the basic
source of energy for virtually all forms of life on Earth. Extensive exploration
of the sea floor since 1977, however, has uncovered the existence of biological
communities that are not based on solar energy. Active areas of sea floor
spreading, such as the centers in the eastern Pacific that lie far below the
limit of light penetration, have chimney like structures known as smokers that
spew mineral-laden water at temperatures of approximately 660 F (350 C).
Observations and studies of these active and inactive hydrothermal vents have
radically altered many views of biological, geological, and geochemical
processes that exist in the deep sea. One of the most significant discoveries is
that the vents and associated chemical constituents provide the energy source
for chemosynthetic bacteria. These bacteria form, in turn, the bottom of the
food chain, sustaining the lush biological communities at the hydrothermal vent
sites. Chemosynthetic bacteria are those that use energy obtained from the
chemical oxidation of inorganic compounds, such as hydrogen sulfide, for the
fixation of carbon dioxide into organic matter. Although the atmosphere allows
sunlight to reach the Earth's surface, it blocks out certain portions of solar
radiation, especially X rays and ultraviolet light. Such radiation is very
harmful, and, if the atmosphere did not filter it out, probably none of the life
forms on Earth could ever have developed. So, the necessary conditions for these
life forms--water, the planet in the solar system known to have all these
"right" conditions. THE EARTH'S PLACE IN SPACE Despite its own special
conditions, the Earth is in some ways similar to the other inner planets--the
group of planets nearer to the sun. Of these planets, Mercury is the closest to
the sun; Venus is second; the Earth is third; and Mars is forth. All of these
planets, including the Earth, are basically balls of rock. Mercury is the
smallest in size. It diameter is about two thirds the greatest width of the
Atlantic Ocean. Mars is larger than Mercury, but its diameter is only a little
more than half that of the Earth. Venus, width a diameter of roughly 7 600 miles
(12 000 kilometers), is almost as large as Earth. Four of the five outer planets
are much bigger than any of the inner planets. The largest, Jupiter, has a
diameter more that 11 times as great as that of the Earth. These four outer
planets are also much less dense than the inner planets. They seem to be balls
of substances that are gases on Earth but chiefly solids at the low temperatures
and high pressures that exist on the outer planets. The exact size or mass of
Pluto, the most distant planet, is not known. Its composition is also a mystery.
All that is known for sure about Pluto is its orbit . Pluto's average distance
from the sun is almost 40 times that of the Earth. At the outer reaches of the
solar system are the comets. A comet consists of nucleus of frozen gases called
ices, water and mineral particles; and a coma of gases and dust particles. Some
comets also have tails. A comet's tail consists of gases and particles of dust
from the coma. As the comet approaches the sun, light from the sun and the solar
wind cause tails to form. For this reason the tails point generally away from
the sun. THE PLANET For several hundred years almost everyone has accepted the
fact that the world is round. Most persons think of it as a sphere, somewhat
like a solid ball. Actually, the diameter is nearly, but not exactly, spherical.
It has a slight bulge around the equator. Measured at sea level, the diameter of
the Earth around the equator is 7 926.7 miles (12 756.8 kilometers). The
distance from the North to the South pole, also measured at sea level, is 7
900.0 miles (12 713.8 kilometers). Compared to overall diameter, the difference
seems small--only 26.7 miles (43 kilometers). But compared to the height of the
Earth's surface features, it is large. For example, the tallest mountain, Mount
Everest, juts less than 6 miles (9 kilometers) above sea level. The Earth's
shape has another slight distortion. It seems slightly fatter around the
Southern Hemisphere than around the Northern Hemisphere. This difference is, at
most, about 100 feet (30 meters). The shape of the Earth was originally
calculated from measurements made by surveyors who worked their way mile by mile
across the continents. Today, artificial satellites, then calculate the
gravitational force that the Earth exerts on the satellites. From these
calculations, they can deduce the shape of the Earth. The slight bulge around
the Southern Hemisphere was discovered from calculations made in this way. The
Earth's Mass, Volume, and Density The mass of the Earth has been found to be, in
numerals, 6 sextillions, 595 quintillions tons. Scientists measure the Earth'
mass by means of a very delicate laboratory experiment. They place heavy lead
weights of carefully measured mass near near other in an apparatus that measures
the force of the gravitational attraction between them. According to Newton's
law of gravitation, the force of gravity is proportional to the products of the
two masses involved. The force of the Earth's gravity on the experimental mass
is easily measured. It is simply the weight of the mass itself. The force of
gravity between two known masses in the laboratory can be measured in the
experiment. The only missing factor is the mass of the Earth, which can easily
be determined by comparison. Scientists can calculate the Earth's volume because
they know the shape of the Earth. They divide the mass of the Earth by the
volume, which gives the average density of the material in the Earth as 3.2
ounces per cubic inch (5.5 grams per cubic centimeter). This average value
includes all the material from the surface of the Earth down to the center of
the Earth. But not all of the material in the Earth has the same density. Most
of the material on the continents is only about half as dense as this average
value. The density of the material at the center of the earth is still somewhat
uncertain, but the best evidence available shows that it is about three times
the average density of the Earth. The Earth's Layers The difference in density
is not the only difference between the Earth's surface and its center. The kinds
of materials at these two locations also seem to be quite different. In fact,
the Earth appears to be built up in a series of layers. The Earth's structure
comprises three basic layers. The outermost layer, which covers the Earth like a
thin skin, is called the crust. Beneath that is a thick layer called the mantle.
Occupying the central region is the core. Each layer is subdivided into other,
more complex, structures. The crust of the Earth varies in thickness from place
to place. The average thickness of the crust under the ocean is 3 miles (5
kilometers), but under the continents the average thickness of the crust is 19
miles (31 kilometers). This difference in thickness under the continents and
under the oceans is an important characteristic of the crust. These two parts of
the crust differ in other ways. Each has different kinds of rocks. Continental
rocks, such as granite, are less dense than rocks in ocean basins, such as
basalt. Each part also has a different structure. The basaltic type of rock that
covers most of the ocean floors also lies underneath the continents. It appears
almost as though the lighter rocks of the continental land masses are floating
on the heavier rocks beneath. Modern theories about the Earth's structure
suggest that this is exactly what is happening. But to understand this theory of
floating rocks, called isostasy, it is necessary to know something about the
Earth's next deeper layer, the mantle. The mantle has never been seen. Men have
drilled deep holes, such as those for oil wells, into the crust of the Earth
both in the continents and in the ocean floor. But no hole has ever been drilled
all the way through the crust in to the mantle. All measurements, scientists can
deduce many characteristics of the mantle. The mantle is about 1 800 miles (2
900 kilometers) thick and is divided into three regions. The rocky mantle
material is quite rigid compared to things encountered in everyday experience.
But if pressure is applied to it over a long period--perhaps millions of
years--it will give a little bit. So, if the distribution of rock in the crust
changes gradually, as it does when material eroded off mountains is deposited in
the ocean, the mantle will slowly give way to make up for the change in the
weight of the rock above it. The core extends outward from the Earth's center to
a radius of about 2 160 miles (3 480 kilometers). Obtaining information about
the Earth's interior is so difficult that may ideas about its structure remain
uncertain. Some evidence indicates that the core is divided into zones. The
inner core, which has a radius of about 780 miles (1 255 kilometers), is quite
rigid, but the outer core surrounding it is almost liquid. scientists disagree
about this description of the core because it is based on incomplete seismic
wave data. The theory suggest that the density of the inner core material is
about 9 to 12 ounces per cubic inch (16 to 20 grams per cubic centimeter). The
density of the outer core material is about 6 to 7 ounces per cubic inch (11 to
12 grams per cubic centimeter). The Earth's Surface Areas Much scientific study
has been devoted to the thin crystal area on which man lives, and most of its
surface features are well known. The oceans occupy 70.8 percent of the surface
area of the Earth, leaving less than a third of the Earth's surface for the
continents. Of course, not all of the Earth's land is dry. A fraction of it is
covered by lakes, streams, and ice. Actually, the dry land portion totals less
than a quarter of the Earth's total surface area. The Salty Oceans The oceans
are salty. Salt is a rather common mineral on the Earth and dissolves easily in
water. Small amounts of salt from land areas dissolve in the water of streams
and rivers and are carried to the sea. This salt has steadily accumulated in the
oceans for billions of years. When water evaporates from the oceans into the
atmosphere, the salt is left behind. The amount of salt dissolved in the oceans
is, on the average, 34.5 percent by weight. About the same percentage can be
obtained if three quarters of a teaspoon of salt is dissolved in eight ounces of
water. Water Supply for the Earth Water that evaporates from the surface of the
oceans into the atmosphere provides most of the rain that falls on the
continents. Steadily moving air currents in the Earth's atmosphere carry the
moist air inland. When the air cools, the vapour condenses to form water
droplets. These are seen most commonly as clouds. Often the droplets come
together to form raindrops. If the atmosphere is cold enough, snowflakes form
instead of raindrops. In either case, water that has traveled from an ocean
hundreds of even thousands of miles away falls to the Earth's surface. There,
except for what evaporates immediately, it gathers into streams or soaks into
the ground and begins its journey back to the sea. Much of the Earth's water
moves underground, supplying trees and other plants with the moisture they need
to live. Most ground water, like surface water, moves toward the sea, but it
moves more slowly. The Balance of Moisture and Temperature The movement of water
in a cycle, from the oceans to the atmosphere to the land and then back to the
oceans, is called the hydrologic cycle. The oceans have a strong balancing force
on this cycle. They interact with the atmosphere to maintain an almost constant
average value of water vapour in the atmosphere. Without the balancing effect of
the oceans, whole continents could be totally dry at some times and completely
flooded at others. The oceans also act as a reservoir of heat. When the
atmosphere above an ocean is cold, heat from the ocean warms it. When the
atmosphere is warmer than the ocean, the ocean cools it. Without it, the
differences between winter and summer temperatures, and even between those of
day and night, probably would be greater. The Food and Water Supply All of man's
food comes from the earth. Very little comes from the sea. Almost all of it
comes from farms on the continents. But man can use only a small portion of the
continents for farming . About 7 percent of the Earth's land is considered
arable, or suitable for farming. The rest is taken up by the swamps and jungles
near the equator, the millions of square miles of desert, the rugged mountains,
and--mostly in the Far North--the frozen tundra. Man has been searching for ways
to produce more food to supply the demands of the Earth's continually increasing
population. Many persons have suggested that the oceans might supply more food.
They point out that the oceans cover more than 70 percent of the Earth's surface
and absorb about 70 percent of sunlight. Since sunlight is a basic requirement
for agriculture, it seems reasonable that the oceans could supply a great deal
of food. But what seems reasonable is not always so. Almost all the plants that
live in the oceans and absorb sunlight as they grow are algae. Algae do not make
very tasty dish for man, but they are an important part of the food pyramid of
the oceans. In this pyramid the algae are eaten by small sea creatures. These,
in turn, are eaten by larger and larger ones. Man now enters the pyramid when he
catches fish, but the fish he catches are near the top of the pyramid. All the
steps between are very inefficient. It takes about a thousand pounds of algae to
produce a pound of codfish, less than a day's supply of food for a man. To feed
the growing population of the world, man must find an efficient way to farm the
sea. He cannot depend simply on catching fish. Much of the Earth's land area is
unusable for agriculture because of the lack of adequate water. Millions of
acres of land have been converted into farmland by damming rives to obtain water
for irrigation. Some scientists have estimated that if all the rivers of the
world were used efficiently, the amount of land suitable for farming might
increase by about 10 percent. Another way to increase the water supply would be
to convert ocean water into fresh water. Man has known how to this for more than
2 000 years. But the process has been slow, and even with modern equipment it is
costly. The distillation plant for the United States navel base at Guantanamo,
Cuba, produces more than 2 million gallons of water a day, but at a cost of
$1.25 for every thousand gallons. In New York City, where fresh water is
available, the cost is about 20 cents per thousand gallons. Scientists have
investigated the use of nuclear-powered distillation plants. One plant would
produce 150 million gallons of water daily at a cost of 35 to 40 cents per
thousand gallons. It also would provide nearly 2 million kilowatts of
electricity. The Atmosphere The Earth's structure consists of the crust, the
mantle, and the core. Another way of defining the Earth's regions, especially
those near the surface, makes it easier to understand important interactions
that take place. In this definition, the regions are called the lithosphere, the
hydrosphere, and the atmosphere. The lithosphere includes all the solid material
of the Earth. Litho refers to stone, and the lithosphere is made up of all the
stone, rock, and the whole interior of the planet Earth. The hydrosphere
includes all the water on the Earth's surface. Hydro means water, and the
hydrosphere is made up of all the liquid water in the crust--the oceans,
streams, lakes, and groundwater--as well as the frozen water in glaciers, on
mountains, and in the Arctic and Antarctic ice sheets. The atmosphere includes
all the gases above the Earth to the beginning of interplanetary space. Atmo
means gas or vapour. The atmosphere extends to a few hundred miles above the
surface, but it has no sharp boundary. At high altitudes it simply gets thinner
and thinner until it becomes impossible to tell where the gas of interplanetary
space begins. The atmosphere contains water vapour and a number of other gases.
Near the surface of the Earth, 78 percent of the atmosphere is nitrogen. Oxygen,
vital for all animal species, including man, makes up 21 percent. The remaining
one percent is composed of a number of different gases, such as argon, carbon
dioxide, helium, and neon. One of these--carbon dioxide--is a vital to plant
life as oxygen is to animal life. But carbon dioxide makes up only about 0.03
percent of the atmosphere. The weight of the atmosphere as it presses on the
Earth's surface is great enough to exert an average force of about 14.7 pounds
per square inch (1.03 kilograms per square centimeter) at sea level. The
pressure changes slightly from place to place and develops the high and low
pressure regions associated with weather patterns. The pressure at 36 000 feet
(11 000 meters)-- a typical cruising altitude for commercial jet planes--is only
about one fifth as great as atmospheric pressure at sea level. The temperature
of the atmosphere also falls at high altitudes. At 36 000 feet (11 000 meters),
the temperature averages -56 C. The average temperature remains steady at --56 C
and up to an altitude of 82 000 feet (25 000 meters). Above this altitude, the
temperature rises. The atmosphere has been divided into regions. The one nearest
the Earth--below 6 miles (10 kilometers)--is called the troposphere. The next
higher region, where the temperature remains steady, is called the stratosphere.
Above that is the mesosphere, and still higher, starting about 50 miles (80
kilometers) above the surface, is the ionosphere. In this uppermost region many
of the molecules and atoms of the Earth's atmosphere are ionized. That is, they
carry either a positive or negative electrical charge. The composition of the
upper atmosphere is different from that of the atmosphere near the Earth's
surface. High in the stratosphere and upward into the mesosphere, chemical
reactions take place among the various molecules. Ozone, a molecule that
contains three atoms of oxygen, is formed. ( A molecule of the oxygen animals
breathe has two atoms.) Other molecules have various combinations of nitrogen
and oxygen. In higher regions the atmosphere is made up almost completely of
nitrogen, and higher still almost completely of oxygen. At the outer most
reaches of the atmosphere, the light gases, helium and hydrogen, predominate.
The Earth's Magnetic Field Scientists explain that another boundary besides the
atmosphere seems to separate the environment of the Earth from the environment
of space. This boundary is known as the magnetopause. It is the boundary between
that region of space dominated by the Earth's magnetic field, called the
magnetosphere, and interplanetary space, where magnetic fields are dominated
primarily by the sun. The Earth has a strong magnetic field. It is as if the
Earth were a huge bar magnet. The magnetic compass used to find directions on
the Earth's surface works because of this magnetic field. This same magnetic
field extends far out into space. The Earth's magnetic field exerts a force on
any electrically charged particle that moves through it. There appears to be a
steady "wind" of charged particles moving outward from the sun. This
solar wind is deflected near the Earth by the Earth's magnetic field. In this
interaction, the Earth's magnetic field is slightly squeezed in on the side that
faces the sun, and pulled out into a long tail on the side away from the sun. In
the magnetosphere, orbiting swarms of charged particles move in huge broad belts
around the Earth. Their movement is regular because they are dominated by the
comparatively constant magnetic field of the Earth. The discovery of these
radiation belts by the first American satellite, Explorer 1, was one of the
earliest accomplishments of the space age. The charged particles within the
radiation belts actually travel in a complex corkscrew pattern. They move back
and forth from north to south while the whole group slowly drifts around the
Earth. When the magnetic field of the sun is especially strong, the
magnetosphere is squeezed. The belts of trapped particles are pushed nearer to
the Earth. Scientists are not certain what causes the famous aurora borealis, or
northern lights, and the aurora australis, or southern lights. According to one
explanation, when the trapped particles are forced down into the Earth's
atmosphere, they collide with particles there and a great deal of energy is
exchanged. This energy is changed into light, and the spectacular auroras
result. The Earth Through Time The Earth's crust formed about 4.5 billion years
ago. Since then the surface features of the land have been shaped, destroyed,
and reshaped, and even the positions of the continents have changed. Over the
years, various kinds of plants and animals have developed. Some thrived for a
time and then died off: others adapted to new conditions and survived. All these
events are recorded in the Earth's rocks, but the record is not continuous in
any region. Geologists can sometimes fill in the gaps by studying sequences of
rocks in various regions of the Earth. The Earth's Motion and Time The Earth
makes one rotation on its axis every 24 hours with reference to the sun. It is
24 hours from high noon on one day to high noon on the next. It takes 365.25
days--one year--from the Earth to travel once around the sun. Calendars mark 365
days for most years, but every fourth year--leap year--has 366 days. When
observed from over the North Pole, the Earth rotates and revolves in a
counterclockwise direction. When observed from the South Pole, the Earth rotates
and revolves in a clockwise direction. The Changing Earth The great features of
the Earth seem permanent and unchanging. The giant mountain ranges, the long
river valleys, and the broad plains have been known throughout recorded history.
All appear changeless, but changes occur steadily. Small ones can be seen almost
any day. The rivulets of mud that form on the side of a hill during a rainstorm
move soil from one place to another. Sudden gusts of wind blow dust and sand
around, redistributing these materials. Occasionally, spectacular changes take
place. A volcano erupts and spreads lava over the surrounding landscape, burying
it under a thick layer of fresh rock. Earthquakes break the Earth's crust,
causing portions of it to slide and move into new positions. In the lifetime of
one man, or even in the generations of recorded history, these changes have been
small compared to the changes that created mountains or the vast expense of the
prairie. But the recorded history of man covers only a short period of the
Earth's history. Scientists believe that the Earth has existed for about 4.5
billion years. Man's recorded history extends back only about 6 000 years, or
0.0000013 percent of the Earth's age. There is ample evidence that the Earth's
surface has changed greatly since its original formation.