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.