Earth Planet
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.