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Реферат: Earth Planet Essay Research Paper The Earth

Название: Earth Planet Essay Research Paper The Earth
Раздел: Топики по английскому языку
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Earth Planet Essay, Research Paper

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.

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