Earth Planet Essay Research Paper The Earth (стр. 1 из 2)

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