The Earth, man’s home, is a planet.
The Earth has special characteristics, and these areimportant to man. It is the only planet known to have the right temperature and the right atmosphere tosupport the kind of environments and natural resources in which plants and man and other animals cansurvive. This fact is so important to man that he has developed a special science called ecology, whichdeals 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 frommicroscopic plant and animals to giant trees and mammoth whales. Distinct types of plants or animals maybe common in many parts of the world or may be limited to a small area. Some kinds thrive underconditions that are deadly for others. So some persons suggest that forms of life quite different fromthose known on Earth might possibly survive on planets with conditions that are far different fromconditions on Earth.
Many persons believe that the Earth is the only planet in the solar system that can support anykind of life. Scientists have theorized that some primitive forms of life may exist on the surface ofMars, but evidence gathered in 1976 by unmanned probes sent to the Martian surface seems to indicate thatthis is unlikely. Scientist at one time also believed that Venus might support life. Clouds always hide thesurface of Venus, so it was thought possible that the temperature and atmosphere on the planet’s surfacemight be suitable for living things. But it is now known that the surface of Venus is too hot–anaverage of 800 F (425 C)–for liquid water to exist there.
The life forms man is familiar with could notpossibly live on Venus. The Earth has excellent conditions for life. The temperature is cool enough so that liquid watercan remain on Earth’s surface. In fact, oceans cover more than two thirds of the surface. But thetemperature is also warm enough so that a small fraction of this water is permanently frozen–near theNorth 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 upthe 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 Earthso that plants flourish. Growing plants convert the energy of sunlight into the chemical energy of theirown bodies. This interaction between plants and the sun is the basic source of energy for virtually allforms of life on Earth. Extensive exploration of the sea floor since 1977, however, has uncovered the existence ofbiological communities that are not based on solar energy.
Active areas of sea floor spreading, such asthe centers in the eastern Pacific that lie far below the limit of light penetration, have chimney likestructures 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 alteredmany views of biological, geological, and geochemical processes that exist in the deep sea. One of themost significant discoveries is that the vents and associated chemical constituents provide the energysource 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 arethose that use energy obtained from the chemical oxidation of inorganic compounds, such as hydrogensulfide, for the fixation of carbon dioxide into organic matter. Although the atmosphere allows sunlight to reach the Earth’s surface, it blocks out certainportions 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 havedeveloped. So, the necessary conditions for these life forms–water, the planet in the solar systemknown 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 innerplanets–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, arebasically balls of rock. Mercury is the smallest in size.
It diameter is about two thirds the greatestwidth of the Atlantic Ocean. Mars is larger than Mercury, but its diameter is only a little more thanhalf that of the Earth. Venus, width a diameter of roughly 7 600 miles (12 000 kilometers), is almost aslarge 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 arealso much less dense than the inner planets.
They seem to be balls of substances that are gases on Earthbut 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 alsoa mystery. All that is known for sure about Pluto is its orbit . Pluto’s average distance from the sunis almost 40 times that of the Earth.
At the outer reaches of the solar system are the comets. A comet consists of nucleus of frozengases called ices, water and mineral particles; and a coma of gases and dust particles. Some comets alsohave tails. A comet’s tail consists of gases and particles of dust from the coma. As the cometapproaches the sun, light from the sun and the solar wind cause tails to form. For this reason the tailspoint generally away from the sun.
THE PLANET For several hundred years almost everyone has accepted the fact that the world is round. Mostpersons think of it as a sphere, somewhat like a solid ball. Actually, the diameter is nearly, but notexactly, spherical. It has a slight bulge around the equator. Measured at sea level, the diameter ofthe 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 (12713. 8 kilometers).
Compared to overall diameter, the difference seems small–only 26. 7 miles (43kilometers). But compared to the height of the Earth’s surface features, it is large. For example, thetallest mountain, Mount Everest, juts less than 6 miles (9 kilometers) above sea level.
The Earth’sshape has another slight distortion. It seems slightly fatter around the Southern Hemisphere than aroundthe 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 workedtheir way mile by mile across the continents. Today, artificial satellites, then calculate thegravitational force that the Earth exerts on the satellites. From these calculations, they can deducethe shape of the Earth.
The slight bulge around the Southern Hemisphere was discovered from calculationsmade 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 heavylead weights of carefully measured mass near near other in an apparatus that measures the force of thegravitational attraction between them.
According to Newton’s law of gravitation, the force of gravity is proportional to the products ofthe 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 thelaboratory can be measured in the experiment. The only missing factor is the mass of the Earth, whichcan easily be determined by comparison.
Scientists can calculate the Earth’s volume because they know the shape of the Earth. Theydivide the mass of the Earth by the volume, which gives the average density of the material in the Earthas 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 ofthe Earth. But not all of the material in the Earth has the same density. Most of the material on thecontinents is only about half as dense as this average value. The density of the material at the centerof the earth is still somewhat uncertain, but the best evidence available shows that it is about threetimes 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 Earthappears to be built up in a series of layers. The Earth’s structure comprises three basic layers. The outermost layer, which covers the Earthlike a thin skin, is called the crust. Beneath that is a thick layer called the mantle. Occupying thecentral 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 thecrust under the ocean is 3 miles (5 kilometers), but under the continents the average thickness of thecrust is 19 miles (31 kilometers). This difference in thickness under the continents and under theoceans 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 partalso has a different structure. The basaltic type of rock that covers most of the ocean floors also liesunderneath the continents. It appears almost as though the lighter rocks of the continental land massesare floating on the heavier rocks beneath. Modern theories about the Earth’s structure suggest that this is exactly what is happening. Butto understand this theory of floating rocks, called isostasy, it is necessary to know something about theEarth’s next deeper layer, the mantle.
The mantle has never been seen. Men have drilled deep holes, such as those for oil wells, intothe crust of the Earth both in the continents and in the ocean floor. But no hole has ever been drilledall the way through the crust in to the mantle. All measurements, scientists can deduce manycharacteristics of the mantle. The mantle is about 1 800 miles (2 900 kilometers) thick and is divided into three regions.
Therocky mantle material is quite rigid compared to things encountered in everyday experience. But ifpressure 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 mountainsis deposited in the ocean, the mantle will slowly give way to make up for the change in the weight of therock above it. The core extends outward from the Earth’s center to a radius of about 2 160 miles (3 480kilometers). Obtaining information about the Earth’s interior is so difficult that may ideas about itsstructure remain uncertain.
Some evidence indicates that the core is divided into zones. The innercore, which has a radius of about 780 miles (1 255 kilometers), is quite rigid, but the outer coresurrounding it is almost liquid. scientists disagree about this description of the core because it is based on incomplete seismicwave data. The theory suggest that the density of the inner core material is about 9 to 12 ounces percubic inch (16 to 20 grams per cubic centimeter). The density of the outer core material is about 6 to 7ounces 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 ofits 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 inwater. Small amounts of salt from land areas dissolve in the water of streams and rivers and are carriedto 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 amountof salt dissolved in the oceans is, on the average, 34.
5 percent by weight. About the same percentagecan 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 therain that falls on the continents. Steadily moving air currents in the Earth’s atmosphere carry themoist air inland. When the air cools, the vapour condenses to form water droplets.
These are seen mostcommonly as clouds. Often the droplets come together to form raindrops. If the atmosphere is coldenough, snowflakes form instead of raindrops. In either case, water that has traveled from an oceanhundreds of even thousands of miles away falls to the Earth’s surface. There, except for what evaporatesimmediately, 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 moisturethey need to live.
Most ground water, like surface water, moves toward the sea, but it moves moreslowly. 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 tothe 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 watervapour in the atmosphere. Without the balancing effect of the oceans, whole continents could be totallydry 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, heatfrom 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, probablywould 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 comesfrom 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 upby 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’scontinually 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 70percent of sunlight. Since sunlight is a basic requirement for agriculture, it seems reasonable that theoceans 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. Algaedo 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 andlarger ones. Man now enters the pyramid when he catches fish, but the fish he catches are near the top of thepyramid.
All the steps between are very inefficient. It takes about a thousand pounds of algae toproduce a pound of codfish, less than a day’s supply of food for a man. To feed the growing populationof 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 forirrigation. 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. Manhas known how to this for more than 2 000 years.
But the process has been slow, and even with modernequipment 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 wouldproduce 150 million gallons of water daily at a cost of 35 to 40 cents per thousand gallons. It alsowould 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 definingthe Earth’s regions, especially those near the surface, makes it easier to understand importantinteractions that take place.
In this definition, the regions are called the lithosphere, thehydrosphere, and the atmosphere. The lithosphere includes all the solid material of the Earth. Litho refers to stone, and thelithosphere 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 thehydrosphere is made up of all the liquid water in the crust–the oceans, streams, lakes, andgroundwater–as well as the frozen water in glaciers, on mountains, and in the Arctic and Antarctic icesheets. 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 nosharp boundary. At high altitudes it simply gets thinner and thinner until it becomes impossible to tellwhere 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 up21 percent. The remaining one percent is composed of a number of different gases, such as argon, carbondioxide, helium, and neon. One of these–carbon dioxide–is a vital to plant life as oxygen is to animallife. 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 anaverage 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 regionsassociated with weather patterns. The pressure at 36 000 feet (11 000 meters)– a typical cruisingaltitude for commercial jet planes–is only about one fifth as great as atmospheric pressure at sealevel.
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 altitudeof 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 (10kilometers)–is called the troposphere. The next higher region, where the temperature remains steady, iscalled the stratosphere. Above that is the mesosphere, and still higher, starting about 50 miles (80kilometers) 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’ssurface. High in the stratosphere and upward into the mesosphere, chemical reactions take place amongthe various molecules.
Ozone, a molecule that contains three atoms of oxygen, is formed. ( A moleculeof the oxygen animals breathe has two atoms. ) Other molecules have various combinations of nitrogen andoxygen. In higher regions the atmosphere is made up almost completely of nitrogen, and higher stillalmost completely of oxygen.
At the outer most reaches of the atmosphere, the light gases, helium andhydrogen, predominate. The Earth’s Magnetic Field Scientists explain that another boundary besides the atmosphere seems to separate the environmentof the Earth from the environment of space. This boundary is known as the magnetopause. It is theboundary 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. Themagnetic 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 throughit. 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 along tail on the side away from the sun. In the magnetosphere, orbiting swarms of charged particles move in huge broad belts around theEarth. Their movement is regular because they are dominated by the comparatively constant magnetic fieldof the Earth.
The discovery of these radiation belts by the first American satellite, Explorer 1, wasone 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. Thebelts of trapped particles are pushed nearer to the Earth.
Scientists are not certain what causes thefamous aurora borealis, or northern lights, and the aurora australis, or southern lights. According toone explanation, when the trapped particles are forced down into the Earth’s atmosphere, they collidewith particles there and a great deal of energy is exchanged. This energy is changed into light, and thespectacular auroras result. The Earth Through Time The Earth’s crust formed about 4.
5 billion years ago. Since then the surface features of theland 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 thendied 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 anyregion.
Geologists can sometimes fill in the gaps by studying sequences of rocks in various regions ofthe 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 24hours from high noon on one day to high noon on the next. It takes 365.
25 days–one year–from the Earthto travel once around the sun. Calendars mark 365 days for most years, but every fourth year–leapyear–has 366 days. When observed from over the North Pole, the Earth rotates and revolves in a counterclockwisedirection. 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, thelong river valleys, and the broad plains have been known throughout recorded history.
All appearchangeless, but changes occur steadily. Small ones can be seen almost any day. The rivulets of mud thatform on the side of a hill during a rainstorm move soil from one place to another. Sudden gusts of windblow dust and sand around, redistributing these materials. Occasionally, spectacular changes take place.
A volcano erupts and spreads lava over thesurrounding landscape, burying it under a thick layer of fresh rock. Earthquakes break the Earth’scrust, 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 havebeen small compared to the changes that created mountains or the vast expense of the prairie. But therecorded history of man covers only a short period of the Earth’s history. Scientists believe that theEarth has existed for about 4. 5 billion years.
Man’s recorded history extends back only about 6 000years, or 0. 0000013 percent of the Earth’s age. There is ample evidence that the Earth’s surface haschanged greatly since its original formation.
