Earth Planet


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.

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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 …


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