Eclipse

When three celestial objects become aligned, an eclipse is said to occur. The many eclipse events known to astronomers are of two different types. In the first, the eclipsing body comes between an observer and the eclipsed object. The eclipsed object appears to the observer to be totally or partially covered by the eclipsing object. Eclipses of the second type affect only planets or natural satellites. In this case, the eclipsing body comes between the sun and the eclipsed object. The eclipsed object remains in view of the observer, but the sun's light no longer shines on any of it or part of it, and it becomes darkened by entering into the shadow of the eclipsing object. Examples of this kind of eclipse event are eclipses of the moon and eclipses of the satellites of Jupiter.

Solar and lunar eclipses have long been of interest because they are easily seen without a telescope and offer an impressive spectacle. Primitive peoples were struck with fear by the falling darkness during a total solar eclipse or by the strange sight of the eclipsed moon. Accounts of such eclipses are found among the oldest records of history

Solar Eclipses

A solar eclipse occurs when the moon, revolving in its orbit around the Earth, moves across the disk of the sun so that the moon's shadow sweeps over the face of the Earth. No sunlight penetrates the inner part of the shadow, or umbra. To observers on the Earth within the umbra, the disk of the sun appears completely covered by that of the moon. Such a solar eclipse is said to be total. Because the umbra is narrow at its intersection with the Earth, a total eclipse can be observed only within a very narrow area called the zone of totality. Because of the relative motion of the Earth and moon, the shadow moves rapidly over the Earth's surface. A total solar eclipse thus lasts only a short time—less than eight minutes at any one place on Earth. To observers located within the outer part of the moon's shadow, or penumbra, the disk of the moon appears to overlap the sun's disk in part. This event is called a partial solar eclipse.Because the Earth revolves around the sun in an elliptical orbit, the distance between Earth and sun changes slightly during the course of a year. Similarly, the apparent size of the lunar disk changes to some degree during a month because of the elliptical shape of the moon's orbit. If a solar eclipse occurs when the sun is closest to the Earth and the moon is farthest away, the moon does not completely cover the sun; the rim of the sun is visible around the edge, or limb, of the moon. This type of solar eclipse is known as an annular eclipse. Eclipses of the sun occur two to four times a year. In rare instances more may occur, as in 1935, when five solar eclipses took place.
Total solar eclipses have helped scientists obtain much knowledge about the nature of the sun's chromosphere and corona, the thin external layers of the sun that are usually lost in the brilliant glare from the shining solar surface, or photosphere. During a total solar eclipse the moon acts as a screen outside the Earth's atmosphere, cutting off the direct rays from the photosphere. The brilliance of the sky is decreased greatly, and the fainter parts of the sun become visible. Scientists no longer need to wait for eclipses to occur naturally in order to study the sun. They can use an instrument called a coronagraph to block the photosphere artificially, making it possible for them to conduct studies of the solar chromosphere and corona.

Lunar Eclipses

When the moon travels through the shadow of the Earth and loses its bright illumination by the sun, a lunar eclipse takes place. It can occur only at the time of the full moon—that is, when the moon is directly opposite the sun—because the Earth's shadow is directed away from the sun. A lunar eclipse can be seen from any place on the Earth where the moon is above the horizon. Such an eclipse can be total, partial, or penumbral, depending on the moon's position. If the moon passes through the center of the Earth's umbra, a total lunar eclipse occurs. Totality may extend up to 100 minutes, with the entire eclipse lasting about 3¹/₂ hours. A partial lunar eclipse is observable when only a part of the moon passes through the umbra. The penumbral type occurs when the moon moves only through the outer part of the shadow, dimming its own illumination only slightly. Lunar eclipses generally occur twice a year. In some years, however, there may be none or as many as three.

Other Types of Eclipses

From the Earth, the moon appears against a background of distant stars. As the moon moves eastward across the constellations, it occasionally passes in front of a star or a planet, causing an occultation. Accurately timed observations of occultations are used to study the orbital motion of the moon. Measurements of the time required for a star to disappear also provide information about the diameters of the stars.
The two planets Mercury and Venus, which are closer to the sun than is the Earth, occasionally pass between the Earth and the sun. At such a time either of these planets appears as a small, dark, circular disk projected on the brilliant disk of the sun, crossing it slowly as the planet makes a transit.
Eclipsing binaries are double-star systems consisting of two stellar bodies that revolve around one another. One star passes periodically in front of or behind the other as seen from the Earth, and two eclipses take place during each revolution. From the way in which the light from the binary system varies, it is possible to calculate the orbit and relative sizes of the two bodies

 

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Light

One of the most familiar and important forms of energy is light. Nothing is visible to humans when light is totally absent. But light is even more important for other reasons. Many scientists believe that millions of years ago light from the sun triggered the chemical reactions that led to the development of life on Earth. Without light the living things now on Earth would be unable to survive. Light from the sun provides energy for life on Earth. Plants change the energy of sunlight into food energy. When light rays strike a green plant, some of their energy is changed to chemical energy, which the plant uses to make food out of air and minerals. This process is called photosynthesis. Very nearly all living organisms on Earth depend directly or indirectly on photosynthesis for their food energy.

Some of the energy of sunlight is absorbed by the Earth's atmosphere or by the Earth itself. Much of this energy is then changed to heat energy, which helps warm the Earth, keeping it in the temperature range that living things have adapted to.

LIGHT AND ELECTROMAGNETIC RADIATION

Different kinds of light are visible to different species. Humans see light in what is called the visible range. It includes all the colors beginning with red and continuing through orange, yellow, green, blue, and violet . Some people can see farther into the violet region or the red region than other people. Some animals have a different sensory range. Pit vipers, for example, have sense organs (pits) that “see” rays that humans feel as heat. These rays are called infrared radiation. Bees, on the other hand, not only see some of the colors that humans see but are also sensitive to ultraviolet radiation, which is beyond the range visible to humans. So, though human eyes cannot detect them, infrared rays and ultraviolet rays are related to visible light. Instruments have been built that can detect and photograph objects by means of infrared rays or ultraviolet rays. X rays, which can also be used to photograph objects, are also related to light.

Scientists have learned that all these forms of energy and many other kinds of energy, such as radio waves, microwaves, and gamma rays, have the same structure. They all consist of electrical and magnetic fields that work together in a special way to form electromagnetic radiation.

SOURCES OF LIGHT

Unlike many other animals, humans depend primarily on sight to learn about the world around them. During the day early peoples could see by the light that came from the sun; but night brought darkness and danger. One of the most important steps people have taken to control their environment occurred when they learned to conquer the dark by controlling fire—a source of light.
Torches, candles, and oil lamps are all sources of light. They depend on a chemical reaction—burning—to release the energy we see as light. Plants and animals that glow in the dark—glowworms, fireflies, and some mushrooms—change the chemical energy stored in their tissues to light energy. Such creatures are called bioluminescent. Electric-light bulbs and neon lights change electrical energy, which may be produced by chemical, mechanical, or atomic energy, into light energy.
Light sources are necessary for vision. An object can be seen only if light travels from the object to an eye that can sense it. When the object is itself a light source, it is called luminous. Electric lights are luminous. The sun is a luminous object because it is a source of light. An object that is not itself a source of light must be illuminated by a luminous object before it can be seen. The moon is illuminated by the sun. It is visible only where the sun's rays hit it and bounce off toward Earth—or to an observer in a spacecraft.
In a completely dark room, nothing is visible. When a flashlight is turned on, its bulb and objects in its beam become visible. If a bright overhead bulb is switched on, its light can bounce off the walls, ceiling, floor, and furniture, making them and other objects in its path visible.
Heating some things causes them to give off visible light rays as well as invisible heat rays. This is the case for electric-light filaments, red-hot burners on electric stoves, and glowing coals. The light of such objects is incandescent. Other light sources emit light energy but no heat energy. They are known as luminescent, or cold light, sources. Neon and fluorescent lights are luminescent.

MEASURING LIGHT

The clarity with which an object can be seen depends in part on the amount of light that falls on it, on how well it is illuminated. The amount of light that a light source gives off (called its intensity) is one factor in determining how well a surface will be illuminated by it. Other factors are the slant of the illuminated surface in relation to the light source and the distance between the surface and the source. As a light beam travels outward from most light sources—the exceptions include lasers and searchlights—the beam spreads to cover a larger area. Distance greatly weakens illumination from such sources. The same amount of light will cover a larger area if the surface it reaches is moved farther away. This results in weaker illumination, following the inverse-square law. If the distance from the source is doubled, the amount of light falling on a given area is reduced to one fourth—the inverse of two squared. If the distance is tripled, the area receives only one ninth of the original illumination—the inverse of three squared.
One way of varying the amount of illumination on a surface is to vary the intensity of the light source. Intensity is measured in candles (or candelas in the international system). A candle used to be the amount of light given off by a carefully constructed wax candle. It is now more precisely defined as one sixtieth of the light intensity of one square centimeter of a perfectly black object at the freezing point of platinum (2,046 K, or 3,223.4° F). Photometers are devices that are used to measure the intensity of light sources.
People are often more interested in measuring the illumination of a surface—a desk top or the floor and walls of a room—than in measuring the light that leaves the light fixture. When distance is measured in feet, the illumination of a surface is measured in footcandles. At a distance of one foot the illumination provided by a light source of 100 candles is 100 footcandles. Under the inverse-square law, the same source gives one fourth as much illumination, or 25 footcandles, at a distance of two feet.
Another measurement that scientists find useful is the total amount of light energy that a source gives off over a certain period of time. This amount of light energy is called the luminous flux of a source and is measured in lumens. An ideal one-candle source gives off 4 π lumens. One footcandle is equal to one lumen per square foot.

LIGHT AND MATTER

The way substances look depends greatly on what happens when light hits them. It is possible to see through transparent substances more or less clearly because light can pass through them without being scattered or stopped. Light that bounces off the objects behind a transparent substance can pass right through it almost as if it were not in the way. Clear window glass and clean water are transparent.
Only the surfaces of opaque substances are visible. Light cannot pass through them, and it is not possible to see through them. Opaque substances either absorb or reflect light. The light energy they absorb usually turns into heat and raises their temperature. Mercury, steel, and wood are examples of opaque substances.
Translucent substances permit some light to pass through them, but the light is scattered, and the images of objects behind them are not retained. Usually, if translucent substances are made thinner they become transparent; if they are made thicker they become opaque. Frosted light bulbs, waxed paper, and some kinds of curtain materials are translucent.

Reflection

Reflection occurs when a light ray hits a surface and bounces off. The angle at which the ray hits the surface is equal to the angle at which it bounces off. If the surface is made very flat and smooth by polishing, all the light rays bounce off in the same direction. This type of reflection is called regular, specular, or mirror reflection. A mirror surface forms an image of things that reflect light onto it. This occurs because the light rays maintain the same pattern, except reversed from left to right, that they possessed before being reflected. Mirrors are usually made of smooth glass with a thin layer of a shiny metal such as silver bonded to the rear side
When an opaque surface is rough, even on the microscopic level, the light rays that hit it are scattered, causing the surface itself to become visible. This is diffuse, or irregular, reflection. If a piece of raw steel with a rough opaque surface is polished smooth and flat, it reflects light rays regularly and takes on the qualities of a mirror.

Refraction and Dispersion

Light travels in a straight line as it passes through a transparent substance. But when it moves from one transparent material to another of different density—for example, from air to water or from glass to air—it bends at the interface (where the two surfaces meet). This bending is called refraction. The amount, or degree, of refraction is related to the difference between the speeds of light in the two materials of different densities—the greater the difference in densities, the more the speed changes, and the greater the bend. A slanting object partly out of water displays refraction. The object appears to bend at the interface of the air and water. Lenses refract light. Those that have concave, or hollowed-out, surfaces spread light rays apart. Those that have convex, or bulging, surfaces bring light rays closer together.
For centuries before the 1600s, scientists had known that when a ray of white light shines on a prism, a broad band containing several colors emerges. Some thought that the colors were caused by variations in lightness and darkness. But in 1672 Isaac Newton published the results of his experiments with light. He showed that a second prism placed in the path of a beam of one color could not add more color to the beam. It did, however, spread the beam farther apart. Newton concluded that the first prism broke white light down into its separate parts by spreading them apart, and he was able to establish that white light is not a pure color but a combination of all the colors in the spectrum.
A prism spreads white light into the spectrum because each color has a slightly different speed within the prism, so each color bends (refracts) a slightly different amount as it enters and again as it leaves the prism. Violet light slows up the most, so it is bent the most; red light slows up the least, so it is bent the least. This spreading apart of white light into a spectrum is called dispersion.
Physicists often define dispersion as the fact that different colors move at different speeds within a substance, not necessarily causing a spectrum. For example, when white light enters a glass block that has parallel faces, the colors all have different speeds and bend different amounts as they travel through the glass. This is also dispersion. But the colors all bend back to form white light as they leave the second parallel face, so separate colors are not observed.
Opaque materials absorb all the colors of white light except their own, which they reflect. A piece of pure red material absorbs orange, yellow, green, blue, and violet but reflects red. Transparent colored materials absorb all colors except their own, which they both transmit and reflect. A piece of pure blue cellophane absorbs red, orange, yellow, green, and violet but transmits blue (it looks blue on the side opposite the light source) and reflects blue (it looks blue on the same side as the light source).

MEASURING THE SPEED OF LIGHT

Light can travel through a vacuum. Stars are easily visible on clear nights, though their light must travel for years through empty space before it reaches Earth. A laboratory experiment demonstrates that light can travel through a vacuum. When air is pumped out of a glass vacuum chamber that contains a ringing bell, the bell remains visible while the sound fades away. The vacuum cannot transmit sound waves, but the light rays continue to pass through it.
It is much easier to describe the interaction of light with matter than to explain what light is. One reason for this is that light cannot be seen until it interacts with matter—a beam of light is invisible unless it strikes an eye or unless there are particles that reflect parts of the beam to an eye. Also, light travels very fast—so fast that for centuries men disputed whether it required any time for light to move from one point to another.
Galileo suggested one of the first experiments to measure the speed of light, and Italian scientists carried out his idea. Two men were stationed on two hilltops. Each had a shaded lantern. The first man was to uncover his lantern. As soon as the second man saw the light, he was to uncover his lantern. The scientists tried to measure the time that elapsed between the moment the first lantern was uncovered and the moment a return beam was detected. The speed of light was much too fast to be measured in this way, and the scientists therefore concluded that light might well travel instantaneously.
Olaus Roemer, a Danish astronomer, was dealing with a different problem when he came across the first workable method for measuring the speed of light. He was timing the eclipses of Jupiter's moons and noticed that the time between eclipses varied by several minutes. As the Earth approached Jupiter, the time between eclipses grew shorter. As the Earth receded from Jupiter, the time between eclipses grew longer. In 1676 Roemer proposed that these discrepancies be used to calculate the time required for light to travel the diameter of the Earth's orbit. Since the exact size of the Earth's orbit was not yet known, and since Jupiter's irregular surface caused errors in timing the eclipses, he did not arrive at an accurate value for the speed of light. But he had demonstrated that light took time to travel and that its speed was too quick to measure on Earth with the instruments then available.
In 1849 Armand Fizeau, a French physicist, devised a way to measure the speed of light on Earth instead of relying on uncertain astronomical measurements. His experimental apparatus included a beam of light that was sent through a notch in a rotating disk, was reflected from a mirror, and returned to the disk. The disk had 720 notches. When the returning light passed through a notch, an observer could detect it; if it hit between notches, the light was eclipsed. The distance light would travel (from the open notch to the mirror and back to the point where a tooth could eclipse the light) was measured. Fizeau timed the eclipses and observed the rotational speed of the disk at the time of the eclipses. With this information he calculated that the speed of light in air was 194,000 miles per second. Later investigators refined this method. Jean Foucault replaced the disk with rotating mirrors and arrived at a value of 186,000 miles per second.
One of the most surprising and confusing facts about light was discovered by Albert Michelson and Edward Morley. They measured the speed of light very accurately as it traveled both in the same direction as the Earth's movement and in the direction opposite to the Earth's movement. They expected to get slightly different values, believing that the speed of the Earth would be added to or subtracted from the speed of light. The situation, as they saw it, was similar to that of a person looking out the window of a car traveling at 60 miles per hour. If another car going 80 miles per hour overtakes it, the second car then seems to be moving at a speed of 20 miles per hour, or its own speed minus the speed of the car it has passed. If a car going 80 miles per hour approaches a car going 60 miles per hour, it seems to be traveling at 140 miles per hour, or its own speed plus the speed of the car that it is approaching. Light, the two men discovered, does not behave that way. Its speed appears to be the same, no matter what the speed or direction of movement of the observer making the measurement. Albert Einstein developed his theory of relativity to help explain this phenomenon .
The accepted value for the speed of light in a vacuum is 2.997924562 × 10⁸ meters per second (about 186,282 miles per second), a fundamental constant of the universe. According to the theory of relativity, time and distance may change as the speed of an object approaches the speed of light (its length shrinks and any changes it regularly undergoes take longer to occur, relative to a stationary observer), but the measured value for the speed of light is constant.

LIGHT—WAVE OR PARTICLE?

By the 17th century enough was known about the behavior of light for two conflicting theories of its structure to emerge. One theory held that a light ray was made up of a stream of tiny particles. The other regarded light as a wave. Both of these views have been incorporated into the modern theory of light.
Newton thought that light was composed of tiny particles given off by light sources. He believed that the different colors into which white light could be broken up were formed by particles of different sizes. He thought refraction resulted from the stronger attraction of the denser of two substances for the particles of light. Since the attraction was greater, the speed of light in denser mediums should also be greater, according to his theory. A basic piece of evidence supporting the particle view of light is that light travels in straight lines. This can be seen when a small, steady light source shines on a relatively large object. The shadow of the object has sharp borders. Newton felt that if light were a wave, it would curve slightly around obstacles, giving fuzzy-edged shadows. He pointed out that water waves curve as they pass an obstacle (for example, dock pilings) and that sound waves curve over hills and around the corners of buildings. Newton realized, however, that simple variations in the size of particles did not explain all light phenomena. When he tried to understand the shimmering coloration of soap bubbles, he had to introduce the idea that the particles vibrated.
Christiaan Huygens, a Dutch physicist, proposed that light was a wave. He postulated that a substance called the ether (not to be confused with the class of chemicals called ethers) filled the universe. Waves were generated in this substance when light traveled through it. Huygens assumed that light waves were like sound waves—the movement of alternately compressed and rarefied ether. Such waves are called longitudinal waves because the vibration of the wave is parallel to the direction in which it is traveling.

Polarized Light

Neither particle nor wave theory could really explain the polarization of light by certain transparent crystals. Both Newton and Huygens knew that when light was directed through certain crystals, it would emerge much dimmer. If a second crystal of this class were placed at a certain angle in the path of the dimmed light, the light could pass through it. Then, as either of the two crystals was slowly turned, the light emerging from the second crystal grew dimmer until it was completely blocked. Evidently, something in the structure of the first crystal allowed only part of the light to pass. When the second crystal was lined up properly with the first, it allowed the same amount of light to pass; when it was at the wrong angle to the first crystal, it screened out the light from the first crystal.
Newton speculated that polarization occurred because light particles had various shapes on their sides, some of which were rejected by the crystal structure. This was not a very satisfactory explanation. However, Huygens had to make even more complicated assumptions to explain how crystals could polarize longitudinal waves. Neither the wave theory nor the particle theory was sufficiently developed to account for all the observed light phenomena, but the weight of Newton's reputation caused the particle theory to be accepted by most scientists.

Light Bends Around Corners

In the early 19th century Thomas Young, a British physician, took the next step in developing the wave theory. He demonstrated that light waves were so short that the amount they curved as they passed an object was too small to be visible. He showed that, though shadows from point sources of light appear to have sharp edges, there are thin light-and-dark bands along their borders that are caused by the bending of some light rays into the shadow. This scattering of light, called diffraction, can be observed under certain conditions. A thin tubular source, such as a fluorescent light, is good. A very thin slit in an opaque material, or even two fingers squeezed loosely together so that light may pass between them, may cause diffraction. The slit is held a foot or two in front of one eye, parallel to the light source; the other eye is closed. Light shines through the slit, and a pattern of colored bands, or a colored glow, can be seen outlining the slit. The outline is colored because diffraction disperses white light into its separate colors in much the same way that a prism does. Young observed diffraction and concluded that it occurred because light was a wave.
Three important measurements describe a wave—speed, frequency, and wavelength. Frequency is a measure of the number of waves that pass a given point in a specified amount of time. Wavelength is the distance from one crest (the highest point) to the next crest, or from one trough (the lowest point) to the next. If all the waves have the same speed, a great many short waves will pass a point in the same time that only a few long waves pass it. Speed equals the wavelength times the frequency.
Young set up an experiment to measure the wavelength of light; using the principle of interference. When two sets of waves meet, they interfere with each other in a predictable way. Water waves, such as those made by the wakes of two boats, illustrate this. When two wakes meet, the water becomes choppy. Parts of the waves are very high and parts are very low; the individual waves can reinforce each other or cancel each other. Where two crests meet, the wave becomes higher. Where two troughs meet, the wave becomes deeper. And if a crest and a trough meet, they cancel each other and the water is level.
In his interference experiment Young used a single light source, a pinhole that admitted a single beam of sunlight. This beam fell on a screen that had two pinholes close together. As light passed through each pinhole, it curved and spread out (diffracted). Because the pinholes were close enough together, the two light beams met and interfered with each other. Their interference pattern was seen on a screen behind the pinholes. With this pattern and knowing the distance between the screens, Young was able to calculate that the wavelength of visible light was about one millionth of a meter.
Subsequent measurements show that the wavelengths of visible light range from 7.60 × 10^–5 centimeters to 3.85 × 10^–5 centimeters (2.99 × 10^–5 inches to 1.51 × 10^–5 inches). Each color is associated with a range of wavelengths. Red has the longest lengths. The wavelengths decrease from orange through yellow, green, blue, and violet.

Transverse Waves Explain Polarization

Young and Augustin Jean Fresnel, a French physicist, cooperated in developing the idea that light waves are transverse, that they resemble the waves made when a rope stretched from a post is jerked up and down rather than longitudinal sound waves. The rope itself moves only up and down, at right angles to the forward travel of the wave. Young and Fresnel suggested the wave motion of light might also be at right angles to the direction in which the wave was traveling. The motion could be in any direction between sideways and up-and-down just so long as it was at right angles to the direction of travel. Wave motion of this kind could explain polarization. If a polarizing crystal admitted only those waves that were vibrating in a certain direction, then a second crystal would block those waves if it were turned at an angle to the first. The second one would be oriented to accept only waves vibrating in a different direction, and the first crystal would have already blocked all those waves. Fresnel made calculations that accounted for all the light behavior he knew of by assuming that light was made up of transverse waves.
Measurements of the speed of light in substances other than air presented additional difficulties for Newton's particle theory of light. The theory had assumed that light travels faster in dense substances than in rarefied substances. Fizeau and Foucault measured the speed of light in various transparent substances and discovered that it was slower in denser materials than in air.

Invisible Light

Around 1800—while Young was developing his wave theory—three scientists discovered that the color spectrum was bordered by invisible rays. Sir William Herschel, a British astronomer, was measuring the temperature of the colors dispersed by a prism. As he moved the thermometer down the spectrum from violet to red, he observed a rise in temperature. As he moved the thermometer beyond the red beam, the temperature grew even higher. Herschel had discovered a hot, invisible radiation that appeared to be a continuation of the spectrum. This radiation is called infrared radiation because it occurs just below red in the spectrum, where there is no visible light.
Ultraviolet rays were discovered by Johann Wilhelm Ritter and by William Hyde Wollaston, who were independently studying the effects of light on silver chloride. Silver chloride placed in violet light grew dark. When the chemical was placed in the area beyond the violet of the spectrum, it darkened even more rapidly. They concluded that a chemically powerful kind of invisible radiation lay beyond the violet end of the spectrum.
In 1864 James Clerk Maxwell, a Scottish physicist, published a theory of electricity and magnetism. He had developed equations that predicted the existence of electromagnetic waves caused by electrical disturbances. He calculated the speed of such waves and found it to be the same as the speed of light. Maxwell concluded that light was an electromagnetic wave. As a single light wave travels through space, its movement consists of the growth and collapse of electrical and magnetic fields. The electrical fields are at right angles to the magnetic fields, and both are at right angles to the direction in which the wave is moving.
Maxwell's theory implied that other electromagnetic radiations with wavelengths longer than infrared or shorter than ultraviolet might be found. In 1887 Heinrich Hertz produced radio waves, which have longer wavelengths than infrared rays, thus confirming Maxwell's theory.

LIGHT: WAVE AND PARTICLE

In 1900 the German physicist Max Planck advanced a theory to account for the behavior of blackbodies. A blackbody is an ideal substance with a perfectly black surface that absorbs all the radiation that falls on it and emits radiation in specific ways dependent on temperature. While such an ideal material does not actually exist, some materials resemble it closely enough to provide experimental tests of blackbody theory. The observed behavior is that blackbodies do not emit all wavelengths in equal amounts. Instead, certain wavelengths are emitted more often than others. As the temperature increases, the wavelengths that are emitted preferentially decrease in length. In other words, the wavelength of maximum emission varies inversely with temperature. Planck explained this behavior by suggesting that matter can handle energy only in specific amounts, called quanta, and that amounts of energy between these quanta cannot be absorbed or emitted.
In 1905 Einstein expanded this idea in his explanation of the photoelectric effect. If light falls on certain metals, electrons in those metals are freed and can form an electric current. Einstein was trying to account for the observation that the energy of the electrons is independent of the amount of radiation falling on the metal. The maximum energy of the electrons was observed to depend on the wavelength of the radiation. Einstein suggested that the photoelectric effect could also be accounted for by assuming that electromagnetic energy, including light, always occurs in these bundles. This reintroduced the particle theory. The results of many subsequent experiments supported the idea that light energy travels in quanta. An individual light particle possesses one quantum of energy and is called a photon.
The way matter becomes a light source can be explained in terms of quantum theory. When certain elements are heated, they give off light of a specific color. This light can be separated into a spectrum that is made up of many distinct bright lines. Each element has its unique spectrum, which can be accurately measured. Since a spectrum positively identifies each element, the chemical composition of astronomical bodies is determined by an analysis of their spectra.
Scientists wondered why the atoms of each element, when provided with a wide range of energies by the heating process, give off only the specific energies in their spectra. The modern theory of atomic structure makes this phenomenon understandable. An atom is made up of a heavy, positively charged nucleus which is surrounded by light, negatively charged electrons.
Modern theory states that the electrons of an atom can assume certain fixed energy relationships, called energy levels, to one another and to the nucleus. These energy levels are the same for all the atoms of an element. An electron must occupy one of the energy levels; it cannot possess any energy between levels.
When an atom is heated, enough energy may be given to one of the electrons to raise it to a higher energy level. But it usually jumps back to a lower level, giving off an electromagnetic wave, which is the energy difference of the two levels. When this energy is in the visible light range, it shows up as one of the lines in the element's visible spectrum. Each element has a different spectrum because each element has a different number of electrons and different energy levels available to these electrons.
In the early 20th century atomic theory had not yet explained why both a wave theory and a particle theory were needed to describe light. Physicists used both, depending on which was more useful in a given situation. The paradox was finally resolved in 1924 by Louis de Broglie. He postulated that matter, which had always been treated as a collection of particles, had a wave aspect as well. This wave nature has been demonstrated in experiments with such particles as electrons.

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Dead Sea

Between Israel and Jordan lies the Dead Sea, a salt lake located on the lowest point of the Earth's surface. Its basin lies some 1,300 feet (400 meters) below sea level, making it the lowest body of water in the world. The lake is about 50 miles (80 kilometers) long and 11 miles (18 kilometers) wide. Its surface area is about 394 square miles (1,020 square kilometers).

The Dead Sea extends from north to south in a great depression between rocky cliffs. The depression is a rift valley, caused by the Earth's crust having slipped down between two parallel fractures. The valley is a part of the Great Rift Valley, which continues northward through the Jordan River valley and the Sea of Galilee, and southward through the Gulf of Aqaba and the Red Sea and across East Africa.
The Jordan River flows into the Dead Sea from the north, and four smaller streams feed the lake from the east. Many small, intermittent streams also flow into the lake. No rivers flow out of the Dead Sea.
Temperatures at the Dead Sea are very hot in summer and mild in winter. Situated in a desert, the lake seldom receives more than 3 inches (7.6 centimeters) of rain a year. Evaporation carries off about the top 55 inches (140 centimeters) of the lake's waters annually. This evaporation often results in a thick mist that hovers over the lake.
Evaporation also helps to concentrate salt and other minerals in the lake. The Dead Sea is the world's saltiest natural lake. Its near-surface waters are more than eight times as saline as the ocean, and the lake's salt concentration increases with depth. The extreme salinity allows human bathers to float easily, but it prevents all living things except bacteria from inhabiting the lake. Several minerals, including salt, potash, bromides, and bitumen, or native asphalt, are commercially extracted from its shores.
The name Dead Sea can be traced back to at least the first century BC. In the Hebrew Bible the lake was variously called the Salt Sea, the Sea of the Plain, and the East Sea. The cities of Sodom and Gomorrah, whose destruction is described in the biblical book of Genesis, were located on its shores. The biblical manuscripts known as the Dead Sea Scrolls were found on the northwest shore, near the ruins known as Khirbat Qumran

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Fungus

Fungi (singular, fungus) are everywhere in the environment—in the soil; in lakes, rivers, and the seas; in the air (some are so tiny that they are carried by currents of wind or on the bodies of insects); and in and on plants and animals, including humans. Along with bacteria, fungi are responsible for the decay of organic matter and the release into the atmosphere of carbon, oxygen, nitrogen, and phosphorus. Many of them are eaten at the dinner table.

One of the most beneficial uses of a fungus came with the virtually accidental discovery of the antibiotic penicillin by Sir Alexander Fleming in 1928. Antibiotics, many of them derived from fungi, helped revolutionize the practice of medicine in the 20th century Not all fungi are beneficial, however. Some can cause serious diseases in plants and wreak havoc on whole segments of an agricultural economy. One of the best-known instances of fungus devastation in the 20th century was the destruction of elm trees in Europe and the United States by Dutch elm disease. The fungus responsible, Ceratocystis ulmi, probably arrived in Europe from Asia about the time of World War I. By the 1930s it had spread throughout Europe and Great Britain and killed thousands of trees. It appeared in the United States in 1930 and has since destroyed millions of elm trees. Overland spread of the disease normally occurs through transmission by elm bark beetles.
The word fungus (plural, fungi) is Latin for mushroom, and indeed, mushrooms are among the most commonly known fungi. For many years, most people—including scientists—considered fungi to be plants, mainly because, like plants, fungi do not move. Fungi also lack such complex plant structures as roots, stems, leaves, and flowers, though these traits are shared by some of the more primitive plants, such as mosses and liverworts. However, fungi lack some of the most important characteristics of plants. For example, fungi do not have chlorophyll and thus cannot undergo photosynthesis, which is a key trait all plants have in common. This factor, along with several other characteristics, led scientists to place the fungi into their own kingdom. Included within the kingdom Fungi, along with mushrooms, are molds, mildews, rusts, smuts, truffles, and yeasts. A general scientific term for fungi is mycota, from the Greek word for mushroom, mykes, and the study of these organisms is called mycology. Scientists estimate there are probably 1.5 million species of fungus worldwide, though only about 70,000 species have been described.

The Structure of Fungi

Although fungi are not uniform in appearance—a mushroom, for example, has a cap and stem while common bread mold grows in a thick mat—all fungi have similar structural elements. In most fungal species, the organism's cells are joined in long strands, or filaments, which are called hyphae (singular, hypha). The system of hyphae produced by an individual fungus may be extensive, with the hyphae accumulating in thick mats. The latter are called mycelia (singular, mycelium). What we see as mold on a piece of fruit or bread is actually the mycelium of the fungus that has colonized the food. The mycelium is considered the thallus, or body, of the fungus.
The yeasts are an exception to this, however. Although they are members of the fungi kingdom, yeasts do not share the same structural elements as most other fungi. Yeast cells do not adjoin one another to form hyphae but rather exist as individual unicellular organisms. Because of this difference, the yeasts are classified in a separate taxon, or group, within the fungi kingdom, while the remaining taxa are sometimes referred to as the “true” fungi.
In order to grow, the fungal mycelium uses the organic matter, either living or dead, in its environment. As the mycelium matures, it forms spores. These are seedlike reproductive bodies, each normally consisting of one cell, that become detached from the parent fungus and start new organisms. As the spore grows, it develops into a hypha that branches out and eventually forms the mycelium of a new fungus. In some fungi the spores may be produced directly by any portion of the mycelium; in others, such as the mushroom, they are formed in a special fruiting section, such as the mushroom cap. This section, normally the only visible or most visible section of the fungus, is called the sporophore.

Where Fungi Live

Fungi are very widely distributed throughout the world, particularly in the temperate and tropical regions where there is sufficient moisture for them to grow. They are less likely to be found in dry areas. Some few types of fungi have been reported in Arctic and Antarctic areas (some molds, after all, thrive on refrigerated food).
Fungi live both on land and in the water. Only a small portion of terrestrial fungi is normally seen above the ground. Most of the fungus consists of the complex network of hyphae, which grows just beneath the surface of the ground. The visible parts of fungi vary greatly in size. Some are so tiny that they cannot be seen without the aid of magnification. Others are quite large. Some mushrooms reach diameters of 8 to 10 inches (20 to 25 centimeters) and heights of 10 to 12 inches (25 to 30 centimeters). Bracket fungi that are 15 inches (38 centimeters) in diameter are fairly common; and mushrooms called puffballs have been known to grow to 60 inches (152 centimeters) in diameter. In 1992 scientists announced the discovery of a giant underground fungus, Armillaria ostoyae, that covered 1,500 acres (600 hectares) in Washington State. The only visible signs of its existence were aboveground mushrooms and a rot deadly to trees. In 2000 a larger Armillaria was found. This individual, which was discovered in the Blue Mountains of Oregon, covers 2,200 acres—about the size of 1,665 football fields. Experts estimate that it is at least 2,400 years old, making it not only the largest but also the oldest living organism on Earth.
Although fungi are distributed worldwide, the distribution of a specific species is limited by the temperature and moisture conditions of an area coupled with the available food supply. The best temperature for most fungi to thrive is from 68° to 86° F (20° to 30° C). Some types of fungi, however, do perfectly well at temperatures as high as 120° F (48° C), while a fairly large number of them do well at freezing temperatures, 32° F (0° C) or below.

How Fungi Reproduce

The reproduction of fungi can be either sexual or asexual. Sexual reproduction, as with other organisms, involves the fusion of two nuclei when two sex cells unite. This joining produces spores that can grow into new organisms. However, the majority of fungi reproduce asexually. The simplest asexual process is direct fragmentation, or breaking up, of the fungus body, or thallus. Each of the fragments develops into a new individual organism if environmental conditions are favorable. Such fragmentation usually is the result of outside natural forces. Some yeasts reproduce by simple cell division, wherein a yeast cell divides into two new yeast cells. Other yeast species reproduce by a method called budding: a bud develops on the surface of the yeast cell, after which the nucleus of the cell divides into two. After one of the nuclei moves into the bud, it is capable of starting a life of its own. Some species of the true fungi also reproduce via this method.

How Fungi Obtain Food

Because they lack chlorophyll, fungi are unable to manufacture food out of the raw materials around them as plants do. Thus fungi are categorized as heterotrophic—they must get nutrition from other organisms. Some fungal species get their food from living organisms, a process that may harm the host or benefit it. The vast majority of fungi obtain their nutrients from dead plant or animal matter. By doing this, fungi are among the organisms that serve as decomposers—an essential role in the natural cycle of ecosystems
To obtain nutrients, a fungus secretes enzymes into the living or dead organism on which it grows. The enzymes digest the material, which is then absorbed through the walls of the hyphae. A common example of this action is the rotting of fruits such as peaches or apples. The brown, softened area on the fruit has been subjected to the enzyme secretions of the hyphae. Some fungi, more specialized in their food-absorbing techniques, produce tiny hyphae called rhizoids. These rootlike structures anchor the fungus to its food source and probably also absorb food. Other fungi produce absorptive structures called haustoria, another type of hypha outgrowth.

Saprophytic Fungi

Because they attack only dead organic matter, saprophytes are in good measure responsible for the decomposition of much plant and animal residue in natural ecosystems. In a forest, saprophytic fungi may be found growing on matter as varied as fallen trees, animal droppings, and dead leaves. These fungi play a vital role in the natural community—without them, the forest floor would accumulate enormous piles of dead matter.
In addition to the saprophytes living in nature, there are many such species that strike much closer to home, including those that attack foodstuffs such as bread, processed meat and cheese, and picked fruits and vegetables. Some saprophytes are responsible for the destruction of timber, textiles, paper, and leather. Most saprophytes need oxygen in order to survive and feed. Some few, such as those that cause fermentation, can survive without it.

Parasitic Fungi

Fungi classified as parasites attack living organisms in order to obtain nutrients, and in doing so cause illness or death to the organism being attacked . The fungi in this group are a leading cause of disease in plants. Some of the most common include the downy mildew found on grapes, onions, and tobacco, and the powdery mildew that infests grapes, apples, cherries, lilacs, peaches, and roses. Cereal grains such as corn and wheat are often plagued with smut, while rust can devastate crops from wheat, oats, and beans to asparagus and flowers.
A number of parasitic fungi cause diseases in animals, including humans. Some of these illnesses, such as ringworm and athlete's foot, are fairly benign and self-limiting. A number of fungal organisms, such as Aspergillis and the yeast Candida, cause illnesses ranging from mild to severe depending upon what organ or part of the body is infected. For example, Candida can cause the mild infection commonly known as thrush when it colonizes tongue and throat tissue. If the same organism gets into the bloodstream, however, it can be carried to the joints or to vital organs such as the liver or spleen, where the resulting illness can be severe and even be fatal in some circumstances. Among the most dreaded fungal diseases in humans and animals are the systemic mycoses. These diseases are caused by fungi that initially colonize the lungs but can soon spread to other organs. The organisms responsible for this type of illness are Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, and Cryptococcus neoformans.
One serious fungus-caused disease that may attack people and animals is ergotism. The fungus ergot develops on grasses, especially on rye. It contains a number of poisons called alkaloids. If the grain is harvested and the ergot is not removed, it will get into bread made from the rye and cause ergotism—also known as St. Anthony's fire—for which there is no known cure. The disease may infect cattle that eat the rye grains left in a field. Ergot also contains lysergic acid, the principle active agent in the drug LSD (lysergic acid diethylamide). The fungus does have some positive uses, however. It has been used to develop medicines that induce labor in pregnant women and curtail hemorrhaging after birth.

Mycorrhizal Fungi

The mycorrhizal fungi form a close, mutually beneficial relationship with certain trees and plants. The hyphae of these fungi grow in networks surrounding the plant and tree roots, allowing the plant to extract certain minerals and other nutrients from the fungus. The fungus in turn obtains moisture and carbohydrates from the tree. Although these tree and plant species can survive without the association, studies have shown that the presence of mycorrhizae helps the plants to grow better relative to similar plants without the benefit of a mycorrhizal partner. Associations such as these, where both partners benefit from the relationship, are called mutualisms

Other Plant and Animal Associations

There are several other examples of symbioses between fungi and plants or animals. Lichens, for example, are combinations of fungi and algae living in such close association that they seem to be a single plant form .
Certain scale insects embed themselves in the bark of trees and remain there sucking sap from the tree for the rest of their lives. A type of fungus will spread itself in a network over the bark of the tree, covering the insects and feeding off them, without killing them. This is a case of double parasitism: the insects live off the tree and the fungus off the insects, both to the disadvantage of the tree.
Types of fungi called sooty molds live on the surfaces of plants in association with scale insects. They do not live as parasites on either the insects or the plants, but they obtain nourishment from the secretions of the insects. The extensive growth of the mold's mycelium, however, may prevent light from reaching the plant's surface and thus cause it to die because photosynthesis is inhibited.
An interesting kind of reverse parasitism occurs in certain ant colonies. The so-called leaf-cutter ants, which are members of the genera Acromyrmex and Atta, cultivate fungi in their underground colonies by feeding the organisms tiny bits of leaves. The ants themselves feed entirely on the fungi.

Some Familiar Fungi

A casual walk through a forest is sufficient to bring one into contact with many familiar fungi, such as varieties of mushrooms and the familiar bracket fungus that grows on wood. Other types of fungi are brought to one's attention largely in association with food: the mold that covers stale bread, the yeast that is used for baking or brewing, and the mushrooms and truffles that are available in supermarkets or offered as delicacies in many restaurants.

Molds

The fuzzy substance we see growing on old bread or fruit is mold, a mass of mycelia sprouting fruiting bodies that are often not immediately visible to the naked eye. The molds we see on dead organic matter such as old food are saprophytic. Many of these molds belong to the genera Aspergillus, Penicillium, and Rhizopus.

Mushrooms

This group of fungi are conspicuous because of the umbrella-shaped fruiting body that grows above the ground. Many mushrooms are edible but some are not. Some people use the term toadstool for poisonous mushrooms, but botanists make no such distinction. Among the mushrooms are the puffballs and earthstars, which grow in soil or on rotting wood in forests and grassy areas. Many of these are edible while young. When they mature, they dry out and become powdery inside. The largest of the puffballs, Calavatia gigantea, may be as large as 4 feet (120 centimeters) or more across. The earthstars are so named because, in addition to the puffball effect, they have a leaflike expanded base that resembles a star. Since some mushrooms are poisonous, only an expert in mushroom identification should collect mushrooms that are intended for people or animals to eat.

Truffles

For centuries this fungus, which grows underground, has been prized as a food delicacy. Truffles are saprophytes and grow in association with the roots of trees, particularly oaks. They range in size from the size of a pea to as large as an orange. Three countries are famed for their truffles: France, Italy, and England. Mature French truffles are black with white veins; Italian truffles are white; and English truffles are either black or brown, depending on the species. Because truffles grow underground, they are not always easy to locate, so hunting them is usually carried out with the aid of pigs or dogs, both of which have a more developed sense of smell than humans.

Yeasts

The yeasts are a group of unicellular fungi. Some yeasts are commercially significant because they are used in baking, brewing, and fermentation. Yeast does its work primarily by interacting with the carbohydrates (sugar and starches) in either dough for bread or liquid for brewing and fermentation Brewers' yeast has long been considered nutritionally useful to humans because it contains a high quantity of B vitamins. Some yeasts cause decay in fruits and vegetables, both of which are a ready source of carbohydrate which the yeast can metabolize via fermentation. Because of their chromosome structure and the ease with which the organisms can be maintained in the laboratory, yeasts have also served as an important tool in genetic and medical research

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Mars

As it is visible from Earth without the aid of a telescope, the planet Mars has intrigued stargazers since ancient times. Babylonians mentioned Mars in records from about 3,000 years ago, associating the red planet with their god of death. The modern name Mars is that of the Roman god of war; the planet's two moons, Phobos (Fear) and Deimos (Terror), are named after the sons of Ares, the Greek god of war.

Mars passes relatively close to the Earth in its orbit. In addition, it generally has no obscuring layer of clouds, unlike the Earth's other neighbor, Venus. Thus Mars is a nearly ideal subject for telescopic observation. Over the centuries observers have noted various phenomena on the planet's surface, including a seasonal growing and shrinking of the polar caps and seasonal changes in the appearance of dark markings. The explanation of many early observations had to await the exploratory space missions by the United States and Soviet Union during the 1960s and 1970s.

Planetary Data

The fourth planet from the sun, Mars is the outermost of the terrestrial, or Earth-like, planets, which are the dense, rocky worlds closest to the sun. Because Mars has an elliptical orbit, its distance from the sun varies, from about 129 million miles (207 million kilometers) at the closest point in its orbit, or perihelion, to some 155 million miles (249 million kilometers) at its farthest point, or aphelion. The planet completes one revolution around the sun in about 687 Earth days, almost twice the time it takes the Earth to complete its orbit of about 365 days. Its distance from the Earth varies considerably, from less than 35 million miles (56 million kilometers) to nearly 250 million miles (400 million kilometers). The best time to view Mars from Earth is when it is at its closest to both the sun and the Earth so that it appears both bright and large.
With a mean diameter of about 4,219 miles (6,790 kilometers), Mars is the third smallest planet in the solar system. It is about half the size of Earth and is much less dense. Its gravity is about a third of the Earth's, its surface area about a fourth, and its mass only about a 10th. Like the Earth, Mars is roughly spherical, with a slight bulging at its equator and flattening at its poles.
Mars rotates on its axis at roughly the same rate as Earth; a Martian day, called a sol, lasts about 24.7 hours. The red planet is also tilted on its rotational axis at an angle similar to that of the Earth. Consequently, like Earth, it is subject to seasonal variations in climate as first one hemisphere and then the other receives more sunlight during the planet's orbit around the sun. Because of its more elliptical orbit, the seasons on Mars are not as even as they are on Earth. The spring and summer in the north, for example, last about 382 days, or more than half the 687-day year. In the south the summer is shorter.

Atmosphere, Surface, and Interior

The Martian atmosphere is composed mostly of carbon dioxide and is very thin, exerting about ¹/₁₀₀ the surface pressure that the Earth's atmosphere exerts. The thin atmosphere on Mars does not insulate the planet as well as the thicker one does on Earth. The surface of Mars is thus colder than Earth's would be if the two planets were the same distance from the sun. The temperature at the Martian surface varies widely during the course of a day, from about −118 ° F (−83 ° C) just before dawn to about −28 ° F (−33 ° C) in the afternoon. The atmosphere also does not shield the surface from ultraviolet radiation from the sun. This intense radiation bombardment is one reason why scientists believe that no living things currently exist on the surface of Mars.The Martian surface is dry and dusty. The question of whether liquid water has existed on the surface of Mars is of particular interest to scientists trying to determine if life ever existed on the planet. Liquid water is a requirement for all known forms of life. (But of course water does not in itself indicate the presence of living things.) Water currently exists on Mars as ice deposited at the poles, as ice trapped below the surface, and as vapor in the atmosphere. Rivers, lakes, and seas may have been present on Mars in its remote past, when temperatures may have been warmer and the atmospheric pressure higher. Because of the current low temperatures and pressure, it has been thought that liquid water has not existed at or near the surface in modern times. Recent findings, however, have prompted a reassessment of this view. Photographs taken in 2000 by the orbiting spacecraft Mars Global Surveyor showed hundreds of gullies that seemed to have formed relatively recently. Some planetary scientists believe that the gullies were carved by flowing water. They theorize that, episodically, small amounts of liquid water have flowed on and just below the surface in geologically recent times and that liquid water may still be present in parts of the planet's subsurface. But this theory has been disputed.

 Also challenged by recent findings was the view of Mars as a geologically “dead” planet on which volcanic activity had last occurred a few billion years ago. Data from the Mars Global Surveyor indicates that volcanoes may have erupted some 40 million years ago, which is considered recent in geologic time. The planet has the largest volcano in the solar system, Olympus Mons. At a height of 17 miles (27 kilometers), the volcano is three times higher than Earth's Mount Everest and covers an area the size of the state of Arizona. It sits on the Tharsis Plateau, a broad, elevated plain dotted with large volcanoes and fractures. The largest fracture system is Valles Marineris, a huge valley about 2,500 miles (4,000 kilometers) long and varying from 2¹/₂ to 6 miles (4 to 10 kilometers) in depth. The Tharsis Plateau may have been formed by a rising plume of hot mantle material, possibly accompanied by plate tectonic activity.

Other regions on Mars include smooth plains, densely cratered areas, mesas, and rolling hills formed by various combinations of fracturing, volcanism, and atmospheric-related erosion and deposition. The planet's hemispheres show striking geologic differences, with ancient, heavily cratered highlands in the south and younger, flat lowlands in the north.
Dark markings have been observed on the Martian surface for hundreds of years. These markings cover about a third of the surface and change in a seasonal pattern in both extent and color. Once thought to be vast seas or vegetation, the dark areas are now known to result from the accumulation of dust, which shifts along with seasonal winds.
Dust storms occur frequently on the planet, especially in the southern hemisphere in spring and summer. About every two or three years, Mars is engulfed by global dust storms. Local temperature differences generate strong winds that lift the dust from the surface to form thick clouds. The clouds block the sunlight, gradually causing the surface temperatures to even out and the winds to subside. Some of the atmospheric dust is deposited in a snowfall of dust and ice in the polar regions.
Ice caps form over both the north and south poles according to seasonal changes. Each cap grows larger when its hemisphere experiences fall and shrinks during the spring. During summer, most of the ice cap melts. The south pole's ice cap is larger. Its permanent ice cap, the portion that survives the summer melting, is covered with mesas, holes, and troughs and appears to be formed of carbon dioxide ice. Smaller pits and cracks mark the flatter permanent cap at the north pole, which is made of water ice.Scientists do not have direct information about the Martian interior, but they have developed a model based on the planet's known characteristics, such as its size, mass, gravity signature, and surface elevations. Mars most likely has a metal-rich core with a diameter of about 2,100 miles (3,400 kilometers). Surrounding the core is a molten rocky mantle that is probably more dense than the Earth's mantle. The thickness of the Martian crust is thought to vary from about only 2 miles (3 kilometers) in some places to more than 60 miles (90 kilometers) in others.
Unlike most of the other planets (except for Venus and maybe Pluto), Mars has no global magnetic field, though there are indications that the planet once had a strong field. Ancient rocks in the crust of the southern hemisphere are highly magnetized in striped patterns that suggest that the planet's magnetic field may have reversed polarity several times, as has the Earth's. The rocks are about 10 times as magnetized as any on Earth.

Satellites

Mars has two small satellites, Phobos and Deimos, which may be captured asteroids. Both are so small that they do not have enough internal gravity to draw them into spherical shapes; instead, they are shaped more or less like potatoes. Phobos is about 17 miles (27 kilometers) long; Deimos is about 9¹/₂ miles (15 kilometers) long. Both have rotational periods equal to their orbital periods, so that they always point the same face toward Mars. The surface of Deimos appears smooth because its craters are almost buried in regolith, a layer of fine rubble generated by repeated impacts with other bodies. Phobos is also covered with regolith, but its surface is far more rugged and very heavily cratered.Phobos is very close to Mars, and its orbit is gradually decaying, so that it is drawing closer to the planet with each orbit. Astronomers estimate that Phobos may fall to the Martian surface sometime in the next 100 million years. Deimos is in a more distant orbit and is gradually moving away from the planet.
Both satellites are very dark and are probably made of a carbonaceous chondrite material. This is a primitive substance that includes many of the first materials to precipitate out of the solar nebula during the creation of the solar system. It is found on many satellites, asteroids, and meteorites.

Observation and Exploration

For centuries astronomers have considered the possibility that life might exist on Mars, the most Earth-like of the planets. In the 1600s astronomers began to observe the planet with the aid of newly developed telescopes. In 1877 the Italian astronomer Giovanni Schiaparelli described what he believed was a system of interconnecting, straight-edged channels on the planet. He called these features canali, meaning “channels,” or “canals.” U.S. astronomer Percival Lowell thought that these features were not naturally occurring waterways but structures that had been built by an advanced but dying Martian civilization. Most astronomers could see no canals, however, and many doubted their reality. The controversy was finally resolved only when pictures sent from the United States Mariner probes showed many craters but nothing resembling manufactured channels or canals.Four of the Mariner series of unmanned space probes launched by the National Aeronautics and Space Administration (NASA) investigated Mars. The first craft to successfully fly by Mars was Mariner 4, which photographed the planet as it passed it in 1965. It was followed by Mariners 6 and 7, which analyzed the atmosphere and captured images as they flew by in 1969. The first spacecraft to orbit a planet other than Earth was Mariner 9, which circled Mars for nearly a year in 1971–72. The Soviet Union also sent a series of unmanned space probes to Mars in the 1960s and 1970s. Its Mars 3 lander, the first craft to successfully soft-land on the planet, touched down in 1971 amid a global dust storm. Its communications systems failed after about 20 seconds.
The United States Viking probes, consisting of two orbiting spacecraft and two landers, were intended in part to search for evidence of past or present forms of life on Mars. The two landers touched down on the planet in 1976 and performed numerous experiments, including a detailed chemical analysis of the Martian atmosphere and soil. No trace of any organic material was found.The next United States probe, the Mars Observer, was launched in September 1992 and was programmed to land on Mars in August 1993. The 980-million-dollar spacecraft lost contact with Earth, however, and was presumed lost in September 1993.
A team of scientists announced in 1996 that a meteorite from Mars that fell to Earth 13,000 years ago contained organic molecules, minerals, and carbonate globules that are all associated with bacterial life. The team believed that this provided the first evidence of life on early Mars. Most scientists, however, were skeptical of this claim.
In 1996 the United States launched the Mars Global Surveyor, the first in a new series of unmanned spacecraft designed to explore the planet. The probe began to orbit Mars in September 1997. After a delay due to an equipment malfunction, in March 1999 the craft began mapping a variety of the planet's properties, including its gravity and magnetic fields and the topography and mineral composition of the surface. It also took more than 100,000 photographs of the surface.
In 1997 the next Martian mission—the Pathfinder—landed on Mars to study the geology and atmosphere of the planet. On board the Mars Pathfinder was a small roving vehicle called Sojourner that collected and analyzed samples of the Martian soil and beamed images back to Earth. Evidence collected during Pathfinder's 83 days of surface operations indicated that the surface of Mars bears signs of an ancient Earth-like atmosphere and geology. Water-worn rock conglomerates and other sand and surface features that could only have been created by flowing, liquid water mark the surface of the planet. A tremendous amount of differentiation—heating, cooling, and recycling of crust material—appeared to have taken place at some point in the planet's history.Mars exploration suffered two major setbacks in the late 1990s. NASA's unmanned Mars Climate Orbiter, launched in December 1998, was designed to transmit daily weather images and other atmospheric data for a full Martian year, or 687 days. However, confusion of English and metric units in key navigation figures sent the craft off course as it attempted to enter orbit around Mars in September 1999. Less than three months later, NASA lost contact with another craft, the Mars Polar Lander, as it approached touchdown on the Mars surface.
Another global mapping orbiter, NASA's 2001 Mars Odyssey, reached the planet in October 2001. In addition to mapping the chemical composition of the surface, the orbiter confirmed the presence of water ice in the subsurface and revealed its distribution. Along with the Mars Global Surveyor, Odyssey also acted as a communications relay for two robotic wheeled rovers, Spirit and Opportunity. NASA's twin rovers landed on Mars in January 2004 to collect geologic data to help determine whether the planet's environment was suitable for life in the past. Spirit landed in Gusev Crater, which some scientists believe may have been an ancient lake bed. Opportunity arrived at Meridiani Planum. In the gravel at its landing site, Opportunity confirmed the presence of gray hematite, a mineral that on Earth usually forms in association with water.
The European Space Agency (ESA) sent its first mission to Mars in 2003. The orbiter Mars Express, designed to map a variety of properties in the Martian atmosphere, surface, and subsurface and to photograph surface features using a high-resolution stereoscopic camera, arrived at the planet in December 2003. The orbiter carried on board the British lander Beagle 2. After being released at Mars, however, the lander failed to return communications signals and was declared lost.

 

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Vitamin

All living things, plant or animal, need vitamins for health, growth, and reproduction. Yet vitamins are not a source of calories and do not contribute significantly to body mass. The plant or animal uses vitamins as tools in processes that regulate chemical activities in the organism and that use basic food elements—carbohydrates, fats, and proteins—to form tissues and to produce energy.

Vitamins can be used over and over, and only tiny amounts are needed to replace those that are lost. Nevertheless, most vitamins are essential in the diet because the body does not produce enough of them or, in many cases, does not produce them at all.
Thirteen different vitamins have been identified by nutritionists: A, eight B-complex vitamins, C, D, E, and K. Some substances, such as carnitine and choline, behave like vitamins but are made in adequate amounts in the human body.
Vitamins were originally placed in categories based on their function in the body and were given letter names. Later, as their chemical structures were revealed, they were also given chemical names. Today, both naming conventions are used.

Daily Requirements

With a few exceptions, the body is unable to make vitamins; they must be supplied in the daily diet or through supplements. One exception is vitamin D, which can be produced in the skin when the skin is exposed to sunlight. Another vitamin, vitamin K, is not made by the human body but is formed by microorganisms that normally flourish in the intestinal tract only when green, leafy vegetables and vegetable oils are ingested.
The body's vitamin requirements are expressed in terms of recommended dietary allowances, or RDA. These allowances are the amount of essential nutrients that, if acquired daily, are considered to be sufficient to meet the known nutritional needs of most healthy persons. In the United States, the RDA values are established by the Food and Nutrition Board of the National Academy of Sciences/National Research Council (NAS/NRC). In addition, two agencies of the United Nations—the Food and Agriculture Organization and the World Health Organization— develop RDA for different, worldwide population groups.
In the past, the strength of a vitamin or the amount of the vitamin necessary to produce a certain effect in the body was often expressed in terms of international units, abbreviated IU. The unit corresponds to a weight of the purified vitamin, and its value differs from one vitamin to another. Today, the strength of a vitamin is generally expressed directly in metric weights— micrograms or milligrams.

How Vitamins Work

In the body, proteins, carbohydrates, and fats combine with other substances to yield energy and build tissues. These chemical reactions are catalyzed, or accelerated, by enzymes produced from specific vitamins, and they take place in specific parts of the body.
The vitamins needed by humans are divided into two categories: water-soluble vitamins (the B vitamins and vitamin C) and fat-soluble vitamins (A, D, E, and K). The water-soluble vitamins are absorbed by the intestine and carried by the circulatory system to the specific tissues where they will be put into use. The B vitamins act as coenzymes, compounds that unite with a protein component called an apoenzyme to form an active enzyme The enzyme then acts as a catalyst in the chemical reactions that transfer energy from the basic food elements to the body. It is not known whether vitamin C acts as a coenzyme.
When a person takes in more water-soluble vitamins than are needed, small amounts are stored in body tissue, but most of the excess is excreted in urine. Because water-soluble vitamins are not stored in the body in appreciable amounts, a daily supply is essential to prevent depletion.
Fat-soluble vitamins seem to have highly specialized functions. The intestine absorbs fat-soluble vitamins, and the lymph system carries these vitamins to the different parts of the body. Fat-soluble vitamins are involved in maintaining the structure of cell membranes. It is also believed that fat-soluble vitamins are responsible for the synthesis of certain enzymes.
The body can store larger amounts of fat-soluble vitamins than of water-soluble vitamins. The liver provides the chief storage tissue for vitamins A and D, while vitamin E is stored in body fat and to a lesser extent in reproductive organs. Relatively little vitamin K is stored. Excessive intake of fat-soluble vitamins, particularly vitamins A and D, can lead to toxic levels in the body.
Many vitamins work together to regulate several processes within the body. A lack of vitamins or a diet that does not provide adequate amounts of certain vitamins can upset the body's internal balance or block one or more metabolic reactions.

Sources of Vitamins

Vitamins, though they are available from a variety of sources, are unevenly distributed in natural sources. For example, some vitamins, such as vitamin D, are produced only by animals, whereas other vitamins are found only in plants. (For natural sources of vitamins, see table.) All vitamins can be synthesized, or produced commercially, from foods and other sources, and there is no evidence that natural vitamins are superior to those that are synthetically derived.
Some foods are fortified with vitamins—that is, vitamins that are not normally present in the food, or that have been removed during processing, are added to the food before it is sold. Milk, for example, is fortified with vitamin D, and vitamins that have been lost from flour during processing are often replaced.
Although vitamin supplementation is generally unnecessary for otherwise well-nourished persons, there are times when the body's vitamin requirements may increase and when vitamin supplementation may be essential. Those likely to require such supplements include pregnant women, the elderly, and the chronically ill. Excessive intakes of supplemental vitamins should be avoided, however, because of the possibility of toxicity.

Kinds of Vitamins

Vitamin A,

also called retinol, is a fat-soluble vitamin that is readily destroyed upon exposure to heat, light, or air. The vitamin has a direct role in vision and is a component of a pigment present in the retina of the eye. It is essential for the proper functioning of most body organs and also affects the functioning of the immune system.
Vitamin A deficiency results in various disorders that most commonly involve the eye and the epithelial tissues—the skin and the mucous membranes lining the internal body surfaces. An early symptom of vitamin A deficiency is the development of night blindness, and continued deficiency eventually results in loss of sight. If deficiency is prolonged, the skin may become dry and rough. Vitamin A deficiency may also result in defective bone and teeth formation.
Excessive intake of vitamin A causes a toxic condition. The symptoms may include nausea, coarsening and loss of hair, drying and scaling of the skin, bone pain, fatigue, and drowsiness. There may also be blurred vision and headache in adults, and growth failure, enlargement of the liver, and nervous irritability in children.

Vitamin B complex

consists of several vitamins that are grouped together because of the loose similarities in their properties, distribution in natural sources, and physiological functions. All the B vitamins are soluble in water. Most of the B vitamins have been recognized as coenzymes, and they all appear to be essential in facilitating the metabolic processes of all forms of animal life. The complex includes B₁ (thiamine), B₂ (riboflavin), niacin (nicotinic acid), B₆ (a group of related pyridines), B₁₂ (cyanocobalamin), folic acid, pantothenic acid, and biotin.
Vitamin B₁, or thiamine, helps the body convert carbohydrates into energy and helps in the metabolism of proteins and fats. Vitamin B₁ deficiency affects the functioning of gastrointestinal, cardiovascular, and peripheral nervous systems. Beriberi and Wernicke-Korsakoff syndrome (often seen in alcoholics) are the primary diseases related to thiamine deficiency. General symptoms of beriberi include loss of appetite and overall lassitude, digestive irregularities, and a feeling of numbness and weakness in the limbs and extremities.
Vitamin B₂, or riboflavin, is required to complete several reactions in the energy cycle. Reddening of the lips with cracks at the corners of the mouth, inflammation of the tongue, and a greasy, scaly inflammation of the skin are common symptoms of deficiency.
Niacin, or nicotinic acid, helps the metabolism of carbohydrates. Prolonged deprivation leads to pellagra, a disease characterized by skin lesions, gastrointestinal disturbance, and nervous symptoms.
A form of Vitamin B₆ is a coenzyme for several enzyme systems involved in the metabolism of proteins, carbohydrates, and fats. No human disease has been found to be caused by a deficiency of this vitamin. Chronic use of large doses of vitamin B₆ can create dependency and cause complications in the peripheral nervous system.
Vitamin B₁₂, or cyanocobalamin, is a complex crystalline compound that functions in all cells, but especially in those of the gastrointestinal tract, the nervous system, and the bone marrow. It is known to aid in the development of red blood cells in higher animals. Deficiency most commonly results in pernicious anemia
Folic acid is necessary for the synthesis of nucleic acids and the formation of red blood cells. Folic-acid deficiency most commonly causes folic-acid-deficiency anemia. Symptoms include gastrointestinal problems, such as sore tongue, cracks at the corners of the mouth, diarrhea, and ulceration of the stomach and intestines. Large doses of folic acid can cause convulsions and other nervous-system problems.
Pantothenic acid promotes a large number of metabolic reactions essential for the growth and well- being of animals. Deficiency in experimental animals leads to growth failure, skin lesions, and graying of the hair. A dietary deficiency severe enough to lead to clear-cut disease has not been described in humans.
Biotin plays a role in metabolic processes that lead to the formation of fats and the utilization of carbon dioxide. Biotin deficiency results in anorexia, nausea, vomiting, inflammation of the tongue, pallor, depression, and dermatitis.

Vitamin C,

or ascorbic acid, is water-soluble and easily destroyed. It is essential in wound healing and in the formation of collagen, a protein important in the formation of healthy skin, tendons, bones, and supportive tissues. Deficiency results in defective collagen formation and is marked by joint pains, irritability, growth retardation, anemia, shortness of breath, and increased susceptibility to infection. Scurvy is the classic disease related to deficiency. Symptoms peculiar to infantile scurvy include swelling of the lower extremities, pain upon flexing them, and bone lesions. Excessive ascorbic-acid intake can cause kidney stones, gastrointestinal disturbances, and red-blood-cell destruction.

Vitamin D

is a fat-soluble compound essential for calcium metabolism in animals and therefore important for normal mineralization of bone and cartilage. The skin forms vitamin D when exposed to sunlight, but in some circumstances sunlight may lack sufficient amounts of ultraviolet rays to bring about adequate production of the vitamin.
Deficiencies cause many biochemical and physiological imbalances. If uncorrected, faulty mineralization of bones and teeth causes rickets in growing children and osteomalacia (progressive loss of calcium and phosphorus from the bones) in adults. Common early symptoms of rickets include restlessness, profuse sweating, lack of muscle tone in the limbs and abdomen, and delay in learning to sit, crawl, and walk. Rickets may produce such conditions as bowlegs and knock-knees. Deficiency may also cause osteoporosis, a bone condition characterized by an increased tendency of the bones to fracture. Large doses of vitamin D are toxic, and symptoms include weakness, loss of appetite, nausea, vomiting, diarrhea, excessive thirst, and weight loss.

Vitamin E

is a fat-soluble compound. The metabolic roles of this vitamin are poorly understood. Its primary role appears to be as an inhibitor of oxidation processes in body tissues. Deficiency is rare but may impair neuromuscular function. Although serious toxicity has not been attributed to large doses of vitamin E, adverse effects have been reported.

Vitamin K

is fat-soluble and essential for the synthesis of certain proteins necessary for the clotting of blood. Deficiency, though relatively uncommon, results in impaired clotting of the blood and internal bleeding.

Vitamin-like substances

include a number of compounds that resemble vitamins in their activity but are normally synthesized in the human body in adequate amounts. They are often classified with the B vitamins because of similarities in function and distribution in foods. Their status as essential nutrients remains uncertain. Choline is found in all living cells and plays a role in nerve function and various metabolic processes. Myoinositol is a water-soluble compound; its significance in human nutrition is not established. Para-aminobenzoic acid is an integral part of folic acid but its role in human nutrition has not been documented. Carnitine has an essential role in the transport of fatty substances. Lipoic acid seems to have a coenzyme function similar to that of thiamine; however, because it is synthesized in the human liver and kidneys, it is not considered a vitamin. Bioflavinoids are a group of substances that affect the permeability of capillaries but do not normally have to be added to human diets.

History

The value of certain foods in maintaining health was recognized long before the first vitamins were actually identified. In the 18th century, for example, it had been demonstrated that the addition of citrus fruits to the diet would prevent the development of scurvy. In the 19th century it was shown that substituting unpolished for polished rice in a rice-based diet would prevent the development of beriberi.
In 1906 the British biochemist Frederick Hopkins demonstrated that foods contained necessary “accessory factors” in addition to proteins, carbohydrates, fats, minerals, and water. In 1911 the Polish chemist Casimir Funk discovered that the anti-beriberi substance in unpolished rice was an amine (a type of nitrogen-containing compound), so Funk proposed that it be named vitamine—for “vital amine.” This term soon came to be applied to the accessory factors in general. It was later discovered that many vitamins do not contain amines at all. Because of its widespread use, Funk's term continued to be applied, but the final letter e was dropped.
In 1912 Hopkins and Funk advanced the vitamin hypothesis of deficiency, a theory that postulates that the absence of sufficient amounts of a particular vitamin in a system may lead to certain diseases. During the early 1900s, through experiments in which animals were deprived of certain types of foods, scientists succeeded in isolating and identifying the various vitamins recognized today.

 

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Water

Nearly three fourths of the Earth's surface is covered with water. Perhaps the most important liquid in the world, water is usually easy to get from rain, springs, wells, streams, rivers, ponds, and lakes. It fills the vast ocean beds. As vapor, water is also present in the air, where it often condenses into clouds. The bodies of most living things contain a large proportion of water. For example, water comprises about 60 percent of the weight of the human body.

Water is necessary for life. Millions of years ago the first forms of life on Earth grew in the sea. Although today many plants and animals are able to live on land, they still need water. This life-sustaining liquid makes up most of the animal blood or plant sap that nourishes living tissues.
Used but never used up, water constantly circulates throughout the world. A person taking a drink of water today may be drinking the same water that gave refreshment to a Stone Age man. Although water constantly replenishes the Earth, many areas almost entirely lack this prized liquid.
The location of water helps determine where man can settle. People who dwell in cities obtain their water from faucets. If city water stopped flowing, everyone would have to leave the city and seek water elsewhere. The growth of crops that man needs for his food depends upon water. One fully grown corn plant uses more than a gallon of water a day. It takes about 800,000 gallons of water to grow an acre of cotton. The climate of the Earth is affected by water  and through erosion and the scraping action of glaciers (rivers of ice), water changes the surface of the land
Water's physical properties make it vastly different from most other liquids. Water, for example, has the rare property of being lighter as a solid than as a liquid. If ice (solid water) were heavier than water, frozen water in a lake would sink to the bottom and pile up to the top, killing all the marine life. Water's ability to store great amounts of heat helps living things survive through wide changes in temperature. The amount of heat produced by a man during one day's activity would be enough to raise his body temperature by as much as 300°  F were it not for the water in his tissues.

Water in Daily Life

Human tissues require about 2¹/₂quarts of water a day. Most persons drink about a quart of water each day. The water content of foods supplies the rest. An egg, for example, is about 74 percent water; a watermelon, 92 percent; and a piece of lean meat, about 70 percent. Beverages such as milk, coffee, tea, and soft drinks are mainly water.
Some packaged foods are dehydrated; others are freeze-dried. In both processes the water is removed from them to prevent spoilag. Water is necessary for the preparation of many other foods.
On the average, each person who lives in a city or on a modern farm in the United States requires more than 50 gallons of water a day for personal and household uses. These include drinking, washing, preparing meals, and removing waste. A bath in a tub consumes perhaps 25 gallons of water. About five gallons of water flow each minute a shower runs. Large amounts of water are also used in sprinkling lawns and gardens and in operating the air-conditioning units and heating systems of many homes, shops, and office buildings. More than 110 billion gallons of water are consumed—that is, lost to immediate reuse—every day in the United States. This total includes water used in irrigation, in industry, and in fire fighting and street cleaning.
Water is very important to industry. It turns the turbines of hydroelectric plants that produce electricity for light, heat, and power for many factories and communities . Some industries, such as the petroleum industry, need water to prepare their products. Ten gallons (38 liters) of water, for example, are needed to refine one gallon (4 liters) of gasoline. Lakes, rivers, and oceans are important water highways for shipping industry's products .
In dry areas, farmers must irrigate their land to grow crops. Irrigation projects have produced fertile areas in regions that once were deserts. More than 100 billion gallons (380 billion liters) of fresh water are used each day to irrigate cropland in the United States
Although water is usually helpful to people, it can be destructive too. Floods, sleet, hail, snow, and heavy rains cause millions of dollars worth of damage each year. These destructive floods and storms also cause human injuries and deaths

Origin

Billions of years ago the Earth was a mass of hot, swirling gases and dust. Hydrogen and oxygen, the builders of water, were among the gases. When the Earth began to cool, atoms of hydrogen and oxygen joined to form water. The Earth, however, was still too hot for water to exist in the liquid state. Steam, which is water in the gaseous state, rose from the Earth and cooled to form thick clouds above it. Whenever some of the water droplets in these clouds fell to Earth, they immediately boiled back into the clouds.
Finally, the Earth cooled enough for rocks to form and for some of the water to remain liquid. When this happened, vast amounts of water vapor in the clouds condensed and fell to Earth. Scientists think that the first rain may have fallen for hundreds of years. Depressions in the Earth's surface began to fill with water. Torrents of water flowed over the rocks of the Earth and began to shape the continents. 

Composition and Physical States

A molecule of water (chemical formula, H₂O) contains two atoms of hydrogen and one atom of oxygen. Because it is much heavier than hydrogen, oxygen provides about 89 percent of the weight of a water molecule. Whether water is in a liquid state, a solid state (ice), or a gaseous state (water vapor or steam), its chemical makeup remains the same. The three physical states of water depend upon the motion of water molecules, which in turn depends upon heat. In ice, for example, the water molecules have lost so much heat that they move slowly. Electrical attraction between the molecules then becomes strong enough to bind them together in a fixed arrangement with little molecular motion; thus ice holds its shape.
When water is in the liquid form, its molecules have acquired enough heat to keep them moving more rapidly than those in ice. This increased motion is enough to overcome much of the electrical attraction between molecules and allow them to move about rather freely. Since the molecules of water in the liquid state are not held in a rigid pattern, the water takes the shape of whatever container holds it. When water exists as steam or vapor, its molecules are moving so swiftly—because of further increased heat—that attraction is fully overcome.
Atmospheric pressure also affects the changes in water's physical state. At the sea-level pressure of one standard atmosphere (760 millimeters of mercury), pure water freezes into ice at 32°  F (0°  C) and boils into steam at 212°  F (100°  C). Above sea level, where pressure is reduced, water boils at lower temperatures and freezes at higher temperatures.

Density and Weight

Water reaches its greatest density (weight per unit volume) at 39.2°  F (4°  C). The density of pure water at 39.2°  F is one gram per cubic centimeter. This value is the basis for determining the specific gravity of a substance. The specific gravity of any substance is defined as the ratio of its density to the density of water at 39.2° F. The density of gold, for example, is 19.3 grams per cubic centimeter; thus its specific gravity is 19.3. This means that gold is 19.3 times more dense (heavier) than water. Substances with specific gravities greater than 1.000 sink in water; those with less than 1.000 float on water.
Each cubic foot of water weighs 62.4 pounds. A gallon (231 cubic inches) of water weighs about 8¹/₃ pounds. Seawater is usually some 3¹/₂ percent heavier than fresh water because it contains about 35 pounds of salts in each 1,000 pounds of water. The weight of water, of course, causes pressure to increase with depth. In the oceans pressure increases more than 4¹/₃pounds per square inch for every ten feet of depth. At this rate the pressure a mile down in the ocean is more than 2,300 pounds per square inch.

How Water Freezes and Expands

At sea-level pressure, fresh water freezes at 32°  F (0°  C). Seawater freezes at about 28°  F (–2°  C) because the salts in this water lower its freezing point. As the temperature descends to the freezing point, the movement of water molecules slows. While turning to ice, water remains at 32°  F (0°  C) but continues to yield heat. When ice melts, the resulting mixture of ice and water remains at 32°  F (0°  C) until all the ice has melted. By then the water has absorbed the same amount of heat that it lost while freezing. The amount of heat that is given off or absorbed without temperature change is called the latent heat of fusion. It amounts to about 80 calories for each gram of water.
Water expands by nearly one tenth of its volume when it freezes. Thus, 1 cubic foot of water becomes 1.09 cubic feet of ice. The ice therefore becomes less dense (lighter) than water at the same temperature, and the ice floats.
Freezing water expands with enormous force—up to tons per square inch depending on the rate of freeze and other factors. Unprotected water pipes often burst on cold nights because of this tremendous expansive force. Heavier water pipes would be useless because scientists have shown that a water-filled cast-iron vessel with sides many inches thick will still burst when the water freezes. If faucets are allowed to run at a trickling rate, often the friction of the moving water produces enough heat to prevent pipe bursts.

How Water Evaporates and Boils

Heat transforms water from a liquid to a gas. All substances hold some heat, and their molecules are all in motion . The molecules in liquid water do not move fast enough to escape. At the water's surface, however, some molecules are bumped by molecules below them and thus acquire enough speed to break loose and fly into the air. This constant escape of surface molecules is called evaporation.
As water temperature rises, evaporation speeds up because the molecules move more rapidly. If the rise in temperature is great enough, even molecules deep beneath the surface will break loose from their neighbors and form bubbles of vapor. These bubbles then rise to the surface and fly away as steam. The temperature that is high enough to cause this activity is called the boiling point. The boiling point of water at sea level is 212°  F (100°  C).
When liquid water turns to steam or vapor, the water absorbs heat without a rise in temperature. When two equal amounts of water turn into vapor, one slowly by ordinary evaporation and the other rapidly by boiling, the amount of heat finally absorbed by each is about equal. In the absence of a flame or other applied-heat source, evaporating water draws heat from its surroundings. In doing so, it cools whatever is near it. People in warm climates often keep their water cool by placing it in a large canvas bag or a porous pottery jug which becomes moist as some of the water seeps through it. As evaporation takes place from the moist surface, heat is drawn from water farther inside, and thus the water is cooled.
Water that turns into vapor has absorbed heat. The amount of heat needed to turn one gram of water at 212°  F (100°  C) and at sea-level pressure into steam is about 540 calories. Called the latent heat of vaporization, this is a useful property of water. Its effect is large. When a cubic foot of water at sea-level pressure boils away, it becomes about 1,700 cubic feet of steam. As the quickly moving water molecules fly off as steam, they can transfer considerable energy to surrounding objects. This energy is used in heating systems, in steam engines, and in turbines

Pressure Affects the Boiling Point

Atmospheric pressure influences the boiling point of water. When atmospheric pressure increases, the boiling point becomes higher, and when atmospheric pressure decreases (as it does when elevation increases), the boiling point becomes lower.
Pressure on the surface of water tends to keep the water molecules contained. As pressure increases, water molecules need additional heat to gain the speed necessary for escape. Pressure cookers work on this principle. When a pressure cooker gauge shows 100 pounds pressure per square inch, the temperature inside the cooker is more than 300° F (149°  C).
Lowering the pressure lowers the boiling point because the molecules need less speed to escape. The low atmospheric pressure on high mountains lowers the boiling point to such an extent that water cannot get hot enough to boil eggs satisfactorily.

More Than One Kind of Water

Scientists at first thought that all water molecules were alike. They later learned that hydrogen has three isotopes and oxygen has six isotopes . These nine isotopes can combine in a number of ways to form water molecules of different weights. Only one of oxygen's isotopes, however, is usually involved in the formation of water because this isotope makes up more than 99 percent of the world's oxygen. The isotopes of hydrogen are far more important. Chemists call these isotopes protium (single-weight hydrogen), deuterium (double-weight hydrogen), and tritium (triple-weight hydrogen). Protium combines with oxygen to form light water; deuterium and oxygen form heavy water; and tritium and oxygen produce superheavy water.
Ordinary water found in nature consists mostly of the light variety and has the formula H₂O. Heavy water is called deuterium oxide (D₂O) by chemists. It is about 10 percent heavier than H₂O. Only one part of heavy water is found in about 5,000 parts of ordinary water. Heavy water can be separated from light water by evaporation, but chemists commonly use a more efficient process called electrolysis  Because D₂O reacts more slowly to electrolysis than does H₂O, the heavy water remains after the light water disappears. Scientists use heavy water to slow down fast-moving neutrons in nuclear reactors
Superheavy water is called tritium oxide (T₂O). Little is known about its properties because it is difficult to separate and is highly unstable. Since tritium is radioactive, scientists use traces of T₂O to observe the effect of water upon various organic compounds. The radioactive tritium can be detected and followed by special instruments.
Pure water is never found in nature because water is an excellent solvent for many minerals. It also picks up bits of matter wherever it flows. Chemists must distill water to obtain pure water for delicate chemical processes. Chemical terms containing the prefix hydr- (from the Greek word hydor, meaning “water”), such as hydrate, hydride, and hydroxide, show that water is contained in a substance. Anhydrous and dehydrated mean that water usually present in a substance has been removed.

How Water Circulates Throughout the World

Water must be readily available to support life and its activities. At first thought it may seem that water is always available, since the Earth is literally surrounded by water. At times, up to 4 percent of the atmosphere near ground level is water vapor. Also, many thousands of lakes, rivers, and streams are scattered over the Earth's surface. The vast oceans, almost an unending source of water, cover some 140,500,000 square miles and contain almost 330,000,000 cubic miles of water. Yet, with all this water, there are parts of the Earth that are scorched and arid. The manner in which water circulates between Earth and atmosphere determines where ample water supplies can be found and used.

The Water Cycle

If no forces except gravity were at work, the world's water would settle into the ocean basins and remain there. The land surfaces would become lifeless deserts. Water, however, does not stagnate in the oceans. It is continually evaporated from the oceans and other bodies of water by the heat of the sun and blown by the winds across sea and land. Thus an immense amount of water is always suspended in the atmosphere in the form of vapor. When certain weather conditions prevail in the atmosphere, some of the water vapor forms clouds. When such clouds accumulate more water vapor than they can hold, the water is returned to the land as rain or snow  This process of moving water out of the oceans, into the atmosphere, and back to the land and oceans is called the water cycle, or hydrologic cycle.
Sun, air, water, and the force of gravity work together to keep the water cycle going. Major steps in the cycle include: the evaporation of water by the sun's heat and the transpiration of water by plants; the condensation of water vapor by cold air; the precipitation of water by gravity; and the return of water by gravity to the oceans. Some water evaporates into the air from rivers, lakes, moist soil, and plants, but most of the water that moves over the surface of the Earth comes from the oceans and eventually returns to the oceans.

Surface Water and Groundwater

The soil covering the Earth acts as a giant sieve. Soil particles have tiny spaces between them that allow water to trickle down into the soil. When a heavy rainfall occurs, these tiny spaces in soil quickly fill with water, and the excess water, called surface water, runs over the top of the soil. Such surface runoff flows as a thin, hardly noticeable sheet of water until it reaches a depression in the land, such as a gutter or a streambed, where the water can be contained. There, it no longer flows as a sheet of water but as a clear-cut channel of water, moving downward to the ocean.
Water that infiltrates the soil trickles slowly downward, or percolates, through pores and cracks in soil and rocks. Rock strata, or layers, and soil capable of holding water are called aquifers. Eventually, the water reaches a level where it can go no farther because bedrock forms a base. As more and more water accumulates, the aquifer becomes saturated (filled) with water and cannot hold any more. Water held in aquifers is called groundwater. The depth at which groundwater is found varies because the hard bedrock base exists at varying levels. Groundwater is a major source of fresh water. Scientists estimate that there may be enough groundwater in North America to cover the continent with a sheet of water almost 100 feet (30 meters) thick. By means of wells, humans bring this water to the surface to satisfy their need for water. Some of the groundwater moves toward the surface of the soil by capillary action and is evaporated into the air. Plants draw their water from ground so moistened. Water is drawn through the roots of a plant to its leaves, from which it evaporates. This process is called transpiration A fully grown oak tree may transpire about 100 gallons (380 liters) of water a day. In summer an acre of corn transpires from 3,000 to 4,000 gallons (11,360 to 15,140 liters) of water each day.

The Water Table

The topmost level of groundwater is called the water table; below this level the soil is waterlogged. If a hole is dug deep enough in the soil, it may reach the water table. The water table is not at the same level everywhere. It may be close to the surface in some places and hundreds of feet beneath the soil in others. Sometimes a deep cut in the land will expose the water table. Then the groundwater runs off as a stream or river. Changes in climatic conditions and in the amount of precipitation used by vegetation may cause the water table to rise or fall. Heavy rainfall can raise the water table. If the level becomes too high, damage can occur to plants. During times of sparse rainfall, the soil becomes extremely dry, and groundwater that seeps to the surface and evaporates is not replaced. The water table then becomes lower. If much of the lost water is not soon replaced, a drought may occur
Water that is drawn from wells may affect the level of the water table in a given area. When groundwater is pumped to the surface, the water level in the well becomes slightly lower than the surrounding water table. Groundwater then flows downward to the level of water in the well, causing a cone of depression in the water table. This lowers the water table slightly. If water is rapidly drawn from a number of wells in the same area, the water table may be lowered considerably. The water table may rise again when sufficient rainfall occurs or when there is a decrease in the amount of water taken from wells.

Water Movement

Both groundwater and surface water move downslope. Some groundwater may become trapped in hard rock. It remains there—under pressure because groundwater above the trapped water weighs down upon it. Wells drilled into the pool of trapped water release the water, and it rushes to the surface without being pumped. Such wells are called artesian wells.
Normally, groundwater moves slowly down sloping land, spreading and flattening itself in porous soil. It eventually empties into permanent, steadily flowing streams, which in turn drain into large rivers that flow into the ocean.

How Communities Are Supplied with Water

Humans require a supply of fresh water to sustain life. Water-supply systems provide water for irrigation, homes, businesses, industry, and waste removal. Water is also necessary for public needs, such as fire fighting, hydrant flushing, and street cleaning. City water-supply systems usually include works for the collection, transmission, purification, storage, and distribution of water.
Some cities get water by pumping it from a lake, from a river, or from ponds. Other communities pump their water from wells. Storage reservoirs or dams are sometimes constructed at or near points of water collection to ensure a dependable supply of water. Many reservoirs have multiple uses, including public water supply, irrigation, navigation, hydroelectric power, flood control, and recreation. Water is often transported to waterworks by canals, aqueducts, or tunnels. Pipelines, through which water flows either by gravity or under pressure, are also used. Another method of obtaining fresh water is by desalting seawater, commonly referred to as desalination. Desalination facilities are usually located along coastal areas.
Before water is distributed for use, it is usually treated to make it hygienically safe, attractive, and palatable. The pumping station, which regulates the amount of water distributed, and the water-treatment system are called waterworks.
Different cities furnish differing amounts of water to their citizens, but the average amount of water used by a city dweller in the United States is about 150 gallons (570 liters) each day. This figure includes water used for such purposes as fire fighting, waste disposal, street cleaning, and industry. Most cities cannot pay cash to build expensive waterworks, so they issue bonds to raise the money. To repay these bonds and maintain the water system, cities once taxed property owners. Today, most cities require meters in each building and charge the user for the amount of water used. Major improvements and additions to the system are frequently financed by revenue bonds, which are paid for by the water users.

Water Purification and Other Treatments

Simple water systems—those that transmit water directly from source to user without treatment—work well if the source provides relatively pure water. Few cities, however, can find a supply of such water. Sewage or barnyard wastes may carry disease-causing organisms into the water supply. Untreated industrial wastes often pollute the supply. The water may contain mud, silt, and dissolved minerals. Waterworks remove such impurities before sending the water into the mains. Waterworks process the water in different ways, depending on the water source and the intended use. Before purification, water is usually pumped through coarse screens that catch large objects. Pumps then force the screened water into a mixing tank. There, chemicals called coagulants are stirred into the water. The coagulants combine with bacteria, mud, and silt to form sticky clumps called flocs. Then the water passes into deep, broad sedimentation tanks, or settling basins. As the water passes slowly through the tanks, the flocs settle to the bottom. They are removed from the tank bottom by mechanical scrapers.
Water from the sedimentation tanks is filtered through sand or other porous material. The filter catches all remaining suspended matter. Rapid sand filters are most commonly used. Sand is spread from 24 to 36 inches (61 to 91 centimeters) deep in the filter basin, which may cover several acres. Each acre (0.4 hectare) of filter can handle as much as 125 million gallons (473 million liters) of water a day.
The filter sand does more than mechanically strain the water. Gradually the impurities form a jellylike surface mat on the sand. Bacteria and suspended matter stick to the surface mat as water passes through. Reversing the water flow washes away the accumulated wastes. Some filtration plants use finely crushed anthracite, or hard coal, as a filter in place of sand.
Many water-supply systems do not have elaborate filtration plants. But even in systems that have elaborate filtration plants, bacteria may get past the purification devices. Water is therefore usually sterilized with a chemical to ensure that it is safe to drink. Chlorine is the most common sterilizer. It takes only slight amounts of chlorine to kill bacteria. Where water is sediment-free, only one or two parts of chlorine need be added to 10 million parts of water. Sometimes water is forced under pressure into the air in a process called aeration. Oxygen in the air purifies the water somewhat.

Fluoridation.

Many communities add small amounts of fluorides to the water supply, though such actions have in some cases provoked controversy. A correctly regulated amount of fluorides in water has been shown to be safe and to reduce dental decay in children by making tooth enamel more resistant to the acids produced by bacteria in the mouth and by interfering with bacterial growth. Excessive amounts of fluorine, however, may cause mottling of the teeth, which, although it presents no health problems, causes an unattractive appearance.

Hard water.

In some areas extra soap is needed for washing objects such as clothing because the water is hard. Hard water contains certain dissolved minerals, such as calcium bicarbonate, magnesium bicarbonate, and calcium sulfate, which make it difficult for soap to lather.
One method of softening water, the lime-soda process, takes the hardening materials out of the water. Lime (an oxide of calcium) and soda ash (a salt of carbonic acid) are added to the water. They combine with the hardening materials to form compounds that precipitate, such as calcium carbonate. Another method, the cation-exchange, or zeolite, process, also chemically changes the water-hardening materials. Hard water runs into a tank of zeolite, a mineral that contains sodium ions (electrically charged particles). These ions change places with calcium or magnesium ions, forming sodium compounds that do not harden water. Brine, which contains sodium and chlorine ions, is then pumped into the zeolite to replace lost sodium ions. The calcium and magnesium ions are freed and combine with the chlorine ions to form chlorides, which are drained off.

Desalination.

As the competition for water resources becomes more intense, increasing attention is being given to waters that are widely available but unusable because of their salt content. Desalination is a process by which fresh water can be made from seawater. The first land-based seawater-desalting plant was built in Kuwait in 1949. Since then, the cost of desalting has been substantially lowered because of larger plant construction and use of improved materials and processes by individual plants. There are now more than 1,500 land-based desalting plants in the world. In the United States, California and other arid western states are facing the need to build such plants.
There are several different ways to remove salt from salt water. Distillation is the most widely used process. The process of distillation involves heating the seawater until the fresh water evaporates, leaving behind the solid salts. The fresh water is then obtained by inducing the freshwater vapor to condense. In flash evaporation, heated seawater is sprayed into a tank that contains air under reduced pressure. Since liquids boil at increasingly lower temperatures as the pressure on them is reduced, less heat and thus less fuel are required.
The membrane processes for desalting are used mostly in the Middle East, where about half of the world's desalinated water is made. One membrane process is called reverse osmosis. In this process salt water is forced under pressure against a membrane. Fresh water passes through the membrane, while the concentrated mineral salts remain behind.

Distribution

To distribute water from waterworks, large pipes called mains are used. They carry the water underground to all parts of the city or town. Distribution pipes are made of cast iron, ductile iron, steel, or concrete; metal pipes are often coated to protect against corrosion. Smaller pipes or service lines carrying water to consumers may be made of copper or tough plastic. Because lead in even very small quantities is harmful to humans—especially to children—its use in pipe joints is now illegal in many locations. Fire hydrants along streets are supplied by pipes from the mains.
The city or a privately owned water company must provide a way of forcing water through the mains and up to the buildings. A city or town may place a tank on a hill or atop a high tower and pump water into it. Water in the tank is released to the mains, flowing downward by gravity. The greater height and weight of the water still in the tank creates pressure in the mains. This action supplies water to fire hydrants and to all faucets lower than the tank or tower.
The waterworks send water into the mains at a pressure of 30 to 100 pounds per square inch (2 to 7 kilograms per square centimeter). This pressure carries water up to many buildings without further pumping.

Water for Waste Removal

A water system must also remove wastes from homes and industries. Huge pipes and sewers, partially filled with water, transport these wastes and dump them far from drinking-water intakes. Before being dumped, wastes are also usually treated to remove poisonous substances.
Sewers also carry away storm water to prevent street and home flooding. Water in sewers is rarely pumped, because wastes are often so bulky that they would clog pumps. Instead, sewer pipes are laid at such an angle that sewer water will flow downward by gravity to the outlet.

Early Water Supply and Distribution Systems

The nomads of prehistoric times wandered to find good watering places and green pastures. They pitched their camps beside water and moved on when the nearby pastures were exhausted. In deserts such as the Sahara they settled near oases, dependable water sources. Nomads today live in much the same way. Rivers or lakes were probably humankind's first constant supplies of water. Small villages rose near the water, and people drew the water with hollow shells, animal skulls, or leather bags.
People learned that when small ponds and streams dried some water lingered under the water beds and could be reached by digging shallow holes. Deeper holes, which reached more stable water tables, resulted in permanent wells.
People eventually learned to dam streams to form reservoirs, ensuring permanent water supplies. Many of the world's first cities built open tanks to catch and store rainwater. When surface water became scarce, the people used the stored rainwater.
As the populations of early cities grew, water supplies became inadequate. In Egypt, Assyria, and Babylonia, open canals were dug to bring river water to the cities. When cities were attacked, they often fell because stored water gave out. In the 7th century BC a ruler of the Greek island of Samos ordered a tunnel dug through a mountain to bring water into his fortified city. At that time it was an enormous engineering achievement . Many early cities developed some type of aqueduct system, and Rome became famous for its extensive and well-built aqueducts. At one time Rome had eleven major brick or stone aqueducts to supply the city's fountains, public baths, and public buildings.
During the Middle Ages many of Europe's water-supply systems, originally built by the Romans, fell into ruin. City water supplies were limited and often contaminated. Such water was often responsible for typhoid, dysentery, and cholera. In 1550 a resident of Paris, France, could expect only 1 quart (0.9 liter) of water a day. By 1700 the supply had increased—but only to 2.5 quarts (2.4 liters) per person per day.
Historians think that the first modern waterworks were built in London, England, in 1582. In that system pumps filled a reservoir and gravity forced the water through wooden mains. Later, in 1613, the New River water company brought water into London from various sources located outside the city. America's first waterworks, privately owned, were built in Boston, Mass., in 1652.
Early distribution systems used hollow logs as mains. The tapered end of one log fit into the hollow end of the next. These early waterworks pumped water for only a part of each day. Pressure was so low that water could not be raised above the ground floors of houses. Stagnant water could seep into the mains and contaminate the supply. By 1800, iron pipes were replacing wooden mains. The invention of the steam engine and its application to water pumps brought great improvements. Today, giant pumps in waterworks are driven by electricity or turbines.

Conservation

Unlike many of the world's natural resources, water is a replenishable resource. However, it is vitally important that humans conserve water and help to maintain the quality of water by discontinuing practices that contaminate and pollute the supply faster than it can replenish itself.
While some areas—such as the states and provinces bordering the Great Lakes—have ample water, other areas must depend upon rivers, small lakes, and wells. The problem of getting enough water is serious in many parts of the world. Many areas, for example, have long, dry summers and short seasons of heavy rain or snow. The surface runoff resulting from these heavy rains or snows floods the rivers, and engineers must speed the runoff to the sea to prevent widespread damage
Inadequately treated sewage, agricultural runoff, and industrial wastes that flow into water supplies lower the quality of water. Radioactive substances in water, from industry or research centers, emit potentially harmful radiation. Products such as detergents, artificial fertilizers, and insecticides may become pollutants when they enter water-supply systems. Increasing the effectiveness of waste-treatment plants and developing relatively environmentally safe products, such as biodegradable detergents, can help eliminate these pollutants. To assist projects for control of water pollution in the United States, Congress passed the Safe Drinking Water Act in 1974 and Congress amended it in 1986.

 

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