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  1. 1. Armstrong, Neil Neil A. Armstrong was an American astronaut. He was the first person to set foot on the moon. Neil A. Armstrong was an American astronaut. He was the first person to set foot on the moon. Image credit: NASA Born in 1930, Neil A. Armstrong, a United States astronaut, was the first person to set foot on the moon. On July 20, 1969, Armstrong and Buzz Aldrin landed the Apollo 11 lunar module Eagle on the moon. Armstrong left the module and explored the lunar surface. Upon taking his first step onto the moon, he said: "That's one small step for a man, one giant leap for mankind." But the word a was lost in radio transmission. Armstrong was born on Aug. 5, 1930, on his grandparents' farm in Auglaize County, Ohio. He moved with his family to several Ohio communities before they settled in Wapakoneta when Neil was 13 years old. Armstrong developed an interest in flying at an early age. His love of airplanes grew when he went for his first plane ride in a Ford Tri- Motor, a "Tin Goose," at the age of 6. From then on, he was fascinated by aviation. In 1947, Armstrong entered Purdue University. He began studies in aeronautical engineering. But in 1949, the United States Navy called him to active duty. Armstrong became a Navy pilot and was sent to Korea in 1950, near the start of the Korean War. In Korea, he flew 78 combat missions in Navy Panther jets. In 1952, Armstrong returned to Purdue. He earned a bachelor's degree in aeronautical engineering there in 1955. Armstrong was a civilian test pilot assigned to test the X-15 rocket airplane before becoming an astronaut in 1962. He made his first space flight in 1966 on Gemini 8 with David R. Scott. The two men performed the first successful docking of two vehicles in space -- the Gemini 8 and an uninhabited Agena rocket. Armstrong resigned from the United States astronaut program in 1970. Also in 1970, he earned a master's degree in aerospace engineering at the University of Southern California. From 1971 to 1979, Armstrong was a professor of aerospace engineering at the University of Cincinnati. In 1986, he was named vice chairman of a presidential commission investigating the breakup of the space shuttle Challenger. From 1982 to 1992, Armstrong served as chairman of the board of Computing Technologies for Aviation, a company that develops software for flight scheduling. Aurora An aurora is a natural display of light in the sky that can be seen with the unaided eye only at night. An auroral display in the Northern Hemisphere is called the aurora borealis,
  2. 2. or the northern lights. A similar phenomenon in the Southern Hemisphere is called the aurora australis. Auroras are the most visible effect of the sun's activity on the earth's atmosphere. Most auroras occur in far northern and southern regions. They appear chiefly as arcs, clouds, and streaks. Some move, brighten, or flicker suddenly. The most common color in an aurora is green. But displays that occur extremely high in the sky may be red or purple. Most auroras occur about 60 to 620 miles (97 to 1,000 kilometers) above the earth. Some extend lengthwise across the sky for thousands of miles or kilometers. A bar magnet has a magnetic field like Auroral displays are associated with the solar that of the sun. Field lines, which wind, a continuous flow of electrically charged represent the field, exit the north pole particles from the sun. When these particles reach and enter the south pole. Image credit: the earth's magnetic field, some get trapped. World Book diagram by Precision Many of these particles travel toward the earth's Graphics magnetic poles. When the charged particles strike atoms and molecules in the atmosphere, energy is released. Some of this energy appears in the form of auroras. Auroras occur most frequently during the most intense phase of the 11-year sunspot cycle. During this phase, dark patches on the sun's surface, called sunspots, increase in number. Violent eruptions on the sun's surface, known as solar flares, are associated with sunspots. Electrons and protons released by solar flares add to the number of solar particles that interact with the earth's atmosphere. This increased interaction produces extremely bright auroras. It also results in sharp variations in the earth's magnetic field called magnetic storms. During these storms, auroras may shift from the polar regions toward the equator.
  3. 3. Comet A comet (KOM iht) is an icy body that releases gas or dust. Most of the comets that can be seen from Earth travel around the sun in long, oval orbits. A comet consists of a solid nucleus (core) surrounded by a cloudy atmosphere called the coma and one or two tails. Most comets are too small or too faint to be seen without a telescope. Some comets, however, become visible to the unaided eye for several weeks as they pass close to the sun. We can see comets because the gas and dust in their comas and tails reflect sunlight. Also, the gases release Halley's Comet becomes visible to energy absorbed from the sun, causing them to the unaided eye about every 76 years glow. as it nears the sun. Image credit: Lick Observatory Astronomers classify comets according to how long they take to orbit the sun. Short- period comets need less than 200 years to complete one orbit, while long-period comets take 200 years or longer. Astronomers believe that comets are leftover debris from a collection of gas, ice, rocks, and dust that formed the outer planets about 4.6 billion years ago. Some scientists believe that comets originally brought to Earth some of the water and the carbon-based molecules that make up living things. Parts of a comet The nucleus of a comet is a ball of ice and rocky dust particles that resembles a dirty snowball. The ice consists mainly of frozen water but may include other frozen substances, such as ammonia, carbon dioxide, carbon monoxide, and methane. Scientists believe the nucleus of some comets may be fragile because several comets have split apart for no apparent reason. As a comet nears the inner solar system, heat from the sun vaporizes some of the ice on the surface of the nucleus, spewing gas and dust particles into space. This gas and dust forms the comet's coma. Radiation from the sun pushes dust particles away from the coma. These particles form a tail called the dust tail. At the same time, the solar wind -- that is, the flow of high-speed electrically charged particles from the sun-converts some of the comet's gases into ions (charged particles). These ions also stream away from the
  4. 4. coma, forming an ion tail. Because comet tails are pushed by solar radiation and the solar wind, they always point away from the sun. Most comets are thought to have a nucleus that measures about 10 miles (16 kilometers) or less across. Some comas can reach diameters of nearly 1 million miles (1.6 million kilometers). Some tails extend to distances of 100 million miles (160 million kilometers). The life of a comet Comets that pass near the sun come from Scientists think that short-period comets two groups of comets near the outer edge of come from a band of objects called the the solar system, according to astronomers. Kuiper belt, which lies beyond the orbit of The disk-shaped Kuiper belt contributes Pluto. The gravitational pull of the outer comets that orbit the sun in fewer than 200 planets can nudge objects out of the Kuiper years. The Kuiper belt lies beyond Pluto's belt and into the inner solar system, where orbit, which extends to about 4.6 billion they become active comets. Long-period miles (7.4 billion kilometers) from the sun. comets come from the Oort cloud, a nearly The Oort cloud provides comets that take spherical collection of icy bodies about longer to complete their orbits. The outer 1,000 times farther away from the sun than edge of the Oort cloud may be 1,000 times Pluto's orbit. Gravitational interactions with farther than the orbit of Pluto. Image credit: passing stars can cause icy bodies in the World Book diagram by Terry Hadler, Oort cloud to enter the inner solar system Bernard Thornton Artists and become active comets. Comets lose ice and dust each time they return to the inner solar system, leaving behind trails of dusty debris. When Earth passes through one of these trails, the debris become meteors that burn up in the atmosphere. Eventually, some comets lose all their ices. They break up and dissipate into clouds of dust or turn into fragile, inactive objects similar to asteroids. The long, oval-shaped orbits of comets can cross the almost circular orbits of the planets. As a result, comets sometimes collide with planets and their satellites. Many of the impact craters in the solar system were caused by collisions with comets. Studying comets Scientists learned much about comets by studying Halley's Comet as it passed near Earth in 1986. Five spacecraft flew past the comet and gathered information about its appearance and chemical composition. Several probes flew close enough to study the
  5. 5. nucleus, which is normally concealed by the comet's coma. The spacecraft found a roughly potato-shaped nucleus measuring about 9 miles (15 kilometers) long. The nucleus contains equal amounts of ice and dust. About 80 percent of the ice is water ice, and frozen carbon monoxide makes up another 15 percent. Much of the remainder is frozen carbon dioxide, methane, and ammonia. Scientists believe that other comets are chemically similar to Halley's Comet. Scientists unexpectedly found the nucleus of Halley's Comet to be extremely dark black. They The space probe Giotto passed near now believe that the surface of the comet, and Halley's Comet on March 14, 1986. perhaps most other comets, is covered with a Giotto returned dramatic close-up black crust of dust and rock that covers most of images of the comet, including this the ice. These comets release gas only when holes one. Image credit: European Space in this crust rotate toward the sun, exposing the Agency interior ice to the warming sunlight. Another comet nucleus that has been seen by spacecraft cameras is that of Comet Borrelly. During a flyby in 2001, the Deep Space 1 spacecraft observed a nucleus about half the size of the nucleus of Halley's Comet. Borrelly's nucleus was also potato-shaped and had a dark black surface. Like Halley's Comet, Comet Borrelly only released gas from small areas where holes in the crust exposed the ice to sunlight. In 1994, astronomers observed a comet named Shoemaker-Levy 9, which had split into more than two dozen pieces, crashing into the planet Jupiter. One of the most active comets seen in more than 400 years was Comet Hale-Bopp, which came within 122 million miles (197 million kilometers) of Earth in 1997. This was not an especially close approach for a comet. However, Hale-Bopp appeared bright to the unaided eye because its unusually large nucleus gave off a great deal of dust and gas. The nucleus was estimated to be about 18 to 25 miles (30 to 40 kilometers) across. In 2004, the U.S. spacecraft Stardust passed near the nucleus of Comet Wild 2 and gathered samples from the comet's coma. Stardust was scheduled to return the samples to Earth in 2006. Also in 2004, the European Space Agency launched the Rosetta spacecraft, which was to go into orbit around Comet Churyumov-Gerasimenko in 2014. Rosetta carried a small probe designed to land on the comet's nucleus.
  6. 6. Europa Europa, (yu ROH puh), is a large moon of Jupiter. Its surface is made of ice, which may have an ocean of water beneath it. Such an ocean could provide a home for living things. The surface layer of ice or ice and water is 50 to The surface of Europa, a moon 100 miles (80 to 160 kilometers) deep. The satellite has of Jupiter, consists mostly of an extremely thin atmosphere. Electrically charged huge blocks of ice that have particles from Jupiter's radiation belts continuously cracked and shifted about, bombard Europa. suggesting that there may be an ocean of liquid water Europa is one of the smoothest bodies in the solar system. underneath. Image credit: Its surface features include shallow cracks, valleys, NASA ridges, pits, blisters, and icy flows. None of them extend more than a few hundred yards or meters upward or downward. In some places, huge sections of the surface have split apart and separated. The surface of Europa has few impact craters (pits caused by collisions with asteroids or comets). The splitting and shifting of the surface and disruptions from below have destroyed most of the old craters. Europa's interior is hotter than its surface. This internal heat comes from the gravitational forces of Jupiter and Jupiter's other large satellites, which pull Europa's interior in different directions. As a result, the interior flexes, producing heat in a process known as tidal heating. The core of Europa may be rich in iron, but most of the satellite is made of rock. Europa's diameter is 1,940 miles (3,122 kilometers), slightly smaller than Earth's moon. Europa takes 3.55 days to orbit Jupiter at a distance of 416,900 miles (670,900 kilometers). The Italian astronomer Galileo discovered Europa in 1610. Much of what is known about it comes from data gathered by a space probe, also named Galileo, that orbited Jupiter from 1995 to 2003.
  7. 7. Global Warming Global warming is an increase in the average temperature of Earth's surface. Since the late 1800's, the global average temperature has increased about 0.7 to 1.4 degrees F (0.4 to 0.8 degrees C). Many experts estimate that the average temperature will rise an additional 2.5 to 10.4 degrees F (1.4 to 5.8 degrees C) by 2100. That rate of increase would be much larger than most past rates of increase. Scientists worry that human societies and natural ecosystems might not adapt to rapid climate changes. An ecosystem consists of the living organisms and physical environment in a particular area. Global warming could cause much harm, so countries throughout the world drafted an agreement called the Kyoto Protocol to help limit it. Causes of global warming Climatologists (scientists who study climate) have analyzed the global warming that has occurred since the late 1800's. A majority of climatologists have concluded that human activities are responsible for most of the warming. Human activities contribute to global warming by enhancing Earth's natural greenhouse effect. The greenhouse effect warms Earth's surface through a complex process involving sunlight, gases, and particles in the atmosphere. Gases that trap heat in the atmosphere are known as greenhouse gases. The main human activities that contribute to global warming are the burning of fossil fuels (coal, oil, and natural gas) and the clearing of land. Most of the burning occurs in automobiles, in factories, and in electric power plants that provide energy for houses and office buildings. The burning of fossil fuels creates carbon dioxide, whose chemical formula is CO2. CO2 is a greenhouse gas that slows the escape of heat into space. Trees and other plants remove CO2 from the air during photosynthesis, the process they use to produce food. The clearing of land contributes to the buildup of CO2 by reducing the rate at which the gas is removed from the atmosphere or by the decomposition of dead vegetation. A small number of scientists argue that the increase in greenhouse gases has not made a measurable difference in the temperature. They say that natural processes could have caused global warming. Those processes include increases in the energy emitted (given off) by the sun. But the vast majority of climatologists believe that increases in the sun's energy have contributed only slightly to recent warming. The impact of global warming
  8. 8. Continued global warming could have many damaging effects. It might harm plants and animals that live in the sea. It could also force animals and plants on land to move to new habitats. Weather patterns could change, causing flooding, drought, and an increase in damaging storms. Global warming could melt enough polar ice to raise the sea level. In certain parts of the world, human disease could spread, and crop Thousands of icebergs float off the yields could decline. coast of the Antarctic Peninsula after 1,250 square miles (3,240 square Harm to ocean life kilometers) of the Larsen B ice shelf disintegrated in 2002. The area of the Through global warming, the surface waters of ice was larger than the state of Rhode the oceans could become warmer, increasing the Island or the nation of Luxembourg. stress on ocean ecosystems, such as coral reefs. Antarctic ice shelves have been High water temperatures can cause a damaging shrinking since the early 1970's process called coral bleaching. When corals because of climate warming in the bleach, they expel the algae that give them their region. Image credit: NASA/Earth color and nourishment. The corals turn white and, Observatory unless the water temperature cools, they die. Added warmth also helps spread diseases that affect sea creatures. Changes of habitat Widespread shifts might occur in the natural habitats of animals and plants. Many species would have difficulty surviving in the regions they now inhabit. For example, many flowering plants will not bloom without a sufficient period of winter cold. And human occupation has altered the landscape in ways that would make new habitats hard to reach or unavailable altogether. Weather damage Extreme weather conditions might become more frequent and therefore more damaging. Changes in rainfall patterns could increase both flooding and drought in some areas. More hurricanes and other tropical storms might occur, and they could become more powerful. Rising sea level Continued global warming might, over centuries, melt large amounts of ice from a vast sheet that covers most of West Antarctica. As a result, the sea level would rise throughout the world. Many coastal areas would experience flooding, erosion, a loss of wetlands, and an entry of seawater into freshwater areas. High sea levels would submerge some coastal cities, small island nations, and other inhabited regions.
  9. 9. Threats to human health Tropical diseases, such as malaria and dengue, might spread to larger regions. Longer- lasting and more intense heat waves could cause more deaths and illnesses. Floods and droughts could increase hunger and malnutrition. Changes in crop yields Canada and parts of Russia might benefit from an increase in crop yields. But any increases in yields could be more than offset by decreases caused by drought and higher temperatures -- particularly if the amount of warming were more than a few degrees Celsius. Yields in the tropics might fall disastrously because temperatures there are already almost as high as many crop plants can tolerate. Limited global warming Climatologists are studying ways to limit global warming. Two key methods would be (1) limiting CO2 emissions and (2) carbon sequestration -- either preventing carbon dioxide from entering the atmosphere or removing CO2 already there. Limiting CO2 emissions Two effective techniques for limiting CO2 emissions would be (1) to replace fossil fuels with energy sources that do not emit CO2, and (2) to use fossil fuels more efficiently. Alternative energy sources that do not emit CO2 include the wind, sunlight, nuclear energy, and underground steam. Devices known as wind turbines can convert wind energy to electric energy. Solar cells can convert sunlight to electric energy, and various devices can convert solar energy to useful heat. Geothermal power plants convert energy in underground steam to electric energy. Alternative sources of energy are more expensive to use than fossil fuels. However, increased research into their use would almost certainly reduce their cost. Carbon sequestration could take two forms: (1) underground or underwater storage and (2) storage in living plants. Underground or underwater storage would involve injecting industrial emissions of CO2 into underground geologic formations or the ocean. Suitable underground formations include natural reservoirs of oil and gas from which most of the oil or gas has been removed. Pumping CO2 into a reservoir would have the added benefit of making it easier to remove the remaining oil or gas. The value of that product could offset the cost of sequestration. Deep deposits of salt or coal could also be suitable.
  10. 10. The oceans could store much CO2. However, scientists have not yet determined the environmental impacts of using the ocean for carbon sequestration. Storage in living plants Green plants absorb CO2 from the atmosphere as they grow. They combine carbon from CO2 with hydrogen to make simple sugars, which they store in their tissues. After plants die, their bodies decay and release CO2. Ecosystems with abundant plant life, such as forests and even cropland, could tie up much carbon. However, future generations of people would have to keep the ecosystems intact. Otherwise, the sequestered carbon would re-enter the atmosphere as CO2. Agreement on global warming Delegates from more than 160 countries met in Kyoto, Japan, in 1997 to draft the agreement that became known as the Kyoto Protocol. That agreement calls for decreases in the emissions of greenhouse gases. Emissions targets Thirty-eight industrialized nations would have to restrict their emissions of CO2 and five other greenhouse gases. The restrictions would occur from 2008 through 2012. Different countries would have different emissions targets. As a whole, the 38 countries would restrict their emissions to a yearly average of about 95 percent of their 1990 emissions. The agreement does not place restrictions on developing countries. But it encourages the industrialized nations to cooperate in helping developing countries limit emissions voluntarily. Industrialized nations could also buy or sell emission reduction units. Suppose an industrialized nation cut its emissions more than was required by the agreement. That country could sell other industrialized nations emission reduction units allowing those nations to emit the amount equal to the excess it had cut. Several other programs could also help an industrialized nation earn credit toward its target. For example, the nation might help a developing country reduce emissions by replacing fossil fuels in some applications. Approving the agreement The protocol would take effect as a treaty if (1) at least 55 countries ratified (formally approved) it, and (2) the industrialized countries ratifying the protocol had CO2 emissions in 1990 that equaled at least 55 percent of the emissions of all 38 industrialized countries in 1990.
  11. 11. In 2001, the United States rejected the Kyoto Protocol. President George W. Bush said that the agreement could harm the U.S. economy. But he declared that the United States would work with other countries to limit global warming. Other countries, most notably the members of the European Union, agreed to continue with the agreement without United States participation. By 2004, more than 100 countries, including nearly all the countries classified as industrialized under the protocol, had ratified the agreement. However, the agreement required ratification by Russia or the United States to go into effect. Russia ratified the protocol in November 2004. The treaty was to come into force in February 2005. Analyzing global warming Scientists use information from several sources to analyze global warming that occurred before people began to use thermometers. Those sources include tree rings, cores (cylindrical samples) of ice drilled from Antarctica and Greenland, and cores drilled out of sediments in oceans. Information from these sources indicates that the temperature increase of the 1900's was probably the largest in the last 1,000 years. Computers help climatologists analyze past climate changes and predict future changes. First, a scientist programs a computer with a set of mathematical equations known as a climate model. The equations describe how various factors, such as the amount of CO2 in the atmosphere, affect the temperature of Earth's surface. Next, the scientist enters data representing the values of those factors at a certain time. He or she then runs the program, and the computer describes how the temperature would vary. A computer's representation of changing climatic conditions is known as a climate simulation. In 2001, the Intergovernmental Panel on Climate Change (IPCC), a group sponsored by the United Nations (UN), published results of climate simulations in a report on global warming. Climatologists used three simulations to determine whether natural variations in climate produced the warming of the past 100 years. The first simulation took into account both natural processes and human activities that affect the climate. The second simulation took into account only the natural processes, and the third only the human activities. The climatologists then compared the temperatures predicted by the three simulations with the actual temperatures recorded by thermometers. Only the first simulation, which took into account both natural processes and human activities, produced results that corresponded closely to the recorded temperatures. The IPCC also published results of simulations that predicted temperatures until 2100. The different simulations took into account the same natural processes but different patterns of human activity. For example, scenarios differed in the amounts of CO2 that would enter the atmosphere due to human activities.
  12. 12. The simulations showed that there can be no "quick fix" to the problem of global warming. Even if all emissions of greenhouse gases were to cease immediately, the temperature would continue to increase after 2100 because of the greenhouse gases already in the atmosphere. Hurricane A hurricane is a powerful, swirling storm that begins over a warm sea. Hurricanes form in waters near the equator, and then they move toward the poles. The winds of a hurricane swirl around a calm central zone called the eye surrounded by a band Hurricane winds swirl about the eye, a of tall, dark clouds called the eyewall. The eye is calm area in the center of the storm. usually 10 to 40 miles (16 to 64 kilometers) in The main mass of clouds shown in this diameter and is free of rain and large clouds. In photograph measures almost 250 miles the eyewall, large changes in pressure create the (400 kilometers) across. The hurricane's strongest winds. These winds can hurricane, named Andrew, struck the reach nearly 200 miles (320 kilometers) per hour. Bahamas, Florida, and Louisiana in Damaging winds may extend 250 miles (400 1992, killing 65 people and causing kilometers) from the eye. billions of dollars in damage. Image credit: NASA Hurricanes are referred to by different labels, depending on where they occur. They are called hurricanes when they happen over the North Atlantic Ocean, the Caribbean Sea, the Gulf of Mexico, or the Northeast Pacific Ocean. Such storms are known as typhoons if they occur in the Northwest Pacific Ocean, west of an imaginary line called the International Date Line. Near Australia and in the Indian Ocean, they are referred to as tropical cyclones. Hurricanes are most common during the summer and early fall. In the Atlantic and the Northeast Pacific, for example, August and September are the peak hurricane months. Typhoons occur throughout the year in the Northwest Pacific but are most frequent in summer. In the North Indian Ocean, tropical cyclones strike in May and November. In the South Indian Ocean, the South Pacific Ocean, and off the coast of Australia, the hurricane season runs from December to March. Approximately 85 hurricanes, typhoons, and tropical cyclones occur in a year throughout the world. In the rest of this article, the term hurricane refers to all such storms. Hurricane conditions Hurricanes require a special set of conditions, including ample heat and moisture, that exist primarily over warm tropical oceans. For a hurricane to form, there must be a warm
  13. 13. layer of water at the top of the sea with a surface temperature greater than 80 degrees F (26.5 degrees C). Warm seawater evaporates and is absorbed by the surrounding air. The warmer the ocean, the more water evaporates. The warm, moist air rises, lowering the atmospheric pressure of the air beneath. In any area of low atmospheric pressure, the column of air that extends from the surface of the water -- or land -- to the top of the atmosphere is relatively less dense and therefore weighs relatively less. Air tends to move from areas of high pressure to areas of low pressure, creating wind. In the Northern Hemisphere, the earth's rotation causes the wind to swirl into a low-pressure area in a counterclockwise direction. In the Southern Hemisphere, the winds rotate clockwise around a low. This effect of the rotating earth on wind flow is called the Coriolis effect. The Coriolis effect increases in intensity farther from the equator. To produce a hurricane, a low-pressure area must be more than 5 degrees of latitude north or south of the equator. Hurricanes seldom occur closer to the equator. For a hurricane to develop, there must be little wind shear -- that is, little difference in speed and direction between winds at upper and lower elevations. Uniform winds enable the warm inner core of the storm to stay intact. The storm would break up if the winds at higher elevations increased markedly in speed, changed direction, or both. The wind shear would disrupt the budding hurricane by tipping it over or by blowing the top of the storm in one direction while the bottom moved in another direction. The life of a hurricane Meteorologists (scientists who study weather) divide the life of a hurricane into four stages: (1) tropical disturbance, (2) tropical depression, (3) tropical storm, and (4) hurricane. Tropical disturbance is an area where rain clouds are building. The clouds form when moist air rises and becomes cooler. Cool air cannot hold as much water vapor as warm air can, and the excess water changes into tiny droplets of water that form clouds. The clouds in a tropical disturbance may rise to great heights, forming the towering thunderclouds that meteorologists call cumulonimbus clouds. Cumulonimbus clouds usually produce heavy rains that end after an hour or two, and the weather clears rapidly. If conditions are right for a hurricane, however, there is so much heat energy and moisture in the atmosphere that new cumulonimbus clouds continually form from rising moist air. Tropical depression is a low-pressure area surrounded by winds that have begun to blow in a circular pattern. A meteorologist considers a depression to exist when there is low pressure over a large enough area to be plotted on a weather map. On a map of surface pressure, such a depression appears as one or two circular isobars (lines of equal
  14. 14. pressure) over a tropical ocean. The low pressure near the ocean surface draws in warm, moist air, which feeds more thunderstorms. The winds swirl slowly around the low-pressure area at first. As the pressure becomes even lower, more warm, moist air is drawn in, and the winds blow faster. Tropical storm When the winds exceed 38 miles (61 kilometers) per hour, a tropical storm has developed. Viewed from above, the storm clouds now have a well-defined circular shape. The seas have become so rough that ships must steer clear of the area. The strong winds near the surface of the ocean draw more and more heat and water vapor from the sea. The increased warmth and moisture in the air feed the storm. A tropical storm has a column of warm air near its center. The warmer this column becomes, the more the pressure at the surface falls. The falling pressure, in turn, draws more air into the storm. As more air is pulled into the storm, the winds blow harder. Each tropical storm receives a name. The names help meteorologists and disaster planners avoid confusion and quickly convey information about the behavior of a storm. The World Meteorological Organization (WMO), an agency of the United Nations, issues four alphabetical lists of names, one for the North Atlantic Ocean and the Caribbean Sea, and one each for the Eastern, Central, and Northwestern Pacific. The lists include both men's and women's names that are popular in countries affected by the storms. Except in the Northwestern and Central Pacific, the first storm of the year gets a name beginning with A -- such as Tropical Storm Alberto. If the storm intensifies into a hurricane, it becomes Hurricane Alberto. The second storm gets a name beginning with B, and so on through the alphabet. The lists do not use all the letters of the alphabet, however, since there are few names beginning with such letters as Q or U. For example, no Atlantic or Caribbean storms receive names beginning with Q, U, X, Y, or Z. Because storms in the Northwestern Pacific occur throughout the year, the names run through the entire alphabet instead of starting over each year. The first typhoon of the year might be Typhoon Nona, for example. The Central Pacific usually has fewer than five named storms each year. The system of naming storms has changed since 1950. Before that year, there was no formal system. Storms commonly received women's names and names of saints of both genders. From 1950 to 1952, storms were given names from the United States military alphabet -- Able, Baker, Charlie, and so on. The WMO began to use only the names of women in 1953. In 1979, the WMO began to use men's names as well.
  15. 15. Hurricane A storm achieves hurricane status when its winds exceed 74 miles (119 kilometers) per hour. By the time a storm reaches hurricane intensity, it usually has a well-developed eye at its center. Surface pressure drops to its Hurricane winds on the ocean surface swirl lowest in the eye. counterclockwise around a calm eye in the Northern Hemisphere. Image credit: World In the eyewall, warm air spirals upward, Book illustrations by Bruce Kerr creating the hurricane's strongest winds. The speed of the winds in the eyewall is related to the diameter of the eye. Just as ice skaters spin faster when they pull their arms in, a hurricane's winds blow faster if its eye is small. If the eye widens, the winds decrease. Heavy rains fall from the eyewall and bands of dense clouds that swirl around the eyewall. These bands, called rainbands, can produce more than 2 inches (5 centimeters) of rain per hour. The hurricane draws large amounts of heat and moisture from the sea. The path of a hurricane Hurricanes last an average of 3 to 14 days. A long-lived storm may wander 3,000 to 4,000 miles (4,800 to 6,400 kilometers), typically moving over the sea at speeds of 10 to 20 miles (16 to 32 kilometers) per hour. Hurricanes in the Northern Hemisphere usually begin by traveling from east to west. As the storms approach the coast of North America or Asia, however, they shift to a more northerly direction. Most hurricanes turn gradually northwest, north, and finally northeast. In the Southern Hemisphere, the storms may travel westward at first and then turn southwest, south, and finally southeast. The path of an individual hurricane is irregular and often difficult to predict. All hurricanes eventually move toward higher latitudes where there is colder air, less moisture, and greater wind shears. These conditions cause the storm to weaken and die out. The end comes quickly if a hurricane moves over land, because it no longer receives heat energy and moisture from warm tropical water. Heavy rains may continue, however, even after the winds have diminished. Hurricane damage Hurricane damage results from wind and water. Hurricane winds can uproot trees and tear the roofs off houses. The fierce winds also create danger from flying debris. Heavy rains may cause flooding and mudslides.
  16. 16. The most dangerous effect of a hurricane, however, is a rapid rise in sea level called a storm surge. A storm surge is produced when winds drive ocean waters ashore. Storm surges are dangerous because many coastal areas are densely populated and lie only a few feet or meters above sea level. A 1970 cyclone in East Pakistan (now Bangladesh) produced a surge that killed about 266,000 people. A hurricane in Galveston, Texas, in 1900 produced a surge that killed about 6,000 people, the worst natural disaster in United States history. Hurricane watchers rate the intensity of storms on a scale called the Saffir-Simpson scale, developed by American engineer Herbert S. Saffir and meteorologist Robert H. Simpson. The scale designates five levels of hurricanes, ranging from Category 1, described as weak, to Category 5, which can be devastating. Category 5 hurricanes have included Hurricane Camille, which hit the United States in 1969; Hurricane Gilbert, which raked the West Indies and Mexico in 1988; and Hurricane Andrew, which struck the Bahamas, Florida, and Louisiana in 1992. Forecasting hurricanes Meteorologists use weather balloons, satellites, and radar to watch for areas of rapidly falling pressure that may become hurricanes. Specially equipped airplanes called hurricane hunters investigate budding storms. If conditions are right for a hurricane, the National Weather Service issues a hurricane watch. A hurricane watch advises an area that there is a good possibility of a hurricane within 36 hours. If a hurricane watch is issued for your location, check the radio or television often for official bulletins. A hurricane warning means that an area is in danger of being struck by a hurricane in 24 hours or less. Keep your radio tuned to a news station after a hurricane warning. If local authorities recommend evacuation, move quickly to a safe area or a designated hurricane shelter.
  17. 17. Moon Moon is Earth's only natural satellite and the only astronomical body other than Earth ever visited by human beings. The moon is the brightest object in the night sky The moon's surface shows but gives off no light of its own. Instead, it reflects light striking contrasts of light and from the sun. Like Earth and the rest of the solar system, dark. The light areas are the moon is about 4.6 billion years old. rugged highlands. The dark zones were partly flooded by The moon is much smaller than Earth. The moon's lava when volcanoes erupted average radius (distance from its center to its surface) is billions of years ago. The lava 1,079.6 miles (1,737.4 kilometers), about 27 percent of froze to form smooth rock. the radius of Earth. Image credit: Lunar and Planetary Institute The moon is also much less massive than Earth. The moon has a mass (amount of matter) of 8.10 x 1019 tons (7.35 x 1019 metric tons). Its mass in metric tons would be written out as 735 followed by 17 zeroes. Earth is about 81 times that massive. The moon's density (mass divided by volume) is about 3.34 grams per cubic centimeter, roughly 60 percent of Earth's density. Because the moon has less mass than Earth, the force due to gravity at the lunar surface is only about 1/6 of that on Earth. Thus, a person standing on the moon would feel as if his or her weight had decreased by 5/6. And if that person dropped a rock, the rock would fall to the surface much more slowly than the same rock would fall to Earth. Despite the moon's relatively weak gravitational force, the moon is close enough to Earth to produce tides in Earth's waters. The average distance from the center of Earth to the center of the moon is 238,897 miles (384,467 kilometers). That distance is growing -- but extremely slowly. The moon is moving away from Earth at a speed of about 1 1/2 inches (3.8 centimeters) per year. The distance to the moon is measured to an The temperature at the lunar equator ranges accuracy of 5 centimeters by a laser beam from extremely low to extremely high -- sent from Earth. The beam bounces off a from about -280 degrees F (-173 degrees C) laser reflector placed on the moon by at night to +260 degrees F (+127 degrees C) astronauts, and returns to Earth. Image credit: World Book diagram by Bensen Studios
  18. 18. in the daytime. In some deep craters near the moon's poles, the temperature is always near -400 degrees F (-240 degrees C). The moon has no life of any kind. Compared with Earth, it has changed little over billions of years. On the moon, the sky is black -- even during the day -- and the stars are always visible. A person on Earth looking at the moon with the unaided eye can see light and dark areas on the lunar surface. The light areas are rugged, cratered highlands known as terrae (TEHR ee). The word terrae is Latin for lands. The highlands are the original crust of the moon, shattered and fragmented by the impact of meteoroids, asteroids, and comets. Many craters in the terrae exceed 25 miles (40 kilometers) in diameter. The largest is the South Pole-Aitken Basin, which is 1,550 miles (2,500 kilometers) in diameter. The dark areas on the moon are known as maria (MAHR ee uh). The word maria is Latin for seas; its singular is mare (MAHR ee). The term comes from the smoothness of the dark areas and their resemblance to bodies of water. The maria are cratered landscapes that were partly flooded by lava when volcanoes erupted. The lava then froze, forming rock. Since that time, meteoroid impacts have created craters in the maria. The moon has no substantial atmosphere, but small amounts of certain gases are present above the lunar surface. People sometimes refer to those gases as the lunar atmosphere. This "atmosphere" can also be called an exosphere, defined as a tenuous (low-density) zone of particles surrounding an airless body. Mercury and some asteroids also have an exosphere. In 1959, scientists began to explore the moon with robot spacecraft. In that year, the Soviet Union sent a spacecraft called Luna 3 around the side of the moon that faces away from Earth. Luna 3 took the first photographs of that side of the moon. The word luna is Latin for moon. On July 20, 1969, the U.S. Apollo 11 The first people on the moon were U.S. lunar module landed on the moon in the astronauts Neil A. Armstrong, who took this first of six Apollo landings. Astronaut picture, and Buzz Aldrin, who is pictured next Neil A. Armstrong became the first to a seismograph. A television camera and a human being to set foot on the moon. United States flag are in the background. Their lunar module, Eagle, stands at the right. In the 1990's, two U.S. robot space Image credit: NASA probes, Clementine and Lunar Prospector, detected evidence of frozen water at both of the moon's poles. The ice came from comets that hit the moon over the last 2 billion to 3 billion years. The ice apparently has lasted in areas that are always in the shadows of
  19. 19. crater rims. Because the ice is in the shade, where the temperature is about -400 degrees F (-240 degrees C), it has not melted and evaporated. This article discusses Moon (The movements of the moon) (Origin and evolution of the moon) (The exosphere of the moon) (Surface features of the moon) (The interior of the moon) (History of moon study). The movements of the moon The moon moves in a variety of ways. For example, it rotates on its axis, an imaginary line that connects its poles. The moon also orbits Earth. Different amounts of the moon's lighted side become visible in phases because of the moon's orbit around Earth. During events called eclipses, the moon is positioned in line with Earth and the sun. A slight motion called libration enables us to see about 59 percent of the moon's surface at different times. Rotation and orbit The moon rotates on its axis once every 29 1/2 days. That is the period from one sunrise to the next, as seen from the lunar surface, and so it is known as a lunar day. By contrast, Earth takes only 24 hours for one rotation. The moon's axis of rotation, like that of Earth, is tilted. Astronomers measure axial tilt relative to a line perpendicular to the ecliptic plane, an imaginary surface through Earth's orbit around the sun. The tilt of Earth's axis is about 23.5 degrees from the perpendicular and accounts for the seasons on Earth. But the tilt of the moon's axis is only about 1.5 degrees, so the moon has no seasons. Another result of the smallness of the moon's tilt is that certain large peaks near the poles are always in sunlight. In addition, the floors of some craters -- particularly near the south pole -- are always in shadow. The moon completes one orbit of Earth with respect to the stars about every 27 1/3 days, a period known as a sidereal month. But the moon revolves around Earth once with respect to the sun in about 29 1/2 days, a period known as a synodic month. A sidereal month is slightly shorter than a synodic month because, as the moon revolves around Earth, Earth is revolving around the sun. The moon needs some extra time to "catch up" with Earth. If the moon started on its orbit from a spot between Earth and the sun, it would return to almost the same place in about 29 1/2 days. A synodic month equals a lunar day. As a result, the moon shows the same hemisphere -- the near side -- to Earth at all times. The other hemisphere -- the far side -- is always turned away from Earth.
  20. 20. People sometimes mistakenly use the term dark side to refer to the far side. The moon does have a dark side -- it is the hemisphere that is turned away from the sun. The location of the dark side changes constantly, moving with the terminator, the dividing line between sunlight and dark. The lunar orbit, like the orbit of Earth, is shaped like a slightly flattened circle. The distance between the center of Earth and the moon's center varies throughout each orbit. At perigee (PEHR uh jee), when the moon is closest to Earth, that distance is 225,740 miles (363,300 kilometers). At apogee (AP uh jee), the farthest position, the distance is 251,970 miles (405,500 kilometers). The moon's orbit is elliptical (oval-shaped). Phases As the moon orbits Earth, an observer on Earth can see the moon appear to change shape. It seems to change from a crescent to a circle and back again. The shape looks different from one day to the next because the observer sees different parts of the moon's sunlit surface as the moon orbits Earth. The different appearances are known as the phases of the moon. The moon goes through a complete cycle of phases in a synodic month. The moon has four phases: (1) new moon, (2) first quarter, (3) full moon, and (4) last quarter. When the moon is between the sun and Earth, its sunlit side is turned away from Earth. Astronomers call this darkened phase a new moon. The next night after a new moon, a thin crescent of light appears along the moon's eastern edge. The remaining portion of the moon that faces Earth is faintly visible because of earthshine, sunlight reflected from Earth to the moon. Each night, an observer on Earth can see more of the sunlit side as the terminator, the line between sunlight and dark, moves westward. After about seven days, the observer can see half a full moon, commonly called a half moon. This phase is known as the first quarter because it occurs one quarter of the way through the synodic month. About seven days later, the moon is on the side of Earth opposite the sun. The entire sunlit side of the moon is now visible. This phase is called a full moon. About seven days after a full moon, the observer again sees a half moon. This phase is the last quarter, or third quarter. After another seven days, the moon is between Earth and the sun, and another new moon occurs. As the moon changes from new moon to full moon, and more and more of it becomes visible, it is said to be waxing. As it changes from full moon to new moon, and less and less of it can be seen, it is waning. When the moon appears smaller than a half moon, it is called crescent. When it looks larger than a half moon, but is not yet a full moon, it is called gibbous (GIHB uhs). Like the sun, the moon rises in the east and sets in the west. As the moon progresses through its phases, it rises and sets at different times. In the new moon phase, it rises with
  21. 21. the sun and travels close to the sun across the sky. Each successive day, the moon rises an average of about 50 minutes later. Eclipses occur when Earth, the sun, and the moon are in a straight line, or nearly so. A lunar eclipse occurs when Earth gets directly -- or almost directly -- between the sun and the moon, and Earth's shadow falls on the moon. A lunar eclipse can occur only during a full moon. A solar eclipse occurs when the moon gets directly -- or almost directly -- between the sun and Earth, and the moon's shadow falls on Earth. A solar eclipse can occur only during a new moon. During one part of each lunar orbit, Earth is between the sun and the moon; and, during another part of the orbit, the moon is between the sun and Earth. But in most cases, the astronomical bodies are not aligned directly enough to cause an eclipse. Instead, Earth casts its shadow into space above or below the moon, or the moon casts its shadow into space above or below Earth. The shadows extend into space in that way because the moon's orbit is tilted about 5 degrees relative to Earth's orbit around the sun. Libration People on Earth can sometimes see a small part of the far side of the moon. That part is visible because of lunar libration, a slight rotation of the moon as viewed from Earth. There are three kinds of libration: (1) libration in longitude, (2) diurnal (daily) libration, and (3) libration in latitude. Over time, viewers can see more than 50 percent of the moon's surface. Because of libration, about 59 percent of the lunar surface is visible from Earth. Libration in longitude occurs because the moon's orbit is elliptical. As the moon orbits Earth, its speed varies according to a law discovered in the 1600's by the German astronomer Johannes Kepler. When the moon is relatively close to Earth, the moon travels more rapidly than its average speed. When the moon is relatively far from Earth, the moon travels more slowly than average. But the moon always rotates about its own axis at the same rate. So when the moon is traveling more rapidly than average, its rotation is too slow to keep all of the near side facing Earth. And when the moon is traveling more slowly than average, its rotation is too rapid to keep all of the near side facing Earth. Diurnal libration is caused by a daily change in the position of an observer on Earth relative to the moon. Consider an observer who is at Earth's equator when the moon is full. As Earth rotates Diurnal libration enables an observer on Earth to see around one edge of the moon, then the other, during a single night. The libration occurs because Earth's rotation changes the observer's viewpoint by a distance equal to the diameter of Earth. Image credit: World Book illustration
  22. 22. from west to east, the observer first sees the moon when it rises at the eastern horizon and last sees it when it sets at the western horizon. During this time, the observer's viewpoint moves about 7,900 miles (12,700 kilometers) -- the diameter of Earth -- relative to the moon. As a result, the moon appears to rotate slightly to the west. While the moon is rising in the east and climbing to its highest point in the sky, the observer can see around the western edge of the near side. As the moon descends to the western horizon, the observer can see around the eastern edge of the near side. Libration in latitude occurs because the moon's axis of rotation is tilted about 6 1/2 degrees relative to a line perpendicular to the moon's orbit around Earth. Thus, during each lunar orbit, the moon's north pole tilts first toward Earth, then away from Earth. When the lunar north pole is tilted toward Earth, people on Earth can see farther than normal along the top of the moon. When that pole is tilted away from Earth, people on Earth can see farther than normal along the bottom of the moon. Origin and evolution of the moon Scientists believe that the moon formed as a result of a collision known as the Giant Impact or the "Big Whack." According to this idea, Earth collided with a planet-sized object 4.6 billion years ago. As a result of the impact, a cloud of vaporized rock shot off Earth's surface and went into orbit around Earth. The cloud cooled and condensed into a ring of small, solid bodies, which then gathered together, forming the moon. The rapid joining together of the small bodies released much energy as heat. Consequently, the moon melted, creating an "ocean" of magma (melted rock). The magma ocean slowly cooled and solidified. As it cooled, dense, iron-rich materials sank deep into the moon. Those materials also cooled and solidified, forming the mantle, the layer of rock beneath the crust. As the crust formed, asteroids bombarded it heavily, shattering and churning it. The largest impacts may have stripped off the entire crust. Some collisions were so powerful that they almost split the moon into pieces. One such collision created the South Pole-Aitken Basin, one of the largest known impact craters in the solar system. About 4 billion to 3 billion years ago, melting occurred in A basalt rock that astronauts the mantle, probably caused by radioactive elements deep brought to Earth from the in the moon's interior. The resulting magma erupted as moon formed from lava that dark, iron-rich lava, partly flooding the heavily cratered erupted from a lunar volcano. surface. The lava cooled and solidified into rocks known Escaping gases created the as basalts (buh SAWLTS). holes before the lava solidified into rock. Image credit: Lunar and Planetary Institute
  23. 23. Small eruptions may have continued until as recently as 1 billion years ago. Since that time, only an occasional impact by an asteroid or comet has modified the surface. Because the moon has no atmosphere to burn up meteoroids, the bombardment continues to this day. However, it has become much less intense. Impacts of large objects can create craters. Impacts of micrometeoroids (tiny meteoroids) grind the surface rocks into a fine, dusty powder known as the regolith (REHG uh lihth). Regolith overlies all the bedrock on the moon. Because regolith forms as a result of exposure to space, the longer a rock is exposed, the thicker the regolith that forms on it. The exosphere of the moon The lunar exosphere -- that is, the materials surrounding the moon that make up the lunar "atmosphere" -- consists mainly of gases that arrive as the solar wind. The solar wind is a continuous flow of gases from the sun -- mostly hydrogen and helium, along with some neon and argon. The remainder of the gases in the exosphere form on the moon. A continual "rain" of micrometeoroids heats lunar rocks, melting and vaporizing their surface. The most common atoms in the vapor are atoms of sodium and potassium. Those elements are present in tiny amounts -- only a few hundred atoms of each per cubic centimeter of exosphere. In addition to vapors produced by impacts, the moon also releases some gases from its interior. Most gases of the exosphere concentrate about halfway between the equator and the poles, and they are most plentiful just before sunrise. The solar wind continuously sweeps vapor into space, but the vapor is continuously replaced. During the night, the pressure of gases at the lunar surface is about 3.9 x 10-14 pound per square inch (2.7 x 10-10 pascal). That is a stronger vacuum than laboratories on Earth can usually achieve. The exosphere is so tenuous -- that is, so low in density -- that the rocket exhaust released during each Apollo landing temporarily doubled the total mass of the entire exosphere. The surface of the moon is covered with bowl-shaped holes called craters, shallow depressions called basins, and broad, flat plains known as maria. A powdery dust called the regolith overlies much of the surface of the moon. Craters
  24. 24. The vast majority of the moon's craters are formed by the impact of meteoroids, asteroids, and comets. Craters on the moon are named for famous scientists. For example, Euler Crater has central peaks Copernicus Crater is named for Nicolaus Copernicus, a and slumped walls. The peaks Polish astronomer who realized in the 1500's that the almost certainly formed planets move about the sun. Archimedes Crater is named quickly after the impact that for the Greek mathematician Archimedes, who made produced the crater many mathematical discoveries in the 200's B.C. compressed the ground. The ground rebounded upward, The shape of craters varies with their size. Small craters forming the peaks. The crater with diameters of less than 6 miles (10 kilometers) have walls are slumped because the relatively simple bowl shapes. Slightly larger craters original walls were too steep cannot maintain a bowl shape because the crater wall is to withstand the force of too steep. Material falls inward from the wall to the floor. gravity. Material fell inward, As a result, the walls become scalloped and the floor away from the walls. This becomes flat. crater, in Mare Imbrium (Sea of Rains), is about 17 1/2 Still larger craters have terraced walls and central peaks. miles (28 kilometers) across. Terraces inside the rim descend like stairsteps to the Image credit: Lunar and floor. The same process that creates wall scalloping is Planetary Institute responsible for terraces. The central peaks almost certainly form as did the central peaks of impact craters on Earth. Studies of the peaks on Earth show that they result from a deformation of the ground. The impact compresses the ground, which then rebounds, creating the peaks. Material in the central peaks of lunar craters may come from depths as great as 12 miles (19 kilometers). Surrounding the craters is rough, mountainous material -- crushed and broken rocks that were ripped out of the crater cavity by shock pressure. This material, called the crater ejecta blanket, can extend about 60 miles (100 kilometers) from the crater. Farther out are patches of debris and, in many cases, irregular secondary craters, also known as secondaries. Those craters come in a range of shapes and sizes, and they are often clustered in groups or aligned in rows. Secondaries form when material thrown out of the primary (original) crater strikes the surface. This material consists of large blocks, clumps of loosely joined rocks, and fine sprays of ground-up rock. The material may travel thousands of miles or kilometers. Crater rays are light, wispy deposits of powder that can extend thousands of miles or kilometers from the crater. Rays slowly vanish as micrometeoroid bombardment mixes the powder into the upper surface layer. Thus, craters that still have visible rays must be among the youngest craters on the moon. Craters larger than about 120 miles (200 kilometers) across tend to have central mountains. Some of them also have inner rings of peaks, in addition to the central peak.
  25. 25. The appearance of a ring signals the next major transition in crater shape -- from crater to basin. Basins are craters that are 190 miles (300 kilometers) or more across. The smaller basins have only a single inner ring of peaks, but the larger ones typically have multiple rings. The rings are concentric -- that is, they all have the same center, like the rings of a dartboard. The spectacular, multiple-ringed basin called the Eastern Sea (Mare Orientale) is almost 600 miles (1,000 kilometers) across. Other basins can be more than 1,200 miles (2,000 kilometers) in diameter -- as large as the entire western United States. Basins occur equally on the near side and far side. Most basins have little or no fill of basalt, particularly those on the far side. The difference in filling may be related to variations in the thickness of the crust. The far side has a thicker crust, so it is more difficult for molten rock to reach the surface there. In the highlands, the overlying ejecta blankets of the basins make up most of the upper few miles or kilometers of material. Much of this material is a large, thick layer of shattered and crushed rock known as breccia (BREHCH ee uh). Scientists can learn about the original crust by studying tiny fragments of breccia. Maria, the dark areas on the surface of the moon, make up about 16 percent of the surface area. Some maria are named in Latin for weather terms -- for example, Mare Imbrium (Sea of Rains) and Mare Nubium (Sea of Clouds). Others are named for states of mind, as in Mare Serenitatus (Sea of Serenity) and Mare Tranquillitatis (Sea of Tranquility). Landforms on the maria tend to be smaller than those of the highlands. The small size of mare features relates to the scale of the processes that formed them -- volcanic eruptions and crustal deformation, rather than large impacts. The chief landforms on the maria include wrinkle ridges and rilles and other volcanic features. Wrinkle ridges are blisterlike humps that wind across the surface of almost all maria. The ridges are actually broad folds in the rocks, created by compression. Many wrinkle ridges are roughly circular, aligned with small peaks that stick up through the maria and outlining interior rings. Circular ridge systems also outline buried features, such as rims of craters that existed before the maria formed. Rilles are snakelike depressions that wind across A lunar rover is parked near the edge many areas of the maria. Scientists formerly of Hadley Rille, a long channel thought the rilles might be ancient riverbeds. probably formed by lava 4 billion to 3 However, they now suspect that the rilles are billion years ago. The slopes in the background are part of a formation called the Swann Hills. This photo was taken during the Apollo 15 mission in 1971. Astronaut David R. Scott is reaching under a seat to get a camera. Image credit: NASA
  26. 26. channels formed by running lava. One piece of evidence favoring this view is the dryness of rock samples brought to Earth by Apollo astronauts; the samples have almost no water in their molecular structure. In addition, detailed photographs show that the rilles are shaped somewhat like channels created by flowing lava on Earth. Volcanic features Scattered throughout the maria are a variety of other features formed by volcanic eruptions. Within Mare Imbrium, scarps (lines of cliffs) wind their way across the surface. The scarps are lava flow fronts, places where lava solidified, enabling lava that was still molten to pile up behind them. The presence of the scarps is one piece of evidence indicating that the maria consist of solidified basaltic lava. Small hills and domes with pits on top are probably little volcanoes. Both dome-shaped and cone-shaped volcanoes cluster together in many places, as on Earth. One of the largest concentrations of cones on the moon is the Marius Hills complex in Oceanus Procellarum (Ocean of Storms). Within this complex are numerous wrinkle ridges and rilles, and more than 50 volcanoes. Large areas of maria and terrae are covered by dark material known as dark mantle deposits. Evidence collected by the Apollo missions confirmed that dark mantling is volcanic ash. Much smaller dark mantles are associated with small craters that lie on the fractured floors of large craters. Those mantles may be cinder cones -- low, broad, cone-shaped hills formed by explosive volcanic eruptions. The interior of the moon The moon, like Earth, has three interior zones -- crust, mantle, and core. However, the composition, structure, and origin of the zones on the moon are much different from those on Earth. Most of what scientists know about the interior of Earth and the moon has been learned by studying seismic events -- earthquakes and moonquakes, respectively. The data on moonquakes come from scientific equipment set up by Apollo astronauts from 1969 to 1972. Crust The average thickness of the lunar crust is about 43 miles (70 kilometers), compared with about 6 miles (10 kilometers) for Earth's crust. The outermost part of the moon's crust is broken, fractured, and jumbled as a result of the large impacts it has endured. This shattered zone gives way to intact material below a depth of about 6 miles. The bottom of
  27. 27. the crust is defined by an abrupt increase in rock density at a depth of about 37 miles (60 kilometers) on the near side and about 50 miles (80 kilometers) on the far side. Mantle The mantle of the moon consists of dense rocks that are rich in iron and magnesium. The mantle formed during the period of global melting. Low-density minerals floated to the outer layers of the moon, while dense minerals sank deeper into it. Later, the mantle partly melted due to a build-up of heat in the deep interior. The source of the heat was probably the decay (breakup) of uranium and other radioactive elements. This melting produced basaltic magmas -- bodies of molten rock. The magmas later made their way to the surface and erupted as the mare lavas and ashes. Although mare volcanism occurred for more than 1 billion years -- from at least 4 billion years to fewer than 3 billion years ago -- much less than 1 percent of the volume of the mantle ever remelted. Core Data gathered by Lunar Prospector confirmed that the moon has a core and enabled scientists to estimate its size. The core has a radius of only about 250 miles (400 kilometers). By contrast, the radius of Earth's core is about 2,200 miles (3,500 kilometers). The lunar core has less than 1 percent of the mass of the moon. Scientists suspect that the core consists mostly of iron, and it may also contain large amounts of sulfur and other elements. Earth's core is made mostly of molten iron and nickel. This rapidly rotating molten core is responsible for Earth's magnetic field. A magnetic field is an influence that a magnetic object creates in the region around it. If the core of a planet or a satellite is molten, motion within the core caused by the rotation of the planet or satellite makes the core magnetic. But the small, partly molten core of the moon cannot generate a global magnetic field. However, small regions on the lunar surface are magnetic. Scientists are not sure how these regions acquired magnetism. Perhaps the moon once had a larger, more molten core. There is evidence that the lunar interior formerly contained gas, and that some gas may still be there. Basalt from the moon contains holes called vesicles that are created during a volcanic eruption. On Earth, gas that is dissolved in magma comes out of solution during an eruption, much as carbon dioxide comes out of a carbonated beverage when you shake the drink container. The presence of vesicles in lunar basalt indicates that the deep interior contained gases, probably carbon monoxide or gaseous sulfur. The existence of volcanic ash is further evidence of interior gas; on Earth, volcanic eruptions are largely driven by gas.
  28. 28. History of moon study Ancient ideas Some ancient peoples believed that the moon was a rotating bowl of fire. Others thought it was a mirror that reflected Earth's lands and seas. But philosophers in ancient Greece understood that the moon is a sphere in orbit around Earth. They also knew that moonlight is reflected sunlight. Some Greek philosophers believed that the moon was a world much like Earth. In about A.D. 100, Plutarch even suggested that people lived on the moon. The Greeks also apparently believed that the dark areas of the moon were seas, while the bright regions were land. In about A.D. 150, Ptolemy, a Greek astronomer who lived in Alexandria, Egypt, said that the moon was Earth's nearest neighbor in space. He thought that both the moon and the sun orbited Earth. Ptolemy's views survived for more than 1,300 years. But by the early 1500's, the Polish astronomer Nicolaus Copernicus had developed the correct view -- Earth and the other planets revolve about the sun, and the moon orbits Earth. Early observations with telescopes The Italian astronomer and physicist Galileo wrote the first scientific description of the moon based on observations with a telescope. In 1609, Galileo described a rough, mountainous surface. This description was quite different from what was commonly believed -- that the moon was smooth. Galileo noted that the light regions were rough and hilly and the dark regions were smoother plains. The presence of high mountains on the moon fascinated Galileo. His detailed description of a large crater in the central highlands -- probably Albategnius -- began 350 years of controversy and debate about the origin of the "holes" on the moon. Other astronomers of the 1600's mapped and cataloged every surface feature they could see. Increasingly powerful telescopes led to more detailed records. In 1645, the Dutch engineer and astronomer Michael Florent van Langren, also known as Langrenus, published a map that gave names to the surface features of the moon, mostly its craters. A map drawn by the Bohemian-born Italian astronomer Anton M. S. de Rheita in 1645 correctly depicted the bright ray systems of the craters Tycho and Copernicus. Another effort, by the Polish astronomer Johannes Hevelius in 1647, included the moon's libration zones. By 1651, two Jesuit scholars from Italy, the astronomer Giovanni Battista Riccioli and the mathematician and physicist Francesco M. Grimaldi, had completed a map of the moon. That map established the naming system for lunar features that is still in use.
  29. 29. Determining the origin of craters Until the late 1800's, most astronomers thought that volcanism formed the craters of the moon. However, in the 1870's, the English astronomer Richard A. Proctor proposed correctly that the craters result from the collision of solid objects with the moon. But at first, few scientists accepted Proctor's proposal. Most astronomers thought that the moon's craters must be volcanic in origin because no one had yet described a crater on Earth as an impact crater, but scientists had found dozens of obviously volcanic craters. In 1892, the American geologist Grove Karl Gilbert argued that most lunar craters were impact craters. He based his arguments on the large size of some of the craters. Those included the basins, which he was the first to recognize as huge craters. Gilbert also noted that lunar craters have only the most general resemblance to calderas (large volcanic craters) on Earth. Both lunar craters and calderas are large circular pits, but their structural details do not resemble each other in any way. In addition, Gilbert created small craters experimentally. He studied what happened when he dropped clay balls and shot bullets into clay and sand targets. Gilbert was the first to recognize that the circular Mare Imbrium was the site of a gigantic impact. By examining photographs, Gilbert also determined which nearby craters formed before and after that event. For example, a crater that is partially covered by ejecta from the Imbrium impact formed before the impact. A crater within the mare formed after the impact. Describing lunar evolution Gilbert suggested that scientists could determine the relative age of surface features by studying the ejecta of the Imbrium impact. That suggestion was the key to unraveling the history of the moon. Gilbert recognized that the moon is a complex body that was built up by innumerable impacts over a long period. In his book The Face of the Moon (1949), the American astronomer and physicist Ralph B. Baldwin further described lunar evolution. He noted the similarity in form between craters on the moon and bomb craters created during World War II (1939-1945) and concluded that lunar craters form by impact. Baldwin did not say that every lunar feature originated with an impact. He stated correctly that the maria are solidified flows of basalt lava, similar to flood lava plateaus on Earth. Finally, independently of Gilbert, he concluded that all circular maria are actually huge impact craters that later filled with lava. In the 1950's, the American chemist Harold C. Urey offered a contrasting view of lunar history. Urey said that, because the moon appears to be cold and rigid, it has always been so. He then stated -- correctly -- that craters are of impact origin. However, he concluded
  30. 30. falsely that the maria are blankets of debris scattered by the impacts that created the basins. And he was mistaken in concluding that the moon never melted to any significant extent. Urey had won the 1934 Nobel Prize in chemistry and had an outstanding scientific reputation, so many people took his views seriously. Urey strongly favored making the moon a focus of scientific study. Although some of his ideas were mistaken, his support of moon study was a major factor in making the moon an early goal of the U.S. space program. In 1961, the U.S. geologist Eugene M. Shoemaker founded the Branch of Astrogeology of the U.S. Geological Survey (USGS). Astrogeology is the study of celestial objects other than Earth. Shoemaker showed that the moon's surface could be studied from a geological perspective by recognizing a sequence of relative ages of rock units near the crater Copernicus on the near side. Shoemaker also studied the Meteor Crater in Arizona and documented the impact origin of this feature. In preparation for the Apollo missions to the moon, the USGS began to map the geology of the moon using telescopes and pictures. This work gave scientists their basic understanding of lunar evolution. Apollo missions Beginning in 1959, the Soviet Union and the United States sent a series of robot spacecraft to examine the moon in detail. Their ultimate goal was to land people safely on the moon. The United States finally reached that goal in 1969 with the landing of the Apollo 11 lunar module. The United States conducted six more Apollo missions, including five landings. The last of those was Apollo 17, in December 1972. The Apollo missions revolutionized the understanding of the moon. Much of the knowledge gained about the moon also applies to Earth and the other inner planets -- Mercury, Venus, and Mars. Scientists learned, for example, that impact is a fundamental geological process operating on the planets and their satellites. After the Apollo missions, the Soviets sent four Luna robot craft to the moon. The last, Luna 24, returned samples of lunar soil to Earth in August 1976. Recent exploration The Clementine orbiter used No more spacecraft went to the moon until January 1994, radar signals to find evidence when the United States sent the orbiter Clementine. From of a large deposit of frozen February to May of that year, Clementine's four cameras water on the moon. The orbiter took more than 2 million pictures of the moon. A laser sent radar signals to various device measured the height and depth of mountains, target points on the lunar craters, and other features. Radar signals that Clementine surface. The targets reflected some of the signals to Earth, where they were received by large antennas and analyzed. Image credit: Lunar and Planetary Institute
  31. 31. bounced off the moon provided evidence of a large deposit of frozen water. The ice appeared to be inside craters at the south pole. The U.S. probe Lunar Prospector orbited the moon from January 1998 to July 1999. The craft mapped the concentrations of chemical elements in the moon, surveyed the moon's magnetic fields, and found strong evidence of ice at both poles. Small particles of ice are apparently part of the regolith at the poles. The SMART-1 spacecraft, launched by the European Space Agency in 2003, went into orbit around the moon in 2004. The craft's instruments were designed to investigate the moon's origin and conduct a detailed survey of the chemical elements on the lunar surface. Planets A planet is a large, round heavenly body that orbits a star and shines with light reflected from the star. Eight planets orbit the sun in our solar system. In order of increasing distance from the sun, they are: (1) Mercury, (2) Venus, (3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus, and (8) Neptune. Many nearly planet-sized objects, called dwarf planets, also orbit the sun. Dwarf planets include The sun blazes with energy. Pluto and Ceres. Since 1992, astronomers have On its surface, magnetic forces discovered many planets orbiting other stars. create loops and streams of gas Traditionally, the term planet has had no formal definition that extend tens of thousands in astronomy. Millions of objects orbit the sun—the most of miles or kilometers into basic characteristic of a planet. But scholars have space. This image was made struggled to devise a simple classification system that by photographing ultraviolet distinguishes the smallest worlds from the largest comets, radiation given off by atoms of asteroids, and other bodies. iron gas that are hotter than 9 million degrees F (5 million The International Astronomical Union (IAU), the degrees C). Image credit: recognized authority in naming heavenly bodies, divides NASA/Transition Region & Coronal Explorer
  32. 32. objects that orbit the sun into three major classes: (1) planets, (2) dwarf planets, and (3) small solar system bodies. A planet orbits the sun and no other body. It has so much mass (amount of matter) that its own gravitational pull compacts it into a round shape. In addition, a planet has a strong enough gravitational pull to sweep the region of its orbit relatively free of other objects. A dwarf planet also orbits the sun and is large enough to be round. However, it does not have a strong enough gravitational pull to clear the region of its orbit. Small solar system bodies, including most asteroids and comets, have too little mass for gravity to round their irregular shapes. Many planets, dwarf planets, and other bodies have smaller objects orbiting them called satellites or moons. The planets in our solar system can be divided into two groups. The innermost four planets—Mercury, Venus, Earth, and Mars—are small, rocky worlds. They are called the terrestrial (earthlike) planets, from the Latin word for Earth, terra. Earth is the largest terrestrial planet. The other Earthlike planets have from 38 to 95 percent of Earth's diameter and from 5.5 to 82 percent of Earth's mass. The outer four planets—Jupiter, Saturn, Uranus, and Neptune—are called gas giants or Jovian (Jupiterlike) planets. They have gaseous atmospheres and no solid surfaces. All four Jovian planets consist mainly of hydrogen and helium. Smaller amounts of other materials also occur, including traces of ammonia and methane in their atmospheres. They range from 3.9 times to 11.2 times Earth's diameter and from 15 times to 318 times Earth's mass. Jupiter, Saturn, and Neptune give off more energy than they receive from the sun. Most of this extra energy takes the form of infrared radiation, which is felt as heat, instead of visible light. Scientists think the source of some of the energy is probably the slow compression of the planets by their own gravity. From its discovery in 1930, Pluto was generally considered a planet. However, its small size and irregular orbit led many astronomers to question whether Pluto should be grouped with worlds such as Earth and Jupiter. Pluto more closely resembles other icy objects found in a region of the outer solar system called the Kuiper belt. In the early 2000’s, astronomers found several such Kuiper belt objects (KBO’s) comparable in size to Pluto. The IAU created the “dwarf planet” classification to describe Pluto and other nearly planet-sized objects. Observing the planets People have known the inner six planets of our solar system for thousands of years because they are visible from Earth without a telescope. The outermost three planets— Uranus and Neptune—were discovered by astronomers, beginning in the 1780's. These planets can be seen from Earth with a telescope. To the unaided eye, the planets look much like the background stars in the night sky. However, the planets move slightly from night to night in relation to the stars. The name
  33. 33. planet comes from a Greek word meaning to wander. The planets and the moon follow the same apparent path through the sky. This path, known as the zodiac, is about 16° wide. At its center is the ecliptic, the apparent path of the sun. If you see a bright object near the ecliptic at night or near sunrise or sunset, it is most likely a planet. You can even see the brightest planets in the daytime, if you know where to look. Planets and stars also differ in the steadiness of their light when viewed from Earth's surface. Planets shine with a steady light, but stars seem to twinkle. The twinkling is due to the moving layers of air that surround Earth. Stars are so far away that they are mere points of light in the sky, even when viewed through a telescope. The atmosphere bends the starlight passing through it. As small regions of the atmosphere move about, the points of light seem to dance and change in brightness. Planets, which are much closer, look like tiny disks through a telescope. The atmosphere scatters light from different points on a planet's disk. However, enough light always arrives from a sufficient number of points to provide a steady appearance. Orbits Viewed from Earth's surface, the planets of the solar system and the stars appear to move around Earth. They rise in the east and set in the west each night. Most of the time, the planets move westward across the sky slightly more slowly than the stars do. As a result, the planets seem to drift eastward relative to the background stars. This motion is called prograde. For a while each year, however, the planets seem to reverse their direction. This backward motion is called retrograde. In ancient times, most scientists thought that the moon, sun, planets, and stars actually moved around Earth. One puzzle that ancient scientists struggled to explain was the annual retrograde motion of the planets. In about A.D. 150, the Greek astronomer Ptolemy developed a theory that the planets orbited in small circles, which in turn orbited Earth in larger circles. Ptolemy thought that retrograde motion was caused by a planet moving on its small circle in an opposite direction from the motion of the small circle around the big circle. In 1543, the Polish astronomer Nicolaus Copernicus showed that the sun is the center of the orbits of the planets. Our term solar system is based on Copernicus's discovery. Copernicus realized that retrograde motion occurs because Earth moves faster in its orbit than the planets that are farther from the sun. The planets that are closer to the sun move faster in their orbits than Earth travels in its orbit. Retrograde motion occurs whenever Earth passes an outer planet traveling around the sun or an inner planet passes Earth. In the 1600's, the German astronomer Johannes Kepler used observations of Mars by the Danish astronomer Tycho Brahe to figure out three laws of planetary motion. Although
  34. 34. Kepler developed his laws for the planets of our solar system, astronomers have since realized that Kepler's laws are valid for all heavenly bodies that orbit other bodies. Kepler's first law says that planets move in elliptical (oval-shaped) orbits around their parent star—in our solar system, the sun. An ellipse is a closed curve formed around two fixed points called foci. The ellipse is formed by the path of a point moving so that the sum of its distances from the two foci remains the same. The orbital paths of the planets form such curves, with the parent star at one focus of the ellipse. Before Kepler, scientists had assumed that the planets moved in circular orbits. Kepler's second law says that an imaginary line joining the parent star to its planet sweeps across equal areas of space in equal amounts of time. When a planet is close to its star, it moves relatively rapidly in its orbit. The line therefore sweeps out a short, fat, trianglelike figure. When the planet is farther from its star, it moves relatively slowly. In this case, the line sweeps out a long, thin figure that resembles a triangle. But the two figures have equal areas. Kepler's third law says that a planet's period (the time it takes to complete an orbit around its star) depends on its average distance from the star. The law says that the square of the planet's period—that is, the period multiplied by itself—is proportional to the cube of the planet's average distance from its star—the distance multiplied by itself twice—for all planets in a solar system. The English scientist, astronomer, and mathematician Isaac Newton presented his theory of gravity and explained why Kepler's laws work in a treatise published in 1687. Newton showed how his expanded version of Kepler's third law could be used to find the mass of the sun or of any other object around which things orbit. Using Newton's explanation, astronomers can determine the mass of a planet by studying the period of its moon or moons and their distance from the planet. Rotation Planets rotate at different rates. One day is defined as how long it takes Earth to rotate once. Jupiter and Saturn spin much faster, in only about 10 hours. Venus rotates much slower, in about 243 Earth days. Most planets rotate in the same direction in which they revolve around the sun, with their axis of rotation standing upright from their orbital path. A law of physics holds that such rotation does not change by itself. So astronomers think that the solar system formed out of a cloud of gas and dust that was already spinning. Uranus is tipped on its side, however, so that its axes lies nearly level with its paths around the sun. Venus is tipped all the way over. Its axis is almost completely upright, but the planet rotates in the direction opposite from the direction of its revolution around
  35. 35. the sun. Most astronomers think that some other objects in the solar system must have collided with Uranus, Pluto, and Venus and tipped them. The planets of our solar system The planet Astronomers measure distances within the solar system in Mercury was first astronomical units (AU). One astronomical unit is the average photographed in distance between Earth and the sun, which is about 93 million miles detail on March (150 million kilometers). The inner planets have orbits whose 29, 1974, by the diameters are 0.4, 0.7, 1.0, and 1.5 AU, respectively. The orbits of the U.S. probe gas giants are much larger: 5, 10, 20, and 30 AU, respectively. Mariner 10. Image Because of their different distances from the sun, the temperature, credit: NASA surface features, and other conditions on the planets vary widely. Mercury, the innermost planet, has no moon and almost no atmosphere. It orbits so close to the sun that temperatures on its surface can climb as high as 800 degrees F (430 degrees C). But some regions near the planet's poles may be always in shadow, and astronomers speculate that water or ice may remain there. No spacecraft has visited Mercury since the 1970's, when Mariner 10 photographed about half the planet's surface at close range. The Messenger spacecraft, launched in 2004, was scheduled to fly by Mercury three times before going into orbit around the planet in 2011. Venus is known as Earth's twin because it resembles Earth in size and mass, though it has no moon. Venus has a dense atmosphere that consists primarily of carbon dioxide. The pressure of the atmosphere on Venus's surface is 90 times that of Earth's atmosphere. Venus's thick atmosphere traps energy from the sun, raising the surface temperature on Venus to about 870 degrees F (465 degrees C), hot enough to melt lead. This trapping of heat is Thick clouds of known as the greenhouse effect. Scientists have sulfuric acid cover warned that a similar process on Earth is causing Venus. Image permanent global warming. Several spacecraft credit: NASA have orbited or landed on Venus. In the 1990's, the Magellan spacecraft used radar -- radio waves bounced off the planet -- to map Venus in detail. Earth, our home Earth, our home planet, has an atmosphere that is planet, has oceans mostly nitrogen with some oxygen. Earth has of liquid water, oceans of liquid water and continents that rise and continents above sea level. Many measuring devices on the that rise above sea surface and in space monitor conditions on our level. Image planet. In 1998, the National Aeronautics and credit: Space Administration (NASA) launched the first of NASA/Goddard a series of satellites called the Earth Observing Space Flight Center
  36. 36. System (EOS). The EOS satellites will carry remote-sensing instruments to measure climate changes and other conditions on Earth's surface. Mars is known as the red planet because of its reddish-brown appearance, caused by rusty dust on the Martian surface. Mars is a cold, dry world with a thin atmosphere. The atmospheric pressure (pressure exerted by the weight of the gases in the atmosphere) on the Martian surface is less than 1 percent the atmospheric pressure on Earth. This low surface pressure has enabled most of the water that Mars may once have had to escape into space. The planet Mars has clouds in its The surface of Mars has giant volcanoes, a huge system of canyons, atmosphere and a and stream beds that look as if water flowed through them in the past. deposit of ice at its Mars has two tiny moons, Phobos and Deimos. Many spacecraft have north pole. Image landed on or orbited Mars. credit: NASA/JPL/Malin Jupiter, the largest planet in our solar system, has more mass than the Space Science other planets combined. Like the other Jovian planets, it has gaseous The layers of Systems outer layers and may have a rocky core. A huge storm system called dense clouds the Great Red Spot in Jupiter's atmosphere is larger than Earth and has raged for around Jupiter hundreds of years. appear in a photograph of the Jupiter's four largest moons -- Io, Europa, Ganymede, and Callisto -- are larger than planet taken by Pluto, and Ganymede is also bigger than Mercury. Circling Jupiter's equator are three the Voyager 1 thin rings, consisting mostly of dust particles. A pair of Voyager spacecraft flew by space probe. Jupiter in 1979 and sent back close-up pictures. In 1995, the Galileo spacecraft dropped a Image credit: JPL probe into Jupiter's atmosphere. Galileo orbited Jupiter from 1995 to 2003. Saturn, another giant planet, has a magnificent set of gleaming rings. Its gaseous atmosphere is not as colorful as Jupiter's, however. One reason Saturn is relatively drab is that its hazy upper atmosphere makes the cloud patterns below difficult to see. Another Saturn is encircled reason is that Saturn is farther than Jupiter from the sun. Because of by seven major the difference in distance, Saturn is colder than Jupiter. Due to the rings. Image credit: temperature difference, the kinds of chemical reactions that color NASA/JPL/Space Jupiter's atmosphere occur too slowly to do the same on Saturn. Science Institute Saturn's moon Titan is larger than Pluto and Mercury. Titan has a thick atmosphere of nitrogen and methane. In 1980 and 1981, the Voyager 2 spacecraft sent back close-up views of Saturn and its rings and moons. The Cassini spacecraft began orbiting Saturn in 2004. It carried a small probe that was designed to be dropped into Titan's atmosphere.
  37. 37. Uranus was the first planet discovered with a telescope. German- born English astronomer William Herschel found it in 1781. He Uranus appears in true at first thought he had discovered a comet. Almost 200 years colors, left, and false later, scientists detected 10 narrow rings around Uranus when the colors, right, in images planet moved in front of a star and the rings became visible. produced by Voyager 2 studied Uranus and its rings and moons close-up in combining numerous 1986. pictures taken by the Voyager 2 spacecraft. Neptune was first observed in 1846 by German Image credit: JPL astronomer Johann G. Galle after other astronomers predicted its position by studying how it affected Uranus's orbit. In 1989, Voyager 2 found that Neptune had a storm system called the Great Dark Spot, similar to Jupiter's Great Red Spot. But five years later, in 1994, the Hubble Space Telescope found that the Great Dark Spot had vanished. The blue clouds Neptune has four narrow rings, one of which has clumps of of Neptune are matter. Neptune's moon Triton is one of the largest in the solar mostly frozen system and has volcanoes that emit plumes of frozen nitrogen. methane. The other object The Dwarf Planets shown is Pluto is so far from Neptune's moon The solar system’s dwarf planets consist primarily of rock and ice Earth that even Triton. Image and feature little or no atmosphere. They lack the mass to sweep powerful telescopes credit: their orbits clear, so they tend to be found among populations of reveal little detail of its NASA/JPL similar, smaller bodies. surface. The Hubble Space Telescope Ceres ranks as the largest of millions of asteroids found between the orbits of Mars gathered the light for and Jupiter. Ceres has a rocky composition and resembles a slightly squashed the pictures of Pluto sphere. Its longest diameter measures 596 miles (960 kilometers). The Italian shown here. Image astronomer Giuseppe Piazzi discovered Ceres in 1801. As with Pluto, people once credit: NASA widely considered Ceres a planet. The outer dwarf planets generally lie beyond the orbit of Neptune. Astronomers have had difficulty studying bodies in this region because they are extremely far from Earth. Dozens of them probably fit the IAU’s definition of a dwarf planet. Most of these bodies belong to the Kuiper belt. Some astronomers have suggested calling the outer dwarf planets plutonians in honor of Pluto, the first one discovered. A body designated 2003 UB313 ranks as the largest dwarf planet, with a diameter of around 1,500 miles (2,400 kilometers). Quaoar «KWAH oh wahr» , a KBO discovered in 2002, measures roughly half the size of Pluto. Sedna, discovered in 2004, measures about three-fourths the size of Pluto and lies nearly three times as far from the sun. Some scientists think Sedna belongs to population of cometlike objects called the Oort cloud, which lies beyond the Kuiper belt.