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Old Tuesday, January 17, 2006
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Default Solar System

Solar System


Solar System, the Sun and everything that orbits the Sun, including the nine planets and their satellites; the asteroids and comets; and interplanetary dust and gas. The term may also refer to a group of celestial bodies orbiting another star. In this article, solar system refers to the system that includes Earth and the Sun.
The dimensions of the solar system are specified in terms of the mean distance from Earth to the Sun, called the astronomical unit (AU). One AU is 150 million km (about 93 million mi). The most distant known planet, Pluto, orbits about 39 AU from the Sun. Estimates for the boundary where the Sun’s magnetic field ends and interstellar space begins—called the heliopause—range from 86 to 100 AU.

The most distant known planetoid orbiting the Sun is Sedna, whose discovery was reported in March 2004. A planetoid is an object that is too small to be a planet. At the farthest point in its orbit, Sedna is about 900 AU from the Sun. Comets known as long-period comets, however, achieve the greatest distance from the Sun; they have highly eccentric orbits ranging out to 50,000 AU or more.

The solar system was the only planetary system known to exist around a star similar to the Sun until 1995, when astronomers discovered a planet about 0.6 times the mass of Jupiter orbiting the star 51 Pegasi. Jupiter is the most massive planet in our solar system. Soon after, astronomers found a planet about 8.1 times the mass of Jupiter orbiting the star 70 Virginis, and a planet about 3.5 times the mass of Jupiter orbiting the star 47 Ursa Majoris. Since then, astronomers have found planets and disks of dust in the process of forming planets around many other stars. Most astronomers think it likely that solar systems of some sort are numerous throughout the universe


The Sun is a typical star of intermediate size and luminosity. Sunlight and other radiation are produced by the conversion of hydrogen into helium in the Sun’s hot, dense interior (see Nuclear Energy). Although this nuclear fusion is transforming 600 million metric tons of hydrogen each second, the Sun is so massive (2 × 1030 kg, or 4.4 × 1030 lb) that it can continue to shine at its present brightness for 6 billion years. This stability has allowed life to develop and survive on Earth.

For all the Sun’s steadiness, it is an extremely active star. On its surface, dark sunspots bounded by intense magnetic fields come and go in 11-year cycles and sudden bursts of charged particles from solar flares can cause auroras and disturb radio signals on Earth. A continuous stream of protons, electrons, and ions also leaves the Sun and moves out through the solar system. This solar wind shapes the ion tails of comets and leaves its traces in the lunar soil, samples of which were brought back from the Moon’s surface by piloted United States Apollo spacecraft.

3) Planets in Our Solar System

Nine known planets revolve around the Sun in our solar system. The planets, shown here in order of their distance from the Sun, vary greatly in size, rotation, color, and composition. For instance, Mercury, a small, hot planet, is, on average, 58 million km (36 million mi) from the Sun, while icy Pluto is 5.9 billion km (3.67 billion mi) away. Venus rotates relatively slowly around its axis, so that one day on the planet equals 58 Earth days. Jupiter is the largest planet in the system, with a volume 1,400 times greater than that of Earth. Saturn has a broad set of rings and features more than twenty satellites, the most of any planet. Mars is characterized by orange coloration and distinct polar ice caps, while methane in the atmospheres of Uranus and Neptune makes these planets a bright blue-green. In addition to being the farthest planet from the Sun, Pluto has the longest period of revolution: 247.7 years.

Nine major planets are currently known. They are commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets (Jupiter, Saturn, Uranus, and Neptune). The inner planets are small and are composed primarily of rock and iron. The outer planets are much larger and consist mainly of hydrogen, helium, and ice. Pluto does not belong to either group, and there is an ongoing debate as to whether Pluto should be categorized as a major planet.

Mercury is surprisingly dense, apparently because it has an unusually large iron core. With only a transient atmosphere, Mercury has a surface that still bears the record of bombardment by asteroidal bodies early in its history. Venus has a carbon dioxide atmosphere 90 times thicker than that of Earth, causing an efficient greenhouse effect by which the Venusian atmosphere is heated. The resulting surface temperature is the hottest of any planet—about 477°C (about 890°F).

Earth is the only planet known to have abundant liquid water and life. However, in 2004 astronomers with the National Aeronautics and Space Administration’s Mars Exploration Rover mission confirmed that Mars once had liquid water on its surface. Scientists had previously concluded that liquid water once existed on Mars due to the numerous surface features on the planet that resemble water erosion found on Earth. Mars’s carbon dioxide atmosphere is now so thin that the planet is dry and cold, with polar caps of frozen water and solid carbon dioxide, or dry ice. However, small jets of subcrustal water may still erupt on the surface in some places.

Jupiter is the largest of the planets. Its hydrogen and helium atmosphere contains pastel-colored clouds, and its immense magnetosphere rings, and satellites make it a planetary system unto itself. One of Jupiter’s largest moons, Io, has volcanoes that produce the hottest surface temperatures in the solar system. At least four of Jupiter’s moons have atmospheres, and at least three show evidence that they contain liquid or partially frozen water. Jupiter’s moon Europa may have a global ocean of liquid water beneath its icy crust.

Saturn rivals Jupiter, with a much more intricate ring structure and a similar number of satellites. One of Saturn’s moons, Titan, has an atmosphere thicker than that of any other satellite in the solar system. Uranus and Neptune are deficient in hydrogen compared with Jupiter and Saturn; Uranus, also ringed, has the distinction of rotating at 98° to the plane of its orbit. Pluto seems similar to the larger, icy satellites of Jupiter or Saturn. Pluto is so distant from the Sun and so cold that methane freezes on its surface


The asteroids are small rocky bodies that move in orbits primarily between the orbits of Mars and Jupiter. Numbering in the thousands, asteroids range in size from Ceres, which has a diameter of 1,003 km (623 mi), to microscopic grains. Some asteroids are perturbed, or pulled by forces other than their attraction to the Sun, into eccentric orbits that can bring them closer to the Sun. If the orbits of such bodies intersect that of Earth, they are called meteoroids. When they appear in the night sky as streaks of light, they are known as meteors, and recovered fragments are termed meteorites. Laboratory studies of meteorites have revealed much information about primitive conditions in our solar system. The surfaces of Mercury, Mars, and several satellites of the planets (including Earth’s Moon) show the effects of an intense bombardment by asteroidal objects early in the history of the solar system. On Earth that record has eroded away, except for a few recently found impact craters.

Some meteors and interplanetary dust may also come from comets, which are basically aggregates of dust and frozen gases typically 5 to 10 km (about 3 to 6 mi) in diameter. Comets orbit the Sun at distances so great that they can be perturbed by stars into orbits that bring them into the inner solar system. As comets approach the Sun, they release their dust and gases to form a spectacular coma and tail. Under the influence of Jupiter’s strong gravitational field, comets can sometimes adopt much smaller orbits. The most famous of these is Halley’s Comet, which returns to the inner solar system at 75-year periods. Its most recent return was in 1986. In July 1994 fragments of Comet Shoemaker-Levy 9 bombarded Jupiter’s dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the tremendous kinetic energy of the fragments was released through massive explosions, some resulting in fireballs larger than Earth.

Comets circle the Sun in two main groups, within the Kuiper Belt or within the Oort cloud. The Kuiper Belt is a ring of debris that orbits the Sun beyond the planet Neptune. Many of the comets with periods of less than 500 years come from the Kuiper Belt. In 2002 astronomers discovered a planetoid in the Kuiper Belt, and they named it Quaoar.

The Oort cloud is a hypothetical region about halfway between the Sun and the heliopause. Astronomers believe that the existence of the Oort cloud, named for Dutch astronomer Jan Oort, explains why some comets have very long periods. A chunk of dust and ice may stay in the Oort cloud for thousands of years. Nearby stars sometimes pass close enough to the solar system to push an object in the Oort cloud into an orbit that takes it close to the Sun.

The first detection of the long-hypothesized Oort cloud came in March 2004 when astronomers reported the discovery of a planetoid about 1,700 km (about 1,000 mi) in diameter. They named it Sedna, after a sea goddess in Inuit mythology. Sedna was found about 13 billion km (about 8 billion mi) from the Sun. At its farthest point from the Sun, Sedna is the most distant object in the solar system and is about 130 billion km (about 84 billion mi) from the Sun.

Many of the objects that do not fall into the asteroid belts, the Kuiper Belt, or the Oort cloud may be comets that will never make it back to the Sun. The surfaces of the icy satellites of the outer planets are scarred by impacts from such bodies. The asteroid-like object Chiron, with an orbit between Saturn and Uranus, may itself be an extremely large inactive comet. Similarly, some of the asteroids that cross the path of Earth’s orbit may be the rocky remains of burned-out comets. Chiron and similar objects called the Centaurs probably escaped from the Kuiper Belt and were drawn into their irregular orbits by the gravitational pull of the giant outer planets, Jupiter, Saturn, Neptune, and Uranus.

The Sun was also found to be encircled by rings of interplanetary dust. One of them, between Jupiter and Mars, has long been known as the cause of zodiacal light, a faint glow that appears in the east before dawn and in the west after dusk. Another ring, lying only two solar widths away from the Sun, was discovered in 1983.


If one could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction. All of the planets except Venus and Uranus rotate on their axes in this same direction. The entire system is remarkably flat—only Mercury and Pluto have obviously inclined orbits. Pluto’s orbit is so elliptical that it is sometimes closer than Neptune to the Sun.

The satellite systems mimic the behavior of their parent planets and move in a counterclockwise direction, but many exceptions are found. Jupiter, Saturn, and Neptune each have at least one satellite that moves around the planet in a retrograde orbit (clockwise instead of counterclockwise), and several satellite orbits are highly elliptical. Jupiter, moreover, has trapped two clusters of asteroids (the so-called Trojan asteroids) leading and following the planet by 60° in its orbit around the Sun. (Some satellites of Saturn have done the same with smaller bodies.) The comets exhibit a roughly spherical distribution of orbits around the Sun.

Within this maze of motions, some remarkable patterns exist: Mercury rotates on its axis three times for every two revolutions about the Sun; no asteroids exist with periods (intervals of time needed to complete one revolution) 1/2, 1/3, …, 1/n (where n is an integer) the period of Jupiter; the three inner Galilean satellites of Jupiter have periods in the ratio 4:2:1. These and other examples demonstrate the subtle balance of forces that is established in a gravitational system composed of many bodies.


Despite their differences, the members of the solar system probably form a common family. They seem to have originated at the same time; few indications exist of bodies joining the solar system, captured later from other stars or interstellar space.

Early attempts to explain the origin of this system include the nebular hypothesis of the German philosopher Immanuel Kant and the French astronomer and mathematician Pierre Simon de Laplace, according to which a cloud of gas broke into rings that condensed to form planets. Doubts about the stability of such rings led some scientists to consider various catastrophic hypotheses, such as a close encounter of the Sun with another star. Such encounters are extremely rare, and the hot, tidally disrupted gases would dissipate rather than condense to form planets.

Current theories connect the formation of the solar system with the formation of the Sun itself, about 4.7 billion years ago. The fragmentation and gravitational collapse of an interstellar cloud of gas and dust, triggered perhaps by nearby supernova explosions, may have led to the formation of a primordial solar nebula. The Sun would then form in the densest, central region. It is so hot close to the Sun that even silicates, which are relatively dense, have difficulty forming there. This phenomenon may account for the presence near the Sun of a planet such as Mercury, having a relatively small silicate crust and a larger than usual, dense iron core. (It is easier for iron dust and vapor to coalesce near the central region of a solar nebula than it is for lighter silicates to do so.) At larger distances from the center of the solar nebula, gases condense into solids such as are found today from Jupiter outward. Evidence of a possible preformation supernova explosion appears as traces of anomalous isotopes in tiny inclusions in some meteorites. This association of planet formation with star formation suggests that billions of other stars in our galaxy may also have planets. The high frequency of binary and multiple stars, as well as the large satellite systems around Jupiter and Saturn, attest to the tendency of collapsing gas clouds to fragment into multibody systems.


Last edited by Sureshlasi; Thursday, August 09, 2007 at 08:43 PM.
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Old Tuesday, January 17, 2006
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Default Earth (planet)

Earth (planet)


Earth (planet), one of nine planets in the solar system, the only planet known to harbor life, and the “home” of human beings. From space Earth resembles a big blue marble with swirling white clouds floating above blue oceans. About 71 percent of Earth’s surface is covered by water, which is essential to life. The rest is land, mostly in the form of continents that rise above the oceans.

Earth’s surface is surrounded by a layer of gases known as the atmosphere, which extends upward from the surface, slowly thinning out into space. Below the surface is a hot interior of rocky material and two core layers composed of the metals nickel and iron in solid and liquid form.

Unlike the other planets, Earth has a unique set of characteristics ideally suited to supporting life as we know it. It is neither too hot, like Mercury, the closest planet to the Sun, nor too cold, like distant Mars and the even more distant outer planets—Jupiter, Saturn, Uranus, Neptune, and tiny Pluto. Earth’s atmosphere includes just the right amount of gases that trap heat from the Sun, resulting in a moderate climate suitable for water to exist in liquid form. The atmosphere also helps block radiation from the Sun that would be harmful to life. Earth’s atmosphere distinguishes it from the planet Venus, which is otherwise much like Earth. Venus is about the same size and mass as Earth and is also neither too near nor too far from the Sun. But because Venus has too much heat-trapping carbon dioxide in its atmosphere, its surface is extremely hot—462°C (864°F)—hot enough to melt lead and too hot for life to exist.

Although Earth is the only planet known to have life, scientists do not rule out the possibility that life may once have existed on other planets or their moons, or may exist today in primitive form. Mars, for example, has many features that resemble river channels, indicating that liquid water once flowed on its surface. If so, life may also have evolved there, and evidence for it may one day be found in fossil form. Water still exists on Mars, but it is frozen in polar ice caps, in permafrost, and possibly in rocks below the surface.

For thousands of years, human beings could only wonder about Earth and the other observable planets in the solar system. Many early ideas—for example, that the Earth was a sphere and that it traveled around the Sun—were based on brilliant reasoning. However, it was only with the development of the scientific method and scientific instruments, especially in the 18th and 19th centuries, that humans began to gather data that could be used to verify theories about Earth and the rest of the solar system. By studying fossils found in rock layers, for example, scientists realized that the Earth was much older than previously believed. And with the use of telescopes, new planets such as Uranus, Neptune, and Pluto were discovered.

In the second half of the 20th century, more advances in the study of Earth and the solar system occurred due to the development of rockets that could send spacecraft beyond Earth. Human beings were able to study and observe Earth from space with satellites equipped with scientific instruments. Astronauts landed on the Moon and gathered ancient rocks that revealed much about the early solar system. During this remarkable advancement in human history, humans also sent unmanned spacecraft to the other planets and their moons. Spacecraft have now visited all of the planets except Pluto. The study of other planets and moons has provided new insights about Earth, just as the study of the Sun and other stars like it has helped shape new theories about how Earth and the rest of the solar system formed.

As a result of this recent space exploration, we now know that Earth is one of the most geologically active of all the planets and moons in the solar system. Earth is constantly changing. Over long periods of time land is built up and worn away, oceans are formed and re-formed, and continents move around, break up, and merge.

Life itself contributes to changes on Earth, especially in the way living things can alter Earth’s atmosphere. For example, Earth at one time had the same amount of carbon dioxide in its atmosphere as Venus now has, but early forms of life helped remove this carbon dioxide over millions of years. These life forms also added oxygen to Earth’s atmosphere and made it possible for animal life to evolve on land.

A variety of scientific fields have broadened our knowledge about Earth, including biogeography, climatology, geology, geophysics, hydrology, meteorology, oceanography, and zoogeography. Collectively, these fields are known as Earth science. By studying Earth’s atmosphere, its surface, and its interior and by studying the Sun and the rest of the solar system, scientists have learned much about how Earth came into existence, how it changed, and why it continues to change.


Earth is the third planet from the Sun, after Mercury and Venus. The average distance between Earth and the Sun is 150 million km (93 million mi). Earth and all the other planets in the solar system revolve, or orbit, around the Sun due to the force of gravitation. The Earth travels at a velocity of about 107,000 km/h (about 67,000 mph) as it orbits the Sun. All but one of the planets orbit the Sun in the same plane—that is, if an imaginary line were extended from the center of the Sun to the outer regions of the solar system, the orbital paths of the planets would intersect that line. The exception is Pluto, which has an eccentric (unusual) orbit.

Earth’s orbital path is not quite a perfect circle but instead is slightly elliptical (oval-shaped). For example, at maximum distance Earth is about 152 million km (about 95 million mi) from the Sun; at minimum distance Earth is about 147 million km (about 91 million mi) from the Sun. If Earth orbited the Sun in a perfect circle, it would always be the same distance from the Sun.

The solar system, in turn, is part of the Milky Way Galaxy, a collection of billions of stars bound together by gravity. The Milky Way has armlike discs of stars that spiral out from its center. The solar system is located in one of these spiral arms, known as the Orion arm, which is about two-thirds of the way from the center of the Galaxy. In most parts of the Northern Hemisphere, this disc of stars is visible on a summer night as a dense band of light known as the Milky Way.

Earth is the fifth largest planet in the solar system. Its diameter, measured around the equator, is 12,756 km (7,926 mi). Earth is not a perfect sphere but is slightly flattened at the poles. Its polar diameter, measured from the North Pole to the South Pole, is somewhat less than the equatorial diameter because of this flattening. Although Earth is the largest of the four planets—Mercury, Venus, Earth, and Mars—that make up the inner solar system (the planets closest to the Sun), it is small compared with the giant planets of the outer solar system—Jupiter, Saturn, Uranus, and Neptune. For example, the largest planet, Jupiter, has a diameter at its equator of 143,000 km (89,000 mi), 11 times greater than that of Earth. A famous atmospheric feature on Jupiter, the Great Red Spot, is so large that three Earths would fit inside it.

Earth has one natural satellite, the Moon. The Moon orbits the Earth, completing one revolution in an elliptical path in 27 days 7 hr 43 min 11.5 sec. The Moon orbits the Earth because of the force of Earth’s gravity. However, the Moon also exerts a gravitational force on the Earth. Evidence for the Moon’s gravitational influence can be seen in the ocean tides. A popular theory suggests that the Moon split off from Earth more than 4 billion years ago when a large meteorite or small planet struck the Earth. As Earth revolves around the Sun, it rotates, or spins, on its axis, an imaginary line that runs between the North and South poles. The period of one complete rotation is defined as a day and takes 23 hr 56 min 4.1 sec. The period of one revolution around the Sun is defined as a year, or 365.2422 solar days, or 365 days 5 hr 48 min 46 sec. Earth also moves along with the Milky Way Galaxy as the Galaxy rotates and moves through space. It takes more than 200 million years for the stars in the Milky Way to complete one revolution around the Galaxy’s center.

Earth’s axis of rotation is inclined (tilted) 23.5° relative to its plane of revolution around the Sun. This inclination of the axis creates the seasons and causes the height of the Sun in the sky at noon to increase and decrease as the seasons change. The Northern Hemisphere receives the most energy from the Sun when it is tilted toward the Sun. This orientation corresponds to summer in the Northern Hemisphere and winter in the Southern Hemisphere. The Southern Hemisphere receives maximum energy when it is tilted toward the Sun, corresponding to summer in the Southern Hemisphere and winter in the Northern Hemisphere. Fall and spring occur in between these orientations.


The atmosphere is a layer of different gases that extends from Earth’s surface to the exosphere, the outer limit of the atmosphere, about 9,600 km (6,000 mi) above the surface. Near Earth’s surface, the atmosphere consists almost entirely of nitrogen (78 percent) and oxygen (21 percent). The remaining 1 percent of atmospheric gases consists of argon (0.9 percent); carbon dioxide (0.03 percent); varying amounts of water vapor; and trace amounts of hydrogen, nitrous oxide, ozone, methane, carbon monoxide, helium, neon, krypton, and xenon.

A) Layers of the Atmosphere

The layers of the atmosphere are the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The troposphere is the layer in which weather occurs and extends from the surface to about 16 km (about 10 mi) above sea level at the equator. Above the troposphere is the stratosphere, which has an upper boundary of about 50 km (about 30 mi) above sea level. The layer from 50 to 90 km (30 to 60 mi) is called the mesosphere. At an altitude of about 90 km, temperatures begin to rise. The layer that begins at this altitude is called the thermosphere because of the high temperatures that can be reached in this layer (about 1200°C, or about 2200°F). The region beyond the thermosphere is called the exosphere. The thermosphere and the exosphere overlap with another region of the atmosphere known as the ionosphere, a layer or layers of ionized air extending from almost 60 km (about 50 mi) above Earth’s surface to altitudes of 1,000 km (600 mi) and more.

Earth’s atmosphere and the way it interacts with the oceans and radiation from the Sun are responsible for the planet’s climate and weather. The atmosphere plays a key role in supporting life. Almost all life on Earth uses atmospheric oxygen for energy in a process known as cellular respiration, which is essential to life. The atmosphere also helps moderate Earth’s climate by trapping radiation from the Sun that is reflected from Earth’s surface. Water vapor, carbon dioxide, methane, and nitrous oxide in the atmosphere act as “greenhouse gases.” Like the glass in a greenhouse, they trap infrared, or heat, radiation from the Sun in the lower atmosphere and thereby help warm Earth’s surface. Without this greenhouse effect, heat radiation would escape into space, and Earth would be too cold to support most forms of life.

Other gases in the atmosphere are also essential to life. The trace amount of ozone found in Earth’s stratosphere blocks harmful ultraviolet radiation from the Sun. Without the ozone layer, life as we know it could not survive on land. Earth’s atmosphere is also an important part of a phenomenon known as the water cycle or the hydrologic cycle.

B) The Atmosphere and the Water Cycle

The water cycle simply means that Earth’s water is continually recycled between the oceans, the atmosphere, and the land. All of the water that exists on Earth today has been used and reused for billions of years. Very little water has been created or lost during this period of time. Water is constantly moving on Earth’s surface and changing back and forth between ice, liquid water, and water vapor.

The water cycle begins when the Sun heats the water in the oceans and causes it to evaporate and enter the atmosphere as water vapor. Some of this water vapor falls as precipitation directly back into the oceans, completing a short cycle. Some of the water vapor, however, reaches land, where it may fall as snow or rain. Melted snow or rain enters rivers or lakes on the land. Due to the force of gravity, the water in the rivers eventually empties back into the oceans. Melted snow or rain also may enter the ground. Groundwater may be stored for hundreds or thousands of years, but it will eventually reach the surface as springs or small pools known as seeps. Even snow that forms glacial ice or becomes part of the polar caps and is kept out of the cycle for thousands of years eventually melts or is warmed by the Sun and turned into water vapor, entering the atmosphere and falling again as precipitation. All water that falls on land eventually returns to the ocean, completing the water cycle.


Earth’s surface is the outermost layer of the planet. It includes the hydrosphere, the crust, and the biosphere.

A) Hydrosphere

The hydrosphere consists of the bodies of water that cover 71 percent of Earth’s surface. The largest of these are the oceans, which contain over 97 percent of all water on Earth. Glaciers and the polar ice caps contain just over 2 percent of Earth’s water in the form of solid ice. Only about 0.6 percent is under the surface as groundwater. Nevertheless, groundwater is 36 times more plentiful than water found in lakes, inland seas, rivers, and in the atmosphere as water vapor. Only 0.017 percent of all the water on Earth is found in lakes and rivers. And a mere 0.001 percent is found in the atmosphere as water vapor. Most of the water in glaciers, lakes, inland seas, rivers, and groundwater is fresh and can be used for drinking and agriculture. Dissolved salts compose about 3.5 percent of the water in the oceans, however, making it unsuitable for drinking or agriculture unless it is treated to remove the salts.

B) Crust

The crust consists of the continents, other land areas, and the basins, or floors, of the oceans. The dry land of Earth’s surface is called the continental crust. It is about 15 to 75 km (9 to 47 mi) thick. The oceanic crust is thinner than the continental crust. Its average thickness is 5 to 10 km (3 to 6 mi). The crust has a definite boundary called the Mohorovičić discontinuity, or simply the Moho. The boundary separates the crust from the underlying mantle, which is much thicker and is part of Earth’s interior.

Oceanic crust and continental crust differ in the type of rocks they contain. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks form when molten rock, called magma, cools and solidifies. Sedimentary rocks are usually created by the breakdown of igneous rocks. They tend to form in layers as small particles of other rocks or as the mineralized remains of dead animals and plants that have fused together over time. The remains of dead animals and plants occasionally become mineralized in sedimentary rock and are recognizable as fossils. Metamorphic rocks form when sedimentary or igneous rocks are altered by heat and pressure deep underground.

Oceanic crust consists of dark, dense igneous rocks, such as basalt and gabbro. Continental crust consists of lighter-colored, less dense igneous rocks, such as granite and diorite. Continental crust also includes metamorphic rocks and sedimentary rocks.

C) Biosphere

The biosphere includes all the areas of Earth capable of supporting life. The biosphere ranges from about 10 km (about 6 mi) into the atmosphere to the deepest ocean floor. For a long time, scientists believed that all life depended on energy from the Sun and consequently could only exist where sunlight penetrated. In the 1970s, however, scientists discovered various forms of life around hydrothermal vents on the floor of the Pacific Ocean where no sunlight penetrated. They learned that primitive bacteria formed the basis of this living community and that the bacteria derived their energy from a process called chemosynthesis that did not depend on sunlight. Some scientists believe that the biosphere may extend relatively deep into Earth’s crust. They have recovered what they believe are primitive bacteria from deeply drilled holes below the surface.

D) Changes to Earth’s Surface

Earth’s surface has been constantly changing ever since the planet formed. Most of these changes have been gradual, taking place over millions of years. Nevertheless, these gradual changes have resulted in radical modifications, involving the formation, erosion, and re-formation of mountain ranges, the movement of continents, the creation of huge supercontinents, and the breakup of supercontinents into smaller continents.

The weathering and erosion that result from the water cycle are among the principal factors responsible for changes to Earth’s surface. Another principal factor is the movement of Earth’s continents and seafloors and the buildup of mountain ranges due to a phenomenon known as plate tectonics. Heat is the basis for all of these changes. Heat in Earth’s interior is believed to be responsible for continental movement, mountain building, and the creation of new seafloor in ocean basins. Heat from the Sun is responsible for the evaporation of ocean water and the resulting precipitation that causes weathering and erosion. In effect, heat in Earth’s interior helps build up Earth’s surface while heat from the Sun helps wear down the surface.

D-1) Weathering

Weathering is the breakdown of rock at and near the surface of Earth. Most rocks originally formed in a hot, high-pressure environment below the surface where there was little exposure to water. Once the rocks reached Earth’s surface, however, they were subjected to temperature changes and exposed to water. When rocks are subjected to these kinds of surface conditions, the minerals they contain tend to change. These changes constitute the process of weathering. There are two types of weathering: physical weathering and chemical weathering.

Physical weathering involves a decrease in the size of rock material. Freezing and thawing of water in rock cavities, for example, splits rock into small pieces because water expands when it freezes.

Chemical weathering involves a chemical change in the composition of rock. For example, feldspar, a common mineral in granite and other rocks, reacts with water to form clay minerals, resulting in a new substance with totally different properties than the parent feldspar. Chemical weathering is of significance to humans because it creates the clay minerals that are important components of soil, the basis of agriculture. Chemical weathering also causes the release of dissolved forms of sodium, calcium, potassium, magnesium, and other chemical elements into surface water and groundwater. These elements are carried by surface water and groundwater to the sea and are the sources of dissolved salts in the sea.

D-2) Erosion

Erosion is the process that removes loose and weathered rock and carries it to a new site. Water, wind, and glacial ice combined with the force of gravity can cause erosion.

Erosion by running water is by far the most common process of erosion. It takes place over a longer period of time than other forms of erosion. When water from rain or melted snow moves downhill, it can carry loose rock or soil with it. Erosion by running water forms the familiar gullies and V-shaped valleys that cut into most landscapes. The force of the running water removes loose particles formed by weathering. In the process, gullies and valleys are lengthened, widened, and deepened. Often, water overflows the banks of the gullies or river channels, resulting in floods. Each new flood carries more material away to increase the size of the valley. Meanwhile, weathering loosens more and more material so the process continues.

Erosion by glacial ice is less common, but it can cause the greatest landscape changes in the shortest amount of time. Glacial ice forms in a region where snow fails to melt in the spring and summer and instead builds up as ice. For major glaciers to form, this lack of snowmelt has to occur for a number of years in areas with high precipitation. As ice accumulates and thickens, it flows as a solid mass. As it flows, it has a tremendous capacity to erode soil and even solid rock. Ice is a major factor in shaping some landscapes, especially mountainous regions. Glacial ice provides much of the spectacular scenery in these regions. Features such as horns (sharp mountain peaks), arêtes (sharp ridges), glacially formed lakes, and U-shaped valleys are all the result of glacial erosion.

Wind is an important cause of erosion only in arid (dry) regions. Wind carries sand and dust, which can scour even solid rock.

Many factors determine the rate and kind of erosion that occurs in a given area. The climate of an area determines the distribution, amount, and kind of precipitation that the area receives and thus the type and rate of weathering. An area with an arid climate erodes differently than an area with a humid climate. The elevation of an area also plays a role by determining the potential energy of running water. The higher the elevation the more energetically water will flow due to the force of gravity. The type of bedrock in an area (sandstone, granite, or shale) can determine the shapes of valleys and slopes, and the depth of streams.

A landscape’s geologic age—that is, how long current conditions of weathering and erosion have affected the area—determines its overall appearance. Relatively young landscapes tend to be more rugged and angular in appearance. Older landscapes tend to have more rounded slopes and hills. The oldest landscapes tend to be low-lying with broad, open river valleys and low, rounded hills. The overall effect of the wearing down of an area is to level the land; the tendency is toward the reduction of all land surfaces to sea level.

D-3) Plate Tectonics

Opposing this tendency toward leveling is a force responsible for raising mountains and plateaus and for creating new landmasses. These changes to Earth’s surface occur in the outermost solid portion of Earth, known as the lithosphere. The lithosphere consists of the crust and another region known as the upper mantle and is approximately 65 to 100 km (40 to 60 mi) thick. Compared with the interior of the Earth, however, this region is relatively thin. The lithosphere is thinner in proportion to the whole Earth than the skin of an apple is to the whole apple.

Scientists believe that the lithosphere is broken into a series of plates, or segments. According to the theory of plate tectonics, these plates move around on Earth’s surface over long periods of time. Tectonics comes from the Greek word, tektonikos, which means “builder.”

According to the theory, the lithosphere is divided into large and small plates. The largest plates include the Pacific plate, the North American plate, the Eurasian plate, the Antarctic plate, the Indo-Australian plate, and the African plate. Smaller plates include the Cocos plate, the Nazca plate, the Philippine plate, and the Caribbean plate. Plate sizes vary a great deal. The Cocos plate is 2,000 km (1,000 mi) wide, while the Pacific plate is nearly 14,000 km (nearly 9,000 mi) wide.

These plates move in three different ways in relation to each other. They pull apart or move away from each other, they collide or move against each other, or they slide past each other as they move sideways. The movement of these plates helps explain many geological events, such as earthquakes and volcanic eruptions as well as mountain building and the formation of the oceans and continents.

D-3a) When Plates Pull Apart

When the plates pull apart, two types of phenomena occur depending on whether the movement takes place in the oceans or on land. When plates pull apart on land, deep valleys known as rift valleys form. An example of a rift valley is the Great Rift Valley that extends from Syria in the Middle East to Mozambique in Africa. When plates pull apart in the oceans, long, sinuous chains of volcanic mountains called mid-ocean ridges form, and new seafloor is created at the site of these ridges. Rift valleys are also present along the crests of the mid-ocean ridges.

Most scientists believe that gravity and heat from the interior of the Earth cause the plates to move apart and to create new seafloor. According to this explanation, molten rock known as magma rises from Earth’s interior to form hot spots beneath the ocean floor. As two oceanic plates pull apart from each other in the middle of the oceans, a crack, or rupture, appears and forms the mid-ocean ridges. These ridges exist in all the world’s ocean basins and resemble the seams of a baseball. The molten rock rises through these cracks and creates new seafloor.

D3b) When Plates Collide

When plates collide or push against each other, regions called convergent plate margins form. Along these margins, one plate is usually forced to dive below the other. As that plate dives, it triggers the melting of the surrounding lithosphere and a region just below it known as the asthenosphere. These pockets of molten crust rise behind the margin through the overlying plate, creating curved chains of volcanoes known as arcs. This process is called subduction.

If one plate consists of oceanic crust and the other consists of continental crust, the denser oceanic crust will dive below the continental crust. If both plates are oceanic crust, then either may be subducted. If both are continental crust, subduction can continue for a while but will eventually end because continental crust is not dense enough to be forced very far into the upper mantle.

The results of this subduction process are readily visible on a map showing that 80 percent of the world’s volcanoes rim the Pacific Ocean where plates are colliding against each other. The subduction zone created by the collision of two oceanic plates—the Pacific plate and the Philippine plate—can also create a trench. Such a trench resulted in the formation of the deepest point on Earth, the Mariana Trench, which is estimated to be 11,033 m (36,198 ft) below sea level.

On the other hand, when two continental plates collide, mountain building occurs. The collision of the Indo-Australian plate with the Eurasian plate has produced the Himalayan Mountains. This collision resulted in the highest point of Earth, Mount Everest, which is 8,850 m (29,035 ft) above sea level.

D-3c) When Plates Slide Past Each Other

Finally, some of Earth’s plates neither collide nor pull apart but instead slide past each other. These regions are called transform margins. Few volcanoes occur in these areas because neither plate is forced down into Earth’s interior and little melting occurs. Earthquakes, however, are abundant as the two rigid plates slide past each other. The San Andreas Fault in California is a well-known example of a transform margin.

The movement of plates occurs at a slow pace, at an average rate of only 2.5 cm (1 in) per year. But over millions of years this gradual movement results in radical changes. Current plate movement is making the Pacific Ocean and Mediterranean Sea smaller, the Atlantic Ocean larger, and the Himalayan Mountains higher.


The interior of Earth plays an important role in plate tectonics. Scientists believe it is also responsible for Earth’s magnetic field. This field is vital to life because it shields the planet’s surface from harmful cosmic rays and from a steady stream of energetic particles from the Sun known as the solar wind.

A) Composition of the Interior

Earth’s interior consists of the mantle and the core. The mantle and core make up by far the largest part of Earth’s mass. The distance from the base of the crust to the center of the core is about 6,400 km (about 4,000 mi).
Scientists have learned about Earth’s interior by studying rocks that formed in the interior and rose to the surface. The study of meteorites, which are believed to be made of the same material that formed the Earth and its interior, has also offered clues about Earth’s interior. Finally, seismic waves generated by earthquakes provide geophysicists with information about the composition of the interior. The sudden movement of rocks during an earthquake causes vibrations that transmit energy through the Earth in the form of waves. The way these waves travel through the interior of Earth reveals the nature of materials inside the planet.

The mantle consists of three parts: the lower part of the lithosphere, the region below it known as the asthenosphere, and the region below the asthenosphere called the lower mantle. The entire mantle extends from the base of the crust to a depth of about 2,900 km (about 1,800 mi). Scientists believe the asthenosphere is made up of mushy plastic-like rock with pockets of molten rock. The term asthenosphere is derived from Greek and means “weak layer.” The asthenosphere’s soft, plastic quality allows plates in the lithosphere above it to shift and slide on top of the asthenosphere. This shifting of the lithosphere’s plates is the source of most tectonic activity. The asthenosphere is also the source of the basaltic magma that makes up much of the oceanic crust and rises through volcanic vents on the ocean floor.

The mantle consists of mostly solid iron-magnesium silicate rock mixed with many other minor components including radioactive elements. However, even this solid rock can flow like a “sticky” liquid when it is subjected to enough heat and pressure.

The core is divided into two parts, the outer core and the inner core. The outer core is about 2,260 km (about 1,404 mi) thick. The outer core is a liquid region composed mostly of iron, with smaller amounts of nickel and sulfur in liquid form. The inner core is about 1,220 km (about 758 mi) thick. The inner core is solid and is composed of iron, nickel, and sulfur in solid form. The inner core and the outer core also contain a small percentage of radioactive material. The existence of radioactive material is one of the sources of heat in Earth’s interior because as radioactive material decays, it gives off heat. Temperatures in the inner core may be as high as 6650°C (12,000°F).

B) The Core and Earth’s Magnetism

Scientists believe that Earth’s liquid iron core is instrumental in creating a magnetic field that surrounds Earth and shields the planet from harmful cosmic rays and the Sun’s solar wind. The idea that Earth is like a giant magnet was first proposed in 1600 by English physician and natural philosopher William Gilbert. Gilbert proposed the idea to explain why the magnetized needle in a compass points north. According to Gilbert, Earth’s magnetic field creates a magnetic north pole and a magnetic south pole. The magnetic poles do not correspond to the geographic North and South poles, however. Moreover, the magnetic poles wander and are not always in the same place. The north magnetic pole is currently close to Ellef Ringnes Island in the Queen Elizabeth Islands near the boundary of Canada’s Northwest Territories with Nunavut. The south magnetic pole lies just off the coast of Wilkes Land, Antarctica.

Not only do the magnetic poles wander, but they also reverse their polarity—that is, the north magnetic pole becomes the south magnetic pole and vice versa. Magnetic reversals have occurred at least 170 times over the past 100 million years. The reversals occur on average about every 200,000 years and take place gradually over a period of several thousand years. Scientists still do not understand why these magnetic reversals occur but think they may be related to Earth’s rotation and changes in the flow of liquid iron in the outer core.

Some scientists theorize that the flow of liquid iron in the outer core sets up electrical currents that produce Earth’s magnetic field. Known as the dynamo theory, this theory appears to be the best explanation yet for the origin of the magnetic field. Earth’s magnetic field operates in a region above Earth’s surface known as the magnetosphere. The magnetosphere is shaped somewhat like a teardrop with a long tail that trails away from the Earth due to the force of the solar wind.

Inside the magnetosphere are the Van Allen radiation belts, named for the American physicist James A. Van Allen who discovered them in 1958. The Van Allen belts are regions where charged particles from the Sun and from cosmic rays are trapped and sent into spiral paths along the lines of Earth’s magnetic field. The radiation belts thereby shield Earth’s surface from these highly energetic particles. Occasionally, however, due to extremely strong magnetic fields on the Sun’s surface, which are visible as sunspots, a brief burst of highly energetic particles streams along with the solar wind. Because Earth’s magnetic field lines converge and are closest to the surface at the poles, some of these energetic particles sneak through and interact with Earth’s atmosphere, creating the phenomenon known as an aurora.


A) Origin of Earth

Most scientists believe that the Earth, Sun, and all of the other planets and moons in the solar system formed about 4.6 billion years ago from a giant cloud of gas and dust known as the solar nebula. The gas and dust in this solar nebula originated in a star that ended its life in a violent explosion known as a supernova. The solar nebula consisted principally of hydrogen, the lightest element, but the nebula was also seeded with a smaller percentage of heavier elements, such as carbon and oxygen. All of the chemical elements we know were originally made in the star that became a supernova. Our bodies are made of these same chemical elements. Therefore, all of the elements in our solar system, including all of the elements in our bodies, originally came from this star-seeded solar nebula.
Due to the force of gravity tiny clumps of gas and dust began to form in the early solar nebula. As these clumps came together and grew larger, they caused the solar nebula to contract in on itself. The contraction caused the cloud of gas and dust to flatten in the shape of a disc. As the clumps continued to contract, they became very dense and hot. Eventually the atoms of hydrogen became so dense that they began to fuse in the innermost part of the cloud, and these nuclear reactions gave birth to the
Sun. The fusion of hydrogen atoms in the Sun is the source of its energy.

Many scientists favor the planetesimal theory for how the Earth and other planets formed out of this solar nebula. This theory helps explain why the inner planets became rocky while the outer planets, except for Pluto, are made up mostly of gases. The theory also explains why all of the planets orbit the Sun in the same plane.

According to this theory, temperatures decreased with increasing distance from the center of the solar nebula. In the inner region, where Mercury, Venus, Earth, and Mars formed, temperatures were low enough that certain heavier elements, such as iron and the other heavy compounds that make up rock, could condense out—that is, could change from a gas to a solid or liquid. Due to the force of gravity, small clumps of this rocky material eventually came together with the dust in the original solar nebula to form protoplanets or planetesimals (small rocky bodies). These planetesimals collided, broke apart, and re-formed until they became the four inner rocky planets. The inner region, however, was still too hot for other light elements, such as hydrogen and helium, to be retained. These elements could only exist in the outermost part of the disc, where temperatures were lower. As a result two of the outer planets—Jupiter and Saturn—are mostly made of hydrogen and helium, which are also the dominant elements in the atmospheres of Uranus and Neptune.

B) The Early Earth

Within the planetesimal Earth, heavier matter sank to the center and lighter matter rose toward the surface. Most scientists believe that Earth was never truly molten and that this transfer of matter took place in the solid state. Much of the matter that went toward the center contained radioactive material, an important source of Earth’s internal heat. As heavier material moved inward, lighter material moved outward, the planet became layered, and the layers of the core and mantle were formed. This process is called differentiation.

Not long after they formed, more than 4 billion years ago, the Earth and the Moon underwent a period when they were bombarded by meteorites, the rocky debris left over from the formation of the solar system. The impact craters created during this period of heavy bombardment are still visible on the Moon’s surface, which is unchanged. Earth’s craters, however, were long ago erased by weathering, erosion, and mountain building. Because the Moon has no atmosphere, its surface has not been subjected to weathering or erosion. Thus, the evidence of meteorite bombardment remains.
Energy released from the meteorite impacts created extremely high temperatures on Earth that melted the outer part of the planet and created the crust. By 4 billion years ago, both the oceanic and continental crust had formed, and the oldest rocks were created. These rocks are known as the Acasta Gneiss and are found in the Canadian territory of Nunavut. Due to the meteorite bombardment, the early Earth was too hot for liquid water to exist and so it was impossible for life to exist.

C) Geologic Time

Geologists divide the history of the Earth into three eons: the Archean Eon, which lasted from around 4 billion to 2.5 billion years ago; the Proterozoic Eon, which lasted from 2.5 billion to 543 million years ago; and the Phanerozoic Eon, which lasted from 543 million years ago to the present. Each eon is subdivided into different eras. For example, the Phanerozoic Eon includes the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. In turn, eras are further divided into periods. For example, the Paleozoic Era includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian Periods.

The Archean Eon is subdivided into four eras, the Eoarchean, the Paleoarchean, the Mesoarchean, and the Neoarchean. The beginning of the Archean is generally dated as the age of the oldest terrestrial rocks, which are about 4 billion years old. The Archean Eon ended 2.5 billion years ago when the Proterozoic Eon began. The Proterozoic Eon is subdivided into three eras: the Paleoproterozoic Era, the Mesoproterozoic Era, and the Neoproterozoic Era. The Proterozoic Eon lasted from 2.5 billion years ago to 543 million years ago when the Phanerozoic Eon began. The Phanerozoic Eon is subdivided into three eras: the Paleozoic Era from 543 million to 248 million years ago, the Mesozoic Era from 248 million to 65 million years ago, and the Cenozoic Era from 65 million years ago to the present.

Geologists base these divisions on the study and dating of rock layers or strata, including the fossilized remains of plants and animals found in those layers. Until the late 1800s scientists could only determine the relative ages of rock strata. They knew that in general the top layers of rock were the youngest and formed most recently, while deeper layers of rock were older. The field of stratigraphy shed much light on the relative ages of rock layers.

The study of fossils also enabled geologists to determine the relative ages of different rock layers. The fossil record helped scientists determine how organisms evolved or when they became extinct. By studying rock layers around the world, geologists and paleontologists saw that the remains of certain animal and plant species occurred in the same layers, but were absent or altered in other layers. They soon developed a fossil index that also helped determine the relative ages of rock layers.
Beginning in the 1890s, scientists learned that radioactive elements in rock decay at a known rate. By studying this radioactive decay, they could determine an absolute age for rock layers. This type of dating, known as radiometric dating, confirmed the relative ages determined through stratigraphy and the fossil index and assigned absolute ages to the various strata. As a result scientists were able to assemble Earth’s geologic time scale from the Archean Eon to the present. See also Geologic Time.

C-1) Precambrian

The Precambrian is a time span that includes the Archean and Proterozoic eons and began about 4 billion years ago. The Precambrian marks the first formation of continents, the oceans, the atmosphere, and life. The Precambrian represents the oldest chapter in Earth’s history that can still be studied. Very little remains of Earth from the period of 4.6 billion to about 4 billion years ago due to the melting of rock caused by the early period of meteorite bombardment. Rocks dating from the Precambrian, however, have been found in Africa, Antarctica, Australia, Brazil, Canada, and Scandinavia. Some zircon mineral grains deposited in Australian rock layers have been dated to 4.2 billion years.

The Precambrian is also the longest chapter in Earth’s history, spanning a period of about 3.5 billion years. During this timeframe, the atmosphere and the oceans formed from gases that escaped from the hot interior of the planet as a result of widespread volcanic eruptions. The early atmosphere consisted primarily of nitrogen, carbon dioxide, and water vapor. As Earth continued to cool, the water vapor condensed out and fell as precipitation to form the oceans. Some scientists believe that much of Earth’s water vapor originally came from comets containing frozen water that struck Earth during the period of meteorite bombardment.

By studying 2-billion-year-old rocks found in northwestern Canada, as well as 2.5-billion-year-old rocks in China, scientists have found evidence that plate tectonics began shaping Earth’s surface as early as the middle Precambrian. About a billion years ago, the Earth’s plates were centered around the South Pole and formed a supercontinent called Rodinia. Slowly, pieces of this supercontinent broke away from the central continent and traveled north, forming smaller continents.

Life originated during the Precambrian. The earliest fossil evidence of life consists of prokaryotes, one-celled organisms that lacked a nucleus and reproduced by dividing, a process known as asexual reproduction. Asexual division meant that a prokaryote’s hereditary material was copied unchanged. The first prokaryotes were bacteria known as archaebacteria. Scientists believe they came into existence perhaps as early as 3.8 billion years ago, but certainly by about 3.5 billion years ago, and were anaerobic—that is, they did not require oxygen to produce energy. Free oxygen barely existed in the atmosphere of the early Earth.

Archaebacteria were followed about 3.46 billion years ago by another type of prokaryote known as cyanobacteria or blue-green algae. These cyanobacteria gradually introduced oxygen in the atmosphere as a result of photosynthesis. In shallow tropical waters, cyanobacteria formed mats that grew into humps called stromatolites. Fossilized stromatolites have been found in rocks in the Pilbara region of western Australia that are more than 3.4 billion years old and in rocks of the Gunflint Chert region of northwest Lake Superior that are about 2.1 billion years old.

For billions of years, life existed only in the simple form of prokaryotes. Prokaryotes were followed by the relatively more advanced eukaryotes, organisms that have a nucleus in their cells and that reproduce by combining or sharing their heredity makeup rather than by simply dividing. Sexual reproduction marked a milestone in life on Earth because it created the possibility of hereditary variation and enabled organisms to adapt more easily to a changing environment. The very latest part of Precambrian time some 560 million to 545 million years ago saw the appearance of an intriguing group of fossil organisms known as the Ediacaran fauna. First discovered in the northern Flinders Range region of Australia in the mid-1940s and subsequently found in many locations throughout the world, these strange fossils appear to be the precursors of many of the fossil groups that were to explode in Earth's oceans in the Paleozoic Era. See also Evolution; Natural Selection.

C-2) Paleozoic Era

At the start of the Paleozoic Era about 543 million years ago, an enormous expansion in the diversity and complexity of life occurred. This event took place in the Cambrian Period and is called the Cambrian explosion. Nothing like it has happened since. Almost all of the major groups of animals we know today made their first appearance during the Cambrian explosion. Almost all of the different “body plans” found in animals today—that is, the way an animal’s body is designed, with heads, legs, rear ends, claws, tentacles, or antennae—also originated during this period.

Fishes first appeared during the Paleozoic Era, and multicellular plants began growing on the land. Other land animals, such as scorpions, insects, and amphibians, also originated during this time. Just as new forms of life were being created, however, other forms of life were going out of existence. Natural selection meant that some species were able to flourish, while others failed. In fact, mass extinctions of animal and plant species were commonplace.

Most of the early complex life forms of the Cambrian explosion lived in the sea. The creation of warm, shallow seas, along with the buildup of oxygen in the atmosphere, may have aided this explosion of life forms. The shallow seas were created by the breakup of the supercontinent Rodinia. During the Ordovician, Silurian, and Devonian periods, which followed the Cambrian Period and lasted from 490 million to 354 million years ago, some of the continental pieces that had broken off Rodinia collided. These collisions resulted in larger continental masses in equatorial regions and in the Northern Hemisphere. The collisions built a number of mountain ranges, including parts of the Appalachian Mountains in North America and the Caledonian Mountains of northern Europe.

Toward the close of the Paleozoic Era, two large continental masses, Gondwanaland to the south and Laurasia to the north, faced each other across the equator. Their slow but eventful collision during the Permian Period of the Paleozoic Era, which lasted from 290 million to 248 million years ago, assembled the supercontinent Pangaea and resulted in some of the grandest mountains in the history of Earth. These mountains included other parts of the Appalachians and the Ural Mountains of Asia. At the close of the Paleozoic Era, Pangaea represented over 90 percent of all the continental landmasses. Pangaea straddled the equator with a huge mouthlike opening that faced east. This opening was the Tethys Ocean, which closed as India moved northward creating the Himalayas. The last remnants of the Tethys Ocean can be seen in today’s Mediterranean Sea.

The Paleozoic came to an end with a major extinction event, when perhaps as many as 90 percent of all plant and animal species died out. The reason is not known for sure, but many scientists believe that huge volcanic outpourings of lavas in central Siberia, coupled with an asteroid impact, were joint contributing factors.

C-3) Mesozoic Era

The Mesozoic Era, beginning 248 million years ago, is often characterized as the Age of Reptiles because reptiles were the dominant life forms during this era. Reptiles dominated not only on land, as dinosaurs, but also in the sea, in the form of the plesiosaurs and ichthyosaurs, and in the air, as pterosaurs, which were flying reptiles. See also Dinosaur; Plesiosaur; Ichthyosaur; Pterosaur.

The Mesozoic Era is divided into three geological periods: the Triassic, which lasted from 248 million to 206 million years ago; the Jurassic, from 206 million to 144 million years ago; and the Cretaceous, from 144 million to 65 million years ago. The dinosaurs emerged during the Triassic Period and were one of the most successful animals in Earth’s history, lasting for about 180 million years before going extinct at the end of the Cretaceous Period. The first birds and mammals and the first flowering plants also appeared during the Mesozoic Era. Before flowering plants emerged, plants with seed-bearing cones known as conifers were the dominant form of plants. Flowering plants soon replaced conifers as the dominant form of vegetation during the Mesozoic Era.

The Mesozoic was an eventful era geologically with many changes to Earth’s surface. Pangaea continued to exist for another 50 million years during the early Mesozoic Era. By the early Jurassic Period, Pangaea began to break up. What is now South America began splitting from what is now Africa, and in the process the South Atlantic Ocean formed. As the landmass that became North America drifted away from Pangaea and moved westward, a long subduction zone extended along North America’s western margin. This subduction zone and the accompanying arc of volcanoes extended from what is now Alaska to the southern tip of South America. Much of this feature, called the American Cordillera, exists today as the eastern margin of the Pacific Ring of Fire.

During the Cretaceous Period, heat continued to be released from the margins of the drifting continents, and as they slowly sank, vast inland seas formed in much of the continental interiors. The fossilized remains of fishes and marine mollusks called ammonites can be found today in the middle of the North American continent because these areas were once underwater. Large continental masses broke off the northern part of southern Gondwanaland during this period and began to narrow the Tethys Ocean. The largest of these continental masses, present-day India, moved northward toward its collision with southern Asia. As both the North Atlantic Ocean and South Atlantic Ocean continued to open, North and South America became isolated continents for the first time in 450 million years. Their westward journey resulted in mountains along their western margins, including the Andes of South America.

C-4) Cenozoic Era

The Cenozoic Era, beginning about 65 million years ago, is the period when mammals became the dominant form of life on land. Human beings first appeared in the later stages of the Cenozoic Era. In short, the modern world as we know it, with its characteristic geographical features and its animals and plants, came into being. All of the continents that we know today took shape during this era.
A single catastrophic event may have been responsible for this relatively abrupt change from the Age of Reptiles to the Age of Mammals. Most scientists now believe that a huge asteroid or comet struck the Earth at the end of the Mesozoic and the beginning of the Cenozoic eras, causing the extinction of many forms of life, including the dinosaurs. Evidence of this collision came with the discovery of a large impact crater off the coast of Mexico’s Yucatán Peninsula and the worldwide finding of iridium, a metallic element rare on Earth but abundant in meteorites, in rock layers dated from the end of the Cretaceous Period. The extinction of the dinosaurs opened the way for mammals to become the dominant land animals.
The Cenozoic Era is divided into the Tertiary and the Quaternary periods. The Tertiary Period lasted from about 65 million to about 1.8 million years ago. The Quaternary Period began about 1.8 million years ago and continues to the present day. These periods are further subdivided into epochs, such as the Pleistocene, from 1.8 million to 10,000 years ago, and the Holocene, from 10,000 years ago to the present.
Early in the Tertiary Period, Pangaea was completely disassembled, and the modern continents were all clearly outlined. India and other continental masses began colliding with southern Asia to form the Himalayas. Africa and a series of smaller microcontinents began colliding with southern Europe to form the Alps. The Tethys Ocean was nearly closed and began to resemble today’s Mediterranean Sea. As the Tethys continued to narrow, the Atlantic continued to open, becoming an ever-wider ocean. Iceland appeared as a new island in later Tertiary time, and its active volcanism today indicates that seafloor spreading is still causing the country to grow.
Late in the Tertiary Period, about 6 million years ago, humans began to evolve in Africa. These early humans began to migrate to other parts of the world between 2 million and 1.7 million years ago.
The Quaternary Period marks the onset of the great ice ages. Many times, perhaps at least once every 100,000 years on average, vast glaciers 3 km (2 mi) thick invaded much of North America, Europe, and parts of Asia. The glaciers eroded considerable amounts of material that stood in their paths, gouging out U-shaped valleys. Anatomically modern human beings, known as Homo sapiens sapiens, became the dominant form of life in the Quaternary Period. Most anthropologists (scientists who study human life and culture) believe that anatomically modern humans originated only recently in Earth’s 4.6-billion-year history, about 130,000 years ago. See also Human Evolution.


With the rise of human civilization about 8,000 years ago and especially since the Industrial Revolution in the mid-1700s, human beings began to alter the surface, water, and atmosphere of Earth. In doing so, they have become active geological agents, not unlike other forces of change that influence the planet. As a result, Earth’s immediate future depends to a great extent on the behavior of humans. For example, the widespread use of fossil fuels is releasing carbon dioxide and other greenhouse gases into the atmosphere and threatens to warm the planet’s surface. This global warming could melt glaciers and the polar ice caps, which could flood coastlines around the world and many island nations. In effect, the carbon dioxide that was removed from Earth’s early atmosphere by the oceans and by primitive plant and animal life, and subsequently buried as fossilized remains in sedimentary rock, is being released back into the atmosphere and is threatening the existence of living things. See also Global Warming.
Even without human intervention, Earth will continue to change because it is geologically active. Many scientists believe that some of these changes can be predicted. For example, based on studies of the rate that the seafloor is spreading in the Red Sea, some geologists predict that in 200 million years the Red Sea will be the same size as the Atlantic Ocean is today. Other scientists predict that the continent of Asia will break apart millions of years from now, and as it does, Lake Baikal in Siberia will become a vast ocean, separating two landmasses that once made up the Asian continent.
In the far, far distant future, however, scientists believe that Earth will become an uninhabitable planet, scorched by the Sun. Knowing the rate at which nuclear fusion occurs in the Sun and knowing the Sun’s mass, astrophysicists (scientists who study stars) have calculated that the Sun will become brighter and hotter about 3 billion years from now, when it will be hot enough to boil Earth’s oceans away. Based on studies of how other Sun-like stars have evolved, scientists predict that the Sun will become a red giant, a star with a very large, hot atmosphere, about 7 billion years from now. As a red giant the Sun’s outer atmosphere will expand until it engulfs the planet Mercury. The Sun will then be 2,000 times brighter than it is now and so hot it will melt Earth’s rocks. Earth will end its existence as a burnt cinder.
Three billion years is the life span of millions of human generations, however. Perhaps by then, humans will have learned how to journey beyond the solar system to colonize other planets in the Milky Way Galaxy and find another place to call “home.


Last edited by Sureshlasi; Thursday, August 09, 2007 at 08:52 PM.
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Jupiter (planet)

Jupiter (planet), fifth planet from the Sun and the largest planet in the solar system. The fourth brightest object in Earth’s sky, after the Sun, the Moon, and Venus, Jupiter is more than three times brighter than Sirius, the brightest star. Due to its prominence in the sky, the Romans named the planet for their chief god, Jupiter.
Jupiter orbits the Sun at an average distance of 780 million km (480 million mi), which is about five times the distance from Earth to the Sun. Jupiter’s year, or the time it takes to complete an orbit about the Sun, is 11.9 Earth years, and its day, or the time it takes to rotate on its axis, is about 9.9 hours, less than half an Earth day.
Unlike the rocky inner planets of the solar system (Mercury, Venus, Earth, and Mars), Jupiter is a ball of dense gas and has no solid surface. Jupiter may have a core composed of rock-forming minerals like those trapped in comet ices, but the core makes up less than 5 percent of the planet’s mass. The force of gravity at the level of the highest clouds in Jupiter’s atmosphere is about 2.5 times the force of gravity at Earth’s surface.
Gas and clouds in Jupiter’s atmosphere travel at high speeds. This phenomenon is not fully understood but it is related to the planet’s high rate of rotation. These gases and clouds travel faster at the equator than at higher latitudes. The gases and clouds of the atmosphere are thrown outward as the planet rotates, similar to the manner in which mud is thrown outward from a spinning wheel. The balance between gravity and this outward force, which is proportional to the rotational speed of the atmosphere, noticeably distorts the planet’s round shape. Higher speed at the equator produces greater outward force, causing an equatorial bulge, whereas lower speed at the poles gives gravity the edge, leading to polar flattening. Jupiter’s equatorial diameter is 143,000 km (89,000 mi), 6.5 percent larger that the polar diameter of 133,700 km (83,000 mi).


In 1610, when Italian philosopher and scientist Galileo Galilei began the first telescopic study of Jupiter, the commonly held view of the universe was one developed by 2nd-century Alexandrian astronomer Ptolemy. Ptolemy’s model assumed that all of the stars and planets moved in orbits around Earth. When Galileo discovered four satellites, or moons, revolving around Jupiter, he recognized that he had discovered evidence in support of the competing Copernican theory. This theory, proposed by Polish astronomer Nicolaus Copernicus in the early 1500s, held that the planets, including Earth, revolve around the Sun. Galileo strongly supported the Copernican model and played a major role in advancing this theory and creating a more modern view of the solar system. In recognition of Galileo’s contribution, the four largest of Jupiter’s moons are collectively known as the Galilean satellites.
When viewed through a modern telescope, the oblate (flattened) disk of Jupiter has a pearly color with bands of pastel browns and blues. Earth-based observers can best view Jupiter when it is near solar opposition—that is, when both planets are aligned on the same side of the Sun. At opposition, Jupiter rises at sunset and sets at sunrise, which means that it is visible all night long. In addition, the distance from Earth to Jupiter is at its minimum at opposition, making Jupiter appear nearly one and a half times larger than it does when it is farthest from Earth. Because Jupiter orbits the Sun in the same direction as Earth, Earth has to travel a little more than a full year to catch up to Jupiter from one opposition to the next. The time interval between oppositions is about 399 days. In the year 2002, this opposition occurred on January 1.
In the mid-1950s radio astronomers discovered that Jupiter emitted strong radio waves at many frequencies (see Radio Astronomy). This radio data indicated that Jupiter has a magnetic field—that is, a surrounding area of magnetic force. Jupiter, in other words, acts like a giant magnet. Earth has a similar but much weaker magnetic field. Just above the clouds Jupiter’s magnetic field is 10 times more intense than Earth’s field is at Earth’s surface. Like Earth’s field, Jupiter’s field is tipped about 10° relative to its axis of rotation. The interaction of Jupiter’s magnetic field with charged particles ejected from the Sun creates radio noise near the poles and auroras similar to Earth’s aurora borealis, or northern lights. As Jupiter rotates, its north and south magnetic poles become obscured to different extents, which makes the intensity of the planet’s radio noise as detected from Earth vary in a regular pattern. The pattern repeats at intervals of 9 hours 55.5 minutes, indicating the rate of rotation of Jupiter’s interior where the magnetic field is generated.


Astronomers were able to accurately determine Jupiter’s mass even before 1900. They calculated the gravitational force that Jupiter exerts on its satellites by measuring their movements around the planet over an extended period. Because the gravitational force exerted by a planet is proportional to its mass, they could deduce Jupiter’s mass. Spacecraft flying by Jupiter have made more detailed studies of Jupiter’s gravitational field possible, giving clues about the planet’s inner structure. These spacecraft have also relayed close-up images of the clouds and information about the composition of Jupiter’s outer layers. Putting all of this data together, astronomers have assembled a detailed picture of Jupiter’s composition and structure.

A) Composition of Jupiter
The fact that Jupiter’s radius is 11.2 times larger than Earth’s means that its volume is more than 1,300 times the volume of Earth. The mass of Jupiter, however, is only 318 times the mass of Earth. Jupiter’s density (1.33 g/cm3) is therefore less than one-fourth of Earth’s density (5.52 g/cm3). Jupiter’s low density indicates that the planet is composed primarily of the lightest elements—hydrogen and helium.
Galileo, a National Aeronautics and Space Administration (NASA) spacecraft composed of an orbiter and a planetary probe, arrived at Jupiter in 1995. The probe, which entered the atmosphere near 6° north, measured high winds and a puzzling lack of water molecules deep in Jupiter’s atmosphere. It also found that the ratio of the amount of hydrogen present to the amount of helium present was similar to the ratio that has been determined for the outer envelope of the Sun. This similarity in the hydrogen-helium ratio supports the theory that Jupiter and the Sun formed from the same cloud of material (See also Planetary Science).

B) Structure of Jupiter
When a spacecraft flies by a planet, the gravitational field of the planet causes the spacecraft to accelerate. This change in speed and direction can be detected as a slight shift in the frequency of the radio signals that the spacecraft is sending back to Earth (see Doppler Effect). Scientists have analyzed radio signals from several spacecraft that have passed Jupiter and have combined their results with studies of Jupiter's composition to create computer models of the planet. The computer models predict that Jupiter's outer layer, composed of a gaseous mixture of hydrogen, helium, and traces of hydrogen-rich compounds such as ammonia, methane, and water vapor, is about 1,000 km (about 600 mi) thick. Beneath this layer, the pressure is so great and the atmosphere is so hot and compressed that the hydrogen and helium atoms do not behave as a gas, but as what physicists call a supercritical fluid. Supercritical fluids form at high temperatures and pressures and have properties similar to those of both gases and liquids. The supercritical zone extends 20,000 to 30,000 km (12,000 to 19,000 mi) into Jupiter, which is about one-fourth to one-third of the radius of the planet.
Beneath the supercritical fluid zone, the pressure reaches 3 million Earth atmospheres. At this depth, the atoms collide so frequently and violently that the hydrogen atoms are ionized—that is, the negatively charged electrons are stripped away from the positively charged protons of the hydrogen nuclei. This ionization results in a sea of electrically charged particles that resembles a liquid metal and gives rise to Jupiter’s magnetic field. This liquid metallic hydrogen zone is 30,000 to 40,000 km (19,000 to 25,000 mi) thick—about half the radius of the planet—and extends to the molten rock core at Jupiter's center. The molten rock core occupies a sphere with a radius of about 10,000 km (about 6,000 mi)—about one-fourth of Jupiter's total radius—and has a mass perhaps 10 to 15 times the mass of Earth.

C) Evolution of Jupiter
According to current theories, an enormous disk of dust and gas encircled the Sun as it formed more than 4.5 billion years ago. The material in this disk eventually formed the planets, moons, and asteroids of the solar system. Mineral particles and metal-rich grains in this disk combined with icy comet-like fragments to form seeds for larger bodies. The largest fragments swept up the most dust and surrounding gases and became the planets. Planets such as Jupiter and Saturn that attained masses greater than 14 times the mass of Earth had sufficient gravity to attract and hold hydrogen and helium atoms, which constituted most of the disk material. These planets became gas giants. Planets with weaker gravity, such as Earth and Mars, could not hold hydrogen and helium and so remained smaller and mainly rocky. Eventually, nearly all of the matter of the disk was concentrated in a few bodies: the planets and their moons. Jupiter was the largest of these bodies.
Despite the planet’s large size, Jupiter is far too small to become a star. The pressure and temperature at Jupiter’s core are not high enough to cause sustained fusion of hydrogen—the process that makes a star shine. Even though Jupiter contains more than twice as much mass as all the other planetary bodies in the solar system combined, it would need to have about 80 times its current mass for sustained fusion to occur.


As light travels outward from the Sun it spreads equally in all directions, decreasing in intensity. Because Jupiter is five times more distant from the Sun than Earth is, the light that falls on Jupiter is 25 times less intense than the light that strikes Earth, and the intensity of solar energy reaching Jupiter is therefore only about 4 percent of that reaching Earth. Studies of infrared radiation (energy radiated as heat) from Jupiter reveal that the planet gives off 1.67 times as much energy as it receives from the Sun. The source of the excess radiated energy is apparently stored heat that was created by the energy of impacts that occurred during Jupiter’s formation and the subsequent gravitational compression of the planet’s material. The difference in temperature between the top of Jupiter’s atmosphere and its deepest layers drives the circulation that transports heat from deep within the planet outward.

A) Banded Appearance
From a distance Jupiter appears to have horizontal stripes, which result from winds that shear its cloud layers into sharply defined bands. These bands circle the planet, with winds along the edges of adjacent bands blowing in opposite directions. Earth’s trade winds form a similar pattern, but Jupiter’s winds are much stronger and more stable. The strongest winds, at low latitudes near Jupiter’s equator, drive individual cloud systems 11° eastward every 24 hours. At higher latitudes the clouds alternately shift westward and eastward corresponding to the banded structure of the atmosphere, which is sculpted by these wind jets. This cloud motion indicates winds of 600 km/h (370 mph) at low latitudes with winds decreasing to tens of kilometers per hour at high latitudes.
Some of the cloud bands appear whitish, while others are orangey or brown. Scientists believe that the colors result from the presence of trace gases in Jupiter’s atmosphere. In the upper reaches of the atmosphere, the temperature drops below the freezing point of ammonia, one of the trace gases. In regions where warmer gases are carried up from below, the fresh ammonia freezes to form highly reflective white ice crystals. The ice crystals are swept horizontally by prevailing winds, causing the formation of bands that appear bright from reflected sunlight. Ultraviolet radiation from the Sun interacts with molecules of other trace gases in the upper atmosphere and generates yellow-brown smog. This smog settles down on the clouds causing those that are deeper in the atmosphere to appear darker brown. Within the darker bands, the atmosphere tends to sink and the ammonia ice crystals melt, exposing more brown smog particles and causing further darkening.

B) Storms
Major storms often appear suddenly on Jupiter. Evidence suggests that, unlike storms on Earth, which are driven by solar heating of the atmosphere, Jupiter’s storms are caused by bubbles of warmer gas rising through the atmosphere from deep within the planet. These bubbles, carrying varying amounts of heat, create cloud systems that are constrained on the north and south by bands of strong wind blowing in opposite directions. Unable to move north or south, and with no solid landmasses to create friction, the storms roll in the winds and feed off smaller storm systems for weeks or longer.
Jupiter’s most famous storm, the Great Red Spot, has persisted for centuries. The Great Red Spot is so enormous that if three Earths were placed side by side in front of it, they would scarcely span it. The earliest report of a red spot was by Robert Hooke in 1664, although scientists are not sure if the current spot has existed continuously since that time. The cause of the Great Red Spot is not yet known, but its motion is such that it must sustain itself on energy gained from the upper atmosphere, perhaps by absorbing the energy of smaller atmospheric disturbances. It cannot be linked to a heat source deep in the atmosphere, because it moves slowly westward at an irregular rate. The red color of the spot appears to be caused by impurities such as sulfur or phosphorus compounds that absorb ultraviolet, violet, and blue light.
In 1938, three smaller, separate storms formed in a belt near 30° south latitude. Because of their color and shape, these storms were called white ovals. In 1998 astronomers observed that two of these white ovals had merged to form a slightly larger storm system, visible as a single white oval. In 2000 the remaining two storm systems combined into a single storm. Although this storm is still smaller than the Great Red Spot, the east-west dimension of the remaining white oval is roughly equal to the diameter of Earth. The storm rotates in a counter-clockwise direction as seen from above. Weather systems on Earth that behave in this manner have air masses rising near their centers. Analysis of infrared light that the white ovals on Jupiter emit reveals that they are composed of ammonia ice and that their temperature is -157°C (-251°F). At this temperature ammonia forms white crystals. Thus, the data contributes to a consistent picture of rising ammonia gas expanding, freezing, and forming a fresh white ice cloud above the weather system.

C) Comet Shoemaker-Levy
In 1994 the comet Shoemaker-Levy 9 provided a unique opportunity to study Jupiter’s atmosphere. The comet was torn apart by Jupiter’s gravitational field as it approached the planet. The resulting fragments collided with Jupiter’s upper atmosphere at speeds of up to 216,000 km/h (134,000 mph). The collisions generated huge explosions in Jupiter’s stratosphere. About a minute after the fragments entered Jupiter’s upper atmosphere, an explosion ejected a rapidly expanding cloud of material about 3,000 km (1,900 mi) above Jupiter’s cloud layer. When this material fell back into Jupiter’s stratosphere, it generated shock waves and discharged enough energy to heat an area several thousand kilometers in diameter from its normally frigid -100°C (-150°F) to more than 700°C (1,300°F). The resulting debris cooled and formed a dark layer in Jupiter’s stratosphere that slowly settled into the deeper atmosphere. Winds then swept the debris around the planet and removed all trace of the event within months.


The thick layer of liquid metallic hydrogen created by the high pressures and temperatures deep within Jupiter generates an enormous magnetic field. The interaction between the rotation of the planet and cooling of the outer region drives circulation within this liquid metallic hydrogen zone. The circulation of the metallic hydrogen generates electrical currents. These electrical currents, rotating with the planet, create a magnetic field that is similar in shape to Earth’s field but far stronger. Out beyond the orbits of Jupiter’s four large Galilean moons, charged particles emitted by the Sun greatly distort the weak outer envelope of the field, pushing it in toward Jupiter on the side facing the Sun and dragging it out in a long tail on the opposite side. Closer to Jupiter the strong field traps the charged particles. The entire region of particle-field interactions is known as the magnetosphere.
Particles that are trapped by the strong inner field of Jupiter’s magnetosphere move in helical, or spiral, paths along the magnetic field lines toward the poles of Jupiter’s field. Because the magnetic field is more concentrated near the poles, the particles frequently collide with one another and with molecules in Jupiter’s upper atmosphere. These collisions create auroras over the poles that are similar to Earth’s aurora borealis and aurora australis—the northern lights and southern lights.


Jupiter, encircled by at least 61 satellites and a series of thin rings, is similar to a miniature solar system. For this reason, Jupiter is of great interest to planetary scientists and others who are concerned with the formation of planetary systems. Sixteen of Jupiter's moons are discussed in this section; the remaining 45 are relatively recent discoveries and have not yet been extensively studied.

A) Jupiter’s Rings and Inner Satellites
In 1979, a camera on the Voyager 1 spacecraft used a long exposure with the line of sight passing through the equatorial region to determine that Jupiter has a thin ring. Three inner moons of Jupiter were also discovered from images taken by the Voyager spacecraft. These moons, named Metis, Adrastea, and Thebe, along with Amalthea, discovered in 1892, revolve around Jupiter at average distances of 128,000 km (79,500 mi), 129,000 km (80,000 mi), 222,000 km (138,000 mi), and 181,000 km (112,000 mi), respectively. They are dark and irregularly shaped. Amalthea is 135 km (84 mi) across its largest dimension, and the other three moons range from 10 to 50 km (6 to 31 mi) in diameter.

The ring is composed of three parts: a main ring, a halo, and an outer ring. The main ring is flat, about 7,000 km (4,300 mi) wide, and extends out to 128,500 km (79,800 mi), about twice the radius of Jupiter. A halo of charged particles, which are spread poleward by magnetic interactions, overlaps the main ring. A faint, outer, gossamer ring begins beyond the main ring and extends to the orbits of Amalthea and Thebe.
The ring and the four inner moons form a closely related system. In 1998 astronomers at Cornell University concluded that material scattered from the four inner moons is the source of the ring particles, and that the structure of the rings is determined by the dimensions and tilts of the orbits of the moons relative to Jupiter's equator. Dust is knocked off when micrometeoroids strike Jupiter's four innermost moons. Metis and Adrastea orbit Jupiter at the outer edge of the inner rings and sweep up material in their paths, acting as "shepherds" to keep the outer edge of the ring sharp. Amalthea and Thebe, orbiting farther from Jupiter, supply material to sustain the outer gossamer ring.

B) The Galilean Moons
Beyond the rings and small inner satellites are Jupiter’s famous Galilean moons. Galileo discovered these satellites in 1610. These four moons are much larger than Jupiter’s other satellites. They range from the size of Earth’s Moon to the size of the planet Mercury. The closer a moon is to Jupiter, the more dense it tends to be, just as the closer a planet is to the Sun, the more dense it tends to be. Planetary scientists believe that these parallel trends reveal much about how the planets and the solar system formed and evolved over the intervening ages. The innermost satellites, Io and Europa, which orbit Jupiter at 421,000 and 671,000 km (262,000 and 417,000 mi), are dense and rocky like Mercury, Venus, Earth, and Mars, the innermost planets of the solar system. Ganymede and Callisto, at greater distances from Jupiter—1,070,000 and 1,883,000 km (660,000 and 1,117,000 mi)—are composed of lower-density, icy materials.
Tidal stresses—fluctuations in gravitational forces—repeatedly flex the moons Io and Europa. The resulting expansion and contraction of the moons causes internal friction that heats them up. Both satellites exhibit forms of volcanic activity as a result. Io is dominated by active sulfur volcanism, while Europa is covered with a blanket of water ice that cracks and vents the tidally generated heat. Exobiologists, scientists who study the possibility of life on other planets, speculate that conditions within the ices on Europa might support primitive forms of life. The Galileo spacecraft began orbiting Jupiter in December 1995 and initiated an in-depth examination of the Galilean moons in December 1997. With data sent back from the spacecraft, scientists have determined that Ganymede has its own magnetic field and Callisto has patterns in its surface structures that show the moon has slowly been modified by its environment. Europa has a complex, glacially active surface, and Io is much more volcanically active than originally believed. Galileo continued to gather data into 2003, focusing primarily on Io and Europa, but also engaging in several close passes by Ganymede and Callisto.

C The Outer Satellites
Prior to 1999, two additional families of small satellites, located in inclined elliptical orbits at large distances from Jupiter, were known. The first family, Leda, Himalia, Lysithea, and Elara, orbit at average distances of about 11 million km (about 6.6 million mi). These satellites, along with the inner and Galilean satellites and Jupiter’s rings, revolve about Jupiter in the same direction that the planet rotates on its axis. The second family, Ananke, Carme, Pasiphae, and Sinope, orbit at average distances of about 21 to 23 million km (about 13 to 14 million mi) and revolve in the opposite direction. Since 1999, 45 more distant small moons have been found, bringing the total number of known satellites to 61. Most of these new members are also in elongated, tilted orbits and are less than 10 kilometers (6 miles) in diameter. The nature of the orbits of the outer moons suggests that they are trapped asteroids or fragments of larger bodies that were broken up by collisions with asteroids or comets. Eleven of these newly discovered moons have been named: Themisto, Iocaste, Harpalyke, Praxidike, Taygete, Chalden, Kalyke, Callirrhoe, Megaclite, Isonoe, and Erinome. The rest are referred to by numbers that reflect the year and order in which they were discovered.


An era of detailed observations of Jupiter began with NASA’s Pioneer 10 spacecraft, launched in March 1972. Pioneer 10 was followed in April 1973 by Pioneer 11. These simple spinning spacecraft carried instruments that provided excellent information on Jupiter’s gravitational field, magnetosphere, and upper stratosphere. The next NASA spacecraft explorations of Jupiter were the Voyager 1 and Voyager 2 missions of 1979. The Voyager craft were designed to maintain a stable orientation in space, so that onboard cameras and other imaging instruments could be used to map Jupiter in ultraviolet (UV), visible, and infrared (IR) light. The visual images provided detailed maps of Jupiter’s cloud deck, the IR data produced information about how heat escaped and the relative abundance of materials in Jupiter’s upper atmosphere, and the UV data provided information on the interaction of Jupiter’s magnetic field with the solar wind and the upper atmosphere. In 1990 NASA launched the spacecraft Ulysses from an orbiting space shuttle to study the Sun from an orbit passing over its poles. To get Ulysses into that unusual orbit, astronomers aimed the spacecraft to swing twice around Jupiter, using the planet as a gravitational slingshot. While flying by Jupiter in 1992 and 1998 Ulysses took measurements of Jupiter’s magnetosphere and gravitational field.
In 1989, prior to the launch of Ulysses, NASA launched the Galileo spacecraft on a mission to Jupiter. The Galileo spacecraft took a slower route to Jupiter, reaching the planet in 1995. Unlike previous spacecraft that merely passed by Jupiter, Galileo entered orbit around the planet in order to engage in longer-term study. The spacecraft also launched a remote probe into the planet. The probe plunged through Jupiter’s opaque cloud deck, and the orbiting Galileo spacecraft relayed information the probe gathered to Earth. The probe transmitted its readings until it reached a depth in Jupiter’s atmosphere where the pressure was 20 Earth atmospheres, at which point high temperatures caused its transmitter to fail. Galileo’s remote probe provided direct measurement of the relative abundance of the elements in Jupiter’s outer atmosphere and the strength of its winds, revealing an unexpected low level of water in the clouds and high wind speeds. The Galileo spacecraft continued to gather and transmit information about Jupiter’s magnetic field, atmosphere, and moons until 2003. NASA dove the spacecraft into Jupiter’s atmosphere when Galileo’s fuel dwindled in September 2003. Galileo was traveling so fast that friction with the atmosphere burned up the spacecraft.
More data on Jupiter was collected by the Cassini/Huygens spacecraft, which flew by Jupiter in December 2000 on its way to a rendezvous with Saturn in 2004. Cassini’s mission to Saturn was similar to Galileo’s Jupiter mission: to orbit Saturn and drop the Huygens probe, built by the European Space Agency, onto Saturn’s moon Titan.


Planetary scientists are interested in studying Jupiter further to learn about its interior structure, chemical composition, atmospheric circulation, heat loss, and aging processes. Astronomers have detected more than 80 planets orbiting other stars. Jupiter can serve as an accessible laboratory as scientists try to understand the limited data that can be obtained from these distant worlds. Further exploration calls for a careful study of the most important factors that can be measured, the technology required to do the job, and realistic budgetary projections. New ways to explore more efficiently are needed, including improved spacecraft power systems, ion drive engines, miniaturization of instruments, and upgrades to Earth-based radio receiving equipment. Small craft could be used to map Jupiter’s gravitational and magnetic fields, sample its atmosphere, or perform other tasks. As data comes in, new probes can be constructed and launched in less time and at less expense than larger spacecraft such as Galileo.

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Default Saturn (planet)

Saturn (planet)


Saturn (planet), sixth planet in order of distance from the Sun, and the second largest in the solar system. Saturn's most distinctive feature is its ring system, which was first seen in 1610 by Italian scientist Galileo, using one of the first telescopes. He did not understand that the rings were separate from the body of the planet, so he described them as handles (ansae). The Dutch astronomer Christiaan Huygens was the first to describe the rings correctly. In 1655, desiring further time to verify his explanation without losing his claim to priority, Huygens wrote a series of letters in code, which when properly arranged formed a Latin sentence that read in translation, “It is girdled by a thin flat ring, nowhere touching, inclined to the ecliptic.” The rings are named in order of their discovery, and from the planet outward they are known as the D, C, B, A, F, G, and E rings. These rings are now known to comprise more than 100,000 individual ringlets, each of which circles the planet.


As seen from Earth, Saturn appears as a yellowish object—one of the brightest in the night sky. Observed through a telescope, the A and B rings are easily visible, whereas only under optimal conditions can the D and E rings be seen. Sensitive Earth-based telescopes have detected nine satellites, and in the haze of Saturn's gaseous envelope, pale belts and zones parallel to the equator can be distinguished.
Three United States spacecraft have enormously increased knowledge of the Saturnian system. The Pioneer 11 (see Pioneer) probe flew by in September 1979, followed by Voyager 1 in November 1980 and Voyager 2 (see Voyager) in August 1981. These spacecraft carried cameras and instruments for analyzing the intensities and polarizations of radiation in the visible, ultraviolet, infrared, and radio portions of the electromagnetic spectrum (see Electromagnetic Radiation). The spacecraft were also equipped with instruments for studying magnetic fields and for detecting charged particles and interplanetary grains.
The National Aeronautics and Space Administration (NASA) launched an orbiter called the Cassini spacecraft toward Saturn in October 1997. It should reach Saturn in 2004, when it will begin studying Saturn and its moons, launching a probe (the Huygens probe) into the atmosphere of Saturn's moon Titan.


The mean density of Saturn is eight times less than that of Earth because the planet consists mainly of hydrogen. The enormous weight of Saturn's atmosphere causes the atmospheric pressure to increase rapidly toward the interior, where the hydrogen gas condenses into a liquid. Closer to the center of the planet, the liquid hydrogen is compressed into metallic hydrogen, which is an electrical conductor. Electrical currents in this metallic hydrogen are responsible for the planet's magnetic field. At the center of Saturn, heavy elements have probably settled into a small rocky core with a temperature close to 15,000°C (27,000°F). Both Jupiter and Saturn are still settling gravitationally, following their original accretion from the gas and dust nebula from which the solar system was formed more than 4.7 billion years ago. This contraction generates heat, causing Saturn to radiate into space three times as much heat as it receives from the Sun.

Saturn's atmospheric constituents are, in order by mass, hydrogen (88 percent) and helium (11 percent); and traces of methane, ammonia, ammonia crystals, and such other gases as ethane, acetylene, and phosphine comprise the remainder. Voyager images showed whirls and eddies of clouds occurring deep in a haze that is much thicker than that of Jupiter because of Saturn's lower temperature. The temperatures of Saturn's cloud tops are close to –176°C (-285°F), about 27 degrees Celsius (49 degrees Fahrenheit) lower than such locations on Jupiter.
Based on the movements of Saturnian storm clouds, the period of rotation of the atmosphere near the equator is about 10 hr 11 min. Radio emissions that have been detected coming from the body of the planet indicate that the body of Saturn and its magnetosphere rotate with a period of 10 hr 39 min 25 sec. The approximately 28.5-min difference between these two times indicates that Saturnian equatorial winds have velocities close to 1,700 km/h (1,060 mph).
In 1988, from studies of Voyager photos, scientists determined an odd atmospheric feature around Saturn's north pole. What may be a standing-wave pattern (see Wave Motion), repeated six times around the planet, makes cloud bands some distance from the pole appear to form a huge, permanent hexagon.


Saturn's magnetic field is substantially weaker than that of Jupiter, and Saturn's magnetosphere is about one-third the size of Jupiter's. Saturn's magnetosphere consists of a set of doughnut-shaped radiation belts in which electrons and atomic nuclei are trapped. The belts extend to more than 2 million km (1.3 million mi) from the center of Saturn and even farther in the direction away from the Sun, although the size of the magnetosphere fluctuates, depending on the intensity of the solar wind (the flow of charged particles from the Sun). The solar wind and Saturn's rings and satellites supply the particles that are trapped in the radiation belts. The rotation period of 10 hr 39 min 25 sec for Saturn's interior was measured by Voyager 1 while passing through the magnetosphere, which rotates in synchrony with the interior of Saturn. The magnetosphere interacts with the ionosphere, the topmost layer of Saturn's atmosphere, causing auroral emissions of ultraviolet radiation.
Surrounding the Saturnian satellite Titan and Titan's orbit, and extending to the orbit of Saturn's moon Rhea, is an enormous doughnut-shaped cloud of neutral hydrogen atoms. A disk of plasma, composed of hydrogen and possibly oxygen ions, extends from outside the orbit of the moon Tethys almost to the orbit of Titan. The plasma rotates in nearly perfect synchrony with Saturn's magnetic field.


The visible rings stretch out to a distance of 136,200 km (84,650 mi) from Saturn's center, but in many regions they may be only 5 m (16.4 ft) thick. They are thought to consist of aggregates of rock, frozen gases, and water ice ranging in size from less than 0.0005 cm (0.0002 in) in diameter to about 10 m (33 ft) in diameter—from dust to boulder size. An instrument aboard Voyager 2 counted more than 100,000 ringlets in the Saturnian system.
The apparent separation between the A and B rings is called Cassini's division, after its discoverer, the French astronomer Giovanni Cassini. Voyager's television showed five new faint rings within Cassini's division. The wide B and C rings appear to consist of hundreds of ringlets, some slightly elliptical, that have ripples of varying density. The gravitational interaction between rings and satellites, which causes these density waves, is still not completely understood. The B ring appears bright when viewed from the side illuminated by the Sun, but dark on the other side because it is dense enough to block most of the sunlight. Voyager images have also revealed radial, rotating spokelike patterns in the B ring.

Saturn has 18 confirmed moons and as many as 14 proposed new, unconfirmed moons. In the past many proposed new moons have turned out to be just dense spots in Saturn's rings, but the Cassini spacecraft should be able to definitively catalog Saturn's moons. The diameters of Saturn's satellites range from 20 to 5,150 km (12 to 3,200 mi). They consist mostly of the lighter, icy substances that prevailed in the outer parts of the gas and dust nebula from which the solar system was formed and where radiation from the distant Sun could not evaporate the frozen gases. The five larger inner satellites—Mimas, Enceladus, Tethys, Dione, and Rhea—are roughly spherical in shape and composed mostly of water ice. Rocky material may constitute up to 40 percent of Dione's mass. The surfaces of the five are heavily cratered by meteorite impacts. Enceladus has a smoother surface than the others, the least cratered area on its surface being less than a few hundred million years old. (Possibly Enceladus is still undergoing tectonic activity; see Plate Tectonics.) Astronomers suspect that Enceladus supplies particles to the E ring, which neighbors Enceladus's orbit. Mimas, far from being smooth, displays an impact crater the diameter of which is one-third of the diameter of the satellite itself. Tethys also bears a large crater and a valley 100 km (62 mi) in width that stretches more than 2,000 km (1,200 mi) across the surface. Both Dione and Rhea have bright, wispy streaks on their already highly reflective surfaces. Some scientists conjecture these were caused either by ice ejected from craters by meteorites, or by fresh ice that has migrated from the interior.
Several small satellites have been discovered immediately outside the A ring and close to the F and G rings. Possibly four so-called Trojan satellites of Tethys and one of Dione have also been discovered. Trojan satellites occur in regions of stability that lead or follow a body in its orbit around a massive central body, in this case, Saturn.
The outer satellites Hyperion and Iapetus also consist mainly of water ice. Iapetus has a very dark region in contrast to most of its surface, which is bright. This dark region and the rotation of the satellite are the cause of the variations of brightness that were noticed by Cassini in 1671. Phoebe, the farthest satellite, moves in a retrograde orbit (in the opposite direction of the orbits of the other satellites) that is at a sharp angle to Saturn's equator. Phoebe is probably a cometary body captured by Saturn's gravitational field.
Between the inner and outer satellites orbits Titan, Saturn's largest moon. Scientists believe its diameter is 5,150 km (3,200 mi), larger even than the planet Mercury. The exact diameter of Titan, exclusive of its atmosphere, is not known, because a dense orange haze hides the surface. The thickness of Titan's atmosphere is probably about 300 km (about 186 mi). Titan has a nitrogen atmosphere with traces of methane, ethane, acetylene, ethylene, hydrogen cyanide, and carbon monoxide and dioxide. On the surface, the temperature is about -182°C (-296°F), and methane or ethane may be present in the forms of rain, snow, ice, and vapor. The interior of Titan probably consists of equal amounts of rock and water ice. No magnetic fields have been detected. Titan appears nearly featureless to telescopes and space probes alike, but astronomers have glimpsed distinct methane clouds over the moon’s south pole and a bright highland area on its surface. Much more will be known about Titan after the visit of the Cassini spacecraft and the Huygens probe in 2004.

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Old Wednesday, January 18, 2006
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Default Uranus (planet)

Uranus (planet)


Uranus (planet), major planet in the solar system, seventh planet from the Sun. Uranus revolves outside the orbit of Saturn and inside the orbit of Neptune. The average distance from Uranus to the Sun is 2.87 billion km (1.78 billion mi). Uranus has an inner rocky core that is surrounded by a vast ocean of water mixed with rocky material. From the core, this ocean extends upward until it meets an atmosphere of hydrogen, helium, and methane. Uranus has 11 known rings and 27 confirmed moons. The mass of Uranus is 14.5 times greater than the mass of Earth, and its volume is 67 times greater than that of Earth. The force of gravity at the surface of Uranus is 1.17 times the force of gravity on Earth. Because of its great size and mass, scientists classify Uranus as one of the giant or Jovian (like Jupiter) planets—along with Jupiter, Saturn, and Neptune
Uranus was the first planet that people discovered by using a telescope. Sir William Herschel, a German-born British musician and astronomer, discovered the planet in 1781. Herschel accidentally discovered it while measuring shifts in the positions of stars in the constellation Gemini. He observed that Uranus is a moving object, so he first reported his discovery to the British Royal Society as a comet. However, people had observed and plotted Uranus on star charts dating back to 1690 (believing it was a star). Astronomers used these earlier observations to identify the object as a planet and to establish its orbit. Herschel originally named the planet Georgium Sidus (Star of George) in honor of King George III of Great Britain. Later, astronomers named the planet after Uranus, a figure who embodied the heavens and was the father of Saturn and the grandfather of Jupiter in Greek and Roman mythology.


Uranus orbits the Sun, varying from 2.74 x 109 km (1.70 x 109 mi) to 3.00 x 109 km (1.86 x 109 mi) in distance from the Sun. The orbit of Uranus traces out a flat region of space called the planet’s orbital plane. The orbital plane of Uranus lies close to Earth’s orbital plane. As a result, Uranus always crosses the same region of Earth’s sky. Uranus, which appears to be a star to the naked eye, is so faint that people did not consider it important enough to include among the stars outlining the familiar constellations. Through a large telescope, the planet appears as a blue-green disk with a diameter of about 3.5 arc seconds. Arc seconds describe the size of objects in the night sky by giving the size of the angle that the objects block out in the sky (a quarter held at arm’s length is approximately 7,000 arc seconds).
Because Uranus is so far from Earth (2.84 × 109 km/1.76 × 109 mi), only one spacecraft has visited the planet. During a rare alignment of the four giant planets, the spacecraft Voyager 2, which was launched on August 20, 1977, was able to pass by Jupiter (in 1979), Saturn (in 1981), Uranus (in 1986), and Neptune (in 1989). Scientists launched Voyager 2 with just enough energy to pass Jupiter. However, the strong gravitational pull of Jupiter accelerated the spacecraft as it passed by the planet so that Voyager 2 had enough energy to reach Saturn. As Voyager 2 successively passed each of the four giant planets, the gravitational pull of each planet accelerated the spacecraft enough to help it reach the next planet.
As Voyager 2 passed by Uranus, the spacecraft recorded and transmitted images of the planet, its rings, and some of its moons. Astronomers studying these images discovered five previously undetected rings and ten previously undiscovered moons. In addition to discovering these inner moons, Voyager 2 passed close to Miranda, the 11th satellite from Uranus, and mapped the moon’s surface in detail. Surface features of Miranda include craters, canyons, and geologically young systems of ridges and grooves. Because the other large satellites were more distant from the spacecraft’s path, Voyager 2 was unable to make detailed images of their surfaces.


Uranus takes 84 years to complete a single revolution around the Sun, so a year on Uranus is 84 times longer than a year on Earth. Uranus spins in place around its axis (an imaginary line that runs down the middle of the planet) once every 17.25 hours, just as Earth spins once every 24 hours. The ends of the axis mark the north and south poles of Uranus, just as Earth’s axis marks the North Pole and the South Pole on Earth. Uranus rotates about an axis (the way a plastic globe spins on a rod) that tilts 98° into its orbital plane (the plane created by Uranus’s orbit around the Sun). Because of this tilt, one pole of Uranus points almost directly toward the Sun during half of Uranus’s 84-year orbit, and the other pole points toward the Sun during the second half. This pattern creates 42-year-long seasons of lightness and darkness, alternately, on each end of Uranus. Despite these long seasons, the difference in temperature between the two poles is not great (the planet’s average temperature in its upper atmosphere is about -212°C/-350°F). This uniform temperature indicates that heat is conducted efficiently, or travels easily, throughout the planet.
As Uranus spins about its axis, material near the planet’s equator must travel farther to make one rotation than material near the poles must travel. This equatorial material must then move faster than material at the poles. All material has inertia (the tendency of a moving mass to continue moving in a straight line), and this property makes the fast-moving material near the equator want to fly off from the planet in a straight line. The rest of the planet’s mass gravitationally attracts the material and keeps it glued to the planet, but the material’s inertia makes the planet bulge out at the equator. The bulge around the equator of Uranus is about 2 percent of the radius, or about 500 km (about 300 mi).


Uranus contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Uranus formed from the same material—principally frozen water and rock—that composes most of the planet’s moons. As the planet grew, pressures and temperatures in the planet’s interior increased, heating the planet’s frozen water into a hot liquid.
Uranus probably has a relatively small rocky core (smaller in size than Earth’s core), with a radius no larger than 2,000 km (1,240 mi) and a temperature of about 6650°C (12,000°F). Uranus’s core may be small because most of the rock composing the planet remains mixed with the body of water that surrounds the core and extends upward to the planet’s atmosphere.
The vast body of liquid on Uranus accounts for most of the planet’s volume. Scientists think this ocean consists mostly of water molecules, which are mixed with silicate, magnesium, nitrogen-bearing molecules, and hydrocarbons (molecules composed of carbon and hydrogen). Uranus’s ocean is extremely hot (about 6650°C/about 12,000°F). Water at the surface of Earth evaporates, or boils, at 100°C (212°F). The ocean on Uranus remains liquid at such a high temperature, however, because the pressure deep in Uranus is about five million times stronger than the atmospheric pressure on Earth at sea level. Higher pressure holds molecules in liquids close together and prevents them from spreading out to form vapor.

The atmosphere of Uranus, which contains hydrogen, helium, and trace amounts of methane, extends about 5,000 km (about 3,100 mi) above the planet’s ocean. At the time of the Voyager 2 flyby in 1986, the atmosphere was relatively calm and inactive, with few storms or clouds, but Hubble Space Telescope images showed more activity in 2001. Winds blow parallel to the equator of Uranus, moving in the same direction as the planet’s rotation at high latitudes, and opposite to the rotation at low latitudes. These winds layer Uranus’s clouds into bands. Light reflected from Uranus’s deep atmosphere is blue-green, because the atmospheric methane absorbs red and orange light. Unlike the other giant planets, Uranus radiates little heat into space from its deep interior.

Although Uranus is one of the giant planets, it is smaller and has a different chemical composition than Saturn and Jupiter. While Saturn and Jupiter are made of mostly hydrogen and helium, Uranus captured a much smaller amount of these elements as the solar system formed. Instead, Uranus captured mostly water. Because water is more dense than hydrogen and helium, Uranus is more compact than Jupiter or Saturn. Jupiter, for example, has a radius of 71,355 km (44,338 mi) while Uranus has a radius of 25,548 km (15,875 mi). If Uranus had the same mass it has now but consisted of the lighter elements hydrogen and helium, the planet would be larger but much less dense than Jupiter.


Astronomers have identified 11 rings of debris encircling Uranus’s equator. These extremely dark, narrow rings orbit the planet in the plane of its equator at distances from 3.8 × 104 km (2.4 × 104 mi) to 5.1 × 104 km (3.2 × 104 mi). Many of these rings are made of ice and rock boulders about the size of large beach balls. Several observatories first detected five of the ten rings in 1977. Starting from the innermost ring, these five rings were called Alpha, Beta, Gamma, Delta, and Epsilon. In 1986 images taken by the Voyager 2 spacecraft helped scientists discover five more rings encircling Uranus.
Astronomers believe that at least 27 moons orbit Uranus. Uranus’s moons are named for characters in the works of English playwright William Shakespeare and English poet Alexander Pope. The two largest and brightest moons, Oberon and Titania, were discovered by Sir William Herschel in 1787. British astronomer William Lassell detected the two next largest moons, Umbriel and Ariel. The surfaces of these four largest moons are old, heavily cratered, and geologically inactive. Astronomers believe that these four moons consist of half ice and half rock. American astronomer Gerard Peter Kuiper discovered a smaller fifth moon, Miranda, in 1948.
Voyager 2 helped scientists discover Uranus’s 11 innermost moons, each with a diameter of less than 100 km (60 mi). The tenth moon from Uranus was discovered in 1999 from photos that Voyager 2 took in 1986. This moon does not yet have an official name but is known as 1986U10. In order of their distance from Uranus, these 11 inner moons are Cordelia (which is closest), Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, 1986U10, and Puck.
Two more distant moons were discovered in 1997 by Canadian astronomer Brett Gladman and collaborators using the 200-inch telescope and a special camera at the Palomar Observatory on Mount Palomar in California. These moons were subsequently named Caliban and Sycorax. In 1999 the same group reported the discovery of three additional small, distant moons: Prospero, Setebos, and Stephano. Prospero and Setebos are even more distant from Uranus than Sycorax, while Stephano’s average orbital distance lies between those of Caliban and Sycorax. Unlike the planet’s 16 other moons, these 5 outer moons orbit Uranus in the direction opposite that in which the planet rotates and follow highly eccentric orbits that are inclined to the plane of Uranus’s equator. Astronomers believe that these oddball satellites are captured asteroids rather than satellites that formed from the same planetary nebula (cloud of dust and gases that condenses into planets) that formed Uranus (see Hale Observatories).


Uranus, like Earth, is surrounded by a magnetic field, a region of space that exerts a small force on electrically charged or magnetic material. Uranus’s deep oceans contain electrically charged particles called ions. Ocean currents on Uranus circulate these charged particles, which in turn creates a magnetic field. Scientists believe that ocean currents in the other Jovian planets—Neptune, Saturn, and Jupiter—are created by heat released from the planets’ cores. The core of Uranus releases less heat than the other three Jovian planets, however, and astronomers are unsure about what causes ocean currents in Uranus’s fluid interior. Uranus’s magnetic field is similar in strength to Earth’s magnetic field. Uranus’s magnetic axis (the line joining the north and south poles of its magnetic field) is aligned with the planet’s strongly tilted rotational axis, although the magnetic field is offset from the center of the planet. The influence of Uranus’s magnetic field extends for several hundred thousand kilometers above the planet.

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Old Wednesday, January 18, 2006
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Default Neptune (planet)

Neptune (planet)


Neptune (planet), major planet in the solar system, eighth planet from the Sun and fourth largest in diameter. Neptune maintains an almost constant distance, about 4.5 billion km (about 2.8 billion mi), from the Sun. Neptune revolves outside the orbit of Uranus and for most of its orbit moves inside the elliptical path of the outermost planet Pluto (see Solar System). Every 248 years, Pluto’s elliptical orbit brings the planet inside Neptune’s nearly circular orbit for about 20 years, temporarily making Neptune the farthest planet from the Sun. The last time Pluto’s orbit brought it inside Neptune’s orbit was in 1979. In 1999 Pluto’s orbit carried it back outside Neptune’s orbit.
Astronomers believe Neptune has an inner rocky core that is surrounded by a vast ocean of water mixed with rocky material. From the inner core, this ocean extends upward until it meets a gaseous atmosphere of hydrogen, helium, and trace amounts of methane. Neptune has four rings and 11 known moons. Even though Neptune's volume is 72 times Earth’s volume, its mass is only 17 times Earth’s mass. Because of its size, scientists classify Neptune—along with Jupiter, Saturn, and Uranus—as one of the giant or Jovian planets (so-called because they resemble Jupiter).
Mathematical theories of astronomy led to the discovery of Neptune. To account for wobbles in the orbit of the planet Uranus, British astronomer John Couch Adams and French astronomer Urbain Jean Joseph Leverrier independently calculated the existence and position of a new planet in 1845 and 1846, respectively. They theorized that the gravitational attraction of this planet for Uranus was causing the wobbles in Uranus’s orbit. Using information from Leverrier, German astronomer Johann Gottfried Galle first observed the planet in 1846.
After its discovery, Leverrier proposed that the new planet be named after the sea god Neptune from Greek and Roman mythology. The appropriateness of this name was confirmed in the 20th century when astronomers learned about Neptune’s watery interior.


The orbit of Neptune traces out a flat region of space called the planet’s orbital plane. The orbital plane of Neptune (and of all the planets in our solar system except for Pluto) lies close to Earth’s orbital plane. As a result, Neptune always crosses the same region of Earth’s sky. Neptune is barely visible to the naked eye and is so faint that even through binoculars it appears as a dim star. Through a large telescope, the planet appears from Earth as a small greenish disk with a diameter of about 2.3 arc seconds. Astronomers use the unit arc second to describe the size of objects in the night sky. Arc seconds give the angle an object blocks out in the sky (a quarter held at arm’s length is approximately 7,000 arc seconds).
Because Neptune is so far from Earth (4.49 x 109 km/2.79 x 109 mi), only one spacecraft has visited the planet. During a rare alignment of the four giant planets, the spacecraft Voyager 2, launched on August 20, 1977, was able to pass by Jupiter (in 1979), Saturn (in 1981), Uranus (in 1986), and Neptune (in 1989). Scientists launched Voyager 2 with just enough energy to pass Jupiter. However, the strong gravitational pull of Jupiter accelerated the spacecraft as it passed by the planet so that Voyager 2 had enough energy to reach Saturn. As Voyager 2 successively passed each of the four giant planets, the gravitational pull of the planet accelerated the spacecraft enough to help it reach the next planet, until it reached Neptune more than ten years after its launch.
As Voyager 2 passed by Neptune, it recorded and transmitted images of the planet, its rings, and its moons. Astronomers studying these images discovered four rings and five previously undiscovered moons. Four of these newly discovered moons are the innermost moons of Neptune, the largest of which measures only 180 km (112 mi) in diameter—small enough to fit in a large crater of Earth’s Moon.


Neptune takes 164.79 years to complete a single revolution around the Sun, so a year on Neptune is 164.79 times longer than a year on Earth. The planet spins in place once every 16 hours, just as Earth spins once every 24 hours. Neptune spins around its axis, an imaginary line that runs down the middle of the planet. The ends of the line mark the north and south poles of Neptune, much like Earth’s axis marks the North Pole and South Pole on Earth. The axis of rotation on Neptune tilts 29.6° into its orbital plane (the plane created by Neptune’s orbit around the Sun). This tilt gives Neptune almost Earthlike seasons. (Seasons on Earth result from our planet’s 23.5° tilt into its orbital plane.)


Neptune contains mostly rock and water, with hydrogen and helium (and trace amounts of methane) in its dense atmosphere. Astronomers believe that Neptune formed from frozen water and rock supplied by icy comet-like material found in the outer regions of the solar system. As the planet grew in size, pressures and temperatures in the planet’s interior increased, heating the planet’s frozen water into hot liquid.
Astronomers believe that Neptune has a solid core no larger than Earth (Earth’s diameter is 12,756 km/7,926 mi) and that this core is composed primarily of iron and silicon. Neptune’s core may be small because most of the rock composing the planet remains mixed with the vast ocean that extends upward from the core to the atmosphere.
Neptune’s vast body of liquid accounts for most of its volume. Scientists think this ocean is composed mostly of water as well as molecules of methane and ammonia. Neptune’s ocean is extremely hot (about 4700°C/about 8500°F). The ocean remains liquid at this temperature instead of evaporating because the pressure deep in Neptune is several million times higher than the atmospheric pressure on Earth. Higher pressure holds molecules in liquid closer together and prevents them from spreading apart to form vapor.
The gaseous atmosphere of Neptune contains hydrogen, helium, and about 3 percent methane. It extends about 5,000 km (about 3,000 mi) above the planet’s ocean. Light reflected from Neptune’s deep atmosphere is blue, because the atmospheric methane absorbs red and orange light but scatters blue light. In 1998 astronomers also identified molecules of methyl in Neptune’s atmosphere. Methyl molecules each contain one carbon atom and three hydrogen atoms. Methyl molecules are known as hydrocarbon radicals because they are short-lived and highly reactive. They combine with each other to form ethane (C2H6), a flammable, colorless gas. The discovery of methyl in Neptune’s atmosphere marked the first observation of a hydrocarbon radical in the atmosphere of the outer planets. Astronomers hypothesize that great storm systems on Neptune eject methane into the upper atmosphere. Once in the upper atmosphere, the Sun’s energy breaks the methane down into methyl molecules. Below Neptune's methane clouds, at levels where the pressure rises to more than four times the atmospheric pressure at sea level on Earth, there may be a dense cloud layer composed of hydrogen sulfide particles.
Neptune emits about 2.7 times the amount of heat it absorbs from the Sun. Astronomers believe the excess heat that Neptune radiates comes from comet-like material that crashed into Neptune as the planet formed. Due to the force of gravity in the planet’s interior, the material in Neptune’s core is continually being pulled inward. As the material compacts, the molecules strike each other more frequently and with more force, releasing energy in the form of heat. Neptune’s core, which reaches temperatures of 5149°C (9300°F), is hotter than the Sun’s surface.
Neptune has the fastest winds in the solar system, reaching speeds of 2,000 km/h (1,200 mph).
Neptune has an active atmosphere, with winds and massive storms that may be caused by heat escaping the planet’s interior. Neptune’s winds, which blow in a latitude direction, are faster in the planet’s polar regions than they are at Neptune’s equator. Neptune has the fastest winds in the solar system, reaching speeds of 2,000 km/h (1,200 mph).
Using the Hubble Space Telescope, astronomers have observed storms thousands of kilometers across in Neptune’s atmosphere. These storms are often visible as dark spots that appear and disappear in Neptune’s atmosphere over many months. The largest storm, known as the Great Dark Spot, appeared in the planet’s southern hemisphere and was photographed extensively in 1989 by the Voyager 2 spacecraft. Scientists estimated that the Great Dark Spot was as large in diameter as Earth is. By 1994 images transmitted to Earth by the Hubble Space Telescope showed that the Great Dark Spot had disappeared. Scientists believe this dark spot was an immense storm that either dissipated or was covered by other atmospheric features. From 1994 through 1998, astronomers used the Hubble Space Telescope to observe the emergence of additional large dark spots in Neptune’s northern hemisphere, indicating that the planet’s atmosphere changes rapidly. The chemical makeup of the cloud particles that form Neptune's Great Dark Spots is not known. Some scientists believe that the bright clouds rimming the poleward edges of the dark spots are composed of condensed methane particles.
Although Neptune is one of the giant planets, it is smaller and has a different chemical composition than those of Saturn and Jupiter. While Saturn and Jupiter are made of mostly hydrogen and helium, Neptune captured a much smaller amount of these elements as the solar system formed. Instead, Neptune captured mostly water. Because water is more dense than hydrogen or helium, Neptune is more compact than either Jupiter or Saturn. Jupiter, for example, has a radius of 71,355 km (44,338 mi), while Neptune has a radius of 24,766 km (15,389 mi).


From Voyager 2 spacecraft images, astronomers identified four rings of debris encircling Neptune’s equator. These rings range in width from 15 km (9.3 mi) to 5,800 km (3,600 mi). All of these rings completely encircle the planet, but the outermost ring includes three or more arcs of concentrated debris, some of which had been detected from Earth before the Voyager 2 encounter. In 1998 a new infrared camera on the Hubble Space Telescope obtained the first new images of Neptune's mysterious ring-arcs since the 1989 Voyager 2 encounter. Astronomers had speculated that the gravitational pull from nearby moons caused smaller particles to form the concentrated debris arcs, but the new images showed that this theory is incorrect.
Eleven moons are known to orbit Neptune. Only two of these moons—Triton and Nereid—were large enough to be directly observed from Earth prior to the 1990s. Triton was discovered in 1846 by British astronomer William Lassell, and Nereid was discovered in 1949 by Dutch-born American astronomer Gerard Kuiper. Scientists discovered another moon, Larissa, in 1981 when the moon occulted (moved in front of) a star, and they discovered five more moons of Neptune from images transmitted to Earth by the Voyager 2 spacecraft. Searches carried out with large Earth-based telescopes led astronomers to announce in 2003 that Neptune has at least three additional moons. These three are the smallest and most distant from Neptune of all the planet’s moons, and astronomers know little else about them.

The four innermost moons of Neptune are quite small, ranging in diameter from 58 km (36 mi) to 180 km (110 mi). From the closest to Neptune outward, these moons are Naiad, Thalassa, Despina, and Galatea. Larissa is the fifth moon in distance from Neptune. It is heavily cratered and irregular in shape. Its density, chemical composition, and internal structure are unknown. Proteus is the sixth moon out from Neptune. This satellite is the largest irregularly shaped moon in the solar system, measuring 436 km (262 mi) through its widest diameter and 402 km (241 mi) through its narrowest diameter. Triton is the seventh moon from Neptune and is the largest of the planet’s moons, measuring 2,700 km (1,700 mi) in diameter. Triton, which consists of about one-quarter ice and three-quarters rock, has few craters on its surface, which suggests that this moon is relatively young. Nereid is the eighth moon from Neptune and has the most elliptical orbit of any planet or moon in the solar system, varying in distance around Neptune from 1.4 x 106 km (8.7 x 105 mi) to 9.6 x 106 km (6.0 x 106 mi Eleven moons are known to orbit Neptune. Only two of these moons—Triton and Nereid—were large enough to be directly observed from Earth prior to the 1990s. Triton was discovered in 1846 by British astronomer William Lassell, and Nereid was discovered in 1949 by Dutch-born American astronomer Gerard Kuiper. Scientists discovered another moon, Larissa, in 1981 when the moon occulted (moved in front of) a star, and they discovered five more moons of Neptune from images transmitted to Earth by the Voyager 2 spacecraft. Searches carried out with large Earth-based telescopes led astronomers to announce in 2003 that Neptune has at least three additional moons. These three are the smallest and most distant from Neptune of all the planet’s moons, and astronomers know little else about them.

The four innermost moons of Neptune are quite small, ranging in diameter from 58 km (36 mi) to 180 km (110 mi). From the closest to Neptune outward, these moons are Naiad, Thalassa, Despina, and Galatea. Larissa is the fifth moon in distance from Neptune. It is heavily cratered and irregular in shape. Its density, chemical composition, and internal structure are unknown. Proteus is the sixth moon out from Neptune. This satellite is the largest irregularly shaped moon in the solar system, measuring 436 km (262 mi) through its widest diameter and 402 km (241 mi) through its narrowest diameter. Triton is the seventh moon from Neptune and is the largest of the planet’s moons, measuring 2,700 km (1,700 mi) in diameter. Triton, which consists of about one-quarter ice and three-quarters rock, has few craters on its surface, which suggests that this moon is relatively young. Nereid is the eighth moon from Neptune and has the most elliptical orbit of any planet or moon in the solar system, varying in distance around Neptune from 1.4 x 106 km (8.7 x 105 mi) to 9.6 x 106 km (6.0 x 106 mi ).
Neptune, like Earth, is surrounded by a magnetic field, a region of space that exerts a small force on electrically charged or magnetic material. Scientists believe that the slow escape of heat from the planet’s core circulates currents of electrically charged particles in Neptune’s deep ocean, generating a magnetic field. Neptune’s magnetic axis, the line indicating the direction of the force the planet’s magnetic field exerts, is aligned at an angle of 47° to Neptune’s axis of rotation. The influence of Neptune’s magnetic field extends for several hundred thousand kilometers above the planet.

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Old Thursday, December 14, 2006
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Thumbs up Real facts about Sun

Our Sun is a normal main-sequence G2 star, one of more than 100 billion stars in our galaxy.

diameter: 1,390,000 km.
mass: 1.989e30 kg
temperature: 5800 K (surface)
15,600,000 K (core)
The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.

The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else ("metals") amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core.

The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.

Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun's density is more than 150 times that of water.

The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.

The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.

A small region known as the chromosphere lies above the photosphere.

The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse (left). Temperatures in the corona are over 1,000,000 K.

It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey.

The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere (also known as the heliosphere) extends well beyond Pluto.

In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis.

Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, that it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed.

Further study of the solar wind will be done by the recently launched Wind, ACE and SOHO spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth.

The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft.

Spectacular loops and prominences are often visible on the Sun's limb (left).

The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%.

The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).

The Sun's satellites
There are eight planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should be classified as planets and which as "smaller objects" has been the source of some controversy, but in the end it is really only a matter of definition. Pluto is no longer officially a planet but we'll keep it here for history's sake.)

Distance Radius Mass
Planet (000 km) (km) (kg) Discoverer Date
--------- --------- ------ ------- ---------- -----
Mercury 57,910 2439 3.30e23

Venus 108,200 6052 4.87e24

Earth 149,600 6378 5.98e24

Mars 227,940 3397 6.42e23

Jupiter 778,330 71492 1.90e27

Saturn 1,426,940 60268 5.69e26

Uranus 2,870,990 25559 8.69e25 Herschel 1781

Neptune 4,497,070 24764 1.02e26 Galle 1846

Pluto 5,913,520 1160 1.31e22 Tombaugh 1930


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Old Wednesday, April 11, 2007
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Default Stardust Memories

Stardust Memories

After a seven-year, three-billion-mile expedition through the solar system, NASA’s Stardust spacecraft capsule landed in the Arizona desert on Jan. 15, 2006, with an impressive bounty: a canister full of tens of thousands of comet particles and a smattering of interstellar dust, the first such samples ever collected.

Stardust captured the comet particles from Comet Wild-2 (pronounced Vilt) when the spacecraft flew within 149 miles of it on Jan. 2, 2004, roughly in the vicinity of Jupiter. The spacecraft’s collector swept up particles left in the comet’s wake, preserving them in a silicon material called aerogel, which cushioned and protected the particles on their long journey to Earth.

The Stardust samples offer a time capsule to our primordial past. Comets contain some of the oldest material in the solar system, formed out of the remaining dust and gases left over after the solar system’s creation 4.6 billion years ago. Principal investigator Donald Brownlee has commented that “this has been a fantastic opportunity to collect the most primitive material in the solar system. We fully expect some of the comet particles to be older than the Sun.” Michael Zolensky, another Stardust scientist, offers a vivid sense of how intimately connected comets are to an understanding of life on Earth: “It’s like looking at our great-great grandparents. Much of Earth's water and organics—you know, the molecules in our bodies—perhaps came from comets. So these samples will tell us…basically, where our atoms and molecules came from, and how they were delivered to Earth, and in what amount.”

The samples have already defied their expectations. Some contain minerals that could only have been formed at enormously high temperatures. But comets are icy balls thought to have formed far from the Sun, in the outer, frigid regions of the solar system. Brownlee noted that “when these minerals formed they were either red-hot or white-hot grains, and yet we collected them at a comet [from] the Siberia of the solar system.” According to Zolensky, “It suggests that, if these are really from our own sun, they've been ejected out—ballistically out—all the way across the entire solar system and landed out there.…We can't give you all the answers right now. It's just great we have new mysteries to worry about now.”

About 150 scientists around the world are currently studying the comet samples, while an army of amateur scientists have turned their attention to the stardust also collected during the mission. About 65,000 volunteers from the general public will be enlisted to help find images of interstellar dust embedded in the aerogel. The volunteers of Stardust@home will search images delivered to them on the Internet, using so-called virtual microscopes. While the comet dust is visible to the naked eye, the bits of stardust are only a few microns in diameter. Just 40 to 100 grains of stardust are thought to have been collected, and searching for these particles has been described by the NASA team like “tracking down 45 ants on a football field.”


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Default Asteroids

Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets. Asteroids range in size from Ceres, which has a diameter of about 1000 km, down to the size of pebbles. Sixteen asteroids have a diameter of 240 km or greater. They have been found inside Earth's orbit to beyond Saturn's orbit. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter. Some have orbits that cross Earth's path and some have even hit the Earth in times past. One of the best preserved examples is Barringer Meteor Crater near Winslow, Arizona.

Asteroids are material left over from the formation of the solar system. One theory suggests that they are the remains of a planet that was destroyed in a massive collision long ago. More likely, asteroids are material that never coalesced into a planet. In fact, if the estimated total mass of all asteroids was gathered into a single object, the object would be less than 1,500 kilometers (932 miles) across -- less than half the diameter of our Moon.

Much of our understanding about asteroids comes from examining pieces of space debris that fall to the surface of Earth. Asteroids that are on a collision course with Earth are called meteoroids. When a meteoroid strikes our atmosphere at high velocity, friction causes this chunk of space matter to incinerate in a streak of light known as a meteor. If the meteoroid does not burn up completely, what's left strikes Earth's surface and is called a meteorite.

Of all the meteorites examined, 92.8 percent are composed of silicate (stone), and 5.7 percent are composed of iron and nickel; the rest are a mixture of the three materials. Stony meteorites are the hardest to identify since they look very much like terrestrial rocks.

Because asteroids are material from the very early solar system, scientists are interested in their composition. Spacecraft that have flown through the asteroid belt have found that the belt is really quite empty and that asteroids are separated by very large distances. Before 1991 the only information obtained on asteroids was though Earth based observations. Then on October 1991 asteroid 951 Gaspra was visited by the Galileo spacecraft and became the first asteroid to have hi-resolution images taken of it. Again on August 1993 Galileo made a close encounter with asteroid 243 Ida. This was the second asteroid to be visited by spacecraft. Both Gaspra and Ida are classified as S-type asteroids composed of metal-rich silicates.

On June 27, 1997 the spacecraft NEAR made a high-speed close encounter with asteroid 253 Mathilde. This encounter gave scientists the first close-up look of a carbon rich C-type asteroid. This visit was unique because NEAR was not designed for flyby encounters. NEAR is an orbiter destined for asteroid Eros in January of 1999.

Astronomers have studied a number of asteroids through Earth-based observations. Several notable asteroids are Toutatis, Castalia, Geographos and Vesta. Astronomers studied Toutatis, Geographos and Castalia using Earth-based radar observations during close approaches to the Earth. Vesta was observed by the Hubble Space Telescope.
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Default Comets

Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.

Comet structures are diverse and very dynamic, but they all develop a surrounding cloud of diffuse material, called a coma, that usually grows in size and brightness as the comet approaches the Sun. Usually a small, bright nucleus (less than 10 km in diameter) is visible in the middle of the coma. The coma and the nucleus together constitute the head of the comet.

As comets approach the Sun they develop enormous tails of luminous material that extend for millions of kilometers from the head, away from the Sun. When far from the Sun, the nucleus is very cold and its material is frozen solid within the nucleus. In this state comets are sometimes referred to as a "dirty iceberg" or "dirty snowball," since over half of their material is ice. When a comet approaches within a few AU of the Sun, the surface of the nucleus begins to warm, and volatiles evaporate. The evaporated molecules boil off and carry small solid particles with them, forming the comet's coma of gas and dust.

When the nucleus is frozen, it can be seen only by reflected sunlight. However, when a coma develops, dust reflects still more sunlight, and gas in the coma absorbs ultraviolet radiation and begins to fluoresce. At about 5 AU from the Sun, fluorescence usually becomes more intense than reflected light.

As the comet absorbs ultraviolet light, chemical processes release hydrogen, which escapes the comet's gravity, and forms a hydrogen envelope. This envelope cannot be seen from Earth because its light is absorbed by our atmosphere, but it has been detected by spacecraft.

The Sun's radiation pressure and solar wind accelerate materials away from the comet's head at differing velocities according to the size and mass of the materials. Thus, relatively massive dust tails are accelerated slowly and tend to be curved. The ion tail is much less massive, and is accelerated so greatly that it appears as a nearly straight line extending away from the comet opposite the Sun.

Each time a comet visits the Sun, it loses some of its volatiles. Eventually, it becomes just another rocky mass in the solar system. For this reason, comets are said to be short-lived, on a cosmological time scale. Many scientists believe that some asteroids are extinct comet nuclei, comets that have lost all of their volatiles.
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