Earth

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.

Topics:

• Earth, The Solar System, and the Galaxy
• Earth's Atmosphere
• Earth's Surface
• Earth's Interior
• Earth's Future

Earth, The Solar System, and the Galaxy

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.

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.

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.

Solar System: The Sun and the Solar Wind

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 (see Space Exploration; Apollo program).

The Sun’s activity also influences the heliopause, a region of space that astronomers believe marks the boundary between the solar system and interstellar space. The heliopause is a dynamic region that expands and contracts due to the constantly changing speed and pressure of the solar wind. In November 2003 a team of astronomers reported that the Voyager 1 spacecraft appeared to have encountered the outskirts of the heliopause at about 86 AU from the Sun. They based their report on data that indicated the solar wind had slowed from 1.1 million km/h (700,000 mph) to 160,000 km/h (100,000 mph). This finding is consistent with the theory that when the solar wind meets interstellar space at a turbulent zone known as the termination shock boundary, it will slow abruptly. However, another team of astronomers disputed the finding, saying that the spacecraft had neared but had not yet reached the heliopause.

Solar System: The Major Planets

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. See also Planetary Science.

Solar System: Other Orbiting Bodies

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.

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.

Solar System: Movements of the Planets and Their Satellites

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.

Sun

Sun, closest star to Earth. The Sun is a huge mass of hot, glowing gas. The strong gravitational pull of the Sun holds Earth and the other planets in the solar system in orbit. The Sun’s light and heat influence all of the objects in the solar system and allow life to exist on Earth.

The Sun is an average star—its size, age, and temperature fall in about the middle of the ranges of these properties for all stars. Astronomers believe that the Sun is about 4.6 billion years old and will keep shining for about another 7 billion years.

For humans, the Sun is beautiful and useful, but also powerful and dangerous. As Earth turns, the Sun rises over the eastern horizon in the morning, passes across the sky during the day, and sets in the west in the evening. This movement of the Sun across the sky marks the passage of time during the day (see Sundial). The Sun’s movement can produce spectacular sunrises and sunsets under the right atmospheric conditions. At night, reflected sunlight makes the Moon and planets bright in the night sky.

The Sun provides Earth with vast amounts of energy every day. The oceans and seas store this energy and help keep the temperature of Earth at a level that allows a wide variety of life to exist. Plants use the Sun’s energy to make food, and plants provide food for other organisms. The Sun’s energy also creates wind in Earth’s atmosphere. This wind can be harnessed and used to produce power.

While it lights our day and provides energy for life, sunlight can also be harmful to people. Human skin is sensitive to ultraviolet light emitted from the Sun. Earth’s atmosphere blocks much of the harmful light, but sunlight is still strong enough to burn skin under some conditions. Sunburn is one of the most important risk factors in the development of skin cancers, which can be fatal. Sunlight is also very harmful to human eyes. A person should never look directly at the Sun, even with sunglasses or during an eclipse. The Sun influences Earth with more than just light. Particles flowing from the Sun can disrupt Earth’s magnetic field, and these disruptions can interfere with electronic communications.

Topics:

• Characteristics of the Sun
• Interior of the Sun
• The Sun's Atmosphere

Characteristics of the Sun

The distance between the sun and the earth varies because the earth travels in an elliptical rather than circular orbit. The distance is roughly 100 times the sun’s diameter. Turbulence in the photosphere forms granules of various sizes and sunspots. Temperature is a measure of kinetic energy. The dense plasma in the center of the sun is roughly 2500 times hotter than the surface. Gases in the corona have escaped from the sun’s surface and have a very high velocity. The sun’s spectral type, G2, indicates that it is composed of hydrogen, helium, calcium, iron and other metals.

Interior of the Sun

Regions of the Sun include the core, radiation zone, convection zone, and photosphere. Gases in the core are about 150 times as dense as water and reach temperatures as high as 16 million degrees C (29 million degrees F). The Sun’s energy is produced in the core through nuclear fusion of hydrogen atoms into helium. In the radiation zone, heat flows outward through gases that are about as dense as water. The radiation zone is cooler than the core, about 2.5 million degrees C (4.5 million degrees F). In the convection zone, churning motions of the gases carry the Sun’s energy further outward. The convection zone is slightly cooler, about 2 million degrees C (3.6 million degrees F), and less dense, about one-tenth as dense as water. The photosphere is much cooler, about 5500° C (10,000° F) and much less dense, about one-millionth that of water. The turbulence of this region is visible from earth in the form of sunspots, solar flares, and small patches of gas called granules.

The Sun's Atmosphere

The material in the Sun farther out from the center than the photosphere makes up the Sun’s atmosphere. The atmosphere extends far beyond the disk we see in the sky. Very diffuse solar gases extend all the way to Earth and beyond.

The solar atmosphere consists of, from the innermost part outward, the photosphere, the chromosphere, the corona, and the expanding outer layers of the corona that astronomers call the solar wind. The photosphere is the visible part of the Sun. We look right through the chromosphere, the corona, and the solar wind, just as we see through Earth’s atmosphere at night.

The chromosphere and corona are visible during total solar eclipses, when the Moon lines up between the Sun and Earth, completely blocking the main disk of the Sun from view. The thin chromosphere becomes visible a few seconds before or after a solar eclipse, creating a narrow pink, rose, or ruby-colored band at the edge of the Sun. For up to eight minutes during an eclipse, the corona is visible to the unaided eye as a faint, shimmering halo of pearl-white light spreading out from the lunar silhouette. Although the light of the chromosphere and corona is not bright enough to be dangerous, and can be viewed safely without filters during the total phase of an eclipse, the partial phases of a solar eclipse are very hazardous to human eyes and can only be viewed indirectly or through special filters. Scientists can study all layers of the Sun’s atmosphere at any time using special instruments.

Mercury

Mercury (planet), one of the planets in the solar system. Mercury orbits closest to the Sun of all the planets, at an average distance of approximately 58 million km (about 36 million mi). The planet’s diameter is 4,879 km (3,032 mi), and its volume and mass are about one-eighteenth that of Earth. Mercury’s mean density is approximately equal to that of Earth and is higher than that of any of the other planets. The force of gravity on the planet's surface is about one-third of that on Earth's surface or about twice the surface gravity on the Moon.

Mercury revolves once about the Sun every 88 days. Radar observations of the planet show that it rotates only once every 58.7 days, two-thirds of its period of revolution. Only three of the planet’s days, therefore, occur during every two of its years. The side facing the Sun gets very hot, while the side facing away quickly cools to frigid temperatures. The point in Mercury's orbit at which the planet is closest to the Sun (called the planet’s perihelion) moves a tiny amount every orbit, too much to be accounted for by the gravitational influence of other planets. The observation of these changes in Mercury's perihelion was one of the first confirmations of Einstein’s theory of relativity, which predicted their existence.

SURFACE

The Mariner 10 spacecraft passed Mercury twice in 1974 and once in 1975. It sent back pictures of a moonlike, crater-pocked surface and reported temperatures to be about 430°C (about 810°F) on the sunlit side and about -180°C (about -290°F) on the dark side. Although Mercury’s surface appears very similar to the surface of the Moon, there are some significant differences. The smooth, lava-like plains on Mercury, for example, are not as dark as the smooth plains (maria) of the Moon. Also unlike the surface of the Moon, the surface of Mercury is crisscrossed by long escarpments, or cliffs, indicating a period of contraction as the planet cooled early in its history.

Mercury is a poor reflector of sunlight because its surface consists of rough, porous, dark-colored rock. The planet’s albedo, or the amount of sunlight it reflects, is only about 12 percent. Earth, in contrast, reflects about 37 percent of the sunlight that strikes it, while Venus, the most reflective planet in the solar system, reflects 65 percent.

COMPOSITION

Mercury’s high density indicates that the relatively dense and abundant element iron accounts for a large proportion of the planet’s composition. The surface of Mercury, however, contains little iron, suggesting that most of Mercury’s iron is now concentrated in a large iron core. Collisions with other protoplanets early in the history of the solar system may have stripped away much of Mercury’s low-density crust, leaving behind a dense, iron-rich core.

In 1991 powerful radio telescopes on Earth revealed signs of vast deposits of ice in the polar regions of Mercury. These ice deposits occur in areas where sunlight never falls, such as crater bottoms near both of the planet’s poles. Similar ice deposits were found during the 1990s near the poles of the Moon by the Clementine and Lunar Prospector spacecrafts.

Scientists use a technique called spectroscopy to conduct studies of the light that Mercury reflects. These studies indicate that planet has only an extremely thin atmosphere, containing sodium and potassium. Apparently these elements slowly escape as gases from the crust of the planet.

MAGNETIC FIELD

Mercury is the only rocky planet other than Earth to have a global magnetic field, which is about 1 percent as strong as Earth's. The presence of the field and its global extent together suggest that the core of the planet is largely liquid iron, which produces a magnetic field as it moves. Scientists believe Mercury's crust insulates the planet's outer core, keeping it liquid despite the very cold temperatures on the dark side of the planet.

Venus

Venus (planet), one of the planets in the solar system, the second in distance from the Sun. Except for the Sun and the Moon, Venus is the brightest object in the sky. The planet is called the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancient times the evening star was called Hesperus and the morning star Phosphorus or Lucifer. Because of the distances of the orbits of Venus and Earth from the Sun, Venus is never visible more than three hours before sunrise or three hours after sunset.

When viewed through a telescope, the planet exhibits phases like the Moon. Maximum brilliance (a stellar magnitude of -4.4, 15 times as bright as the brightest star) is seen in the crescent phase when Venus is closer to Earth. Venus’s full phase appears smaller and dimmer because it occurs when the planet is on the far side of the Sun from Earth. The phases and positions of Venus in the sky repeat every 1.6 years (see Time; Year). Transits of Venus (when the planet moves across the face of the Sun as seen from Earth) are rare, occurring in pairs at intervals of a little more than a century. The next two transits will be in 2004 and 2012.

Topics:

• Exploration
• Atmosphere
• Surface Features

Exploration to Venus

Venus's complete cloud cover and deep atmosphere make it difficult to study from Earth. Most knowledge of the planet has been obtained through the use of space vehicles, particularly those carrying probes that descend through the atmosphere. The first flyby was that of Mariner 2, launched by the United States in 1962, followed by Mariner 5 in 1967 and Mariner 10 in 1974. The former Union of Soviet Socialist Republics developed several entry probes, some combined with flybys or orbiters: Venera 4 and 5 (1967), 6 (1969), 7 (1970), 8 (1972), 9 and 10 (1975), 11 and 12 (1978), 13 and 14 (1981), and 15 and 16 (1983); Vega 1 and 2, sent toward Halley's comet in 1984, also flew by Venus and released descent capsules. Several of these probes successfully reached the planet's surface. The United States sent two Pioneer Venus missions in 1978. Pioneer Venus 2 sent four probes to the surface, while the remaining craft explored the upper atmosphere. Pioneer Venus 1, an orbiter, measured the upper atmosphere for 14 years. The Magellan probe, launched toward Venus in 1989, transmitted radar images of the planet from 1990 to 1994.

Venus Atmosphere

The atmosphere of the planet consists of 97 percent carbon dioxide and is so thick that the surface pressure is 96 bars (compared with 1 bar on Earth). The surface temperature on Venus varies little from place to place and is extremely hot, about 462°C (736 K/864°F). The high surface temperature is explained by an intense greenhouse effect. Even though only a small percentage of the solar energy that falls on Venus reaches the surface, the planet stays hot because the thick atmosphere prevents the energy from escaping.

That nearly all of Venus's atmosphere is carbon dioxide is not as strange as it might seem; in fact, the crust of Earth contains almost as much carbon dioxide chemically bound in the form of limestone. About 3 percent of the Venusian atmosphere is nitrogen gas. By contrast, 78 percent of Earth's atmosphere is nitrogen. Water and water vapor are extremely rare on Venus. Many scientists argue that Venus, being closer to the Sun, was subjected to a so-called runaway greenhouse effect, which caused any oceans to evaporate into the atmosphere. The hydrogen atoms of the water molecules could have been lost to space and the oxygen atoms to the crust. Another possibility is that Venus had very little water to begin with.

Cloud particles on Venus mostly consist of concentrated sulfuric acid. Earth’s atmosphere also contains a very thin haze of sulfuric acid particles in the stratosphere. On Earth, however, sulfuric acid does not build up because rain carries it down to react with surface materials. On Venus the acid evaporates at the cloud base, which lies about 50 km (31 mi) above the surface, and so remains in the atmosphere. The upper parts of the clouds, visible from Earth and from Pioneer Venus 1, extend as haze 70 to 80 km (44 to 50 mi) above the surface. The clouds contain a pale yellow impurity, better detected at near-ultraviolet wavelengths. Variations in the sulfur dioxide content of the atmosphere may indicate active volcanism on the planet.

Certain cloud patterns and weather features that can be discerned in the cloud tops give some information about wind motion in the atmosphere. The upper-level winds circle the planet at 360 km/h (225 mph). These winds cover the planet completely, blowing toward the east at virtually every latitude from equator to pole. The motions of descending probes, however, have shown that the bulk of Venus's tremendously dense atmosphere, closer to the planet's surface, is almost stagnant. From the surface up to 10 km (6 mi) altitude, wind speeds are only about 3 to 18 km/h (2 to 11 mph). The high-speed winds probably result from the transfer of momentum from Venus's slow-moving, massive lower atmosphere to higher altitudes where the atmosphere is less massive, so that the same momentum results in a much higher velocity.

The upper atmosphere and ionosphere were studied in great detail by Pioneer Venus 1, which passed through them once each day. On Earth this region is very hot; on Venus it is not, even though Venus is closer to the Sun. Surprisingly, on the night side of Venus the upper atmosphere is extremely cold. (Day-side temperatures are 40°C/104°F, compared to night-side temperatures of -170°C/-274°F.) Scientists believe that strong winds blow from the day side toward the near vacuum that is caused by the low temperatures on the night side. Such winds would carry along light gases, such as hydrogen and helium, which are concentrated in a night-side “bulge.”

On Earth the ionosphere is isolated from the solar wind by the magnetosphere. Venus lacks a magnetic field of its own, but the solar wind seems to generate an induced magnetosphere around the planet, probably by a dynamo action involving its own magnetic field.

Venus Surface Features

Venus rotates very slowly on its axis, and the direction is retrograde (opposite to that of Earth). Curiously, the periods of Venus's orbit and rotation cause the same side of the planet to always face Earth when the two planets are closest. At such times, the side facing Earth can be viewed and mapped by Earth-based radar.

In contrast to the very large antenna needed for Earth-based radar mapping of Venus, a modest instrument on Pioneer Venus 1 was able to conduct a nearly global survey. Combined with data from the Soviet probes, the Magellan orbiter, and Earth-based radar, the survey shows that the surface of Venus is primarily a rolling plain interrupted by two continent-sized highland areas, which have been named Ishtar Terra and Aphrodite Terra after two manifestations of the goddess Venus. Aphrodite Terra, although not as high as Ishtar Terra, extends nearly halfway around the equatorial region; it occupies the planet's far side as viewed from Earth at closest approach.

The more powerful radar aboard the Magellan spacecraft has revealed huge volcanoes, large solidified lava flows, and a large array of meteorite craters. The largest impact crater is almost 300 km (190 mi) across—the smallest about 5 km (3 mi). Although the probe's radar could resolve even smaller craters, if any were present, Venus's dense atmosphere apparently prevents smaller meteorites from impacting the surface of the planet. It is believed that all craters older than about 500 million years have been obliterated, while the more recent ones show little sign of modification.

The global survey and other probes have also revealed evidence that a great deal of tectonic activity has taken place on Venus, at least in the past. Such evidence includes ridges, canyons, a troughlike depression that extends across 1400 km (870 mi) of the surface, and a gigantic volcanic cone whose base is more than 700 km (435 mi) wide. The Soviet probes sent back photographs of the areas in which they set down, and also measured the natural radioactivity of the rocks. The radioactivity resembles that of granite and strongly suggests that the material of Venus is differentiated, or chemically separated, by volcanic activity. Angular rocks that are visible in the Soviet pictures also suggest the existence of geologic activity that would counteract the forces of erosion.

Moon

Moon, name given to the only natural satellite of Earth. The Moon is the second brightest object in Earth’s sky, after the Sun, and has accordingly been an object of wonder and speculation for people since earliest times. The natural satellites of the other planets in the solar system are also sometimes referred to as moons.

Telescopes have revealed a wealth of lunar detail since their invention in the 17th century, and spacecraft have contributed further knowledge since the 1950s. Earth’s Moon is now known to be a slightly egg-shaped ball composed mostly of rock and metal. It has no liquid water, virtually no atmosphere, and is lifeless. The Moon shines by reflecting the light of the Sun. Although the Moon appears bright to the eye, it reflects on average only 7 percent of the light that falls on it. This reflectivity, called albedo, of 0.07 is similar to that of coal dust.

The diameter of the Moon is about 3,480 km (about 2,160 mi), or about one-fourth that of Earth. The Moon’s mass is only 1.2 percent of Earth’s mass. The average density of the Moon is only three-fifths that of Earth, and gravity at the lunar surface is only one-sixth as strong as gravity at sea level on Earth. The Moon moves in an elliptical (oval-shaped) orbit around Earth at an average distance of 384,403 km (238,857 mi) and at an average speed of 3,700 km/h (2,300 mph). It completes one revolution in 27 days 7 hours 43 minutes. For the Moon to go from one phase to the next similar phase—as seen from Earth—requires 29 days 12 hours 44 minutes. This period is called a lunar month. The Moon rotates once on its axis in the same period of time that it circles Earth, accounting for the fact that virtually the same portion of the Moon (the “near side”) is always turned toward Earth.

articles:

• The Moon Seen From Earth
• Surface of the Moon
• Origin of the Moon
• Magnetic Properties of the Moon

The Moon Seen From Earth

The Moon shows progressively different phases as it moves along its orbit around Earth. Half the Moon is always in sunlight, just as half of Earth has day while the other half has night. Thus, there is no permanent “dark side of the Moon,” which is sometimes confused with the Moon’s far side—the side that always faces away from Earth. The phases of the Moon depend on how much of the sunlit half can be seen at any one time. In the phase called the new moon, the near side is completely in shadow. About a week after a new moon, the Moon is in first quarter, resembling a luminous half-circle; another week later, the full moon shows its fully lighted near side; a week afterward, in its last quarter, the Moon appears as a half-circle again. The entire cycle is repeated each lunar month. The Moon is full when it is farther away from the Sun than Earth; it is new when it is closer. When it is more than half illuminated, it is said to be in gibbous phase. The Moon is said to be waning as it progresses from full to new, and to be waxing as it proceeds from new to full.

At any one time, an observer on Earth can see only 50 percent of the Moon’s entire surface. However, an additional 9 percent can be seen from time to time around the edges because the viewing angle from Earth changes slightly as the Moon moves through its elliptical orbit. This slight relative motion is called libration.

Surface of the Moon

Ancient observers of the Moon believed that the dark regions on its face were oceans, giving rise to their name maria (Latin for “seas”). This term is still used today although these regions are now known to be completely dry. The brighter regions were held to be continents. Modern observation and exploration of the Moon has yielded far more comprehensive and specific knowledge.

The Moon has no movement of wind or water to alter its surface, yet it was geologically active in the past and is still not totally unchanging. Craters cover the surface, and meteors continue to create new craters. Billions of years ago volcanic eruptions sculpted large areas of the surface. Volcanic features such as maria, domes (low, rounded, circular hills), and rilles (channels or grooves) are still discernable. Scientists have also recently discovered evidence of ice in permanently shadowed areas of the surface.

See: Craters; Volcanic Features; Ice

Moon's Craters

The Moon’s surface is covered with craters overlain by a layer of soil called regolith. Nearly all the craters were formed by explosive impacts of high-velocity meteorites. Billions of years of this meteorite bombardment ground up the Moon’s surface rocks to produce the finely divided rock fragments that compose the regolith. Craters range in size from microscopic to the South Pole-Aitken Basin, which measures over 2,500 km (1560 mi) in diameter and would nearly span the continental United States. The highest mountains on the Moon, in the Leibnitz and Doerfel ranges near the south pole, make up the rim crest of the South Pole-Aitken Basin and have peaks up to 6,100 m (20,000 ft) in height, comparable to the Himalayas on Earth. At full moon long bright streaks that radiate from certain craters can be seen. These streaks are called ray systems. Ray systems are created when bright material ejected from the craters by meteorites splashes out onto the darker surrounding surface.

The biggest of the Moon’s craters were created by the impacts of large remnants from the formation of the planets billions of years ago when the young solar system still contained many such remnants. Astronomers, however, have directly observed meteorites forming small craters on the Moon’s surface. Seismometers operating on the lunar surface have also recorded signals indicating between 70 and 150 meteorite impacts per year, with projectile masses from 100 g to 1,000 kg (4 oz to 2,200 lb). Hence the Moon is still being bombarded by meteorites, although neither as often nor as violently as in the distant past.

Origin of the Moon

Measuring the ages of lunar rocks has revealed that the Moon is about 4.6 billion years old, or about the same age as Earth and probably the rest of the solar system. Before the modern age of space exploration, scientists had three major models for the origin of the Moon. The fission from Earth model proposed that the young, molten Earth rotated so fast that it flung off some material that became the Moon. The formation in Earth orbit model claimed that the Moon formed independently, but close enough to Earth to orbit the planet. The formation far from Earth model proposed that the Moon formed independently in orbit around the Sun but was subsequently captured by Earth’s gravity when it passed close to the planet. None of these three models, however, is entirely consistent with current knowledge of the Moon. In 1975, having studied moon rocks and close-up pictures of the Moon, scientists proposed what has come to be regarded as the most probable of the theories of formation: a giant, planetary impact.

The giant impact model proposes that early in Earth’s history, well over 4 billion years ago, Earth was struck by a large planet-sized body. Early estimates for the size of this object were comparable to the size of Mars, but a computer simulation by American scientists in 1997 suggested that the body would have to have been at least 2.5 to 3 times the size of Mars. The catastrophic impact blasted portions of Earth and the impacting body into Earth orbit, where debris from the impact eventually coalesced to form the Moon. After years of research on lunar rocks during the 1970s and 1980s, this model became the most widely accepted one for the Moon’s origin. The giant impact model seems to account for all of the available evidence: the similarity in composition between Earth and Moon indicated by analysis of lunar samples, the near-complete global melting of the Moon (and possibly Earth) in the distant past , and the simple fact that the other models are all inadequate to one degree or another. Research continues on the ramifications of such a violent lunar origin to the early history of Earth and the other planets.

Magnetic Properties of the Moon

The Moon has no global magnetic field as does Earth. Some lunar rocks are weakly magnetic, indicating that they solidified in the presence of a magnetic field. The Moon has small, local magnetic fields that seem to be strongest in areas that are on opposite hemispheres from large basins. The origin of these local magnetic fields is unknown. Some scientists speculate that the magnetic fields were induced by the extreme shock pressures associated with the large asteroid collisions that created the basins. Others believe that the Moon originally had a global magnetic field generated by the movement of liquid metal in the core as on Earth. This global field shut down for some reason and only remnants of it exist in certain places on the lunar surface, preserved in material ejected by the asteroid collisions. The “fossil” magnetism found in some lunar rocks supports the former global field model, whereas the regional distribution of the magnetic surface anomalies tends to support the local field model. Regions of strong magnetic fields repel the charged particles that stream from the Sun in the solar wind. Scientists believe that interaction with the solar wind darkens the Moon, and that some lighter swirl-shaped regions of the Moon are protected by local magnetic fields.

Moon Volcanic Features

Maria, domes, rilles, and a few craters display indisputable characteristics of volcanic origin. Maria are plains of dark-colored rock that cover approximately 40 percent of the Moon's visible hemisphere. The maria formed when molten rock erupted onto the surface and solidified between 3.16 billion and 3.96 billion years ago. This rock resembles terrestrial basalt, a volcanic rock type widely distributed on Earth, but the rock that formed the maria has a higher iron content and contains unusually large amounts of titanium. The largest of the maria is Oceanus Procellarum, an oval-shaped plain on the near side of the Moon 2,500 km by 1,500 km wide. Photographs of the side of the Moon not visible from Earth have revealed a startling fact: The far side generally lacks the maria that are so conspicuous a feature of the visible side. This probably reflects the fact that the Moon's crust is thicker on the far side than on the near side, and therefore the lavas that form the maria were more easily erupted through the thinner crust. Rilles are of two basic types: sinuous and straight. Sinuous rilles are meandering channels that are probably lava drainage channels or collapsed lava tubes formed by large lava flows. Straight rilles are large shallow troughs caused by movement of the Moon’s crust; they may be up to a thousand kilometers long and several kilometers wide. Domes are small rounded features that range from 8 to 16 km (5 to 10 mi) in diameter and from 60 to 90 m (200 to 300 ft) in height. Domes, thought to be small inactive volcanoes, often contain a small rimless pit on their tops.

Magnetic and other measurements indicate a current temperature at the Moon’s core as high as 1600°C (2900°F), above the melting point of most lunar rocks. Evidence from seismic recordings suggests that some regions near the lunar center may be liquid. However, no evidence of recent volcanic activity has been observed.

Ice Surface of the Moon

Temperatures on most of the Moon’s surface are too extreme for water or ice to exist, ranging from a maximum of 127°C (261°F) at lunar noon to a minimum of -173°C (-279°F) just before lunar dawn. Temperatures in permanently shadowed areas near the lunar poles, however, may consistently be as low as -220°C (-364°F). In 1996 a team working with data gathered by the Clementine spacecraft announced that frozen water may exist in one of these shadowed areas near the Moon’s south pole. Clementine was a joint venture by the Department of Defense and the National Aeronautics and Space Administration (NASA). The spacecraft’s radar showed what may be an 8,000 sq km (3,000 sq mi) area covered with a mixture of dirt and ice crystals. Clementine was launched in 1994 and gathered data for four months.

NASA launched the Lunar Prospector spacecraft toward the Moon in 1998. Prospector returned data confirming the Clementine discovery and suggesting that a significant amount of water exists in the dark areas near the lunar poles in the form of ice crystals mixed with soil. Estimates of the amount of water on the Moon vary widely, from 10 million to 6 billion metric tons.

In 1999, at the end of the Lunar Prospector’s mission, scientists programmed the spacecraft to crash at a specific spot likely to contain water, hoping that the debris that rose with the impact would contain detectable water vapor. Although no water was detected after the crash, scientists could not conclude that no water existed on the Moon. They acknowledged several other possible explanations for the result: The spacecraft might have missed its target area, the telescopes used to observe the crash might have been aimed incorrectly, or the magnitude of the impact created by the Lunar Prospector spacecraft may have been insufficient to generate a large cloud of water vapor.

Eclipse

Eclipse->> SOLAR ECLIPSES

The length of the moon’s umbra varies from 367,000 to 379,800 km (228,000 to 236,000 mi), and the distance between the earth and the moon varies from 357,300 to 407,100 km (222,000 to 253,000 mi). Total solar eclipses occur when the moon’s umbra reaches the earth. The diameter of the umbra is never greater than 268.7 km (167 mi) where it touches the surface of the earth, so that the area in which a total solar eclipse is visible is never wider than that and is usually considerably narrower. The width of the penumbra shadow, or the area of partial eclipse on the surface of the earth, is about 4828 km (about 3000 mi). At certain times when the moon passes between the earth and the sun, its shadow does not reach the earth. At such times an annular eclipse occurs in which an annulus or bright ring of the solar disk appears around the black disk of the moon.

The shadow of the moon moves across the surface of the earth in an easterly direction. Because the earth is also rotating eastward, the speed of the moon shadow across the earth is equal to the speed of the moon traveling along its orbit, minus the speed of the earth’s rotation. The speed of the shadow at the equator is about 1706 km/h (about 1060 mph); near the poles, where the speed of rotation is virtually zero, it is about 3380 km/h (about 2100 mph). The path of a total solar eclipse and the time of totality can be calculated from the size of the moon’s shadow and from its speed. The maximum duration of a total solar eclipse is about 7.5 minutes, but these are rare, occurring only once in several thousand years. A total eclipse is usually visible for about 3 minutes from a point in the center of the path of totality.

In areas outside the band swept by the moon’s umbra but within the penumbra, the sun is only partly obscured, and a partial eclipse occurs.

At the beginning of a total eclipse, the moon begins to move across the solar disk about 1 hour before totality. The illumination from the sun gradually decreases and during totality (and near totality) declines to the intensity of bright moonlight. This residual light is caused largely by the sun’s corona, the outermost part of the sun’s atmosphere. As the surface of the sun narrows to a thin crescent, the corona becomes visible. At the moment before the eclipse becomes total, brilliant points of light, called Baily’s beads, flash out in a crescent shape. These points are caused by the sun shining through valleys and irregularities on the lunar surface. Baily’s beads are also visible at the instant when totality is ending, called emersion. Just before, just after, and sometimes during totality, narrow bands of moving shadows can be seen. These shadow bands are not fully understood but are thought to be caused by irregular refraction of light in the atmosphere of the earth. Before and after totality, an observer located on a hill or in an airplane can see the moon’s shadow traveling eastward across the earth’s surface like a swiftly moving cloud shadow.

Eclipse

Eclipse->> FREQUENCY OF ECLIPSES

If the earth’s orbit, or the ecliptic, were in the same plane as the moon’s orbit, two total eclipses would occur during each lunar month, a lunar eclipse at the time of each full moon, and a solar eclipse at the time of each new moon. The two orbits, however, are inclined, and, as a result, eclipses occur only when the moon or the sun is within a few degrees of the two points, called the nodes, where the orbits intersect.

Periodically both the sun and the moon return to the same position relative to one of the nodes, with the result that eclipses recur at regular intervals. The time of the interval, called the saros, is a little more than 6585.3 days or about 18 years, 9 to 11 days, depending on the number of intervening leap years, and 8 hours. The saros, known since the time of ancient Babylonia, corresponds almost exactly to 19 returns of the sun to the same node, 242 returns of the moon to the same node, and 223 lunar months. The disparity between the number of returns of the moon and the number of lunar months is the result of the nodes moving westward at the rate of 19.5° per year. An eclipse that recurs after the saros will be a duplicate of the earlier eclipse but will be visible 120° farther west on the earth’s surface, because of the rotation of the earth during the third of a day included in the interval. Lunar eclipses recur 48 or 49 times and solar eclipses 68 to 75 times before slight differences in the motions of the sun and moon eliminate the eclipse.

During one saros about 70 eclipses take place, usually 29 lunar and 41 solar; of the latter, usually 10 are total and 31 partial. The minimum number of eclipses that can occur in a given saros year is 2, the maximum 7, and the average is 4.

During the 20th century 375 eclipses took place: 228 solar and 147 lunar. The last total eclipse of the sun visible in the United States occurred over the state of Hawaii on July 11, 1991. The prior such eclipse occurred over the state of Washington on February 26, 1979. The next total eclipse will be visible from the U.S. in 2017.

Eclipse

Eclipse->> OBSERVATION OF ECLIPSES

Many problems of astronomy can be studied only during a total eclipse of the sun. Among these problems are the size and composition of the solar corona and the bending of light rays passing close to the sun because of the sun’s gravitational field (see Relativity). The great brilliance of the solar disk and the sun-induced brightening of the earth’s atmosphere make observations of the corona and nearby stars impossible except during a solar eclipse. The coronagraph, a photographic telescope, permits direct observation of the edge of the solar disk at all times. Today, scientific solar eclipse observations are extremely valuable, particularly when the path of the eclipse traverses large land areas. An elaborate network of special observatories may provide enough data for months of analysis by scientists. Such data may provide information on how minute variations in the sun affect weather on earth, and how scientists can improve predictions of solar flares.