Mercury

Mercury

The planet Mercury was first photographed in detail on March 29, 1974, by the U.S. probe Mariner 10. The probe was about 130,000 miles (210,000 kilometers) from Mercury.
The planet Mercury was first photographed in detail on March 29, 1974, by the U.S. probe Mariner 10. The probe was about 130,000 miles (210,000 kilometers) from Mercury. Image credit: NASA

Mercury is the planet nearest the sun. It has a diameter of 3,032 miles (4,879 kilometers), about two-fifths of Earth’s diameter. Mercury orbits the sun at an average distance of about 36 million miles (58 million kilometers), compared with about 93 million miles (150 million kilometers) for Earth.

Because of Mercury’s size and nearness to the brightly shining sun, the planet is often hard to see from the Earth without a telescope. At certain times of the year, Mercury can be seen low in the western sky just after sunset. At other times, it can be seen low in the eastern sky just before sunrise.

Orbit

Mercury travels around the sun in an elliptical (oval-shaped) orbit. The planet is about 28,580,000 miles (46,000,000 kilometers) from the sun at its closest point, and about 43,380,000 miles (69,820,000 kilometers) from the sun at its farthest point. Mercury is about 48,000,000 miles (77,300,000 kilometers) from Earth at its closest approach.

Mercury moves around the sun faster than any other planet. The ancient Romans named it Mercury in honor of the swift messenger of their gods. Mercury travels about 30 miles (48 kilometers) per second, and goes around the sun once every 88 Earth days. The Earth goes around the sun once every 365 days, or one year.

Rotation

As Mercury moves around the sun, it rotates on its axis, an imaginary line that runs through its center. The planet rotates once about every 59 Earth days — a rotation slower than that of any other planet except Venus. As a result of the planet’s slow rotation on its axis and rapid movement around the sun, a day on Mercury — that is, the interval between one sunrise and the next — lasts 176 Earth days.

Until the mid-1960′s, astronomers believed that Mercury rotated once every 88 Earth days, the same time the planet takes to go around the sun. If Mercury did this, one side of the planet would always face the sun, and the other side would always be dark. However, radar studies conducted in 1965 showed that the planet rotates once in about 59 days.

Phases

When viewed through a telescope, Mercury can be seen going through “changes” in shape and size. These apparent changes are called phases, and resemble those of the moon. They result from different parts of Mercury’s sunlit side being visible from the Earth at different times.

As Mercury and the Earth travel around the sun, Mercury can be seen near the other side of the sun about every 116 days. At this point, almost all its sunlit area is visible from the Earth. It looks like a bright, round spot with almost no visible marks. As Mercury moves around the sun toward the Earth, less and less of its sunlit area can be seen. After about 36 days, only half its surface is visible. After another 22 days, it nears the same side of the sun as the Earth, and only a thin sunlit area is visible. The amount of sunlit area that can be seen increases gradually after Mercury passes in front of the sun and begins moving away from the Earth.

When Mercury is on the same side of the sun as the Earth is, its dark side faces the Earth. The planet is usually not visible at this point because Mercury and the Earth orbit the sun at different angles. As a result, Mercury does not always pass directly between the Earth and the sun. Sometimes Mercury is directly between the Earth and the sun. When this occurs, every 3 to 13 years, the planet is in transit and can be seen as a black spot against the sun.

Surface and atmosphere

The surface of Mercury consists of cratered terrain and smooth plains.
The surface of Mercury consists of cratered terrain and smooth plains. Image credit: NASA

Mercury’s surface appears to be much like that of the moon. It reflects approximately 6 percent of the sunlight it receives, about the same as the moon’s surface reflects. Like the moon, Mercury is covered by a thin layer of minerals called silicates in the form of tiny particles. It also has broad, flat plains; steep cliffs; and many deep craters similar to those on the moon. The craters formed when meteors or small comets crashed into the planet. Mercury does not have enough atmosphere to slow down meteoroids and burn them up by friction. The Caloris Basin, Mercury’s largest crater, measures about 800 miles (1,300 kilometers) across.

Mercury’s interior appears to resemble that of the Earth. Both planets have a rocky layer called a mantle beneath their crust, and both planets have an iron core. Based on Mercury’s size and mass, scientists believe the planet’s core makes up about three-fourths of its radius. Earth’s core makes up about half of its radius. The discovery of a magnetic field around Mercury led some scientists to believe that the planet’s outer core, like Earth’s, consists of liquid iron.

Mercury is dry, extremely hot, and almost airless. The sun’s rays are approximately seven times as strong on Mercury as they are on the Earth. The sun also appears about 2 1/2 times as large in Mercury’s sky as in the Earth’s.

Mercury does not have enough gases in its atmosphere to reduce the amount of heat and light it receives from the sun. The temperature on the planet may reach 840 degrees F (450 degrees C) during the day. But at night, the temperature may drop as low as -275 degrees F (-170 degrees C). Because of the lack of atmosphere, Mercury’s sky is black. Stars probably would be visible from the surface during the day.

Scans of Mercury made by Earth-based radar indicate that craters at Mercury’s poles contain water ice. The floors of the craters are permanently shielded from sunlight, so the temperature never gets high enough to melt the ice.

Mercury is surrounded by an extremely small amount of helium, hydrogen, oxygen, and sodium. This envelope of gases is so thin that the greatest possible atmospheric pressure (force exerted by the weight of gases) on Mercury would be about 0.00000000003 pound per square inch (0.000000000002 kilogram per square centimeter). The atmospheric pressure on the Earth is about 14.7 pounds per square inch (1.03 kilograms per square centimeter).

The plant and animal life of the Earth could not live on Mercury because of the lack of oxygen and the intense heat. Scientists doubt that the planet has any form of life.

Density and mass

Mercury’s density is slightly less than the Earth’s (see Density). That is, a portion of Mercury would weigh slightly less than an equal portion of the Earth. Mercury is smaller than the Earth and therefore has much less mass (see Mass). Mercury’s smaller mass makes its force of gravity only about a third as strong as that of the Earth. An object that weighs 100 pounds on the Earth would weigh only about 38 pounds on Mercury.

Flights to Mercury

Mariner 10 is the only space probe that has visited the planet Mercury. It flew past Venus in 1974, then made three passes near Mercury in 1974 and 1975. A probe called Messenger, launched in 2004, was scheduled to make its first visit to Mercury in 2008.
Mariner 10 is the only space probe that has visited the planet Mercury. It flew past Venus in 1974, then made three passes near Mercury in 1974 and 1975. A probe called Messenger, launched in 2004, was scheduled to make its first visit to Mercury in 2008. Image credit: NASA

The United States Mariner 10 became the first and only spacecraft to reach Mercury. The remotely controlled spacecraft flew to within 460 miles (740 kilometers) of Mercury on March 29, 1974. It swept past the planet again on Sept. 24, 1974, and on March 16, 1975. During those flights, the spacecraft photographed portions of the surface of Mercury. It also detected Mercury’s magnetic field.

Mariner 10 became the first spacecraft to study two planets. The probe photographed and made scientific measurements of Venus while traveling to Mercury. As the probe flew near Venus, the planet’s gravity pulled on the spacecraft, causing it to move faster. Thus, Mariner 10 reached Mercury in less time and by using less fuel than if it had flown directly from the Earth.

In 2004, the United States launched the Messenger probe to Mercury. Messenger was scheduled to fly by Mercury twice in 2008 and once in 2009 before going into orbit around the planet in 2011. The probe was then to orbit Mercury for one Earth year while mapping Mercury’s surface and studying its composition, interior structure, and magnetic field.

Contributor: Maria T. Zuber, Ph.D., Professor of Geophysics and Planetary Science, Massachusetts Institute of Technology.

How to cite this article: To cite this article, World Book recommends the following format: Zuber, Maria T. “Mercury.” World Book Online Reference Center. 2004. World Book, Inc. (http://www.worldbookonline.com/wb/Article?id=ar356240.)

Neptune

Neptune

The blue clouds of Neptune are mostly frozen methane, the main chemical in natural gas -- a fuel for heating and cooking on Earth. The other object shown is Neptune's moon Triton.
The blue clouds of Neptune are mostly frozen methane, the main chemical in natural gas — a fuel for heating and cooking on Earth. The other object shown is Neptune’s moon Triton. Image credit: NASA/JPL

Neptune is one of the two planets that cannot be seen without a telescope. The other is Pluto. Neptune is about 30 times as far from the sun as is Earth. Pluto is the only planet farther from the sun than Neptune. But every 248 years Pluto moves inside Neptune’s orbit for about a 20-year period, during which it is closer to the sun than Neptune. Pluto last crossed Neptune’s orbit on Jan. 23, 1979, and remained within it until Feb. 11, 1999.

Neptune’s diameter at the equator is 30,775 miles (49,528 kilometers), or almost 4 times that of Earth. It is about 17 times as massive (heavy) as Earth, but is not so dense as Earth. Neptune has 11 satellites (moons) and several rings around it.

Neptune travels around the sun in an elliptical (oval-shaped) orbit. Its average distance from the sun is about 2,793,100,000 miles (4,495,060,000 kilometers). Neptune goes around the sun once about every 165 Earth years, compared with once a year for Earth. As Neptune orbits the sun, it spins on its axis, an imaginary line through its center. Neptune’s axis is not perpendicular (at an angle of 90 degrees) to the planet’s path around the sun. The axis tilts about 28 degrees from the perpendicular position. Neptune spins around once in about 16 hours and 7 minutes.

Surface and atmosphere

Scientists believe that Neptune is made up chiefly of hydrogen, helium, water, and silicates. Silicates are the minerals that make up most of Earth’s rocky crust, though Neptune does not have a solid surface like Earth. Thick clouds cover Neptune’s surface. Its interior begins with a region of heavily compressed gases. Deep in the interior, these gases blend into a liquid layer that surrounds the planet’s central core of rock and ice. The tilt of its axis causes the sun to heat the Neptune’s northern and southern halves alternately, resulting in seasons and temperature changes.

Bright blue clouds that surround the planet Neptune consist mainly of frozen methane. Winds that carry these clouds may reach speeds up to 700 miles (1,100 kilometers) per hour.
Bright blue clouds that surround the planet Neptune consist mainly of frozen methane. Winds that carry these clouds may reach speeds up to 700 miles (1,100 kilometers) per hour. Image credit: NASA/JPL

Neptune is surrounded by thick layers of clouds in rapid motion. Winds blow these clouds at speeds up to 700 miles (1,100 kilometers) per hour. The clouds farthest from Neptune’s surface consist mainly of frozen methane. Scientists believe that Neptune’s darker clouds, which lie below the clouds of methane, are composed of hydrogen sulfide.

In 1989, the Voyager 2 spacecraft found that Neptune had a dark area made up of violently swirling masses of gas resembling a hurricane. This area, called the Great Dark Spot, was similar to the Great Red Spot on Jupiter. But in 1994, the Hubble Space Telescope found that the Great Dark Spot had vanished.

The icy crust of Triton, Neptune's largest satellite, has ridges and valleys that were revealed in photographs taken by the U.S. space probe Voyager 2.
The icy crust of Triton, Neptune’s largest satellite, has ridges and valleys that were revealed in photographs taken by the U.S. space probe Voyager 2. Image credit: NASA/JPL

Satellites and rings

Neptune has 11 known satellites. Triton, Neptune’s largest satellite, is about 1,681 miles (2,705 kilometers) in diameter and about 220,440 miles (354,760 kilometers) from Neptune. It is the only major satellite in the solar system that orbits in a direction opposite to that of its planet. Triton has a circular orbit and travels once around Neptune every six days. Triton may once have been a large comet that traveled around the sun. At some point, Neptune’s gravity probably captured the comet, and it became a satellite of Neptune. Scientists have discovered evidence that volcanoes on Triton once spewed a slushy mixture of water and ammonia. This mixture is now frozen on Triton’s surface. Triton has a surface temperature of -390 degrees F (-235 degrees C), the coldest known temperature in the solar system. Some volcanoes on Triton remain active, shooting crystals of nitrogen ice as high as 6 miles (10 kilometers) above the moon’s surface.

In Neptune's outermost ring, 39,000 miles (63,000 kilometers) from the planet, material mysteriously clumps into three bright, dense arcs.
In Neptune’s outermost ring, 39,000 miles (63,000 kilometers) from the planet, material mysteriously clumps into three bright, dense arcs. Image credit: NASA

Neptune has three conspicuous rings and one faint ring. All of these rings are much fainter and darker than the rings of Saturn. They appear to consist of particles of dust. Neptune’s outer ring is unlike any other planetary ring in the solar system. It has three curved segments that are brighter and denser than the rest of the ring. Scientists do not know why the dust is spread unevenly in the ring.

Discovery

Neptune was discovered by means of mathematics before being seen through a telescope. Astronomers had noticed that Uranus, which they thought was the most distant planet, was not always in the position they predicted for it. The force of gravity of some unknown planet seemed to be influencing Uranus.

In 1843, John C. Adams, a young English astronomer and mathematician, began working to find the location of the unknown planet. Adams predicted the planet would be about 1 billion miles (1.6 billion kilometers) farther from the sun than Uranus. He completed his remarkably accurate work in September 1845. Adams sent it to Sir George B. Airy, the Astronomer Royal of England. However, Airy did not look for the planet with a telescope. Apparently, he lacked confidence in Adams.

Meanwhile, Urbain J. J. Leverrier, a young French mathematician unknown to Adams, began working on the project. By mid-1846, Leverrier also had predicted Neptune’s position. He sent his predictions, which were similar to those of Adams, to the Urania Observatory in Berlin, Germany. Johann G. Galle, the director of the observatory, had just charted the fixed stars in the area where the planet was believed to be. On Sept. 23, 1846, Galle and his assistant, Heinrich L. d’Arrest, found Neptune near the position predicted by Leverrier. Today, both Adams and Leverrier are credited with the discovery. The planet was named for Neptune, the Roman sea god. In August 1989, the Voyager 2 spacecraft provided the first close-up views of Neptune and most of its moons. The spacecraft also discovered the planet’s rings and six of its moons — Despina, Galatea, Larissa, Naiad, Proteus, and Thalassa.

Contributor: Bradford A. Smith, Ph.D., Astronomer, Institute for Astronomy, University of Hawaii.

How to cite this article: To cite this article, World Book recommends the following format: Smith, Bradford A. “Neptune.” World Book Online Reference Center. 2004. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar386900.

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Saturn

Saturn is encircled by seven major rings. In this photograph, a section of the rings is hidden by the planet's shadow.
Saturn is encircled by seven major rings. In this photograph, a section of the rings is hidden by the planet’s shadow. The Cassini spacecraft, launched in 1997 to study Saturn and its rings and satellites, captured this natural color image as it approached the planet in 2004. Image credit: NASA/JPL/Space Science Institute

Saturn is the second largest planet. Only Jupiter is larger. Saturn has seven thin, flat rings around it. The rings consist of numerous narrow ringlets, which are made up of ice particles that travel around the planet. The gleaming rings make Saturn one of the most beautiful objects in the solar system. Jupiter, Neptune, and Uranus are the only other planets known to have rings. Their rings are much fainter than those around Saturn.

Saturn’s diameter at its equator is about 74,900 miles (120,540 kilometers), almost 10 times that of Earth. The planet can be seen from Earth with the unaided eye, but its rings cannot. Saturn was the farthest planet from Earth that the ancient astronomers knew about. They named it for the Roman god of agriculture.

Saturn travels around the sun in an elliptical (oval-shaped) orbit. Its distance from the sun varies from about 941,070,000 miles (1,514,500,000 kilometers) at its farthest point to about 840,440,000 miles (1,352,550,000 kilometers) at its closest point. The planet takes about 10,759 Earth days, or about 29 1/2 Earth years, to go around the sun, compared with 365 days, or one year, for Earth.

Rotation

As Saturn travels around the sun, it spins on its axis, an imaginary line drawn through its center. Saturn’s axis is not perpendicular (at an angle of 90 degrees) to the planet’s path around the sun. The axis tilts at an angle of about 27 degrees from the perpendicular position.

Saturn rotates faster than any other planet except Jupiter. Saturn spins around once in only 10 hours 39 minutes, compared to about 24 hours, or one day, for Earth. The rapid rotation of Saturn causes the planet to bulge at its equator and flatten at its poles. The planet’s diameter is 8,000 miles (13,000 kilometers) larger at the equator than between the poles.

Surface and atmosphere

Most scientists believe Saturn is a giant ball of gas that has no solid surface. However, the planet seems to have a hot solid inner core of iron and rocky material. Around this dense central part is an outer core that probably consists of ammonia, methane, and water. A layer of highly compressed, liquid metallic hydrogen surrounds the outer core. Above this layer lies a region composed of hydrogen and helium in a viscous (syruplike) form. The hydrogen and helium become gaseous near the planet’s surface and merge with its atmosphere, which consists chiefly of the same two elements.

Bands of clouds circle the planet Saturn. The large swirling spot is a hurricane-like mass of gas 1,900 miles (3,000 kilometers) across.
Bands of clouds circle the planet Saturn. The large swirling spot is a hurricane-like mass of gas 1,900 miles (3,000 kilometers) across. Image credit: NASA

A dense layer of clouds covers Saturn. Photographs of the planet show a series of belts and zones of varied colors on the cloud tops. This banded appearance seems to be caused by differences in the temperature and altitude of atmospheric gas masses.

The plants and animals that live on Earth could not live on Saturn. Scientists doubt that any form of life exists on the planet.

Temperature

The tilt of Saturn’s axis causes the sun to heat the planet’s northern and southern halves unequally, resulting in seasons and temperature changes. Each season lasts about 7 1/2 Earth years, because Saturn takes about 29 times as long to go around the sun as Earth does. Saturn’s temperature is always much colder than Earth’s, because Saturn is so far from the sun. The temperature at the top of Saturn’s clouds averages -285 degrees F (-175 degrees C).

The temperatures below Saturn’s clouds are much higher than those at the top of the clouds. The planet gives off about 2 1/2 times as much heat as it receives from the sun. Many astronomers believe that much of Saturn’s internal heat comes from energy generated by the sinking of helium slowly through the liquid hydrogen in the planet’s interior.

Density and mass

Saturn has a lower density than any other planet. It is only about one-tenth as dense as Earth, and about two-thirds as dense as water. That is, a portion of Saturn would weigh much less than an equal portion of Earth, and would float in water.

Although Saturn has a low density, it has a greater mass than any other planet except Jupiter. Saturn is about 95 times as massive as Earth. The force of gravity is a little higher on Saturn than on Earth. A 100-pound object on Earth would weigh about 107 pounds on Saturn.

Rings

The rings of Saturn surround the planet at its equator. They do not touch Saturn. As Saturn orbits the sun, the rings always tilt at the same angle as the equator.

The seven rings of Saturn consist of thousands of narrow ringlets. The ringlets are made up of billions of pieces of ice. These pieces range from ice particles that are the size of dust to chunks of ice that measure more than 10 feet (3 meters) in diameter.

Saturn’s major rings are extremely wide. The outermost ring, for example, may measure as much as 180,000 miles (300,000 kilometers) across. However, the rings of Saturn are so thin that they cannot be seen when they are in direct line with Earth. They vary in thickness from about 660 to 9,800 feet (200 to 3,000 meters). A space separates the rings from one another. Each of these gaps is about 2,000 miles (3,200 kilometers) or more in width. However, some of the gaps between the major rings contain ringlets.

The dark side of Saturn's rings was photographed by Voyager 1 as it flew by the side opposite the sun. The dense B-ring -- the reddish-brown band -- appears dark because it blocks much of the sunlight. It is the brightest ring when viewed from earth.
The dark side of Saturn’s rings was photographed by Voyager 1 as it flew by the side opposite the sun. The dense B-ring — the reddish-brown band — appears dark because it blocks much of the sunlight. It is the brightest ring when viewed from earth. Image credit: JPL

Saturn’s rings were discovered in the early 1600′s by the Italian astronomer Galileo. Galileo could not see the rings clearly with his small telescope, and thought they were large satellites. In 1656, after using a more powerful telescope, Christiaan Huygens, a Dutch astronomer, described a “thin, flat” ring around Saturn. Huygens thought the ring was a solid sheet of some material. In 1675, Giovanni Domenico Cassini, an Italian-born French astronomer, announced the discovery of two separate rings made up of swarms of satellites. Later observations of Saturn resulted in the discovery of more rings. The ringlets were discovered in 1980.

Satellites

In addition to its rings, Saturn has 25 satellites that measure at least 6 miles (10 kilometers) in diameter, and several smaller satellites. The largest of Saturn’s satellites, Titan, has a diameter of about 3,200 miles (5,150 kilometers) — larger than the planets Mercury and Pluto. Titan is one of the few satellites in the solar system known to have an atmosphere. Its atmosphere consists largely of nitrogen.

Many of Saturn’s satellites have large craters. For example, Mimas has a crater that covers about one-third the diameter of the satellite. Another satellite, Iapetus, has a bright side and a dark side. The bright side of this satellite reflects about 10 times as much sunlight as the dark side. The satellite Hyperion is shaped somewhat like a squat cylinder rather than like a sphere. Unlike Saturn’s other satellites, Hyperion’s axis does not point toward the planet.

Flights to Saturn

In 1973, the United States launched a space probe to study both Saturn and Jupiter. This craft, called Pioneer-Saturn, sped by Jupiter in 1974 and flew within 13,000 miles (20,900 kilometers) of Saturn on Sept. 1, 1979. The probe sent back scientific data and close-up photographs of Saturn. The data and photographs led to the discovery of two of the planet’s outer rings.

Pioneer-Saturn also found that the planet has a magnetic field, which is 1,000 times as strong as that of Earth. This field produces a large magnetosphere (zone of strong magnetic forces) around Saturn. In addition, data from the probe indicated the presence of radiation belts inside the planet’s magnetosphere. The belts consist of high-energy electrons and protons, and are comparable to Earth’s Van Allen belts.

The Cassini probe, launched in 1997, began orbiting Saturn in 2004. Cassini was designed to study Saturn, its rings, and its moons and to drop a probe called Huygens into the atmosphere of the moon Titan.
The Cassini probe, launched in 1997, began orbiting Saturn in 2004. Cassini was designed to study Saturn, its rings, and its moons and to drop a probe called Huygens into the atmosphere of the moon Titan. Image credit: NASA

In 1977, the United States launched two space probes — Voyager 1 and Voyager 2 — to study Saturn and other planets. Voyager 1 flew within 78,000 miles (126,000 kilometers) of Saturn on Nov. 12, 1980. On Aug. 25, 1981, Voyager 2 flew within 63,000 miles (101,000 kilometers) of the planet.

The Voyager probes confirmed the existence of Saturn’s seventh ring. They also found that the planet’s rings are made up of ringlets. In addition, the probes sent back data and photographs that led to the discovery or confirmation of the existence of nine satellites. The Voyager probes also determined that the atmosphere of Titan consists chiefly of nitrogen. In 1997, the United States launched the Cassini probe to study Saturn, its rings, and its satellites. The probe began orbiting Saturn in 2004. Cassini also carried a probe called Huygens, which was to separate from Cassini and land on Titan. Huygens was built by the European Space Agency, an organization of European nations.

Contributor: Hyron Spinrad, Ph.D., Professor of Astronomy, University of California, Berkeley.

How to cite this article: To cite this article, World Book recommends the following format: Spinrad, Hyron. “Saturn.” World Book Online Reference Center. 2004. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar492440.

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