SIGHT ON PLANETS IN SOLAR SYSTEM
| PLANET | DISTANCE FROM SUN(MILLION KM) |
MASS(IN KG) | TEMPERATURE(K) | NUMBER OF MOONS |
|---|---|---|---|---|
| MERCURY | 57.9 | 3.3x1023 | 100-700 | 0 |
| VENUS | 108.2 | 4.87x1024 | 726 | 0 |
| EARTH | 149.6 (1AU) | 5.98x1024 | 260-310 | 1 |
| MARS | 227.9 | 6.42x1023 | 150-310 | 2 |
| JUPITER | 778.3 | 1.90x1027 | 120 | 67(18 NAMED+SMALLER ONES) |
| SATURN | 1427 | 5.69x1026 | 88 | 62(32 NAMED) |
| URANUS | 2871 | 8.68x1025 | 59 | 27(21 NAMED) |
| NEPTUNE | 4497.1 | 1.02x1026 | 48 | 13 |
DWARF PLANETS
- PLUTO
- ORCUS
- HAUMEA
- QUAOAR
- MAKEMAKE
- 2007 OR10
- ERIS
- SEDNA
MERCURY
Period of revolution around the sun:87.96 Earth days
Period of Rotation:58.7 Earth days
Diametre:4878 km
Moons:No Moons
Mercury's elliptical orbit takes the small planet as close as 47 million km (29 million miles) and as far as 70 million km (43 million miles) from the sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, our star would appear more than three times as large as it does when viewed from Earth. Because Mercury is so close to the sun, it is hard to directly observe from Earth except during twilight.
mercury makes an appearance indirectly, however -- 13 times each century, Earth observers can watch Mercury pass across the face of the sun, an event called a transit. The transits fall within several days of May 8 and November 10. The first two transits of Mercury in the 21st century occurred 7 May 2003 and 8 November 2006. The next will occur on 9 May 2016.
Temperatures on Mercury's surface can reach 800 degrees Fahrenheit (427 degrees Celsius). Because Mercury's atmosphere is so thin, the surface cannot retain that heat so nighttime temperatures can drop to -290 degrees Fahrenheit (-179 degrees Celsius).
Mercury's thin atmosphere, or exosphere, is made up of atoms blasted off the surface by the solar wind and micrometeoroid impacts. Because of solar radiation pressure, the atoms quickly escape into space and form a tail of neutral particles. Though Mercury's magnetic field has just 1 percent the strength of Earth's, the field is very active. The magnetic field in the solar wind episodically connects to Mercury's field, creating intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface. When these ions strike the surface, they knock off neutral atoms and send them on a loop high into the sky where other processes may fling them back to the surface or accelerate them away from Mercury.
Mercury's surface resembles that of Earth's Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. While there are areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, formed by contraction of the crust. The Caloris Basin, one of the largest features on Mercury, is about 1,550 km (960 miles) in diameter. It was the result of an asteroid impact on the planet's surface early in the solar system's history. Over the next several billion years, Mercury shrank in radius about 1 to 2 km (0.6 to 1.2 miles) as the planet cooled after its formation. The outer crust contracted and grew strong enough to prevent magma from reaching the surface, ending the period of volcanic activity.
Mercury is the second densest planet after Earth, with a large metallic core having a radius of 1,800 to 1,900 km (1,100 to 1,200 miles), about 75 percent of the planet's radius. In 2007, researchers using ground-based radars to study the core found evidence that it is molten (liquid). Mercury's outer shell, comparable to Earth's outer shell (called the mantle), is only 500 to 600 km (300 to 400 miles) thick.
The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface. In 1991, astronomers on Earth using radar observations showed that Mercury may have water ice at its north and south poles inside deep craters that are perpetually cold. Infalling comets or meteorites might have brought ice to these regions of Mercury, or water vapor might have outgassed from the interior and frozen out at the poles.
In 2008 and 2009, NASA's MESSENGER mission performed two close flybys of Mercury. By the second flyby, the spacecraft had imaged about 80 percent of the surface at useful resolution and made discoveries about the magnetic field and how Mercury's crust was formed. The flybys employed Mercury's gravity to help ease the spacecraft into orbit in March 2011. The spacecraft is studying and imaging Mercury from orbit and will map nearly the entire planet in color. MESSENGER is the first spacecraft to orbit Mercury.
How Mercury Got its Name
Mercury is appropriately named for the swiftest of the ancient Roman gods. Mercury, the god of commerce, is the Roman counterpart to the ancient Greek god Hermes, the messenger of the gods.
Significant Dates
1631: Pierre Gassendi uses a telescope to watch from Earth as Mercury crosses the face of the sun.
1965: Though it was thought for centuries that the same side of Mercury always faces the sun, astronomers find the planet rotates three times for every two orbits
1974-1975: Mariner 10 photographs roughly half of Mercury's surface in three flybys.
1991: Scientists using Earth-based radar find signs of ice locked in permanently shadowed areas of craters in Mercury's polar regions.
2008: MESSENGER's first flyby of Mercury initiates the most comprehensive study yet of the innermost planet. The three flybys revealed the side of the planet not seen by Mariner 10. Also, many more images and discoveries were obtained by these flybys.
MORE DETAILS ON MERCURY
VENUS
Period of revolutionaround the sun:224.68 Earth days.
Period of Rotation:243 Earth days
Diametre:12104 km
Moons:No Moons
Venus and Earth are similar in size, mass, density, composition and gravity. There, however, the similarities end. Venus is covered by a thick, rapidly spinning atmosphere, creating a scorched world with temperatures hot enough to melt lead and a surface pressure 90 times that of Earth.
The Venusian year (orbital period) is about 225 Earth days long, while the planet's sidereal rotation period is 243 Earth days, making a Venus solar day (measured noon to noon) about 117 Earth days long. Resulting from this slow rotation Venus cannot generate a magnetic field similar to Earth's -- though its core iron content is similar to that of the Earth. (Venus' iron core is approximately 3,000 km [1,900 miles] in radius.) Venus rotates retrograde (east to west) compared with Earth's prograde (west to east) rotation. Seen from Venus, the sun would rise in the west and set in the east.
Because of its proximity to Earth and the way its clouds reflect sunlight, Venus appears to be the brightest planet in the sky. Although we cannot normally see through Venus' thick atmosphere, NASA's Magellan mission to Venus during the early 1990s used radar to image 98 percent of the surface, and the Galileo spacecraft used infrared mapping to view mid-level cloud structure as it passed by Venus in 1990 on its way to Jupiter.
Like Mercury, Venus can be seen periodically passing across the face of the sun. These transits of Venus occur in pairs with more than a century separating each pair. Since the telescope was invented, transits were observed in 1631, 1639; 1761, 1769; and 1874, 1882. On 8 June 2004, astronomers worldwide saw the tiny dot of Venus crawl across the sun; the second in this pair of early 21st-century transits occurred 6 June 2012.
Venus' atmosphere consists mainly of carbon dioxide, with clouds of sulfuric acid droplets. Only trace amounts of water have been detected in the atmosphere. The thick atmosphere traps the sun's heat, resulting in surface temperatures higher than 880 degrees Fahrenheit (471 degrees Celsius). Probes that have landed on Venus survived only a few hours before being destroyed by the incredible temperatures. Sulfur compounds are abundant in Venus' clouds. The corrosive chemistry and dense, moving atmosphere cause significant surface weathering and erosion.
As Venus moves forward in its solar orbit while slowly rotating backwards on its axis, the top level of cloud layers zips around the planet every four Earth days, driven by hurricane-force winds traveling at about 360 km (224 miles) per hour. The wind speeds within the clouds decrease with cloud height, and winds at the surface are estimated to be just a few kilometers per hour. How this atmospheric super-rotation forms and is maintained continues to be a topic of scientific investigation.
Atmospheric lightning bursts, long suspected by scientists, were finally confirmed in 2007 by the European Venus Express orbiter. On Earth, Jupiter and Saturn, lightning is associated with water clouds, but on Venus, it is associated with clouds of sulfuric acid.
Radar images of the surface show wind streaks and sand dunes. Craters smaller than 1.5 to 2 km (0.9 to 1.2 miles) across do not exist on Venus because small meteors burn up in the dense atmosphere before they reach the surface.
It is thought that Venus was completely resurfaced by volcanic activity 300 to 500 million years ago. More than 1,000 volcanoes or volcanic centers larger than 20 km (12 miles) in diameter dot the surface. Volcanic flows have produced long, sinuous channels extending for hundreds of kilometers. Venus has two large highland areas: Ishtar Terra, about the size of Australia, in the north polar region; and Aphrodite Terra, about the size of South America, straddling the equator and extending for almost 10,000 km (6,000 miles). Maxwell Montes, the highest mountain on Venus and comparable to Mount Everest on Earth, is at the eastern edge of Ishtar Terra.
How Venus Got its Name
Venus is named for the ancient Roman goddess of love and beauty. (Venus is the Roman counterpart to the Greek goddess Aphrodite.) It is believed Venus was named for the most beautiful of the ancient gods because it shone the brightest of the five planets known to ancient astronomers. Other civilizations have named it for their god or goddess of love/war as well.
Significant Dates:
650 AD: Mayan astronomers make detailed observations of Venus, leading to a highly accurate calendar.
1761-1769: Two European expeditions to watch Venus cross in front of the sun lead to the first good estimate of the sun's distance from Earth.
1962: NASA's Mariner 2 reaches Venus and reveals the planet's extreme surface temperatures. It is the first spacecraft to send back information from another planet.
1970: The Soviet Union's Venera 7 sends back 23 minutes of data from the surface of Venus. It is the first spacecraft to successfully land on another planet.
1990-1994: NASA's Magellan spacecraft, in orbit around Venus, uses radar to map 98 percent of the planet's surface.
2005: The European Space Agency launches Venus Express to study the atmosphere and plasma environment of Venus from orbit.
MORE DETAILS ON VENUS
EARTH
Period of revolutionaround the sun:365.26 Earth days.
Period of Rotation:24 Hours
Diametre:12756 km
Moons:Luna
Earth, our home planet, is the only planet in our solar system known to harbor life: life that is incredibly diverse. All the things we need to survive exist under a thin layer of atmosphere that separates us from the cold, airless void of space.
Earth is made up of complex, interactive systems that create a constantly changing world that we are striving to understand. From the vantage point of space we are able to observe our planet globally, using sensitive instruments to understand the delicate balance among its oceans, air, land and life. Satellite observations help study and predict weather, drought, pollution, climate change and many other phenomena that affect the environment, economy and society.
Earth is the third planet from the sun and the fifth largest in our solar system. Earth's diameter is just a few hundred kilometers larger than that of Venus.
The four seasons are a result of Earth's axis of rotation being tilted 23.45 degrees with respect to the plane of Earth's orbit around the sun. During part of the year, the northern hemisphere is tilted toward the sun and the southern hemisphere is tilted away, producing summer in the north and winter in the south. Six months later, the situation is reversed. During March and September, when spring and fall begin in the northern hemisphere, both hemispheres receive roughly equal amounts of solar illumination.
Earth's global ocean, which covers nearly 70 percent of the planet's surface, has an average depth of about 4 km (2.5 miles). Fresh water exists in the liquid phase only within a narrow temperature span: 32 to 212 degrees Fahrenheit (0 to 100 degrees Celsius). This span is especially narrow when contrasted with the full range of temperatures found within the solar system. The presence and distribution of water vapor in the atmosphere is responsible for much of Earth's weather.
We are enveloped by an atmosphere that consists of 78 percent nitrogen, 21 percent oxygen and 1 percent other ingredients. The atmosphere affects Earth's long-term climate and short-term local weather, shields us from much of the harmful radiation coming from the sun and protects us from meteors as well: most of which burn up before they can strike the surface as meteorites. Earth-orbiting satellites have revealed that the upper atmosphere actually swells by day and contracts by night due to solar heating during the day and cooling at night.
Our planet's rapid rotation and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape in space. (The solar wind is a stream of charged particles continuously ejected from the sun.) The Earth's magnetic field does not fade off into space, but has definite boundaries. When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above our planet's magnetic poles. These air molecules then begin to glow, and are known as the aurorae -- the northern and southern lights.
Earth's lithosphere, which includes the crust (both continental and oceanic) and the upper mantle, is divided into huge plates that are constantly moving. For example, the North American plate moves west over the Pacific Ocean basin, roughly at a rate equal to the growth of our fingernails. Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate. The theory of motion of the large plates of the lithosphere is known as plate tectonics. Developed within the last 40 years, this explanation has unified the results of centuries of study of our planet.
How Earth Got its Name
The name Earth is at least 1,000 years old. All of the planets, except for Earth, were named after Greek and Roman gods and goddesses. However, the name Earth is an English/German word, which simply means the ground: eor(th)e and ertha (Old English) and erde (German).
Significant Dates
1960: NASA launches the Television Infrared Observation Satellite (TIROS), the first weather satellite.
1972: The Earth Resources Technology Satellite 1 (renamed Landsat 1) is launched, the first in a series of Earth-imaging satellites that continues today.
1987: NASA's Airborne Antarctic Ozone Experiment helps determine the cause of the Antarctic ozone hole.
1992: TOPEX/Poseidon, a U.S.-France mission, begins measuring sea-surface height. Jason 1 continues these measurements in 2001.
1997: TOPEX/Poseidon captures the evolution of El Nino (cold ocean water in the equatorial Pacific Ocean) and La Nina (warm ocean water in the equatorial Pacific Ocean).
1997: The U.S.-Japan Tropical Rainfall Measuring Mission is launched to provide 3-D maps of storm structure.
1999: Quick Scatterometer (QuikScat) launches in June to measure ocean surface wind velocity; in December the Active Cavity Irradiance Monitor Satellite launches to monitor the total amount of the sun's energy reaching Earth.
1999-2006: A series of satellites are launched to provide global observations of the Earth system -- simultaneously studying land, oceans, atmosphere, water cycles, gravity, clouds and aerosols.
2006: The Antarctic ozone hole was the largest yet observed.
2007: Arctic sea ice reaches the all-time minimum since satellite records began.
2008: The third U.S.-France mission to measure sea-level height, Ocean Surface Topography Mission/Jason 2, is launched, doubling global data coverage.
2009: NASA and Japan release the most accurate topographic map of Earth.
And many more by indians,europeans and japanese.
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MARS
Period of revolution around the sun:686.98 Earth days.
Period of Rotation:1.026 Earth days
Diametre:6787 km
Moons:Diemos,Phobos
Though details of Mars' surface are difficult to see from Earth, telescope observations show seasonally changing features and white patches at the poles. For decades, people speculated that bright and dark areas on Mars were patches of vegetation, that Mars could be a likely place for life-forms and that water might exist in the polar caps. When the Mariner 4 spacecraft flew by Mars in 1965, many were shocked to see photographs of a bleak, cratered surface. Mars seemed to be a dead planet. Later missions, however, have shown that Mars is a complex member of the solar system and holds many mysteries yet to be solved.
Mars is a rocky body about half the size of Earth. As with the other terrestrial planets -- Mercury, Venus and Earth -- the surface of Mars has been altered by volcanism, impacts, crustal movement, and atmospheric effects such as dust storms.
Mars often appears reddish due to a combination of the fact that its surface is comprised of iron-rich minerals that essentially rust (or oxidize) and that the dust made of these minerals is kicked up into the atmosphere, giving the atmosphere a reddish hue as well.
Mars has no global magnetic field, but NASA's Mars Global Surveyor orbiter found that areas of the Martian crust in the southern hemisphere are highly magnetized. Evidently, these are traces of a magnetic field that remain in the planet's crust from about 4 billion years ago.
Mars has two small moons, Phobos and Deimos, that may be captured asteroids. Potato-shaped, they have too little mass for gravity to make them spherical. Phobos, the innermost moon, is heavily cratered, with deep grooves on its surface.
Like Earth, Mars experiences seasons because of the tilt of its rotational axis (in relation to the plane of its orbit). Mars' orbit is slightly elliptical, so its distance to the sun changes, affecting the Martian seasons. Mars' seasons last longer than those of Earth. The polar ice caps on Mars grow and recede with the seasons; layered areas near the poles suggest that the planet's climate has changed more than once. Volcanism in the highlands and plains was active more than 3 billion years ago, but some of the giant shield volcanoes are younger, having formed between 1 and 2 billion years ago. Mars has the largest volcanic mountain in the solar system, Olympus Mons, as well as a spectacular equatorial canyon system, Valles Marineris.
Scientists believe that Mars experienced huge floods about 3.5 billion years ago. Though we do not know where the ancient flood water came from, how long it lasted or where it went, recent missions to Mars have uncovered intriguing hints. In 2002, NASA's Mars Odyssey orbiter detected hydrogen-rich polar deposits, indicating large quantities of water ice close to the surface. Further observations found hydrogen in other areas as well. If water ice permeated the entire planet, Mars could have substantial subsurface layers of frozen water. In 2004, the Mars Exploration Rover named Opportunity found structures and minerals indicating that liquid water was once present at its landing site. The rover's twin, Spirit, also found the signature of ancient water near its landing site halfway around Mars from Opportunity's location.
The cold temperatures and thin atmosphere on Mars don't allow liquid water to exist at the surface for long, and the quantity of water required to carve Mars' great channels and flood plains is not evident today. Unraveling the story of water on Mars is important to unlocking its climate history, which will help us understand the evolution of all the planets. Water is believed to be an essential ingredient for life; evidence of past or present water on Mars is expected to hold clues about whether Mars could ever have been a habitat for life.
In 2008, NASA's Phoenix Mars Lander found water ice in the Martian arctic, which was expected. Phoenix also observed precipitation -- snow falling from clouds. This and soil chemistry experiments have led scientists to believe that the Phoenix landing site had a wetter and warmer climate in the recent past (the last few million years). It is unsettled whether Phoenix's soil samples contained any carbon-based organic compounds. More extensive surveys must wait until NASA's Mars Science Laboratory mission, with its large rover (named Curiosity), examines Martian rocks and soils to determine the geologic processes that formed them and learn more about the present and past habitability of the planet.
How Mars Got its Name
Mars was named by the Romans for their god of war because of its red, bloodlike color. Other civilizations also named this planet from this attribute; for example, the Egyptians named it "Her Desher," meaning "the red one."
Significant Dates
1877: Asaph Hall discovers the two moons of Mars, Phobos and Deimos.
1965: NASA's Mariner 4 sends back 22 photos of Mars, the world's first close-up photos of a planet beyond Earth.
1976: Viking 1 and 2 land on the surface of Mars.
1997: Mars Pathfinder lands and dispatches Sojourner, the first wheeled rover to explore the surface of another planet.
2002: Mars Odyssey begins its mission to make global observations and find buried water ice on Mars.
2004: Twin Mars Exploration Rovers named Spirit and Opportunity land on Mars and find the strongest evidence yet obtained that the Red Planet once had underground liquid water and water flowing on the surface.
2006: Mars Reconnaissance Orbiter begins returning high-resolution images as it studies the history of water on Mars.
2008: Phoenix lands on Mars to study the history of water and search for complex organic molecules; confirms the presence of water ice near the north pole.
2012 Mars Science Laboratory's Curiosity rover lands on Mars to study whether the Red Planet ever was -- or is still today -- an environment suitable for life.
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JUPITER
Period of revolution around the sun:11.862 Earth days.
Period of Rotation:9.84 Earth Hours
Diametre:142796 km
Moons:Ganymede,Callisto.Lo,Europa,Himalia,Amathea,Thebe,Elara,Metis,Pasiphae,Carne,Sinope,Lysithea,Ananke,Lede,Thermisto,Callirrhoe,Praxidike,Megaclite,locaste,Taygete,kalyke,autonoe,
harpalyke,thyone,hermippe,chaldene,aoeda,eukallde,isonoe,helika,carpo,S/2003 J5,S/2000 J11,Aitne,Eurydome,Hegomone,Arche,Euanthe,Sponde,S/2003 J2,S/2003 J9,EEuporie,Pasthee,Kore,Cyllene,Mneme,Kale,Kallinchore,S/2003 J16,S/2003 J19,S/2003 J15,S/2003 J10,S/2003 J23,S/2011 J2,S/2010 J1,S/2003 J4,S/2011 J1,S/2010 J2
upiter is the most massive planet in our solar system; with four large moons and many smaller moons it forms a kind of miniature solar system. In fact, Jupiter resembles a star in composition, and if it had been about 80 times more massive, it would have become a star rather than a planet.
On 7 January 1610, using his primitive telescope, astronomer Galileo Galilei saw four small "stars" near Jupiter. He had discovered Jupiter's four largest moons, now called Io, Europa, Ganymede, and Callisto. These four moons are known today as the Galilean satellites.
Newly discovered moons of Jupiter are reported by astronomers and acknowledged with a temporary designation by the International Astronomical Union; once their orbits are confirmed, they are included in Jupiter's large moon count. Not including the "temporary" moons, Jupiter has 50 total.
Galileo would be astonished at what we have learned about Jupiter and its moons, largely from the NASA mission named after him. Io is the most volcanically active body in our solar system. Ganymede is the largest planetary moon and the only moon in the solar system known to have its own magnetic field. A liquid ocean may lie beneath the frozen crust of Europa, and icy oceans may also lie beneath the crusts of Callisto and Ganymede. Jupiter's appearance is a tapestry of beautiful colors and atmospheric features. Most visible clouds are composed of ammonia. Water vapor exists deep below and can sometimes be seen through clear spots in the clouds. The planet's "stripes" are dark belts and light zones created by strong east-west winds in Jupiter's upper atmosphere. Dynamic storm systems rage on Jupiter. The Great Red Spot, a giant spinning storm, has been observed since the 1800s. In recent years, three storms merged to form the Little Red Spot, about half the size of the Great Red Spot.
The composition of Jupiter's atmosphere is similar to that of the sun -- mostly hydrogen and helium. Deep in the atmosphere, the pressure and temperature increase, compressing the hydrogen gas into a liquid. At depths of about a third of the way down, the hydrogen becomes metallic and electrically conducting. In this metallic layer, Jupiter's powerful magnetic field is generated by electrical currents driven by Jupiter's fast rotation. At the center, the immense pressure may support a solid core of rock about the size of Earth.
Jupiter's enormous magnetic field is nearly 20,000 times as powerful as Earth's. Trapped within Jupiter's magnetosphere (the area in which magnetic field lines encircle the planet from pole to pole) are swarms of charged particles. Jupiter's rings and moons are embedded in an intense radiation belt of electrons and ions trapped by the magnetic field. The Jovian magnetosphere, comprising these particles and fields, balloons 1 to 3 million km (600,000 to 2 million miles) toward the sun and tapers into a windsock-shaped tail extending more than 1 billion km (600 million miles) behind Jupiter as far as Saturn's orbit.
Discovered in 1979 by NASA's Voyager 1 spacecraft, Jupiter's rings were a surprise: a flattened main ring and an inner cloud-like ring, called the halo, are both composed of small, dark particles. A third ring, known as the gossamer ring because of its transparency, is actually three rings of microscopic debris from three small moons: Amalthea, Thebe and Adrastea. Data from the Galileo spacecraft indicate that Jupiter's ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet's four small inner moons. The main ring probably is composed of material from the moon Metis. Jupiter's rings are more easily visible when backlit by the sun but have been captured by Hubble Space Telescope images.
In December 1995, NASA's Galileo spacecraft dropped a probe into Jupiter's atmosphere, which made the first direct measurements of the planet's atmosphere. The spacecraft then began a multiyear study of Jupiter and the largest moons. As Galileo began its 29th orbit, the Cassini-Huygens spacecraft was nearing Jupiter for a gravity-assist maneuver on the way to Saturn. The two spacecraft made simultaneous observations of the magnetosphere, solar wind, rings, and Jupiter's auroras.
NASA launched a mission named Juno in 2011 to conduct an in-depth study of Jupiter from a polar orbit. Juno will examine Jupiter's chemistry, atmosphere, interior structure, and magnetosphere.
How Jupiter Got its Name
The largest and most massive of the planets was named Zeus by the Greeks and Jupiter by the Romans; he was the most important deity in both pantheons.
Significant Dates
1610: Galileo Galilei makes the first detailed observations of Jupiter.
1973: Pioneer 10 becomes the first spacecraft to cross the asteroid belt and fly past Jupiter.
1979: Voyager 1 and 2 discover Jupiter's faint rings, several new moons and volcanic activity on Io's surface.
1994: Astronomers observe as pieces of comet Shoemaker-Levy 9 collide with Jupiter's southern hemisphere.
1995-2003: The Galileo spacecraft drops a probe into Jupiter's atmosphere and conducts extended observations of Jupiter and its moons and rings.
2007: Images by NASA's New Horizons spacecraft, on the way to Pluto, show new perspectives on Jupiter's atmospheric storms, the rings, volcanic Io, and icy Europa.
2009: On July 20, almost exactly 15 years after fragments of comet Shoemaker-Levy slammed into Jupiter, a comet or asteroid crashes into the giant planet's southern hemisphere.
2011: Juno launches to examine Jupiter's chemistry, atmosphere, interior structure, and magnetosphere.
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SATURN
Period of revolution around the sun:29.456 Earth years.
Period of Rotation:10.2 Earth Hours
Diametre:120660 km
Moons:moons
Saturn was the most distant of the five planets known to the ancients. In 1610, Italian astronomer Galileo Galilei was the first to gaze at Saturn through a telescope. To his surprise, he saw a pair of objects on either side of the planet. He sketched them as separate spheres, thinking that Saturn was triple-bodied. Continuing his observations over the next few years, Galileo drew the lateral bodies as arms or handles attached to Saturn. In 1659, Dutch astronomer Christiaan Huygens, using a more powerful telescope than Galileo's, proposed that Saturn was surrounded by a thin, flat ring. In 1675, Italian-born astronomer Jean-Dominique Cassini discovered a division between what are now called the A and B rings. It is now known that the gravitational influence of Saturn's moon Mimas is responsible for the Cassini Division, which is 4,800 km (3,000 miles) wide.
Like Jupiter, Saturn is made mostly of hydrogen and helium. Its volume is 755 times greater than that of Earth. Winds in the upper atmosphere reach 500 m (1,600 feet) per second in the equatorial region. (In contrast, the strongest hurricane-force winds on Earth top out at about 110 m, or 360 feet per second.) These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in the atmosphere.
In the early 1980s, NASA's Voyager 1 and Voyager 2 spacecraft revealed that Saturn's rings are made mostly of water ice, and they imaged "braided" rings, ringlets and "spokes" -- dark features in the rings that circle the planet at different rates from that of the surrounding ring material. Saturn's ring system extends hundreds of thousands of kilometers from the planet, yet the vertical depth is typically about 10 m (30 feet) in the main rings. During Saturn's equinox in autumn 2009, when sunlight illuminated the rings edge-on, Cassini spacecraft images showed vertical formations in some of the rings; the particles seem to pile up in bumps or ridges more than 3 km (2 miles) tall.
Saturn's largest moon, Titan, is a bit bigger than the planet Mercury. (Titan is the second-largest moon in the solar system; only Jupiter's moon Ganymede is bigger.) Titan is shrouded in a thick, nitrogen-rich atmosphere that might be similar to what Earth's was like long ago. Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth. Saturn also has many smaller icy satellites. From Enceladus, which shows evidence of recent (and ongoing) surface changes, to Iapetus, with one hemisphere darker than asphalt and the other as bright as snow, each of Saturn's satellites is unique.
Though Saturn's magnetic field is not as huge as Jupiter's, it is still 578 times as powerful as the Earth's. Saturn, its rings and many of its satellites lie totally within Saturn's own enormous magnetosphere -- the region of space in which the behavior of electrically charged particles is influenced more by Saturn's magnetic field than by the solar wind. While the Hubble Space Telescope imaged Saturn's aurora in the ultraviolet, the Cassini spacecraft found that Saturn has a unique secondary aurora at the north pole, imaged in the infrared in 2008. Aurorae occur when charged particles spiral into a planet's atmosphere along magnetic field lines. On Earth, these charged particles come from the solar wind. Cassini showed that at least some of Saturn's aurorae are like Jupiter's and are largely unaffected by the solar wind.
The next chapter in our knowledge of Saturn is being written right now by the Cassini mission, which carried Europe's Huygens probe to Saturn. The Huygens probe descended through Titan's atmosphere in January 2005, collecting data on the atmosphere and surface. The Cassini spacecraft, orbiting Saturn since 2004, continues to explore the planet and its moons, rings and magnetosphere. By July 2009, Cassini had returned more than 200,000 images. The Cassini Equinox Mission is studying the rings during Saturn's autumnal equinox, when the sun shines directly on Saturn's equator. Three-dimensional features are visible in the rings during this time of year, such as moonlets that protrude above and below the ring plane and cast shadows, revealing their sizes and shapes.
How Saturn Got its Name
Saturn is named for the Roman god of agriculture. The Greek equivalent was Cronos, father of Zeus/Jupiter. Other civilizations have given different names to Saturn, which is the farthest planet from Earth that can be observed by the unaided human eye.
Significant Dates
1610: Galileo Galilei reports seeing odd appendages on either side of Saturn; he did not realize he was viewing Saturn's rings.
1979: Pioneer 11 is the first spacecraft to reach Saturn, flying within 22,000 km (13,700 miles) of the cloud tops.
1981: Using Saturn's powerful gravity as an interplanetary slingshot, Voyager 2 is placed on a path toward Uranus, then Neptune, then out of the solar system.
2004: After a seven-year journey, Cassini becomes the first spacecraft to orbit Saturn.
2005: The Huygens probe successfully lands on Titan, returning images of the complex surface.
2008: Cassini's four-year mission is extended for two years and designated the Cassini Equinox Mission.
2010: Cassini is extended until September 2017 and designated the Cassini Solstice Mission.
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URANUS
Period of revolution around the sun:84.07 Earth Years.
Period of Rotation:17.9 Earth hours
Diametre:51118 km
Moons:moons
Uranus, discovered in 1781 by astronomer William Herschel, was the first planet found with the aid of a telescope. As the seventh planet from the sun, Uranus takes 84 Earth years to complete one orbit.
Like Venus, Uranus rotates east to west. Uranus' rotation axis is tilted almost parallel to its orbital plane, so Uranus appears to be rotating on its side. This situation may be the result of a collision with a planet-sized body early in the planet's history, which apparently radically changed Uranus' rotation. Because of Uranus' unusual orientation, the planet experiences extreme variations in sunlight during each 20-year-long season.
Voyager 2, the only spacecraft to visit Uranus, imaged a bland-looking sphere in 1986. When Voyager flew by, the south pole of Uranus pointed almost directly at the sun because Uranus was near its southern summer solstice, with the southern hemisphere bathed in continuous sunlight and the northern hemisphere radiating heat into the blackness of space.
Uranus reached equinox in December 2007, when it was fully illuminated as the sun passed over the planet's equator. By 2028, the north pole will point directly at the sun, a reversal of the situation when Voyager flew by. Equinox also brings ring-plane crossing, when Uranus' rings appear to move more and more edge-on as seen from Earth.
The Hubble Space Telescope and the Keck Observatory in Hawaii captured detailed images of Uranus as the planet approached equinox. While Voyager 2 saw only a few discrete clouds, more recent observations reveal that Uranus exhibits dynamic clouds as it approaches equinox, including rapidly evolving bright features and a new Great Dark Spot like those seen on Neptune.
Uranus is one of the two ice giants of the outer solar system (the other is Neptune). Uranus' atmosphere is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas in the atmosphere. Sunlight passes through the atmosphere and is reflected back out by Uranus' cloud tops. Methane gas absorbs the red portion of the light, resulting in a blue-green color. The bulk (80 percent or more) of the mass of Uranus is contained in an extended liquid core consisting mostly of icy materials (water, methane and ammonia).
For nearly a quarter of the Uranian year, the sun shines directly over each pole, plunging the other half of the planet into a long, dark winter.
While magnetic fields are typically in alignment with a planet's rotation, Uranus' magnetic field is tipped over: the magnetic axis is tilted nearly 60 degrees from the planet's axis of rotation, and is also offset from the center of the planet by one-third of the planet's radius. The magnetic fields of both Uranus and Neptune are very irregular.
Uranus has two sets of rings. The inner system of nine rings, discovered in 1977, consists mostly of narrow, dark rings. Voyager found two additional inner rings. An outer system of two more-distant rings was discovered in Hubble Space Telescope images in 2003. In 2006, Hubble observations and observations made at the Keck Observatory showed that the outer rings are brightly colored.
Uranus has 27 known moons, named for characters from the works of William Shakespeare or Alexander Pope. Miranda is the strangest-looking Uranian moon: its complex surface may indicate partial melting of the interior, with icy material drifting to the surface.
How Uranus Got its Name
William Herschel tried unsuccessfully to name his discovery Georgian Sidus after King George III; but instead the planet was named for Uranus, the Greek god of the sky.
Significant Dates
1781: Astronomer William Herschel discovers Uranus.
1787-1851: Four Uranian moons are discovered and named Titania, Oberon, Ariel, and Umbriel.
1977: Scientists discover nine faint rings of Uranus while observing a distant star pass behind the planet.
1986: Voyager 2 discovers 10 moons and two additional rings during its historic flyby.
2003-2005: The Hubble Space Telescope images two delicate rings far from the planet, and two new moons.
2007: Uranus reaches equinox.
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NEPTUNE
Period of revolution around the sun:164.81 Earth Years.
Period of Rotation:19.1 Earth hours
Diametre:48600 km
Moons:moons
The ice giant Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. (Galileo had recorded it as a fixed star during observations with his small telescope in 1612 and 1613.) When Uranus didn't travel exactly as astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus' orbit.
After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory. Galle found Neptune on his first night of searching in 1846. Seventeen days later, its largest moon, Triton, was also discovered.
Nearly 4.5 billion km (2.8 billion miles) from the sun, Neptune orbits the sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth. Interestingly, the unusual elliptical orbit of the dwarf planet Pluto brings Pluto inside Neptune's orbit for a 20-year period out of every 248 Earth years. Pluto can never crash into Neptune, though, because for every three laps Neptune takes around the sun, Pluto makes two. This repeating pattern prevents close approaches of the two bodies.
The main axis of Neptune's magnetic field is tipped over by about 47 degrees compared with the planet's rotation axis. Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation, Neptune's magnetosphere undergoes wild variations during each rotation because of this misalignment. The magnetic field of Neptune is about 27 times more powerful than that of Earth.
Neptune's atmosphere extends to great depths, gradually merging into water and other melted ices over a heavier, approximately Earth-size solid core. Neptune's blue color is the result of methane in the atmosphere. Uranus' blue-green color is also the result of atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color.
Despite its great distance and low energy input from the sun, Neptune's winds can be three times stronger than Jupiter's and nine times stronger than Earth's. In 1989, Voyager 2 tracked a large, oval-shaped, dark storm in Neptune's southern hemisphere. This "Great Dark Spot," which was large enough to contain the entire Earth, spun counterclockwise, and moved westward at almost 1,200 km (750 miles) per hour. Subsequent images taken by the Hubble Space Telescope showed no sign of this Great Dark Spot, but did reveal the appearance and then fading of two other Great Dark Spots over the last decade. Voyager 2 also imaged clouds casting shadows on a lower cloud deck, enabling scientists to visually measure the altitude differences between the upper and lower cloud decks.
Neptune has six known rings. Voyager 2's observations confirmed that these unusual rings are not uniform, but have four thick regions (clumps of dust) called arcs. The rings are thought to be relatively young and short-lived.
Neptune has 13 known moons, six of which were discovered by Voyager 2. Triton, Neptune's largest moon, orbits the planet in the opposite direction compared with the rest of the moons, suggesting that it may have been captured by Neptune in the distant past. Triton is extremely cold -- temperatures on its surface are about -391degrees Fahrenheit (-235 degrees Celsius). Despite this deep freeze at Triton, Voyager 2 discovered geysers spewing icy material upward more than 8 km (5 miles). Triton's thin atmosphere, also discovered by Voyager, has been detected from Earth several times since, and is growing warmer -- although scientists do not yet know why.
How Neptune Got its Name
Neptune was predicted by John Couch Adams and Urbain Le Verrier. The men independently accounted for the irregularities in the motion of Uranus by correctly predicting the orbital elements of a trans-Uranian planet. Using the predicted parameters of Le Verrier (Adams never published his predictions), Johann Galle discovered the planet in 1846. Galle wanted to name the planet for Le Verrier, but that was not acceptable to the international astronomical community. Instead, this planet is named for the Roman god of the sea.
Significant Dates
1846: Using mathematical calculations, astronomers discover Neptune, increasing the number of known planets to eight. Neptune's largest moon, Triton, is found the same year.
1984: Astronomers find evidence for the existence of a ring system around Neptune.
1989: Voyager 2 becomes the first and only spacecraft to visit Neptune, passing about 4,800 km (2,983 miles) above the planet's north pole.
1998: Scientists using telescopes on Earth and in space image Neptune's rings and ring arcs for the first time.
2003: Using improved observing techniques, astronomers discover five new moons orbiting Neptune.
2005: Scientists using the Keck Observatory image the outer rings and find that some of the ring arcs have deteriorated.
2011: Neptune completes its first 165-year orbit of the sun since its discovery in 1846.
MORE ON NEPTUNE
MOHAMMAD ANWAR M
EMAIL ME at mhdanwar487@gmail.com
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- PLUTO
- ORCUS
- HAUMEA
- QUAOAR
- MAKEMAKE
- 2007 OR10
- ERIS
- SEDNA
MERCURY
Period of revolution around the sun:87.96 Earth days
Period of Rotation:58.7 Earth days
Diametre:4878 km
Moons:No Moons
Mercury's elliptical orbit takes the small planet as close as 47 million km (29 million miles) and as far as 70 million km (43 million miles) from the sun. If one could stand on the scorching surface of Mercury when it is at its closest point to the sun, our star would appear more than three times as large as it does when viewed from Earth. Because Mercury is so close to the sun, it is hard to directly observe from Earth except during twilight. mercury makes an appearance indirectly, however -- 13 times each century, Earth observers can watch Mercury pass across the face of the sun, an event called a transit. The transits fall within several days of May 8 and November 10. The first two transits of Mercury in the 21st century occurred 7 May 2003 and 8 November 2006. The next will occur on 9 May 2016. Temperatures on Mercury's surface can reach 800 degrees Fahrenheit (427 degrees Celsius). Because Mercury's atmosphere is so thin, the surface cannot retain that heat so nighttime temperatures can drop to -290 degrees Fahrenheit (-179 degrees Celsius). Mercury's thin atmosphere, or exosphere, is made up of atoms blasted off the surface by the solar wind and micrometeoroid impacts. Because of solar radiation pressure, the atoms quickly escape into space and form a tail of neutral particles. Though Mercury's magnetic field has just 1 percent the strength of Earth's, the field is very active. The magnetic field in the solar wind episodically connects to Mercury's field, creating intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface. When these ions strike the surface, they knock off neutral atoms and send them on a loop high into the sky where other processes may fling them back to the surface or accelerate them away from Mercury. Mercury's surface resembles that of Earth's Moon, scarred by many impact craters resulting from collisions with meteoroids and comets. While there are areas of smooth terrain, there are also lobe-shaped scarps or cliffs, some hundreds of miles long and soaring up to a mile high, formed by contraction of the crust. The Caloris Basin, one of the largest features on Mercury, is about 1,550 km (960 miles) in diameter. It was the result of an asteroid impact on the planet's surface early in the solar system's history. Over the next several billion years, Mercury shrank in radius about 1 to 2 km (0.6 to 1.2 miles) as the planet cooled after its formation. The outer crust contracted and grew strong enough to prevent magma from reaching the surface, ending the period of volcanic activity. Mercury is the second densest planet after Earth, with a large metallic core having a radius of 1,800 to 1,900 km (1,100 to 1,200 miles), about 75 percent of the planet's radius. In 2007, researchers using ground-based radars to study the core found evidence that it is molten (liquid). Mercury's outer shell, comparable to Earth's outer shell (called the mantle), is only 500 to 600 km (300 to 400 miles) thick. The first spacecraft to visit Mercury was Mariner 10, which imaged about 45 percent of the surface. In 1991, astronomers on Earth using radar observations showed that Mercury may have water ice at its north and south poles inside deep craters that are perpetually cold. Infalling comets or meteorites might have brought ice to these regions of Mercury, or water vapor might have outgassed from the interior and frozen out at the poles. In 2008 and 2009, NASA's MESSENGER mission performed two close flybys of Mercury. By the second flyby, the spacecraft had imaged about 80 percent of the surface at useful resolution and made discoveries about the magnetic field and how Mercury's crust was formed. The flybys employed Mercury's gravity to help ease the spacecraft into orbit in March 2011. The spacecraft is studying and imaging Mercury from orbit and will map nearly the entire planet in color. MESSENGER is the first spacecraft to orbit Mercury. How Mercury Got its Name Mercury is appropriately named for the swiftest of the ancient Roman gods. Mercury, the god of commerce, is the Roman counterpart to the ancient Greek god Hermes, the messenger of the gods.
Significant Dates
1631: Pierre Gassendi uses a telescope to watch from Earth as Mercury crosses the face of the sun.
1965: Though it was thought for centuries that the same side of Mercury always faces the sun, astronomers find the planet rotates three times for every two orbits
1974-1975: Mariner 10 photographs roughly half of Mercury's surface in three flybys.
1991: Scientists using Earth-based radar find signs of ice locked in permanently shadowed areas of craters in Mercury's polar regions.
2008: MESSENGER's first flyby of Mercury initiates the most comprehensive study yet of the innermost planet. The three flybys revealed the side of the planet not seen by Mariner 10. Also, many more images and discoveries were obtained by these flybys.
MORE DETAILS ON MERCURY
VENUS
Period of revolutionaround the sun:224.68 Earth days.
Period of Rotation:243 Earth days
Diametre:12104 km
Moons:No Moons
Venus and Earth are similar in size, mass, density, composition and gravity. There, however, the similarities end. Venus is covered by a thick, rapidly spinning atmosphere, creating a scorched world with temperatures hot enough to melt lead and a surface pressure 90 times that of Earth.
The Venusian year (orbital period) is about 225 Earth days long, while the planet's sidereal rotation period is 243 Earth days, making a Venus solar day (measured noon to noon) about 117 Earth days long. Resulting from this slow rotation Venus cannot generate a magnetic field similar to Earth's -- though its core iron content is similar to that of the Earth. (Venus' iron core is approximately 3,000 km [1,900 miles] in radius.) Venus rotates retrograde (east to west) compared with Earth's prograde (west to east) rotation. Seen from Venus, the sun would rise in the west and set in the east. Because of its proximity to Earth and the way its clouds reflect sunlight, Venus appears to be the brightest planet in the sky. Although we cannot normally see through Venus' thick atmosphere, NASA's Magellan mission to Venus during the early 1990s used radar to image 98 percent of the surface, and the Galileo spacecraft used infrared mapping to view mid-level cloud structure as it passed by Venus in 1990 on its way to Jupiter. Like Mercury, Venus can be seen periodically passing across the face of the sun. These transits of Venus occur in pairs with more than a century separating each pair. Since the telescope was invented, transits were observed in 1631, 1639; 1761, 1769; and 1874, 1882. On 8 June 2004, astronomers worldwide saw the tiny dot of Venus crawl across the sun; the second in this pair of early 21st-century transits occurred 6 June 2012. Venus' atmosphere consists mainly of carbon dioxide, with clouds of sulfuric acid droplets. Only trace amounts of water have been detected in the atmosphere. The thick atmosphere traps the sun's heat, resulting in surface temperatures higher than 880 degrees Fahrenheit (471 degrees Celsius). Probes that have landed on Venus survived only a few hours before being destroyed by the incredible temperatures. Sulfur compounds are abundant in Venus' clouds. The corrosive chemistry and dense, moving atmosphere cause significant surface weathering and erosion. As Venus moves forward in its solar orbit while slowly rotating backwards on its axis, the top level of cloud layers zips around the planet every four Earth days, driven by hurricane-force winds traveling at about 360 km (224 miles) per hour. The wind speeds within the clouds decrease with cloud height, and winds at the surface are estimated to be just a few kilometers per hour. How this atmospheric super-rotation forms and is maintained continues to be a topic of scientific investigation. Atmospheric lightning bursts, long suspected by scientists, were finally confirmed in 2007 by the European Venus Express orbiter. On Earth, Jupiter and Saturn, lightning is associated with water clouds, but on Venus, it is associated with clouds of sulfuric acid. Radar images of the surface show wind streaks and sand dunes. Craters smaller than 1.5 to 2 km (0.9 to 1.2 miles) across do not exist on Venus because small meteors burn up in the dense atmosphere before they reach the surface. It is thought that Venus was completely resurfaced by volcanic activity 300 to 500 million years ago. More than 1,000 volcanoes or volcanic centers larger than 20 km (12 miles) in diameter dot the surface. Volcanic flows have produced long, sinuous channels extending for hundreds of kilometers. Venus has two large highland areas: Ishtar Terra, about the size of Australia, in the north polar region; and Aphrodite Terra, about the size of South America, straddling the equator and extending for almost 10,000 km (6,000 miles). Maxwell Montes, the highest mountain on Venus and comparable to Mount Everest on Earth, is at the eastern edge of Ishtar Terra.
How Venus Got its Name
Venus is named for the ancient Roman goddess of love and beauty. (Venus is the Roman counterpart to the Greek goddess Aphrodite.) It is believed Venus was named for the most beautiful of the ancient gods because it shone the brightest of the five planets known to ancient astronomers. Other civilizations have named it for their god or goddess of love/war as well.
Significant Dates:
650 AD: Mayan astronomers make detailed observations of Venus, leading to a highly accurate calendar.
1761-1769: Two European expeditions to watch Venus cross in front of the sun lead to the first good estimate of the sun's distance from Earth.
1962: NASA's Mariner 2 reaches Venus and reveals the planet's extreme surface temperatures. It is the first spacecraft to send back information from another planet.
1970: The Soviet Union's Venera 7 sends back 23 minutes of data from the surface of Venus. It is the first spacecraft to successfully land on another planet.
1990-1994: NASA's Magellan spacecraft, in orbit around Venus, uses radar to map 98 percent of the planet's surface.
2005: The European Space Agency launches Venus Express to study the atmosphere and plasma environment of Venus from orbit.
MORE DETAILS ON VENUS
EARTH
Period of revolutionaround the sun:365.26 Earth days.
Period of Rotation:24 Hours
Diametre:12756 km
Moons:Luna
Earth, our home planet, is the only planet in our solar system known to harbor life: life that is incredibly diverse. All the things we need to survive exist under a thin layer of atmosphere that separates us from the cold, airless void of space. Earth is made up of complex, interactive systems that create a constantly changing world that we are striving to understand. From the vantage point of space we are able to observe our planet globally, using sensitive instruments to understand the delicate balance among its oceans, air, land and life. Satellite observations help study and predict weather, drought, pollution, climate change and many other phenomena that affect the environment, economy and society. Earth is the third planet from the sun and the fifth largest in our solar system. Earth's diameter is just a few hundred kilometers larger than that of Venus. The four seasons are a result of Earth's axis of rotation being tilted 23.45 degrees with respect to the plane of Earth's orbit around the sun. During part of the year, the northern hemisphere is tilted toward the sun and the southern hemisphere is tilted away, producing summer in the north and winter in the south. Six months later, the situation is reversed. During March and September, when spring and fall begin in the northern hemisphere, both hemispheres receive roughly equal amounts of solar illumination. Earth's global ocean, which covers nearly 70 percent of the planet's surface, has an average depth of about 4 km (2.5 miles). Fresh water exists in the liquid phase only within a narrow temperature span: 32 to 212 degrees Fahrenheit (0 to 100 degrees Celsius). This span is especially narrow when contrasted with the full range of temperatures found within the solar system. The presence and distribution of water vapor in the atmosphere is responsible for much of Earth's weather. We are enveloped by an atmosphere that consists of 78 percent nitrogen, 21 percent oxygen and 1 percent other ingredients. The atmosphere affects Earth's long-term climate and short-term local weather, shields us from much of the harmful radiation coming from the sun and protects us from meteors as well: most of which burn up before they can strike the surface as meteorites. Earth-orbiting satellites have revealed that the upper atmosphere actually swells by day and contracts by night due to solar heating during the day and cooling at night. Our planet's rapid rotation and molten nickel-iron core give rise to a magnetic field, which the solar wind distorts into a teardrop shape in space. (The solar wind is a stream of charged particles continuously ejected from the sun.) The Earth's magnetic field does not fade off into space, but has definite boundaries. When charged particles from the solar wind become trapped in Earth's magnetic field, they collide with air molecules above our planet's magnetic poles. These air molecules then begin to glow, and are known as the aurorae -- the northern and southern lights. Earth's lithosphere, which includes the crust (both continental and oceanic) and the upper mantle, is divided into huge plates that are constantly moving. For example, the North American plate moves west over the Pacific Ocean basin, roughly at a rate equal to the growth of our fingernails. Earthquakes result when plates grind past one another, ride up over one another, collide to make mountains, or split and separate. The theory of motion of the large plates of the lithosphere is known as plate tectonics. Developed within the last 40 years, this explanation has unified the results of centuries of study of our planet.
How Earth Got its Name
The name Earth is at least 1,000 years old. All of the planets, except for Earth, were named after Greek and Roman gods and goddesses. However, the name Earth is an English/German word, which simply means the ground: eor(th)e and ertha (Old English) and erde (German).
Significant Dates
1960: NASA launches the Television Infrared Observation Satellite (TIROS), the first weather satellite.
1972: The Earth Resources Technology Satellite 1 (renamed Landsat 1) is launched, the first in a series of Earth-imaging satellites that continues today.
1987: NASA's Airborne Antarctic Ozone Experiment helps determine the cause of the Antarctic ozone hole.
1992: TOPEX/Poseidon, a U.S.-France mission, begins measuring sea-surface height. Jason 1 continues these measurements in 2001.
1997: TOPEX/Poseidon captures the evolution of El Nino (cold ocean water in the equatorial Pacific Ocean) and La Nina (warm ocean water in the equatorial Pacific Ocean).
1997: The U.S.-Japan Tropical Rainfall Measuring Mission is launched to provide 3-D maps of storm structure.
1999: Quick Scatterometer (QuikScat) launches in June to measure ocean surface wind velocity; in December the Active Cavity Irradiance Monitor Satellite launches to monitor the total amount of the sun's energy reaching Earth.
1999-2006: A series of satellites are launched to provide global observations of the Earth system -- simultaneously studying land, oceans, atmosphere, water cycles, gravity, clouds and aerosols.
2006: The Antarctic ozone hole was the largest yet observed.
2007: Arctic sea ice reaches the all-time minimum since satellite records began.
2008: The third U.S.-France mission to measure sea-level height, Ocean Surface Topography Mission/Jason 2, is launched, doubling global data coverage.
2009: NASA and Japan release the most accurate topographic map of Earth.
And many more by indians,europeans and japanese.
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MARS
Period of revolution around the sun:686.98 Earth days.
Period of Rotation:1.026 Earth days
Diametre:6787 km
Moons:Diemos,Phobos
Though details of Mars' surface are difficult to see from Earth, telescope observations show seasonally changing features and white patches at the poles. For decades, people speculated that bright and dark areas on Mars were patches of vegetation, that Mars could be a likely place for life-forms and that water might exist in the polar caps. When the Mariner 4 spacecraft flew by Mars in 1965, many were shocked to see photographs of a bleak, cratered surface. Mars seemed to be a dead planet. Later missions, however, have shown that Mars is a complex member of the solar system and holds many mysteries yet to be solved. Mars is a rocky body about half the size of Earth. As with the other terrestrial planets -- Mercury, Venus and Earth -- the surface of Mars has been altered by volcanism, impacts, crustal movement, and atmospheric effects such as dust storms. Mars often appears reddish due to a combination of the fact that its surface is comprised of iron-rich minerals that essentially rust (or oxidize) and that the dust made of these minerals is kicked up into the atmosphere, giving the atmosphere a reddish hue as well. Mars has no global magnetic field, but NASA's Mars Global Surveyor orbiter found that areas of the Martian crust in the southern hemisphere are highly magnetized. Evidently, these are traces of a magnetic field that remain in the planet's crust from about 4 billion years ago. Mars has two small moons, Phobos and Deimos, that may be captured asteroids. Potato-shaped, they have too little mass for gravity to make them spherical. Phobos, the innermost moon, is heavily cratered, with deep grooves on its surface. Like Earth, Mars experiences seasons because of the tilt of its rotational axis (in relation to the plane of its orbit). Mars' orbit is slightly elliptical, so its distance to the sun changes, affecting the Martian seasons. Mars' seasons last longer than those of Earth. The polar ice caps on Mars grow and recede with the seasons; layered areas near the poles suggest that the planet's climate has changed more than once. Volcanism in the highlands and plains was active more than 3 billion years ago, but some of the giant shield volcanoes are younger, having formed between 1 and 2 billion years ago. Mars has the largest volcanic mountain in the solar system, Olympus Mons, as well as a spectacular equatorial canyon system, Valles Marineris. Scientists believe that Mars experienced huge floods about 3.5 billion years ago. Though we do not know where the ancient flood water came from, how long it lasted or where it went, recent missions to Mars have uncovered intriguing hints. In 2002, NASA's Mars Odyssey orbiter detected hydrogen-rich polar deposits, indicating large quantities of water ice close to the surface. Further observations found hydrogen in other areas as well. If water ice permeated the entire planet, Mars could have substantial subsurface layers of frozen water. In 2004, the Mars Exploration Rover named Opportunity found structures and minerals indicating that liquid water was once present at its landing site. The rover's twin, Spirit, also found the signature of ancient water near its landing site halfway around Mars from Opportunity's location. The cold temperatures and thin atmosphere on Mars don't allow liquid water to exist at the surface for long, and the quantity of water required to carve Mars' great channels and flood plains is not evident today. Unraveling the story of water on Mars is important to unlocking its climate history, which will help us understand the evolution of all the planets. Water is believed to be an essential ingredient for life; evidence of past or present water on Mars is expected to hold clues about whether Mars could ever have been a habitat for life. In 2008, NASA's Phoenix Mars Lander found water ice in the Martian arctic, which was expected. Phoenix also observed precipitation -- snow falling from clouds. This and soil chemistry experiments have led scientists to believe that the Phoenix landing site had a wetter and warmer climate in the recent past (the last few million years). It is unsettled whether Phoenix's soil samples contained any carbon-based organic compounds. More extensive surveys must wait until NASA's Mars Science Laboratory mission, with its large rover (named Curiosity), examines Martian rocks and soils to determine the geologic processes that formed them and learn more about the present and past habitability of the planet.
How Mars Got its Name
Mars was named by the Romans for their god of war because of its red, bloodlike color. Other civilizations also named this planet from this attribute; for example, the Egyptians named it "Her Desher," meaning "the red one."
Significant Dates
1877: Asaph Hall discovers the two moons of Mars, Phobos and Deimos.
1965: NASA's Mariner 4 sends back 22 photos of Mars, the world's first close-up photos of a planet beyond Earth.
1976: Viking 1 and 2 land on the surface of Mars.
1997: Mars Pathfinder lands and dispatches Sojourner, the first wheeled rover to explore the surface of another planet.
2002: Mars Odyssey begins its mission to make global observations and find buried water ice on Mars.
2004: Twin Mars Exploration Rovers named Spirit and Opportunity land on Mars and find the strongest evidence yet obtained that the Red Planet once had underground liquid water and water flowing on the surface.
2006: Mars Reconnaissance Orbiter begins returning high-resolution images as it studies the history of water on Mars.
2008: Phoenix lands on Mars to study the history of water and search for complex organic molecules; confirms the presence of water ice near the north pole.
2012 Mars Science Laboratory's Curiosity rover lands on Mars to study whether the Red Planet ever was -- or is still today -- an environment suitable for life.
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JUPITER
Period of revolution around the sun:11.862 Earth days.
Period of Rotation:9.84 Earth Hours
Diametre:142796 km
Moons:Ganymede,Callisto.Lo,Europa,Himalia,Amathea,Thebe,Elara,Metis,Pasiphae,Carne,Sinope,Lysithea,Ananke,Lede,Thermisto,Callirrhoe,Praxidike,Megaclite,locaste,Taygete,kalyke,autonoe,
harpalyke,thyone,hermippe,chaldene,aoeda,eukallde,isonoe,helika,carpo,S/2003 J5,S/2000 J11,Aitne,Eurydome,Hegomone,Arche,Euanthe,Sponde,S/2003 J2,S/2003 J9,EEuporie,Pasthee,Kore,Cyllene,Mneme,Kale,Kallinchore,S/2003 J16,S/2003 J19,S/2003 J15,S/2003 J10,S/2003 J23,S/2011 J2,S/2010 J1,S/2003 J4,S/2011 J1,S/2010 J2
upiter is the most massive planet in our solar system; with four large moons and many smaller moons it forms a kind of miniature solar system. In fact, Jupiter resembles a star in composition, and if it had been about 80 times more massive, it would have become a star rather than a planet. On 7 January 1610, using his primitive telescope, astronomer Galileo Galilei saw four small "stars" near Jupiter. He had discovered Jupiter's four largest moons, now called Io, Europa, Ganymede, and Callisto. These four moons are known today as the Galilean satellites. Newly discovered moons of Jupiter are reported by astronomers and acknowledged with a temporary designation by the International Astronomical Union; once their orbits are confirmed, they are included in Jupiter's large moon count. Not including the "temporary" moons, Jupiter has 50 total. Galileo would be astonished at what we have learned about Jupiter and its moons, largely from the NASA mission named after him. Io is the most volcanically active body in our solar system. Ganymede is the largest planetary moon and the only moon in the solar system known to have its own magnetic field. A liquid ocean may lie beneath the frozen crust of Europa, and icy oceans may also lie beneath the crusts of Callisto and Ganymede. Jupiter's appearance is a tapestry of beautiful colors and atmospheric features. Most visible clouds are composed of ammonia. Water vapor exists deep below and can sometimes be seen through clear spots in the clouds. The planet's "stripes" are dark belts and light zones created by strong east-west winds in Jupiter's upper atmosphere. Dynamic storm systems rage on Jupiter. The Great Red Spot, a giant spinning storm, has been observed since the 1800s. In recent years, three storms merged to form the Little Red Spot, about half the size of the Great Red Spot. The composition of Jupiter's atmosphere is similar to that of the sun -- mostly hydrogen and helium. Deep in the atmosphere, the pressure and temperature increase, compressing the hydrogen gas into a liquid. At depths of about a third of the way down, the hydrogen becomes metallic and electrically conducting. In this metallic layer, Jupiter's powerful magnetic field is generated by electrical currents driven by Jupiter's fast rotation. At the center, the immense pressure may support a solid core of rock about the size of Earth. Jupiter's enormous magnetic field is nearly 20,000 times as powerful as Earth's. Trapped within Jupiter's magnetosphere (the area in which magnetic field lines encircle the planet from pole to pole) are swarms of charged particles. Jupiter's rings and moons are embedded in an intense radiation belt of electrons and ions trapped by the magnetic field. The Jovian magnetosphere, comprising these particles and fields, balloons 1 to 3 million km (600,000 to 2 million miles) toward the sun and tapers into a windsock-shaped tail extending more than 1 billion km (600 million miles) behind Jupiter as far as Saturn's orbit. Discovered in 1979 by NASA's Voyager 1 spacecraft, Jupiter's rings were a surprise: a flattened main ring and an inner cloud-like ring, called the halo, are both composed of small, dark particles. A third ring, known as the gossamer ring because of its transparency, is actually three rings of microscopic debris from three small moons: Amalthea, Thebe and Adrastea. Data from the Galileo spacecraft indicate that Jupiter's ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet's four small inner moons. The main ring probably is composed of material from the moon Metis. Jupiter's rings are more easily visible when backlit by the sun but have been captured by Hubble Space Telescope images. In December 1995, NASA's Galileo spacecraft dropped a probe into Jupiter's atmosphere, which made the first direct measurements of the planet's atmosphere. The spacecraft then began a multiyear study of Jupiter and the largest moons. As Galileo began its 29th orbit, the Cassini-Huygens spacecraft was nearing Jupiter for a gravity-assist maneuver on the way to Saturn. The two spacecraft made simultaneous observations of the magnetosphere, solar wind, rings, and Jupiter's auroras. NASA launched a mission named Juno in 2011 to conduct an in-depth study of Jupiter from a polar orbit. Juno will examine Jupiter's chemistry, atmosphere, interior structure, and magnetosphere.
How Jupiter Got its Name
The largest and most massive of the planets was named Zeus by the Greeks and Jupiter by the Romans; he was the most important deity in both pantheons.
Significant Dates
1610: Galileo Galilei makes the first detailed observations of Jupiter.
1973: Pioneer 10 becomes the first spacecraft to cross the asteroid belt and fly past Jupiter.
1979: Voyager 1 and 2 discover Jupiter's faint rings, several new moons and volcanic activity on Io's surface.
1994: Astronomers observe as pieces of comet Shoemaker-Levy 9 collide with Jupiter's southern hemisphere.
1995-2003: The Galileo spacecraft drops a probe into Jupiter's atmosphere and conducts extended observations of Jupiter and its moons and rings.
2007: Images by NASA's New Horizons spacecraft, on the way to Pluto, show new perspectives on Jupiter's atmospheric storms, the rings, volcanic Io, and icy Europa.
2009: On July 20, almost exactly 15 years after fragments of comet Shoemaker-Levy slammed into Jupiter, a comet or asteroid crashes into the giant planet's southern hemisphere.
2011: Juno launches to examine Jupiter's chemistry, atmosphere, interior structure, and magnetosphere.
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SATURN
Period of revolution around the sun:29.456 Earth years.
Period of Rotation:10.2 Earth Hours
Diametre:120660 km
Moons:moons
Saturn was the most distant of the five planets known to the ancients. In 1610, Italian astronomer Galileo Galilei was the first to gaze at Saturn through a telescope. To his surprise, he saw a pair of objects on either side of the planet. He sketched them as separate spheres, thinking that Saturn was triple-bodied. Continuing his observations over the next few years, Galileo drew the lateral bodies as arms or handles attached to Saturn. In 1659, Dutch astronomer Christiaan Huygens, using a more powerful telescope than Galileo's, proposed that Saturn was surrounded by a thin, flat ring. In 1675, Italian-born astronomer Jean-Dominique Cassini discovered a division between what are now called the A and B rings. It is now known that the gravitational influence of Saturn's moon Mimas is responsible for the Cassini Division, which is 4,800 km (3,000 miles) wide. Like Jupiter, Saturn is made mostly of hydrogen and helium. Its volume is 755 times greater than that of Earth. Winds in the upper atmosphere reach 500 m (1,600 feet) per second in the equatorial region. (In contrast, the strongest hurricane-force winds on Earth top out at about 110 m, or 360 feet per second.) These super-fast winds, combined with heat rising from within the planet's interior, cause the yellow and gold bands visible in the atmosphere. In the early 1980s, NASA's Voyager 1 and Voyager 2 spacecraft revealed that Saturn's rings are made mostly of water ice, and they imaged "braided" rings, ringlets and "spokes" -- dark features in the rings that circle the planet at different rates from that of the surrounding ring material. Saturn's ring system extends hundreds of thousands of kilometers from the planet, yet the vertical depth is typically about 10 m (30 feet) in the main rings. During Saturn's equinox in autumn 2009, when sunlight illuminated the rings edge-on, Cassini spacecraft images showed vertical formations in some of the rings; the particles seem to pile up in bumps or ridges more than 3 km (2 miles) tall. Saturn's largest moon, Titan, is a bit bigger than the planet Mercury. (Titan is the second-largest moon in the solar system; only Jupiter's moon Ganymede is bigger.) Titan is shrouded in a thick, nitrogen-rich atmosphere that might be similar to what Earth's was like long ago. Further study of this moon promises to reveal much about planetary formation and, perhaps, about the early days of Earth. Saturn also has many smaller icy satellites. From Enceladus, which shows evidence of recent (and ongoing) surface changes, to Iapetus, with one hemisphere darker than asphalt and the other as bright as snow, each of Saturn's satellites is unique. Though Saturn's magnetic field is not as huge as Jupiter's, it is still 578 times as powerful as the Earth's. Saturn, its rings and many of its satellites lie totally within Saturn's own enormous magnetosphere -- the region of space in which the behavior of electrically charged particles is influenced more by Saturn's magnetic field than by the solar wind. While the Hubble Space Telescope imaged Saturn's aurora in the ultraviolet, the Cassini spacecraft found that Saturn has a unique secondary aurora at the north pole, imaged in the infrared in 2008. Aurorae occur when charged particles spiral into a planet's atmosphere along magnetic field lines. On Earth, these charged particles come from the solar wind. Cassini showed that at least some of Saturn's aurorae are like Jupiter's and are largely unaffected by the solar wind. The next chapter in our knowledge of Saturn is being written right now by the Cassini mission, which carried Europe's Huygens probe to Saturn. The Huygens probe descended through Titan's atmosphere in January 2005, collecting data on the atmosphere and surface. The Cassini spacecraft, orbiting Saturn since 2004, continues to explore the planet and its moons, rings and magnetosphere. By July 2009, Cassini had returned more than 200,000 images. The Cassini Equinox Mission is studying the rings during Saturn's autumnal equinox, when the sun shines directly on Saturn's equator. Three-dimensional features are visible in the rings during this time of year, such as moonlets that protrude above and below the ring plane and cast shadows, revealing their sizes and shapes.
How Saturn Got its Name
Saturn is named for the Roman god of agriculture. The Greek equivalent was Cronos, father of Zeus/Jupiter. Other civilizations have given different names to Saturn, which is the farthest planet from Earth that can be observed by the unaided human eye.
Significant Dates
1610: Galileo Galilei reports seeing odd appendages on either side of Saturn; he did not realize he was viewing Saturn's rings.
1979: Pioneer 11 is the first spacecraft to reach Saturn, flying within 22,000 km (13,700 miles) of the cloud tops.
1981: Using Saturn's powerful gravity as an interplanetary slingshot, Voyager 2 is placed on a path toward Uranus, then Neptune, then out of the solar system.
2004: After a seven-year journey, Cassini becomes the first spacecraft to orbit Saturn.
2005: The Huygens probe successfully lands on Titan, returning images of the complex surface.
2008: Cassini's four-year mission is extended for two years and designated the Cassini Equinox Mission.
2010: Cassini is extended until September 2017 and designated the Cassini Solstice Mission.
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URANUS
Period of revolution around the sun:84.07 Earth Years.
Period of Rotation:17.9 Earth hours
Diametre:51118 km
Moons:moons
Uranus, discovered in 1781 by astronomer William Herschel, was the first planet found with the aid of a telescope. As the seventh planet from the sun, Uranus takes 84 Earth years to complete one orbit. Like Venus, Uranus rotates east to west. Uranus' rotation axis is tilted almost parallel to its orbital plane, so Uranus appears to be rotating on its side. This situation may be the result of a collision with a planet-sized body early in the planet's history, which apparently radically changed Uranus' rotation. Because of Uranus' unusual orientation, the planet experiences extreme variations in sunlight during each 20-year-long season. Voyager 2, the only spacecraft to visit Uranus, imaged a bland-looking sphere in 1986. When Voyager flew by, the south pole of Uranus pointed almost directly at the sun because Uranus was near its southern summer solstice, with the southern hemisphere bathed in continuous sunlight and the northern hemisphere radiating heat into the blackness of space. Uranus reached equinox in December 2007, when it was fully illuminated as the sun passed over the planet's equator. By 2028, the north pole will point directly at the sun, a reversal of the situation when Voyager flew by. Equinox also brings ring-plane crossing, when Uranus' rings appear to move more and more edge-on as seen from Earth. The Hubble Space Telescope and the Keck Observatory in Hawaii captured detailed images of Uranus as the planet approached equinox. While Voyager 2 saw only a few discrete clouds, more recent observations reveal that Uranus exhibits dynamic clouds as it approaches equinox, including rapidly evolving bright features and a new Great Dark Spot like those seen on Neptune. Uranus is one of the two ice giants of the outer solar system (the other is Neptune). Uranus' atmosphere is mostly hydrogen and helium, with a small amount of methane and traces of water and ammonia. Uranus gets its blue-green color from methane gas in the atmosphere. Sunlight passes through the atmosphere and is reflected back out by Uranus' cloud tops. Methane gas absorbs the red portion of the light, resulting in a blue-green color. The bulk (80 percent or more) of the mass of Uranus is contained in an extended liquid core consisting mostly of icy materials (water, methane and ammonia). For nearly a quarter of the Uranian year, the sun shines directly over each pole, plunging the other half of the planet into a long, dark winter. While magnetic fields are typically in alignment with a planet's rotation, Uranus' magnetic field is tipped over: the magnetic axis is tilted nearly 60 degrees from the planet's axis of rotation, and is also offset from the center of the planet by one-third of the planet's radius. The magnetic fields of both Uranus and Neptune are very irregular. Uranus has two sets of rings. The inner system of nine rings, discovered in 1977, consists mostly of narrow, dark rings. Voyager found two additional inner rings. An outer system of two more-distant rings was discovered in Hubble Space Telescope images in 2003. In 2006, Hubble observations and observations made at the Keck Observatory showed that the outer rings are brightly colored. Uranus has 27 known moons, named for characters from the works of William Shakespeare or Alexander Pope. Miranda is the strangest-looking Uranian moon: its complex surface may indicate partial melting of the interior, with icy material drifting to the surface.
How Uranus Got its Name
William Herschel tried unsuccessfully to name his discovery Georgian Sidus after King George III; but instead the planet was named for Uranus, the Greek god of the sky.
Significant Dates
1781: Astronomer William Herschel discovers Uranus.
1787-1851: Four Uranian moons are discovered and named Titania, Oberon, Ariel, and Umbriel.
1977: Scientists discover nine faint rings of Uranus while observing a distant star pass behind the planet.
1986: Voyager 2 discovers 10 moons and two additional rings during its historic flyby.
2003-2005: The Hubble Space Telescope images two delicate rings far from the planet, and two new moons.
2007: Uranus reaches equinox.
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NEPTUNE
Period of revolution around the sun:164.81 Earth Years.
Period of Rotation:19.1 Earth hours
Diametre:48600 km
Moons:moons
The ice giant Neptune was the first planet located through mathematical predictions rather than through regular observations of the sky. (Galileo had recorded it as a fixed star during observations with his small telescope in 1612 and 1613.) When Uranus didn't travel exactly as astronomers expected it to, a French mathematician, Urbain Joseph Le Verrier, proposed the position and mass of another as yet unknown planet that could cause the observed changes to Uranus' orbit. After being ignored by French astronomers, Le Verrier sent his predictions to Johann Gottfried Galle at the Berlin Observatory. Galle found Neptune on his first night of searching in 1846. Seventeen days later, its largest moon, Triton, was also discovered. Nearly 4.5 billion km (2.8 billion miles) from the sun, Neptune orbits the sun once every 165 years. It is invisible to the naked eye because of its extreme distance from Earth. Interestingly, the unusual elliptical orbit of the dwarf planet Pluto brings Pluto inside Neptune's orbit for a 20-year period out of every 248 Earth years. Pluto can never crash into Neptune, though, because for every three laps Neptune takes around the sun, Pluto makes two. This repeating pattern prevents close approaches of the two bodies. The main axis of Neptune's magnetic field is tipped over by about 47 degrees compared with the planet's rotation axis. Like Uranus, whose magnetic axis is tilted about 60 degrees from the axis of rotation, Neptune's magnetosphere undergoes wild variations during each rotation because of this misalignment. The magnetic field of Neptune is about 27 times more powerful than that of Earth. Neptune's atmosphere extends to great depths, gradually merging into water and other melted ices over a heavier, approximately Earth-size solid core. Neptune's blue color is the result of methane in the atmosphere. Uranus' blue-green color is also the result of atmospheric methane, but Neptune is a more vivid, brighter blue, so there must be an unknown component that causes the more intense color. Despite its great distance and low energy input from the sun, Neptune's winds can be three times stronger than Jupiter's and nine times stronger than Earth's. In 1989, Voyager 2 tracked a large, oval-shaped, dark storm in Neptune's southern hemisphere. This "Great Dark Spot," which was large enough to contain the entire Earth, spun counterclockwise, and moved westward at almost 1,200 km (750 miles) per hour. Subsequent images taken by the Hubble Space Telescope showed no sign of this Great Dark Spot, but did reveal the appearance and then fading of two other Great Dark Spots over the last decade. Voyager 2 also imaged clouds casting shadows on a lower cloud deck, enabling scientists to visually measure the altitude differences between the upper and lower cloud decks. Neptune has six known rings. Voyager 2's observations confirmed that these unusual rings are not uniform, but have four thick regions (clumps of dust) called arcs. The rings are thought to be relatively young and short-lived. Neptune has 13 known moons, six of which were discovered by Voyager 2. Triton, Neptune's largest moon, orbits the planet in the opposite direction compared with the rest of the moons, suggesting that it may have been captured by Neptune in the distant past. Triton is extremely cold -- temperatures on its surface are about -391degrees Fahrenheit (-235 degrees Celsius). Despite this deep freeze at Triton, Voyager 2 discovered geysers spewing icy material upward more than 8 km (5 miles). Triton's thin atmosphere, also discovered by Voyager, has been detected from Earth several times since, and is growing warmer -- although scientists do not yet know why.
How Neptune Got its Name
Neptune was predicted by John Couch Adams and Urbain Le Verrier. The men independently accounted for the irregularities in the motion of Uranus by correctly predicting the orbital elements of a trans-Uranian planet. Using the predicted parameters of Le Verrier (Adams never published his predictions), Johann Galle discovered the planet in 1846. Galle wanted to name the planet for Le Verrier, but that was not acceptable to the international astronomical community. Instead, this planet is named for the Roman god of the sea.
Significant Dates
1846: Using mathematical calculations, astronomers discover Neptune, increasing the number of known planets to eight. Neptune's largest moon, Triton, is found the same year.
1984: Astronomers find evidence for the existence of a ring system around Neptune.
1989: Voyager 2 becomes the first and only spacecraft to visit Neptune, passing about 4,800 km (2,983 miles) above the planet's north pole.
1998: Scientists using telescopes on Earth and in space image Neptune's rings and ring arcs for the first time.
2003: Using improved observing techniques, astronomers discover five new moons orbiting Neptune.
2005: Scientists using the Keck Observatory image the outer rings and find that some of the ring arcs have deteriorated.
2011: Neptune completes its first 165-year orbit of the sun since its discovery in 1846.
MORE ON NEPTUNE
MOHAMMAD ANWAR M
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