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Jupiter

Jupiter

Jupiter is the largest planet in the solar system and is easily visible in the night sky. Jupiter's mass (1.9 × 1027kilograms [4.2 × 1027 pounds]) is nearly two and a half times the mass of the rest of the solar system's planets combined. Jupiter's volume, filled mostly with gas, is 1,316 times that of Earth. The fifth planet from the Sun, Jupiter's year is 11.86 Earth years but its day is short, only nine hours and fifty-five minutes. Jupiter resembles a small star: its composition, like the Sun's, is mostly hydrogen and helium. It emits about twice the energy that it receives from the Sun and puts out over 100 times more heat than Earth. If Jupiter had been about 50 to 100 times larger, it might have evolved into a star rather than a planet.

Historic Observations of Jupiter

Jupiter has intrigued humans since antiquity. It is named for the king of the Roman gods, and most of its twenty-eight moons are named after the god's many lovers. In 1609 and 1610, Italian mathematician and astronomer Galileo Galilei and German astronomer Simon Marius began telescopic studies of Jupiter and its system. Galileo is credited with the discovery of Jupiter's four largest moons: Io, Europa, Ganymede, and Callisto, now called the Galilean satellites in his honor. These moons had an impact on the thinking of those times. It was believed then that Earth was the center of the universe and that all the planets and moons revolved around Earth. Galileo's observations showed that the four moons revolved around Jupiter, not Earth. This discovery contributed to Galileo's doom. He was condemned by the Catholic Church, forced to recant his discovery, and only in 1992 did Pope John Paul II agree that Galileo was right to support Copernicanism.

As telescopes improved, other astronomers continued to observe Jupiter and to study its colorful bands and the long-lived storm known as the Great Red Spot. Twenty-four other smaller satellites have been discovered, from Amalthea in 1892 to Leda in 1974 to twelve new moons in 2001. Observations from Earth showed that Jupiter has a massive magnetosphere and that the planet emits radiation at radio wavelengths . From this, astronomers deduced that Jupiter is surrounded by radiation belts , similar to Earth's Van Allen radiation belts , and that the planet must have a strong magnetic field.

Spacecraft Explorations

Space missions allowed scientists to make great leaps forward in the exploration of Jupiter and its moons. The first spacecraft to fly by Jupiter were Pioneer 10 (in 1973) and Pioneer 11 (in 1974). They passed as close as 43,000 kilometers (26,660 miles) from Jupiter. Their suite of instruments made important observations of the atmosphere, magnetosphere, and space environment around the planet. In 1979 the spacecraft Voyager 1 and Voyager 2 passed close to Jupiter and its moons, making startling discoveries that included auroras on Jupiter, a ring system surrounding the planet, and active volcanoes on the moon Io.

In 1995, the Galileo spacecraft became the first to orbit Jupiter. It dropped a probe into the planet that survived for 57.6 minutes, until it was crushed by Jupiter's enormous pressure. The probe's instruments sent back valuable information on the temperature, pressure, composition, and density of the upper atmosphere.

The Galileo probe provided scientists with their first glimpse inside the top layers of the atmosphere. One surprising discovery was that Jupiter has thunderstorms that are many times larger than those on Earth. The cause of the thunderstorms is the vertical circulation of water vapor in the top layers of Jupiter's atmosphere.

The main Galileo spacecraft has been making observations of Jupiter, its moons, and its environment since 1995, and these were slated to continue until 2002. Scientific observations continue to be made using Earth-based telescopes and the Hubble Space Telescope, which is in orbit around Earth. The combination of many sets of observations over time is extremely valuable for understanding Jupiter and its system.

The Atmosphere and Interior of Jupiter

Jupiter's atmosphere has alternating patterns of dark and light belts and zones. Within these belts and zones are gigantic storm systems such as the Great Red Spot. The locations and sizes of the belts and zones change gradually over time, and many of them can be seen through a telescope. The Great Red Spot has lasted for at least 100 years, and probably as long as 300 years. It rotates counterclockwise every six days, and this direction, plus its location in the southern hemisphere, indicates that it is a high-pressure zone. This differs from the cyclones that occur on Earth, which are low-pressure zones. The red color of the spot is something of a mystery. Several chemicals, including phosphorus, have been suggested as the cause of the red color but, on the whole, the reasons for Jupiter's different colors are not yet understood.

The atmosphere of Jupiter consists of about 81 percent hydrogen and 18 percent helium, with small amounts of methane, ammonia, phosphorus, water vapor, and various hydrocarbons. Observations by Galileo showed a cloud of fresh ammonia ice downstream from the Great Red Spot. Jupiter's atmosphere has strong winds, but the mechanisms that drive them are not well understood. There are at least twelve different streams of prevailing winds, and they can reach velocities of up to 150 meters per second (492 feet per second) at the equator. On Earth, winds are driven by large differences in temperature, differences that do not exist, at least not on the top part of Jupiter's atmosphere, where the temperature at the poles is about the same as that at the equator (-130°C [-202°F]).

The cloud layer, which is thought to be only about 50 kilometers (31 miles) thick, comprises only a small part of the planet. What is the interior of Jupiter like? The pressure inside Jupiter, which increases with depth, is enormousit may reach about 100 million times the pressure on Earth's surface. Although we cannot directly observe Jupiter's interior, theory plus observations of the atmosphere and the surrounding environment suggest that below the cloud layer there is a 21,000-kilometer-thick (13,000-mile-thick) layer of hydrogen and helium. This layer gradually changes from gas to liquid as the pressure increases. Beneath this layer is a sea of liquid metallic hydrogen about 40,000 kilometers (24,800 miles) deep. Metallic hydrogen does not form on Earth, because our planet lacks the extreme pressures necessary to break up the hydrogen molecules and pack them so tightly that they break up and become electrically conductive. This electrically conductive metallic hydrogen is what drives Jupiter's strong magnetic field. Deeper still in Jupiter's interior is the core, which may be solid and rocky. It is estimated that the core is about one and a half times Earth's diameter, but ten to thirty times more massive. It is also very hot: about 30,000°C (54,000°F). This heat comes up through the layers and is detected at "hot spots" in the atmosphere, which are cloud-free holes.

Magnetic Field and Rings

Jupiter's sea of metallic hydrogen causes it to have the strongest magnetic field of any planet in the solar system. The field is inverted relative to Earth's, that is, a compass there would always point south. The region around the planet that is dominated by the magnetic field is called the magnetosphere. The stream of charged particles sent by the solar wind causes Jupiter's magnetosphere to be shaped like a teardrop, pointing directly away from the Sun. Inside the magnetosphere is a swarm of ions, protons , and electrons , which are called plasma. The plasma rotates along with Jupiter's magnetic field, blasting off charged particles. Some of them impact on the surfaces of the moons. On Io, volcanoes eject material into space, and the particles get caught up in Jupiter's magnetosphere. This creates a doughnut-shaped region of charged particles at about the distance from Jupiter of Io's orbit. This is called the Io plasma torus. It was first observed by the Pioneer spacecraft.

The Voyager missions showed that Jupiter is surrounded by faint rings. Unlike Saturn's rings, which are made up of icy particles, Jupiter's rings are made up of small dust particles. Two small satellites, Adrastea and Metis, are embedded within the rings. Observations by Galileo spacecraft showed that the dust comes from meteoroids impacting the satellites closest to Jupiter.

The Galilean Satellites

The Galilean satellites are all different from one another. Io and Europa have greater densities than Ganymede and Callisto, suggesting that the two inner moons (Io and Europa) contain more rock, and the outer moons more water ice.

Io.

Io is the most volcanically active body in the solar system. It is the only place outside Earth where eruptions of hot magma have been observed. Other planets and moons in the solar system have been volcanically active in the distant past. Io is about the same size as Earth's Moon and, had it not been for its peculiar orbit, it too would have cooled down and volcanism would have ceased. Tidal stresses are produced within Io as a result of the gravitational pull of Jupiter, Europa, and Ganymede. These stresses cause the interior of Io to heat up, leading to active volcanism. About 100 active volcanoes have been seen so far on Io, many of which were discovered from their thermal signature in infrared observations made by the Galileo spacecraft. Some of the active volcanoes have plumes that can reach 300 kilometers (186 miles) high. Io's surface is very young as a result of many continuous volcanic eruptions, and no impact craters have been seen. The colors of the surfacevivid reds, yellows, greens, and blackare different from those seen on other solid bodies in the solar system. These colors are a result of sulfur and silicates on the surface. Io's lavas are hotter than those seen on Earth today, reaching temperatures of 1,500°C (2,700°F). They may be similar in composition to ultramafic lavas on Earth, which erupted millions of years ago.

Europa.

Europa is particularly intriguing because of the possibility that it might harbor life. Observations by Galileo spacecraft showed that Europa's cracked surface resembles the ice floes seen in Earth's polar regions. High-resolution images show that some of the broken pieces of the ice crust have shifted away from one another, but that they fit together like a jigsaw puzzle. This suggests that the crust has been, or still is, lubricated from underneath by warm ice or liquid water. The two most basic ingredients for life are water and heat. Like Io, Europa is subject to tidal stresses because of

THE GALILEAN SATELLITES
Name Radius Distance from Jupiter Density
Io 1,821 km 421,600 km 3.53 gm/cm-3
Europa 1,565 km 670,900 km 2.97 gm/cm-3
Ganymede 2,634 km 1,070,000 km 1.94 gm/cm-3
Callisto 2,403 km 1,883,000 km 1.85 gm/cm-3

Jupiter and Ganymede's gravitational pull. While Europa has no evidence of current active silicate volcanism, the tidal stresses may cause heating of the interior, providing the other key ingredient for life. Europa's surface does show evidence of ice volcanism. There are places where material appears to have come up from underneath as slushy ice and flowed on the surface. Europa has very few impact craters, indicating that its surface is young. Slushy ice flowing over the surface probably erased many impact craters. Europa's surface composition is dominated by water, but Galileo detected other compounds, including hydrogen peroxide (H2O2) on the surface and a thin oxygen atmosphere. The behavior of Jupiter's magnetic field around Europa implies that there may be ions circulating globally beneath the icy surface.

Ganymede.

Larger than the planets Mercury and Pluto, Ganymede was the first moon known to have a magnetic field, one of the earliest discoveries made by the Galileo mission. The field is stronger than that of Mercury. Ganymede has a core made up of metallic iron or iron sulfides. If the core is molten and moving, it would produce the strong magnetic field observed by Galileo. Ganymede's surface shows a complex geologic history. The surface is characterized by large dark areas and by bright grooved terrains. The grooves are thought to have formed when the crust separated along lines of weakness. Other images showed hillcrests and crater rims capped by ice, and old terrain cut by furrows and marked by impact craters. Observations in the ultraviolet made from the Hubble Space Telescope showed the presence of oxygen on Ganymede, and Galileo observations detected hydrogen escaping from Ganymede into space. These results indicate that Ganymede has a thin oxygen atmosphere. Astronomers believe that the atmosphere is produced when charged particles trapped in Jupiter's magnetic field come down to Ganymede's surface. The charged particles penetrate the icy surface, disrupting the water ice. The hydrogen escapes into space, whereas the heavier oxygen atoms are left behind.

Callisto.

About the same size as the planet Mercury, Callisto is Jupiter's second largest moon. Its surface is heavily cratered, implying that it is extremely old, probably dating from about 4 billion years ago, which is close to the time when the solar system formed. Callisto's surface is icy and has some large impact craters and basins surrounded by concentric rings. The largest impact basin is called Valhalla, and it has a bright central region 600 kilometers (372 miles) in diameter, with rings extending to 3,000 kilometers (1,860 miles) in diameter. Galileo observations showed that Callisto has a magnetic field. Underneath its icy crust, Callisto may have a liquid ocean, which, if it is as salty as Earth's oceans, could carry enough electrical currents to produce the magnetic field. A major discovery made by the Galileo mission is that Callisto has a thin atmosphere of carbon dioxide.

see also Exploration Programs (volume 2); Galilei, Galileo (volume 2); NASA (volume 3); Planetary Exploration, Future of (volume 2); Robotic Exploration of Space (volume 2); Shoemaker, Eugene (volume 2); Small Bodies (volume 2).

Rosaly M. C. Lopes

Bibliography

Beatty, J. Kelly, Carolyn Colins Petersen, and Andrew Chaikin, eds. The New Solar System, 4th ed. Cambridge, UK: Sky Publishing Corporation and Cambridge University Press, 1999.

Shirley, James. H., and Rhodes. W. Fairbridge, eds. Encyclopedia of Planetary Sciences. London: Chapman & Hall, 1997.

Internet Resources

NASA Jet Propulsion Laboratory, California Institute of Technology. <http://jpl.nasa.gov/>.

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Jupiter (in astronomy)

Jupiter (jōō´pətər), in astronomy, 5th planet from the sun and largest planet of the solar system.

Astronomical and Physical Characteristics

Jupiter's orbit lies beyond the asteroid belt at a mean distance of 483.6 million mi (778.3 million km) from the sun; its period of revolution is 11.86 years. In order from the sun it is the first of the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—very large, massive planets of relatively low density, having rapid rotation and a thick, opaque atmosphere. Jupiter has a diameter of 88,815 mi (142,984 km), more than 11 times that of the earth. Its mass is 318 times that of the earth and about 21/2 times the mass of all other planets combined.

The atmosphere of Jupiter is composed mainly of hydrogen, helium, methane, and ammonia. However, the concentration of nitrogen, carbon, sulfur, argon, xenon, and krypton—as measured by an instrument package dropped by the space probe Galileo during its 1995 flyby of the planet—is more than twice what was expected, raising questions about the accepted theory of Jupiter's formation. The atmosphere appears to be divided into a number of light and dark bands parallel to its equator and shows a range of complex features, including a storm called the Great Red Spot. Located in the southern hemisphere and varying from c.15,600 to 25,000 mi (25,000 to 40,000 km) in one direction and 7,500 to 10,000 mi (12,000 to 16,000 km) in the other, the storm rotates counterclockwise and has been observed ever since 1664, when Robert Hooke first noted it. Also in the southern hemisphere is the Little Red Spot, c.8,000 mi (13,000 km) across. It formed from three white-colored storms that developed in the 1940s, merged in 1998–2000, and became clearly red by 2006. Analysis of the data obtained when massive pieces of the comet Shoemaker Levy 9 plunged into Jupiter in 1994 has extended our knowledge of the Jovian atmosphere.

Jupiter has no solid rock surface. One theory pictures a gradual transition from the outer ammonia clouds to a thick layer of frozen gases and finally to a liquid or solid hydrogen mantle. Beneath that Jupiter probably has a core of rocky material with a mass 10–15 times that of the earth. The spot and other markings of the atmosphere also provide evidence for Jupiter's rapid rotation, which has a period of about 9 hr 55 min. This rotation causes a polar flattening of over 6%. The temperature ranges from about -190°F (-124°C) for the visible surface of the atmosphere, to 9°F (-13°C) at lower cloud levels; localized regions reach as high as 40°F (4°C) at still lower cloud levels near the equator. Jupiter radiates about four times as much heat energy as it receives from the sun, suggesting an internal heat source. This energy is thought to be due in part to a slow contraction of the planet. Jupiter is also characterized by intense nonthermal radio emission; in the 15-m range it is the strongest radio source in the sky. Jupiter has a huge asymetrical magnetic field, extending past the orbit of Saturn in one direction but far less in the direction of the sun. This magnetosphere traps high levels of energetic particles far more intense than those found within earth's Van Allen radiation belts. Six space probes have encountered the Jovian system: Pioneers 10 and 11 (1973 and 1974), Voyagers 1 and 2 (both 1979), Ulysses (1992), and Galileo (1995–2003).

Its Moons and Rings

At least 63 natural satellites orbit Jupiter. They are conveniently divided into six main groups (in order of increasing distance from the planet): Amalthea, Galilean, Himalia, Ananke, Carme, and Pasiphae. The first group is comprised of the four innermost satellites—Metis, Adrastea, Amalthea, and Thebe. The red color of Amalthea (diameter: 117 mi/189 km), a small, elongated satellite discovered (1892) by Edward Barnard, probably results from a coating of sulfur particles ejected from Io. Metis (diameter: 25 mi/40 km), Adrastea (diameter: 12 mi/20 km), and Thebe (diameter: 62 mi/100 km) are all oddly shaped and were discovered in 1979 in photographs returned to earth by the Voyager 1 space probe. Metis and Adrastea orbit close to Jupiter's thin ring system; material ejected from these moons helps maintain the ring.

The four largest satellites—Io, Europa, Ganymede, and Callisto—were discovered by Galileo in 1610, shortly after he invented the telescope, and are known as the Galilean satellite group. Io (diameter: 2,255 mi/3,630 km), the closest to Jupiter of the four, is the most active geologically, with 30 active volcanoes that are probably energized by the tidal effects of Jupiter's enormous mass. Europa (diameter: 1,960 mi/3,130 km) is a white, highly reflecting body whose smooth surface is covered with dark streaks up to 43 mi/70 km in width and from several hundred to several thousand miles in length. Ganymede (diameter: 3,268 mi/5,262 km), second most distant of the four and the largest satellite in the solar system, has heavily cratered regions, tens of miles across, that are surrounded by younger, grooved terrain. Callisto (diameter: 3,000 mi/4,806 km), the most distant and the least active geologically of the four, has a heavily cratered surface. Themisto (diameter: 5 mi/8 km) orbits Jupiter midway between the Galilean and next main group of satellites, the Himalias. The Himalia group consists of five tightly clustered satellites with orbits outside that of Callisto—Leda (diameter: 6 mi/10 km), Himalia (diameter: 106 mi/170 km), Lysithea (diameter: 15 mi/24 km), Elara (diameter: 50 mi/80 km), and S/2000 J11 (diameter: 2.5 mi/4 km). These 14 inner satellites are regular, that is, their orbits are relatively circular, near equatorial, and prograde, i.e., moving in the same orbital direction as the planet. Almost all of the remainder are irregular in that their orbits are large, elliptical, inclined to that of the planet, and usually retrograde, i.e., motion opposite to that of the planet's rotation. (Jupiter's irregular satellites are distinguished from the regular by the spelling of their names, which all end in the letter "e" .)

Situated between the Himalia and Ananke groups is Carpo (diameter: 2 mi/3 km), which like Thermisto doesn't seem to fit into any of the main groups. The Ananke group comprises 17 satellites, which share similar orbits and range from 1.2 to 2.5 mi (2–4 km) in diameter except for two: S/2003 J12,Euporie,Orthosie,Euanthe,Thyone,Mneme,Harpalyke,Hermippe,Praxidike (diameter: 4.5 mi/7 km), Thelexinoe,Iocaste,Ananke (diameter: 12.5 mi/20 km), S/2003 J16,S/2003 J3,S/2003 J18,Helike, and S/2003 J15.

Like the Ananke group, the Carme group is remarkably homogeneous. It comprises 17 satellites, which share similar orbits and, except for one, range from 1.2 to 3 mi (2–5 km) in diameter: Arche,Pasithee,Chaldene,Kale,Isonoe,Aitne,Erinome,Taygete,Carme (diameter: 28 mi/46 km), Kalyke,Eukelade,Kallichore,S/2003 J17,S/2003 J10,S/2003 J9,S/2003 J5, and S/2003 J19. The most distant of the groups from the planet is the Pasiphae, which comprises 14 widely dispersed satellites that, except for two, range from 1.2 to 4.5 mi (2–7 km) in diameter: S/2000 J12,Eurydome,Autonoe,Sponde,Pasiphae (diameter: 36 mi/58 km), Megaclite,Sinope (diameter: 23 mi/38 km), Hegemone,Aode,Callirrhoe,Cyllene,S/2000 J23,S/2000 J4, and S/2000 J14. The odd orbits of the irregular satellites indicate that they were captured after Jupiter's formation. Because they are small, irregularly shaped, and clustered into groups, it is believed that they originated as parts of a larger body that either shattered due to Jupiter's enormous gravity or broke apart in a collision with another body.

Jupiter has three ringsHalo,Main, and Gossamer—similar to those of Saturn but much smaller and fainter. An intense radiation belt lies between the rings and Jupiter's uppermost atmospheric layers.

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Jupiter

Jupiter

Jupiter, the fifth planet from the Sun, is the largest and most massive planet in our solar system. It is 1,300 times larger than Earth, with more than 300 times the mass of Earth and 2.5 times the mass of all the other planets combined. It has a diameter over 88,000 miles (142,000 kilometers), more than eleven times Earth's diameter of 7,900 miles (12,700 kilometers). Lying about 480 million miles (770 million kilometers) from the Sun, Jupiter takes almost 12 years to complete one revolution.

With its 28 moons, Jupiter is considered a mini-solar system of its own. Before the twenty-first century, astronomers believed Jupiter had only 16 moons. But a rash of discoveries soon put the total at 28. The newly discovered satellites are highly different from Jupiter's more well-known moons. They are much smaller, with estimated diameters ranging from 1.8 to 5 miles (3 to 8 kilometers). Also, they have large and eccentric orbits. Some go around Jupiter in a clockwise direction, while others

orbit counter-clockwise. Astronomers speculate that Jupiter, while it was still young, captured the newly discovered moons from a group of small icy and rocky objects that orbit the Sun.

Jupiter is often the brightest object in the sky after the Sun and Venus. For some unknown reasons, it reflects light that is twice as intense as the sunlight that strikes it.

Jupiter has rings that are composed of small particles. Saturn, Uranus, and Neptune also have ring systems. It was only in late 1998 that

Dr. Joseph Burns, astronomy professor at Cornell University, and a team of researchers figured out how Jupiter's rings are formed. After studying photos taken by the unmanned spacecraft Galileo, astronomers announced that Jupiter's rings are formed when cosmic debris (such as asteroids or particles of comets) are pulled and smashed into Jupiter's moons by the planet's powerful magnetic field. The resulting collision produces dust clouds that become the rings around the planet.

Through a telescope, Jupiter appears as a globe of colorful swirling bands. These bands may be a result of the planet's fast rotation. One day on Jupiter lasts only 10 hours (compared to a rotational period of 24 hours on Earth).

Jupiter's most outstanding feature is its Great Red Spot. The spot is actually a swirling, windy storm measuring 16,000 miles (25,700 kilometers) long and 8,700 miles (14,000 kilometers) wide, an area large enough to cover two Earths. The spot may get its red color from sulfur or phosphorus, but no one is sure. Beneath it lie three white oval areas. Each is a storm about the size of Mars.

The planet's origin

One theory about Jupiter's origin is that the planet is made of the original gas and dust that came together to form the Sun and planets. Since it so far from the Sun, its components may have undergone little or no change. A more recent theory, however, states that Jupiter was formed from ice and rock from comets, and that it grew by attracting other matter around it.

Astronomers have been observing Jupiter since the beginning of recorded time. In 1610, Italian astronomer Galileo Galilei (15641642) looked through his recently developed telescope and discovered the planet's four largest moons: Io, Europa, Ganymede, and Callisto.

Discoveries by the Galileo probe

In 1989, the 2.5-ton (2.3-metric ton) Galileo space probe was launched aboard the space shuttle Atlantis. On December 7, 1995, Galileo began orbiting Jupiter and dropped a mini-probe the size of an average backyard barbecue grill. The probe entered Jupiter's atmosphere at a speed of 106,000 miles (170,500 kilometers) per hour. Soon after, the probe released a parachute and floated down to the planet's hot surface. As it fell, intense winds blew it 300 miles (480 kilometers) horizontally. The probe spent 58 minutes taking extremely detailed pictures of Jupiter until its cameras stopped working at an altitude of about 100 miles (160 kilometers) below the top of the planet's cloud cover. Eight hours later, the probe was completely vaporized as temperatures reached 3,400°F (1,870°C).

What the probe discovered first was a belt of radiation 31,000 miles (49,900 kilometers) above Jupiter's clouds, containing the strongest radio waves in the solar system. It next encountered Jupiter's swirling clouds and found that they contain water, helium, hydrogen, carbon, sulfur, and neon, but in much smaller quantities than expected. It also found gaseous krypton and xenon, but in greater amounts than previously estimated.

Scientists had predicted the probe would encounter three or four dense cloud layers of ammonia, hydrogen, sulfide, and water, but instead it found only thin, hazy clouds. The probe detected only faint signs of lightning at least 600 miles (965 kilometers) away, far less than expected. It also discovered that lightning on Jupiter occurs only one-tenth as often as it does on Earth. Perhaps the biggest surprise uncovered by the probe was the lack of water on the planet.

The probe did not survive long enough to gather information on Jupiter's core. Astronomers believe the planet has a rocky core made of material similar to that of Earth's core. The temperature of the core may be as hot as 18,000°F (9,820°C), with pressures two million times those

at Earth's surface. Scientists believe a layer of compressed hydrogen surrounds the core. Hydrogen in this layer may act like a metal and may be the cause of Jupiter's intense magnetic field (five times greater than the Sun's).

At the beginning of 2001, Galileo was still making valuable scientific observations about the planet and its moons, more than three years after its original two-year mission in orbit around Jupiter. The craft had already received three times the cumulative radiation exposure it was designed to withstand.

Comet Shoemaker-Levy 9 collides with Jupiter

In early 1993, Eugene Shoemaker (19281997), Carolyn Shoemaker (1948 ), and David Levy discovered a comet moving across the night sky. They were surprised at its appearance, since it seemed elongated compared to other comets they had seen. Further observations showed that the comet consisted of a large number of fragments, apparently torn apart during a close encounter with Jupiter during a previous orbit. Calculations showed that this "string of pearls" would collide with Jupiter in July 1994.

A global effort was mounted to observe the impacts with nearly all ground-based and space-based telescopes available. Although astronomers could not predict what effect the collisions would have on Jupiter, or even whether they would be visible, the results turned out to be spectacular. Observatories around the world and satellite telescopes such as the Hubble Space Telescope observed the impacts and their effects. Galileo, en route to Jupiter at the time, provided astronomers with a frontrow seat of the event. Even relatively small amateur telescopes were able to see some of the larger impact sites. Dark regions were visible in the atmosphere for months.

The data collected from the impact event will help scientists to understand the atmosphere of Jupiter, since the collisions dredged up material from parts of the atmosphere that are normally hidden. The wealth of information provided by Galileo, added to the Shoemaker-Levy impact data, is giving astronomers their best understanding yet of the biggest planet in our solar system.

[See also Comet; Solar system; Space probe ]

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Jupiter

Jupiter Fifth major planet from the Sun and the largest of the giant planets. It is one of the brightest objects in the sky. Through a telescope, Jupiter's yellowish elliptical disk is seen to be crossed by brownish-red bands, known as belts and zones. The most distinctive feature is the Great Red Spot (GRS), first observed (1664) by Robert Hooke. Jupiter's rapid rotation and turbulent atmosphere prouduces spots, streaks, and bands. Eddies give rise to the spots, which are cyclones or (like the GRS) anticyclones. Hydrogen accounts for nearly 90% of Jupiter's atmosphere and helium for most of the rest. The pressure at the cloud tops is c.0.5 bar. At 1000km (600mi) below the cloud tops there is an ocean of liquid molecular hydrogen. At a depth of 20,000–25,000km (12,500–15,000mi), under a pressure of 3 million bars, the hydrogen becomes so compressed that it behaves as a metal. At the centre of Jupiter, there is probably a massive iron-silicate core surrounded by an ice mantle. The core temperature is estimated to be 30,000K. The deep, metallic hydrogen ‘mantle’ gives Jupiter a powerful magnetic field. It traps a large quantity of plasma (charged particles); high-energy plasma funnels into the radiation belts. Jupiter's magnetosphere is huge, several times the size of the Sun, and is the source of the planet's powerful radio emissions. Jupiter has 16 known satellites, the four major ones being the Galilean satellites. Knowledge of the planet owes much to visits by space probes: Pioneers 10 and 11, Voyagers 1 and 2, Ulysses, and Galileo.

http://lpl.arizona.edu/nineplanets/nineplanets/jupiter.html; http://wr.usgs.gov

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Jupiter

Jupiter in Roman mythology, the chief god of the Roman state religion, giver of victory, originally a sky god associated with thunder and lightning. His wife was Juno. Also called Jove. His Greek equivalent is Zeus. The name is Latin, from Jovis pater, literally ‘Father Jove’.

In astronomy, Jupiter is the name given to the largest planet in the solar system, a gas giant which is the fifth in order from the sun and one of the brightest objects in the night sky.
Jupiter Ammon a deity worshipped in the Egyptian western desert, where the cult of the Egyptian god Amun was linked with Jupiter.

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Jupiter

Jupiter The fifth and largest planet in the solar system, distant 5.203 AU from the Sun. It has a radius of 71 900 km, and a mass 318 times and a volume 1403 times that of the Earth. Its density is 1310 kg/m3 and it is comprised mainly of hydrogen and helium. The atmosphere is 0.9H-0.1 He (with traces of H2O, CH4, and NH3) which grades down into a liquid shell, overlying a zone of metallic hydrogen. In the centre is a small rock-ice core of about ten Earth masses. Jupiter has at least 16 satellites, including the four Galilean satellites.

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Jupiter (in Roman religion and mythology)

Jupiter, in Roman religion and mythology, the supreme god, also called Jove. Originally a sky deity associated with rain and agriculture, he developed into the great father god, prime protector of the state, concerned, like the Greek Zeus (with whom he is identified), with all aspects of life. At his temple on the Capitol, triumphant generals honored him with their spoils and magistrates paid homage to him with sacrifices. Jupiter was the son of Saturn and Ops and the brother and husband of Juno.

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Jupiter

Ju·pi·ter / ˈjoōpitər/ 1. Roman Mythol. the chief god of the Roman state religion, originally a sky god associated with thunder and lightning. His wife was Juno. Also called Jove. Greek equivalent Zeus. 2. Astron. the largest planet in the solar system, a gas giant that is the fifth in order from the sun and one of the brightest objects in the night sky.

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Jupiter

Jupiter supreme deity of the ancient Romans XIII, (in earliest use Iubiter); largest of the planets XIII; (alch.) †tin XIV; †(her., in blazoning by the names of heavenly bodies) azure XVI. — L.; see JOVIAL.

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Jupiter

Jupiter King of the Roman gods, identified with the Greek god Zeus. He could take on various forms: the light-bringer (Lucetius), god of lightning and thunderbolts (Fulgur), and god of rain (Jupiter Elicius).

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Jupiter

Jupiterbitter, committer, critter, embitter, emitter, fitter, flitter, fritter, glitter, gritter, hitter, jitter, knitter, litter, permitter, pitta, quitter, remitter, sitter, skitter, slitter, spitter, splitter, submitter, titter, transmitter, twitter, witter •drifter, grifter, lifter, shifter, sifter, snifter, uplifter •constrictor, contradictor, depicter, dicta, evictor, inflicter, predictor, victor •filter, kilter, philtre (US philter), quilter, tilter •Jacinta, midwinter, Minter, Pinta, Pinter, printer, splinter, sprinter, tinter, winter •sphincter •assister, ballista, bistre (US bister), blister, enlister, glister, lister, mister, resistor, Sandinista, sister, transistor, tryster, twister, vista •trickster •minster, spinster •hipster, quipster, tipster •cohabiter • arbiter • presbyter •exhibitor, inhibitor, prohibiter •Manchester • Chichester • Silchester •Rochester • Colchester •creditor, editor, subeditor •auditor • Perdita • taffeta • shopfitter •forfeiter • outfitter • counterfeiter •register • marketer •cricketer, picketer •Alistair • weightlifter • filleter •fillister • shoplifter •diameter, heptameter, hexameter, parameter, pentameter, tetrameter •Axminster • Westminster •limiter, perimeter, scimitar, velocimeter •accelerometer, anemometer, barometer, gasometer, geometer, manometer, micrometer, milometer, olfactometer, optometer, pedometer, photometer, pyrometer, speedometer, swingometer, tachometer, thermometer •Kidderminster • janitor •banister, canister •primogenitor, progenitor, senator •administer, maladminister, minister, sinister •monitor • per capita • carpenter •spanakopita • Jupiter • trumpeter •character • barrister • ferreter •teleprinter •chorister, forester •interpreter, misinterpreter •capacitor • ancestor • Exeter •stepsister •elicitor, solicitor •babysitter • house-sitter • bullshitter •competitor • catheter • harvester •riveter • banqueter • non sequitur •loquitur •inquisitor, visitor •compositor, expositor

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Jupiter

Jupiter

The Jovian system

Observations from Earth and space

In situ measurements

The planet
Formation and composition

Atmosphere

The Jovian system Satellites

Io

Europa, Ganymede, and Callisto

Rings

Magnetosphere

Comet ShoemakerLevy 9 collision

Resources

Jupiter, the fifth planet from the sun, is the largest and most massive planet in the solar system. One of the gas giants, it is composed of mostly hydrogen and helium. The Jovian atmosphere provides a rich laboratory for the study of planetary atmospheres. Its most famous feature, the Great Red Spot, has been visible for hundreds of years, and many smaller features are visible in its atmosphere. Thirtynine satellites (or moons) of Jupiter have been discovered (the most of any known planet), ranging in size from larger than Mercury totiny bodies with radii of less than 6 mi (10 km); in addition, the planet has thin rings composed of small particles.

Planetary probes, such as the two Pioneer and Voyager spacecrafts, have flown through the Jovian system and provided enormous detail on its physical and chemical properties, while groundbased and spacebased observations have been used to monitor the planet for centuries. Launched in 1989, the Galileo spacecraft arrived at Jupiter in 1995, and began sending back pictures and data from the Jovian system for the next seven years. In 1994, fragments of Comet ShoemakerLevy 9 crashed into Jupiter, giving a onceinalifetime opportunity to study the atmosphere of the planet, and to learn about the effects of impacts on planets in general.

The Jovian system

The planet Jupiter, due to its size and brightness, was known to the ancients. It is named after Jupiter, the chief god in Roman mythology. This name is appropriate, since Jupiter is the largest and most

massive planet in the solar system. It has a mass of (1.8988 x 1027 kg), more than three times that of Saturn, and more than 300 times that of Earth. Its equatorial radius 44.4 mi (71,492 km) is more than 11 times that of Earths radius.

Jupiter is at a distance of 7.783 x 108 km from the sun, or about 5.2 times the distance of Earth. Because the planet is so far from the sun, it receives much less solar radiation than Earth, and is consequently much colder, with a temperature of only 184°F (120°C) at the top of its clouds. This temperature is actually higher than would be expected from the input of solar energy alone, since the planet generates some of its own heat internally, most likely due to the energy from its gravitational collapse.

Jupiter is one of four gas giants in the outer solar system. These planets differ substantially from the rocky bodies found closer to the sun. Jupiter does not have a solid surface at all, and is hypothesized to have a lower atmosphere of molecular (gaseous) hydrogen that is 8.69912,427 mi (14,00020,000 km) thick, which is underlain by a mantle of metallic (liquid) hydrogen that is 18,641 mi to 24,855 mi (30,000 to 40,000 km) thick. This mantle surrounds an inner core of rock and ice thought to be 6,214 mi (10,000 km) thick. When astronomers look at the planet, they see only the ammonia ice clouds in a hydrogenhelium mixture at the top of the atmosphere. These clouds rotate with different periods. The Jovian cloud structure consists of bands (divided into zones [light color] and belts [dark color]) and interband shear zones (characterized by streaks, loops, plumes, and spots [storms]). Because of the atmospheric motion, there is no single rotation period that can be associated with visible features on this gaseous planet like there would be with a terrestrial planet.

The most commonly used rotation period, referred to as System III, corresponds to the period of the planets periodic radio emissions, which is the rate of rotation of the interior of the planet. This period is 9 hours, 55 minutes, 30 seconds. The Jovian year, which is the time for the planet to complete an orbit about the sun, is 11.86 Earth years long.

Observations from Earth and space

Although Jupiter is visible to the naked eye, and has thus been known for thousands of years, much more detail is visible through even a small telescope. Italian astronomer and physicist Galileo Galilei (15641642) constructed one of the first telescopes of sufficient quality to make astronomical observations, and turned it toward the Jupiter in 1610. By doing so, he was the first to see the band structure in the planets atmosphere, and was the first to see its satellites, or moons. In even a lowpower telescope, these bands are visible. In a typical pair of binoculars, the four Galilean satellites (described below) are visible.

Today, astronomers observe the Jovian system regularly using both ground and spacebased telescopes. These observations are made in many wavelength regions, since each reveals distinct details. Ultraviolet wavelengths, for instance, are particularly useful for observing phenomena such as the aurora, while infrared observations are used to monitor the temperature of the planet.

In situ measurements

The first artificially made objects to travel to Jupiter were Pioneer 10 in December 1973 and Pioneer 11 in December 1974. These were very simple spacecraft, which provided preliminary information about the Jovian system. They made measurements of the radiation belts, the magnetic field, and obtained rather crude images of the planet and its satellites.

The more detailed measurements were made by the two Voyager spacecraft, which passed by Jupiter in March and July, 1979. These very capable spacecraft included a large array of scientific instruments to measure properties of the planet and its environment. Although it is the photographs taken by the Voyagers that are most widely known, much more information was acquired. Ultraviolet and infrared spectra, charged particle counts, and magnetic field measurements were also obtained, in order to provide a more complete view

of the entire Jovian system. Since the encounters were separated by several months, information obtained from Voyager 1 was used in the planning of the Voyager 2 observations, in order to maximize the scientific return. Similarly, the results of Pioneer and groundbased measurements were used to plan the Voyager observations. Thus the observations of the future build on the measurements of the past.

In 1992, Ulysses performed a flyby of the planet, at which time is took measurements of the magnetosphere of Jupiter. It returned to the vicinity of the planet again in 2004.

In 1989, the Galileo spacecraft was launched, with objective of orbiting and studying Jupiter. The mission suffered many setbacks, however, including a malfunction that prevented the main antenna from deploying. Nonetheless, NASA scientists were able to reprogram Galileos software remotely, allowing the probe to meet approximately 70% of mission objectives. In late 1995, the spacecraft reached Jupiter, settling into orbit to study the planet and its satellites at close range. Galileo also released a probe carrying six instruments into the Jovian atmosphere to make direct measurements of the planets composition. It measured highvelocity winds. It also found fewer water molecules than earlier theorized deep in its atmosphere. However, the probe did find that the ratio of hydrogen to helium was similar to that of the outer reaches of the sun; which supports a theory that the sun and Jupiter were formed from the same cloud of interstellar matter. Galileo has had a remarkable record of success while functioning in the Jovian system from 1995 to 2003. Many thousands of photographs of Jupiter and its moons have been returned along with additional data. Galileo has been instrumental in helping find over 20 new moons of Jupiter. Galileo, up to that time, was the only spacecraft to have descended into the atmosphere of Jupiter when it sent a probe and, later, itself into the atmosphere (on September 21, 2003).

The Cassini spacecraft flew by Jupiter in 2000 while on a visit to Saturn. It took highresolution images of the planet. The Juno spacecraft is planned to be launched in 2010 for a detailed study of Jupiter from a polar orbit. NASAs New Horizons craft will visit Jupiter in 2007 while on its way to the dwarf planet Pluto.

The planet
Formation and composition

Jupiter is believed to have formed in a manner similar to the other gas giants in the outer solar system. Rocky and/or icy planetesimals condensed from the solar nebula when the solar system was formed more than 4.5 billion years ago. The condensation process continued until the planetesimals were roughly Earthsized. Although the temperatures close to the sun in the inner solar system drove off the volatile (easily vaporized) gases from bodies formed there, at the much greater distance of Jupiter this did not occur, and the planetesimals continued to collect gas. As the force of gravity slowly crushed the material in the center of the planet, energy was released, which is still seen today as extra heat released from the planet as it cools.

The current composition of the whole of Jupiter is believed to be 24% helium and otherwise primarily molecular hydrogen, with less than 1% of other constituents, including methane (CH4), ammonia (NH3) and water (H20). This composition is similar to the

primordial composition of the solar system, and similar to the makeup of the sun today. Arocky and/or icy core is thought to exist at the center of the planet, with a size similar to that of Earth. Hydrogen and helium in metallic and molecular form make up the majority of the atmosphere further from the center.

Atmosphere

The most striking feature in Jupiters atmosphere is the Great Red Spot, a huge storm several times larger than Earths diameter, which has been observed for more than 160 years (or perhaps much longerbecause the observations were not continuous, it is unclear if spots observed before then were of the presentday spot). Jupiters atmosphere has many storm systems, but the Red Spot is the most prominent. Early theories suggested that it was due to clouds colliding with a feature on the surface of the planet, but since there are no such features (and no solid surface, except perhaps at great depths in the core of the planet), this has since been abandoned. The Spot is also not static, but varies over both short and long timescales. Winds on both the north and south sides of the spot prevent it from varying in latitude, but variations in longitude are seen.

In 2000, three smaller storm spots merged into one, like the Great Red Spot is theorized to have formed around 300 years ago. In late 2005, the spot was white in color, but turned brown, and then red by the end of the year. As of 2006, it is the same color as the Great Red Spot. The official name of the spot is Oval BA, being about half the size of the Great Red Spot, which is twice the diameter of Earth.

Jupiters storm activity is quite violent. Although data from Galileo showed lightning activity is only about 10% as frequent per given area as on Earth, the intensity is ten times as high. The strong winds dominate the atmosphere of Jupiter, however. These winds are found in bands, with the speed and direction varying greatly with latitude. Both the northern and southern hemisphere have at least ten bands of alternating wind direction. The strongest of these winds is more than about 400 mph (600 km/hr). The winds are very stable, although the fine details they cause, such as small white spots, can come and go in just days. Other features, such as the Red Spot, and three nearby white spots that have been visible since 1938, can last months or years. The longlived spots are seen to vary with longitude, but not latitude. Astronomers make computer models of the atmosphere in order to understand the processes occurring there, but they are not yet able to explain the persistence of these features.

The other primary feature visible in the atmosphere are the clouds. Three main cloud layers are seen, composed of ices of water, ammonium hydrosulfide, and ammonia. Since the different cloud layers are at different heights in the atmosphere, each represents a different temperature region. The variation in the colors of the clouds are also thought to be due to different chemistry. Theories suggest that the colors are due to sulfur or phosphorus in the atmosphere, but this has not been verified.

The Jovian system Satellites

Jupiter has 63 or more moons, as of the last count in 2006. Satellites of Jupiter have an organizational structure that puts them into one of several classes. The inner satellites have circular orbits, they move in a prograde direction, and they lie in Jupiters equatorial plane (e.g., Adrastea and Metis [which share one orbit]), Amalthea, and Thebe. The Galilean satellites are slightly more distant than the inner satellites and occupy nearly circular orbits, which are prograde. All Galilean satellites (Io, Europa, Ganymede, and Callisto) are locked in a 1:1 spin orbit couple with Jupiter (the same face toward Jupiter all the time, like Earths moon to Earth). An inner group of outer satellites are farther out and are all prograde, but have inclined orbits with respect to Jupiters equatorial plane (e.g., Leda, Himalia, Lysithea, Elara). An outer group of outer satellites have a mixture of prograde and retrograde orbital motions and also have highly inclined orbits.

When Galileo turned his telescope toward Jupiter in 1610, he discovered what he called the four new Medician stars, after Cosimo of Medici, a former pupil. These four satellites of Jupiter, now known as the Galilean satellites in his honor, today carry the names Io, Europa, Ganymede, and Callisto, after Jupiters (the Greek god) lovers. These bodies are planets in their own right, with sizes that rival or exceed those of the inner planets.

Galileos discovery of a solar system in miniature lent strong support to the idea of the Copernican system, which postulated that the planets were in orbit about the sun, rather than Earth. Until Galileos observations, there was no physical evidence for such a system. By finding a planet with its own satellites, the possibility that there can be centers of motion other than Earth appeared much more likely.

Since Galileos time, astronomers have discovered at least 59 additional satellites of Jupiter. All of these are much smaller than the moonsized Galilean satellites, and hence are much more difficult to observe. Most were not discovered until the Voyager spacecraft reached Jupiter. These smaller objects are commonly irregularly shaped and in inclined (tilted), eccentric orbits. It is likely that many of them have been captured by the planets gravitational field, rather than having formed in the vicinity of Jupiter like the Galilean satellites.

Io

Io is the most striking of the Jovian satellites and it is the most volcanically active body in the solar system. The Voyager spacecraft discovered active volcanoes on the surface, with plumes as high as 186 mi (300 km); eight were recorded by Voyager 1, and six of these were seen to be still active when Voyager 2 arrived several months afterward. Based on the colors of the surface of the moon (yellow, orange, red, and black), scientists believe that the surface consists mostly of sulfur and sulfur compounds.

Most of the surface of the satellite had been transformed by the time the Galileo spacecraft arrived 17 years later. The color and contours of the surface in the southern hemisphere had changed significantly, giving evidence of nearly continuous volcanic activity. Io has the most rapid planetary resurfacing process in the solar system, and hence the youngest overall surface of any known planetary body. Volcanic features abound on Io, including lava flows, ash falls, and volcanic vents and caldera.

Only in a few, isolated spots is there a hint of older (perhaps not volcanic) crust on Io. These spots are called massifs and plateaus, and consist of highly fractured rocky crust. Impact craters are absent on Io, suggestingagainthat the crust is very young.

Current theories suggest that the volcanism on Io is caused by its proximity to Jupiter, and hence the strong gravitational forces, which continually squeeze and stretch the satellite. Because of this continual reprocessing of the surface, impact craters are not seen as on the other satellites. Io also has an important interaction with the Jovian magnetosphere.

It is hypothesized that Io has an internal structure consisting of a liquid ironsulfur core, a mantle of silicate rock that may be partially molten, a lithosphere of brittle silicate rock, a thiosphere (sulfur layer) of liquid or plastic sulfur, and a crust of solid sulfur and sulfur compounds.

Europa, Ganymede, and Callisto

Europa is an ivory, gray, and brown world with a water ice surface. Few craters are visible, and the surface is relatively smooth, but dissected by global networks of fractures. Some areas of Europa (bright plains) have a high density of fractures, whereas other areas (mottled terrains) have less obvious fractures. Icy lava from below Europas ice crust may have flowed out and resurfaced parts of the mottled terrains. This suggests that processes have been (and may be still) at work to renew and recoat the surface of Europa.

Even more exciting is evidence suggesting that Europa may possess the right combination of conditions to sustain life. Based on data from Galileo, paleontologists now believe that the satellite features liquid (or at least slushy) oceans beneath its icy crust, warmed by volcanic activity, geysers, and other thermal outflow generated by tidal stretching and squeezing (like Io, on a smaller scale). In particular, the surface in many areas appears fractured and segmented, indicating ice flows moving over liquid water. In conjunction with organic compounds, the water and heat may have created a biologically viable environment, similar to the hydrothermal vents that on Earth have been shown to support organic activity.

Europa has but five clearly defined impact craters, but there are many other impactcrater like features called crater palimpsests, which look like flat, circular, brown spots that may be craters filled by water lavas. The largest of these features is called Tyre Macula, and is 62 mi (100 km) in diameter.

It is hypothesized that Europa has a solid ironsulfur core, a solid silicate mantle, an asthenosphere of silicates, ice, and water, and a cryosphere (ice crust) of water ice that is approximately 62 mi (100 km) thick.

Ganymede, the largest of the moons, is an ivory and gray waterice world. Impact craters are visible on the surface, but there is a mixture of both dark heavily cratered (older terrains) and sparsely cratered but highly fractured regions (bright and grooved terrains). This suggests that part of the surface is very old (heavily cratered regions) and part has experienced resurfacing since its formation (bright and grooved terrains). Ganymede has a density that suggests a silicate rock core surrounded by a mantle and cryosphere somewhat like that of Europa (but perhaps not as warm). Ganymede has a large, very old impact craterbasin that has been partially covered by ice lavas, known as Memphis Facula (about 311 mi [500 km] in diameter).

Callisto is the outermost of the Galilean satellites. Callisto is another gray waterice world, but it is darker than the Europa and Ganymede. Impact craters are visible over the entire surface, suggesting that little resurfacing has occurred. The density of craters on Callisto is higher than on Earths moon, suggesting most of the surface is very old. There is a huge, multiring impact crater basin on Callisto called Valhalla, which has 25 concentric rings extending out to a diameter of 621 mi (1,000 km). Valhalla may be the largest impact crater in the solar system. Callisto has an internal core of silicate rock and a mantle and cryosphere like the other Galilean satellites.

Several other small satellites of Jupiter have been imaged (e.g., Sinope, Amalthea, and Thebe) and these appear to be captured chondritic asteroids. This lends credence to the theory that all irregularly shaped (nonGalilean) satellites of Jupiter are captured asteroids of one kind or another.

Rings

A remarkable result of the Voyager spacecrafts encounter with Jupiter was the discovery of a ring of particles orbiting Jupiter. This ring is much smaller and simpler than the familiar rings of Saturn, and they are not visible directly from Earth. Since their discovery, however, astronomers have made measurements of them by watching the light from stars as they pass behind the rings. In this way they can measure their extent and density.

The rings extends to approximately 1.7 Jovian radii from the center of the planet. Small shepherd moons (Metis and Adrastea) may act to help gravitationally confine the ring material, creating a sharp outer boundary. Indeed, debris lost from these shepherd satellites is thought to be the rings origin. The rings are about 3,728 mi (6,000 km) wide and about 19 mi (30 km) thick. They are dark and there is some internal band structure to the rings. The size of material is very small, a few microns in size (like fine dust).

Magnetosphere

Jupiter has a magnetic field more than ten times stronger than that of Earth. The magnetosphere, which is created due to this field and its interaction with the solar wind, has major effects on the Jovian system. The size of the magnetosphere is larger than the sun; since the innermost Jovian satellites are embedded deep in the magnetosphere, they are particularly affected by the magnetic field. One of the most complex results is the interaction of Io with this field.

Atoms such as sodium and sulfur have been discovered in a cloud around Io as it orbits Jupiter. This material is believed to originate on the surface of the satellite, and then reach space after being driven from the surface due to collisions with highenergy particles. Through a not yet understood process, these atoms can become charged, and thus get trapped by the magnetic field of Jupiter. The result is a plasma torus, a doughnutshaped region of charged atoms, which rotates with the planets magnetic field. Observations of these emissions in the ultraviolet since the 1970s have shown both a time variability and a spatial asymmetry in brightness from the torus.

Particles from the region around Io are also thought to be responsible for the aurora (similar to Earths northern lights) seen at Jupiters poles, but once again, the process is not clearly understood. The auroras appear to be caused by particles from the torus region, which rain down on the atmosphere at the poles of the planet, creating emissions.

KEY TERMS

Comet ShoemakerLevy 9 A comet that crashed into Jupiter in 1994.

Copernican system The description of the solar system that has the planets orbiting the sun, proposed by Nicholas Copenicus.

Galilean satellites The four largest satellites of Jupiter, discovered by Galileo in 1610.

Gas giant One of the large outer planets, including Jupiter, composed primarily of hydrogen and helium.

Great Red Spot A large, storm in Jupiters atmosphere, which has been visible for more than 300 years.

Io plasma torus A region of charged particles that are trapped in Jupiters magnetic field.

Planetesimal Small bodies from which planets formed.

Voyager Two unmanned planetary probes that flew by Jupiter and its satellites in 1979.

Radio telescopes first detected emissions from Jupiter in the 1955. These radio waves are created when electrons travel through the planets magnetosphere. Measurements show both short and longterm variability of the radio emissions.

Comet ShoemakerLevy 9 collision

In early 1993, Eugene and Carolyn Shoemaker and David Levy discovered a comet moving across the night sky. They were surprised at its appearance, since it seemed elongated compared to other comets they had seen. Further observations showed that the comet consisted of a large number of fragments, apparently torn apart during a close encounter with Jupiter during a previous orbit. Calculations showed that this string of pearls would collide with Jupiter in July, 1994.

A worldwide effort was mounted to observe the impacts with nearly all groundbased and spacebased telescopes available. Although astronomers could not predict what effect the collisions would have on Jupiter, or even whether they would be visible, the results turned out to be spectacular. Observatories around the world, and satellite telescopes such as the Hubble Space Telescope observed the impacts and their effects. Galileo, en route to Jupiter at the time, provided astronomers with a frontrow seat at proceedings. Even relatively small amateur telescopes were able to see some of the larger impact sites. Dark regions were visible in the atmosphere for months.

See also Space probe.

Resources

BOOKS

de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Morrison, D., and Tobias Owen.The Planetary System. 3rd ed. AddisonWesley Publishing, 2002.

Sobel, Dava. The Planets. New York: Viking, 2005.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.

OTHER

Jet Propulsion Laboratory, National Aeronautics and Space Administration. Planet Quest: The Search for Another Earth. <http://planetquest.jpl.nasa.gov/gallery/gallery_index.cfm>(accessed October 14, 2006).

TeachNetlab.org. A Tour of the Planets. <http://www.teachnetlab.org/miami/2001/salidoi2/a_tour_of_the_planets.htm>(accessed October 14, 2006).

SpaceKids, National Aeronautics and Space Administration. Tour the Solar System and Beyond. <http://spacekids.hq.nasa.gov/osskids/animate/mac.html> (accessed October 14, 2006).

Jet Propulsion Laboratory, National Aeronautics and Space Administration. Welcome to the Planets: Jupiter. <http://pds.jpl.nasa.gov/planets/choices/jupiter1.htm>(accessed October 14, 2006).

David Sahnow
David T. King, Jr.

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Jupiter

JUPITER

JUPITER . The name Iuppiter is made up of two elements: the first, Iou-, stems from the Indo-European *dyeu, the root of dies, or "day"; while in the second element we find the Latin word pater, meaning "father." Iuppiter therefore identifies the "god of heavenly light." Many of the ancients were aware of this meaning; Paulus-Festus for example, describes the epithet Lucetius, referring to the god, this way: "Lucetium Iovem appellabant quod eum lucis esse causam credebant" (Jupiter was called Lucetius since he was believed to be the author of light). The name Iuppiter belongs to the Indo-European domain, and is semantically related to the Greek Zeus, which stems from *dyeus.

All Italians recognized Jupiter as their god, particularly the Latins, who honored him under the title of Jupiter Latiaris during the Feriae Latinae. This feast was celebrated each year under the auspices of Alba Longa on the summit of the Alban Hills. It continued in this location as a movable feast (feriae conceptivae ) after Rome replaced Alba as the supervisor of this federal ceremony. Moreover, Jupiter is present in the Umbrian ritual of the Bronze Tables of Iguvium. This feast celebrates Jupiter, Mars, and Vofionus; Table VI lists the three identical prayers accompanying the sacrifices of three oxen, one to each of the deities. Tinia, the great god of the Etruscan pantheon, was equivalent to Zeus-Jupiter; he controlled three kinds of lightning. This supreme deity was often represented on mirrors, accompanied by Uni (Juno) and Menrva (Minerva), with background images from Greek mythology.

Ancient Rome honored Jupiter as the supreme god. His preeminence was never called into question, not even when syncretism brought in the ritus Graecus (Greek rite): in the lectisternium of 217 bce, Jupiter, along with Juno, held the highest rank. Jupiter served as the keystone in the ancient triad of gods, along with Mars and Quirinus; later, in the Capitoline triad, his companions became Juno and Minerva. According to Latin and Greek historiography, and also according to archaeological evidence, the Capitoline temple of Jupiter, Juno, and Minerva with its three cellae was built at the end of the sixth century by Etruscan kings, the Tarquinii. Some scholars have also speculated on an Etruscan origin for the Latin triads, or another triad associating Ceres, Liber, and Libera (whose temple was dedicated on the Aventine at the beginning of the Republic).

The Jupiter of the archaic epoch, specified as Jupiter Feretrius an epithet that the ancients traced to either ferre (to bear, carry) or ferire (to slay, strike)was venerated in a chapel located on the summit of the Capitoline thought to have been built by Romulus. There, the first king of Rome consecrated the first spolia opima (spoils seized from a slain enemy commander) to Jupiter. According to Paulus-Festus, the sanctuary of Jupiter Feretrius contained "sceptrum per quod iurarent et lapidem silicem quo foedus ferirent" ("a sword for swearing oaths and a flintstone for concluding treaties"). Livy records that the Roman Fetialis, the college of priestly officials responsible for ritual declarations of war or peace, concluded a treaty in the name of the Roman people with the Alban people they prayed precisely to Jupiter Feretrius to smite (ferire ) the Roman people if they should deviate from the treaty, similar to the way the priest would strike the sacrificial pig with the flintstone. Plutrach records that near the sanctuary of Jupiter Feretrius, the Ludi Capitolini (Capitoline games) were celebrated each year on the ides of October; they consisted of hand-to-hand combat and foot races.

Jupiter is linked with Roman triumph. Romans believed their victories arose from two factors: the excellence of the general and from the favor of the supreme god who, as optimus and maximus, ensured the prosperity of the Roman Empire. Victory processions were directed towards Capitolium, where the victor would present a solemn offering to Iuppiter Optimus Maximus ; the triumphator (victor) appeared to have achieved the rank of a god. The triumphator was clothed in the vestis triumphalis : the tunica palmata, thus called because of the palm-branches embroidered on it; and the toga picta, a name also owed to its rich embroidery; on his head, he wore the corona laurea, or crown of laurels, the symbol of the triumph. Scholars differ in their interpretations of the figure of the triumphator and the meaning of his insignia: some scholars saw the personification and embodiment of the god Jupiter, but others traced back the insignia triumphalia to the regal robes; consequently, they recognized the former rex in the triumphator. By wearing the ornatus Iovis and the corona Etrusca, by having his face painted with the red lead, and by exclaiming triumpe, the triumphator is viewed as the god manifesting himself. This idea may have originated in Etruscan kingship and thus can be explained against the background of Etruscan religion. These robes, which had originally become the state robes of the king when they had turned into ornatus Iovis, were, on the very day of the triumph, taken back from the god by the king, who then was characterized as both Jupiter and king.

Being god of heaven, Jupiter protected all the ides, or "days of full light," so called because those days were prolonged by a full moon. The ides were the thirteenth day of most months, but the fifteenth day in March, May, July, and October. On these days, the Romans offered Jupiter a sacrificial lamb (ovis Idulis ). His cult was maintained by the flamen Dialis, who was "in the god's permanent service," and was "celebrated every day." His principal feast was celebrated on the Vinalia, which were divided into the Vinalia Rustica (August 19), marked by the consecration of grapes, and the Vinalia Priora (April 23), marked by the offering of wine. The feast of the Meditrinalia (October 11), celebrating the magical and medical power of the vinum novum also honored Jupiter. Together with Venus, he was venerated as the sovereign god, protector of the Romans. From a naturalistic point of view, scholars point to the gift of the wine to Jupiter as a proof of an "agrarian" god; in fact, Latin peasants honored Jupiter because his powers could endanger their crops and the vineyards. The link between Jupiter and wine and Jupiter and Venus (who is associated with Jupiter in the Vinalia) indicates his sovereignty and preeminence.

Many other epithets illustrate different aspects of Jupiter. Some correspond to his atmospheric manifestations, such as Jupiter Tonans (the thunderer) or Jupiter Fulgur (he who throws lightning). Others refer to his magical or juridical interventions, such as Jupiter Stator (he who immobilizes or Jupiter Fidius (loyalty warranter). During the historical epoch, however, his principal title was officially Jupiter Optimus Maximus. The anniversary of the founding of his temple on the Capitoline fell on the ides of September, and it was followed by the Ludi Romani (Roman games). On the calends of January, the new consuls would go there, accompanied by senators, magistrates, priests, and common people. The consul named to lead a military expedition would pronounce the vota, which were prayers and promises for gaining a victory. Upon his triumphant return, he would go to the temple to give thanks to the sovereign god.

See Also

Fides; Flamen; Indo-European Religions, overview article; Juno; Minerva; Quirinus; Roman Religion, article on the Early Period.

Bibliography

Banti, Laura. "Il culto del cosidetto tempio di Apollo a Veii e il problema delle triadi etrusco-italiche." Studi Etruschi 17 (1943): 187224.

Brelich, Angelo. "Juppiter e le idus." In Ex orbe religionum. Studia Geo Windengren oblata, pp. 299306. Leiden, 1972.

Cazeneuve, O. "Jupiter, Liber et le vin latin." Revue d'Histoire des Religions 205 (1988): 245265.

Dumézil, Georges. Jupiter, Mars, Quirinus, Essai sur la conception indo-européenne de la société et sur les origines de Rome. Paris, 1941.

Dumézil, Georges. "Quaestiones indo-italicae, 14 (Jupiter et les Vinalia), 15 (Le mythe des Vinalia priora), 16 (Inter exta caesa et porrecta)." Revue des Études Latines 39 (1961): 261274.

Dumézil, Georges. La religion romaine archaïque. 2d ed. Paris, 1974. Translated from the first edition by Philip Krapp as Archaic Roman Religion, 2 vols. (Chicago, 1970).

Fowler, W. Warde. "Juppiter and the Triumphator." Classical Review 30 (1916): 153.

Gantz, Timothy. "Divine Triads on an Archaic Etruscan Frieze Plaque from Poggio Civitate (Murlo)." Studi Etruschi 39 (1971): 122.

Koch, Carl. Der römischen Juppiter (Frankfurter Studien zur Religion und Kultur der Antike). Frankfurt-am-Main, Germany, 1937.

Montanari, Enrico. Identità culturale e conflitti religiosi nella Roma reppublicana. Rome, 1988.

Rufus, Fear Julius. "The Cult of Jupiter and Roman Imperial Ideology." Aufstieg und Nidergang der römischen Welt, II, 17, no. 1, 1981: 3143.

Schilling, Robert. Rites, cultes, dieux de Rome. Paris, 1979.

Schilling, Robert. La religion romaine de Vénus. 2d ed. Paris, 1982.

Versnel, Henobrik Simon. Triumphus. An Inquiry into the Origin, Development, and Meaning of the Roman Triumph. Leiden, 1970.

Wissowa, Georg. Religion und Kultus der Römer. 2d ed. Munich, 1912.

Robert Schilling (1987)

Charles Guittard (2005)

Translated from French by Paul C. Duggan

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Jupiter

Jupiter

Jupiter, the fifth planet from the Sun , is the largest and most massive planet in our solar system . One of the gas giants, it is composed of mostly hydrogen and helium. The Jovian atmosphere provides a rich laboratory for the study of planetary atmospheres . Its most famous feature, the Great Red Spot, has been visible for hundreds of years, and many smaller features are visible in its atmosphere. Thirty-nine satellites (or moons) of Jupiter have been discovered (the most of any known planet), ranging in size from larger than Mercury to tiny bodies with radii of less than 6 mi (10 km); in addition, the planet has thin rings composed of small particles.

Planetary probes, such as the two Pioneer and Voyager spacecrafts, have flown through the Jovian system and provided enormous detail on its physical and chemical properties, while ground-based and space-based observations have been used to monitor the planet for centuries. Launched in 1989, the Galileo spacecraft arrived at Jupiter in 1995, and began sending back pictures and data from the Jovian system for the next seven years. In 1994, fragments of Comet Shoemaker-Levy 9 crashed into Jupiter, giving a once-in-a-lifetime opportunity to study the atmosphere of the planet, and to learn about the effects of impacts on planets in general.


The Jovian system

The planet Jupiter, due to its size and brightness, was known to the ancients. It is named after Jupiter, the chief god in Roman mythology. This name is appropriate, since Jupiter is the largest and most massive planet in our solar system. It has a mass of (1.8988 × 1027 kg), more than three times that of Saturn , and more than 300 times that of Earth . Its equatorial radius 44.4 mi (71,492 km) is more than 11 times that of Earth.

Jupiter is at a distance of 7.783 × 108 km from the Sun, or about 5.2 times the distance of Earth. Because the planet is so far from the Sun, it receives much less solar radiation than Earth, and is consequently much colder, with a temperature of only -184°F (-120°C) at the top of its clouds . This temperature is actually higher than would be expected from the input of solar energy alone, since the planet generates some of its own heat internally, most likely due to the energy from its gravitational collapse.

Jupiter is one of four gas giants in the outer solar system. These planets differ substantially from the rocky bodies found closer to the Sun. Jupiter does not have a solid surface at all, and is hypothesized to have a lower atmosphere of molecular (gaseous) hydrogen that is 8.699–12,427 mi (14,000–20,000) km thick, which is underlain by a mantle of metallic (liquid) hydrogen that is 18,641 mi–24,855 mi (30,000–40,000 km) thick. This mantle surrounds an inner core of rock and ice thought to be 6,214 mi (10,000 km) thick. When we look at the planet, we see only the ammonia ice clouds in a hydrogen-helium mixture at the top of the atmosphere. These clouds rotate with different periods. The Jovian cloud structure consists of bands (divided into zones [light color ] and belts [dark color]) and inter-band shear zones (characterized by streaks, loops, plumes, and spots [storms]). Because of the atmospheric motion , there is no single rotation period that can be associated with visible features on this gaseous planet like there would be with a terrestrial planet.

The most commonly used rotation period, referred to as System III, corresponds to the period of the planet's periodic radio emissions, which is the rate of rotation of the interior of the planet. This period is 9 hours, 55 minutes, 30 seconds. The Jovian year, which is the time for the planet to complete an orbit about the Sun, is 11.86 Earth years long.


Observations from Earth and space

Although Jupiter is visible to the naked eye , and has thus been known for thousands of years, much more detail is visible through even a small telescope . Galileo Galilei constructed one of the first telescopes of sufficient quality to make astronomical observations, and turned it toward the Jupiter in 1610. By doing so, he was the first to see the band structure in the planet's atmosphere, and was the first to see its satellites, or moons. In even a low-power telescope, these bands are visible. In a typical pair of binoculars, the four Galilean satellites (described below) are visible.

Today, astronomers observe the Jovian system regularly using both ground- and space-based telescopes. These observations are made in many wavelength regions, since each reveals distinct details. Ultraviolet wavelengths, for instance, are particularly useful for observing phenomena such as the aurora, while infrared observations are used to monitor the temperature of the planet.


In situ measurements

The first man-made objects to travel to Jupiter were Pioneer 10 in December 1973 and Pioneer 11 in December 1974. These were very simple spacecraft, which provided preliminary information about the Jovian system. They made measurements of the radiation belts, the magnetic field, and obtained rather crude images of the planet and its satellites.

The more detailed measurements were made by the two Voyager spacecraft , which passed by Jupiter in March and July, 1979. These very capable spacecraft included a large array of scientific instruments to measure properties of the planet and its environment. Although it is the photographs taken by the Voyagers which are most widely known, much more information was acquired. Ultraviolet and infrared spectra, charged particle counts, and magnetic field measurements were also obtained, in order to provide a more complete view of the entire Jovian system. Since the encounters were separated by several months, information obtained from Voyager 1 was used in the planning of the Voyager 2 observations, in order to maximize the scientific return. Similarly, the results of Pioneer and ground-based measurements were used to plan the Voyager observations. Thus the observations of the future build on the measurements of the past.

In 1989, the Galileo spacecraft was launched, with objective of orbiting and studying Jupiter. The mission suffered many setbacks, however, including a malfunction that prevented the main antenna from deploying. Nonetheless, NASA scientists were able to reprogram Galileo's software remotely, allowing the probe to meet approximately 70% of mission objectives. In late 1995, the spacecraft reached Jupiter, settling into orbit to study the planet and its satellites at close range. Galileo also released a probe carrying six instruments into the Jovian atmosphere to make direct measurements of the planet's composition. Galileo has had a remarkable record of success while functioning in the Jovian system from 1995-2002. Many thousands of photographs of Jupiter and its moons have been returned along with much additional data. Galileo has been instrumental in helping find over 20 new moons of Jupiter.


The planet

Formation and composition

Jupiter is believed to have formed in a manner similar to the other gas giants in the outer solar system. Rocky and/or icy planetesimals condensed from the solar nebula when the solar system was formed more than 4.5 billion years ago. The condensation process continued until the planetesimals were roughly Earth-sized. Although the temperatures close to the Sun in the inner solar system drove off the volatile (easily-vaporized) gases from bodies formed there, at the much greater distance of Jupiter this did not occur, and the planetesimals continued to collect gas. As the force of gravity slowly crushed the material in the center of the planet, energy was released, which is still seen today as extra heat released from the planet as it cools.

The current composition of the whole of Jupiter is believed to be 24% helium and otherwise primarily molecular hydrogen, with less than 1% of other constituents, including methane (CH4), ammonia (NH3) and water (H20). This composition is similar to the primordial composition of the solar system, and similar to the makeup of the Sun today. A rocky and/or icy core is thought to exist at the center of the planet, with a size similar to that of Earth. Hydrogen and helium in metallic and molecular form make up the majority of the atmosphere further from the center.


Atmosphere

The most striking feature in Jupiter's atmosphere is the Great Red Spot, a huge storm several times larger than Earth's diameter, which has been observed for more than 150 years (or perhaps much longer—because the observations were not continuous, it is unclear if spots observed before then were of the present-day spot). Jupiter's atmosphere has many storm systems, but the Red Spot is the most prominent. Early theories suggested that it was due to clouds colliding with a feature on the surface of the planet, but since there are no such features (and no solid surface, except perhaps at great depths in the core of the planet), this has since been abandoned. The Spot is also not static, but varies over both short and long timescales. Winds on both the north and south sides of the spot prevent it from varying in latitude, but variations in longitude are seen.

Jupiter's storm activity is quite violent. Although data from Galileo showed lightning activity is only about 10% as frequent per given area as on Earth, the intensity is 10 times as high. It is the strong winds that dominate the atmosphere of Jupiter, however. These winds are found in bands, with the speed and direction varying greatly with latitude. Both the northern and southern hemisphere have at least 10 bands of alternating wind direction. The strongest of these winds is more than about 400 miles/hour (600 km/hr). The winds are very stable, although the fine details they cause, such as small white spots, can come and go in just days. Other features, such as the Red Spot, and three nearby white spots which have been visible since 1938, can last months or years. The long-lived spots are seen to vary with longitude, but not latitude. Astronomers make computer models of the atmosphere in order to understand the processes occurring there, but they are not yet able to explain the persistence of these features.

The other primary feature visible in the atmosphere are the clouds. Three main cloud layers are seen, composed of ices of water, ammonium hydrosulfide, and ammonia. Since the different cloud layers are at different heights in the atmosphere, each represents a different temperature region. The variation in the colors of the clouds are also thought to be due to different chemistry . Theories suggest that the colors are due to sulfur or phosphorus in the atmosphere, but this has not been verified.


The Jovian system

Satellites

Satellites of Jupiter have an organizational structure that puts them into one of several classes. The inner satellites have circular orbits, they move in a prograde direction, and they lie in Jupiter's equatorial plane (e.g., Adrastea and Metis (which share one orbit), Amalthea, and Thebe. The Galilean satellites are slightly more distant than the inner satellites and occupy nearly circular orbits, which are prograde. All Galilean satellites (Io, Europa, Ganymede, and Callisto) are locked in a 1:1 spin orbit couple with Jupiter (the same face toward Jupiter all the time, like Earth's Moon to Earth). An inner group of outer satellites are farther out and are all prograde, but have inclined orbits with respect to Jupiter's equatorial plane (e.g., Leda, Himalia, Lysithea, Elara). An outer group of outer satellites have a mixture of prograde and retrograde orbital motions and also have highly inclined orbits.

When Galileo turned his telescope toward Jupiter in 1610, he discovered what he called the four new "Medician stars," after Cosimo of Medici, a former pupil. These four satellites of Jupiter, now known as the Galilean satellites in his honor, today carry the names Io, Europa, Ganymede, and Callisto, after Jupiter's (the Greek god's) lovers. These bodies are planets in their own right, with sizes that rival or exceed those of the inner planets.

Galileo's discovery of a solar system in miniature lent strong support to the idea of the Copernican system, which postulated that the planets were in orbit about the Sun, rather than Earth. Until Galileo's observations, there was no physical evidence for such a system. By finding a planet with its own satellites, the possibility that there can be centers of motion other than Earth appeared much more likely.

Since Galileo's time, astronomers have discovered 35 additional satellites of Jupiter. All of these are much smaller than the Moon-sized Galilean satellites, and hence are much more difficult to observe. Most were not discovered until the Voyager spacecraft reached Jupiter. These smaller objects are commonly irregularly shaped and in inclined (tilted), eccentric orbits. It is likely that many of them have been captured by the planet's gravitational field, rather than having formed in the vicinity of Jupiter like the Galilean satellites.


Io

Io is the most striking of the Jovian satellites and it is the most volcanically active body in the solar system. The Voyager spacecraft discovered active volcanoes on the surface, with plumes as high as 186 mi (300 km); eight were recorded by Voyager 1, and six of these were seen to be still active when Voyager 2 arrived several months afterward. Based on the colors of the surface of the moon (yellow, orange, red, and black), scientists believe that the surface consists mostly of sulfur and sulfur compounds.

Most of the surface of the satellite had been transformed by the time Galileo arrived, 17 years later. The color and contours of the surface in the southern hemisphere had changed significantly, giving evidence of nearly continuous volcanic activity. Io has the most rapid planetary resurfacing process in the solar system, and hence the youngest overall surface of any known planetary body. Volcanic features abound on Io, including lava flows, ash falls, and volcanic vents and caldera.

Only in a few, isolated spots is there a hint of older (perhaps not volcanic) crust on Io. These spots are called massifs and plateaus, and consist of highly fractured rocky crust. Impact craters are absent on Io, suggesting—again—that the crust is very young.

Current theories suggest that the volcanism on Io is caused by its proximity to Jupiter, and hence the strong gravitational forces, which continually squeeze and stretch the satellite. Because of this continual reprocessing of the surface, impact craters are not seen as on the other satellites. Io also has an important interaction with the Jovian magnetosphere .

It is hypothesized that Io has an internal structure consisting of a liquid iron-sulfur core, a mantle of silicate rock that may be partially molten, a lithosphere of brittle silicate rock, a "thiosphere" (sulfur layer) of liquid or plastic sulfur, and a crust of solid sulfur and sulfur compounds.


Europa, Ganymede, and Callisto

Europa is an ivory, gray, and brown world with a water ice surface. Few craters are visible, and the surface is relatively smooth, but dissected by global networks of fractures. Some areas of Europa (bright plains) have a high density of fractures, whereas other areas (mottled terrains) have less obvious fractures. Icy "lava" from below Europa's ice crust may have flowed out and resurfaced parts of the mottled terrains. This suggests that processes have been (and may be still) at work to renew and re-coat the surface of Europa.

Even more exciting is evidence suggesting that Europa may possess the right combination of conditions to sustain life. Based on data from Galileo, paleontologists now believe that the satellite features liquid (or at least slushy) oceans beneath its icy crust, warmed by volcanic activity, geysers, and other thermal outflow generated by tidal stretching and squeezing (like Io, on a smaller scale). In particular, the surface in many areas appears fractured and segmented, indicating ice floes moving over liquid water. In conjunction with organic compounds, the water and heat may have created a biologically viable environment, similar to the hydrothermal vents that on Earth have been shown to support organic activity.

Europa has but five clearly defined impact craters, but there are many other impact-crater-like features called crater palimpsests, which look like flat, circular, brown spots that may be craters filled by water lavas. The largest of these features is called Tyre Macula, and is 62 mi (100 km) in diameter.

It is hypothesized that Europa has a solid iron-sulfur core, a solid silicate mantle, an asthenosphere of silicates, ice, and water, and a cryosphere (ice crust) of water ice that is approximately 62 mi (100 km) thick.

Ganymede, the largest of the moons, is an ivory and grey water-ice world. Impact craters are visible on the surface, but there is a mixture of both dark heavily cratered (older terrains) and sparsely cratered but highly fractured regions (bright and grooved terrains). This suggests that part of the surface is very old (heavily cratered regions) and part has experienced resurfacing since its formation (bright and grooved terrains). Ganymede has a density that suggests a silicate rock core surrounded by a mantle and cryosphere somewhat like that of Europa (but perhaps not as warm). Ganymede has a large, very old impact crater basin that has been partially covered by ice lavas, known as Memphis Facula (about 311 mi [500 km] in diameter).

Callisto is the outermost of the Galilean satellites. Callisto is another gray water-ice world, but it is darker than the Europa and Ganymede. Impact craters are visible over the entire surface, suggesting that little resurfacing has occurred. The density of craters on Callisto is higher than on Earth's moon, suggesting most of the surface is very old. There is a huge, multi-ring impact crater basin on Callisto called Valhalla, which has 25 concentric rings extending out to a diameter of 621 mi (1,000 km). Valhalla may be the largest impact crater in the solar system. Callisto has an internal core of silicate rock and a mantle and cryosphere like the other Galilean satellites.

Several other small satellites of Jupiter have been imaged (e.g., Sinope, Amalthea, and Thebe) and these appear to be captured chondritic asteroids. This lends credence to the theory that all irregularly shaped (non-Galilean) satellites of Jupiter are captured asteroids of one kind or another.


Rings

A remarkable result of the Voyager spacecraft's encounter with Jupiter was the discovery of a ring of particles orbiting Jupiter. This ring is much smaller and simpler than the familiar rings of Saturn, and they are not visible directly from Earth. Since their discovery, however, astronomers have made measurements of them by watching the light from stars as they pass behind the rings. In this way they can measure their extent and density.

The rings extends to approximately 1.7 Jovian radii from the center of the planet. Small "shepherd" moons (Metis and Adrastea) may act to help gravitationally confine the ring material, creating a sharp outer boundary. Indeed, debris lost from these shepard satellites is thought to be the ring's origin. The rings are about 3,728 mi (6,000 km) wide and about 19 mi (30 km) thick. They are dark and there is some internal band structure to the rings. The size of material is very small, a few microns in size (like fine dust).


Magnetosphere

Jupiter has a magnetic field more than ten times stronger than that of Earth. The magnetosphere, which is created due to this field and its interaction with the solar wind , has major effects on the Jovian system. The size of the magnetosphere is larger than the Sun; since the innermost Jovian satellites are embedded deep in the magnetosphere, they are particularly affected by the magnetic field. One of the most complex results is the interaction of Io with this field.

Atoms such as sodium and sulfur have been discovered in a cloud around Io as it orbits Jupiter. This material is believed to originate on the surface of the satellite, and then reach space after being driven from the surface due to collisions with high energy particles. Through a not yet understood process, these atoms can become charged, and thus get trapped by the magnetic field of Jupiter. The result is a "plasma torus," a doughnut-shaped region of charged atoms, which rotates with the planet's magnetic field. Observations of these emissions in the ultraviolet since the 1970s have shown both a time variability and a spatial asymmetry in brightness from the torus .

Particles from the region around Io are also thought to be responsible for the aurora (similar to Earth's northern lights) seen at Jupiter's poles, but once again, the process is not clearly understood. The auroras appear to be caused by particles from the torus region, which rain down on the atmosphere at the poles of the planet, creating emissions.

Radio telescopes first detected emissions from Jupiter in the 1955. These radio waves are created when electrons travel through the planet's magnetosphere. Measurements show both short- and long-term variability of the radio emissions.

Comet Shoemaker-Levy 9 collision

In early 1993, Eugene and Carolyn Shoemaker and David Levy discovered a comet moving across the night sky. They were surprised at its appearance, since it seemed elongated compared to other comets they had seen. Further observations showed that the comet consisted of a large number of fragments, apparently torn apart during a close encounter with Jupiter during a previous orbit. Calculations showed that this "string of pearls" would collide with Jupiter in July, 1994.

A worldwide effort was mounted to observe the impacts with nearly all ground-based and space-based telescopes available. Although astronomers could not predict what effect the collisions would have on Jupiter, or even whether they would be visible, the results turned out to be spectacular. Observatories around the world, and satellite telescopes such as the Hubble Space Telescope observed the impacts and their effects. Galileo, en route to Jupiter at the time, provided astronomers with a front-row seat at proceedings. Even relatively small amateur telescopes were able to see some of the larger impact sites. Dark regions were visible in the atmosphere for months.

See also Space probe.


Resources

books

Christiansen, E.H., and W.K. Hamblin. Exploring the Planets. 2nd ed. Englewood Cliffs, NJ: Prentice-Hall, 1995.

de Pater, Imke, and Jack J. Lissauer. Planetary Sciences. Cambridge, UK: Cambridge University Press, 2001.

Morrison, D., and Tobias Owen. The Planetary System. 3rd ed. Addison-Wesley Publishing, 2002.

Taylor, F.W. The Cambridge Photographic Guide to the Planets. Cambridge University Press, 2002.

periodicals

Beebe, Reta F. "Queen of the Giant Storms." Sky & Telescope (1990): 359-364.

O'Meara, Stephen James. "The Great Dark Spots of Jupiter." Sky & Telescope (1994): 30–35.

other

Arnett, B. SEDS, University of Arizona. "The Nine Planets, a Multimedia Tour of the Solar System." November 6, 2002 [cited February 8, 2003]. <http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html>.


David Sahnow David T. King, Jr.

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Comet Shoemaker-Levy 9

—A comet which crashed into Jupiter in 1994.

Copernican system

—The description of the solar system which has the planets orbiting the Sun, proposed by Nicholas Copenicus.

Galilean satellites

—The four largest satellites of Jupiter, discovered by Galileo in 1610.

Gas giant

—One of the large outer planets, including Jupiter, composed primarily of hydrogen and helium.

Great Red Spot

—A large, storm in Jupiter's atmosphere, which has been visible for more than 150 years.

Io plasma torus

—A region of charged particles which are trapped in Jupiter's magnetic field.

Planetesimal

—Small bodies from which planets formed.

Voyager

—Two unmanned planetary probes which flew by Jupiter and its satellites in 1979.

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