|
For the god, see Jupiter (mythology). For other uses of this term, see Jupiter (disambiguation). Jupiter (IPA: ) is the fifth planet from the Sun and the largest within the solar system. Jupiter and the other gas giants—Saturn, Uranus, and Neptune—are sometimes referred to as "Jovian planets". Overview
Historical observations The planet Jupiter has been known since ancient times and is visible to the naked eye in the night sky. The Romans named the planet after the Roman god Jupiter (also called Jove). The astronomical symbol for the planet is a stylized representation of the god's lightning bolt. The Chinese, Korean, Japanese, and Vietnamese refer to the planet as the wood star, 木星,• based on the Chinese Five Elements. In Vedic Astrology, Hindu astrologers refer to Jupiter as Brihaspati, or "Guru" which means the "Big One". In Hindi, Thursday is referred to as Guruvaar (day of Jupiter). In the English language Thursday is rendered as Thor's day, with Thor being identified with the Roman god Jupiter. In 1610, Galileo Galilei discovered the four largest moons of Jupiter, Io, Europa, Ganymede and Callisto (now known as the Galilean moons) using a telescope, the first observation of moons other than Earth's. This was also the first discovery of a celestial motion not apparently centered on the Earth. It was a major point in favor of Copernicus' heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory placed him under the threat of the Inquisition. In 1892, E. E. Barnard observed a fifth satellite of Jupiter with the 36-inch refractor at Lick Observatory in California. The discovery, a testament to his extraordinary eyesight, made him quickly famous. The moon was later named Amalthea. Planetary composition Jupiter is composed of a relatively small rocky core, surrounded by metallic hydrogen, with further layers of liquid hydrogen and gaseous hydrogen. There is no clear boundary or surface between these different phases of hydrogen; the conditions blend smoothly from gas to liquid as one descends. Atmosphere Jupiter's atmosphere is composed of ~90% hydrogen and ~10% helium by number of atoms. The atmosphere is ~75%/24% by mass; with ~1% of the mass accounted for by other substances - the interior contains denser materials such that the distribution is ~71%/24%/5%. The atmosphere contains trace amounts of methane, water vapor, ammonia, and "rock". There are also traces of carbon, ethane, hydrogen sulphide, neon, oxygen, phosphine, and sulphur. The outermost layer of the atmosphere contains crystals of frozen ammonia. Through IR and UV measurements benzene (at a relative mixing ratio of 2x10-9 to hydrogen) and other hydrocarbons have also been found.• This atmospheric composition is very close to the composition of the solar nebula. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. Jupiter's upper atmosphere undergoes differential rotation, an effect first noticed by Giovanni Cassini (1690). The rotation of Jupiter's polar atmosphere is ~5 minutes longer than that of the equatorial atmosphere. In addition, bands of clouds of different latitudes, known as tropical regions flow in opposing directions on the prevailing winds. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 600 km/h are not uncommon. The only spacecraft to have descended into Jupiter's atmosphere to take scientific measurements is the Galileo probe (see Galileo mission). It sent an atmospheric probe into Jupiter upon arrival in 1995, then itself entered Jupiter's atmosphere and burned up in 2003. The Great Red Spot The Great Red Spot is a persistent anticyclonic storm on the planet Jupiter, 22° south of the equator, which has lasted at least 340 years. The storm is large enough to be visible through Earth-based telescopes. It was probably first observed by Giovanni Domenico Cassini, who described it around 1665. This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on February 25, 1979, when the spacecraft was 9.2 million km (5.7 million miles) from Jupiter. Cloud details as small as 160 km (100 miles) across can be seen here. The colorful, wavy cloud pattern to the left of the Red Spot is a region of extraordinarily complex and variable wave motion. To give a sense of Jupiter's scale, the white oval storm directly below the Great Red Spot is approximately the same diameter as Earth. The oval object rotates counterclockwise, with a period of about 6 days. The Great Red Spot's dimensions are 24–40,000 km × 12–14,000 km. It is large enough to contain two or three planets of Earth size. The cloudtops of this storm are about 8 km above the surrounding cloudtops. Storms such as this are not uncommon within the turbulent atmospheres of gas giants. Jupiter also has white ovals and brown ovals, which are lesser unnamed storms. White ovals tend to consist of relatively cool clouds within the upper atmosphere. Brown ovals are warmer and located within the "normal cloud layer". Such storms can last hours or centuries. Before the Voyager missions, astronomers were highly uncertain of its nature. Many believed it to be a solid or liquid feature on Jupiter's surface. Planetary rings Jupiter has a faint planetary ring system composed of smoke-like dust particles knocked from its moons by energetic meteor impacts. The innermost doughnut-shaped ring, called the halo, is almost as thick (20,000km) as it is wide (22,800km). This is followed by the thinnest and brightest main ring, which is made of dust from the satellites Adrastea and Metis. Metis orbits within its fluid Roche limit with Jupiter, and objects not rigidly attached to it may freely fall away from it and into Jupiter's gravitational field. Two wide gossamer rings encircle the main ring, originating from Thebe and Amalthea. Finally, there is a distant and very faint outer ring circling Jupiter backwards—retrograde of its spin. It is not known for certain where the material for this outer ring comes from, but it may be captured interplanetary dust. Magnetosphere Jupiter has a very large and powerful magnetosphere. In fact, if one could see Jupiter's magnetic field from Earth, it would appear five times as large as the full moon in the sky despite being so much farther away. The magnetic field is generated by eddy currents in Jupiter's metallic hydrogen core. This magnetic field collects a large flux of particle radiation in Jupiter's radiation belts, as well as producing a dramatic gas torus and flux tube associated with Io. Jupiter's magnetosphere is the largest planetary structure in the solar system. The ''Pioneer'' probes confirmed that Jupiter's enormous magnetic field is 10 times stronger than Earth's and contains 20,000 times as much energy. The sensitive instruments aboard found that the Jovian magnetic field's "north" magnetic pole is at the planet’s geographic south pole, with the axis of the magnetic field tilted 11 degrees from the Jovian rotation axis and offset from the center of Jupiter in a manner similar to the axis of the Earth's field. The Pioneers measured the bow shock of the Jovian magnetosphere to the width of 26 million kilometres (16 million miles), with the magnetic tail extending beyond Saturn’s orbit. The data showed that the magnetic field fluctuates rapidly in size on the sunward side of Jupiter because of pressure variations in the solar wind, an effect studied in further detail by the two ''Voyager'' spacecraft. It was also discovered that streams of high-energy atomic particles are ejected from the Jovian magnetosphere and travel as far as the orbit of the Earth. Energetic protons were found and measured in the Jovian radiation belt and electric currents were detected flowing between Jupiter and some of its moons, particularly Io. Exploration of Jupiter A number of probes have visited Jupiter. Pioneer flyby missions Pioneer 10 flew past Jupiter in December of 1973, followed by Pioneer 11 exactly one year later. Pioneer 10 obtained the first ever close up images of Jupiter and the Galilean moons, studied its atmosphere, detected its magnetic field, observed its radiation belts and found that Jupiter is mainly liquid. Voyager flyby missions Voyager 1 flew by in March 1979 followed by Voyager 2 in July of the same year. The Voyagers vastly improved the understanding of the Galilean moons and discovered Jupiter's rings. They also took the first close up images of the planet's atmosphere. Ulysses flyby mission In February 1992, Ulysses solar probe performed a flyby of Jupiter at a distance of 450,000 km (6.3 Jovian radii). The flyby was required to attain a polar orbit around the Sun. The probe conducted studies on Jupiter's magnetosphere. Since there are no cameras onboard the probe, no images were taken. In February 2004, the probe came again in the vicinity of Jupiter. This time the distance was much greater, about 240 million km. Galileo mission
Cassini flyby mission In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever made of the planet. On December 19, 2000, the Cassini spacecraft, captured a very low resolution image of the moon Himalia, but it was too distant to show any surface details. New Horizons flyby mission The New Horizons probe and its Atlas V launcher lifted off from Pad 41 at Cape Canaveral Air Force Station, Florida, directly south of Space Shuttle Launch Complex 39, at 2:00 p.m. EST (1900 UTC) on January 19, 2006. New Horizons passed Lunar orbit before midnight EST on the same day, and is scheduled to reach Jupiter in February 2007. It will pass through the Jupiter system at 21 km/s (47,000 mph), with closest approach to Jupiter occurring at approximately 06:00 UTC February 28, 2007. The flyby will come within about 32 Jovian radii (3 Gm) of Jupiter and will be the center of a 4-month intensive observation campaign. New Horizons also has instruments built twenty years after Galileo's - particularly Galileo's cameras, which were evolved versions of Voyager cameras which, in turn, were evolved Mariner cameras. Because of the much shorter distance from Jupiter to Earth, the communications link can transmit multiple loadings of the memory buffer. The mission will actually return more data from Jupiter than Pluto. Imaging of Jupiter began on September 4, 2006. Future probes NASA is planning a mission to study Jupiter in detail from a polar orbit. Named Juno, the spacecraft is planned to launch by 2010. Because of the possibility of a liquid ocean on Jupiter's moon Europa, there has been great interest to study the icy moons in detail. A mission proposed by NASA was dedicated to study them. The JIMO (Jupiter Icy Moons Orbiter) was expected to be launched sometime after 2012. However, the mission was deemed too ambitious and its funding was cancelled. Natural satellites
Galilean moons The orbits of Io, Europa, and Ganymede, the largest moon in the solar system, form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three moons to distort their orbits into elliptical shapes, since each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularize their orbits. This constant tug of war causes regular flexing of the three moons' shapes, Jupiter's gravity stretches the moons more strongly during the portion of their orbits that are closest to it and allowing them to spring back to more spherical shapes when they're farther away. This flexing causes tidal heating of the three moons' cores. This is seen most dramatically in Io's extraordinary volcanic activity, and to a somewhat less dramatic extent in the geologically young surface of Europa indicating recent resurfacing. Classification of Jupiters moons
Life on Jupiter It is considered highly unlikely that there is any Earth-like life on Jupiter, as there is little water in the atmosphere and any possible solid surface deep within Jupiter would be under extraordinary pressures. However, in 1976, before the Voyager missions, Carl Sagan hypothesized (with Edwin Ernest Salpeter) that ammonia-based life could evolve in Jupiter's upper atmosphere. Sagan and Salpeter based this hypothesis on the ecology of terrestrial seas which have simple photosynthetic plankton at the top level, fish at lower levels feeding on these creatures, and marine predators which hunt the fish. The Jovian equivalents Sagan and Salpeter hypothesized were "sinkers", "floaters", and "hunters". The "sinkers" would be plankton-like organisms which fall through the atmosphere, existing just long enough that they can reproduce in the time they are kept afloat by convection. The "floaters" would be giant bags of gas functioning along the lines of hot air balloons, using their own metabolism (feeding off sunlight and free molecules) to keep their gas warm. The "hunters" would be almost squid-like creatures, using jets of gas to propel themselves into "floaters" and consume them.• Trojan asteroids In addition to its moons, Jupiter's gravitational field controls numerous asteroids which have settled into the regions of the Lagrangian points preceding and following Jupiter in its orbit around the sun. These are known as the Trojan asteroids, and are divided into Greek and Trojan "camps" to commemorate the Iliad. The first of these, 588 Achilles, was discovered by Max Wolf in 1906; since then hundreds more have been discovered. The largest is 624 Hektor. Cometary impact
See also | |||||||||||||||||
|
| ||||||||||||||||||
 |
|
| |