Structure
Jupiter is one of the four gas giants; that is, it is not
primarily composed of solid matter. It is the largest planet
in the Solar System, having a diameter of 142,984 km at its
equator. Jupiter's density, 1.326 g/cm³, is the second
highest of the gas giant planets, but lower than any of the
four terrestrial planets. (Of the gas giants, Neptune has
the highest density.)
Composition
Jupiter's upper atmosphere is composed of about 90% hydrogen and 10% helium by number of atoms,[4] or 86% H2 and 13% He by fraction of gas molecules—see table to the right. Since a helium atom has about four times as much mass as a hydrogen atom, the composition changes when described in terms of the proportion of mass contributed by different atoms. Thus the atmosphere is approximately 75% hydrogen and 24% helium by mass, with the remaining 1% of the mass consisting of other elements. The interior contains denser materials such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds. There are also traces of carbon, ethane, hydrogen sulphide, neon, oxygen, phosphine, and sulfur. The outermost layer of the atmosphere contains crystals of frozen ammonia.[11][12] Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have also been found.[13]
The atmospheric proportions of hydrogen and helium are very close to the theoretical composition of the primordial solar nebula. However, neon in the upper atmosphere only consists of 20 parts per million by mass, which is about a tenth as abundant as in the Sun.[14] Helium is also depleted, although to a lesser degree. This depletion may be a result of precipitation of these elements into the interior of the planet.[15] Abundances of heavier inert gases in Jupiter's atmosphere are about two to three times that of the sun.
Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other gas giants Uranus and Neptune have relatively much less hydrogen and helium.[16] However, because of the lack of atmospheric entry probes, high quality abundance numbers of the heavier elements are lacking for the outer planets beyond Jupiter.
Mass
Jupiter is 2.5 times more massive than all the other planets in our solar system combined; so massive that its barycenter with the Sun actually lies above the Sun's surface (1.068 solar radii from the Sun's center). Although this planet dwarfs the Earth (with a diameter 11 times as great) it is considerably less dense. A volume equal to 1,317 Earths only contains 318 times as much mass.[17][18]
Theoretical models indicate that if Jupiter had much more mass than it does at present, the planet would shrink. For small changes in mass, the radius would not change appreciably, and above about four Jupiter masses the interior would become so much more compressed under the increased gravitation force that the planet's volume would actually decrease despite the increasing amount of matter. As a result, Jupiter is thought to have about as large a diameter as a planet of its composition and evolutionary history can achieve. The process of further shrinkage with increasing mass would continue until appreciable stellar ignition is achieved in high-mass brown dwarfs around 50 Jupiter masses.[19] This has led some astronomers to term it a "failed star", although it is unclear whether or not the processes involved in the formation of planets like Jupiter are similar to the processes involved in the formation of multiple star systems.
Although Jupiter would need to be about seventy-five times as massive to fuse hydrogen and become a star, the smallest red dwarf is only about 30% larger in radius than Jupiter.[20][21] In spite of this, Jupiter still radiates more heat than it receives from the Sun. The amount of heat produced inside the planet is nearly equal to the total solar radiation it receives.[22] This additional heat radiation is generated by the Kelvin-Helmholtz mechanism through adiabatic contraction. This process results in the planet shrinking by about 2 cm each year.[23] When it was first formed, Jupiter was much hotter and was about twice its current diameter.[24]
Internal structure
Jupiter is thought to have a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen[23]. Beyond this basic outline, there is still considerable uncertainty]]. The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths (see below). The existence of the core is inferred from gravitational measurements[23] indicating a mass of up to twelve times the Earth's mass or roughly 3% of the total mass.[25][22]
The core region is surrounded by dense metallic hydrogen, which extends outward to about 78% of the radius of the planet.[22] It is not known how sharp the boundary is between the core and this layer; the concentration of heavy elements may simply decrease gradually[23]. Rain-like droplets of helium and neon precipitate downward through this layer, depleting the abundance of these elements in the upper atmosphere.[15][26]
Above the layer of metallic hydrogen lies a transparent interior atmosphere of liquid hydrogen and gaseous hydrogen, with the gaseous portion extending downward from the cloud layer to a depth of about 1,000 km.[22] Instead of a clear boundary or surface between these different phases of hydrogen, there may be a smooth gradation from gas to liquid as one descends.[27][28]
The temperature and pressure inside Jupiter increase steadily toward the core. At the phase transition region where liquid hydrogen (heated beyond its critical point) becomes metallic, it is believed the temperature is 10,000 K and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K and the interior pressure is roughly 3,000–4,500 GPa.[22]
Cloud layers
upiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulphide. The clouds are located in the tropopause and are arranged into bands of different latitudes, known as tropical regions. These are sub-divided into lighter-hued zones and darker belts. The interactions of these conflicting circulation patterns cause storms and turbulence. Wind speeds of 100 m/s (360 km/h) are common in zonal jets.[29] The zones have been observed to vary in width, color and intensity from year to year, but they have remained sufficiently stable for astronomers to give them identifying designations.[18]
The cloud layer is only about 50 km deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region. There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning detected in the atmosphere of Jupiter. (Water is a polar molecule that can carry a charge, so it is capable of creating the charge separation needed to produce lightning.)[22] These electrical discharges can be up to a thousand times as powerful as lightning on the Earth.[30] The water clouds can form thunderstorms driven by the heat rising from the interior.[31]
The orange and brown coloration in the clouds of Jupiter are caused by upwelling compounds that change color when they are exposed to ultraviolet light from the Sun. The exact makeup remains uncertain, but the substances are believed to be phosphorus, sulfur or possibly hydrocarbons.[32][22] These colorful compounds, known as chromophores, mix with the warmer, lower deck of clouds. The zones are formed when rising convection cells form crystallizing ammonia that masks out these lower clouds from view.[17]
Jupiter's low axial tilt means that the poles constantly receive less solar radiation than at the planet's equatorial region. Convection within the interior of the planet transports more energy to the poles, however, balancing out the temperatures at the cloud layer.
Great Red Spot and other storms
The best known feature of Jupiter is the Great Red Spot, a persistent anticyclonic storm located 22° south of the equator that is larger than Earth. It is known to have been in existence since at least 1831,[33] and possibly since 1665.[34] Mathematical models suggest that the storm is stable and may be a permanent feature of the planet.[35] The storm is large enough to be visible through Earth-based telescopes.
The oval object rotates counterclockwise, with a period of about 6 days.[36] 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's diameter.[37] The maximum altitude of this storm is about 8 km above the surrounding cloudtops.[38]
Storms such as this are common 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 for hours or centuries.
Even before Voyager proved that the feature was a storm, there was strong evidence that the spot could not be associated with any deeper feature on the planet's surface, as the Spot rotates differentially with respect to the rest of the atmosphere, sometimes faster and sometimes more slowly. During its recorded history it has traveled several times around the planet relative to any possible fixed rotational marker below it.
In 2000, an atmospheric feature formed in the southern hemisphere that is similar in appearance to the Great Red Spot, but smaller in size. This was created when several smaller, white oval-shaped storms merged to form a single feature—these three smaller white ovals were first observed in 1938. The merged feature was named Oval BA, and has been nicknamed Red Spot Junior. It has since increased in intensity and changed color from white to red
Orbit and rotation
The average distance between Jupiter and the Sun is 778 million km (about 5.2 times the average distance from the Earth to the Sun, or 5.2 AU) and it completes an orbit every 11.86 years. The elliptical orbit of Jupiter is inclined 1.31° compared to the Earth. Because of an eccentricity of 0.048, the distance from Jupiter and the Sun varies by 75 million km between perihelion and aphelion, or the nearest and most distant points of the planet along the orbital path respectively.
The axial tilt of Jupiter is relatively small: only 3.13°. As a result this planet does not experience significant seasonal changes, in contrast to Earth and Mars for example.[46]
Jupiter's rotation is the solar system's fastest, completing a rotation on its axis in slightly less than ten hours; this creates an equatorial bulge easily seen through an Earth-based amateur telescope. This rotation produces a centripetal acceleration at the equator of about 1.67 m/s², compared to the equatorial surface gravity of 24.79 m/s²; thus the net acceleration felt at the equatorial surface is only about 23.12 m/s². The planet is shaped as an oblate spheroid, meaning that the diameter across its equator is longer than the diameter measured between its poles. On Jupiter, the equatorial diameter is 9275 km longer than the diameter measured through the poles.[28]
Because Jupiter is not a solid body, its upper atmosphere undergoes differential rotation. The rotation of Jupiter's polar atmosphere is about 5 minutes longer than that of the equatorial atmosphere; three "systems" are used as frames of reference, particularly when graphing the motion of atmospheric features. System I applies from the latitudes 10° N to 10° S; its period is the planet's shortest, at 9h 50m 30.0s. System II applies at all latitudes north and south of these; its period is 9h 55m 40.6s. System III was first defined by radio astronomers, and corresponds to the rotation of the planet's magnetosphere; its period is Jupiter's "official" rotation.
Observation
Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus);[17] however at times Mars appears brighter than Jupiter. Depending on Jupiter's position with respect to the Earth, it can vary in visual magnitude from as bright as −2.8 at opposition down to −1.6 during conjunction with the Sun. The angular diameter of Jupiter likewise varies from 47.1 to 30.6 arc seconds.[48]
Earth overtakes Jupiter every 398.9 days as it orbits the Sun, a duration called the synodic period. As it does so, Jupiter appears to undergo retrograde motion with respect to the background stars. That is, for a period of time Jupiter seems to move backward in the night sky, performing a looping motion.
Jupiter's 12-year orbital period corresponds to the dozen constellations in the zodiac.[18] As a result, each time Jupiter reaches opposition it has advanced eastward by about the width of a zodiac constellation. The orbital period of Jupiter is also about two-fifths the orbital period of Saturn, forming a 5:2 orbital resonance between the two largest planets in the Solar System.
Because the orbit of Jupiter is outside the Earth's, the phase angle of Jupiter as viewed from the Earth never exceeds 11.5°, and is almost always close to zero. That is, the planet always appears nearly fully illuminated when viewed through Earth-based telescopes. It was only during spacecraft missions to Jupiter that crescent views of the planet were obtained.
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