Atmosphere of Jupiter
The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide, and water. Although water is thought to reside deep in the atmosphere, its directly-measured concentration is very low. The nitrogen, sulfur, and noble gas abundances in Jupiter's atmosphere exceed solar values by a factor of about three.
The atmosphere of Jupiter lacks a clear lower boundary and gradually transitions into the liquid interior of the planet. From lowest to highest, the atmospheric layers are the troposphere, stratosphere, thermosphere and exosphere. Each layer has characteristic temperature gradients. The lowest layer, the troposphere, has a complicated system of clouds and hazes composed of layers of ammonia, ammonium hydrosulfide, and water. The upper ammonia clouds visible at Jupiter's surface are organized in a dozen zonal bands parallel to the equator and are bounded by powerful zonal atmospheric flows known as jets, exhibiting a phenomenon known as atmospheric super-rotation. The bands alternate in color: the dark bands are called belts, while light ones are called zones. Zones, which are colder than belts, correspond to upwellings, while belts mark descending gas. The zones' lighter color is believed to result from ammonia ice; what gives the belts their darker colors is uncertain. The origins of the banded structure and jets are not well understood, though a "shallow model" and a "deep model" exist.
The Jovian atmosphere shows a wide range of active phenomena, including band instabilities, vortices, storms and lightning. The vortices reveal themselves as large red, white or brown spots. The largest two spots are the [|Great Red Spot] and Oval BA, which is also red. These two and most of the other large spots are anticyclonic. Smaller anticyclones tend to be white. Vortices are thought to be relatively shallow structures with depths not exceeding several hundred kilometers. Located in the southern hemisphere, the GRS is the largest known vortex in the Solar System. It could engulf two or three Earths and has existed for at least three hundred years. Oval BA, south of GRS, is a red spot a third the size of GRS that formed in 2000 from the merging of three white ovals.
Jupiter has powerful storms, often accompanied by lightning strikes. The storms are a result of moist convection in the atmosphere connected to the evaporation and condensation of water. They are sites of strong upward motion of the air, which leads to the formation of bright and dense clouds. The storms form mainly in belt regions. The lightning strikes on Jupiter are hundreds of times more powerful than those seen on Earth, and are assumed to be associated with the water clouds. Recent Juno observations suggest Jovian lightning strikes occur above the altitude of water clouds. A charge separation between falling liquid ammonia-water droplets and water ice particles may generate higher-altitude lightning. Upper-atmospheric lightning has also been observed above the level.
Vertical structure
The atmosphere of Jupiter is classified into four layers, by increasing altitude: the troposphere, stratosphere, thermosphere and exosphere. Unlike the Earth's atmosphere, Jupiter's lacks a mesosphere. Jupiter does not have a solid surface, and the lowest atmospheric layer, the troposphere, smoothly transitions into the planet's fluid interior. This is a result of having temperatures and the pressures well above those of the critical points for hydrogen and helium, meaning that there is no sharp boundary between gas and liquid phases. Hydrogen is considered a supercritical fluid when the temperature is above and the pressure is above.Since the lower boundary of the atmosphere is ill-defined, the pressure level of, at an altitude of about below with a temperature of around, is commonly treated as the base of the troposphere. In scientific literature, the pressure level is usually chosen as a zero point for altitudes—a "surface" of Jupiter. As is generally the case, the top atmospheric layer, the exosphere, does not have a specific upper boundary. The density gradually decreases until it smoothly transitions into the interplanetary medium approximately above the "surface".
The vertical temperature gradients in the Jovian atmosphere are similar to those of the atmosphere of Earth. The temperature of the troposphere decreases with height until it reaches a minimum at the tropopause, which is the boundary between the troposphere and stratosphere. On Jupiter, the tropopause is approximately above the visible clouds. The pressure and temperature at the tropopause are about and. In the stratosphere, the temperatures rise to about at the transition into the thermosphere, at an altitude and pressure of around and. In the thermosphere, temperatures continue to rise, eventually reaching at about, where pressure is about.
Jupiter's troposphere contains a complicated cloud structure. The upper clouds, located in the pressure range, are made of ammonia ice. Below these ammonia ice clouds, denser clouds made of ammonium hydrosulfide,, or ammonium sulfide, , and water are thought to exist. There are no methane clouds as the temperatures are too high for it to condense. The water clouds form the densest layer of clouds and have the strongest influence on the dynamics of the atmosphere. This is a result of the higher condensation heat of water and higher water abundance as compared to the ammonia and hydrogen sulfide. Various tropospheric and stratospheric haze layers reside above the main cloud layers. The stratospheric haze layers are made from condensed heavy polycyclic aromatic hydrocarbons or hydrazine, which are generated in the upper stratosphere from methane under the influence of the solar ultraviolet radiation. The methane abundance relative to molecular hydrogen in the stratosphere is about, while the abundance ratio of other light hydrocarbons, like ethane and acetylene, to molecular hydrogen is about.
Jupiter's thermosphere is located at pressures lower than and demonstrates such phenomena as airglow, polar aurorae and X-ray emissions. Within it lie layers of increased electron and ion density that form the ionosphere. The high temperatures prevalent in the thermosphere have not been explained yet; existing models predict a temperature no higher than about. They may be caused by absorption of high-energy solar radiation, by heating from the charged particles precipitating from the Jovian magnetosphere, or by dissipation of upward-propagating gravity waves. The thermosphere and exosphere at the poles and at low latitudes emit X-rays, which were first observed by the Einstein Observatory in 1983. The energetic particles coming from Jupiter's magnetosphere create bright auroral ovals, which encircle the poles. Unlike their terrestrial analogs, which appear only during magnetic storms, aurorae are permanent features of Jupiter's atmosphere. The thermosphere was the first place outside the Earth where the trihydrogen cation was discovered. This ion emits strongly in the mid-infrared part of the spectrum, at wavelengths between ; this is the main cooling mechanism of the thermosphere.
Chemical composition
| Element | Sun | Jupiter/Sun |
| / | 0.0975 | |
| / | ||
| / | ||
| / | ||
| / | ||
| / | ||
| / | ||
| / | ||
| / | 0.82 | |
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The composition of Jupiter's atmosphere is similar to that of the planet as a whole. Jupiter's atmosphere is the most comprehensively understood of those of all the giant planets because it was observed directly by the Galileo atmospheric probe when it entered the Jovian atmosphere on December 7, 1995. Other sources of information about Jupiter's atmospheric composition include the Infrared Space Observatory, the Galileo and Cassini orbiters, and Earth-based observations.
The two main constituents of the Jovian atmosphere are molecular hydrogen and helium. The helium abundance is relative to molecular hydrogen by number of molecules, and its mass fraction is, which is slightly lower than the Solar System's primordial value. The reason for this low abundance is not entirely understood, but some of the helium may have condensed into the core of Jupiter. This condensation is likely to be in the form of helium rain: as hydrogen turns into the metallic state at depths of more than, helium separates from it forming droplets which, being denser than the metallic hydrogen, descend towards the core. This can also explain the severe depletion of neon, an element that easily dissolves in helium droplets and would be transported in them towards the core as well.
The atmosphere contains various simple compounds such as water, methane, hydrogen sulfide, ammonia and phosphine. Their abundances in the deep troposphere imply that the atmosphere of Jupiter is enriched in the elements carbon, nitrogen, sulfur and possibly oxygen by a factor of 2–4 relative to the Sun. The noble gases argon, krypton and xenon also appear in abundance relative to solar levels, while neon is scarcer. Other chemical compounds such as arsine and germane are present only in trace amounts. The upper atmosphere of Jupiter contains small amounts of simple hydrocarbons such as ethane, acetylene, and diacetylene, which form from methane under the influence of the solar ultraviolet radiation and charged particles coming from Jupiter's magnetosphere. The carbon dioxide, carbon monoxide and water present in the upper atmosphere are thought to originate from impacting comets, such as Shoemaker-Levy 9. The water cannot come from the troposphere because the cold tropopause acts like a cold trap, effectively preventing water from rising to the stratosphere.
Earth- and spacecraft-based measurements have led to improved knowledge of the isotopic ratios in Jupiter's atmosphere. As of July 2003, the accepted value for the deuterium abundance is, which probably represents the primordial value in the protosolar nebula that gave birth to the Solar System. The ratio of nitrogen isotopes in the Jovian atmosphere, to, is, a third lower than that in the Earth's atmosphere. The latter discovery is especially significant since the previous theories of Solar System formation considered the terrestrial value for the ratio of nitrogen isotopes to be primordial.