Francesco Ficoroni

Cloud pattern on Jupiter

The Jovian atmosphere is the largest planetary atmosphere in the Solar System. It is primarily made of hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide and water. The latter two are not directly observed and are thought to reside deep in the atmosphere. The abundances of the oxygen, nitrogen and sulfur as well as noble gases in the atmosphere of Jupiter are thought to exceed solar values by a factor of about three.

The atmosphere of Jupiter is divided in a number of layers by altitude (the troposphere, stratosphere, thermosphere and exosphere) which are distinguished by different temperature trends. It lacks a clear lower boundary and gradually transitions into the fluid interior of the planet. The lowest layer, the troposphere, has a complicated system of clouds and hazes, which comprises cloud layers of ammonia, ammonium hydrosulfide and water. The upper ammonia clouds visible at Jupiter's "surface" are organized in a dozen of zonal bands, parallel to the equator, that are bounded by powerful zonal atmospheric flows (winds) known as jets. 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 generally warmer belts mark descending air. The origins of the banded structure and jets are not well understood, though two models exist. The shallow model holds that they are surface phenomena overlaying stable interior. In the deep model, the bands and jets are just surface manifestations of the deep circulation in the outer molecular envelope, which is organized in a number of cylinders.

The Jovian atmosphere shows a wide range of active phenomena, including band instabilities, vortices (cyclones and anticyclones), storms and lightning. The vortices reveal themselves as large red, white or brown spots (ovals). The largest of them are predominately anticyclonic and include such structures as the Great Red Spot (GRS) and Oval BA, which are red in color. Other anticyclones tend to be white. GRS located to the south of the equator is the largest known vortex in the Solar System and has existed for at least three hundred years. Oval BA, which formed only in 2000 by merge of three white ovals, is a smaller relative of GRS. It is located in southern mid latitudes. Vortices are thought to be relatively shallow structures with depths not exceeding several hundred kilometers.

Jupiter demonstrates powerful storm activity which is always 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 processes within them lead to lightning activity. The storms form mainly in belt regions. The lightning strikes on Jupiter are more powerful than those on Earth. However, there are fewer of them and the average levels of lightning activity are comparable on two planets.

Chemical composition

Time-lapse sequence from the approach of Voyager I to Jupiter

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 gas giants 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 (ISO), the Galileo and Cassini orbiters, and ground-based observations.

The two main constituents of the Jovian atmosphere are molecular hydrogen and helium. The helium abundance is 0.157 ± 0.0036 relative to the molecular hydrogen by number of molecules, and its mass fraction is 0.234 ± 0.005, which is slightly lower than the primordial value. The atmosphere contains various simple compounds such as water, methane (CH₄), hydrogen sulfide (H₂S), ammonia (NH₃) and phosphine (PH₃). Their abundances in the deep (below 10 bar) troposphere imply that the atmosphere of Jupiter is enriched in the elements nitrogen, sulfur and possibly oxygen by factor of 2–4 as compared to the Sun's values. The noble gases argon, krypton and xenon appear to be enriched relative to solar abundances as well (see table), while neon is scarcer. Other chemical compounds such as AsH₃, GeH₄ are present only in trace amounts. The upper atmosphere of Jupiter contains small amounts of simple hydrocarbons such as ethane, acetylene, and diacetylene, which are thought to 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 part of the atmosphere is thought originate from comets crashing into the planet, such as comet Shoemaker-Levy 9. The water can not come from the troposphere because the cold tropopause acts like a cold trap, effectively preventing water from reaching the stratosphere (see Vertical structure below).

Earth- and spacecraft-based measurements have led to an improved knowledge about isotopic ratios in Jupiter's atmosphere. As of July 2008, the accepted value for the deuterium abundance is 2.25 ± 0.35, which is thought to represent the primordial value in the protosolar nebula that gave birth to the Solar System. The ratio of nitrogen isotopes in the Jovian atmosphere, N¹⁵ to N¹⁴, is 2.3, a third lower than in Earth's atmosphere (3.5). 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.

Vertical structure

Vertical structure of the atmosphere of Jupiter

The atmosphere of Jupiter can be divided into four layers: 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 above those of the critical points, meaning that there is no boundary to differentiate between gases and liquids.

Since the lower boundary of the atmosphere is ill-defined, the pressure level of 10 bars, at an altitude of about 90 km below the 1 bar pressure level with a temperature of around 340 K, is commonly treated as the base of the troposphere. In scientific literature, the 1 bar pressure level is usually chosen as a zero point for altitudes—a “surface” of Jupiter. The top atmospheric layer, the exosphere, does not have a well defined upper boundary either. The density gradually decreases until one typical for the interplanetary space is reached about 5,000 km above 1 bar pressure level.

The temperature variations in the Jovian atmosphere have similar behaviors to the atmosphere of Earth. The temperature of the troposphere decreases with height until it reaches a minimum called tropopause, which is the boundary between the troposphere and stratosphere. On Jupiter, the tropopause is located at the altitude of approximately 50 km above the visible clouds (or 1 bar level), where the pressure is about 0.1 bars and temperature 110 K. In the stratosphere, the temperatures rise to about 200 K at the transition into the thermosphere, transition which is located at an altitude of around 320 km and has a pressure of about 1 μbar. In the thermosphere, temperatures continue to rise, eventually reaching 1000 K at the altitude of about 1000 km, where pressure is about 1 nbar.

Jupiter's troposphere contains a complicated cloud structure. The visible clouds, located in the pressure range 0.7–1.5 bar, are made of ammonia ice. Below these ammonia ice clouds, clouds made of ammonium hydrosulfide or ammonium sulfide (between 2–4 bar) and water (5–7 bar) 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. The latter is caused by the higher condensation heat of the water and the higher water abundance as compared to the ammonia and hydrogen sulfide (the oxygen is more abundant chemical element than either nitrogen or sulfur). Various tropospheric (at 0.2 bar) and stratospheric (10 mbar) haze layers reside above the main cloud layers. The latter are made from condensed heavy hydrocarbons, which are generated in the upper stratosphere (1–100 μbar) from methane under the influence of the solar ultraviolet radiation. The methane abundance relative to molecular hydrogen in the stratosphere is about 10⁻⁴, while the abundances (to molecular hydrogen) of other light hydrocarbons like ethane and acetylene are of order of 10⁻⁶.

Jupiter's thermosphere is located at the pressures lower than 1 mubar and demonstrates such phenomena as airglow, polar aurorae and X-ray emissions. Within it lie layers of increased electron and ion density which form the ionosphere. The high temperatures prevalent in the thermosphere (800–1000 K) have not been fully explained yet; existing models predict a temperature no higher than about 400 K. They may be caused by absorption of high-enegy solar radiation (UV or X-ray), by heating from the extrajovian particles captured by the magnetosphere, or by dissipation of upward-propagating gravity waves. The thermosphere and exosphere at poles as well as low latitudes emit X-rays, which were first observed by the Einstein Observatory in 1983. The falling particles captured by Jupiter's magnetosphere create bright aurorae which encircle the poles. Unlike their terrestrial analogs which appear only during magnetic storms, the aurorae are permanent features of the Jupiter's atmosphere. The thermosphere was the first place outside Earth where the trihydrogen cation (H₃⁺) was discovered. This ion produces strong emissions in the mid-infrared part of the spectrum, at the wavelengths between 3–5 μm, and is the main cooler of the thermosphere.

Zones, belts and jets

See also: Atmospheric circulation

The visible surface of Jupiter is divided in a number of bands parallel to the equator. There are two types of bands: lightly colored zones and relatively dark belts. The wide Equatorial Zone (EZ) extends between latitudes of approximately 7°S to 7°N and. Above and below the EZ, the North and South Equatorial belts (NEB and SEB) extend to 18°N and 18°S, respectively. Farther from the equator lie the North and South Tropical zones (NtrZ and STrZ). The alternating pattern of belts and zones continues until the polar regions at approximately 50 degrees latitude, where their visible appearance becomes somewhat muted. The basic belt-zone structure probably extends well towards the poles or at least to 80° North or South.

The difference in the appearance between zones and belts is caused by differences in the opacity of the clouds. Ammonia concentration is higher in zones, which leads to the appearance of denser clouds of ammonia ice at higher altitudes, which in turn leads to their lighter color. On the other hand, in belts clouds are thinner and are located at lower altitudes. The upper troposphere is colder in zones and warmer in belts. The exact nature of chemicals that make Jovian zones and bands so colorful is not known, but they may include complicated compounds of sulfur, phosphorus and carbon.

The Jovian bands are bounded by zonal atmospheric flows (winds), called jets. The westward (retrograde) jets are found at the transition from zones to belts (going away from the equator), whereas eastward (prograde) jets mark the transition from belts to zones. Such flow velocity patterns mean that the zonal winds decrease in belts and increase in zones from the equator to the pole. Therefore wind shear in belts is cyclonic, while in zones it is anticyclonic. The EZ is an exception to this rule, showing a strong eastward (prograde) jet and has a local minimum of the wind speed exactly at the equator. The jet speeds are high on Jupiter, reaching more than 100 m/s. These speeds correspond to ammonia clouds located in the pressure range 0.7–1 bar. The prograde jets are generally more powerful than the retrograde jets. The vertical extent of jets is not known. They decay over two to three scale heights above the clouds, while below the cloud level, winds increase slightly and than remain constant down to at least 22 bar—the maximum operational depth reached by the Galileo probe.

The origin of Jupiter's banded structure is not completely clear. The simplest interpretation is that zones are sites of atmospheric upwelling, whereas belts are manifestations of downwelling. When air enriched in ammonia rises in zones, it expands and cools, forming high and dense clouds. In belts, however, the air descends, warming adiabatically, and white ammonia clouds evaporate, revealing lower, darker clouds. The band-jet structure on Jupiter is remarkably stable, having changed only rarely between 1980 and 2000. One example of change is a slight decrease of the speed of the strongest eastward jet located at the boundary between the North Tropical zone and North Temperate belts at 23°N.

Idealized illustration of Jupiter's cloud bands; they are labeled with their official abbreviations. Lighter zones are on the right, and darker zones are indicated on the left. The Great Red Spot and Oval BA are shown in the South Tropical Zone and South Temperate Belt, respectively.

Specific bands

The belts and zones that divide Jupiter's atmosphere each have their own names and unique characteristics.

The North and South Polar Regions extend from the poles to roughly 40–48° N/S. These bluish-gray regions are usually featureless.

The North North Temperate Region rarely shows more detail than the polar regions, due to limb darkening, foreshortening, and the general diffuseness of features. That said, the North-North Temperate Belt (NNTB) is the northernmost distinct belt, though it occasionally "disappears". Disturbances tend to be minor and short-lived. The North-North Temperate Zone (NNTZ) is perhaps more prominent, but also generally quiet. Other minor belts and zones in the region are occasionally observed.

The North Temperate Region is part of a latitudinal region easily observable from Earth, and thus has a superb record of observation. It also features the strongest prograde jet stream on the planet—a westerly current that forms the southern boundary of the North Temperate Belt (NTB). The NTB fades roughly once a decade (this was the case during the Voyager encounters), making the North Temperate Zone (NTZ) apparently merge into the North Tropical Zone (NTropZ). Other times, the NTZ is divided by a narrow belt into northern and southern components.

The North Tropical Region is comprised of the NTropZ and the North Equatorial Belt (NEB). The NTropZ is generally stable in coloration, changing in tint only in tandem with activity on the NTB's southern jet stream. Like the NTZ, it too is sometimes divided by a narrow band, the NTropB. On rare occasions, the southern NTropZ plays host to "Little Red Spots". As the name suggests, these are northern equivalents of the Great Red Spot. Unlike the GRS, they tend to occur in pairs and are always short-lived, lasting a year on average; one was present during the Pioneer 10 encounter.

The NEB is one of the most active belts on the planet. It is characterized by anticyclonic white ovals and cyclonic "barges" (also known as "brown ovals"), with the former usually forming farther north than the latter; as in the NTropZ, most of these features are relatively short-lived. Like the South Equatorial Belt (SEB), the NEB has sometimes dramatically faded and "revived".

Zones, belts and vortices on Jupiter. The wide equatorial zone is visible in the center surrounded by two dark equatorial belts (SEB and NEB). The large grayish-blue irregular "hot spots" at the northern edge of the white Equatorial Zone change over the course of time as they march eastward across the planet. At the south margin of the SEB Great Red Spot can be found. Strings of small storms rotate around northern-hemisphere ovals. Small, very bright features appear quickly and randomly in turbulent regions, candidates for lightning storms. The smallest visible features at the equator are about 600 kilometers across. This 14-frame animation spans 24 Jovian days, or about 10 Earth days. Motion occurs at about 600,000 times actual speed.

The Equatorial Region (EZ) is one of the more stable regions of the planet, in latitude and in activity. The northern edge of the EZ hosts spectacular plumes that trail southwest from the NEB, which are bounded by dark, warm (in infrared) features known as festoons (hot spots). Though the southern boundary of the EZ is usually quiescent, observations from the late 19th into the early 20th century show that this pattern was then reversed relative to today. The EZ varies considerably in coloration, from pale to an ocher, or even coppery hue; it is occasionally divided by an Equatorial Band (EB). Features in the EZ move roughly 390 km/h (240 mph) relative to the other latitudes.

The South Tropical Region includes the SEB and the South Tropical Zone. It is by far the most active on the planet, as it is home to its strongest retrograde jet stream. The SEB is usually the broadest, darkest belt on Jupiter; however, it is sometimes split by a zone (the SEBZ), and can fade entirely during a SEB Revival cycle. Another characteristic of the SEB is a long train of cyclonic disturbances following the Great Red Spot. Similar to the NTropZ, the STropZ is one of the most prominent zones on the planet; not only does it contain the GRS, but it is occasionally rent by a South Tropical Disturbance (STropD), a division of the zone that can be very long-lived; the most famous one lasted from 1901 to 1939.

The South Temperate Region, or South Temperate Belt (STB), is yet another dark, prominent belt, more so than the NTB; until March 2000, its most famous features were the long-lived white ovals BC, DE, and FA, which have since merged to form Oval BA ("Red Jr."). The oval actually were part of South Temperate Zone, but they extended into STB partially blocking it. The STB has occasionally faded, apparently due to complex interactions between the white ovals and the GRS. The appearance of the South Temperate Zone (STZ)—the zone in which the white ovals originated—is highly variable.

The South South Temperate Region is difficult to observe from Earth, even more so than the NNTR; detail is subtle and can only be studied well by large telescopes or spacecraft.

Many zones and belts are more transient in nature and are not always visible. These include Equatorial band (EB); North Equatorial belt zone (a white zone within the belt) (NEBZ); South Equatorial belt zone (SEBZ); and North Tropical zone belt (an additional belt inside the white zone) (NTropZB).

When a disturbance divides a normally singular belt or zone, a N or an S is added to indicate whether the component is a northern or southern one; e.g., NEB(N) and NEB(S).

Dynamics

Circulation in Jupiter's atmosphere is markedly different from that in the atmosphere of Earth. The interior of Jupiter is fluid and lacks any solid surface. Therefore, convection may occur throughout the planet's outer molecular envelope. As of 2008, a comprehensive theory of the dynamics of the Jovian atmosphere has not been developed. Any such theory needs to explain the following facts: the existence of narrow stable bands and jets that are symmetric relative the equator of the planet, the strong prograde jet observed at the equator, the difference between zones and belts, and the origin of large vortices like the Great Red Spot.

The theories regarding the dynamics of the Jovian atmosphere can be broadly divided into two classes: shallow and deep. The former hold that the observed circulation is largely confined to a thin outer (weather) layer of the planet, which overlays the stable interior. The latter hypothesis postulates that the observed atmospheric flows are only a surface manifestation of deeply rooted circulation in the outer molecular envelop of Jupiter. As both theories have their own successes and failures, many planetary scientists actually think that the true theory will include elements of both models.

Shallow models

The first attempts to explain Jovian atmospheric dynamics date back to the 1960s. They were partly based on terrestrial meteorology, which was well developed at that time. Those shallow models assumed that the jets on Jupiter are driven by small scale turbulence, which is in turn maintained by the moist convection in the outer layer of the atmosphere (above the water clouds). The moist convection is phenomenon related to the condensation and evaporation of water and is one of the major drivers of terrestrial weather. The production of the jets in this model is related to a well-known property of two dimensional turbulence—the so-called inverse cascade, in which small turbulent structures (vortices) merge to form larger ones. The finite size of the planet means that the cascade can not produce structures larger than some characteristic scale, which for Jupiter is called the Rhines scale. Its existence is connected to production of Rossby waves. This process works as follows: when the largest turbulent structures reach a certain size, the energy begins to flow into Rossby waves instead of larger structures, and the inverse cascade stops. Since on the spherical rapidly rotating planet the dispersion relation of the Rossby waves is anisotropic, the Rhines scale in the direction parallel to the equator is larger than in the direction orthogonal to it. The ultimate result of the process described above is production of large scale elongated structures, which are parallel to the equator. The meridional extent of them appears to match the actual width of jets. Therefore in shallow models vortices actually feed the jets and should disappear by merging into them.

While these weather–layer models can successfully explain the existence of a dozen narrow jets, they have serious problems. A glaring failure of the the model is the prograde (super-rotating) equatorial jet: with some rare exceptions shallow models produce a strong retrograde (subrotating) jet, contrary to observations. In addition, the jets tend to be unstable and can disappear over the time. Shallow models cannot explain how the observed atmospheric flows on Jupiter violate stability criteria. More elaborated multilayer versions of weather–layer models produce more stable circulation, but many problems persist. Meanwhile, the Galileo probe found that the winds on Jupiter extend well below the water clouds at 5–7 bar and do not show any evidence of decay down to 22 bar pressure level, which implies that circulation in the Jovian atmosphere may in fact be deep.

Deep models

The deep model was first proposed by Busse in 1976. His model was based on another well-known feature of fluid mechanics, the Taylor-Proudman theorem. It holds that in any fast-rotating barotropic ideal liquid, the flows are organized in a series of cylinders parallel to the rotational axis. The conditions of the theorem are probably met in the fluid Jovian interior. Therefore the outer molecular envelope of the planet may be divided into a number of cylinders, each cylinder having a circulation independent of the others. Those latitudes where the cylinders' outer and inner boundaries intersect with the visible surface of the planet correspond to the jets; the cylinders themselves are observed as zones and belts. The deep model easily explains the strong prograde jet observed at the equator of Jupiter; the jets it produces are stable and do not obey the 2D stability criterion. However it has major difficulties; it produces a very small number of broad jets, and realistic simulations of 3D flows are not possible as of 2008, meaning that the simplified models used to justify deep circulation may fail to catch important aspects of the fluid dynamics within Jupiter. One model published in 2004 successfully reproduced the Jovian band-jet structure. It assumed that the molecular envelope is thinner than in all other models occupying only the outer 10% of the Jupiter’s radius; in standard models of the Jovian interior, the envelop extends over the outer 20–30%. The driving of the deep circulation is another problem. In fact, the deep flows can be caused both by shallow forces (moist convection, for instance) or by deep planet wide convection that transports heat out of the Jovian interior. Which of these mechanisms is more important is not clear yet.

Discrete features

Vortices

New Horizons IR view of Jupiter's atmosphere

The atmosphere of Jupiter is the home to hundreds of vortices—circular rotating structures. Like in the Earth’s atmosphere, vortices can be divided into two classes: cyclones and anticyclones. The former rotate in the direction similar to the rotation of the planet (counterclockwise in the northern hemisphere and clockwise in the southern), while the latter—in the reverse direction. However a major difference from the terrestrial atmosphere is that in Jovian atmosphere anticyclones dominate over cyclones, as more than 90% of vorteces larger than 2000 km in diameter are anticyclones. The lifetime of vortices varies from several days to hundreds years depending on their size. For instance, the average lifetime of anticyclones with diameters from 1000 to 6000 km is 1–3 years. Vortices have been never observed in the equatorial region of Jupiter (within 10° of latitude), where they are unstable. As on any rapidly rotating planet, Jupiter's anticyclones are high pressure centers, while cyclones are low pressure.

The anticyclones in Jupiter's atmosphere are always confined within zones, where the wind speed increases in direction from the equator to the poles. They are usually bright and are identified with white ovals. They can move in longitude, but stay at approximately the same latitude unable to escape from the confining zone. The wind speeds at their periphery are about 100 m/s. Different anticyclones located in one zone tend to merge, when they approach each other. However Jupiter has two anticyclones that are somewhat different from all others. They are Great Red Spot (GRS) and Oval BA; the latter formed only in 2000. In contrast to white ovals, these structures are red in color, arguably due to dredging up of red material from the depths. On Jupiter the anticyclones usually form through the merges of smaller structures including convective storms (see below), although large ovals can result from the instability of jets. The latter was observed in 1938–1940, when a few white ovals appeared as a result of instability of the southern temperate zone; they later merged to form Oval BA.

In contrast to anticyclones, the Jovian cyclones tend to be small, dark and irregular structures. Some of the darker and more regular features are known as brown ovals (or badges). However the existence of a few long–lived large cyclones has been suggested. In addition to compact cyclones, Jupiter has several large irregular filamentary patches, which demonstrate cyclonic rotation. One of them is located to the west of the GRS (in its wake region) in the southern equatorial belt. These patches are called cyclonic regions (CR). The cyclones are always located in the belts and tend to merge when they encounter each other much like anticyclones.

The deep structure of vortices is not completely clear. They are thought to be relatively thin, as any thickness greater than about 500 km will lead to instability. The large anticyclones are known to extend only a few tens of kilometers above the visible clouds. The early hypothesis that the vortices are deep convective plumes (or convective columns) as of 2008 is not shared by the majority of planetary scientists.

Great Red Spot

Image of Jupiter obtained by Pioneer 10 in 1974

The Great Red Spot (GRS) is a persistent anticyclonic vortex on the south border of the South Equatorial belt. It appears to be a remarkably stable feature, and most sources concur that it has been continuously observed for 300 years.

The GRS rotates counterclockwise, with a period of about six Earth days or 14 Jovian days. Its dimensions are 24–40,000 km west to east and 12–14,000 km south to north. The spot is large enough to contain two or three planets the size of Earth. At the start of 2004, the Great Red Spot had approximately half the longitudinal extent it had a century ago, when it was 40,000 km in diameter. At the present rate of reduction it would become circular by 2040, although this is unlikely because of the distortion effect of the neighboring jet streams. It is not known how long the spot will last, or whether the change is a result of normal fluctuations.

Approximate size comparison of Earth and the GRS

Infrared data has long indicated that the Great Red Spot is colder (and thus, higher in altitude) than most of the other clouds on the planet; the cloudtops of the GRS are about 8 km above the surrounding clouds. Furthermore, careful tracking of atmospheric features revealed the spot's counterclockwise circulation as far back as 1966—observations dramatically confirmed by the first time-lapse movies from the Voyager flybys. The spot is spatially confined by a modest eastward jet stream (prograde) to its south and a very strong westward (retrograde) one to its north. Though winds around the edge of the spot peak at about 120 m/s (430 km/h), currents inside it seem stagnant, with little inflow or outflow. The rotation period of the spot has decreased with time, perhaps as a direct result of its steady reduction in size.

The Great Red Spot's latitude has been stable for the duration of good observational records, typically varying by about a degree. Its longitude, however, is subject to constant variation. Because Jupiter does not rotate uniformly at all latitudes, astronomers have defined three different systems for defining the latitude. System II is used for latitudes of more than 10°, and was originally based on the average rotation rate of the Great Red Spot of 9h 55m 42s. Despite this, the spot has "lapped" the planet in System II at least 10 times since the early nineteenth century. Its drift rate has changed dramatically over the years and has been linked to the brightness of the South Equatorial Belt, and the presence or absence of a South Tropical Disturbance.

An animation of the Great Red Spot

It is not known exactly what causes the Great Red Spot's reddish color. Theories supported by laboratory experiments suppose that the color may be caused by complex organic molecules, red phosphorus, or yet another sulphur compound. The GRS varies greatly in hue, from almost brick-red to pale salmon, or even white. The spot occasionally "disappears", becoming evident only through the Red Spot Hollow, which is its niche in the South Equatorial Belt (SEB). The visibility of GRS is apparently coupled to the appearance of the SEB; when the belt is bright white, the spot tends to be dark, and when it is dark, the spot is usually light. The periods when the spot is dark or light occur at irregular intervals; as of 1997, during the preceding 50 years, the spot was darkest in the periods 1961–66, 1968–75, 1989–90, and 1992–93.

The Great Red Spot should not be confused with the Great Dark Spot, a feature observed near the northern pole of Jupiter in 2000 by the Cassini–Huygens spacecraft. Note that a feature in the atmosphere of Neptune was also called the Great Dark Spot. The latter feature was imaged by Voyager 2 in 1989, and may have been an atmospheric hole rather than a storm and it was no longer present as of 1994 (although a similar spot had appeared farther to the north).

Oval BA

A feature in the South Temperate Belt, Oval BA, was first seen in 2000 after the collision of three small white storms, and has intensified since then. Ovals BC and DE merged in 1998, forming Oval BE. Then, in March 2000, BE and FA joined together, forming Oval BA.

File:Jovian--OvalBA.jpg

Formation of Oval BA

File:Jupiter little red spot NH.png

New Horizons IR view of Oval BA

Oval BA slowly began to turn red in August 2005. The change was not noticed at the time because it was slight and Jupiter was close to solar conjunction. However the change was prominent in December of the same year after the conjunction. On February 24, 2006, Filipino amateur astronomer Christopher Go discovered the change in color and alerted the ALPO Jupiter Section. Richard Schmude Jr., ALPO Jupiter Section coordinator, using the archives of ALPO Japan Kansai division confirmed the change. The color intensified further during these months. In March 2006, it had apparently the same color as the GRS.

In April 2006, a group of professional astronomers led by Dr. Amy Simon-Miller (NASA GSFC), Dr Imke de Pater and Dr Phil Marcus (UC Berkeley) used the Hubble Space Telescope to image both the Great Red Spot and Oval BA. Other professional and amateur astronomers have collaborated with this project as well. The team discovered that the Great Red Spot and Oval BA might converge in 2006. The storms pass each other about every two years, but the passings of 2002 and 2004 did not produce anything exciting. Dr. Amy Simon-Miller, of the Goddard Space Flight Center, predicted the storms would have their closest passing on July 4, 2006. On July 20, the two storms were photographed passing each other by the Gemini Observatory without converging.

Oval BA is getting stronger according to observations made with the Hubble Space Telescope. The wind speeds have reached the 645 km/h (400 mph) mark, which, is about the same as in the Great Red Spot. As of July 2008, its size is about the diameter of Earth—approximately half the size of the Great Red Spot, and it has turned red. As a result, some scientists have begun calling it "Red Spot Jr." or "Red Jr." Dr. Tony Phillips coined this term, but professionals still call it Oval BA. The New Horizons team refer to it as the "Little Red Spot" Oval BA should not be confused with another major storm on Jupiter, the Little Red Spot (or Baby Red Spot) which turned red before the GRS and Oval BA shredded it in late June/early July of 2008.

Storms and lightning

The storms on Jupiter are similar to thunderstorms on Earth. They reveal themselves via bright clumpy clouds about 1000 km in size, which appear from time to time in the belts' cyclonic regions, especially within the strong westward (retrograde) jets. In contrast to vortices, storms are short-lived phenomena; the strongest of them may exist for several months, while the mean lifetime is only 3–4 days. They are believed to be due mainly to moist convection within Jupiter's troposphere. Storms are actually tall convective columns (plumes), which bring the wet air from the depths to the upper part of the atmosphere, where it condenses in clouds. A typical vertical extent of Jovian storms is about 100 km; as they extend from a pressure level of about 5–7 bar, where a hypothetical water cloud layer is located, to as high as 0.2–0.5 bar.

Storms on Jupiter are always associated with lightning. The imaging of the night–side hemisphere of Jupiter by Galileo and Cassini spacecrafts revealed regular light flashes particularly at 51°N, 56°S and 23°N latitudes; higher latitude flashes concentrated near the locations of the westward jets. The lightning strikes on Jupiter are on average more powerful than those on Earth. However they are less frequent and the light power emitted from a given area is similar to that on Earth. A few flashes were detected in polar regions making Jupiter the second planet after Earth to demonstrate polar lightning.

Every 15–17 years Jupiter is rattled by especially powerful storms. They appear at 23°N latitude, where the strongest eastward jet is located. The last such an event was observed in March–June 2007. Two storms appeared in the northern temperate belt 55° apart in longitude. They caused a significant disturbance to the belt. The dark material that was shed by the storms mixed with clouds and changed the belt’s color. The storms moved with the speed as high as 170 m/s, slightly faster than the jet itself, hinting at the existence of strong winds deep in the atmosphere.

Disturbances

The normal pattern of bands and zones is sometimes disrupted for periods of time. One particular class of disruption are long-lived darkenings of the South Tropical Zone, normally referred to as "South Tropical Disturbances" (STD). The longest lived STD in recorded history was followed from 1901 until 1939, having been first seen by Percy B. Molesworth on February 28, 1901. It took the form of darkening over part of the normally bright South Tropical zone. Several other similar disturbances in the South Tropical Zone have been recorded since then.

Observational history

Early astronomers, using small telescopes with their eyes as detectors, recorded the changing appearance of Jupiter’s atmosphere. Their descriptive terms—belts and zones, brown spots and red spots, plumes, barges, festoons, and streamers—are still used. Other terms such as vorticity, vertical motion, cloud heights have entered in use later, in the 20th century.

Traditional Earth-based telescopic resolution is 3000 km and is enough to image the major atmospheric features. Pioneer 10 and Pioneer 11 improved on Earth-based resolution, but Voyagers 1 and 2 provided a breakthrough. The most important data for cloud tracking were the “approach” movies that were recorded during the three months prior to each of the two encounters (in March and July of 1979). The spacecraft obtained a view of each feature every 10 hours as the resolution improved from 500 km to 60 km, while occasional views of selected features continued down to a resolution of about 5 km. The Voyager infrared spectrometer (IRIS) viewed the entire planet at a resolution of several thousand kilometers and obtained spectra of all the major dynamical features.

Galileo obtained less data than Voyager, but the imaging resolution, usually 25 km, and the wavelength coverage were better. In particular, the near-infrared response of the Galileo camera allowed imaging in the absorption bands of methane, from which one separates clouds at different altitudes. Cassini combined the high data rate of Voyager with the broad spectral coverage of Galileo, yielding a best resolution of 60 km (the Cassini data were still being analyzed at the time of this writing).

Today, ground-based telescopes and the Hubble Space Telescope provide a continuous record of Jupiter’s cloud features at several-month intervals. These data document the major events and also the extreme steadiness of the atmosphere. Ground-based telescopes provide the highest spectral resolution. Several trace gases, which provide important diagnostics of vertical motion, were discovered from the ground. Earth-based radio observations probe the deep atmosphere. Hubble was essential during the collisions of comet Shoemaker-Levy 9 with Jupiter in 1994. Besides recording the waves and debris from the collisions, it defined the prior dynamical state of the atmosphere.

At this resolution, cloud tracking over a few hours yields wind estimates with errors of a few m/s. In contrast, the winds around the GRS and many of the zonal jets exceed 100 m/s. Winds are measured relative to a uniform rotation rate (with period 9h 55m 29.71s) defined by radio emissions that are presumably tied to the magnetic field and thus to the planet’s interior. The Galileo probe provided profiles of wind, temperature, composition, clouds, and radiation as functions of pressure down to the 22 bar level, but only at one point on the planet. Except at the Galileo probe site, these quantities are uncertain below the 1 bar level.

Great Red Spot studies

A wider view of Jupiter and the Great Red Spot as seen from Voyager 1 in 1979

The first sighting of GRS is often credited to Robert Hooke, who described a spot on the planet in May 1664; however, it is likely that Hooke's spot was in the wrong belt altogether (the North Equatorial Belt, versus the current location in the South Equatorial Belt). Much more convincing is Giovanni Cassini's description of a "permanent spot" in the following year. With fluctuations in visibility, Cassini's spot was observed from 1665 to 1713.

A minor mystery concerns a Jovian spot depicted in 1711 on a canvas by Donato Creti, which is exhibited in the Vatican. It is a part of a series of panels in which different (magnified) heavenly bodies serve as backdrops for various Italian scenes; the creation of all of them overseen by the astronomer Eustachio Manfredi for accuracy. The Creti's painting is the first known to depict the GRS as red. No Jovian feature was officially described as red before the late 1800s.

The present GRS was first seen only after 1830 and well-studied only after a prominent apparition in 1879. A long 118-year gap separates the observations made after 1830 from its seventeenth-century discovery; whether the original spot dissipated and re-formed, whether it faded, or even if the observational record was simply poor are unknown. The older spots had a short observational history and slower motion than that the modern spot, which make their identity unlikely.

On February 25, 1979, when the Voyager 1 spacecraft was 9.2 million km (5.7 million miles) from Jupiter it transmitted the first detailed image of the Great Red Spot back to Earth. Cloud details as small as 160 km (100 miles) across were visible. The colorful, wavy cloud pattern seen to the west (left) of the GRS is the spot's wake region, where extraordinarily complex and variable cloud motions are observed.

White ovals

File:Red spot jr in color.jpg

An image of Oval BA obtained by New Horizons in February 2007

The formation of the three white ovals, which later merged into Oval BA, can be traced to 1939, when the South Temperate Zone was rent by dark features that effectively split the zone into three long sections. Jovian observer Elmer J. Reese labeled the dark sections AB, CD, and EF. The rifts expanded, shrinking the remaining segments of the STZ into the white ovals FA, BC, and DE.

The white ovals covered almost 90 degrees of longitude shortly after their formation, but contracted rapidly during their first decade; their length stabilized at 10 degrees or less after 1965. Although they originated as segments of the STZ, they evolved to become completely embedded in the South Temperate Belt, suggesting that they moved north, "digging" a niche into the STB. Indeed, much like the GRS, their circulations were confined by two opposing jet streams on their northern and southern boundaries, with an eastward jet to their north and a retrograde westward one to the south.

The longitudinal movement of the ovals seemed to be influenced by two factors: Jupiter's position in its orbit—they became faster at aphelion, and by proximity to the GRS accelerating when ovals were within 50 degrees of the Spot. The overall trend of the white oval drift rate was deceleration, with a decrease by half between 1940 and 1990.

During the Voyager fly-bys, the ovals extended roughly 9000 km from east to west, 5000 km from north to south, and rotated every five days (cp. six for the GRS at the time).

References

• Beebe, Reta (1997). Jupiter the Giant Planet (2nd edition ed.). Washington: Smithsonian Books. ISBN 1560986859. OCLC 224014042 35559012. {{cite book}}: |edition= has extra text (help); Check |oclc= value (help)
• Ingersoll, Andrew P. (2004). "Dynamics of Jupiter's Atmosphere" (pdf). In Bagenal, F.; Dowling, T.E.; McKinnon, W.B. (ed.). Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. {{cite encyclopedia}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: multiple names: editors list (link)
• Yelle, R.V. (2004). "Jupiter's Thermosphere and Ionosphere" (pdf). In Bagenal, F.; Dowling, T.E.; McKinnon, W.B. (ed.). Jupiter: The Planet, Satellites and Magnetosphere. Cambridge University Press. {{cite encyclopedia}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: multiple names: editors list (link)
• Rogers, John H. (1995). The Giant Planet Jupiter. Cambridge: Cambridge University Press. ISBN 0521410088. OCLC 219591510 30357355. {{cite book}}: Check |oclc= value (help)
• Smith, B. A.; et al. (1979). "The Jupiter system through the eyes of Voyager 1". Science. 204: 951–957, 960–972. doi:10.1126/science.204.4396.951. PMID 17800430. Retrieved 2007-06-14. {{cite journal}}: Explicit use of et al. in: |author= (help)
• Vasavada, Ashvin R. (2005). "Jovian atmospheric dynamics: an update after Galileo and Cassini". Reports on Progress in Physiscs. 68: 1935–1996. doi:10.1088/0034-4885/68/8/R06. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

Further reading

• [Numerous authors] (1999). Beatty, Kelly J.; Peterson, Carolyn Collins; Chaiki, Andrew (ed.). The New Solar System (4th edition ed.). Massachusetts: Sky Publishing Corporation. ISBN 0933346867. OCLC 39464951. {{cite book}}: |edition= has extra text (help)CS1 maint: multiple names: editors list (link)
• Hockey, Thomas (1999). Galileo's Planet: Observing Jupiter Before Photography. Bristol, Philadelphia: Institute of Physics Publishing. ISBN 0750304480. OCLC 39733730.
• Peek, Bertrand M. (1981). The Planet Jupiter: The Observer's Handbook (Revised edition ed.). London: Faber and Faber Limited. ISBN 0571180264. OCLC 8318939 9042200. {{cite book}}: |edition= has extra text (help); Check |oclc= value (help)

External links

• Damian Peach. "A Guide to Jovian Activity."
• NASA's Astronomy Picture of the Day—"Jupiter's Two Largest Storms Nearly Collide"
• NASA: Jupiter's New Red Spot
• NASA: Huge Storms About To Converge
• Hubble Space Telescope Image Gallery
• Jupiter 2006
• Website of Red Spot Jr.
• SPACE.com: Surprise! Jupiter Has A New Red Spot
• New Red Spot Growing Fast On Jupiter
• Yang, Sarah (April 21, 2004). "Researcher predicts global climate change on Jupiter as giant planet's spots disappear". UC Berkeley News. Retrieved 2007-06-14.
• Phillips, Tony (March 3, 2006). "Jupiter's New Red Spot". Science at NASA. Retrieved 2007-06-14.
• Phillips, Tony (June 5, 2006). "Huge Storms Converge". Science at NASA. Retrieved 2007-06-14.
• Youssef, Ashraf; Marcus, Philip S. (2003). "The dynamics of jovian white ovals from formation to merger". Icarus. 162 (1): 74–93. doi:10.1016/S0019-1035(02)00060-X. Retrieved 2007-06-15.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Williams, Gareth P. (May 4, 2005). "NOAA Web Page". Geophysical Fluid Dynamics Laboratory. Retrieved 2007-07-21.