The Meteorology of Jupiter the Visible Features of the Giant Planet Reflect the Circulation of Its Atmosphere

Home , Atmosphere of Jupiter, Jupiter radius, Stratosphere, Troposphere

The Meteorology of Jupiter The visible features of the giant planet reflect the circulation of its atmosphere. A model reproducing those features should apply to other planetary atmospheres, including the earth's

by Andrew P. Ingersoll

very feature that is visible in a picture mospheres of the two planets consist chiefly inferred ratio of helium to hydrogen in the of the planet Jupiter is a cloud: the of noncondensable gases: hydrogen and he sun (I 15). It is the abundance ratios, to E : dark belts, the light-colored zones lium on Jupiter, nitrogen and oxygen on the gether with the low density of Jupiter as a and the Great Red Spot. The solid surface, earth; mixed in are small amounts of water whole, that suggest that the planet is very if indeed there is one, lies many thousands vapor and other gases that do condense, much like the sun in its composition. of kilometers below the visible surface. Yet forming clouds. In terms of the temperature The amount of heat Jupiter radiates im most of the atmospheric features of Jupiter changes that would occur on the two plan plies that the interior of the planet is hot. If have an extremely long lifetime and an or ets if the condensable vapors were entirely it were cold, there would not be enough ganized structure that is unknown in atmo converted into liquid or solid form, thus heat in the interior to have lasted until the spheric features of the earth. Those differ releasing all their latent heat, Jupiter's at present time. A consequence of the hot-in ences, and the fact that atmospheric fea mosphere would be somewhat less affected terior model of Jupiter is that solids cannot tures are easy to observe on Jupiter, make by condensation than the earth's. Clouds, form in it. According to current thinking, the giant planet a laboratory where terres condensation and precipitation are none the planet is mostly liquid, with a gradual trial meteorologists can test theories about theless important in the dynamics of both transition to a gaseous atmosphere in the atmospheric dynamics in ways not possible atmospheres. outermost few thousand kilometers. on the earth. Spectroscopic data at infrared and radio Jupiter's diameter is 11 times larger than ur picture of the average composition wavelengths also yield information about the earth's; its surface gravity is 2.4 times O and vertical structure of the Jovian the temperature and pressure in the Jovian stronger; it rotates on its axis 2.4 times fast atmosphere is based partly on observation atmosphere. At the deepest levels the tem er (once every 10 hours). There is little or no and partly on theory. Spectroscopic studies perature decreases with altitude at the rate change of season on Jupiter because the axis from the earth have established that the of some two degrees Celsius per kilometer. of the planet's rotation is nearly parallel to atmosphere is mostly molecular hydrogen That rate is close to the adiabatic lapse rate, the axis of its orbit around the sun. (Hz), with smaller amounts of methane which is the value of the temperature gradi Jupiter's atmosphere and interior are (CH4), ammonia (NH3) and a growing list ent in a well-mixed atmosphere. In such an mostly hydrogen, with other elements such of other gases. In those studies absorption adiabatic atmosphere parcels of gas move as helium, carbon, oxygen and nitrogen features in the infrared spectrum of Jupiter vertically without exchanging heat with mixed with the hydrogen in the same pro are compared with absorption features in neighboring parcels. They get cooler or portions as they are in the sun. Because that the spectrum of gases in the laboratory. warmer, but they do so only because their mixture does not solidify at the tempera Since each gas has characteristic wave pressure changes as they ascend or descend. tures and pressures that have been calculat lengths at which it absorbs radiation, ab Neighboring parcels at the same level in the ed to exist on Jupiter, the planet is probably sorption spectra of the Jovian atmosphere atmosphere are indistinguisliable from one gaseous or liquid throughout its interior. make it possible to positively identify most another. An adiabatic temperature gradient The mixture is gently stirred at all depths by of the gases in it even in very small concen also usually implies that the atmosphere is convection currents that carry heat from trations. The exception is helium, which ab stirred by convection. the interior to the surface. Theoretical cal sorbs radiation only in the ultraviolet region The temperature in Jupiter's atmosphere culations indicate that the internal heat is of the spectrum at wavelengths that can is 165 degrees Kelvin (degrees C. above ab most likely left over from Jupiter's initial not be observed through the earth's atmo solute zero) at the level where the pressure gravitational contraction, when it con sphere. Helium was recently detected, how is one atmosphere (the pressure of the densed out of the nebula that also gave rise ever, by an ultraviolet spectrometer on the earth's atmosphere at sea level). The tem to the sun and the other planets. The spacecraft Pioneer 10, and the effect of the perature continues to decrease with height amount of internal energy Jupiter releases helium on the infrared spectra of other gas until it reaches a minimum value of 105 at present is approximately equal to the es in the Jovian atmosphere has also been degrees K. at the level where the pressure is amount of energy it absorbs from the sun. observed. .1 atmosphere. At that point it begins to rise From the point of view of meteorology From the relative strengths of the absorp slightly once again. By analogy with the the most important differences between Ju tion spectra of hydrogen, methane and am earth's atmosphere this minimum marks piter and the earth lie in the fact that Jupiter monia the relative abundances of those gas the beginning of the Jovian stratosphere. In has an appreciable internal energy source es can be determined. On Jupiter the ratio that layer of the Jovian atmosphere the tem and probably lacks a solid surface. The oth of the number of carbon atoms to hydro perature is controlled largely by radiation er differences-in radius, gravity, rate of ro gen atoms is about 1 : 3,000 and the ratio rather than by convection. tation and so on-are mainly differences of of nitrogen atoms to hydrogen atoms is There are thin clouds on Jupiter as high degree. Even the chemical composition of I : 10,000. Those abundances are close to as the base of the stratosphere. Broken thick Jupiter's atmosphere is similar to the chem the abundances of the same elements in the clouds, with holes opening into deeper lev ical composition of the earth's as far as its sun. Within wide limits the ratio of helium els, begin to be present where the pressure is effect on meteorology is concerned. The at- to hydrogen in Jupiter is consistent with the between .6 and one atmosphere. The pres-

46 © 1976 SCIENTIFIC AMERICAN, INC sure at the cloud tops on Jupiter is thus height above an arbitrary base level. His University of Arizona detected water on Ju similar to the pressure at the cloud tops on calculations show that the deepest and piter for the first time by observing radia the earth. The temperatures at those levels thickest clouds are condensed water, since tion that emerged through holes in the up on Jupiter are much lower, however, since both oxygen and hydrogen are abundant per cloud layers from water molecules at Jupiter is fivetimes farther from the sun in the atmosphere. Above those clouds deeper levels. than the earth is. are clouds of ammonium hydrosulfide Lewis' model, which assumes that all the (NH4SH) and above those in turn are substances are at chemical equilibrium at he composition of the Jovian cloud par clouds of pure ammonia (NH3)' The level of each level, cannot account for the colors of Tticles and the nature of the material that each cloud depends on the vapor pressure of the clouds. The cloud particles in that mod endows them with their remarkable colors the particular condensate, which is a strong el are white, whereas the Jovian clouds are cannot be determined from spectroscopy function of temperature. The less volatile subtle shades of red, brown, white and blue. alone. Here theoretical calculations by John substances such as water condense at higher With convection bringing up exotic materi S. Lewis of the Massachusetts Institute of temperatures (deeper in the atmosphere) al from deeper levels, however, and with Technology have been useful. Lewis began than the more volatile substances such as ultraviolet radiation from the sun energiz by assuming that Jupiter's atmosphere has ammonia. The relatively low vapor pressure ing chemical reactions at the top of the at the same composition as the sun's at the of water at the temperatures characteristic mosphere, it is not surprising that portions ranges of temperature and pressure ob of the Jovian cloud tops also explains why of Jupiter's atmosphere do depart from served on the planet, and that all its constit water vapor was found there only recently: local chemical equilibrium. Only small uents are at chemical equilibrium. He then water cannot be detected spectroscopically amounts of coloring material are needed calculated the amount of solid and liquid unless it is in the form of a vapor. In 1974 to explain the observations, and an atmo matter in the atmosphere as a function of Harold P. Larson and his co-workers at the sphere with the same composition as the

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TURBULENCE OF JOVIAN ATMOSPHERE is depicted in a the far right. Where the curve travels to the right of the vertical line computer simulation devised by Gareth P. Williams of Princeton the net flow is eastward, or faster than the planet's mean rotational University, who used one model that attempts to account for Jupi velocity. Where the curve travels to the left of the vertical line the net ter's east-west bands. The lines plotted are streamlines along which flow is westward, or slower than the mean rotational velocity. The the atmosphere flows. Flow is fastest where the lines are closest to relative velocity of the flow, which is up to 50 meters per second, is in gether. The relative direction of the flowis indicated by the graph at dicated by the magnitude of the deviation of curve from vertical line.

47 © 1976 SCIENTIFIC AMERICAN, INC sun's would contain all the necessary ele ments in a wide variety of compounds. It is currently believed that the clouds are basi cally as Lewis has predicted, with the colors explained by the addition of small amounts of sulfur, red phosphorus and complex or ganic molecules. Data on the horizontal structure and mo tions of the Jovian atmosphere are obtained mainly from photographs at visible wave lengths. The data include ground-based ob servations beginning at the end of the 19th century, together with the high-resolution images made from the spacecraftPioneer 10 and Pioneer 11. The ground-based observa tions before 1955 are summarized by B. M. Peek in his classic work The Planet Jupiter. It is evident from surveying these observa tions that most of the principal features of Jupiter's atmosphere have existed for dec ades or longer. Such longevity of cloud pat terns is remarkable by terrestrial standards. On the earth cloud patterns rarely last long er than one or two weeks, unless they are directly connected with underlying topog raphy such as mountain ranges. On Jupiter there is no underlying topography as far as anyone knows. Hence it would be of great interest to know why the Jovian cloud pat terns are so long-lived. Perhaps the simplest and most direct explanation of the longevity of Jupiter's clouds is one provided by Peter J. Gierasch, who is now at Cornell University, and Rich ard M. Goody of Harvard University. Gier asch and Goody point out that on Jupiter the radiative time constant is extremely long. The radiative time constant is the time it takes for a mass of air to warm up or cool off by radiating in the infrared portion of the spectrum. On the earth the radiative time constant is a few weeks, which is com parable to the lifetime of flowfeatures in the atmosphere. On Jupiter the radiative time constant is longer than a year. The differ-

INFRARED FEATURES of the atmosphere of Jupiter (top) photographed from the spacecraft Pioneer 10 reveal the distribution of the temperature at the cloud tops. Light areas are regions of low infrared emission and are evidence of thick, high clouds. Dark areas are regions of high infrared emission and are evidence for holes in upper cloud layer expos ing warmer layers below. When infrared im age is compared with an image made in visible light (bottom), it is evident that the regions of high clouds correspond to the light-colored "zones" and that the regions of few clouds correspond to the dark-colored "belts." Since clouds usually form on warm, rising currents in an atmosphere and disperse on cooler, sinking currents, the zones must cor respond to regions of warm rising gas and the belts to cooler sinking gas. Great Red Spot can be seen at the lower left in both images; it is an area of particularly low infrared emis sion and resembles the zones more closely than the belts. On image made in VIsible light the comblike feature at top left, two square black dots near Great Red Spot and apparent discontinuity running across photograph are artifacts of way in which picture was made.

48 © 1976 SCIENTIFIC AMERICAN, INC ence is the result of the lower temperatures in Jupiter's atmosphere, the smaller amount of heat the planet receives from the sun (only 4 percent of the amount the earth receives) and the fact that the gases of Jupi ter's atmosphere emit less infrared radia tion than the gases of the earth's atmo sphere. Furthermore, if the masses of gas in the Jovian atmosphere store their heat at \ I altitudes as low as the 'base of the clouds, \ I then the radiative time constant may be \ I \ I even longer than a year because of the large volume of gas that must be heated or cooled. Thus the long lifetime of atmo spheric phenomena on Jupiter could be largely due to the low rate at which temper ature differences are radiated away. An equally remarkable aspect of Jovian cloud features is their organization and spa tial regularity. At any one time there are usually at least 10 bands, the belts and the zones, each circling the planet on a line of constant latitude. Such a high degree of symmetry around the axis of rotation does REGIONS OF HIGH AND LOW PRESSURE on Jupiter are compared in a croSS section of not appear in satellite photographs of ter the atmosphere made along a meridian of longitude. Atmospheric pressure increases toward the restrial clouds. center of the planet. At great depths, where the atmosphere rotates uniformly with planet's The belts are either brown or brown mean rotation period, "surfaces" of constant pressure (darker colors) in the atmosphere are tinged with blue, and the zones are either horizontal. At intermediate depths the higher temperatures of the zones cause the surfaces of constant pressure (lighter colors) to bulge upward, creating higher pressures in the zones white or white tinged with red. The reddest (right) than in the belts (left). Coriolis forces generated by the rotation of the planet then cause features, such as the Great Red Spot and the atmosphere to flow in a direction out of the page (westward) along the equatorward edges of smaller spots, are usually located in the the zones, and into the page (eastward) along poleward edges of zones. Slower, secondary cir zones. Infrared observations that measure culation is produced in atmosphere as warmer gas rises in zones and cooler gas sinks in belts. the temperature of the cloud tops suggest that there is a basic difference between the zones and the red spots on the one hand and they do not answer the more basic question can be determined from the rotation of the the belts on the other. of why such features exist. Some clues to the planet's magnetic field, which has a period answer to that question are provided by ob of nine hours 55 minutes 29.7 seconds. The n Jupiter, as on the earth, the most serving the motions of the clouds. From the same rotation period is typical of the clouds O dramatic differencesin temperature earth observers can chart the position of in Jupiter's middle latitudes, although the within a limited area are found between one small spots with respect to the belts and winds at the edges of belts and zones cause level of the atmosphere and another. Since zones and other large features. Photograph small regions to depart from the mean rota in general temperature decreases with alti ic records of the features go back about 100 tion period by as much as five minutes. tude, a high infrared temperature indicates years. The smallest spots that can be seen Within a band of latitudes extending some that at high altitudes there are only thin, from the earth are about 3,000 kilometers in eight degrees on each side of the Jovian transparent clouds or no clouds at all, and a diameter, which is about a fiftieth the diam equator the rotation period of the atmo low infrared temperature indicates that at eter of Jupiter or approximately the diame sphere is about five minutes shorter than the high altitudes there are relatively thick, ter of a large storm system on the earth. rotation period of the magnetic field. The opaque clouds. On the earth weather satel Such storm systems on the earth generally shortest rotation period of all is found away lites utilize that principle for photographing move with the mean wind direction; for ex from the equatorial region. It is associated clouds both in the daytime and at night at ample, in North America they move from with the boundary between a prominent infrared wavelengths: a convective storm west to east. As the storm systems evolve belt and a zone in the northern hemisphere. such as a hurricane always appears as a and decay, however, their apparent motion There the winds complete a circuit of the region of low infrared temperature because with respect to the mean motion of the wind planet once every nine hours 49 minutes. At of its extensive cover of high clouds. sometimes changes. Hence there are prob that boundary the velocity of the clouds Pioneer 10 made two infrared images as it lems in using cloud features to track winds relative to the velocity of Jupiter.'s rotating sped past Jupiter on December 3, 1973. Pio on a large scale. Such features are nonethe interior is about 120 meters per second, or neer 11 made four infrared images a year less the best wind data we have for Jupiter more than 270 miles per hour. later. A comparison of one of the infrared at present. images with a similar image made in the Observed over a period of a few weeks the henever atmospheric motions persist visible region of the spectrum reveals that smaller spots around the Great Red Spot Wfor intervals that are long compared the zones are regions of low infrared tem always move counterclockwise; therefore with a planet's period of rotation, the Co perature and the belts are regions of high the winds around the Great Red Spot prob riolis force must play an important role in infrared temperature. The Great Red Spot ably blow in that direction. Moreover, the the dynamics of the atmosphere. In the has a particularly low infrared temperature, direction and magnitude of the winds are northern hemisphere of Jupiter or the earth and therefore it is similar to the zones rather typical of those observed in the same region the Coriolis force acts to the right of the than to the belts. Hence by analogy with the decades earlier. Elsewhere on Jupiter the direction of motion. In order to balance the earth one can infer that the red spots and wind blows along lines of constant latitude; force there must be a pressure force to the white zones are regions of active convection the largest relative velocities are at the left, that is, there must be a high-pressure and rising motion in the atmosphere, boundaries between belts and zones. area to the right of the flow direction. Thus whereas the brown belts are regions of As on the earth, the relative velocities are on the earth the wind blows counterclock cloud dispersal and sinking motion. small compared with the mean velocity of wise around a low-pressure center (forming Although the infrared observations help the atmosphere associated with the planet's a cyclone) in the Northern Hemisphere and to classify Jupiter's atmospheric features, rotation. The rotation of Jupiter's interior clockwise around a high-pressure center

49 © 1976 SCIENTIFIC AMERICAN, INC (forming an anticyclone). The pattern is re versed in the Southern Hemisphere. For winds moving in latitudinal zones (as at the boundaries between Jupiter's belts and zones) the highs and lows are found in bands between the regions of maximum and minimum zonal velocity. On Jupiter the zones and the Great Red Spot are high-pressure regions (anticyclon ic) and the belts are low-pressure regions (cyclonic). That was firstpointed out in 1951 by Seymour L. Hess of Florida State University and Hans A. Panofsky of New York University. Hence the zones and the Great Red Spot seem to be fundamentally different from terrestrial storms, which are usually cyclonic at sea level. The difference is not, however, as fundamental as it seems. Since clouds tend to form in rising air, and since rising air tends to be warm, it is rea sonable to assume that the zones and the Great Red Spot are warmer than their sur roundings at any particular level within the clouds. In that respect they resemble tropi cal cyclones (hurricanes) and mature ex tratropical cyclones on the earth, most of which are also warm. The resemblance is significant because terrestrial air masses that are warm tend to have high-pressure

EARTH'S ATMOSPHERE IS BAROCLINIC on a global scale, that is, there are large hori centers and anticyclonic circulation at high zontal temperature differences between the Equator and the poles_ As a result of the differences altitudes, the altitudes to which the Jovian a longitudinal wave forms in the atmosphere in both hemispheres, traveling from west (left) to observations refer. east (right) and transporting heat from the Equator to the poles. The troughs (equatorward ex cursions) of the wave are associated with low-pressure regions (L) and the crests (poleward ex he reason that such storms are anticy cursions) of the wave are associated with high-pressure regions (II). The flow of air around Tclonic at great heights is that the pres low-pressure regions is cyclonic and flow of air around high-pressure regions is anticyclonic. sure drop with altitude in a warm air mass is less than the pressure drop with altitude in a cold one. This fact is a consequence of the hydrostatic relation between pressure change and density: when the density is low, as it is when the air is warm, the pressure drop with altitude is also low. Thus a warm air mass tends to become a high-pressure (anticyclonic) region with in creasing altitude. If the pressure is very much lower than the surroundings at low altitudes, as it is with a terrestrial cyclonic storm, the transition to anticyclonic circu lation may not arise. In general, terrestrial hurricanes are strongly cyclonic at sea level and weakly anticylonic at high altitudes. In other words, the observed anticyclonic cir culation of the Jovian zones and the Great Red Spot is consistent with the evidence from infrared observations that they are warm centers of rising motion. Hence in one way they are similar to warm convec tive storms on the earth. As we have seen, however, there is a fun damental difference between the earth and Jupiter in that the earth's atmosphere is bounded at the bottom by a relatively unde formable surface of water and land. The surface can support large differences in air pressure at sea level from one place to an other, and so there can be strong winds close to the ground. We have little informa JUPITER'S ATMOSPHERE IS BAROTROPIC on a global scale, that is, there is very little tion about the atmosphere below the visible horizontal temperature difference between the equator and the poles. Regions of high pressure clouds on Jupiter, but it is reasonable to (II) and low pressure (L) are linear features along parallels of latitude: the zones (white) and assume that below some level it rotates uni the belts (gray). High-speed winds flow at the boundaries between the bands; where relative wind motion is westward (toward the left) the winds are known as retrograde jets. Above a lati formly with a period equal to the rotation tude of 45 degrees in both hemispheres banded pattern breaks down (see il/llstratioll 011 page period of the magnetic field. The situation is 54). Color oval in southern hemisphere is Great Red Spot. Winds around it are anticyclonic. equivalent to saying that the atmosphere

BELT HOT LOW CYCLONIC LOW COLD DOWN LOW. THIN DARK

ZONE COLD HIGH ANTICYCLONIC HIGH HOT UP HIGH. THICK LIGHT

GREAT RED SPOT COLD HIGH ANTICYCLONIC HIGH HOT UP HIGH. THICK ORANGE

CHARACI'ERISTICS OF FEATURES in the Jovian atmosphere the zones in all respects. Taken together the data suggest that all are summarized. The zones and the Great Red Spot are similar in the features are not different and isolated phenomena, and that they all important respects except shape. The belts are the reverse of all must be linked together as part of one global weather pattern.

has a fluidlower boundary that deforms ocean, causing the water to evaporate. The toward the center that matters. The conver easily when pressure is applied from above. moist air close to the surface becomes un gence occurs below the base of the clouds, The deformation prevents the buildup of stable, leading to convection and the forma at levels where precipitation is evaporating. pressure differences at deep levels and tion of cumulus clouds. Large-scale, orga It is this convergence that continuously re therefore keeps the circulation at those lev nized motions are present over the Tropics supplies the zones with condensable va els weak. Thus whereas a warm air mass on because the small-scale cumulus convec por. Such a process is one possible mecha the earth can have either cyclonic or anticy tion, which is the principal means by which nism by which the temperature difference clonic circulation at low altitudes, on Jupi heat is transferred from the ocean to the between zones and belts is maintained. ter the low-altitude circulation must be atmosphere, varies in response to the large zero. In that respect the circulation on Jupi scale motions. The maximum horizontal o far I have said nothing about the circu ter may resemble currents in the earth's temperature difference from one place to s lation of the atmosphere in middle lati oceans more closely than currents in its at another in the Tropics is found between air tudes on Jupiter and on the earth. On the mosphere. Ocean currents tend to be weak that has been heated by condensation and earth the basic energy source of the atmo at great depths and are always anticyclonic air that has not. sphere at middle latitudes is the horizontal near the surface if the water between the According to Gierasch and Barcilon, the temperature gradient. The gradient arises two levels is warm. temperature difference between the zones because the sun heats the Equator far more There have been several attempts to put and the belts on Jupiter is due to the release than it heats the poles. An atmosphere with these arguments on a quantitative basis. In of latent heat, as are the temperature differ such horizontal temperature differencesat 1969 Jeffrey Cuzzi, who was then a gradu ences over the earth's Tropics. Lewis' mod any given altitude is said to be baroclinic. It ate student at the California Institute of el of the clouds is assumed to apply to the is usually unstable. The instability mani Technology, and I decided to study the ob Jovian zones; the belts are assumed to be fests itself as a wavelike pattern in the ba served zonal wind patterns on Jupiter. We dry, without any condensable vapors. The sic eastward flow of the atmosphere. Each first rediscovered the qualitative relation zones are then about two degrees hotter trough of the wave is the point where the air discussed by Hess and Panofsky in 1951. than the belts at all levels above the level makes its maximum excursion toward the We then made a rough estimate of how where water vapor condenses. (Compared Equator and each crest is where it makes its great a difference in temperature between with the clouds composed of water, the maximum excursion toward the poles. The zones and belts would be needed to account clouds of ammonium hydrosulfide and of wave transports heat both upward in the for the observed winds. Actually the quanti ammonia have only a small effect on the atmosphere and toward the poles. The up ty one wants to determine is not the temper temperature.) Thus a temperature differ ward heat transport, in which hot air rises ature difference alone but rather the prod ence of about two degrees extends over a and cold air sinks, tends to lower the center uct of the average temperature difference depth of about 75 kilometers, which is the of mass of the atmosphere, since a given and the depth to which the difference ex total thickness of the water-ammonia cloud volume of cold air is denser and therefore tends. If the observed zonal motions refer to system. Moreover, the product of the tem heavier than the same volume of hot air. the tops of the ammonia clouds in Lewis' perature difference and the depth agrees The heat transport thereby releases gravita model of the Jovian atmosphere, then the with the IS O kilometer-degrees deduced tional potential energy. The troughs and depth must be measured downward from from the wind observations. crests of the baroclinic wave are respective that level. Gierasch and Barcilon did not offer a ly the cyclones and anticyclones that domi On the basis of the observed Jovian winds mechanism by which condensable vapors nate the earth's weather at the middle lati the value of the product of the temperature remain concentrated in the zones. In a trop tudes. difference and the depth is about IS O kilo ical hurricane on the earth the cyclonic An important question is whether or not meter-degrees. In other words, the actual winds exert a cyclonic stress (force per unit such baroclinic waves are present on Jupi depth over which there are appreciable tem area) on the ocean surface. That stress, in ter. If the heat flux from Jupiter's interior perature differences between the zones and teracting with the Coriolis force, causes air were uniform from the equator to the poles, the belts is unknown, but if the depth were to flow inward toward the low-pressure cen the total heat supplied to the atmosphere IS, 150 or 1,500 kilometers, the tempera ter of the hurricane just above the surface from both the sun and the interior would ture difference at those depths would have and causes water to flow outward just below still be twice as great at the equator as at the to be respectively 10 degrees, I or .1 degree the surface. On the earth the flow in the poles, because of the differencein the degree C. in order to account for the observed ocean can be ignored, but the convergence to which those regions are heated by the winds. The magnitude of the temperature of moist air toward the center of the hurri sun. Such a difference in heating could difference is related to the heat sources and cane renews the hurricane's supply of latent cause a considerable amount of baroclinic heat sinks that are maintaining the atmo heat and maintains its circulation. instability if Jupiter were like the earth at spheric circulation. The critical depth is the On Jupiter the cloudy regions are anticy middle latitudes. thickness of the source-sink region. To dis clonic and rotate in a direction opposite to The axially symmetrical, banded appear cuss the situation further we need to consid that of cloudy regions on the earth. There is ance of Jupiter suggests, however, that Jupi er the source-sink mechanism. no ocean; the lower fluid is simply the atmo ter must be quite different from the earth at In 1971 Gierasch, who was then at Flori sphere below the clouds. An anticyclonic those latitudes. Mixing across circles of lati da State University, and Albert I. Barcilon stress from above on the lower fluid causes tude is an essential aspect of baroclinic in suggested that Jupiter's atmosphere is simi the lower fluid to flow inward and the upper stability. And although there are some dis lar to the earth's atmosphere over the Trop atmosphere to flow outward. Here, howev turbances resembling baroclinic waves on ics. There the sun heats the surface of the er, it is the convergence of the lower fluid Jupiter, they are confined to narrow bands

51 © 1976 SCIENTIFIC AMERICAN, INC of latitude and do not seem to give rise to heat in the Jovian atmosphere by baroclinic a role in supplying energy for the circula large-scale mixing between one band and instabilities is insignificant. tion observed within the Jovian belts and another. In addition, if heat transport If that is the case, the difference in solar zones. It has been suggested that the belts toward the poles by baroclinic instability heating between the equator and the poles and zones are simply surface manifestations were the only means of balancing the extra of Jupiter must be balanced by heat from of giant convection cells. The giant cells amount of solar heating at Jupiter's equa the planet's interior. Recently I suggested a might be as deep as they are wide, and tor, an appreciable temperature difference mechanism whereby the difference could be therefore they might extend an appreciable between the equator and the poles would equalized by the convective flow of the in fraction of Jupiter's radius downward develop. ternal heat. The mechanism works like a toward the center of the planet. Peter H. Stone of M.I.T. has a general thermostat. Where the actual vertical tem theory of baroclinic instability in any plane perature gradient in the atmosphere is equal here are several arguments against the tary atmosphere. For Jupiter he estimates to the adiabatic value the convective heat Tconvection-cell hypothesis, although that the equator would be 30 degrees hotter flux is zero. Where the temperature falls none of them is conclusive. First, if Jupiter's than the poles if the solar heat were trans with altitude at a rate slightly higher than atmosphere is indeed analogous to the ported poleward by baroclinic instability the adiabatic rate, however, the convective earth's tropical atmosphere, it does not alone. His estimate was recently tested heat flux is large. Thus a slight cooling of need a deep, large-scale circulation below when the infrared radiometers on Pioneer the poles relative to the equator because of the clouds. Condensable vapor could be 10 and Pioneer 11 obtained the first good the difference in amount of solar energy concentrated in the zones and dispersed in heat measurements of the Jovian poles. absorbed soon leads to a large increase in the belts in a shallow layer just below the The measurements showed that the heat the convective heat flux to the poles relative base of the clouds. The temperature differ emitted by Jupiter is about the same over to the flux to the equator. The convective ences arising from the condensation within both the poles and the equator. This implies thermostat ensures that Jupiter is very near the clouds seem fully capable of accounting that the poles are at about the same temper ly adiabatic throughout. Hence the relation for the observed winds. Therefore the belts ature as the equator at the same levels of between temperature and pressure at both and zones could well be a superficial phe pressure in the atmosphere. In fact, the ob the equator and the poles must be very near nomenon, extending only slightly deeper served temperature difference is no greater ly equal. The possibility that there is greater than the base of the deepest clouds. than three degrees, as compared with the convective flow of internal heat at the jovi Second, if the belts and zones were asso value of 30 degrees predicted by Stone's an poles seems to be confirmed by the ap ciated with the large-scale convection of in baroclinic-instability model. The implica pearance of convectionlike patterns north ternal heat, one would expect that the infra tion is that Jupiter is not like the earth at of a latitude of about 45 degrees. red emission from Jupiter would be strong middle latitudes, and that the transport of The internal heat source might also play est in the zones, since the zones are the site

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VERTICAL CLOUD STRUCTURE of the atmosphere of Jupiter is based on an analysis of infrared data made by Glenn S. Orton has been computed from a theoretical model.devised by John S. Lew of the Jet Propulsion Laboratory of the California Institute of Tech is of the Massachusetts Institute of Technology. In an atmosphere nology. The black broken line indicates the theoretical temperature composed primarily of the noncondensable gases hydrogen and heli of the atmosphere at the same depths and pressures if the atmosphere um there are distinct layers of clouds. The color line indicates the were perfectly adiabatic (completely mixed by convection). The temperature of the atmosphere at various depths and pressures; it planet's atmosphere is nearly adiabatic except at uppermost levels.

52 © 1976 SCIENTIFIC AMERICAN, INC of rising motion. It is true that we can ob serve the heat flux only at the top of the atmosphere; nevertheless, the zones do not seem to be the principal regions of infrared (j) emission. In fact, it is the belts that both I Z , emit the greater amount of infrared energy :::> and absorb the greater amount of solar en > c: I� ergy. On the other hand, the net flux, that is, <{ c: ----.... the difference between the amount of energy I m I --.. emitted and the amount absorbed, is about c: --...... equal in the zones and the belts. This equali � -- x ...... , ty implies that the internal heat flux at the -':::> base of the clouds is the same in the belts lL 1 and the zones, provided that there is no > t? � c: latitudinal heat transport within the clouds. w � The point is that the radiative-fluxdata pro z w � vide no evidence to support the convection I Z cell hypothesis, even though they do not <{ o " rule it out. <{ , Third, the convection-cell hypothesis has c: i I more than its share of theoretical difficul ties. Convection is typically a small-scale 1 phenomenon. In the sun or in the earth's 60 70 8090 atmosphere the size of the dominant ener o 10 20 30 40 50 LATITUDE (DEGREES) gy-carrying cells is on the order of one scale height, that is, the vertical distance in which the density and the pressure increase by a factor of e, or 2.718. On Jupiter the scale height is a few tens of kilometers, and the belts and zones are some 10,000 kilometers wide. Moreover, on a rotating body convec tion patterns tend to develop a longitudinal structure, just the opposite of what is ob served on Jupiter. Such arguments suggest that small-scale convection is the dominant mode of heat transfer, at least below the (j) clouds, where radiation and condensation I Z are unimportant. :::> > c: here are other ways, however, in which <{ c: I TJupiter's belts and zones might be part m of deep pattern of convection extending c: a � down a significant fraction of the planet's x radius. Several theoretical and laboratory :::>-' studies have shown that convection in a ro lL > tating sphere usually takes the form of long, t? c: thin columns, the axes of which are parallel w z to the axis of rotation. The two ends of each w column intersect the visible surface of the !z sphere at opposite latitudes in the northern <{ o and southern hemispheres. Friedrich H. <{ Busse of the University of California at Los c: Angeles has suggested that the belts and zones are the surface manifestation of such long, thin columns. So far the columns have been shown to exist only in laboratory ex periments with comparatively viscous liq uids. Extending the hypothesis to cover the compressible gases on a rotating sphere the o 10 20 30 40 50 60 70 8090 size of Jupiter is therefore somewhat specu LATITUDE (DEGREES) lative. Whether the belts and zones are a shal RADIATION BALANCE in the atmosphere of the earth (top) and of Jupiter (bottom) shows low phenomenon confined to the cloud lay dramatic differences between the planets. For both planets the amount of thermal radiation ers or are part of a deep convection pattern, emitted (color) is nearly independent of latitude, whereas the amount of sunlight absorbed there is still no satisfactory explanation of (black) is strongly affectedby the oblique angle of the sun at the poles. The earth has a negligi why they are oriented in the east-west direc ble source of internal heat, and so the areas under the two curves for the earth are equal. The ex tion. The earth's tropical atmosphere also is cess amount of solar heating at the earth's Equator is carried poleward by currents in the at banded, with a cloudy zone around the mosphere and ocean. Jupiter has an appreciable source of internal heat: it emits 1.9 times as much energy as it receives from the sun. Thus the area under the color curve for Jupiter is 1.9 Equator and clear belts to the north and times larger than the area under the black curve. In addition very little heat is carried poleward south. The equatorial intertropical conver by currents in Jupiter's atmosphere. Hence Jupiter's internal heat must reach the surface gence zone is a band of cumulus clouds and preferentially at the poles. The smaller bumps in the curves for Jupiter are created by the belts thunderstorms that generally occupies a and zones; more thermal radiation is emitted and more sunlight is absorbed in the belts than in strip some five to 10 degrees north of the the zones. Curves for both planets are averaged relative to longitude, time of day and season.

53 © 1976 SCIENTIFIC AMERICAN, INC Equator. It is a region of net upwelling in instabilities develop and soon the middle neglect the dependence of velocity on alti the atmosphere, even though heat is carried latitude circulation is dominated by cy tude. The only available energy in that basic upward to the base of the stratosphere by a clones and anticyclones. On Jupiter the state is kinetic, and it is associated with the system of both updrafts and downdrafts. equatorial regions heat up only slightly rel different velocities at the different latitudes. On each side of the intertropical conver ative to the polar regions, and the internal There are no horizontal temperature gradi gence zone there is a dry belt extending to heat flux readjusts itself by an amount that ents from one place to another at the same 30 degrees north and south of the Equator. is sufficient to balance the difference in the level. The atmosphere is said to be baro There the motion is almost everywhere solar heating. tropic, as opposed to baroclinic, where the downward; updrafts are confined to a thin The instability of the belts and zones is an available energy is mainly gravitational and layer close to the ground. The skies are gen interesting question in its own right. Any is associated with horizontal temperature erally clear and heat is transferred vertically equilibrium state of an atmosphere, except gradients. mainly by infrared radiation. The north the state of uniform rotation, has kinetic ward offset of the intertropical convergence energy and gravitational potential energy. he different velocities of the eastward zone is apparently due to the fact that the A small-amplitude disturbance may be able Twinds can be plotted with respect to continents and oceans are distributed differ to extract some of that energy and grow at latitude to yield a velocity profile. For a ently in the Northern and Southern hemi the expense of the basic state. The basic barotropic atmosphere an instability is pos spheres. state is stable only if all the disturbances sible only when the curvature of the velocity The differencebetween Jupiter and the remain small. It is unstable if there is at profile exceeds a certain critical value. That earth seems to come at higher latitudes. On least one kind of disturbance that continues value is equal to twice the cosine of the Jupiter the banded pattern repeats itself sev to grow. In any real situation all kinds of latitude times the planet's rotation rate eral more times; on the earth the pattern disturbances are always present to some ex divided by the radius. In 1969 Cuzzi and breaks up into baroclinic instabilities. The tent, so that an unstable basic state always I applied the stability criterion to Peek's difference may be due to the thermostatic breaks up or evolves in a finitetime. long-term data on Jupiter's rotation period effect of the internal heat source on Jupiter. The simplest basic state is one in which with respect to latitude. We found that ac On the earth baroclinic instabilities provide an atmosphere flows strictly from west to cording to this criterion there are only a few the only way for the atmosphere to balance east and the wind velocity varies only with latitudes on Jupiter where an instability is the solar heating: the air over the Equator latitude. Superficially that is the case on even marginally possible. The instabilities heats up relative to the air over the poles, Jupiter, although it may not be valid to are found at the center of retrograde jets in

PATTERN OF BANDS BREAKS DOWN at high latitudes on Jupi· latitude where the basic flow between a belt and a zone is theoretical ter, as is shown by this high.resolution image of the planet's north· ly expected to be unstable. Farther north such an instability domi ern hemisphere made from Pioneer 11. The equator is the second nates at all latitudes. Borders of picture are distorted because of rota light zone from the bottom. The disturbed region to the north is at a tion of planet and motion of spacecraft during time picture was made.

54 © 1976 SCIENTIFIC AMERICAN, INC Jupiter's atmosphere: latitudes where the rotation period is the longest. The distur bance that grows the fastest is a longitudi nally oscillating wave. Thus we expect the basic east-west flow to break up into a wavy pattern in the vicinity of the retrograde jets whenever the stability criterion is exceeded.

When the stability criterion is not exceed I � ed, the east-west flow pattern remains con a:: stant in time. a z A confirmation of sorts was provided by Pioneer 11 in its closeup pictures of the Jovi- an northern hemisphere. The pictures show several bands where a wavelike pattern sug gests the location of an instability. The most prominent patterns are seen on the equatorward edges of zones (the poleward edges of belts). According to Peek's data, those same positions are the location of the retrograde I ""'� /;"! jets. In other words, the instability seems to ------: / � ------

55 © 1976 SCIENTIFIC AMERICAN, INC around the spot. Once again the simplest case is for a barotropic atmosphere. The aim of such a study would be to see if there are any special conditions under which red spots can or cannot exist, and to see if the model can predict any details of the flow pattern of such sP0ts that can be compared with observation. The Great Red Spot is embedded in a zone in the Jovian southern hemisphere, and it rotates counterclockwise like a wheel between two oppositely moving surfaces. Northward (equatorward) of the spot, along the northern edge of the zone, the flow is to the west. Southward (poleward) of it the flow is to the east. In 1970, working with a computer model, I found that the rolling-wheel configuration was the only one that gave a steady (time-independent) configuration of flow. The configuration shows that anticyclonic spots such as the Great Red Spot can exist only in an anticy clonic linear feature such as a zone. That aspect of the computer model is confirmed in the association between red spots and zones in both the northern and the southern hemisphere. The computer model also pre dicts that the Great Red Spot should have pointed tips at its east and west ends where

SMALLER RED SPOT resembling the Great Red Spot is shown in this image made from Pioneer the opposing wind currents of the zone di 10. Such spots, lasting for about two years, are not uncommon and are usually found in the vide. That aspect is confirmed in the close zones. Their similarity to the Great Red Spot suggests that all such features are purely long up pictures of the spot, where the pointed lived meteorological phenomena and are not associated with any solid feature within Jupiter. tips are clearly visible. Recently Tony Max worthy and Larry G. Redekopp of the Uni versity of Southern California have dupli cated additional features of the flow around the Great Red Spot, including the interac tion of other large spots with it over a long period of time. The basic flowis assumed to be barotropic, so that this model suffers from the weakness common to barotropic models: the actual lapse rate must differ from the adiabatic lapse rate by an amount that is inconsistent with other data on Jupi ter's atmosphere.

All the studies of the Great Red Spot dem I\. onstrate that such flow features are possible in a steady state without a solid surface to define the shape of the pattern. In the absence of energy dissipation such a flow feature will last forever. Introducing a small amount of dissipation, as is appropri ate for Jupiter, will not change the flow patterns appreciably. The energy that would need to be dissipated might be com pletely provided by the convergence be tween moist and dry gas and the release of latent heat. Someday I should like to develop a com puter model that works for both Jupiter and the earth. With the appropriate energy sources and sinks, with more stringent boundary conditions, with allowances made for Jupiter's internal heat source and GREAT RED SPOT, photographed from Pioneer 11, is now believed to resemble a terrestrial so on the model should realistically predict hurricane. The observed flow of the atmosphere in the zone and around the spot is anticyclonic, the behavior of the atmosphere of either implying that the pressure within the spot is high. The flow near the top of a terrestrial hurri planet. Having such a model would mean cane is also anticyclonic, implying that it too is a high-pressure region at high altitudes. The that we understood terrestrial meteorology Great Red Spot has been a feature on Jupiter for some three centuries. Hydrodynamic models of the spot show that in the absence of the dissipation of energy such a flow feature could at a much deeper level, since the universali in principle last forever. Even a small amount of dissipation will not affect it appreciably, ty of the principles involved would have and on Jupiter the energy that would need to be dissipated could be completely provided been tested by their applicability in two in by the release of latent heat from the rising gas within the zone condensing to form clouds. dependent atmospheric systems.

Kenting Earth Sciences limited, Ottawa, Canoda

Silver halide imagery, among other Let them take heart. All existing genetics. For doing science, a disper uses, iUuminates mankind's path from aerial photography has not yet been sion like this of near-perfect AgBr its past towards its future. Few traces drained of ethnographic excitement. cubes-ranging narrowly in edge length of our forebears are as obvious as For the sparks to fly, the photography size around 4 micrometers-has come a Stonehenge. Nor does aerial pho and a specific conceptual framework long way from the wild type of AgX tography merely lead the ethnographers must strike together. If you've got the crystal on which practical photography and archaeologists to interesting sites. concepts, we may have hints on where based itself until rather recently. This is They need it as much to see why the to find the photography. Just please quite aside from the uniquely retentive sites were chosen. Their disciplines are don't explain the concepts. Our mind is memory that such a crystal has for a useful, with pertinence to any regional on a balance sheet. State question to few photons of light. The practical con planning commission caught between E. G. Tibbils, Aerial Mapping Markets, sequences thereof have kept numerous the environmentalists and the booster Kodak, Rochester, N.Y. 14 650. scientists the world around in meat and types. potatoes. So intensely has interest been Homo sapiens uses grass fires or nu focused on this stuff that scientists pur clear fission, a lichen uses rock-dissolv suing pure, fundamental studies in ad ing acids, and both organisms change sorption phenomena, dielectric loss, the surface of Earth in pursuing their electrophoresis, light scatter, etc. like to respective needs. Seeking a longer per work with silver halide and its extensive spective and better guide to understand literature. If you want the name of ing human needs than the bottom line someone whose interests run in that di on an accountant's balance sheet, schol rection, try Dr. A. H. Herz, Kodak ars study photographs of our old plan Research Laboratories, Rochester, N.Y. et's surface for subtle clues left in geo 14650. logically recent times by our kind. The aerial camera is the tool for surface study, just as the shovel and brush are for vertical exploration, but the price of air-survey contracts sometimes frus Silver halide itself is to colloid science trates scholars when dealing with peo and surface chemistry almost what the ple interested in balance sheets. colon bacillus and the fruit fly are to

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