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 five times 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.