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Dynamics of Jupiter's Atmosphere

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Dynamics of Jupiter's Atmosphere 6 Dynamics of Jupiter's Atmosphere Andrew P. Ingersoll California Institute of Technology Timothy E. Dowling University of Louisville P eter J. Gierasch Cornell University GlennS. Orton J et P ropulsion Laboratory, California Institute of Technology Peter L. Read Oxford. University Agustin Sanchez-Lavega Universidad del P ais Vasco, Spain Adam P. Showman University of A rizona Amy A. Simon-Miller NASA Goddard. Space Flight Center Ashwin R . V asavada J et Propulsion Laboratory, California Institute of Technology 6.1 INTRODUCTION occurs on both planets. On Earth, electrical charge separa­ tion is associated with falling ice and rain. On Jupiter, t he Giant planet atmospheres provided many of the surprises separation mechanism is still to be determined. and remarkable discoveries of planetary exploration during The winds of Jupiter are only 1/ 3 as strong as t hose t he past few decades. Studying Jupiter's atmosphere and of aturn and Neptune, and yet the other giant planets comparing it with Earth's gives us critical insight and a have less sunlight and less internal heat than Jupiter. Earth broad understanding of how atmospheres work that could probably has the weakest winds of any planet, although its not be obtained by studying Earth alone. absorbed solar power per unit area is largest. All the gi­ ant planets are banded. Even Uranus, whose rotation axis Jupiter has half a dozen eastward jet streams in each is tipped 98° relative to its orbit axis. exhibits banded hemisphere. On average, Earth has only one in each hemi­ cloud patterns and east- west (zonal) jets. All have long-lived sphere. Jupiter has weather patterns ("storms'') that last storms, although Jupiter's Great Red Spot (GRS), which fo r centuries. Earth has stationary weather patterns fixed may be hundreds of years old, seems to be the oldest. to the topography, but the average lifetime of a traveling storm is rvl week. Jupiter has no topography, i.e. , no con­ tinents or oceans; its atmosphere merges smoothly with the 6.1.1 Data Sets planet·s fluid interior. Absorbed sunlight (power per unit area) at Jupiter is only 3.3% that at Earth, yet Jupiter's Early astronomers, using small telescopes with their eyes winds are 3-4 times stronger. The ratio of Jupiter·s internal as detectors. recorded the changing appearance of Jupiter·s power to absorbed solar power is 0.7. On Earth the ratio atmosphere. Their descriptive terms - belts and zones, 4 is 2 X lQ- . Jupiter's hydrologic cycle is fundamentally dif­ brown spots and red spots, plumes, barges, festoons, and ferent from Earth's because it has no ocean, but lightning streamers - are still used. Other terms - describing vorticity, 106 Ingersoll et al. vertical motion. eddy fluxes. temperature gradient . cloud 6.1.2 Scope of t h e Chapter and Role of Models height . and wind shear - have been added. bringing the This chapter reviews the observations and theory of tudy of Jupiter"s atmospheric dynamics to a level similar Jupiter's atmospheric dynamics. Sections 6.2 and 6.3 cover to that of Earth during the pioneering days of terrestrial the banded structures and discrete features. respectively. meteorology everal decades ago. Section 6.4 covers vertical structure and temperatures. ec­ Jupiter bas what is perhaps the most photogenic at­ tion 6.5 discus es lightning and models of moist convection. mosphere in the solar system. :\1ost of the visible contrast Section 6.6 reviews numerical models of the bands and zonal arises from clouds in the 0.7- to 1.5-bar range (see Chap­ jets, and ection 6. 7 review numerical models of the dis­ ter 5). The clouds come in different colors, and usually have crete features. Finally. Section 6. provides a discussion of textme on scales as small as a few tens of kilometers, which outstanding questions and how they might be answered. The is comparable to thee-folding thickness (scale height) of the chapter is aimed at a general planetary science audience. al­ atmosphere. At this resolution. cloud tracking over a few though some familiarity with atmospheric dynamics is help­ 1 ful for the modeling sections. hours yields wind estimates with errors of a few m s- . In contrast. the winds around the GRS and many of the zonal AI3 in the terrestrial atmospheric sciences, validated nu­ 1 merical model are the key to understanding. :\Iodels of jets exceed 100m s- . Winds are measured relative to Sys­ tem lll. a uniform rotation rate with period 9 h 55 m 29.71 s. Jupiter's atmosphere tend to be less complex than models of which is defined by radio emissions that are presumably tied Earth's atmosphere. They nevertheles contain much of the to the magnetic field and thus to the planet"s interior. nonlinear physics associated with large-scale stratified flows in rotating systems. Ideally. the complexity of the models Traditional Earth-based telescopic resolution is 3000 matches that of the observations. so that hypothese can km, which is enough to image the major atmospheric fea­ be tested cleanly. Some pure fl uid dynamics models, e.g., tures. Pioneers 10 and 11 improved on Earth-based res­ of two-dimensional flows without viscosity, find their best olution, but Voyagers 1 and 2 provided a breakthrough. applications on Jupiter and the other giant planets. Exam­ For cloud tracking. the most important data were the ··ap­ ples include the Kida vortex model. the models of inverse proach·· movies that were recorded during the three months cascades and beta-tUl·bulence. and the statistical mechan­ prior to each of the two encounters in :t\Iarch and July of ical models of two-dimen ional coherent tructures. The e 1979. The spacecraft obtained a view of each feature every models are discu ed in ections 6.6 and 6. 7. "'10 hours as the resolution improved from 500 km to 60 Peek (195 ) is the definitive book for early ob erva­ km. Occasional Yiews of selected features continued down tions of Jupiter· atmosphere. Gehrels (1976) is a collec­ to a resolution (pi.xel ize) of "'5 km. The Voyager infrared tion of chapters by various authors following the Pioneer pectrometer (IRIS) viewed the entire planet at a resolu­ encounters. Rogers (1995) is the modern equivalent of Peek. tion of several thousand kilometers and obtained spectra of There are many review articles (Ingersoll1976b, tone 1976. all the major dynamical features. Galileo obtained les data Williams 19 5. Beebe et al. 19 9. Ingersoll 1990, l\1arcu than Voyager. but the imaging resolution, usually 25 km. 1993, Gierasch and Conrath 1993. Dowling 1995a, Inger oil and the wa,·elengtb coverage were better. In particular. the et al. 1995). As an ensemble. the articles record the variance near-infrared response of the Galileo camera allowed imag­ of expert opinion. As a time series. they record the progress ing in the absorption bands of methane, from which one that has been made and bring clarity to t he remaining unan­ separates cloud at different altitudes. Cassini combined the swered questions. high data rate of Voyager with the broad spectral coverage For a point on the surface of an oblate planet. there are of Galileo. yielding a best resolution of 60 km (the Cassini two definitions of latitude. Planetographic (PG) latitude is data were still being analyzed at the time of thi writing). the elevation angle (relative to the equatorial plane) of the vector along the local vertical. and planetocentric (PC) lati­ Ground-based telescopes and the Hubble pace Tele­ tude is the elevation angle (relative to the equatorial plane) scope (HST) provide a continuous record of Jupiter's cloud of the vector from the planet's center. PG latitude is greater features at everal-month intervals. These data document than PC latitude except at the equator and poles where they the major e\·ents and also the extreme steadiness of the are equal. For Jupiter the maximum difference (4.16°) is at atmosphere. Ground-based telescope provide the highest 46.6° PG latitude. Unless otherwise specified, we use P G pectral resolution. Several trace gases. which provide im­ latitudes in this chapter. portant diagnostics of vertical motion. were discovered from the ground. Earth-based radio observations probe t he deep atmo phere. The HST was essentia l during the collisions of Comet Shoemaker- Levy 9 with Jupiter in 1994. Besides 6.2 BANDED STRUCTURE recording the waves and debris from the collisions, the HST 6.2.1 Belts and Zones defined the prior dynamical state of the atmo phere. Jupiter's visible atmosphere is dominated by banded struc­ The Galileo probe provided profiles of wind. tempera­ tures (Figure 6.1). Traditionally. the white bands are called ture. composition, clouds, and radiation as functions of pre - zones and the dark bands are called belts. The zonal jets sure down to the 22-bar level. but only at one point on the (eastward and westward currents in the atmosphere) are planet. Except at the Galileo probe site. these quantitie are strongest on the boundaries between the belts and zone uncertain below the 1-bar level. The base of the water cloud (Figure 6.2). The zones are anticyclonic, which means they is thought to lie at the 6- or 7-bar level, "'75 km below the have an eastward jet on the poleward side and a westward jet clouds that produce the visible contrast. on the equatorward side (in the reference frame of the planet, Atmospheric Dynamics 107 Figure 6 .1. See Plate 2. Whole disk views of J upiter. T he left image is from Voyager 2 in J une 1979.
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