Atmospheric dynamics on the outer planets and some of their satellites

ICARUS 3 8 , 3 3 3 - 3 4 1

(1979)

Atmospheric Dynamics on the Outer Planets and Some of Their Satellites G. S. G O L I T S Y N

Institute of Atmospheric Physics, Academy of Sciences of USSR, Moscow 109017, USSR Received September 2, 1978; revised January 2, 1979 A short review of the atmospheric dynamics for the outer planets and some of their satellites with atmospheres is presented. Their physical properties are discussed. A survey of observational data for atmospheric motions on the large planets is presented and similarity parameters are given for all objects. General problems of the vertical structure of atmospheres are then considered with some detailed discussion for rarefied atmospheres on Io and Ganymede. The low densities of these atmospheres make their dynamics similar to those of the thermospheres of the terrestrial planets but with a specific boundary layer. The atmospheric temperature regime must be strongly coupled to that of their surface, and so winds should be of the order of the velocity of sound. Similarities and differences are noted between the dynamics of Titan and possibly of Pluto and the circulation on Venus. For large and rapidly rotating planets, some analogies with the oceans are pointed out. The "soliton" hypothesis is discussed in some detail for circulation perturbations observed on Jupiter's disk. Finally, it is noted that the bimodal rotation period found for Neptune [-D. P. Cruikshank, Asirophys. J. 220, 157-159 (1978)J may be interpreted as an indication of an equatorial jet on the planet with a relative velocity of about 140 m sec-1. Several detailed reviews of the dynamics of planetary atmospheres, in particular, for Jupiter and giant planets, have appeared recently (Stone, 1973, 1976; M a x w o r t h y , 1973; Golitsyn, 1973; Hide, 1976; Ingersoll, 1976). Because of this and due to the short length of this paper it seems proper to pay attention only to some general aspects and to applications to some new objects, i.e., satellites with rarefied a t m o spheres: Io and, possibly, Ganymede. I t seems t h a t Pluto m a y belong here also as it m a y have a methane atmosphere (Cruikshank et al., 1976). The study of the satellites of the outer planets is planned b y the Voyager mission (see Space Sci. Rev. 18, Nos. 2 and 3, 1977); therefore, some estimates of characteristics of thermal and dynamical regimes of their atmospheres seem to be useful.

Reviews of physical properties of the planets and their satellites m a y be found in papers b y N e w b u r n and Gulkis (1973), Morrison and Cruikshank (1974), Morrison and Burns (1976), and Burns (1977). Some properties have been established more or less reliably; others m a y be reliable b u t are being recons~idered. This is especially true for small and distant obj ects. A rich volume of observations of largescale atmospheric motions is available now for Jupiter only. The latest review of ground-based observations is b y Smith and H u n t (1976), and the Pioneer 10 and 11 images are discussed b y Gehrels (1976). A large n u m b e r of high-resolution images is expected from the two Voyager missions in 1979 and m a n y more from the Jupiter Orbiter in 1983. This will allow one to see the small-scale structure which is necessary

333

0019-1035 / 79[060333-09502.00 [0 Copyright O 1979 b y Academic P r ~ , Inc. All rights of reproduction in a n y form reserved.

334

G. S. GOLITSYN TABLE I ASTRONOMICAL PARAMETERS FOR PLANETS AND SATELLITES

Object Earth Jupiter Saturn Uranus Neptune Pluto Io Ganymede Titan

R~ (AU)

e

r (km)

tp (years)

t (hr)

~0

g (m sec-~)

1 5.2 9.5 19.2 30.1 39.5 5.2 5.2 9.5

0.0167 0.0484 0.0577 0.0471 0.0087 0.246

6 378 71 600 60 000 25 900 24 100 1 300 1 820 2 635 2 900

1 11.9 29.5 84 165 248

24 9.925 10.6 15-25 19.5 153 42.4 172 383

23 °27' 307 ' 26°44 ' 87.5 ° 29 ° ~75 °

9.8 24 9.5 8.7 12 0.5 1.8 1.3 1.2

for a n u n d e r s t a n d i n g of t h e p e c u l i a r i t i e s of l a r g e - s c a l e d y n a m i c s ; i t will also h e l p resolve t h e p r i m a r y q u e s t i o n of t h e r e l a t i v e roles of t h e d i f f e r e n t i a l h e a t i n g b y t h e S u n a n d t h e i n t e r n a l h e a t s o u r c e in d r i v i n g t h e a t m o s p h e r i c m o t i o n s . T h i s q u e s t i o n also s t a n d s for S a t u r n . I t s v i s i b l e d i s k h a s t h e same banded appearance as Jupiter's, but t h e p e r t u r b a t i o n s in t h e f o r m of s p o t s a r e o b s e r v e d m u c h less o f t e n (Peek, 1958; A l e x a n d e r , 1962). A t t h e s a m e t i m e t h e d i f f e r e n t i a l r o t a t i o n is m u c h s t r o n g e r on S a t u r n . T h e d a t a for S a t u r n a n d its s a t e l l i t e s , T i t a n in p a r t i c u l a r , will s u p p o s e d l y b e o b t a i n e d b y t h e V o y a g e r s in 1980 t o 1981. S i m i l a r d a t a for U r a n u s a r e a l m o s t n o n e x i s t e n t b e c a u s e of t h e g r e a t e r d i s t a n c e a n d s m a l l e r size of t h e p l a n e t ( A l e x a n d e r , 1965). T h i s i n f o r m a t i o n m a y b e o b t a i n e d b y

A 0.3 0.42 0.6 0.35 0.35 0.4 0.62 0.35 0.27

V o y a g e r 2 in 1986. I t is a s t o n i s h i n g t h a t s o m e d a t a n o w exist o n t h e " w e a t h e r " o n N e p t u n e . W e d i s c u s s t h i s below. The objects under consideration may be d i v i d e d i n t o t w o g r o u p s . T h e first c o m prises t h e f o u r l a r g e p l a n e t s . T h e s e c o n d c o n s i s t s of P l u t o , Io, G a n y m e d e , a n d T i t a n , all r a t h e r s m a l l o b j e c t s w i t h r a d i i a n o r d e r of m a g n i t u d e less t h a n t h a t of U r a n u s o r Neptune. Among them, only Titan has a dense atmosphere. In Table I the basic astronomical parameters of t h e o b j e c t s i m p o r t a n t for a t m o s p h e r i c d y n a m i c s a r e p r e s e n t e d , i.e., m e a n d i s t a n c e s f r o m t h e Sun, R~; o r b i t a l ecc e n t r i c i t i e s , e; e q u a t o r i a l r a d i i , r ; o r b i t a l p e r i o d s , Tp; r o t a t i o n a l p e r i o d s , t = 2 ~ / ~ , w h i c h for t h e s a t e l l i t e s is e q u a l t o t h e p e r i o d of r e v o l u t i o n a r o u n d t h e p l a n e t ; angles between the rotation vector ~ and

TABLE II ATMOSPHERIC PARAMETERS FOR PLANETS AND SATELLITES

Object Earth Jupiter Saturn Uranus Neptune Pluto Io Ganymede Titan

p~ (Pa)

~

Cp (10-3 J kg-~ °K-~)

~,, (°K km ~1)

Te (°K)

H (km)

c (m sec-~)

105 7 X 104 106 105 105 ~10 10-s 0.1 103-105

29 2.2 2.2 2.2 2.2 16 30 30 16

1 13 13 13 13 1.8

9.8 2.1 0.72 0.67 0.9 0.8

1

1.8

1 1.8

1.3 0.7

255 130 70-97 57 45 50-63 96 109 84

7.3 20 33 25 15 50 15 20 30

270 700 590 460 380 180 190 200 250

~o (see) 107 3 X 108 2 X l0 s 10l° 3 X 10l° 10e 0.1 1200 101°

CIRCULATION ON PLANETS AND SATELLITES the perpendicular to the orbital plane, ~0; gravity acceleration, g; and integral albedos, A. T h e E a r t h is presented for comparison. These data are taken from the aforementioned reviews, and the data for Pluto are from Cruikshank et al. (1976). Table II contains the atmospheric parameters. In the first column there is the pressure ps at the cloud level for the giant planets and, possibly, for T i t a n and surface pressure estimates for Io and Ganymede. For Pluto, the equilibrium methane vapor pressure is taken for the subsolar t e m p e r a t u r e 60°K. This value is valid at the present time, b u t depending on the position of the orbit, it can change from 50 to 63°K. T h e spherical mean temperature is between the limits 35 to 45°K. One m a y not exclude the presence on Pluto of some other gases, e.g., neon (Hart, 1974; Golitsyn, 1975b). T h e mean molecular weight t~ = 2.2 is adopted for the larger planets; it corresponds to a solar abundance of the elements. T h e specific heat capacity, Cp, adiabatic t e m p e r a t u r e lapse rate, "ra = g / C p , the scale height, H, and the isothermal sound velocity, c, are calculated for a perfect gas, and T = T~, the radiation equilibrium temperature. T h e pressure value for Io is of the order of 10 -6 to 10 -4 Pa, which corresponds to the surface n u m b e r density n, = 109 to 1011 am -a. T h e pressure at the level of the upper cloud deck for all larger planets is of the order of 105 Pa (1 bar). ., In the last column of Table II we present the time of radiative cooling (Golitsyn, 1970 ; Gierasch et al., 1970), ro = C p p s T ~ / g q

= C v M / a T ~ 3,

(1)

where M = p , / g is the mass of the atmospheric unit column and a is the S t e f a n Boltzmann constant. T h e ratio of r~ ( = r / c ) , the time of acquiring the local thermodynamic equilibrium (or the time for gravity waves to pass the distance r), to r0 is an i m p o r t a n t nondimensional simi-

335

larity criterion for the atmospheric dynamics. The ratio re/r0 is a measure (an inverse one) of the thermal inertia of an atmosphere or a measure of diabaticity of an atmosphere (Golitsyn, 1970, 1976; Stone, 1974) : I I M ---- r e / r 0

----

aTe3r/cCpM.

(2)

If IIM ( ( 1 the atmosphere is far from radiative equilibrium and its thermal regime is governed mainly b y dynamics leading to small horizontal temperature contrasts. More detailed and conjectural considerations, taking into account the effects of rotation (Golitsyn, 1973, Sect. 11), show t h a t the temperature contrasts ~ T .~ IIM1/2(o~r/c)Te, reflect a decrease of the poleward heat transport for the fast rotation case. Due to the strong dependence r0 ¢~ T~-3 the time ro could be large even for the rather tenuous atmospheres of cold distant objects. For IIM ~ 1 an atmosphere is close to the state of local radiational balance and the dynamics are tuned to this temperature regime (Golitsyn and Steklov, 1978). Because m a n y bodies have large inclinations of the rotational axis to the orbital planes, seasonal effects are important. These are measured b y the ratio

IIs

=

T,/~o.

(3)

If the ratio is small, seasonal effects are not large and are smeared out b y the dense atmosphere. As we shall see below, this is true even for Uranus and Neptune, at least at the cloud level. This fact was first noticed b y Stone (1973). T h e role of diurnal variations in an atmosphere can be estimated b y the value rid = rd//TO.

(4)

Daily variations of t e m p e r a t u r e near the solid surfaces of the bodies which have them are usually much larger t h a n variations in the atmosphere, due to faster cooling of the surface into space. T h e rate

336

G. S. GOLITSYN

of this cooling is determined b y t h e r m a l properties of the u p p e r layer of the surface. Among other i m p o r t a n t external similarity criteria influencing the dynamics and t h e r m o d y n a m i c s of atmospheres, we note two (Hide, 1966; Golitsyn, 1970, 1973, 1976). T h e role of the rotation can be estimated b y the so-called rotational M a c h number

(5)

II~ = ~,r/c

a n d the relation between horizontal and vertical velocities of motions and spatial scales, a t least from a b o v e for the former and from below for the latter, b y the criterion H~ = H / r . (6) This criterion also plays a role in determining the rate of dissipation of a t m o spheric gases (Golitsyn and Steklov, 1977; Steklov, 1977). T h e similarity criteria (2) to (6) can be obtained from the equations of atmospheric dynamics and bounda r y conditions if one normalizes the spatial scales b y the radius, velocities b y the sound speed c, and t e m p e r a t u r e b y Te.The criteria are external in the sense t h a t t h e y are composed of constant external parameters entering the equations and b o u n d a r y conditions. These criteria are presented in T a b l e I I I . T h e level of emission of outgoing radiation with the t e m p e r a t u r e T. is usually higher for deep a t m o s p h e r e s without a distinct lower b o u n d a r y t h a n for the ob-

served cloud u p p e r deck, which is therefore at a higher t e m p e r a t u r e . Because of this, our times are s o m e w h a t different from values used for the larger planets b y Stone (1973) although the results are qualitatively similar. Criteria (2) to (6) m a y p r o b a b l y be applied locally for various atmospheric levels to obtain the order-ofmagnitude estimates. At least for the E a r t h ' s atmosphere, this m e t h o d does not give unreasonable results. W i t h respect to criterion IIM our objects are separated into two groups : IIM <<~ 1 a n d rim ~ 1. T i t a n and all the planets belong to the first group with the possible exclusion of Pluto. (If only m e t h a n e is present in the atmosphere, then the pressure of the s a t u r a t e d v a p o r over ice falls exponentially with t e m p e r a t u r e . Therefore, even at the subsolar point the value of IIM m a y v a r y b y 1.5 to 2 orders of m a g n i t u d e due to large orbital eccentricity.) Io, G a n y m e d e , and, possibly, Pluto in aphelion belong to the second group if the latter does not h a v e neon or nitrogen (Hart, 1974; Golitsyn, 1975a,b). Considering atmospheric dynamics, it is useful to have information on atmospheric vertical structure, especially if the gas is rarefied. E l e m e n t a r y consideration of some basic features of the structure has been carried out recently b y Golitsyn and Steklov (1977). Some simple procedures h a v e been pointed out for estimates of the levels of the turbopause, thermosphere,

TABLE III SIMILARITYPARAMETERS Object

Earth Jupiter Saturn Uranus Neptune Pluto Io

Ganymede Titan

IIM

1/,

lid

1.2 X 10 -3 3 X 10-4 5 X 10-6 4 X 10-e 5 X 10-6 10-2 105 10 3 X 10-e

3 1 0.12 0.17 0.16 I(P 10g 3 X 106 0.1

10-2 3 X 10-4

I1~

1.7 20

2 X 10 -5

19

10-6 5 × 10-e 0.6 10e 500 4 X 10-4

6-4 6 0.12 0.3 0.1 0.04

Hs

1.2 X 10-a 3 X 10-4 4 X 10-4 10-3 5 X 10-4 10-2 4 X 10-2 10-2 10-2

CIRCULATION ON PLANETS AND SATELLITES and exosphere. These estimates give reasonable results for the terrestrial planets and, one may hope, can give reasonable results in other cases as well. T h e turbopause height is usually determined as the level where molecular diffusivity, which increases with height z as the specific volume V ( z ) = 1/p(z), equals the eddy mixing coefficient K. T h e latter may be estimated by the R i c h a r d s o n - O b o u k h o v law : K ~ 0.1 ~/3L4/3.

Here ~ is the rate of turbulent kinetic energy dissipation per unit mass estimated roughly from balance considerations and L ~ H / I O is the motion scale (after data for E a r t h ; Golitsyn and Steklov, 1977). Somewhat more refined estimates of e in rarefied parts of an atmosphere were made by Izakov (1977). T h e t e m p e r a t u r e distribution with height above the turbopause is described b y the heat conduction equations with the thermodiffusivity k = k/pCp, which also increases rapidly with height (the thermoconductivity k :¢ Tl/2). A theory of solutions of such an equation and several solutions of geophysical application were presented b y Golitsyn and R o m a n o v a (1970). It was shown t h a t the maximum of a temperature perturbation located at time t = 0 at z = zl propagates away to infinity in the finite time Tt = H2(z)/ko exp[-(z -- zo)/H-]. Here z0 is some reference level. Later, the temperature for all z > zl is constant with z and depends only on time (and the intensity of the perturbation). If the heat source is solar, then the time of its action is half a day ~r/~. Equating it to the time vt we obtain an equation for the determination of the thermospheric base zt. Because the scale height H ( z ) is changing with height much more slowly than p(z) in the first approximation, we obtain zt -- z0 = H In (w~-t°/~") = H l n (2vt°/vd),

(7)

337

where Tt° means t h a t vt is taken at the reference level z0. T h e exospheric base is determined (Golitsyn and Steklov, 1977) as the height where the mean free path is equal to the scale height H. Higher up there is no energy equilibrium between all translational degrees of freedom for molecules. As was shown, G a n y m e d e with the reported pressure p8 = 10 -3 mbar, has a " n o r m a l " atmosphere with all three aforementioned levels. Io, however, is an anomaly in this sense. If the n u m b e r density n8 < 1011 cm -3 then it has no stationary turbopause, for ns < 101° cm -3 the turbopause is absent altogether and the isothermy should start right from its surface. Because of the large thermodiffusivity, the atmospheric temperature should follow the temperature of the underlying surface with some phase shift depending on height---of course, if there are no heat sources within the atmosphere such as energetic particles from Jupiter's magnetosphere. If n~ < 109 cm -8 then the exposphere begins right from the surface. Therefore, even for Io at n, > 109 cm -3, the consideration of atmospheric dynamics as in a continuous medium may have meaning. T h e daily variations of the surface temperature of the Galilean satellites have been computed by Richardson and Shum (1968). The temperature changes AT are of the order of half of Te. In the situation when the dynamical role of the rotation is small (the Rossby number Ro = u / ~ r >:> 1) the atmospheric wind speeds may be estimated (Gierasch and Sagan, 1971; Golitsyn and Steklov, 1978) as u ~ ( R A T / ~ ) In = c ( A T / T e ) ~/2.

(8)

We see that the speeds are a substantial part of the sound velocity. A similar situation exists in the E a r t h ' s upper atmosphere, the so-called superrotation (see, e.g., Izakov, 1976). For Io, the density is so small and the kinematic viscosity so large t h a t the atmo-

338

G. S. GOLITSYN

spheric motions must be laminar. On Ganymede one may expect a fully developed turbulent regime with an inertial interval (the estimated pressure 1 tLbar is as in the Earth's atmosphere near 90 km). Therefore, we only briefly consider this satellite. The boundary layer of its atmosphere should have specific peculiarities (Golitsyn and Steklov, 1978). The thickness of the viscous sublayer is of the order of 10 m, and the total depth of the dynamical boundary layer even without thermal factors is of the order of 10 kin. The efficiency of the atmospheric circulation in transforming the solar energy into kinetic energy of atmospheric motions is very low, of the order of 10-6 , against 10-2 for the terrestrial atmosphere and 10-~ for the Martian one (Golitsyn, 1973). We shall now briefly consider the atmospheric dynamics of the giant planets and Titan, which have large values for the radiational cooling time r0 and, consequently, IIM << 1. Because of the absence of local radiative equilibrium and the prevailing role of dynamics upon the temperature regime, the horizontal temperature differences are small compared to Te. The simple similarity arguments give reasonable quantitative estimates only for II~--~ 1. The reasons for this fact were shown by Zilitinkevich (1976) from an analysis of baroclinic instability. Titan and, possibly, Pluto can be grouped with such objects, as shown in Table III. Their atmospheric dynamics have been discussed by Leovy and Pollack (1973) and Golitsyn (1975a,b). In the Titan case, for current atmospheric models, one obtains mean winds of the order of 1 or a few meters per second (u ~ p8-I/~; Golitsyn, 1970, 1973) and characteristic temperature difference between equator and poles of the order of I°K. In this respect Titan is like Venus, whose similarity criteria are close: IIM 10-5, II~--~ 10-2 (Golitsyn, 1973). However, one may expect one important dynamical difference : for Titan the Rossby ~,~

number Ro ~ riM I/2 I I ~ - 1 (see Golitsyn, 1975a) is small (for a deep atmosphere), but for Venus Ro >~ 1. This means that the Coriolis force is important for Titan. The circulation there is evidently a symmetric Hadley cell in which a heated gas parcel comes up in the equatorial region and spirals (Coriolis-force!) to colder polar regions where it descends (Leovy and Pollack, 1973). Qualitatively similar conclusions follow for Pluto, if p8 ~> 0.1 mbar there (Golitsyn, 1975b). For all giant planets the rotational Mach numbers are large, especially for Jupiter and Saturn, which prevents one from obtaining quantitative conclusions from similarity arguments. Some considerations (Golitsyn, 1970, 1973) allow one to reach the conclusion that dynamical time constants may be very large, much larger than the time r0, though this requires additional work. Because the reviews listed at the beginning of this paper present a good account of the meteorology of the giant planets, we consider only the latest developments (since May 1975). For Jupiter and Saturn the important problems still are the banded structure of the disk, equatorial jets, characteristics of turbulent mixing, and large lifetimes of many details of the atmospheric structure, especially on the Great Red Spot (GRS). An interesting conjecture to explain the nature of GRS on the basis of Rossby wave solutions was recently put forth by Maxworthy and Redekopp (1976) and Maxworthy et al. (1978). The solitons are formations in which nonlinear hydrodynamical effects balance exactly the dispersion of the waves of various frequencies which form the "wave packet," so to speak. Using these ideas, it was possible to explain (Maxworthy et al., 1978), at least qualitatively, many details of the Jovian circulation which were known for decades and were hydrodynamical puzzles, e.g., the nature of the GRS and the South Tropical Disturbance (STD) and

CIRCULATION ON PLANETS AND SATELLITES their interaction. T h e idea t h a t these are solitons of different natures allows one to explain qualitatively not only their visible shape but also the acceleration of the S T D when it approaches the GRS, the persistence of the shape of both formations after the S T D travels through the GRS, and so forth. Stone (1976) objected to this soliton concept on the grounds t h a t the conditions for the existence of the S T D and GRS are too severe and lead to unrealistic values of the temperature lapse rate. These conditions ( M a x w o r t h y and Redekopp, 1976; Stone, 1976) for the GRS may be simplified to the requirement S D 2 /> 8.7 X 103 km °K where S is the potential temperature lapse r a t e ' a n d D is the depth of the formation. If one takes (Stone, 1976) D -- H = 20 k m then one gets S /> 20°K k m -~, evidently an unrealistic value. However, if one assumes ( M a x w o r t h y et al., 1978) t h a t the GRS extends downward for several scale heights, which does not seem to be unreasonable for such a huge formation, then it is easy to obtain reasonable values for S. On the other hand, the theory of M a x w o r t h y et al. uses the Bousinesq approximation which is valid if D ~ H, at least. Therefore, there are still inherent difficulties with this attractive conjecture. T o prove or disprove it, it would be necessary to consider nonlinear Rossby waves in a compressible atmosphere. Another qualitative explanation of the GRS may be found in Williams (1978). All this shows the importance of detailed measurements of the Jovian vertical structure for an understanding of m a n y problems of atmospheric dynamics on the planet. One should mention here the need for highresolution images of Jupiter to solve the question of whether the small-scale convection due to the internal heat flux penetrates up to the upper cloud layer or not. If it does then m a n y concepts on the hydrodynamical nature of the observed phenomena must be reconsidered. Some hydrodynamical analogies in the

339

dynamics of the ocean and the Jovian atmosphere have been noted. Shuleykin (1976) pointed out t h a t large elliptical eddies like the GRS exist in the ocean: Williams (1975a,b) carried out numerical experiments with a model of the Jovian atmosphere. He found t h a t the steady circulation, in a statistical sense with respect to the kinetic energy, had a banded structure with perturbations in the form of oval eddies. The persistence of the largest eddy as a GRS in the model took place due to the fact t h a t the large-scale quasigeostrophic turbulence transferred energy from smaller to large scales up to the largest scale L ~ ~(2V/f~) ~/2, where V is a characteristic flow velocity, fl = df/rdO is the change of the Coriolis parameter, f = 2w sin 0, with latitude 0. For the GRS f~ ~ 2 X 10 - g k m - l s e c - ~ , a n d f o r V ~ 2 0 m sec -~ we obtain L ~ 15 000 km, which is just of the order of the size of the Spot. Note t h a t eddies of a similar nature in the ocean, so called mesoscale synoptic eddies, have horizontal scales of about 100 km. A more detailed description of the computations and the physical meaning of these results m a y be found in Williams (1978). Saturn differs from Jupiter in having a larger differential rotation (Alexander, 1962). Its equatorial jet is several times broader than that on Jupiter and reaches 400 m sec -I relative to temperate latitudes. This might be understood if one assumes t h a t the rotation extends to greater depths (Hide, 1966) and t h a t the use of the thermal wind equation (Stone, 1973, 1976) may give such wind speeds. To check this assumption one also needs to know the temperature distribution in the atmosphere of the planet. T h e drawings of Uranus sometimes also reveal a banded structure (Alexander, 1965) which can be expected if one takes into account a rather large value of II~. However, this structure is not always discernible, and verification of its presence or absence will be important. A question to be an-

340

G. S. GOLITSYN

swered evidently concerns the role of the rotation axis in the atmospheric dynamics of Uranus, which lies almost in the orbital plane. However, a large radiation time constant and small value of II8 (see Table I I I ) contrast with large seasonal effects at the level of the cloud layer (Stone, 1973). At the same time, in its "stratosphere" upward of the levels of about 104 Pa (100 mbar), these effects should be substantial, causing, for instance, a change in the direction of circulation. Recently some data on the " w e a t h e r " on N e p t u n e have appeared (Joyce et al., 1977 ; Pilcher, 1977 ; Cruikshank, 1978). This " w e a t h e r " reveals itself in temporal variations of the albedo in p h o t o m e t r y in the near infrared (J, H, K, and L regions of the spectrum). An analysis of a rather long time series of photometric measurements allowed determination of new values for the N e p t u n e rotational period (Cruikshank, 1978), which have been used in Table I. Two values were found: 19h35~.0 ± 0~.3 and 18h10.~4 ± 0~.3. B o t h values are present in the frequency spectrum of the series considered, but the first value is more pronounced and seems to be more plausible. However, if the second period is statistically significant then another interpretation is possible. One may assume that the shorter period reflects the more rapid rotation of the equatorial regions. T h e velocity of the equatorial jet should then be around 140 m sec -1 relative to the atmosphere of temperate latitudes, which does not seem to be unreasonable. Lengthier series of observations are evidently needed here. Note added in proof. Since this paper was reported at the C O S P A R X X I Symposium " T h e Planetary Atmospheres and Surfaces" in M a y 1978, a major finding has occurred in relation to Pluto: a satellite was discovered in June 1978 b y J. W. Christy [-see Christy, J. W., and Harrington, R. S. (1978). The satellite of Pluto. Astron. J. 83, No. 8, 1005-10081. Its period of revolu-

tion around Pluto and its distance from the planet (only 17 000 kin) allow one to determine the total mass of the system. Because the satellite is 2 or 3 orders of magnitude fainter than the planet, the main mass is in the planet. With the mean density of about 1.2 g cm -3, appropriate to lesser bodies on the periphery of the Solar System rsee Consolmagno, G. J. and Lewis, J. S. (1978). T h e evolution of the icy satellites interiors and surfaces. Icarus 34, 280-290], this produces a mean radius of about 1300 km with an albedo of 0.4 according to the Pluton's stellar magnitude (Newburn and Gulkis, 1973). T h e value of g is then only about 0.5 m sec -2, which poses a problem for the existence of an atmosphere there (Golitsyn, 1979). Another important finding by Cruikshank occurred in M a y 1978--the determination of the methane atmosphere on Triton, with a mean pressure of about 10-4 mbar = 10-2 Pa (personal communication). These two objects require special study with respect to their atmospheres. ACKNOWLEDGMENTS The author is grateful to G. P. Williams and D. P. Cruikshank for sending the preprints of their papers and useful discussions of the Jovian dynamics and problems for Neptune and Pluto. REFERENCES ALEXANDER, A. F. O'D. (1962). The Planet Saturn.

Faber & Faber, London. ALEXANDERI A. F. O'D. (1965). The Planet Uranus. Faber & Faber, London.

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