ICARUS 124, 632–644 (1996)ARTICLE NO. 0237

The Structure of the Stratospheric Aerosol Layer in the Equatorialand South Polar Regions of Jupiter1

FERNANDO MORENO

Instituto de Astrofısica de Andalucıa, CSIC, Granada, SpainE-mail: fernando@iaa.es

Received April 17, 1996; revised July 1, 1996

content at these locations in Jupiter is important, as itinfluences the stratospheric heating rate and consequentlyA one-dimensional code to study the microphysical processes

affecting aerosol particles in the jovian stratosphere and upper the dynamics in that region. These previous studies havetroposphere has been developed. The algorithms are based on given important insights in the knowledge of the aerosolthose developed by Turco et al.(Turco, R. P., P. Hamill, O. B. distribution and physical properties. However, to deriveToon, R. C. Whitten, and C. S. Kiang 1979. J. Atmos. Sci. 36, these properties, a detailed study of the microphysical pro-699–717) and include the effects of particle sedimentation, cesses affecting the aerosol particles was not applied, incoagulation, eddy diffusion, and growth by heteromolecular part due to the limited knowledge of the physics involved.condensation. A set of Hubble Space Telescope images in the

The first application of a microphysical model to the uppernear-UV was used to retrieve the aerosol distribution in thehaze layers of Jupiter was made by Tomasko and Smithequatorial and south polar regions of Jupiter. The results ob-(unpublished manuscript). The numerical code presentedtained are consistent with the picture in which in the polarhere is based on ideas by these authors, and most of theregions the aerosols are formed from auroral-related phenom-algorithms were originally developed by Turco et al. (1979),ena, while in the equatorial region the largest component of

the aerosols would be those particles formed after condensation for the study of the terrestrial sulfate layer, and subse-of hydrazine onto condensation nuclei. Concerning the charge quently applied to the structure of Titan’s hazes by Toonof the particles, the particles in the south polar region must be et al. (1980) to the Uranian aerosols by Pollack et al. (1987)considerably more charged than at equatorial latitudes. The and to Neptune’s stratospheric haze by Moses et al. (1995).model solutions give particles smaller than about 0.05 mm in In this work, I applied the results of the microphysicalradius at the equator, and 0.07 mm at the polar regions, with code to the study of two very distinct regions of Jupiter,total optical depths of 0.3 and 1.3 at the equator and the south

namely the south polar region, and the equatorial region.polar region, respectively, at 410 nm wavelength. The differ-A set of Hubble Space Telescope WFPC2 archive imagesences in the derived imaginary refractive indices reflect a com-in the near-UV region have been used to determine thepositional change in the aerosols at low and high latitudes.model structures at these two locations. The data reduction 1996 Academic Press, Inc.

is presented in Section II, the numerical code is presentedin Section III, and the application of the code to the abovementioned jovian regions is given in Section IV. The discus-I. INTRODUCTIONsion and conclusions are given in Section V.

Evidence of the existence of the jovian stratosphericaerosol layer and its variation with latitude has come from II. DATA REDUCTIONa variety of measurements, ranging from ground-based

The HST WFPC2 archive images used are listed in Tablemethane band imagery (Owen 1969, West 1979), GalileanI. At the time of the observations (May 5, 1994), Jupiter’ssatellite eclipse studies (Smith et al. 1977, Smith 1980),equatorial and polar apparent diameters were 44.16 andspacecraft measurements in the UV range (West et al. 1981,41.27 arcsec, respectively, and the phase angle was 3.48. ITomasko et al. 1986), and imaging eclipse observationsused a plate scale of 0.0996 arcsec/pixel (R. West, personalfrom Voyager 2 (West 1988). The study of the aerosolcommunication). The absolute calibration of the imageswas obtained by requiring that the geometric albedoes

1 Based on observations made with the NASA/ESA Hubble Spacematched those of previous measurements by Wallace et al.Telescope, obtained from the data archive at the Space Telescope Science(1972) and Karkoschka (1994), as was done recently byInstitute. STScI is operated by the Association of Universities for Re-

search in Astronomy, Inc. under NASA Contract NAS 5-26555. West et al. (1995). Although these measurements pertain

6320019-1035/96 $18.00Copyright 1996 by Academic Press, Inc.All rights of reproduction in any form reserved.

JOVIAN STRATOSPHERIC AEROSOLS 633

TABLE I ­n(v)­t

5 2­

­z(fs 1 fd)Hubble Space Telescope Images Used in the Present Work

Image Filter Effective Geometric1

12Ev

0dv9K(v9, v 2 v9)n(v9)n(v 2 v9)denomination name wavelength (nm) albedo

U2EB0407 F218W 230 0.312 Ey

0dv9K(v, v9)n(v9)n(v) 2

­

­vgn(v) 1 q(v),U2EB0408 F255W 275 0.28

U2EB0409 F336W 335 0.29U2EB040A F410M 410 0.36

where n(v) is the number density of particles having vol-Note. Effective wavelengths and geometric albedoes are from West etume v, z is the vertical coordinate in the atmosphere, t isal. (1995).time, fs is the sedimentation flux, fd is the diffusion flux,K is the coagulation kernel, which is related to the stickingprobability of particles with different sizes, g is the growth

to larger jovian phase angle (p108), I have assumed thatrate of particles due to condensation of hydrazine onto

the differences due to a different phase angle are negligibletheir surfaces, and q(v) is the production rate of particles

when compared with the uncertainties of the absolute val-of size v by chemical reactions.

ues of the reflectivities. For the radiative transfer analysisThe sedimentation flux is given by

I will consider the same values as derived by West et al.(1995) for the effective wavelength and geometric albedoes

fs 5 2ng,(see Table I).Geometric reductions of the images were applied to

obtain the needed tables of the cosines of the solar inci- where g is the fall velocity, given by (Kasten, 1968)dence, emergent, and azimuthal angles (e0, e, DF) andreflectivity (I/F). The two jovian regions analyzed for the

g 5 (2/9)(rpr 2g/n)B9,limb-to-limb study were defined as to the I/F points in-cluded in the boxes l 5 0 6 18 and l 5 265 6 18 planeto-graphic latitude. The geometric reductions followed similar where rp is the particle density, r is the particle radius, gtechniques to those described by Ortiz et al. (1993). The is the gravity acceleration, and n is the gas viscosity. B9 issubterrestrial point was determined by fitting an ellipse to a correction factor given bya contour intensity plot corresponding to the bright limbin each image. This procedure gave the subterrestrial point

B9 5 1 1 1.246Kn 1 0.42Kne20.87/Kn,and position angle of the planet, although this former pa-rameter is not well constrained by this procedure. Theposition angle was further refined by requiring that the where Kn is the Knudsen number, which is defined asbelt-zone boundaries in the images were coincident withthe derived latitude circles. The image set used here ispresented in Fig. 1. Kn 5

lg

r,

III. THE MICROPHYSICAL MODELwhere lg is the gas mean free path.

The one-dimensional microphysical code developed was The eddy diffusion flux is given bybased on the algorithms given by Turco et al. (1979) forthe terrestrial stratospheric aerosols. I assumed that as aresult of chemical reactions, small particles of a specified fd 5 2KEng F ­

­z S nngDG,

size are produced in an altitude range in the atmosphereat a given production rate. Once formed, these embryosundergo Brownian coagulation and processes of sedimen- where KE is the eddy diffusion coefficient and ng is the gastation, and eddy diffusion. I have allowed also the particles number density.to grow by condensation of hydrazine onto the aerosol For the coagulation kernel, I adopted the expression bynuclei. All of these processes are parameterized in the Fuchs (1964),same way as in Turco et al. (1979). I assumed that theparticles have spherical shape, and I neglected moleculardiffusion of particles. Under these assumptions, the re- Kij 5 2farijDij S rij

ri j 1 di j1

4Dij

Gij 1 rijD21

,sulting continuity equation may be written as

634 FERNANDO MORENO

FIG. 1. The Hubble Space Telescope archive images used in this work. The images correspond to the filters F218W, F255W, F336W, andF410M, as labeled. South is toward the upper right corner in every image frame. The dashed lines at the F218W image correspond to the latitudecircles at 08 and at 2658 planetographic. For the image set denomination, see Table I.

whereG 5 S8KBT

fm D1/2

rij 5 ri 1 rj are particle radii, Dij 5 Di 1 Dj ,is the average kinetic velocity of a particle of mass m, andGij 5 (G 2

i 1 G 2j )1/2, and di j 5 (d 2

i 1 d 2j )1/2,

d 5 (1/6rlb)[(2r 1 lb)3 2 (4r 2 1 l2b)3/2] 2 2r

where

is a correction factor, where r is the particle radius, andD 5 KBT(1/6fnr)(1 1 1.246Kn 1 0.42Kne20.87/Kn)

lb 58DfG

.where KB is the Boltzmann’s constant, T is the absolutetemperature, and n is the gas viscosity.

On the other hand, In the coagulation kernel appears a sticking probability a,

JOVIAN STRATOSPHERIC AEROSOLS 635

which is related to the charge of the aerosols and is given The coagulation processes are implemented computa-tionally in the manner described by Turco et al. (1979)by (Hidy and Brock 1970, Toon et al. 1980)which is based on the work by Kritz (1975). Particles con-tained in a given size bin have a volume which is double

a 5 exp2ZiZje2

KBT(ri 1 rj), those corresponding to the previous size bin. To eliminate

problems of artificial diffusion in the numerical integrationof the continuity equation, a combined diffusion and sedi-where e is the unit of charge and Zi and Zj are the numbermentation flux was considered (Turco et al. 1979).of charges carried by the particles of radii ri and rj .

For the growth rate, I adopted the formulation by Hamill The numerical integration of the continuity equationet al. (1977), which was applied to the growth rate of drop- follows the procedures given by Turco et al. (1979), inlets by sulfuric acid molecules in the terrestrial stratosphere which an implicit algorithm for the finite differencingand is adequate for all values of the Knudsen number. I scheme of the equation is adopted. The altitude grid was

set equal to 2 km. In the model computations, the gravitywill assume an analogous situation in which the hydrazinemolecules play the same role in the jovian stratosphere as acceleration was assumed to be the same at the two lati-

tudes analyzed and equal to 2322 cm sec22. The modelsthe sulfuric acid molecules in the terrestrial one. Thegrowth rate is dependent on the molecular diameter of the can be started with any initial distribution of aerosols in

the atmosphere. Generally, an exponentially increasinghydrazine molecule, taken here to be 5 3 1028 cm.

FIG. 2. Properties of the nominal equatorial model. The variation of the mean radius (labeled as rmed), and cumulative optical depth withpressure, as well as the results on reflectivity (I/F) versus longitude referring to the central meridian of the planet, at the four wavelength channelsindicated in the top labels, are shown. In the panel showing the variation of cumulative optical depth with pressure, the solid line corresponds tothe F218W channel, the dashed line to F255W, the dash-dot line to F336W, and the dotted line to F410M. In the panels showing the reflectivityversus longitude, the dots are the observed I/F values, while the solid line represents the model results at the four wavelengths. The imaginaryrefractive indices are 0.08, 0.03, 0.003, and 0.0001 at F218W, F255W, F336W, and F410M. The lower cloud is represented by a two-term Henyey–Greenstein function after Tomasko et al. (1978), with single scattering albedoes of 0.6, 0.85, 0.92, and 0.96 at the four channels, respectively, inorder of increasing wavelength.

636 FERNANDO MORENO

FIG. 3. Equatorial model with a hydrazine mass production rate as given in West et al. (1986), and a charge of 3 electrons per particle of 0.1em in radius, charge being proportional to radius. For an explanation of the different lines in the panels, see the legend to Fig. 2. The best fitimaginary refractive indices are 0.3, 0.09, 0.03, and 0.02, and the lower cloud single scattering albedoes are 0.5, 0.83, 0.91, and 0.95, in order ofincreasing wavelength at the four channels analyzed.

time step is set, and the models run until no variation solution, a total of 15 layer models are defined by regroup-ing the size distributions from the microphysical code intowithin a few percent of the concentration of particles at

every size bin category occurs, so that steady state is these new 15 layers. These 15 layers are the input layersfor a Mie scattering program which operates in conjunctionreached. As boundary conditions, I assumed that no flux

of particles occurs in the upper boundary, placed at 0.5 with a discrete-ordinates radiative transfer program calledDISORT (Stamnes et al. 1988) to give the output reflectivi-mbar, and that there is an aerosol sink in the lower bound-

ary, placed at 400 mbar, so that the concentration there is ties at the selected e0, e, and DF coordinates. A total of 16streams were used in the radiative transfer computations.zero. The models were checked to conserve mass and vari-

ous numerical validations were done. In particular, I Certain parameters were considered fixed and were notmodified in any of the model calculations described below.checked the coagulation algorithms by shutting off all theThus, I employed the Voyager atmospheric pressure–other processes and assuming that the kernel is a constanttemperature profile by Lindal et al. (1981). The particlefunction. Then, the solution was found to agree with thedensity was set to 1 g cm23, as most kinds of polymers haveanalytical Smoluchowski solution.a density close to 1 (Brandrup and Immergut 1966). Thereal refractive index of the particles was set to 1.4. TheIV. APPLICATION OF THE MICROPHYSICAL MODELreal and imaginary refractive indices are assumed to beTO THE EQUATORIAL AND POLAR REGIONSconstant with depth in the atmosphere.

The structure of the atmosphere below the 400 mbarAfter running the microphysical model for a given selec-tion in its input parameters, a solution is reached in which, region is assumed to consist on an ammonia cloud with a

set of values in the single scattering albedo for the differentat every atmospheric layer between 0.5 and 400 mbar, asize distribution of particles is obtained. Then, for that wavelengths and a two-term Henyey–Greenstein phase

JOVIAN STRATOSPHERIC AEROSOLS 637

FIG. 4. Equatorial model for hydrazine production rate from West et al. (1986), and a decrease in a factor of 10 in the nominal eddy diffusionprofile. The derived imaginary refractive indices of the particles and the single scattering albedoes of the lower cloud are the same as those givenin the legend to Fig. 3. For an explanation of the different lines in the panels, see the legend to Fig. 2.

function as derived by Tomasko et al. (1978) from Pio- production of polyynes as sources of stratospheric aerosolsin equatorial latitudes. In fact, for their standard northneer photometry.

The parameters that mostly affect the output size distri- equatorial belt model, the column production rate is1.9 3 105 cm22 sec21. These authors stated that one-halfbutions in the microphysical code are the following: the

production rate profile (in both integrated value and peak of this amount is due to the reaction of C6H 1 C4H2 Rproducts. In the assumption that all the haze material camealtitude in the atmosphere) of the initial embryos and hy-

drazine, the coagulation efficiency (related to the charge from this reaction, a mass production rate of 4.5 3 10217

g cm22 sec21 is estimated. This is about three orders ofcarried by the aerosols), the particle density, and the eddydiffusion profile. Given the large list in the input parame- magnitude smaller than the value estimated for the south

polar region (see below), and, as these authors recognize,ters for the models and the computational time which isrequired for every model run, it is not practical to try to hydrazine condensation should be a major component for

the low-latitude high-altitude haze layer.find the best-fit model by varying simultaneously all theparameters involved. Instead, starting from estimates of As a nominal model for the equatorial latitudes, I con-

sider a model in which the smallest sized particles arecertain input parameters from other investigations, I set anominal model at the two analyzed latitudes and then I produced at high altitudes according to a Gaussian profile

(Toon et al. 1980)looked for the model which produced the best fit to thedata at the four wavelengths involved by varying thoseparameters one at a time. q(z) 5 C exp{20.5[(p 2 p0)/py]2} g cm23 sec21,

The Equatorial Regionwhere p is the pressure at altitude z. I set p05 1 mbar, andpy 5 0.1 mbar. The constant C is the integrated massThe recent hydrocarbon photochemistry study of Glad-

stone et at. (1996) imposes a severe restriction for the production rate, that I considered to be the above stated

638 FERNANDO MORENO

TABLE II produced very small particles, the maximum mean radiusBest-Fit Model Parameters at the Equator being 0.03 em close to the bottom of the model, and very

small optical depths at the four wavelengths. The modelParameter Best fit computations reveal that to bring the model reflectivities

into agreement with the results, the particles would beMass production rate profile p0 5 1 mbarextremely dark at the F218W channel, with an imaginarypy 5 0.1 mbar

C 5 4.5 3 10217 g cm22 sec21 refractive index of 0.9, which is unrealistic as no indicesabove 0.3 are expected in the jovian atmosphere (Tomasko

Hydrazine production rate profile p0 5 330 mbar et al. 1986). In consequence, to fit the observations morepy 5 30 mbar

aerosol loading is required, provided all the other aerosolC 5 6.36 3 10211 g cm22 sec21

model parameters are kept to their nominal values. If Iaccept the constraint on the polymer production rate fromCharge 0–1 electrons per particle

of 0.1 em, charge being Gladstone et al. (1996), the next step is to consider a largerproportional to radius mass production rate of hydrazine. This would be justified

as the hydrazine concentration estimated from differentImaginary refractive indices at 0.08, 0.03, 0.003, and 0.0001

photochemical schemes differ notably, mainly due to theF218W, F255W, F336W, andconsideration of different chemical reactions and/or differ-F410M channelsent rate constants. For instance, Atreya et al. (1977) esti-mated a hydrazine number density of 2 3 1011 cm23 at 400mbar, while Strobel (1975) estimated 1.7 3 1010 cm23 atthe same altitude. If the hydrazine production rate is in-value of C 5 4.5 3 10217 g cm22 sec21. The embryo size

was set to 0.01 em in radius. For that model, the hydrazine creased by a factor of 3 of the value estimated by West etal. (1986) (i.e., up to a value of 6.36 3 10211 g cm22 sec21),production rate was also set to a Gaussian profile with

p0 5 330 mbar and py 5 30 mbar, with a mass production a satisfactory agreement with the experimental results isobtained (see Fig. 2). The largest particles in that modelrate of 2.12 3 10211 g cm22 sec21, which is the total hydra-

zine mass production rate estimated by West et al. (1986), have a mean radius of about 0.05 em, and have imaginaryindices of 0.08, 0.03, 0.003, and 0.0001 at the four channelson the basis of photochemical models by Kaye and Strobel

(1983). For the nominal model, I considered that the parti- in increasing wavelength.Fits similar to those of Fig. 2 are found if the charge ofcles carried no charge, so that the particles will have a

sticking efficiency of unity, and I set the eddy diffusion the particles are restricted to be less than or equal to 1electron per particle of 0.1 e in radius, while keeping theprofile to be described by KE Y 1/Ïng (see, e.g., Hunten

1975), with a corresponding value at 1 bar of KE 5 hydrazine production rate to be a factor of 3 larger thanthat estimated by West et al. (1986). There are other possi-2 3 104 cm22 sec21. The resulting aerosol size distributions

TABLE IIIAerosol Model Properties at the Equator

Mean Column Cumulativep1 p2 radius abundance Optical depth optical depth

Layer (mbar) (mbar) (em) (3109 cm22) at 410 nm at 410 nm

1 0.5 0.7 0.0106 0.08 0.0001 0.00012 0.7 1.0 0.0106 0.14 0.0001 0.00023 1.0 1.5 0.0108 0.19 0.0002 0.00034 1.5 2.2 0.0111 0.21 0.0003 0.00065 2.2 3.3 0.0114 0.21 0.0004 0.00106 3.3 4.8 0.0119 0.20 0.0006 0.00167 4.8 7.0 0.0124 0.19 0.0010 0.00268 7.0 10.4 0.0130 0.18 0.0019 0.00459 10.4 16.5 0.0138 0.18 0.0041 0.0086

10 16.5 27.7 0.0149 0.17 0.0090 0.017611 27.7 47.7 0.0165 0.15 0.0181 0.035712 47.7 81.7 0.0191 0.12 0.0324 0.068013 81.7 141.4 0.0241 0.09 0.0551 0.123214 141.4 243.4 0.0351 0.06 0.0820 0.205115 243.4 404.4 0.0524 0.03 0.0901 0.2953

JOVIAN STRATOSPHERIC AEROSOLS 639

FIG. 5. Properties of the nominal model at 2658 planetographic latitude. The imaginary refractive indices in this case are 0.022, 0.018, 0.0032,and 0.008, in order of increasing wavelength, at the four channels analyzed. For an explanation of the different lines in the panels, see the legendto Fig. 2.

bilities to bring the modeled I/F’s into agreement with the 0.1 at 230 nm (Tomasko et al. 1986). This imposes a con-strain on the hydrazine production rate, since the solutionsobservations, however, for the nominal hydrazine produc-

tion rate, by considering modifications in the charge of the having indices below 0.1 at 230 nm effective wavelengthrequire models having a production rate of about threeaerosols and the eddy diffusion profile. For a charge of 3

electrons per particle of 0.1 em in radius (charge being times that estimated by West et al. (1986). The parametersof the best-fit equatorial model, which corresponds to theproportional to particle radius), I found a satisfactory

agreement with the observed reflectivities by setting higher model having such a large hydrazine production rate, aregiven in Tables II and III.values in the imaginary refractive indices, specifically 0.3,

0.09, 0.03, and 0.02, in order of increasing wavelength (seeFig. 3). A decrease in the eddy difussion profile by an

The South Polar Regionorder of magnitude also produces a reasonable agreementwith the results, for the same set of high values in the Several lines of evidence indicate that the south polar

region is presumably optically thicker than the equatorialimaginary refractive indices (see Fig. 4). In general, forthe nominal hydrazine production rate, it is possible then regions (West et al. 1986). I will assume that the largest

fraction of aerosols at high latitudes come from auroral-to find solutions by invoking large values in the imaginaryrefractive indices, while smaller indices are necessary to related processes, although hydrazine condensation is also

considered to be a component of the high-latitude haze.match the observations when larger hydrazine productionrates are considered. Laboratory experiments of ultraviolet As the hydrazine forms as a result of the ammonia photo-

chemistry, the larger optical path of the solar light reachingand proton irradiation on hydrocarbons available in thejovian stratosphere produce materials that, with the excep- these latitudes will ultimately result in a decrease in the

hydrazine production rate.tion of tholins, the imaginary indices of which fall below

640 FERNANDO MORENO

FIG. 6. Polar model by varying only the state of charge of the particles with respect to the nominal model, with a charge of 3 electrons perparticle of 0.1 em in radius. The effect is mainly to produce a flatter center to limb profiles at every wavelength (compare the modeled I/F’s withthose in Fig. 5). The derived imaginary indices are 0.03, 0.021, 0.011, and 0.005, in order of increasing wavelength. For an explanation of the differentlines in the panels, see the legend to Fig. 2.

The mass production rate of carbon in the form of hydro- Fig. 5, this model produces reflectivity curves that are toosteep at the F255W, F336W, and F410M channels. Aftercarbons ions in the south auroral zone of Jupiter has been

estimated to be 6.8 3 10212 g cm22 sec21 (Pryor and Hord, trying several models by varying the haze production rate,the hydrazine production rate, and the eddy diffusion pro-1991), and a fraction of this mass is presumably converted

to haze particles at the polar regions. Therefore, this quan- file, one at a time, no improvements in the model solutionswere found. However, when the particles are consideredtity should be an upper limit to the mass production rate

of haze particles. I will assume that 1% of that amount is to be charged, I found a much closer agreement with theobservations. This is illustrated in Fig. 6, in which a chargeconverted to haze particles. I set the same functional form

of the embryo production rate profile as defined above for of 3 electrons per particle of 0.1 em in radius was consid-ered, and keeping the rest of parameters to their nominalthe equator, also with p0 5 1 mbar and py 5 0.1 mbar.

Given the lack of knowledge in the hydrazine production value. As can be seen, the charging effect controls coagula-tion, keeping the mean radius of the resulting particle dis-rate at high latitudes, I will assume initially that the mass

production rate is reduced by a factor of 100 with respect tribution to a small value, and increasing considerably thetotal optical thickness except at the F218W channel. Thisto the nominal model production rate at the equatorial

latitudes, i.e., to 2.12 3 10213 g cm22 sec21, with the same results in flatter center to limb curves, although they aretoo flat at the F336W and F410M channels. The derivedparameters in the Gaussian profile, i.e., p0 5 330 mbar and

py 5 30 mbar. The nominal model considers the same eddy imaginary refractive indices are higher than those reportedfor the nominal case (see captions for Figs. 5 and 6). An-diffusion profile as in the equator, and no charge on the

aerosol particles. The best-fit imaginary indices for that other way to modify the curvature in the I/F curves is tovary the region of the embryos production. For instance,model are 0.022, 0.018, 0.0032, and 0.0008. As shown in

JOVIAN STRATOSPHERIC AEROSOLS 641

FIG. 7. Best fit model at high southern latitudes. In this model, the production region of aerosols has a peak at 3 mbar, and the aerosols arecharged with a charge of 3 electrons per particle of 0.1 em in radius (charge being proportional to radius). The derived imaginary indices are 0.027,0.018, 0.008, and 0.0045, in order of increasing wavelength, at the four channels analyzed. For an explanation of the different lines in the panels,see the legend to Fig. 2.

for p0 5 3 mbar, py 5 0.8 mbar, and considering a charge and 0.007 in order of increasing wavelength (Fig. 8). Forthe south polar region, the best-fit model is that corre-of 3 electrons per particle of 0.1 em in radius, with an

integrated embryos production rate of 1.82 3 10214 g cm22 sponding to Fig. 7, and its parameters are given in TablesIV and V.sec21, I found a still closer agreement with the observations

(Fig. 7). In this case, the imaginary indices must have aflatter wavelength dependence than in previous cases, be- V. DISCUSSION AND CONCLUSIONSing 0.027, 0.018, 0.008, and 0.0045 in order of increasing

The center to limb curves at ultraviolet wavelengthswavelength. No variations in the model results are foundby increasing the hydrazine production rate up to a factor from Hubble Space Telescope imaging provide a better

knowledge of the aerosol processes at various locations inof 5 from the value estimated by West et al. (1986), so thatessentially no constraints on the hydrazine production rate the jovian stratosphere and high troposphere. Considering

two rather different regions in the planet, the equatorialcan be given at these latitudes.Distributions with still smaller mean radius in the parti- region and the south polar region, families of microphysical

models have been developed which fit the center to limbcles are also compatible with the observations. I performeda model test with charge 5 3 and eddy diffusion profile a curves at four wavelengths. These models indicate that the

stratospheric aerosols are limited in size to about 0.05 emfactor of ten larger than the nominal one. In that case, Ifound a close agreement with the observations by setting in radius at the equator and 0.07 em at 2658 planetographic

latitude. This parameter is best determined by the shapea larger than nominal embryo mass production rate of1.4 3 10213 g cm22 sec21, with p0 5 1 mbar and py 5 0.1 of the center to limb curve, the determination of which

is very precise in the case of the HST images. Previousmbar, and imaginary refractive indices of 0.03, 0.022, 0.011,

642 FERNANDO MORENO

FIG. 8. High latitude model with charged particles (3 electrons per particle of 0.1 e in radius), and large eddy diffusion profile (larger in afactor of 10 than the nominal one). The combined effect is to keep the particles to very small radii. In this case, the production region of embryosis at p0 5 1 mbar, with py 5 0.1 mbar. The derived imaginary refractive indices are 0.03, 0.022, 0.011, and 0.007, at the four channels considered,in order of increasing wavelength. For an explanation of the different lines in the panels, see the legend to Fig. 2.

determinations of particle size in the jovian stratospherehave been obtained by a number of authors. Thus, satelliteeclipse observations by Smith (1980) indicate the presenceTABLE IVof aerosols in the south temperate zone and north temper-Best-Fit Model Parameters at 2658 Planetographic Latitudeate belt of sizes 0.2—0.5 em in radius and optical depth

Parameter Best fit at 1.05 em of 0.006 and 0.04, respectively, at these locations,being distributed at altitudes below the 50 mbar level.

Mass production rate profile p0 5 3 mbar Aerosol models from IUE photometry by Tomasko et al.py 5 0.8 mbar (1986) favor hazes with sizes within a factor of 2 of 0.2C 5 1.82 3 10214 g cm22 sec21

em in radius at both equatorial and 408N planetocentriclatitude regions. The analysis of Voyager 2 imaging eclipseHydrazine production rate profile p0 5 330 mbar

py 5 30 mbar observations by West (1988) gives some model solutionsWithin a factor of five of with particle sizes in the range from 0.01 to 0.05 em in

C 5 2.12 3 10213 g cm22 sec21the upper stratosphere. The microphysical models whichproduce particle sizes greater than 0.1 em need imaginaryCharge 3 electrons per particlerefractive indices in the radiative transfer models in excessof 0.1 em, charge being

proportional to radius of 0.1 at 230 nm to be compatible with the observationsat the equator, which is unrealistic from laboratory esti-

Imaginary refractive indices at 0.027, 0.018, 0.008, and 0.0045 mates of expected materials in the jovian atmosphere. ForF218W, F255W, F336W,

the south polar region, there are no model solutions com-and F410M channelspatible with particle sizes greater than 0.1 em, for any set

JOVIAN STRATOSPHERIC AEROSOLS 643

TABLE VAerosol Model Properties at 2658 Planetographic Latitude

Mean Column Cumulativep1 p2 radius abundance Optical depth optical depth

Layer (mbar) (mbar) (em) (3109 cm22) at 410 nm at 410 nm

1 0.5 0.7 0.0132 1.10 0.0006 0.00062 0.7 1.0 0.0138 1.96 0.0020 0.00263 1.0 1.5 0.0142 3.60 0.0060 0.00864 1.5 2.2 0.0143 6.67 0.0192 0.02795 2.2 3.3 0.0144 9.66 0.0442 0.07216 3.3 4.8 0.0163 7.68 0.0712 0.14337 4.8 7.0 0.0234 3.41 0.0716 0.21498 7.0 10.4 0.0354 1.81 0.0702 0.28519 10.4 16.5 0.0473 1.48 0.0855 0.3706

10 16.5 27.7 0.0561 1.48 0.1113 0.481811 27.7 47.7 0.0623 1.55 0.1429 0.624712 47.7 81.7 0.0669 1.54 0.1673 0.792113 81.7 141.4 0.0700 1.53 0.1902 0.982214 141.4 243.4 0.0717 1.30 0.1867 1.169015 243.4 404.4 0.0725 0.50 0.1156 1.2846

of imaginary refractive indices. The present analysis gives profile would be better determined by a coupled aerosol–photochemical model, in which the UV radiation reachinghaze optical depths at 410 nm of about 0.3 and 1.3 at the

equator and south polar region, respectively. In the case the atmosphere is not only attenuated by the ammoniaphotodissociation processes, but also by the haze absorp-of the equator, this is consistent with the fact that the

haze must be nearly transparent in the red region of the tion due to the aerosols existing in the atmosphere at everytime step. This would ultimately lead to a proper estimationspectrum, as required from photometric observations at

those wavelengths. of the hydrazine production rate profile. Another im-portant uncertainty comes from the use of a one-dimen-Regarding microphysics, there are significant differences

between equatorial and south polar region models. The sional model, in which horizontal motions are neglected.It is likely, however, that the results from the Galileo mis-high latitude models require that the aerosols must be

considerably more charged than at equatorial latitudes, sion give important information on that respect and, there-fore, be used in future modeling of the jovian strato-unless very high values in the imaginary refractive index

are accepted at the equator. This means that the charging spheric aerosols.mechanism is more efficient at high latitudes than at theequatorial regions. On the other hand, while hydrazine ACKNOWLEDGMENTScondensation onto aerosol particles dominates the aerosol

The Principal Investigator of the dataset used in this work, Haroldloading at the equator, its influence at high latitudes is notWeaver, and the STScI staff are gratefully acknowledged. I thank K.as clear. At high latitudes, the embryo production rate isRages and E. Karkoschka for carefully reviewing the manuscript. This

much higher than at the equator, and the resulting optical work was supported by the Comision Nacional de Ciencia y Tecnologıadepths are substantially higher in the stratosphere. under contracts ESP94-0719 and ESP94-0803.

For the best-fit models, there is also a difference in thedependence of the refractive imaginary index with wave- REFERENCESlength at the equator compared to that at the south polar

ATREYA, S. K., T. M. DONAHUE, AND W. R. KUHN 1977. The distributionregion, which may reflect a compositional difference. Theof ammonia and its photochemical products on Jupiter. Icarus 31,imaginary index spectrum at the south polar region is flatter348–355.than the equatorial one, which decreases strongly with

BRANDRUP, J., AND E. H. IMMERGUT 1966. Polymer Handbook. Intersci-wavelength. The spectrum at the equator is similar in shapeence, New York.

to that of the resulting material from proton irradiation ofFUCHS, N. A. 1964. The Mechanics of Aerosols. Pergamon Press, New

CH4 1 H2 mixtures, but is more absorbing. York.The presented microphysical models have several uncer- GLADSTONE, G. R., M. ALLEN, AND Y. L. YUNG 1996. Hydrocarbon

tainties in their input parameters. The microphysics of em- photochemistry in the upper atmosphere of Jupiter. Icarus 119, 1–52.bryo production and formation region is not well under- HIDY, G. M., AND J. R. BROCK 1970. The Dynamics of Aerocolloidal

Systems, I. Pergamon Press, Oxford, United Kingdom.stood. On the other hand, the hydrazine production rate

644 FERNANDO MORENO

jovian atmosphere aerosol content determined from a satellite eclipseHAMILL, P., O. B. TOON, AND C. S. KIANG 1977. Microphysical processesaffecting stratospheric aerosol particles. J. Atmos. Sci. 34, 1104–1119. observation. Icarus 30, 697–729.

STAMNES, K., S. TSAY, W. WISCOMBE, AND K. JAYAWEERA 1988. Numeri-HUNTEN, D. M. 1975. Vertical transport in atmospheres. In Atmospheresof Earth and the Planets (B. M. McCormac, Ed.), pp. 59–72. D. cally stable algorithm for discrete-ordinate-method radiative transfer

in multiple scattering and emitting layered media. Appl. Opt. 27, 2502–Reidel, Dordrecht.2509.KARKOSCHKA, E. 1994. Spectrophotometry of the jovian planets and

STROBEL, D. F. 1975. Aeronomy of the major planets: PhotochemistryTitan at 300- to 1000-nm wavelength: The methane spectrum. Icarusof ammonia and hydrocarbons. Rev. Geophys. Space Phys. 13, 372–382.111, 174–192.

TOMASKO, M. G., R. A. WEST, AND N. D. CASTILLO 1978. PhotometryKASTEN, F. 1968. Falling speed of aerosol particles. J. Appl. Meteor. 7,and polarimetry of Jupiter at large phase angles. I. Analysis of imaging944-947.data of a belt and a zone from Pioneer 10. Icarus 33, 558–592.KAYE, J. A., AND D. F. STROBEL 1983. HCN formation on Jupiter. The

TOMASKO, M. G., E. KARKOSCHKA, AND S. MARTINEK 1986. Observationscoupled photochemistry of ammonia and acetylene. Icarus 54, 417–433.of the limb darkening of Jupiter at ultraviolet wavelengths and con-

KRITZ, M. A. 1975. Formation Mechanisms of the Stratospheric Aerosol.straints on the properties and distribution of stratospheric aerosols.

Ph.D. Dissertation, Yale University. Icarus 65, 218–243.LINDAL, G. F., G. E. WOOD, G. S. LEVY, J. D. ANDERSON, D. N. TOON, O. B., R. P. TURCO, AND J. B. POLLACK 1980. A physical model

SWEETNAM, H. B. HOTZ, B. J. BUCKLES, D. P. HOLMES, P. E. HOMS, of Titan’s clouds. Icarus 43, 260–282.V. R. ESHLEMAN, G. L. TYLER, AND T. A.CROFT 1981. The atmosphere

TURCO, R. P., P. HAMILL, O. B. TOON, R. C. WHITTEN, AND C. S. KIANGof Jupiter: An analysis of the Voyager radio occultation measurements.1979. A one-dimensional model describing aerosol formation and evo-J. Geophys. Res. 86, 8721–8727.lution in the stratosphere. I. Physical processes and numerical analogs.

MOSES, J. I., K. RAGES, AND J. B. POLLACK 1995. An analysis of Neptune’s J. Atmos. Sci. 36, 699–717.stratospheric haze using high-phase-angle Voyager images. Icarus

WALLACE, L., J. J. CALDWELL, AND B. D. SAVAGE 1972. Ultraviolet113, 232–266.

photometry from the Orbiting Astronomical Observatory. III. Obser-ORTIZ, J. L., F. MORENO, AND A. MOLINA 1993. Absolutely calibrated vations of Venus, Mars, Jupiter, and Saturn longward of 2000 A.

CCD images of Saturn at methane band and continuum wavelengths Astrophys. J. 172, 755–769.during its 1991 opposition. J. Geophys. Res. 98, 3053–3063. WEST, R. A. 1979. Spatially resolved methane band photometry of Jupiter

OWEN, T. 1969. The spectra of Jupiter and Saturn in the photographic I. Absolute reflectivity and center-to-limb variations in the 6190-,infrared. Icarus 10, 355–364. 7250-, and 8900-A bands. Icarus 38, 12–33.

POLLACK, J. B., K. RAGES, S. K. POPE, M. G. TOMASKO, P. N. ROMANI, WEST, R. A. 1988. Voyager 2 imaging eclipse observations of the jovianAND S. K. ATREYA 1987. Nature of the stratospheric haze on Uranus: high altitude haze. Icarus 75, 381–398.Evidence for condensed hydrocarbons. J. Geophys. Res. 92, 15037– WEST, R. A., D. F. STROBEL, AND M. G. TOMASKO 1986. Clouds, aerosols,15065. and photochemistry in the jovian atmosphere. Icarus 65, 161–217.

PRYOR, W. R., AND C. W. HORD 1991. A study of photopolarimeter WEST, R. A., C. W. HORD, K. E. SIMMONS, D. L. COFFEEN, M. SATO,system UV absorption data on Jupiter, Saturn, Uranus, and Neptune: AND A. L. LANE 1981. Near ultraviolet scattering properties of Jupiter.Implications for auroral haze formation. Icarus 91, 161–172. J. Geophys. Res. 86, 8783–8792.

SMITH, D. W. 1980. Galilean satellite eclipse studies II. Jovian strato- WEST, R. A., E. KARKOSCHKA, A. J. FRIEDSON, M. SEYMOUR, K. H.spheric and tropospheric aerosol content. Icarus 44, 116–133. BAINES, AND H. HAMMEL 1995. Impact debris particles in Jupiter’s

stratosphere. Science 267, 1296–1301.SMITH, D. W., T. F. GREENE, AND R. W. SHORTHILL 1977. The upper