The effect of ammonia ice on the outgoing thermal radiance from the atmosphere of Jupiter

ICARUS

52, 94-l 16 (1982)

The Effect of Ammonia

Ice on the Outgoing

the Atmosphere GLENN

Radiance

from

of Jupiter

S. ORTON, JOHN F. APPLEBY,

Earth and Space

Thermal

AND

JOHN V. MARTONCHIK

Sciences Division, Jet Propulsion Laboratory, California Institute 4800 Oak Grove Drive, Pasadena, California 91109

of Technology,

Received March 22, 1982; revised July 19, 1982 We examine the effects of NH3 ice particle clouds in the atmosphere of Jupiter on outgoing thermal radiances. The cloud models are characterized by a number density at the cloud base, by the ratio of the scale height of the vertical distribution of particles (H,) to the gas scale height (H,), and by an effective particle radius. NH, ice particle-scattering properties are scaled from laboratory measurements. The number density for the various particle radius and scale height models is inferred from the observed disk average radiance at 246 cm-‘, and preliminary lower limits on particle sizes are inferred from the lack of apparent NH2 absorption features in the observed spectral radiances as well as the observed minimum flux near 2100 cm-‘. We find lower limits on the particle size of 3 urn if H,JHg = 0.15, or 10 pm if HplHg = 0.50 or 0.05. NH, ice particles are relatively dark near the far-infrared and 8.5pm atmospheric windows, and the outgoing thermal radiances are not very sensitive to various assumptions about the particle-scattering function as opposed to radiances at 5 urn, where particles are relatively brighter. We examined observations in these three different spectral window regions which provide, in principle, complementary constraints on cloud parameters. Characterization of the cloud scale height is difficult, but a promising approach is the examination of radiances and their center-to-limb variation in spectral regions where there is significant opacity provided by gases of known vertical distribution. A blackbody cloud top model can reduce systematic errors due to clouds in temperature sounding to the level of 1K or less. The NHr clouds provide a substantial influence on the internal infrared flux field near the 600-mbar level.

not changed that expectation substantially. For Jupiter the expectation of ammonia clouds has been translated into an interpretation of the visibly bright regions (zones) as clouds of NH3 ice crystals (e.g., Owen and Terrile, 1981). In the present study we explore characteristics of NH3 clouds in Jupiter to provide a basis for preparing data analysis techniques for Galileo Probe and Orbiter experiments which will detect infrared radiation influenced by NH3 ice or sample their properties in situ. There is an abundance of cloud models for the Jovian atmosphere which are derived from analysis of reflected solar radiation (e.g., Tomasko et al., 1978; Sato and Hansen, 1979; West and Tomasko, 1980; Stoll, 1980). We have not felt compelled to rely on any one of those as a definitive initial baseline model, particu-

INTRODUCTION

We are interested in the effects of NH3 ice clouds on the thermal radiation observed emerging from the atmosphere of Jupiter. For both Jupiter and Saturn NH3 has been positively identified in the gas phase by its spectroscopic signature in sufficient abundance that it is certain to be frozen out in the colder regions of the atmosphere below the temperature minimum level. Weidenschilling and Lewis (1973) have suggested that NH3 would be the major uppermost cloud in the Jovian and Saturnian tropospheres, as well as one of the main cloud systems in the Uranian and Neptunian atmospheres. Since the time of their study a greater understanding of the NH3 abundances and the temperature structures of both Jupiter and Saturn has 94 0019-1035/82/100094-23$02,00/O Copyright All rights

0 1982 by Academic of reproduction

Press,

in any form

Inc. reserved.

NH3 ICE ON JUPITER

larly as we are really interested in interpreting NH3 cloud effects at somewhat longer wavelengths than those addressed by almost all of the studies cited above. In addition, the models in those studies are also quite sensitive to the presence of other overlying particle layers, and some of these models are described by cloud layers which are characterized only in terms of optical depths at specific wavelengths. In the analysis of Jovian thermal radiation accurate cloud models are crucial to the successful interpretation of certain spectral regions where gaseous absorbers tend to be relatively weak. The importance of accounting for clouds in radiative transfer models is also stressed because the NH3 clouds seem to be distributed across the face of the planet quite heterogeneously. This is dramatically illustrated by images of Jupiter at 5 urn, a spectral region characterized by very weak gaseous absorption, where the coldest regions, strongly correlated with the location of areas with visibly bright albedos (Orton et al., 1981; Owen and Terrile, 1981), are spatially differentiated from warmer regions which are freer of cloud obscuration. Were we to assume a clear atmosphere everywhere across the disk, the temperature structure near the 500- to 700-mbar level would vary by up to 4K from region to region in order to match the observed variation in brightness temperature (Orton, 1975a; Pirraglia et al. 1981). Such a variation appears to be contradicted by the smaller temperature differences between the spatially distinct locations sounded by the Voyager Radio Subsystem (RSS) occultation experiment (Linda1 et al., 1981). Furthermore, such differences are somewhat unexpected on dynamical grounds (Ingersoll and Cuzzi, 1969). The apparent temperature differences are more likely to be due to the effect of aerosol opacity in spectral windows in the far infrared near 200 cm-’ (Hanel et al., 1979; Orton et al., 1981), near 1150 cm-’ (Sinton et al., 1980), and near 2100 cm-’ (4.8 pm). The influence of absorbing/scat-

95

tering constituents in the atmosphere is thus present over a wide spectral range of thermal radiation; each of these regions will be examined to some extent in this report. Previous modeling of NH3 cloud effects was done through rather crude approximations. Orton (1975a) used an effective blackbody cloud top with unit emissivity in zones to minimize differences between recovered temperatures in zones and apparently warmer (visibly dark) belts near the 600-mbar pressure level. Another approximation was the straightforward application of NH3 ice absorption coefficients (Taylor, 1973), scaled by the column abundance of NH3 ice particles (Orton, 1975b). Development of more realistic models of NH3 ice clouds was initiated by Marten et al. (1981) using a modified isotropic phase function for scattering by individual particles and a restricted form for the vertical distribution of the particle number density formed primarily from Voyager IRIS experiment observations. Our primary purpose in this study is to examine a large range of possible models in order to understand their influence on various spectral regions. We use only comparisons with disk-averaged data (Gillett et al., 1969; Gillett, 1973; Hanel et al., 1981), which may yield limited insight of a detailed quantitative nature owing to the inhomogeneous cloud distribution over the planet as discussed above. We do, however, plan to consider spatially resolved thermal observations of the planet in future work, and this report also serves to provide a description of the diagnostic techniques which will be used in the analysis of better quality data. Here we examine cloud models with a large range of values for particle size, scale height, and number density. We also examine the propagation of errors through uncertainties in the basic optical data and the particle phase function, as well as through observational errors. In addition, we will discuss the probable contribution of NH1 clouds to the total atmospheric infrared opacity, and their effects on the internal in-

96

ORTON, APPLEBY,

frared radiation field, and we will test the accuracy of efficient approximations to the radiative transfer algorithm used in this work, which inludes the detailed effects of multiple scattering. FUNDAMENTALS

OF THE

CALCULATIONS

Our basic radiative transfer technique is based on the discrete ordinate matrix operator algorithm of Grant and Hunt (1969) in the context of a multiple-layer atmospheric model which uses 20 homogeneous atmospheric layers to approximate the inhomogeneous atmospheric properties within each decade of change in the total atmospheric pressure. In principle, this is a greater degree of generalization than available to Marten et al. (1981), who were restricted to a modified isotropic phase function associated with the radiative transfer method of successive orders of scattering. However, in practice, accurate descriptions of the scattering phase functions of NH3 ice crystals formed in a suspended or free-fall state in low-temperature HZ and He environments have not been the focus of much recent meteorological work related to cloud physics. One of the only studies of relevance is the laboratory investigation of the phase functions of NH3 ice at visible wavelengths reported by Holmes ef al. (1980) and Holmes (1981). We took the NH3 ice indices of refraction from the work of Martonchik et ul. (1982), the infrared portion of which was largely based on data presented by Sill et al. (1980). With these fundamental optical properties we formulated test models for phase functions based on Mie theory and perturbations of Mie properties. One of these is the assumption of isotropy, relevant to comparisons with the results of Marten et al. Two other assumed models are based on the semiempirical form of irregular particle phase functions developed by Pollack and Cuzzi (1980). The first of these is a parameterization of “cubic” particles presented by Pollack and Cuzzi (characterized, using their parameterization scheme, by Y= 1.30,

AND MARTONCHIK

x0 = 4.00, g = 1.50). The second served as our standard model for the phase function, a parameterization which we adjusted to correspond as closely as possible to the measured phase functions of the “Tucson” tetrahedrally shaped NH-( ice particles at the visible wavelengths of Holmes’ (1981) measurements. The phase function of NH3 crystals formed at -98°C measured by Holmes at 0.632-km wavelength is shown in Fig. 1, along with the Mie phase function for the same effective particle size, corresponding to an equivalent volume radius of 7 urn, and a rough measure of its rather narrow variance. Using the scheme of Pollack and Cuzzi (1980) we made a best fit to the observed function with the parameters j 4.00

‘\’

”

’

I1

,

”

\

2.00

-

I .w

-

0.20-

0.10

-

\ \

I ‘\

I

FIG. I. Phase function of NH3 ice crystals. Triangles show laboratory measurements of Holmes (1981). Dashed line shows phase function of equivalent volume spheres. Solid line shows best fit to measurements using the semiempirical function of Pollack and Cuzzi (1980). This scaled fit provides the baseline assumption for NH? particle phase functions in this study.

NH3 ICE ON JUPITER

= 1.49, x0 = 14, g = 0.10. The value of Yis the ratio of the surface area of a tetrahedron to the surface of a sphere of equivalent volume; even substantial variations of this value did not perturb the function significantly and so we saw no reason to change to a better “effective value.” The value of x0 is the particle size parameter serving as the boundary between phase functions described by Mie theory and the phase functions of large irregular particles. The relatively large value of x0 is required to adjust the blending of the Mie-like backscattering lobe with scattering by large particles characterized without this feature for the narrow distribution of particle sizes in the laboratory experiments. The value of g, a characterization of the forward- to backward-scattered radiation, was most sensitive to the location of the phase function minimum for a given choice of x0. Figure 1 also shows the best-fit function; it is in rough agreement with the observations, although it slightly overestimates the observed function for high scattering angles. We bear in mind Tomasko’s caveat (personal communication) that the high phase angle measurements are somewhat less certain than the low ones. With these three “large-particle” parameters fixed, we apply the algorithm given by Pollack and Cuzzi using appropriate values of the NH3 indices of refraction and appropriate scaling in x0 to a variety of wavelengths and particle sizes. Later we show the effects of varying our assumptions about this phase function on scattering and absorption at various wavelengths. The single-scattering albedos quoted later in this article were derived from the ratio of scattering coefficient to extinction coefficient, which are, respectively, based on the scattering and extinction efficiencies defined by Pollack and Cuzzi (1980). The asymmetry parameters quoted were derived from explicit numerical integrations. One additional assumption made in the calculations was that finite spectral bandwidth measurements could be modeled ade-

97

quately by using the monochromatic radiative transfer algorithm at an effective band center and by using a gaseous optical depth equal to the negative logarithm of the gaseous transmission averaged over the spectral interval. Of course this is strictly incorrect, as there is no simple way to approximate accurately the effect of the many changes in the gaseous transparency over the spectral interval on, for example, the effective single-scattering albedo. The problem is well documented in terrestrial scattering applications [for example, see the discussion of Wiscombe and Evans (1979) or Fouquart et al. (1980)]. For the cases discussed in this report where muhiple line absorption and scattering are both computed, our examination of the details of line formation showed less than a 0.8% difference in the outgoing radiance calculation using an average spectral band transmission on the one hand, and the average of monochromatic radiance calculations across a line on the other hand. We feel that this is largely due to the relatively dark nature of the particles; at the frequencies explored, none of the models were characterized by a single-scattering albedo greater than 0.83. For darker particles scattering effects are less significant, and the error associated with this approximation becomes even smaller. At 5 p,m, particles in the models were indeed brighter, but we did not investigate the effect of absorbing gases and so the problem of highly variable gaseous absorption in the spectral interval did not arise. For spectral regions where Hz dominates the opacity the collision-induced dipole absorption varies so slowly over the relevant spectral intervals that we could treat the radiative transfer problem as monochromatic with respect to the variation of spectral properties over the relevant bandpass. Some properties characterizing the cloud were fixed and others were left as variables to be determined. The particle size distribution was described using the standard gamma function of Hansen (1971) with a

ORTON,

APPLEBY,

small effective variance of 0.10. From our standpoint, such a variance describes an effectively monodisperse distribution, for the sole reason of simplicity, but provides some “smoothing” of the phase function to minimize the number of terms required for its accurate description using a Legendre expansion. The base of the cloud is fixed near 630 mbar, coincident with the level of NH3 condensation in our temperature structure if the molar fraction of NH3 in the deep atmosphere is assumed to be 2.2 x 10m4(Linda1 et al., 1981). The assumption of negligible numbers of particles at greater pressures implicitly presumes the rate of particle resublimation to be greater than the rate of fallout in precipitation. No particles are distributed above 100 mbar, where stability against convection is assumed to make further upward convective transport rather unlikely. Our temperature structure model is taken from an average of Voyager RSS occultation experiment results (Linda1 et al., 1981) and is shown in Fig. 2 of our companion article on the far-infrared Jovian spectrum (Orton et al., 1982). Consistent with those data and with the results of Gautier et al. (1981), the uniform mixing ratios of H2 and He were assumed to be 0.89 and 0.11, respectively. While the RSS results are derived from regions at low latitudes, they are quite consistent with a mean of Voyager IRIS results (Conrath, private communication; Pirraglia et al., 1981) below about 200 mbar, and they are much less subject to the influence of NH3 clouds than temperatures recovered from IRIS data. Cloud parameters which are left as variable include (1) the effective particle radius, expressed as the equivalent volume radius to be consistent with the interpretation adopted by Pollack and Cuzzi (1980), (2) the particle number density at the base of the cloud, and (3) the ratio of the particle scale height to the gas scale height. The equivalent volume radius is allowed to vary between 1 and 100 Frn, corresponding to Rossow’s (1978) calculations limiting the particle coagulation size. It is noted that

AND MARTONCHIK

larger sizes may be possible, as Rossow’s calculations are strictly only relevant to coagulating liquid droplets, and ice crystals may be slightly more efficient at self-adhesion. Our work differs significantly from that of Marten et al. (1981) in allowing the scale height to operate as an extra degree of freedom. We wanted to be able to examine the possibilities of (1) high-altitude convective distribution, characterized by a large particle scale height to gas scale height ratio (HdH, = 0.50); (2) level-by-level equilibrium with the local NH3 gas scale height ratio (roughly H,,/H, = 0.15); and (3) largescale precipitation with little convective uplift, characterized by a small particle scale height to gas scale height ratio (H,IH, = 0.05). Consistent with the results of our companion article reporting on the far-infrared Jovian spectrum (Orton et al., 1982), we did not provide a deeper cutoff below the temperature minimum as did Marten et al. (1981). This is consistent with our results which implied a global mean vertical distribution of NH3 gas consistent with saturation equilibrium up to the IOO-mbar level with a subsequent cutoff in the stratosphere. In the sections which follow we examine, in several spectral regions of relative transparency, the influence of various cloud models and attempt to constrain the cloud parameters that we have left variable. FAR-INFRARED THERMAL EMISSION

We use the spectrum in the neighborhood of 250 cm-’ to impose some initial constraints on models within the three dimensions of parameterization available. Our initial constraint is provided by the wholedisk equivalent brightness of Jupiter near 246 cm-i. This spectral point is chosen because it is relatively insensitive to the influence of NH3 rotational line manifolds at higher and lower frequencies (at a spectral resolution equal to or exceeding that of the Voyager IRIS experiment, equivalent to 4.3 cm -I). Also the collision-induced dipole of H2 is extremely weak at this

NH3 ICE ON JUPITER

location in the spectrum, and, while other positions between NH3 manifolds could have served equally well, the 246-cm-’ position appears to provide the least measurement noise in the IRIS spectrum. The brightness temperature at this position is taken from the whole-disk equivalent spectrum shown by Hanel er al. (1981) in their Fig. 5, and its value is 134.5 2 O.lK with the uncertainty assumed from their estimates of the measurement noise associated with the co-addition of the five spectra composing the average. Our approach is to vary the value of the number density at the base of the cloud to match the brightness temperature for various values of H,JH, (0.50, 0.15, and 0.05) and several values of the mean particle radius (1,3, 10,30, and 100 km). The families of solutions for number density vs particle size for each of the scale heights assumed are, respectively, shown in the three panels of Fig. 2. The predominant shape of the curves for large radii is determined by the radius-squared dependence of the mean

IO

16

-

Hp/Hg=0.50

_

cross-sectional area. The uncertainty associated with the determination of the number density is about +23%, and it is derived from the (220%) uncertainty of the imaginary component of the NH3 index of refraction, as well as from measurement error. In Fig. 2 we also plotted a curve corresponding to the cloud bottom number density for each radius which one would expect from integrating through the cloud scale height and equating the total mass to the mass burden of NH3 freezing out of the gas phase in a slowly upwelling fluid. In all cases the number density determined from the 246-cm-’ datum is shown to be below this value. Thus we can state that these families of solutions (1) do not violate mass balance constraints and (2) indicate that precipitation is taking place in an average sense over the apparent disk. This conclusion appears to be consistent with the depletion of NH3 ice required by Marten et al. (1981) regarding the amount of NH3 ice expected from equilibrium with the local NH3 partial pressure. However, their examina-

Hp/Hg=0.15

EFFECTIVE

99

Hp/Hg=O.M

PARTICLE RADIUS (pm)

FIG. 2. Families of cloud models matching the disk-averaged thermal radiance measured at 246 cm-l. Solid lines indicate the models matching the observation. Dashed lines indicate the particle number density at the base of the cloud if all the condensate mass available in a slowly rising parcel of fluid were contained in particles of the effective radii indicated, and distributed vertically according to the respective scale heights as marked. Hatched vertical bars indicate lower particle size limits derived for each scale height model from this study.

100

ORTON, APPLEBY,

tion of data was weighted heavily by observations of the North Equatorial Belt (NEB), which is probably far clearer of NH3 clouds than some other planetary regions such as zones. The suggestion of precipitation on a globally averaged basis is consistent with an average over relatively clear, dry regions (belts) and relatively cloudy, wet regions (zones). The presence of significant local precipitation would, in fact, seem to make substantial upward particle convective transport and large cloud scale heights unlikely. However, the results in Fig. 2 for globally averaged data weakens the value of such a conclusion considerably, as we could be averaging very large individual effects together meaninglessly. Therefore we continue to consider the possibility of large cloud scale heights in our investigation. Another constraint on the family of solutions is the lack of evidence for strong NH3 ice features in the observed spectrum, a consideration also examined by Marten et al. (1981). The constraint discriminates against small particles whose scattering characteristics tend to preserve absorption features present in the NH3 ice spectrum. -We discovered that the frequency most sensitive to the presence of ice absorption in the far infrared is near 400 cm-‘. This is slightly higher in frequency than the NH3 ice absorption maximum near 363 cm-‘, owing to the H2 gas absorption which acts to mask the ice influence on the outgoing spectrum near 370 cm-‘. Using the measurement uncertainty in this part of the Voyager whole-disk spectrum (Hanel et al., 1981), estimated very conservatively at *0.5K in brightness temperature, we can place lower bounds on the value of the mean radius which would cause no absorption feature to appear above the noise level of the spectral measurement. The results of this sensitivity criterion, measured as a function of particle size for the three scale height cases, are shown in Fig. 3. For the particle to gas scale height ratio, HP/H, = 0.50, rminZ 10 pm; for HP/H, = 0.15, Ymin^-

AND MARTONCHIK 1181

,

I

I

I

CLEAR

1131’

1.0

I 3.0 EFFECTIVE

, 10.0 PARTICLE

1

30.0

I 100.0

I

RADIUS (,m)

FIG. 3. Sensitivity of the infrared radiance at 400 cm-’ to the appearance of an NH, ice absorption feature for various cloud models. No feature is detected to within 0.5K brightness temperature, compared with the smooth spectrum of the clear atmosphere. As shown by the vertical hatched line, this rules out cloud models with small particles and large values of the particle scale height to gas scale height ratio (shown adjacent to the appropriate curves).

3 Frn; and for HpIHg = 0.05, values of r,,,,, are acceptable down to the lower limit of 1 p,rn adopted from Rossow’s (1978) study. Figure 2 shows an independent restriction on the size of particles in the HP/H, = 0.05 case, which is described in a later section. For the H,IH, = 0.15 case, this particle size restriction is consistent with the value derived by Marten et ul. (1981) for a cloud model with a similar scale height and for similar considerations of the detection limit of NH3 features. The effect of cloud models in the HP/H, = 0.15 family (Fig. 2) with various particle sizes in the 250- to 750cm-l spectrum is illustrated in Fig. 4. The families of models with the lower size restrictions indicated in Fig. 2 are those adopted in our companion article, which explores the IOO- to 300-cm-’ spectrum at a resolution equivalent to 1.65 cm-‘. As it turns out, radiances at frequencies in between other strong NH3 manifolds are not significantly more diagnostic of the cloud properties than at 246 cm-‘. With the number densities of various cloud models fixed by the 246-cm-’ radiance, differences between the synthetic spectra produced by the models are no greater than 2.5K in

NH3 ICE ON JUPITER

FREQUENCY

-I (cm 1

FIG. 4. Whole-disk

spectrum of Jupiter models with particle size limits adopted in the study. The clear atmosphere spectrum is shown for comparison. Note the values at 400 cm-’ are also shown in Fig. 3.

HdH,

= 0.15 and

brightness temperature. Detection of such differences at the two-standard deviation level would therefore require measurements with signal-to-noise ratios somewhere in excess of 40. The values of the single-scattering albedo associated with the NH3 particles in this family of models, with the particle size restrictions derived above, are no greater than 0.83 in the far-infrared spectral regions where the ice absorption is locally a minimum. The insensitivity to assumptions made about the particle-scattering phase function is displayed in Fig. 5 (a) for isotropically scattering particles with other optical properties defined by Mie theory, (b) for spherical particles with all properties defined by Mie theory, (c) for irregular particles using the Pollack and Cuzzi (1980) empirical phase function for “cubic” particles, and (d) for the baseline particle model whose phase function is defined by fitting Holmes’ (1981) tetrahedrally shaped NH3 ice particles. Figure 5 shows the If,/ Hg = 0.50 case which displayed the largest differences. For 1OO-km-particle models, the single-scattering albedos range from 0.55 to 0.61 and the asymmetry parameter from 0.76 to 0.87 (except for isotropic scattering, where it is 0.00). For IO-pm-particle models the single-scattering albedo and

101

asymmetry parameter are identical to 1% (except for isotropic scattering) with respective values of 0.81 and 0.53. For lOOpm particles, differences in the outgoing radiance at 246 cm-’ are at most 1K; for lokm particles (the minimum allowed particle size, according to a criterion of the kind illustrated by Figs. 3 and 4), differences between various models are barely distinguishable, except for the isotropic phase function which is some 2.5K lower than the rest. These differences in brightness temperature correspond to intensity differences of 2 and 5%, respectively; the differences produced by the suite of possible NH3 ice particle-scattering functions is therefore small, if not entirely negligible. Figure 4 also serves to illustrate the outgoing spectrum in the 250- to 750-cm-’ region, where the atmospheric opacity is dominated by the collision-induced Hz dipole absorption. Obviously the largest influences on the spectrum, compared with that of a clear atmosphere, are in the regions where HZ is least opaque, with the strongest

L EMISSION

ANGLE

COSINE

(p’)

FIG. 5. Sensitivity of computed radiances at 246 cm-r to various assumptions about the particle-scattering phase function for IOO- and IO-urn particles with HdHs = 0.50. Phase function models are coded as follows: baseline (solid line), “cubic” particles (short dashed line; see text), spherical particles (long dashed line), and isotropically scattering particles (intermediate dashed line). Curves not shown are indistinguishable from solid curve.

ORTON, APPLEBY,

102

effects near the lowest frequencies. For the lOO+m particles, a size favored by Marten ef al. (1981), the differences near 480 and 750 cm-’ are small but not completely negligible. The HplHg = 0.50 and HJH, = 0.05 cases also support similar conclusions. We explored the possibility of discriminating between various scale height and particle size models by examining their center-tolimb brightness. For 246 cm-i the intensity of outgoing thermal radiance as a function of emission angle is shown in Fig. 6 for IOOkm particles in the three scale height models examined in this study. As is obvious by inspection of this figure, such a measurement is unlikely to be successful as a major diagnostic tool to differentiate between the various scale height models. The greatest differences among the models are at high emission angles (p = 0. l), where the small scale height model is differentiated from the others by some 5% in radiance. For higher frequencies, such as 270 cm-‘, which is further from any NH3 line absorption and is the effective center of a channel filter in the Galileo Orbiter Photometer-Polarimeter-Radiometer (PPR) experiment,

‘5Or-j

1101

__

I

I 0.0

0.5

1.0 EMISSION

ANGLE

COSINE

I

(~1

FIG. 6. Dependence of thermal radiance at 246 cm-l on emission angle (coss’p) for IOO-pm particles and various scale height models. Model identifications are: HJH, = 0.50 (long dashed line), H&H, = 0.15 (solid line), and HJH, = 0.05 (short dashed line). Alternating dashed line represents the H&H, = 0. IS case computed assuming particle contributions to the atmospheric opacity, but with no scattering assumed.

AND MARTONCHIK

the differences in radiance between various scale height models at any emission angle are less than 2.5%. We reserve for future work the obvious exploitation of angular and spatial resolution available in the far-infrared measurements of the Pioneer Infrared Radiometer (IRR) and Voyager IRIS experiments. This is also true of the 8- to 9- and 5-pm regions, where the IRIS whole-disk spectrum is characterized by much poorer signal-tonoise ratios than the far infrared and is therefore not referenced in the sections which follow. 8- TO

9-pm

THERMAL

EMISSION

Another partial window in the Jovian atmosphere lies between the u2 bands of NH3 and the v4 band of CH4, roughly defined by the frequency range of 1100 to 1200 cm-‘. This is also a spectral region explored by Marten et al. (1981), and a strong NH3 ice feature exists nearby at 1057 cm-‘. In exploring diverse spectral regions, the hope is that some additional information will arise from different spectral properties of the ice or of the atmospheric gaseous constituents, or from the different particle size-to-wavelength ratios which change the nature of the radiation interaction with the particles. This region is more complicated than the far-infrared owing to the additional atmospheric absorptions of CH4, NH3, PHI, and CH3D. The center of the v4 fundamental band of PH3 is near I1 18 cm-’ and its absorption is detectable in the Jovian spectrum even at spectral resolutions corresponding to 20 cm-‘. In fact, Orton (1975b) attributed this feature incorrectly to a small NH3 ice absorption feature in the spectral data summarized by Taylor (1973). Therefore Orton’s (1975b) results regarding NH? ice cloud properties are to be viewed with caution and stand to be corrected in part by this report. Radiative transfer calculations in this region must account for molecular absorption. We used a direct (“line-by-line”) computation of atmospheric transmission

NH?

ICE ON JUPITER

based on the method of Scott (1974) as modified recently by Orton (1981) to include Voigt lineshapes. Molecular spectroscopic line parameters were taken from the GEISA list (Husson et al., 1982). The CH4 intensities in this list, derived from the work of Orton and Robiette (1980), were resealed by Husson et al. We are adopting a further resealing along the recommendations of Orton and Robiette (1982) by applying a factor of 0.96 to the intensities presented by Orton and Robiette (1980) in order to be most consistent with recent high-resolution, high-absolute accuracy laboratory work. The NH3 parameters in the GEISA list were derived by Husson et al. (1979). The PHj parameters are based on the work of Goldman et al. (1980) and Tarrago et al. (1981), and the CH3D parameters on the work of Pinkley et al. (1977). The CH4 mixing ratio was assumed to be 1.96 x 10-j, consistent with the recent work by Gautier et al. (1982) with consideration for the resealing of line strengths from the GEISA list. The vertical distribution of NH? gas was taken from our companion article [Orton et al. (1982); see Fig. 2 of that article]. The CH3D mixing ratio was assumed to be 3.5 x lo-‘, consistent with

1100 FREQUENCY

103

analysis of recent spacecraft and Earthbased measurements (Kunde et al., 1982; Knacke et al., 1982). The abundance of PH3 was set by a constant mixing ratio of 5 x 10eR below the temperature minimum with a cutoff at and above the temperature minimum level. The cutoff is loosely based on the Knacke et al. (1982) analysis of the high-resolution 11OO- to 1200-cm-’ spectrum as well as analysis of earlier data. The PHI mixing ratio was set so that the absorption feature seen in the 20-cm-’ spectra could be reproduced approximately. In fact, this value of the PH3 mixing ratio is between those derived by Knacke et al. (8 X 10m6)and Kunde et al. (decreasing from 1 X lo-’ down to 3 X lo-‘” between 1 bar and 100 mbar) and is of importance to us only insofar as it can provide a credible approximate reproduction of gaseous absorption and the characteristics of the outgoing spectral radiances. Figure 7 shows the 20-cm-’ data used for comparison with the models. The data are from the spectral observations of Gillett et al. (1969), Gillett (1973), and Aitken and Jones (1972). The Aitken and Jones spectrum was degraded to match the roughly 20cm-l -resolution element of the other spec-

1200

1200

-1 (cm )

FIG. 7. Central disk spectrum of Jupiter (assuming a spectral resolution element of 20 cm-‘) for models with various particle sizes and scale height ratios. The models are elements of the families of models shown in Fig. 2, with particle radius values and numbers as described in the text, using 400cm-’ radiance restrictions (Fig. 3). For reference, several Earth-based measurements are also plotted: Aitken and Jones (1972; thin line), Gillett et al. (1969; +), and Gillett (1973; x).

104

ORTON,

APPLEBY,

tral data. The spread of the spectral observations is probably the best indicator of their uncertainty at each frequency. The synthetic spectrum characteristic of a clear atmosphere is shown between 1100 and 1200 cm-’ with radiances exceeding the global data in the 1130- to 1180-cm-’ region: the “isothermal” character of the brightness temperature results from the broad continuum opacity of the Hz collision-induced dipole. We calculated the synthetic spectra for the limiting range of particle sizes as determined by Rossow (1977) and by the preceding section as appropriate to the scale height model displayed. It is clear that the lOO-km-particle mode1 spectra lie above the global observations for H,IH, = 0.50 and 0.15, and that all the HP/H, = 0.05model spectra lie below the observations. Insofar as we are congnizant of the heterogeneous origins of the radiances, i.e., their origin in both clear and cloudy areas of the planet, we must be aware that the number density values derived from globally averaged radiances at 246 cm-’ do not result in the appropriate radiance values at much higher frequencies due to the highly nonlinear nature of the Planck function. While we will not examine separate fits to spatially resolved areas on the planet in this report, we can make some preliminary estimates of the direction and magnitude of the corrections which must be applied to the synthetic “global” spectra shown in Fig. 7. Were the radiances to originate from two types of regions, one clear of NH3 clouds, covering some 40% of the disk (representing belts), and the other including NH3 clouds and covering 60% of the disk (representing zones) (Orton, 1975b), the number densities derived for the clouds would be some 2.5 times higher than the values derived for the global characterization. This value is rather insensitive to the cloud scale heights and particle sizes assumed in each model, and it is derived from fitting a “zone” radiance of 12.75 ergs set-i crnm2 ster-i (cm-i)-‘, which is based, in turn, on the “belt” radiance of 15.24 ergs

AND MARTONCHIK

(cm-‘))’ (taken from a set-’ cm-? ster’ clear atmosphere model calculation) and the global mean of 13.75 ergs sect’ cmm2 ster’ (cm-‘))’ (taken from the observations ) at 246 cm-‘. Near 1150 cm-’ a similarly weighted mean of clear model radiances and cloudy radiances, derived from cloud models with 2.5 times the characteristic number densities of the global models, is generally some 30% higher than the values shown in Fig. 7. Thus, the spectra for the HP/H, = 0.05 cases may not necessarily be ruled out on the basis of being systematically lower than the data. On the other hand, we can probably rule out the large-particle models for the HP/H, = 0.50and 0.15 cases. One caveat remains regarding the models which are ostensibly too bright in this region, however. The absorption due to PH3 gas can always be increased with the effect of decreasing the spectral radiances, although with a greater decrease in the neighborhood of 1118 cm-‘. Our assumptions about the PHj abundances required to fit the low-resolution spectra were not tightly constrained and were heuristic at best. Figure 8 shows the emission angle dependence of the radiance at 1160 cm-’ for the

EMISSION

ANGLE

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(p)

FIG. 8. Dependence of thermal radiation at 1160 cm-’ on emission angle (cos-‘CL) for models with scale height ratios and particle sizes shown in Fig. 7. Limb flattening and brightening are due to the increasing influence of gaseous (CH.,) absorption in the stratosphere.

NH3 ICE ON JUPITER

models producing the spectra shown in Fig. 7. It can be seen for each model that the variation of its spatial intensity from the center to the limb is roughly the same as the variation of its spectral intensity going from 1160 to 1200 cm-r, owing to the increasing dominance of gaseous (CH,) opacity in each situation. The models with the largest scale height ratios and the largest particles produce the steepest limb darkening, and it is possible to differentiate between the models on the basis of their center-to-limb behavior in this spectral region. The few existing center-to-limb observations in this spectral region (e.g., Orton, 1975b; Sinton et al., 1980) show definite limb darkening; these observations included scans with spatial resolution on the order of one-tenth the disk size, oriented parallel to the Jovian equator and centered on wide zone regions. These results apparently discriminate against the models producing the three lower curves in Fig. 8, leaving only the HP/ Hg = 0.50 model or the HP/H, = 0.15 model with 100~p,rnparticles. Increasing the cloud number density by a factor of 2.5, consistent with our crude zone model described above, produces only shallower center-tolimb behavior for each model than is represented by the respective curves in Fig. 8. However, it is possible that the mixture of radiation from clear regions into the scanning aperture could have influenced the center-to-limb observations systematically to produce greater limb darkening than representative of zones or that the filter width is sufficiently broad to allow limb-darkened radiances from outside this spectral window to influence the scan results. We have not computed spectra to correct those shown in Figs. 7 and 8 on the basis of the bimodal cloud distribution because it is too simplistic and because the fraction of planetary cloud coverage assumed is rather arbitrary. In fact, the cloud properties resulting from the bimodal distribution model imply that clear regions should be some 3 times brighter than cloudy regions near 8.6 pm. This is borne out neither by observa-

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tions from the ground (Sinton et al., 1980), which measure only a 30% difference in spatial scans using a medium bandpass filter centered at 8.9 pm, nor by Voyager observations, which show few variations exceeding a factor of 2 [see Fig. 8 of Marten et al. (1981) for the range of 1175-cm-’ radiances]. Improved models will result from the examination of measurements in which regions are resolved spatially and in which higher spectral resolution can better separate the influences of gaseous absorption (PH3 lines, for example) and cloud effects which are spectrally broader. Our brief examination of the influence of NH3 clouds on this spectral region does reveal definitive conclusions however. First, while not plotted in Fig. 7, the limits on the minimum particle size obtained from the examination of 400-cm-’ radiances are apparently sufficient to reduce the effects of the strong NH3 ice absorption feature at 1057 cm-l so that it is not apparent in the Jovian spectrum. Neither the 1057-cm-’ feature nor the small local absorption maximum at 1110 cm-’ (Sill et al., 1980) appear to distort the PH3 v4 band Q-branch feature from its position in the clear atmosphere model at 20 cm-’ resolution. Further, the diagnostic potential of this region complements that of the far-infrared regions. Cloud models constrained by 246-cm-’ (and 400-cm-‘) observations do not produce indistinguishable effects on the 1lOO-to 12000-l radiances. We therefore do not see the same kind of degeneracy which was evident for the far-infrared windows between NH3 manifolds. In fact Fig. 7 implies that the cloud models with IOO-pm particles and H,JH, = 0.50 or 0.15 are inconsistent with the spatially averaged radiance data. This suggests that the 8.5~km region can be very useful in complementing the far-infrared region to discriminate among various models with different scale heights and particle sizes, provided that gaseous absorption in this region can be determined somewhat independently. In Fig. 9 we show the sensitivity of calcu-

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FIG. 9. Sensitivity of computed radiances at I160 cm-r to various assumptions about the particle-scattering phase function for the model with HJH, = 0.50 and effective particle radii of 100 pm. Phase function models are coded as in Fig. 5. The relative lack of sensitivity is due to the low values of the single-scattering albedo, relative to those at 246 cm-r (Fig. 5) or 2100 cm I (Fig. 11).

lated outgoing radiances to various assumptions about the particle-scattering phase function at 1160 cm-l for the HP/HP = 0.50 and IOO-p_m effective radius model for which the greatest variations were seen. It is apparent that very little differentiation among the results appears for a range of particle-scattering phase functions. Over most of the observable disk the model results differ by less than 1K in brightness (or 8% in intensity). We note that the single-scattering albedo for “cubic” particles (0.91) is higher than that for the other models (0.54-0.64), although their asymmetry parameter (0.40) is lower than the rest (0.80-0.97), except for the isotropic case (0.00). 5-km THERMAL

EMISSION

The 5-pm window of the Jovian atmosphere was detected in the spectrum of Gillett er al. (1969) and its heterogeneity across the disk was first demonstrated by Westphal (1969). Later Terrile and Westphal (1977) demonstrated the discrete nature of 5-km radiance levels, lending support to the interpretation that atmospheric

AND MARTONCHIK

opacity in this region was dominated by discrete cloud layers. Detailed radiative transfer models reveal that the known or suspected gaseous constituents in the Jovian atmosphere are unable to provide sufficient absorption in the atmosphere to match the observed “continuum” in this region, even at the highest radiance levels observed across the disk (V. Kunde, private communication). This fact further supports the requirement for the presence of spectrally broad absorbing/scattering effects of clouds. In order to provide some instructive illustrations of cloud effects on upwelling thermal radiation in the 5-pm region, we ignore the weak absorption provided by gases near the center of this window. We justify this on the basis that the weak gaseous absorption present exerts little influence on the outgoing radiance and its angular dependence; we are searching, on the other hand, for cloud properties which modulate the radiation between equivalent temperatures of about 200 and 255K. These extremes are observed both by ground-based measurements (Terrile and Westphal, 1977) and by Voyager IRIS measurements (V. Kunde, T. Encrenaz, private communications). We do include absorption due to the Hz collisioninduced dipole, although it is quite weak, using the formulation and parameters given by Cohen and Birnbaum (1981), together with a downward scaling factor of 0.10, which reproduces more accurately the laboratory measurements of Bachet et al. (1982) in this region. The inclusion of Hz absorption yields clear atmosphere radiances equivalent to 290-300K in brightness temperature, exceeding those observed in the clearest regions of the planet. Therefore a crude model for a cloud which radiates thermally at 255K was employed to enable us to evaluate more closely the radiance observed in the clearest atmospheric regions. A nominal frequency of 2100 cm-’ was chosen as a rough representation of the center of the window region, but no substantial variations in brightness temperature would

NH1 ICE ON JUPITER have resulted from the choice of other frequencies 200 cm-’ higher or lower owing to the very slow changes in H2 absorption and in the optical properties of NH3 ice with frequency in this spectral region. Figure 10 summarizes the results of the outgoing radiances as a function of emission angle for the models whose 8.6-km region radiances were illustrated in Figs. 7 and 8. It is immediately clear that radiances from the H,JH, = 0.05 cloud with l-pmsized particles are lower than those observed anywhere on the planet, including nightside observations of zones by Voyager IRIS. Although not shown, particle sizes of 3 km or more yield radiances corresponding to brightness temperatures in excess of 200K, the approximate lower limit of cur-

EMISSION

ANGLE

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(P’)

FIG. 10. Dependence of thermal radiation at 2100 cm-l on emission angle (COSY’~)for models with scale height ratios and particle radii shown in Fig. 7. For models with HdH, = 0.05, only particle sizes greater than 3 Km (not shown) produce equivalent brightness temperatures greater than 200K. Only gaseous absorption due to the Hz collision-induced dipole has been included in the calculations.

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rent observations. The other models in Fig. 10 all fall within the range of observed radiance values. None of the cloud models appear to be as cold as the 200K characteristic of Jovian zones. However, there are several mechanisms available to accommodate lower radiance values. Appealing to the simple bimodal model for the spatial distribution of clouds, we can increase the particle number density by a factor of 2.5 to simulate better the characteristics of NH3 in zones. We observe that this decreases the brightness temperatures of the models by about 9K without much sensitivity to the angle of emission. Further, proper modeling of the cloud system may require another cloud layer between the NH3 cloud and the deeper cloud top which we model as a 255K blackbody; such a cloud could produce the intermediate flux level observed by Terrile and Westphal (1977). To illustrate the influence of such a cloud we place a blackbody radiating surface at the 235K level. Its effect on the HP/H, = 0.15, lOO-km cloud model radiance, as shown by the appropriately marked curve in Fig. 10, is to drop the outgoing radiance by more than a factor of 2 (equivalent to more than 15K in brightness temperature). Further reduction in the outgoing radiance could be achieved by appealing to unknown (continuum) absorbers, to other overlying clouds or aerosols characterized by smaller particle sizes, or to impurities in the NH3 ice crystals which might reduce their single-scattering albedo and increase their extinction coefficient. Differentiation between the various models in the absence of gaseous absorption, i.e., near the center of the window, on the basis of center-to-limb behavior, unlike the 1160-cm-’ region (Fig. 8) but like the 246-cm-’ region (Fig. 5), does not appear to be a useful diagnostic tool, as little variation is seen between the relative center-tolimb structures of the models. It is possible that this may be different in the presence of gaseous absorption, such as near the PH3or NH3-controlled absorption at the edges

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of the spectral window, since the presence bedo increases the sensitivity of the outgoof simultaneous gaseous absorption at 1160 ing radiance to details of the scattering cm-r has served to help differentiate the process, including the form of the phase center-to-limb structure of the various function. We note here that the single-scatmodels. However, neither PH3 nor NH3 are tering albedos of the models shown in Fig. uniformly mixed constituents of the atmo11 are 0.81 (baseline), 0.79 (“cubic”), and sphere, and their own vertical (and horizon0.74 (spherical and isotropic); asymmetry tal) distribution properties would require parameters cover a wider range and are independent constraints before being relied 0.00 (isotropic), 0.45 (baseline), 0.68 (“cuupon as a diagnostic method to determine bic”), and 0.90 (spherical). properties of the NH3 ice cloud. This spectral region thus offers another The sensitivity of the outgoing radiances constraint on cloud properties as evidenced to the assumed form of the particle-scatterby the elimination of “thin” clouds with ing phase function is shown in Fig. 1I for effective particle radii less than 10 p.m (cf the model with H,IH, = 0.15 and lOO-pm Fig. IO). However, the likely influence of particles. The intensity contrast between deeper cloud layers whose properties are various models in Fig. 11 is on the order of poorly known complicates full use of radi35%; for HP/H, = 0.50 it is 20%, and for HP/ ances in this region as a diagnostic tool. Hg = 0.05 it is 50% using the same particle The lack of sensitivity of the center-to-limb size. This sensitivity is due to the absence radiance variation to different NH3 cloud of any other gases which control atmoproperties, except possibly in the presence spheric absorption, and it is due to the in- of stronger gaseous absorbers, seems to recreased particle brightness and decreased strict the usefulness of such measurements optical depth at this frequency relative to in constraining these cloud properties. 246 and 1160 cm-‘. The greater particle al- Nevertheless, this region appears to be the most sensitive (considering only thermal emission) to changes in the particle microphysics. Finally, a fuller study of this region will have to address the role of reflected solar radiation which is likely to be mixed with the upwelling thermal radiance for sunlit regions of the planet. DISCUSSION

230 -

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[,‘I

FIG. Il. Sensitivity of computed radiances at 2 100 cm-’ to various assumptions about the particle-scattering phase function for the model with HdH, = 0.15 and effective particle radii of 100 km. Phase function models are coded as in Fig. 5. The greater sensitivity to the assumed form of the phase function, relative to radiances at 246 cm-’ (Fig. 5) or 1160 cm-’ (Fig. 9). is due to the higher value of the single-scattering albedo of NH, ice particles relative to those frequencies.

This study has served to constrain the range of possible NH7 ice cloud models to particles no less than 3 pm in volume equivalent effective radius with the probability that very large particles (100 km) near the theoretical upper limit allowed by coagulation (Rossow, 1977) may be most suitable, since they run into the fewest difficulties in comparison with various measurements. The Marten er al. (1981) study strongly favors large NH? ice cloud particle sizes, but the extent to which their conclusion remains independent of their constraint on the vertical distribution of the particles is unknown. Further uncertainty with respect

NH3 ICE ON JUPITER to the suppression of ice absorption features by irregular particles, discussed below, tends to weaken some of the arguments for these limits. A cursory comparison of the angular dependence of the radiance with measurements strongly implies that the H,/H, = 0.05 model is not suitable and that thicker cloud models are preferred. Further, for clouds with lOO-km particles, the particle scale height to gas scale height ratio is probably less than 0.15. Several ancillary questions remain regarding the cloud influence on thermal emission. Among them we must consider what the influence of the cloud models is on the recovery of Jovian tropospheric temperatures using radiance mesurements in the 250- to 750-cm-’ range. Further, as it is not considered efficient to require temperature recovery techniques to employ multiple-scattering calculations with a detailed cloud model at each step, we want to explore the degree of approximation which can be assumed to recover temperatures expeditiously with a minimum of variance from the real temperature structure. Other questions, to be answered here only in part, address (1) the influence of overlying clouds or hazes of smaller particles on the outgoing radiances, and the general correspondence between these models and models derived from analysis of reflected solar radiation, and (2) the influence of the NH3 cloud on the internal radiation field and on radiative-convective equilibrium calculations. First we examine the role of NH3 clouds in generating systematic errors in temperature sounding the Jovian atmosphere. We used our models to generate radiances at 287, 310, 340, 365, 475, 530, and 602 cm-‘; these are among the radiances most often used by the Voyager IRIS team to recover temperatures in the atmosphere of Jupiter (B. Conrath, private communication). We attempted to recover the original model temperatures with these radiances in the limited vertical range of 126-500 mbar total pressure. Figure 12 shows the differences

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(K)

FIG. 12. Errors in remotely sensed temperatures in the Jovian atmosphere. Differences are shown between temperatures recovered using clear atmosphere radiances and using cloudy atmosphere radiances. Clouds are characterized by IOO-km particles and several values for the scale height: HP/H, = 0.50 (long dashed line), HP/H, = 0.15 (intermediate dashed line), Hp/Hg = 0.50 (short dashed line), and HP/H, = 0.15 with a blackbody cloud at the deepest NH3 saturation level (alternating dashed-dotted line).

between temperatures recovered using radiances generated by cloudy atmospheric models and a clear atmospheric model assuming clear atmospheric conditions. We note that there is a systematic difference of some 2-3K at the deepest level (500 mbar) with some dependence on the cloud height and the number density at the base of the cloud. At higher levels in the 125- to 300mbar range, however, differences are less than a degree even with the HplHg = 0.50 cloud. The influence of this systematic error is apparent to some extent even at the 125-mbar level. For Jovian zones, where the cloud influences are probably greatest, we expect that larger systematic errors would be present near 500 mbar. Clouds can be detected in the terrestrial atmosphere by noting that clouds produce higher-frequency spatial variations in upwelling radiances than the more slowly varying temperature fields (Chahine, 1976; McCleese and Wilson, 1976). Presuming this to be true for the Jovian atmosphere, use of this technique with the radiances examined above would not be extremely sensitive to the cloud scale height variation. Furthermore, the lack of sounding capability significantly below the cloud bottom level would rule out the possibility of exploiting additional constraints on other

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cloud parameters such as depth. However, the use of 8.5- or j-km radiances could relieve this situation if there were sufficient constraints on the distribution of relevant gaseous absorbers or characteristics of deeper cloud levels. On the other hand, use of this technique requires a spatial resolution exceeding that characteristic of data sets currently available. The available data imply a strong correlation between temperature and cloud cover, especially in latitudinal variations (Conrath, private communication; Orton et al., 1981). Use of the full multiple-scattering algorithm in temperature sounding (or abundance sounding) is lengthy, and we suspect that it would probably be unsuitable for routine use. We note in Fig. 12 that the assumption of a blackbody emitting at the deepest level of NH3 saturation (near 630 mbar) in the temperature retrieval process is a sufficient approximation to the cloud influences on the outgoing radiance to reduce the systematic errors due to the cloud presence to less than a degree for the case of a cloud with HP/H, = 0.15. A slightly more sophisticated approximation, the use of particle extinction treated without scattering (handled in the radiative transfer model in the same way as gaseous absorption), is also quite rapid. For a cloud of lOOkm NH3 ice particles (single-scattering albedo of 0.56) with HP/H, = 0.15 this approximation results in a radiance which is low by some 4%, or 1.5K in brightness temperature at 246 cm-‘. The approximation is much better close to 363 cm-’ and in the 8.5~km region near the 1057-cm-’ ice absorption features where similar particles are much darker. At 5 km, however, where the particles are brighter (single-scattering albedo of 0.82), this approximation yields radiances which are larger by more than 50% or more than 6K in brightness temperature. Therefore, while potentially useful, the “pure absorber” approximation should not be made without a realistic assessment of the systematic errors likely to result. We suspect (but leave for future work) that a

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simple approximation (e.g., an estimate of the first-order effects of scattering) could reduce these errors. An adaptation of the successive orders of scattering method used by Marten et al. (1981) could also be useful. For each of these techniques, however, some independent criterion must be used to determine whether the cloud exists and what fraction of the field of view it covers to achieve the greatest accuracy. The NH3 ice cloud models discussed here are demonstrably capable of modifying the internal infrared radiance field as well as the outgoing field. While a complete examination of their effects should await more definitive models, some brief comments and the promised demonstration are in order. The rough coincidence of the 363-cm-’ NH3 ice absorption with the 370-cm-’ S(0) HI collision-induced rotational line in the region of greatest internal flux near the cloud level allows us to make a convenient comparison of the ratio of optical depths provided by NH3 and H? using only a few frequencies. The models presented above provide only some 15-20% more opacity in the atmosphere than does Hz. If these models are representative of a global mean in any sense relevant to the characterization of the mean atmospheric radiativeconvective equilibrium, then they are unable to provide the additional thickness required, as estimated by Hunten et a/. (1980), for the support of the temperature structures given by Orton (1977). However, considering only equatorial regions which may have greater cloud cover than the global average and the thermal structure of Linda1 et al. (1981) used here, the clouds may become a significant factor in resolving the apparent discrepancy between equilibrium models and measured temperature structures. To demonstrate the influence of the clouds on the internal flux, we chose 246 cm-‘, a spectral region particularly susceptible to the influence of clouds which is also near the peak of the flux distribution. Figure 13 shows our calculation of the mono-

NH3 ICE ON JUPITER

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0.0

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c4.0 -1 AT 246 cm

(IO-6 erg I -’ .m-3 (cm-‘)-‘)

FIG. 13. Monochromatic flux divergence for the model Jovian atmosphere at 246 cm-‘. The radiative transfer calculations consider absorption by Hz gas and absorption and scattering by NH3 ice particles with effective radii of 100 km. Results for a clear model (solid line) are shown with cloud models characterized by scale height ratios given by: HP/H, = 0.50 (long dashed line), HdH, = 0.15 (intermediate dashed line), and HdH, = 0.05 (short dashed line).

chromatic flux divergence at 246 cm-l, using only Hz gas and NH3 ice particle opacities and including all scattering effects. The graphs portray, for loo-urn NH3 ice particles and a variety of scale height models (plus the clear atmosphere case), the first derivative of the difference between the hemispherically averaged upwelling and downwelling fluxes. This is the net monochromatic flux, computed simply by the net flux difference between adjacent layers divided by the altitude change using 20 discrete layers per decade of pressure change (corresponding to approximately 4 km in the 600-mbar cloud region). Departures from the clear atmosphere case are significant and are most pronounced for thin clouds where the optical thickness required to match the 246-cm-’ radiance measurement is distributed over a shorter interval. These departures could be larger were we to calculate over a finer vertical grid; nevertheless, they simulate the results which would be inferred by direct probe measurements in discrete vertical sampling intervals close to those of our calculations.

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At least for 246-cm-’ “monochromatic” flux a substantial diagnostic tool is apparent, and the direction of the departures from the clear atmosphere case resemble the results of placing a thin, discrete layer of finite optical depth near 600 mbar. Just below the cloud greater net flux convergence results from radiances upwelling from the deep atmosphere and downwelling from the cloud. Just above the cloud greater net flux divergence results from combining radiances upwelling from the deep atmosphere and additional radiances upwelling from the cloud level itself. The net integral of these divergences over altitude, from deeper levels to upper levels, is the same for all cloud models, since we constrained all our cloud models to fit the measured upwelling radiance at this frequency in the initial model constraint. The net flux upwelling from the top of the atmosphere for the clear atmosphere model is somewhat greater. Of obvious importance in future work will be the simulation of the radiation which we would expect the Galileo Probe Net Flux Radiometer to measure in its broadband far-infrared channel, which also includes frequencies where NH3 gaseous absorption makes a substantial contribution to the atmospheric opacity. There are four other cloud models of direct relevance to the infrared region against which we can make useful comparisons. The model with lOO-km particles and H,lH, = 0.15 comes closest to the final model of Marten et al. (1981). In fact, for this model our derived base number density is within about a factor of 2.5 of that of the Marten et al. (1981) “depleted” model. We think this is quite satisfactory, considering the differences in model atmosphere, particle-scattering assumptions, data bases, and radiative transfer algorithms which were used. We note that in the recent examination of the high-resolution Jovian spectrum between 1100 and 1200 cm-’ Knacke ef al. (1982) preferred an atmospheric model with no NH3 ice particles, choosing instead to increase the NH3 gaseous mixing ratio in

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the unsaturated region to 3.3 x IO-“, some 50% more than we assumed. However, we note that their spectrum (see their Fig. 5) is characterized by average brightness temperatures in excess of 150K in the center of this region, whereas our reference to older, lower-resolution data inferred a constraint some 8-10K less for the same spectral region. Our “clear atmosphere” models do, indeed, match quite roughly their assumed radiances, which they indicate correspond closely to Voyager IRIS observations of the North Equatorial Belt, a region which is possibly freer of cloud effects than the planetary average. We conclude, then, that the differences between our results stem primarily from the different data bases, implying that their observations are more characregions of the Jovian teristic of “clear” atmosphere (e.g., belts) than they are of zones or the apparent global mean. Further clarification of this point should be provided by the examination of data with greater spatial resolution than we have selected for this study. We made a brief examination of our cloud models from the viewpoint of visible radiation by examining the optical thickness of each model at visible wavelengths, assuming an index of refraction for NH3 of 1.45 + 0.00 i (Martonchik et al., 1982). The optical thickness of clouds with IO-km particles is less than 1.3. For the HP/H, = 0.15 cloud with 3-p.m particles, the visible optical thickness is greater than 1I. Sato and Hansen (1979) require a cloud at this atmospheric level to have an optical thickness of about 10, somewhat independent of the planetary region; West and Tomasko (1980) require the cloud to have an optical thickness in the 1.5 to 3.0 range, depending on the planetary region. Clouds with lO+m or larger particle radii in nearly monodisperse distributions fail to meet the requirements of the models established on the basis of reflected solar radiation data. Increasing the imaginary index of refraction fails to reconcile these results adequately. The only solution we can offer at this time is

AND MARTONCHIK

that the particle size distribution is wider than we have assumed. There must be a sufficient number of small particles to increase the visible extinction to the requisite values, but the particles must have negligible influence on infrared radiances. We can further estimate the influence of overlying particles on the thermal radiances we examined. Choosing the tropospheric haze layer described in greater microphysical detail by Stall (1980) near the ?OO-mbar level, we can estimate the maximum influence his haze model would have on S-km radiation, which should be more susceptible to the influence of small particles than radiation at 8.5 p,m or 50- 100 km. Stoll describes the particles in this haze as 0.17 IJ-m in effective radius and characterizes the haze as providing an optical thickness of 0.125 to 0.250 at 0.44 km. The particle size description is consistent with that given by the Galilean satellite eclipse light curve studies of Smith et al. (1977) and Smith (1980) for “tropospheric” or “tropopause” particles. If we examine the most pathological case which would maximize the influence of this haze at 5 km, taking the imaginary index of refraction to be zero (all extinction caused by scattering) at 0.44 pm and to be equal to the real index of refraction at 5 km, then the 5-km optical thickness of the haze could be as high as 0.33 times the 0.44-km optical thickness. This result is based on total particle extinction characteristics described by Mie theory and is rather insensitive to the actual value of the real index of refraction assumed. Scaling from Stall’s estimates of the 0.44-km optical thickness, such a haze would attenuate 5-km radiances by 4 to 8% (equivalent to 0.7 to 1.4K in brightness temperature, if a characteristic temperature of 230K is assumed). For indices of refraction more than a factor of 5 different, the attenuation is less than about 2%. On the other hand, if the haze particles have a nonzero visible imaginary index of refraction, their influence on 5-pm radiation is less than that estimated for particles which are nonabsorbing at 0.44

NH3 ICE ON JUPITER pm. For an imaginary index of refraction of 0.1 at 0.44 pm, for example, the 5-urn effective attenuation of the haze is less than 1% in the worst case. Thus, while the influence of particles in this haze (and possibly those in the stratosphere) is not likely to be very large, a significant influence cannot be ruled out entirely. CONCLUSIONS

We deliberately chose not to draw analogies with the behavior of terrestrial water clouds except in constraining the NH3 cloud particles to reside above the level of deepest NH3 vapor condensation and below the 100-mbar level, above which convective transport is unlikely. Our constraint on possible families of models for NH3 clouds, derived from far-infrared measurements, are complemented by lower limits on the particle size of 3 pm if HP/HP = 0.15 (and of 10 pm if HJH, = 0.50 or 0.05). These limits, constrained by the absence of identifiable spectral features of NH, ice, could be improved by using measurements more precise than the conservative 0.5K brightness temperature uncertainty at 400 cm-’ cited herein, which is derived from co-adding only five separate spectra (Hanel et al., 1981). Medium-resolution disk-averaged radiance data appear to be inconsistent with large particle (100 pm) sizes for HJH, = 0.50 or 0.15. Very thin (HdHg = 0.05) clouds with radii less than 10 pm appear to be ruled out by virtue of the low level of 5pm radiance they imply, compared with the minimum observed radiance anywhere on the planet (including the nightside, where confusion by reflected solar radiation need not be considered). Diffuse or “thick” clouds (H,,/H, = 0.50) seem somewhat improbable if Stoll’s (1980) discarding of diffuse cloud models on the basis of polarization and photometric data is to be considered; however, we cannot place very hard limits on the cloud scale height solely on the basis of the thermal infrared measurements which we examine here. A first improvement in the models we

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will undertake is the examination of spatially resolved data. We will do this independently of an examination of the Voyager data base in order to establish whatever differences might exist in cloud properties in the time base preceding and following the 1979 epoch, although such measurements are largely characterized by poorer spatial and spectral resolution than the IRIS experiment spectra. This study has shown that for these and for Voyager data, information returned from several different spectral regions should be complementary, rather than degenerate. Several limitations are present, however, in the information which can be gained. For example, after total optical thickness is characterized, the differences between various particle scale height models using center-to-limb data are likely to be small in the 250- to 750-cm-’ spectral region and in the center of the 5-km window. On the other hand, the examination of radiances and their center-to-limb behavior in the presence of gaseous absorption and cloud extinction between 1100 and 1200 cm-’ and near the edges of the 5-km window is likely to be fruitful, judging by calculations for various models at 1160 cm-’ (Figs. 8 and 10). Use of a crude blackbody cloud top model during the temperature retrieval process appears to be successful in reducing the systematic errors due to the cloud presence to the level of 1K or less. Finally, an examination of spectral regions where the particles are likely to be dark (i.e., where multiple scattering is likely to be small) is considered advantageous to establish the bulk cloud properties, such as vertical distribution and characteristic number density. This is because the ambiguities inherent in assumptions about the form of the particle-scattering phase function are smallest and because efficient approximations to the radiative transfer process can be utilized while minimizing inherent systematic errors. We are concerned about violations of the assumptions regarding cloud properties made at the outset of the study. Of para-

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mount importance is the validity of our particular fit of the Pollack and Cuzzi semiempirical theory to predict a variety of scattering properties of NH3 ice particles. In most of the wavelength regions explored here, the particles are substantially Mielike, owing to the large value of x0 required to reproduce the large backscatter of the visible observations. Dr. R. West has brought to our attention the fact that work by Huffman and Bohren (1980), although confined to the Rayleigh size regime, indicates that irregular particles tend to suppress the magnitude of features in the absorption spectrum relative to the magnitude predicted using Mie theory. Therefore, a major question arises as to whether irregular shape effects on resonant absorption by ice particles are responsible to a large degree for suppressing the NH3 ice features required to match the Jovian spectrum rather than the predominance of large particles (as illustrated in Fig. 4). This question could be addressed by calculating the absorption expected by randomly oriented ellipsoids or by observations of NH3 ice particle infrared absorption in the relevant size regime, following the example of the Huffman and Bohren work. The adequacy of the Pollack and Cuzzi theory in this regard, using a greater mixture of irregular particles to Mie particles (i.e., a lower value for x0), could then be assessed on some independent basis. For the present, however, the question remains open, and the minimum particle size limits shown in Fig. 2 for H,/ Hg = 0.50 and 0.15 [as well as those of Marten et al. (1981)] should therefore be regarded as extremely tentative. A corollary question arises regarding the effects of impurities in ice particles, a common situation for terrestrial cumulus clouds, which is possible for Jovian NH3 ice particles even if we only consider the mixture of material in suspected overlying haze layers. We might expect that one effect of such impurities in real NH3 ice particles would be to suppress the absorption features expected from pure NH3 ice particles.

AND MARTONCHIK

Additional concerns are, for example, the effects of broader particle size distributions, the possible vertical variation of the effective particle size [as shown by Bunting (1980) for terrestrial cirrus clouds], and the presence of significant numbers of particles below the expected deepest level of saturation (e.g., during large-scale precipitation). Some of these concerns will undoubtedly defy elucidation by remote sensing data. However, a more competent determination of the particle size distribution should be possible with simultaneous analysis of thermal and reflected solar radiation. Initial studies should look at frequencies where NH3 ice absorption features appear. Solutions for particle properties, such as the single-scattering albedo and the form of the scattering function, derived independently of compositional assumptions, could be used in comparison with properties expected of NH3 ice crystal particles. Such a comparison will be most meaningful if the properties of “pure” and “impure” NH3 ice in various spectral regions under Jovian environmental conditions can be simulated in the laboratory.

ACKNOWLEDGMENTS We wish to thank several individuals whose advice at various stages of this research and whose criticism of this manuscript were most valuable. Among them are: J.-P. Baluteau, M. Chahine. B. Conrath. T. Encrenaz, D. Gamier, R. Hanel, N. Husson. R. Knacke, V. Kunde, A. Marten, D. McCleese, W. Rossow, R. Samuelson, R. Terrile, A. Tokunaga, M. Tomasko. and R. West. Computing support was kindly supplied by J. Hansen with assistance from N. Habra and D. Sol1 of the Goddard Institute for Space Studies. GSO acknowledges tenure as a visiting scientist at I’Observatoire de Paris (Meudon) during one phase of this research. Primary supportfor this work was provided by the NASA Office of Space Sciences and Applications through the Planetary Atmospheres Program Office, the Jupiter Data Analysis Program, and the Galileo Project. This work represents, in part, one phase of research conducted at the Jet Propulsion Laboratory. California Institute of Technology, under Contract NAS7-100 to the National Aeronautics and Space Administration.

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