Application of methane band-model parameters to the visible and near-infrared spectrum of Uranus

Laboratory band-model absorption coefficients of CH₄ are used to calculate the Uranus spectrum from 5400 to 10,400 Å. A good fit of both strong and weak bands for the Uranus spectrum over the entire wavelength interval is achieved for the first time. Three different atmospheric models are employed: a reflecting layer model, a homogeneous scattering layer model, and a clear atmosphere sandwiched between two scattering layers. The spectrum for the reflecting layer model exhibits serious discrepancies but shows that large amounts of CH₄ (5–10 km-am) are necessary to reproduce the Uranus spectrum. Both scattering models give reasonably good fits. The homogeneous model requires a particle scattering albedo $\text{(}\text{g}\text{?}\text{w}{}_{\mathrm{<mtext>p</mtext>}}\text{) ? 0.998}$ and an abundance per scattering mean free path $\text{(}\text{a}\text{?}\text{)}\text{of}\text{a}\text{?}\text{1}\text{km-am}$. The parameters derived from the sandwich layer model are: forsb the upper scattering layer a continuum single scattering albedo ($\text{g}\text{?}\text{w}{}_0$) of 0.995 and a scattering optical depth variable with wavelength consistent with Rayleigh scattering; for the clear layer they are a CH₄ abundance (a) of 2.2 km-am and an effective pressure (p) ? 0.1 atm; for the lower cloud deck a Lambert reflectivity (L) of 0.9 resulted. A severe depletion of CH₄ in the upper scattering layer is required. An enrichment of CH₄/H₂ over the solar ratio by a factor of 4–14 in the lower atmosphere is, however, indicated.     

Longitudinal variability of methane and ammonia bands on jupiter: II. Temporal variations

A set of spectra was obtained of the Jovian Equatorial Zone central meridian 6190 Å CH₄ and 6450 Å NH₃ bands in February 1980, a year after the data reported by W.D. Cochran and A.L. Cochran ((1980) Icarus42, 102–110). These new data confirm the results of the previous study, and also permit a search for temporal, as well as longitudinal, variability of these molecular absorption bands. The new data set shows a correlation of NH₃ and CH₄ equivalent widths, as well as a lack of any strong correlation of red continuum reflectivity with equivalent width. These trends were also exhibited by the 1979 data. Longitude regions of larger or smaller than average equivalent width seem still to be evident a year later, with some slight drift in longitude. An increase in the average CH₄ and NH₃ equivalent width over the entire planet was detected during the one year interval. This is easily understood as the result of a global decrease in the mean altitude of the NH₃ cloud.     

Methane abundance in the atmosphere of Uranus

Modeling of the geometric albedo of Uranus in and near prominent methane absorption bands between 0.5 and 0.9 μm indicates that the visible atmosphere probably consists of a thin aerosol haze layer (τ_scat ? 0.3?0.5; ω_H ? 0.95) above an optically thick, semi-infinite Rayleigh scattering atmosphere. A significant depletion of methane gas above the haze layer is indicated. The mixing ratio of methane in the lower atmosphere is consistent with a value of CH₄/H₂ ? 3 × 10^?3, comparable to those derived for Jupiter and Saturn.     

Spatially resolved methane band photometry of Saturn: II. Cloud structure models at four latitudes

Spatially resolved measurements of Saturn's reflectivity in the 6190-, 7250-, and 8996-Å methane bands are analyzed to determine cloud vertical structures in the Equatorial Zone, South Equatorial Belt, and North and South Temperate Regions near latitudes ±30°. Radiative transfer models are computed for a simple two-parameter structure. The parameters are A₀, the methane column abundance in an aerosol-free layer at the top of the atmosphere, and A₁, the specific abundance of methane in a semi-infinite homogeneous gas and cloud mixture deep in the atmosphere. For the Equatorial Zone, a model with A₀ = 37 ± 3 m-am and A₁ = 26 ± 2 m-am fits all three bands. For the North Temperate Region, a model with A₀ = 39 m-am and A₁ = 47 m-am comes close to fitting all three bands. For the South Equatorial Belt and South Temperate Region, a single A₀ and A₁ do not fit all three bands. The structure for the South Equatorial Belt resembles that for the North Temperate Region. The level where unit cloud optical depth occurs in the South Temperate Region is deeper than the corresponding level at other latitudes. Some suggestions are proposed to explain differences between model parameters derived using different absorption bands.     

Titan: Far-infrared and microwave remote sensing of methane clouds and organic haze

It is shown that Titan's surface and plausible atmospheric thermal opacity sources—gaseous N₂, CH₄, and H₂, CH₄ cloud, and organic haze—are sufficient to match available Earth-based and Voyager observations of Titan's thermal emission spectrum. Dominant sources of thermal emission are the surface for wavelenghts λ ? 1 cm, atmospheric N₂ for 1 cm ? λ ? 200 μm,, condensed and gaseous CH₄ for 200 μm ? λ ? 20 μm, and molecular bands and organic haze for λ ? 20 μm. Matching computed spectra to the observed Voyager IRIS spectra at 7.3 and 52.7° emission angles yields the following abundances and locations of opacity sources: CH₄ clouds: 0.1 g cm^? at a planetocentric radius of 2610–2625 km, 0.3 g cm^?2 at 2590–2610 km, total 0.4 ± 0.1 g cm^–2 above 2590 km; organic haze: $\text{4 ± 2 × 10}{}^{\mathrm{?6}}\text{,}\text{g cm}\text{,}{}^{\mathrm{?2}}$ above 2750 km; tropospheric H₂: 0.3 ± 0.1 mol%. This is the first quantitative estimate of the column density of condensed methane (or CH₄/C₂H₆) on Titan. Maximum transparency in the middle to far IR occurs at 19 μm where the atmospheric vertical absorption optical depth is $\text{?0.6}$ A particle radius $\text{r}\text{? 2 μ}\text{m}$ in the upper portion of the CH₄ cloud is indicated by the apparent absence of scattering effects.     

Study of the deep cloud structure in the equatorial region of Jupiter from voyager infrared and visible data

High spatial resolution infrared and visible data obtained by the Voyager 1 spacecraft have been analyzed simultaneously to infer properties of the deep cloud structure of the Jovian troposphere in the 1- to 4-bar pressure range. Influence of the ammonia upper cloud layer, in the 5μm Jovian window, has been investigated through a cloud model derived from far ir Voyager IRIS measurements. The attenuation, computed with an anisotropic scattering formulation, is too weak to explain 5-μm measurements and provides evidence for existence of a cloud structure at deeper levels. The main conclusions derived from the present analysis are summarized below: (1) the deep cloud structure appears to be vertically associated with the NH₃ upper layer; (2) the ammonia cloud is mainly responsible for the visible appearance of the Jovian equatorial region; (3) the deep cloud structure exhibits a grey opacity in the 5-μm window; (4) coldest 5-μm spectra can be interpreted by the existence of a thick cloud layer located at levels in the 180–195°K temperature range. Implications of these results are discussed in conjunction with predictions of dynamical and thermochemical models. NH₄SH is shown to be a likely candidate for the main deep cloud constituent. An even deeper thick H₂O cloud may be present too, but should not be responsible for the observed spread in 5-μm brightness temperatures.     

The gas composition of jupiter derived from 5-μm airborne spectroscopic observations

The atmospheric transmission window between 1850 and 2250 cm⁻¹ in Jupiter's atmosphere was observed at a spectral resolution of 0.5 cm⁻¹ from the Kuiper Airborne Observatory. The mole fractions of NH₃, PH₃, CH₄, CH₃D, CO, and GeH₄ were derived for the 1- to 6-bar portion of Jupiter's troposphere using a spectrum synthesis program. Knowledge of the abundances of these gases below the visible clouds is necessary to calculate the global inventory of nitrogen, phosphorus, carbon, and deuterium, which, in turn, may constrain models of Jupiter's formation. The N/H ratio is 1.5 ± 0.2 times the value for the Sun's photosphere. The P/H ratio for the 5-bar level is between 1.0 and 1.6 times the solar abundance. The weak ν₃−ν₄ hot band of CH₄ was detected for the first time on Jupiter, thus providing a deep atmospheric value for C/H of 3.6 ± 1.2 times solar. The Jovian deuterium abundance is comparable to that measured in the interstellar medium (D/H = 1.2 ± 0.5) × 10⁻⁵. CO appears to be well mixed with a mole fraction of (1.0 ± 0.3) × 10⁻⁹. Multiple absorption features confirm that GeH₄ is present on Jupiter with a mole fraction of (7.0_−2.0^+4.0) × 10⁻¹⁰. The observed abundances of CO, GeH₄, and PH₃ are consistent with models of convective transport from Jupiter's deep atmosphere.     

Spatially resolved reflectivities of Jupiter during the 1976 opposition

Spatially resolved absolute reflectivities of several regions of the Jovian disk in the wavelength region 3000 to 10760 Å are presented. Spectra were obtained of the central meridian and limbs of the Equatorial Region, North Equatorial Belt, and North Tropical Zone. Equivalent widths of several CH₄ and NH₃ bands are measured. The spatial variations of continuum reflectivity and absorption band profiles are shown in various ratio spectra.     

Jupiter: An inhomogeneous atmospheric model analysis of spatial variations of the H2 4-0 S(1) line

An analysis of the structure of the Jovian atmosphere, primarily based on center-to-limb variations (CTLV) of the equivalent width of the hydrogen quadrupole 4-0 S(1) line, is presented. These data require that the atmosphere have regions of both long- and short- scattering mean free paths. Two alternative cloud structures which fit the data are developed. The first is a two-cloud model (TCM) consisting of a thin upper cloud and a lower semi-infinite cloud, with absorbing gas between the clouds and above the upper cloud. The second model is a reflecting-scattering model (RSM), in which a gas layer lies above a haze consisting of scattering particles and absorbing gas. The cloud-scattering phase function in both models must have a strong forward peak. The CTLV data require, however, the presence of a backscattering lobe on the phase function, with the backscattering intensity about 4% of the forward scattering. The decrease in reflectivity of all regions from the visible to the ultraviolet is explained by the presence of dust particles mixed with the gas. Most of the ultraviolet absorption in the atmosphere must occur above the upper cloud layer. Particles with a uniform distribution of radii from 0.0 to 0.1 μm with a complex index of refraction varying as λ^?2.5 are used. The contrast in reflectivity between belts and zones may be explained by the larger concentration of dust in the belts than in the zones. Spatially resolved ultraviolet limb-darkening curves will help to determine the dust distribution of the Jovian atmosphere. The visible methane bands at λλ 6190, 5430, and 4860 Å are analyzed in terms of these models. We derive a methane-to-hydrogen mixing ratio of 2.8 × 10^?3, which is about 4.5 times the value for solar composition.     

Long-term changes in Saturn's troposphere

We report the results of monitoring Saturn's H₂ quadrupole and CH₄ band absorptions outside of the equatorial zone over one-half of Saturn's year. This interval covers most of the perihelion half of Saturn's elliptical orbit, which happens to be approximately bounded by the equinoxes. Marked long-term changes occur in the CH₄ absorption accompanied by weakly opposite changes in the H₂ absorption. Around the 1980 equinox, the H₂ and CH₄ absorptions in the northern hemisphere appear to be discontinuous with those in the southern hemisphere. This discontinuity and the temporal variation of the absorptions are evidence for seasonal changes. The absorption variations can be attributed to a variable haze in Saturn's troposphere, responding to changes in temperature and insolation through the processes of sublimation and freezing. Condensed or frozen CH₄ is very unlikely to contribute any haze. The temporal variation of the absorption in the strong CH₄ bands at south temperate latitudes is consistent with a theoretically expected phase lag of 60° between the tropopause temperature and the seasonally variable insolation. We model the vertical haze distribution of Saturn's south temperature latitudes during 1971–1977 in terms of a distribution having a particle scale height equal to a fraction of the atmospheric scale height. The results are a CH₄/H₂ mixing ratio of (4.2 ± 0.4) × 10^?3, a haze particle albedo of $\text{ω}\text{= 0.995 ± 0.003}$, and a range of variation in the particle to gas scale-height ratio of 0.6 ± 0.2. The haze was lowest near the time of maximum temperature. We also report spatial measurements of the absorption in the 6450 Å NH₃ band made annually since the 1980 equinox. A 20 ± 4% increase in the NH₃ absorption at south temperate latitudes has occurred since 1973–1976 and the NH₃ absorption at high northern latitudes has increased during spring. Increasing insolation, and the resulting net sublimation of NH₃ crystals, is probably the cause. Significant long-term changes apparently extend to the deepest visible parts of Saturn's atmosphere. An apparently anomalous ortho-para H₂ ratio in 1978 suggests that the southern temperate latitudes experienced an unusual upwelling during that time. This may have signaled a rise in the radiative-convective boundary from deep levels following maximum tropospheric temperature and the associated maximum radiative stability. This would be further evidence that the deep, visible atmosphere is governed by processes such as dynamics and the thermodynamics of phase changes, which have response times much shorter than the radiative time constant.     

Airborne interferometric measurement of the infrared spectrum of Jupiter from 6 to 14 μm

A new spectrum of Jupiter from 700 to 1600 cm^?1 was obtained with an interferometric experiment using the 91.5 cm telescope of the NASA Airborne Infrared Observatory. The spectral resolution is 10 cm^?1 and the signal-to-noise ratio is 30 at 900 cm^?1. NH₃ absorption lines are observed between 820 and 1020 cm^?1. The 1306 cm^?1ν₄CH₄ band strongly appears in emission at a temperature of at least 145° K. The Jovian brightness temperature between 1400 and 1600 cm^?1, according to our measurement, is lower than 170° K.     

The optical properties of Venus and the Jovian planets I. The atmosphere of Jupiter according to polarimetric observations

Results are given for polarization measurements of both the entire Jupiter disk and its centre for seven wavelength regions in the 0.373–0.800 μm range. Interpretation of these observations is based on two model atmospheres: (A) The cloud layer particles and molecules are mixed with a constant ratio. (B) A gas layer with small optical thickness, τ₀, is situated above the cloud layer which consists of aerosol particles. The aerosol particles are considered to be non-absorbing spheres, their size distribution being normal Gaussian. The index of refraction for the particles is considered to be independent of wavelength in the above spectral range. An approximate method is used for the determination of parameters of the Jovian atmosphere. This method was tested by evaluation of the parameters for the Venus cloud layer: The refractive index was found to be n = 1.435 ± 0.015, the square of the logarithmic dispersion of the radius of particles σ² = 0.12 and the mean geometrical radius of particles r₀ = 0.74 μm which agree well with exact values given by Hansen and Arking (1971). For the atmosphere of Jupiter it was found: n = 1.36 ± 0.01, σ² ? 0.3, r₀ ? 0.2 μm. This refractive index for the particles agrees well with the ammonia cloud layer hypothesis.     

The thermal structure of Jupiter II. Observations and analysis of 8–14 micron radiation

Observations of Jovian limb structure at 8.11 and 8.45 microns are reported. These are used along with other limb structure and spectral data in the 8–14 micron region to derive a model of the thermal and cloud structure within the 1.0-0.01 bar pressure regime. The model is generally consistent with models derived from Pioneer 10 infrared radiometer data reported by Orton (1975b). The temperature is about 165K at 1.00 bar, 108K at 0.01 bar, and 143K at 0.03 bar. In zones, an optically opaque cloud of NH₃ exists near the 143K (0.60 bar) level. A partly transparent haze of solid NH₃ particles overlies the cloud. Belts are free of the cloud and have a much lower abundance of NH₃ haze than the zones. The data are consistent with an NH₃ gas abundance defined by saturation equilibrium, with a mixing ratio of 1.5 × 10^?4 deep in the atmosphere, and with a CH₄ mixing ratio of 2 × 10^?3, about three times the currently accepted value.     

Spectropolarimetry of the methane and ammonia bands of Jupiter between 6800 and 8200 Å

Spectropolarimetry of Jupiter at resolutions between 22 and 35 Å reveals a strong increase of linear polarization in the 7250-$\text{A}\text{?}$ CH₄ band. This is very probably due to the decreasing contribution toward the band center of the higher orders of scattering, which have a smaller net polarization than the first few orders. The linear polarization is also enhanced in the band at 7900 $\text{A}\text{?}$ comprising the 7920-$\text{A}\text{?}$ NH₃ and 7600- to 8200-$\text{A}\text{?}$ CH₄ bands. The normalized circular polarization shows a feature at 7250 $\text{A}\text{?}$ with a dispersion shape. This is most probably produced in a double-scattering process involving either a solid or liquid aerosol with an absorption at 7250 $\text{A}\text{?}$. Methane aerosols, the obvious candidates from a spectroscopic point of view, are, however, forbidden if current estimates of the Jovian atmospheric temperature are correct.     

Shadow mechanism and the opposition effect of brightness of atmosphereless celestial bodies

We consider the Irvine-Yanovistkii modification of the shadow model developed by Hapke for the opposition effect of brightness. The relation between the single scattering albedo ω and the transparency coefficient of particles κ is suggested to be used in the form κ = (1 ? ω) ⁿ , which allows the number of unknowns in the model to be reduced to two parameters (the packing density of particles g and ω) and the single-scattering phase function χ(α). The analysis of spectrophotometric measurements of the moon and Mars showed that the data on the observed opposition effect and the changes in the color index with the phase angle α well agree if the values of n = 0.25 and g = 0.4 (the moon) and 0.6 (Mars) are assumed in calculations. When being applied to asteroids of several types, this method also yielded a satisfactory agreement. For the E-type asteroids, the sets of parameters are [g = 0.6, ω = 0.6, A _g = 0.21, and q = 0.83] or [g = 0.3, ω = 0.4, A _g = 0.15, and q = 0.71] under the Martian single-scattering phase function; for the M-type asteroids, it is [g = 0.4, ω ≤ 0.1, A _g ≤ 0.075, and q ≤ 0.42] under the lunar single-scattering phase function; for the S-type asteroids, it is [g = 0.4, ω = 0.4, A _g = 0.28, and q = 0.49] under the lunar single-scattering phase function; and for the C-type asteroids, it is [g = 0.6, ω ≤ 0.1, A _g ≤ 0.075, and q = 0.43] under the modified lunar single-scattering phase function. The polarization measurements fulfilled by Gehrels et al. (1964) for the bright feature on the lunar surface, Copernicus (L = -20°08′, φ = +10°11′), at a phase angle α = 1.6° revealed the deviations in the position of the polarization plane from that typical for the negative branch. They were 22° and 12° in the G and I filters, respectively. At the same time, the deviation was within the error (±3°) in the U filter and for the dark feature Plato (L = -10°32′, φ = +51°25′), which can be caused by the coherent mechanism of the formation of the polarization peak.     

The mean Jovian temperature structure derived from spectral observations from 105 to 630 cm−1

New far-infrared observations of the NH₃ rotation-inversion manifolds in the spectrum of Jupiter have been inverted with the use oftthe detailed ammonia line opacity. A temperature of 160°K at a 1-bar pressure level and a temperature of 105°K for the minimum temperature of the inversion level at 0.15 bars have been derived for gaseous absorption due to NH₃, H₂, and He. The overall fit to the brightness temperature as a function of frequency σ is within ±1°K for 100 ≤ σ ≤ 400 cm^?1 except for the centers of the NH₃ rotation-inversion manifolds where for J ≥ 7 the fit is about 5°K too high. In the continuum for 400 ≤ σ ≤ 630 cm^?1 the fit is within 2.5°K. Consideration of an ammonia ice haze, photodissociation of NH₃ by uv radiation, NH₃ abundance variation, different He/H₂ ratios, and uncertainties in the data effect the temperatures at 1 bar and the temperature at the inversion layer by <7°K. The presently derived temperature at 1 bar of 160°K is consistent with Jovian interior models which can match the gravitational moment, J₂.     

On the C/H and D/H ratios in the atmospheres of Jupiter and Saturn

Using a method defined in a previous paper [M. Combes and T. Encrenaz, Icarus39 1–27 (1979)], we reestimated the C/H ratio in the atmospheres of Jupiter and Saturn by the measurements of the weak visible CH₄ bands, the CH₄ 3ν₃ band, and the (3-0) and (4-0) quadrupole bands of H₂. In the case of Jupiter we conclude that the C/H ratio is enriched by a factor ranging from 1.7 to 3.6 relative to the solar value. In the case of Saturn, our derived C/H value ranges from 1.2 to 3.2 times the solar value. The Jovian D/H ratio derived from this study is 1.2 × 10^?5 < D/H < 3.1 × 10^?5. The value derived for the D/H ratio on Saturn is not precise enough to be conclusive.     

Detection of a CH4 atmosphere on Pluto

Using a low-resolution spectrograph and a CCD array, a spectrum of Pluto from 0.58 to 1.06 μm was obtained. The spectrum had a resolution of $\text{～25}\text{A}\text{?}$ and a signal-to-noise ratio of ～300. It showed CH₄ absorption bands at 6200, 7200, 7900, 8400, 8600, 8900 and 10,000 Å. The strongest of these bands was at 8900 Å with an absorption depth of 0.23. This band was heavily saturated, compared to the weaker bands, providing proof for the gaseous origin of the observed absorptions. By applying CH₄ band model parameters to our data, a total CH₄ abundance of 80 ± 20 m-am was derived. This translates into a one-way abundance of 27 ± 7 m-am and a CH₄ surface pressure of 1.5 × 10^?4 atm. An upper limit to the total pressure of ～0.05 atm could be set. First-order calculations on atmospheric escape showed that this methane atmosphere would be stable if the mass of Pluto is increased 50% over its current value and its radius is 1400 km. Alternatively a heavier gas mixed with the CH₄ atmosphere would aid its stability. The relatively large amount of gaseous CH₄ observed implies that the absorption bands recently reported at 1.7 and 2.3 μm are likely due to atmospheric CH₄ absorptions rather than surface frost as interpreted earlier.     

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

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_p) to the gas scale height (H_g), and by an effective particle radius. NH₃ 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^?1, and preliminary lower limits on particle sizes are inferred from the lack of apparent NH₃ absorption features in the observed spectral radiances as well as the observed minimum flux near 2100 cm^?1. We find lower limits on the particle size of 3 μm if H_p/H_g = 0.15, or 10μmif H_p/H_g = 0.50 or 0.05. NH₃ ice particles are relatively dark near the far-infrared and 8.5-μm 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 μm, 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 NH₃ clouds provide a substantial influence on the internal infrared flux field near the 600-mbar level.     

Solar radiation scattered in the Venus atmosphere: The Venera 11,12 data

Results of the scattered solar radiation spectrum measurements made deep in the Venus atmosphere by the Venera 11 and 12 descent probes are presented. The instrument had two channels: spectrometric (to measure downward radiation in the range 0.45 < γ < 1.17 μm) and photometric (four filters and circular angle scanning in an almost vertical plane). Spectra and angular scans were made in the height range from 63 km above the planet surface. The integral flux of solar radiation is 90 ± 12 W m^?2 measured on the surface at the subsolar point. The mean value of surface absorbed radiation flux per planetary unit area is 17.5 ± 2.3 W m^?2. For Venera 11 and 12 landing sites the atmospheric absorbed radiation flux is ～15 W m^?2 for H >; 43 km and ～45 W m^?2 for H < 48 km in the range 0.45 to 1.55 μm. At the landing sites of the two probes the investigated portion of the cloud layer has almost the same structure: it consists of three parts with boundaries between them at about 51 and 57 km. The base of clouds is near 48 km above the surface. The optical depth of the cloud layer (below 63 km) in the range 0.5 to 1 μm does not depend on the wavelength and is ～29 and ～38 for the Venera 11 and 12 landing sites, respectively. The single-scattering albedo, ω₀, in the clouds is very close to 1 outside the absorption bands. Below 58 km the parameter (1 ? ω₀) is <10^?3 for 0.49 and 0.7 μm. The parameter (1 ? ω₀) obviously increases above 60 km. Below 48 km some aerosol is present. The optical depth here is a strong function of wavelength. It varies from 1.5 to 3 at λ = 0.49 μm and from 0.13 to 0.4 at 1.0 μm. The mean size of particles below the cloud deck is about 0.1 μm. Below 35 km true absorption was found at λ < 0.55 μm with the (1 ? ω₀) maximum at H ≈ 15 km. The wavelength and height dependence of the absorption coefficient are compatible with the assumption that sulfur with a mixing ratio ～2 × 10^?8 normalized to S₂ molecules is the absorber. The upper limits of the mixing ratio for Cl₂, Br₂, and NO₂ are 4 × 10^?8, 2 × 10^?11, and 4 × 10^?10, respectively. The CO₂ and H₂O bands are confidently identified in the observed spectra. The mean value of the H₂O mixing ratio is 3 × 10^?5 < F_H2O < 10^?4 in the undercloud atmosphere. The H₂O mixing ratio evidently varies with height. The most probable profile is characterized by a gradual increase from F_H2O = 2 × 10^?5 near the surface to a 10 to 20 times higher value in the clouds.