Properties of the clouds of Venus, as inferred from airborne observations of its near-infrared reflectivity spectrum

Tc..~avs 34, 28-45 (197~)

Properties of the Clouds of Venus, as Inferred from Airborne Observations of Its Near-Infrared Reflectivity Spectrum J A M E S B. P O L L A C K , D O N A L D W . S T R E C K E R , 1 F R E D C. W I T T E B O R N , E D W I N F . E R I C K S O N , AND B E T T Y J. B A L D W I N

Space Science Division, N A S A Ames Research Center; Moffett Field, California 9~035 Received May 16, 1977; revised October 20, 1977 We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2- to 4.0-tun spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86 °. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 :t: 2% H~SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 :t: 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 ~m in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res. 14, 637-642 (1976)-] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H~SO, and H20 contained in the cloud material both equal approximately 2 X 10 -8. Employing vapor pressure arguments, we show that the acid concentration equals 84 -l- 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 X 10-4 and 10 -~. INTRODUCTION

c l o u d s a n d t h e n i n d i c a t e in w h a t w a y s t h e current observations represent improvem e n t s o v e r o u r e a r l i e r o b s e r v a t i o n s of V e n u s in t h i s s p e c t r a l r e g i o n ( P o l l a c k et al., 1974, 1975) a n d w h a t n e w i n f o r m a t i o n t h e s e d a t a contain. B y a n a l y z i n g o b s e r v a t i o n s of t h e p o l a r i z a t i o n of V e n u s a t v i s i b l e w a v e l e n g t h s as a f u n c t i o n of p h a s e a n g l e a n d w a v e l e n g t h , H a n s e n a n d H o v e n i e r (1974) a n d H a n s e n a n d A r k i n g (1971) o b t a i n e d v e r y p r e c i s e d e t e r m i n a t i o n s of t h e size d i s t r i b u t i o n , v i s i b l e i n d e x of r e f r a c t i o n , a n d s h a p e of t h e cloud particles. According to these results, the cross-sectional weighted average particle r a d i u s is 1.05 ~m, t h e size d i s t r i b u t i o n is v e r y n a r r o w , t h e v i s i b l e r e f r a c t i v e i n d e x

Much has been learned about the prope r t i e s of t h e V e n u s c l o u d s f r o m a v a r i e t y of g r o u n d - b a s e d a n d s p a c e c r a f t o b s e r v a tions and their analyses. This information i n c l u d e s e s t i m a t e s of t h e size d i s t r i b u t i o n of t h e c l o u d p a r t i c l e s , t h e i r c o m p o s i t i o n , the clouds' altitude range, and their optical d e p t h . I n t h i s p a p e r we p r e s e n t n e w o b s e r v a t i o n s of V e n u s ' r e f l e c t i v i t y s p e c t r u m in t h e 1- to 4-urn s p e c t r a l r e g i o n t h a t p r o v i d e f u r t h e r i n f o r m a t i o n on c l o u d c o m position and optical depth. In the remainder of t h i s section, we b r i e f l y r e v i e w t h e c u r r e n t s t a t u s of o u r k n o w l e d g e a b o u t t h e V e n u s 1 Ames Associate. 28

0019-1035/78/0341-0028502.00/0 Copyright O 1978 by Academic Press, Inc. All righta of reproduction in any form reserved.

CLOUDS OF VENUS is about 1.44 with a slight wavelength dependence, and the particles are spherical. All these properties refer to particles ill the top unit optical depth of the clouds. Sill (1972), Young (1973), and Pollack et al. (1974) independently suggested that the clouds were made of a water solution of concentrated sulfuric acid. Sill and Young based this conclusion primarily upon the agreement between the properties of this material and the inferred refractive index of the cloud particles, their shape, and the observed low partial pressure of water vapor near ~he cloud tops. Pollack et al. (1974) showed that, among a wide variety of proposed cloud candidates, only sulfuric acid matched their observations of a strong cloud spectral feature in the 3- to 4-urn region. Further evidence supporting this compositional inference has been given by Young (1974) and Martonchik (1974) on the basis of cloud spectral features seen in the 8- to 13-urn region and by Hansen and Hovenier (1974) on the basis of their determination of refractive indices. All these analyses refer to the composition of the Venus clouds in their top unit optical depth. Evidence that sulfuric acid is present down to optical depths of several tens was provided by Pollack et al. (1975) from an analysis of their reflectivity observations in the 1.2- to 2.5-~m region. While there seems to be substantial agreement that the clouds are composed of sulfuric acid, there has been some debate about the concentration of the acid. Young (1973, 1974) has favored a value of about 75% H2SO4 because such a concentration would remain in a liquid state at cloudtop temperatures and hence the particles would be spherical there. In addition, Young argued that this concentration gave a much better match to spectral data in the 8- to 13-~m region than did a 90% solution. Opposing this view is that of Sill (1972) and Pollack et al. (1975), who favored a value closer to 85% so as to match measurements of the water vapor abundance near the cloudtops.

29

They pointed out that sulfuric acid had a marked tendency to supercool and hence arguments based on its nominal freezing points might be inexact. In addition, they noted that an 85% solution could equally well match the 8- to 13-~m data. Once again these estimates of acid concentration refer to values appropriate for the top portion of the clouds. A combination of ground-based and spacecraft observations have helped define the altitude range and optical depth of the clouds. According to Hansen and Hovenier's (1974) analysis of the polarization data, optical depth unity at visible wavelengths occurs at a pressure level of 50 =i: 25 mbar, which is located between 65 and 70 km above the surface. Observations of the limb of Venus made with the Mariner 10 cameras indicate that the particle scale height is somewhat smaller than the gas scale height in the upper portion of the clouds and that the clouds extend to an altitude of at least 80 km (Lacis, 1975). The lower-altitude portions of the main cloud layer have been studied with nephelometers and photometers carried aboard the Venera 9 and 10 atmospheric entry probes. Results from the nephelometer experiment imply that the number of particles per unit volume is approximately constant between 62 km and the cloud bottom; that the cloud bottom is located at the 49-km altitude level; and that the optical depth of the clouds at visible wavelengths equals 20 to 25 and 50 to 55 at the Venera 9 and 10 entry points, respectively (Marov et al., 1977). Independently, Moroz et al. (1977) find that the cloud optical depth is approximately 25 at both entry points from an analysis of measurements made with the narrow-band photometer experiment aboard these probes. Our previous observations of Venus' reflectivity spectrum in the 1- to 4-urn region were conducted with the 12" telescope aboard NASA Ames' Lear jet and were made when Venus had phase angles of 40 and 120 ° (Pollack et al., 1974, 1975). The

30

POLLACK E T AL.

new observations reported below were obtained at a phase angle of 86 ° with the 36" telescope of NASA Ames' C-141 aircraft. The new results represent a substantial improvement over the earlier observations : they have a much higher signal-to-noise ratio in the crucial 3- to 4-urn region, where a strong cloud feature is located, and they utilize an improved calibration procedure that places the reflectivity curve on an absolute scale for the first time and defines the shape of the curve in a better manner. The latter improvement was realized by using stellar calibration standards in place of direct observations of the Sun. In practice, direct observation of the Sun is a surprisingly difficult calibration procedure both because the Sun is an extended object, while Venus is not, and because the Sun is many orders of magnitude more luminous than Venus. In the following sections, we briefly describe our newT observations, discuss the theoretical calculations used to analyze them, and present a comparison between theoretical and observed reflectivity curves. The comparison enables us to test the sulfuric acid hypothesis further, to obtain a good determination of acid concentration, and to estimate a globally averaged value for the cloud optical depth. Toward the end of the paper, we examine the significance of these results and their relationship to earlier estimates of the above quantities. OBSERVATIONS In this section, we describe the instrument used to measure Venus' near-infrared reflection spectrum, its mating to the C-141 aircraft from which the observations were obtained, the conditions of the observations, and the calibration procedures. Finally, we present the resultant reflectivity curve. The observations were made with an uncooled circular variable filter wheel (CVFW) of about 1.5% spectral resolution (A),/~) that spans the wavelength region from 1.2 to 4.0 #m. Spectral scans were

generated by continuously rotating the wheel at a rate of about 1 rpm. Thus, many individual spectra were obtained and these were appropriately combined to produce the final spectrum. The instantaneous angular position of the filter wheel was sensed with a potentiometer. Light from the object being observed was focused on a limiting aperture near the filter wheel and, after passing through the filter wheel, was reimaged on a 2-mm-diameter, lead sulfide (PbS) detector. This detector was cooled to liquid nitrogen temperature. Both the output from the detector and the potentiometer's voltage were continuously recorded on magnetic tape. A more complete description of this instrument is given in Pollack et al. (1974, 1975). The instrument package was mounted at the bent Cassegrain focus of the 91-cm telescope of the Kuiper Airborne Observatory (KAO), a C-141 aircraft operated by NASA's Ames Research Center. The field of view" was approximately 40" or about twice the angular size of Venus at the time the observations were taken. Thus, the reflectivity values given below refer to the entire disk of Venus. A dichroic beam splitter was added to the original equipment used previously on the Lear jet aircraft. This beam splitter passed visual wavelengths into either the focal plane TV camera or an eyepiece for guiding purposes, while reflecting infrared wavelengths into the spectrometer system. The secondary of the KAO telescope was oscillated at a rate of 20 Hz so that the detector received light alternately from the object and the sky next to the object. The instrument's electronics filtered out the dc component of the resulting signal, thus effecting a firstorder subtraction of sky noise. Sets of spectra were obtained with the object first in one field of view of the oscillating secondary and then in its other field of view. By appropriately co-adding the two sets of spectra, a second-order subtraction of sky noise was realized. A further description of

CLOUDS OF VENUS this procedure is given in Pollack et al. (1974, 1975). The observational results were obtained on f i g h t s originating from Moffett Field, California, on 9, 10, and 12 June 1975 P D T . Venus was observed on the 9th and 10th, Jupiter on all 3 days, a C M a (Sirius) on the 9th and 10th, and a Boo (Arcturus) oIl the 12th. The latter objects served as calibration objects, as described below. For a given object, integration times generally totaled 20 min per flight. The measurements were made when the aircraft was at an altitude of about 41,000 ft. Typically, about 10 p r . ~ m of water vapor was present along the line of sight, which is a factor of 102 to 103 smaller than values above groundbased sites. The wavelength calibration of the filter wheel was effected b y measuring the transmission of a n u m b e r of narrow-passband interference filters of known wavelength and the emission spectra of several noble gas lamps t h a t had very narrow features in the spectral region of interest. This protocol enabled us to convert the recorded potentiometer voltages to a wavelength scale. We estimate the uncertainty in this scale is about ±0.01 #m, which is somewhat less t h a n a typical spectral resolution element. Absolute and relative brightness calibrations were based on a C M a and a Boo. Because the spectrum of a Boo had a high signal-to-noise ratio over the entire 1.2- to 4.0-gin region, it served as a primary standard. The spectral intensity of a Boo was derived in the following manner. In the 1.2- to 3-gin region, the ratio of a Boo to a C M a was generated and it was degraded from its nominal 1.5% resolution to about 5c7vresolution in order to increase the signal-to-noise ratio. In so doing, we in effect assumed t h a t the ratio was a smooth function of wavelength. This smoothed ratio was then multiplied b y a blackbody function at a t e m p e r a t u r e of 10,200°K, a value appropriate for Sirius (Schild et al., 1971), to obtain the relative spectrum of a Boo. This

31

spectrum was placed on an absolute scale b y utilizing ground-based broadband phot o m e t r y (J, H, K, L bands) of a Boo (Johnson, 1962, 1965; Lee, 1970; Strecker, 1976; Hackwell and Gehrz, 1975; Gillett et al., 1971). At wavelengths longward of 3 ~m, the absolute spectral intensity of a Boo was established b y fitting a blackbody function at a temperature of 4460°K, a value appropriate to a Boo (Johnson, 1966), to the shorter-wavelength spectrum (~< 2.2 ~m) and longer-wavelength groundbased p h o t o m e t r y at 3.5 pro. Because of CO absorption near 2.3 ~m and the H opacity minimum near 1.6 ~m, the above procedure could not be used over the entire 1.2- to 4-~m region. A subsequent set of aircraft observations conducted in June 1976, when a much more sensitive system was used, provided confirmation of the spectrum of a Boo derived from the flight series under discussion. The spectrum of a Boo, as found from observations of a Boo and a L y r in June 1976, agreed to b e t t e r t h a n 5 % with the spect r u m found from the 1975 data. The 1975 data were used in the reduction of the Venus observations reported in this paper. Since a Boo was observed on a different day t h a n Venus, we used observations of Jupiter to determine whether there had been any significant change in either the response of the instrument or the transparency of the atmosphere. To within an uncertainty of + 2 % , the absolute signal levels of the Jovian measurements remained identical for all three flights. In order to remove instrumental and atmospheric effects, the co-added scans of Venus were divided by those of a Boo. This ratio was then multiplied b y the spectral intensity of a Boo to produce an intensity spectrum of Venus. Finally, this spectrum was divided b y the solar intensity at Venus' orbit to produce the desired reflectivity spectrum of Venus. The solar flux at the orbit of Venus was found from an appro-

32

POLLACK ET AL.

priate scaling of its measured value at the E a r t h (Smith and Gottlieb, 1974). T h e absolute reflectivity spectrum of Venus in the 1.2- to 4.0-#m region is shown in Fig. 1. T h e individual points oil this curve were derived b y dividing the observed continuous spectrum into spectral bins t h a t measured half a spectral resolution element across. Aside from the noise of the Venus observations themselves, which can be estimated from the scatter of adjacent data points, we estimate t h a t the relative error is 4 - 5 % and the absolute error is 4-15% shortward of 3 /~m and 4-20% longward of 3 urn. These errors reflect the errors introduced b y our calibration procedure. Relative error refers to the uncertainty in the shape of the curve. Reflectivity, as given in Fig. 1, is defined as the ratio of the flux received from Venus to t h a t which would have been received from a flat, perfectly reflecting L a m b e r t surface at Venus' distance from the Sun, which was normally illuminated and had the same projected area as the illuminated portion of Venus' disk. T h e narrow absorption features in Fig. 1, such as those near 1.4, 1.6, 2.0, and 2.7 urn, are due to absorption b y carbon dioxide in Venus' atmosphere. The low reflectivity in the 2.9- to 4.0-urn region and the overall decline in reflectivity from 1.2 to 2.6 #m are due primarily to the absorption properties of the Venus cloud aerosols and are the focus of our analysis. The reflectivity spectrum shown in Fig. 1 is qualitatively similar to one given b y Moroz (1967) for a time close to superior conjunction (phase angle <<86°). However, our spectrum provides a much b e t t e r definition of the 3-~m cloud feature and encompasses spectral regions obscured b y water vapor close to the ground. THEORETICAL ANALYSIS In order to infer properties of the Venus cloud layer, we have computed synthetic spectra of the clouds. Below we briefly de-

~"~'i!"-~:/- ' ~-:. >. p.

,-'~. /-

.,.,..

.1

,3 UJ

• • ...: •

•

¢

.

...

_..:.:.

,..,..... p .

.011 •001 I~ 1.0

WAVELENGTH,

pm

Fro. 1. Measured reflectivity of the integrated disk of Vemm as a function of wavelength. These observations were obtained at a phase angle of 86°. Reflectivity, as used in this figure as well as in all the other figures, refers to the ratio of the observed flux at a given wavelength to that which would have been received from a perfectly reflecting, flat Lambert surface, located at Venus' orbital distance from the Sun, normally illuminated by the Sun, and having the same projected area as the illuminated portion of Venus. scribe the computational technique, the choice of model parameters, and corrections made for the effects of gas absorption. Reflection spectra of model clouds were comp u t e d b y using an accurate numerical scheme, based on the doubling technique (Hansen, 1969), to solve the multiplescattering problem. A Mie-scattering computer program, coupled to the doubling program, was used to determine the singlescattering properties of the cloud particles. Specific intensities so calculated were appropriately integrated across the illuminated portion of the visible disk at the phase angle of observation so as to yield reflectivities equivalent to and in the same units as the observed values. The model clouds were assumed to be vertically and horizontally homogeneous and the cloud particles were assumed to be spherical. A further discussion of our procedure is given in Pollack et al. (1974, 1975). In order to perform the multiple-scatter-

CLOUDS OF VENUS

33

ing calculations, we must first specify values gases, particularly COs. At wavelengths in for the following parameters: optical con- the 1.2- to 2.6-~m region, the gas absorption stants and size distribution function of the bands of COs are almost entirely confined cloud particles, optical depth of the clouds, to narrow, well-defined spectral regions. In and wavelength. The selection of wave- matching our theoretical curves to the lengths was of course based on the spectral observed curve in this spectral interval, we domain of the observations. For the other ignore these obvious COs bands and conparameters, we selected nominal values, centrate on the regions between them, but considered plausible variations of each which are largely free of gaseous absorption. of them in our analysis of the observations. However, we also use the higher-resolution The composition of the cloud particles spectra of Kuiper and Forbes (1967) to enters the calculations through the specifi- estimate the magnitude of any small, residcation of the values of the optical constants, ual COs absorption in these continuum i.e., the real and imaginary indices of re- regions and factor this consideration into fraction. The latter depend on wavelength. our estimate of error bars for the continuum The optical constants for sulfuric acid solu- levels. According to Fig. 1, continuum tions of varying concentrations at room regions are present in the regions of 1.2 to temperature were obtained from the lab- 1.4, 1.48 to 1.52, 1.70 to 1.82, and 2.20 to oratory measurements of Palmer and Wil- 2.54 ~m. The observations of Kuiper and liams (1975) and Williams (1977); those Forbes imply that the last two spectral for several sulfuric acid solutions at a tem- regions are free of obvious COs bands ; that perature of 250°K were found from the a number of weak CO~ bands are present work of Pinkley and Williams (1976) ; and in the 1.2- to 1.4-~m region, which could those for hydrochloric acid were obtained lower our "continuum" by about 5% in the from the study of Williams (1973). highest-reflectivity portion of this region; As our nominal choice, we adopted the and that moderate-strength COs features particle size distribution of Hansen and are present in or'near the'l.48- to 1.52-~m Hovenier (1974). Our computed spectra interval, which could lower the continuum are determined almost entirely by the cross- by about 15% at our resolution. sectional weighted average mean radius, ~, We have used laboratory transmission of the size distribution function, f equals spectra of CO~, in combination with our 1.05/~m for Hansen and Hovenier's function. observed spectrum, to obtain a first-order Finally, we used a value of 128 for the correction for gas absorption in the 2.9- to optical depth of the clouds when computing 3.9-~m spectral region. Since the singlespectra in the 2.9- to 3.9-~m region. How- scattering albedo of the cloud particles is low ever, as shown by Fig. 8 of Pollack et al. throughout this region, we assumed that (1975) and our own computations, the re- a simple reflecting layer model can be used flectivity is almost independent of optical to describe the formation of gas absorption depth, r, in this spectral region, provided features in the reflected spectrum. Accordthat r ~ 1. Such a condition is consistent ing to this model, absorption took place with the values of 20 to 55 given by the above the cloud layer; i.e., multiple-scatVenera 9 and 10 spacecraft results (Marov tering effects can be neglected. We also et al., 1976). At wavelengths shortward of assumed that the bands lie on the square 2.9 urn, r is considered a free parameter, to root portion of the curve of growth, so that be bounded by the observations. In t ~: IYV -- ( W P e ) 112, where t is transmisThe above calculations allow for absorp- sion, W is the amount of absorbing gas tion and scattering by the cloud particles, along the path of light within the atmobut do not take into account absorption by sphere, and Pe is the effective pressure. To

34

POLLACK E T AL.

determine the value of IT" a p p r o p r i a t e for our observations in the 2.9- to 3.9-t~m region, we studied a weak C02 feature centered at 2.98 ran. We defined a cont i n u u m around this b a n d b y fitting a straight line through the observed reflect i v i t y at 2.92 and 3.07 ~m, spectral positions where gas absorption is a m i n i m u m according to the room t e m p e r a t u r e , labo r a t o r y d a t a of Burch et al. (1968). T h e curve running through the circled points in Fig. 2 shows the transmission of the Venus atmosphere, so derived. We next found a value of 1V b y comparing laboratory spectra of CO2 (Burch et al., 1968) with the inferred transmission value at 2.98 ~m. In so doing, we degraded the spectral resolution of the l a b o r a t o r y results to t h a t of our observations and appropriately interpolated t h e m in In t-l/" space. We found a value of 0.064 (kin atm) (arm) for |T"2. The curve running through the d i a m o n d - s h a p e d points in Fig. 2 shows the transmission curve for this value of 1T~ in the 2.8- to 3.1-~m region, as obtained from the same l a b o r a t o r y data. We see t h a t the c o m p u t e d transmission curve agrees quite well with the observed one over this spectral interval, especially in the crucial region from 2.9 to 3.1 ttm. Utilizing the l a b o r a t o r y d a t a of Burch et al. (1968) and G r y v n a k et al. (1966), we obtained an analogous transmission curw~ in the 3.1- to 3.9-~m spectral region for the derived li "~ value. Over this entire region, the transmission values were essentially unity. According to Fig. 2, the lowest transmission value in the 2.9- to 3.1-~m interval is a b o u t 0.8. Thus, only m o d e r a t e corrections are needed for the effects of CO2 absorption in the 2.9- to 3.9-~m region. We multiplied our c o m p u t e d cloud reflectivities values b y these c o m p u t e d transmission values to obtain our final theoretical reflectivity spectra in the 2.9- to 3.9-ttm interval. T h e derived value of 0.064 (kin a t m ) (arm) for l;~"2 in the 2.9- to 3.9-~m spectral

1.0

.6 z

=<.4

0 2.7

I

2.8

O

I

I

LABORATORY VENUS

I

2.9 3.0 3.1 WAVELENGTH, #m

I

I

3 2

3.3

Fxc-. 2. Comparison between the transmission of Venus' atmosphere above the clouds in the 2.8- to 3.1-tLm spectral region (circles) and the transmission of CO~ with a i~,2 value of 0.064 (kin atm) (atm) (diamonds). The former were derived from the observations shown in Fig. 1 by drawing a continuum in a manner described in the text, while the latter were obtained by interpolating laboratory measurements at surrounding

W v a l u e s ( B u r e h et al., 1968).

region is consistent with values based on our a priori expectations. At shorter wavelengths, Young (1972) finds a value of a b o u t 0.3 (kin a t m ) (arm) from an extensive analysis of the observed equivalent widths of a n u m b e r of CO2 features. Because the single-scattering albedo of the cloud particles is m u c h higher in this shorter-wavelength region, m u c h of the line formation takes place within the clouds and therefore a significantly higher l~"2 would be expected there t h a n in the longer wavelength interval. We can also estimate l~T~ill the 2.9- to 3.9-#m interval b y assuming t h a t all the observed reflected light originates from scattering at the altitude where the cloud optical d e p t h is unity. According to Hansen and Hovenier (1974), the atmospheric pressure is 50 :t= 25 m b a r at this position. Allowing for two-way transmission through the Venus a t m o s p h e r e at a v a r i e t y of slant p a t h angles, we find li72 to lie between 0.008 and 0.07 (kin a t m ) (arm). The derived value of 1~"~ in the 2.9to 3.9-#m interval lies slightly below the upper limit of this estimate. Such a comparison is consistent with the low single-

CLOUDS OF VENUS .07 ~ I~ .06 ~.~ I / 05 [ ~ >'' I \1 > 04~ ~' ~,~]\

O r-] ~ ~ l% I'~

35

H2SO4 CURVES: 25%, R (3.4) = ,0658 50%, R (3.4) = .0309 75%' ~ (3.4) =.0175 84 5%, R" (3.4) = .0148 90.0%, R" (3.4) = .0155 95.6%, R (3.4) = .0155 OBSERVED CURVE: R (3.4) = ,0154

_,i,,~,.~

u~.03 m .02 I

~/OBSERVED

.

0 o

2.9

3.0

~ 1

3.1

3.2

"

.

^

_

3.7

3.8

3.9

~ 3.3 3.4 3.5 3.6 WAVELENGTH, ,um

I

4.0

FIG. 3. Comparison between the observed reflectivity spectrum of Venus in the 2.9- to 3.9-t~m spectral region and synthetic spectra for a variety of concentrations of sulfuric acid. Concentrations are expressed in percentage of H2SO4by weight. The observed curve was obtained by drawing a smooth curve through the data points of Fig. 1. All the theoretical curves have been normalized to agree with the observed value at u wavelength of 3.4 t~m. The observed and theoretical values of the reflectivity at 3.4 tim, /~ (3.4), are also shown in this figure. The theoretical curves were derived with optical constants measured at room temperature. scattering albedo of the cloud particles in the 2.9- to 3.9-urn region. RESULTS We now compare theoretical reflectivity spectra with the observed s p e c t r u m to place bounds on cloud particle composition, m e a n size, and optical depth. Figure 3 displays the observed reflectivity curve in the 2.9- to 3.9-urn region together with theoretical curves for w a t e r solutions of sulfuric acid of various percentages of H2SO4 b y weight. The observed curve shown in this figure was obtained b y fitting a s m o o t h curve to the d a t a points given in Fig. 1. All the theoretical curves h a v e been normalized to agree with the observed curve at a wavelength of 3.4 am. Such a normalization facilitates the comparison of the shapes of the theoretical and observed curves. Also indicated in this figure are the absolute values, /~, of the observed and theoretical curves at 3.4 #m. T h e shape of the observed curve in the 2.9- to 3.4-#m region provides a powerful discriminant of acid concentration. We see t h a t sulfuric acid solutions with concentra-

tions of 9 0 % or more lie significantly above the observed curve in the low-wavelength portion of this spectral region, while those with concentrations of 7 5 % or less lie significantly below the observed values. T h e theoretical curve for an 84.5% concentration tracks the observed curve quite well in this spectral region. F u r t h e r m o r e , it is in satisfactory agreement with the shape of the observed curve in the 3.4- to 3.9-~m spectral region and has an absolute value at 3.4 ~m, which is in good agreement with the observed value. As pointed out in the section dealing with the theoretical analysis, the c o m p u t e d /~ values are independent of optical d e p t h r in the 2.9- to 3.9-#m spectral region, provided t h a t r ~ 1. Thus, the above conclusion is valid for a n y reasonable choice of cloud optical depth. All the theoretical curves of Fig. 3 were c o m p u t e d with optical constants appropriate for r o o m t e m p e r a t u r e conditions. However, the portion of the clouds responsible for m o s t of the reflected light has a s o m e w h a t lower t e m p e r a t u r e - - a b o u t 250°K (Young, 1973). To assess the influence of t e m p e r a t u r e on our results we h a v e per-

POLLACK ET AL.

36

formed analogous calculations with optical constants measured a t a t e m p e r a t u r e of 250°K for the 75 and 9 5 % solutions. Figure 4 shows the resultant curves along with the corresponding r o o m t e m p e r a t u r e curves and the observed curve. Once again, the theoretical curves h a v e been normalized to agree with the observed value at a wavelength of 3.4 um. As m i g h t h a v e been expected, lowering the t e m p e r a t u r e causes the 3-urn sulfuric acid b a n d to be slightly narrower and therefore the reflectivities to increase slightly near 2.9 um. Smaller changes in shape occur elsewhere and the absolute values of the reflectivity at 3.4 ~m change only slightly. Using Figs. 3 and 4, we now m a k e an estimate of acid concentration. We assume t h a t the relative reflectivity values n e a r 2.9 p m h a v e an u n c e r t a i n t y of =t=10°Tv due to the combined effects of calibration errors and scatter in the d a t a points. E m p l o y i n g the curves for the 7 5 % solution in Fig. 4 to estimate the influence of a lower t e m p e r a ture on the results of Fig. 3, we find t h a t the acid concentration is 84 =t= 2 % H~SO4 b y weight. T h e remainder is of course water. I n deriving the estimated uncert a i n t y in the acid concentration, we as"07.[.[.~I

s u m e d t h a t a linear relationship held between concentration a n d reflectivity, for concentrations between 75 and 900/0. As mentioned in the Introduction, we earlier identified sulfuric acid as the principal c o m p o n e n t of the Venus clouds on the basis of comparisons between an observed s p e c t r u m in the 2- to 4-~m region and t h a t c o m p u t e d for a large n u m b e r of possible candidates (Pollack el al., 1974). We concluded t h a t only clouds m a d e of concentrated sulfuric acid ( > 7 5 % ) gave a satisfactory fit to these data. Almost all the rejected candidates gave a v e r y poor fit to these earlier observations. However, the closest rival to sulfuric acid, hydrochloric acid, did not violently disagree with the observations, although it gave a noticeably poorer m a t c h t h a n did the concent r a t e d sulfuric acid solution. (See Fig. 6 of Pollack et al., 1975.) Therefore, in Fig. 5, we examine the comparison between a 6 M HC1 solution and the observations. T h e theoretical curve for the 84.5% H2SO4 solution is also shown in this figure. This choice of concentration for the HC1 solution was m a d e so as to optimize the agreem e n t between its v a p o r pressure curves and the spectroscopically determined a b u n -

H2SO4CURVES:

.06 ~ \

O 75%,300"K,R (3.4)= .0177

I / /

1"1 95.6%, 300° K, ~ (3.4) = .0175

.05 ~ ] \

A_ 95.6%,250°K,R(3.4)= .0150

I \1

<>

~

2ooK <3,,:

=,~o3 . .02

.01 F o 2.9

1 3.0

I 3.1

I 3.2

I I I I 3,3 3.4 3.5 3.6 WAVELENGTH, pm

I 3.7

I 3.8

I 3.9

I 4.0

Fro. 4. Comparison between the observed reflectivity spectrum of Venus in the 2.9- to 3.9-~m spectral region and theoretical reflectivity curves for 75 and 95% sulfuric acid solutions. For each concentration two curves are shown, corresponding to results obtained with optical constants measured at room temperature and at 250°K. The theoretical curves have been normalized as in Fig. 3 and the/~ (3.4) values have the same meaning as in Fig. 3.

CLOUDS OF VENUS

37

THEORETICAL VALUES: .05

O []

I .04

84.5% H2SO4, R (3.4) =.0148 6M RCI, R" (3.4) = .0385 OBSERVED CURVE: R (3.4) =.0154

>__ .o3 I-

.02 .01

o 2.9

3.0

3.1

3.2

I l I 3.3 3.4 3.5 3.6 WAVELENGTH, .um

f 3.7

1 3.8

I 3.9

J 4.0

FIG. 5. Comparison between the observed reflectivity spectrum of Venus in the 2.9- to 3.9-tml spectral region and theoretical reflectivity curves for an 84.5% sulfuric acid solution (circles) and a 6 M HC1 solution (squares). The theoretical curves have been normalized as in Fig. 3 and were derived with optical constants measured at room temperature. T h e / ~ (3.4) values have the same meaning as in Fig. 3.

dances of gaseous HCI and H20 (Lewis, 1972). We see t h a t the curve for the HC1 solution badly disagrees with the shape of the observed curve in the 2.9- to 3.4-~m spectral region. Furthermore, the absolute value of this theoretical curve at 3.4 ~m lies far outside the observed value and its associated error bars. We conclude t h a t plausible HC1 solutions are in conflict with our data in the 2.9- to 3.9-tLm region, while H2SO4 solutions with a concentration of about 84% are not. All the above theoretical calculations have been performed with a value of 1.05 ~m for ~, the cross-sectional weighted average particle radius. This choice was based on Hansen and Hovenier's (1974) analysis of polarization observations. While we consider this determination to be quite good, it is nevertheless of interest to see what bounds our observations might place on ~. In Fig. 6, we show the observed curve together with computed reflectivity curves for several values of ~, including the nominal value. All the theoretical curves refer to an acid concentration of 84.5o-/0. While the shape of the computed curves does not v a r y much in the 2.9- to 3.9-t~m spectral region as the value of ~ is altered, the absolute value of the reflectivity at 3.4 #m,

/~ (3.4), does show a marked dependence on this variable. Both the ~ = 2.10- and 0.2625-~m cases have /~ (3.4) values t h a t lie significantly below the observed value, while the corresponding values of ~ = 1.05 and 0.525 t~m agree with the observed value, to within its uncertainty. On the basis of an uncertainty in the observed value of /~ (3.4) of ± 2 0 % , and the results shown in Fig. 6, we conclude t h a t 0.5 um ~ 1.4 t~m. These values encompass our nominal value of 1.05 t~m. T h e reason t h a t /~ (3.4) decreases for b o t h large and small values of ~ is the following. In the large size domain, we can use geometric optics to assess the qualitarive behavior of /~. As ~ is increased, less light is directly transmitted through an average particle and therefore b o t h the single-scattering albedo a n d / ~ decrease. In the smaller size domain, i.e., X < 1, Mie theory implies t h a t particles become b e t t e r absorbers as ~ decreases (Pollack, 1975). X is the ratio of a particle's circumference to the wavelength. We next turn to the observations in the 1.2- to 2.6-t~m region to obtain estimates of the optical depth of the clouds. In all the calculations presented below, ~ is set equal to its nominal value of 1.05 #m and

38

POLLACK E T AL. H2SO 4 C U R V E S : .05

O

84.5%, r = 1.05, R' (3.4) = .0148

[]

84.5%, r = 2.10, R (3.4) = .00788

/k

84.5%, ~" = .2625, R (3.4) = .00377

84.5%, 7 = .525, R (3.4) = .0140

.04,

OBSERVED

.03 I..02

.01

I 2.9

3.0

I

L

3.1

3.2

I

I

t

3.3 3.4 3.5 WAVELENGTH,/~m

~

L

I

I

3.6

3.7

3.8

3.9

I 4.0

FIG. 6. Comparison between the observed reflectivity spectrum of Venus in the 2.9- to 3.9-~m spectral region and theoretical reflectivity curves for several values of the mean cloud particle radius, ~. In all the theoretical cases, the clouds are made of an 84.5% sulfuric acid solution. The theoretical curves have been normalized as in Fig. 3 and were derived with optical constants measured at room temperature. The/~ (3.4) values have the same meaning as in Fig. 3. an acid c o n c e n t r a t i o n of 8 4 . 5 % HeS04 b y weight is used. I n Fig. 7, we show t h e dep e n d e n c e of R at a w a v e l e n g t h of 1.8 ~ m on t h e optical d e p t h of the cloud T. T h e solid horizontal line flanked b y t w o d a s h e d h o r i z o n t a l lines indicate t h e o b s e r v e d value of /~ (1.8) a n d its associated u n c e r t a i n t y . A c c o r d i n g to this figure, r ~ 12. F u r t h e r b o u n d s on the optical d e p t h can be o b t a i n e d f r o m t h e shape of the reflect i v i t y curve. I n Fig. 8, we c o m p a r e the o b s e r v e d c u r v e w i t h theoretical curves h a v i n g a v a r i e t y of values for t h e optical depth. T h e o b s e r v e d curve was derived b y d r a w i n g a s m o o t h curve t h r o u g h the d a t a

points of Fig. 1. All t h e theoretical curves h a v e b e e n n o r m a l i z e d to agree w i t h t h e o b s e r v e d value at a w a v e l e n g t h of 1.8 ~m. Consider first the shape of t h e c o n t i n u u m in t h e 2.2- to 2.6-~m region. T h e o r e t i c a l curves w i t h r of 16 a n d smaller disagree w i t h t h e o b s e r v e d spectral shape in this region, while those w i t h larger optical d e p t h s show a reasonable a m o u n t of agreem e n t . F r o m this comparison, we conclude t h a t r ~ 25. As m e n t i o n e d in t h e I n t r o d u c t i o n , t h e m a t c h of the sulfuric acid curves t o t h e d a t a in t h e 2.9- t o 3.9-~m spectral region implies t h a t the t o p u n i t optical d e p t h of

OBSERVED

P

>_

~

.2

rHEORETICAL

u. uJ .1

0

I 2Q

I 40

I I 60 80 OPTICAL DEPTH

r lOO

T 12o

r 14o

FIG. 7. Reflectivity at a wavelength of 1.8 ~m as a function of optical depth, r, for a cloud made of an 84.5% sulfuric acid solution, whose optical constants were obtained from room temperature measurements. Also shown as horizontal lines are the nominal observed value (solid line) and its bounds (dashed lines), as determined from the estimated errors.

CLOUDS OF VENUS

39

.4

0

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

WAVELENGTH, .um

FIG. 8. Comparison between the observed reflectivity spectrum of Venus in the 1.2- to 2.6-~m spectral region and theoretical spectra for various values of cloud optical depth. The observed curve was derived by drawing a smooth curve through the data points of Fig. 1. All the theoretical curves were generated for clouds made of an 84.5% sulfuric acid solution, whose optical constants were obtained from room temperature measurements. The theoretical curves have been normalized to agree with the observed value at a wavelength of 1.8 am. Also shown in the figure are the observed and theoretical reflectivity values at 1.8 ~m,/~ (1.8).

the clouds is made of this material. The results of Fig. 8 indicate t h a t concentrated sulfuric acid is present down to an optical depth of at least 25; i.e., it is present t h r o u g h o u t most of the cloud. Next consider the level of the theoretical curves of Fig. 8 in the 1.2- to 1.4-am region. According to the discussion of the previous two sections, the highest continuum levels in this region have a relative uncertainty of +1o% - 5 % • The error estimates are based in part on the uncertainties in the calibration and in part on the a m o u n t of residual CO2 absorption. Comparing the highest continuum levels in the 1.2- to 1.4-am region with the theoretical curves in this spectral region, we therefore infer t h a t r ~ 50.

Combining the above estimates of optical depth, we conclude t h a t r --- 37.5 4- 12.5, a result in satisfactory agreement with the values found b y the Venera 9 and 10 nephelometer and p h o t o m e t e r experiments (Marov et al., 1977; Moroz et al., 1977). It is to be noted t h a t our results refer to a globally averaged value, while the probe results refer to values at two particular locations. We now summarize the results of this section by showing a comparison between the observed data points over the entire 1.2- to 3.9-urn spectral region and a theoreticM curve employing model parameters consistent with those found above. This comparison is shown in Fig. 9 for a cloud model having ~ = 1.05 urn, r = 32, and an

40

POLLACK E T AL.

acid concentration of 84.5% H2S04 b y weight. In this figure, no normalization of the theoretical curve has been made; i.e., its absolute reflectivity values have been directly plotted. Ignoring spectral regions having obvious CO~ features and allowing for the small a m o u n t of CO~ absorption in the regions of 1.2 to 1.4 and 1.5 ~m, as discussed in the last section, we find t h a t there is an excellent overall agreement between the two curves.

DISCUSSION

1.

> .1

:

" \THEORETICAL

0

~. ~:.01

.001

1.o

T

1.,

r

,:,

!

r

WAVELENGTH, ~m

Utilizing the properties of the Venus clouds inferred in the previous section, we obtain estimates of the average concentration of cloud material in the region of the atmosphere they occupy; we study the relationship between the cloud particles and the gases t h a t produce t h e m ; and we comment on the probable physical state of the cloud particles. To estimate the volume mixing ratio of H2SO4 and H20 contained in the cloud material, we first calculate M, the a m o u n t of cloud mass contained in a vertical column of unit cross section, which extends throughout the cloud layer. Assuming t h a t the extinction cross section of a typical cloud particle is twice its geometric cross section (Pollack, 1975, Fig. 32) and using values of 37.5, 2 gm (Marov et al., 1977), and 1.77 g / c m a for the optical depth, mean particle radius, and particle density, we find M to equal approximately 8.9 )< 10 -3 g / c m 2. According to the results of M a r o v et al. (1976), the b o t t o m of the clouds occurs at an altitude of about 49 kin, where the pressure is about 1.3 bars. T h e equation of hydrostatic equilibrium implies t h a t the mass density of atmosphere above the b o t t o m of the clouds is approximately 1.5 X 103 g / c m 2 and therefore the average mass mixing ratio of cloud particles to atmosphere in the region of the clouds is about 5.9 X 10 -6. B y mass, H2SO~ accounts for about 84% of the mass mixing ratio and H20 about 16%. The correspond-

Fro. 9. Comparison between the observed reflectivity spectrum of Venus in the 1.2- to 3.9-~m spectral region and the theoretical spectrum of a cloud having properties consistent with those derived in this paper and elsewhere. The model cloud has an optical depth of 32 and a mean particle radius of 1.05 ~m. It is made of a water solution of sulfuric acid having a concentration of 84.5% H~SO4 by weight. The theoretical curve was derived with optical constants measured at room temperature. No normalization has been done to either curve. ing volume mixing ratios are approximately 2 X 10 -6 for b o t h H~SO4 and H20. In order to make use of vapor pressure relationships at the cloud bottom, we need to estimate the volume mixing ratio of sulfuric acid vapor, au~so4, at this position. At the cloud base all the sulfuric acid is in the form of vapor, b u t above a few kilometers of the base almost all of the sulfuric acid is present in the aerosol phase (Wofsy, 1974; Rossow and Sagan, 1975). Thus, all,so4 can be estimated from the volume mixing ratio of H2SO~ in the aerosol phase near the cloud bottom. We determined the latter in the altitude region from 51 to 53 k m (2 to 4 k m above the cloud base) in a fashion similar to t h a t used to obtain the average volume mixing ratio of cloud material. According to results obtained from the nephelometer experiment of the Venera 9 and 10 entry probes (Marov et al., 1977), the mean particle radius equals approximately 2 ~m in the 51- to 53-km altitud~

CLOUDS OF VENUS range, the aerosol extinction coefficient is approximately constant with altitude between 51 and 61 km, and the clouds dissipate between 49 and 51 km. Allowing for aerosols present above 61 km (Lacis, 1975) and below 51 km (Marov et al., 1977), we estimate th at the optical depth equals about 30 in the altitude range 51 to 61 km or about 6 between 51 and 53 km on the basis of the above-cited nephelometer result. Using this information, we find that the volume mixing ratio of H2SO4 contained in cloud material between 51 and 53 km equals approximately 1.7 X 10-6. This number should also be comparable to the volume mixing ratio of H2SO4 vapor, aHsSO,, at the cloud bottom, which is located at a 49-kin altitude (Marov et al., 1977). We next use this estimate of all,so4 to crudely determine the mixing ratio of water vapor below the cloud bottom. We do this in the following way: Given all,so4 and the total pressure at the cloud bottom, we find the partial pressure of H2SO4 vapor at this position. Next we use the vapor pressure curves of H2SO4 above sulfuric acid solutions to find the concentration of sulfuric acid at the cloud bottoms, given the values of the H2SO4 partial pressure and the cloud bottom temperature. Finally, we employ the vapor pressure curves of H20 above sulfuric acid to find the mixing ratio of water vapor at the cloud bottoms, now given the acid concentration and temperature there. To carry out the above scheme for estimating the water vapor mixing ratio, air,o, in the lower atmosphere, we must specify the vapor pressure relationships for H2SO4 and H20 above sulfuric acid solutions, the cloud bottom temperature and pressure and the mixing ratio of H2SO4 vapor, a~so4, at the base of the clouds. For this purpose, we used the vapor pressure curves of Gmitro and Vermeulen (1964). As mentioned above, the base of the sulfuric acid clouds occurs at an altitude of 49 kin,

41

where the atmospheric pressure and temperature equal 1.3 arm and 100°C, respectively (Avduevsky et al., 1977; Marov, 1972). Allowing for plausible uncertainties in the location of the cloud base and in the temperature measurements, we estimate that the cloud bottom temperature has an uncertainty of ±15°C. The parameter a~so4 was estimated to equal 1.7 X 10-6 earlier in this section. When account is taken of the uncertainty in the parameters used to estimate the volume mixing ratio of H2SO4 in cloud material between 51 and 53 km, in the relationship between this mixing ratio and aH2so~, and in the vapor pressure curves for H2SO4 (Verhoff and Banchero, 1972), we estimate that the net uncertainty resulting from these factors is equivalent to a factor of 5 uncertainty in O~H2S O 4 •

Using these parameter values and the procedure outlined earlier, we obtain estimates of the concentration, C, of the sulfuric acid clouds at their base and the water vapor mixing ratio by volume, a~2o, in the lower atmosphere. For the nominal choices of cloud base temperature, Tb, and a~2so4, we find values of 75% for C and 3 × 10-5 for ai,2o. However, there is a wide range of permissible values for C and a ~ o when account is taken of the uncertainty in the input parameter choices. Holding all,so4 equal to its nominal value but varying Tb from 85 to 115°C, we obtain values for aH~O spanning the interval from 6 ;4 10-3 to 9 X 10-2, while C varies from 80 to 73%. When allowance is made for the variance in both Tb and allison, aH~O and C are found to lie between 6 × 10-4 and 3 × 10-1, and 90 and 59% H2SO4 by weight, respectively. This large spread in C and aH~O can be reduced substantially by considering the bounds placed on ai~,o by radio and radar observations. Assuming that water vapor and carbon dioxide were the only sources of microwave opacity, that the volume mixing ratio of CO2, ac o~, equaled 90 ± 10%,

42

POLLACK E T AL.

and t h a t water vapor condensed into water ice particles in the upper troposphere, Pollack and Morrison (1970) inferred a value of 6.5 4- 3.5 X 10 -3 for aH20 from an analysis of microwave observations. Taking into account newer information about Venus, Rossow and Sagan (1975) reanalyzed the microwave observations so as to place bounds on aH~O. This additional information included radio observations made subsequent to 1970, the radio occultation results from Mariner 5, a nominal value of 97% for aco~, and the determination t h a t the main clouds are composed of a sulfuric acid solution. Their analysis included allowance for the contribution of the sulfuric acid clouds to the microwave opacity. Cloud properties were determined from specific choices of ~H~O and an2so4 in the lower atmosphere along with the assumption that total water, i.e., vapor plus aerosol, and total H2SO4 were constant with altitude. When the microwave data alone are considered, Rossow and Sagan (1975) obtained an upper bound of 10 -3 on aH~O. Allowance for the additional constraints imposed b y the visible index of refraction of the cloud aerosols and the a m o u n t of water vapor detected near the cloudtop led to a lowering of this upper bound to about 2 X 10 -4. The upper bounds on auto obtained by Rossow and Sagan need to be revised as a result of the findings of this paper. As discussed more fully below, total water is not a conserved quantity throughout the clouds. Since the bound on all20 they obtained b y jointly considering the microwave and visible data depended on the assumption of total water conservation, the derived bound is no longer valid. Also, the cloud mass obtained from our analysis, and hence its microwave opacity, is much less t h a n t h a t which would have been found b y assuming total water constancy. In deriving bounds on aH~O from the microwave data None, Rossow and Sagan considered only cases involving clouds more

massive t h a n ours (Rossow, private communication). When this constraint is relaxed, a much larger upper bound on an2o is obtained, one comparable to t h a t of Pollack and Morrison (1970), according to Rossow (private communication). We conclude t h a t microwave observations limit aH2O to values less t h a n 10 -2. When this upper bound is placed on az~o, we find t h a t our vapor pressure arguments lead to values ranging from 6 X 10 -4 to 10 -~ for aH~o and from 90 to 78% for C. These values of the water vapor mixing ratio in the lower atmosphere are consistent with a value of 10-3 determined from infrared observations made from the Venera 9 and 10 probes (Moroz et al., 1977). In addition, our values of aH~o imply t h a t water vapor is a significant source of infrared opacity in the lower atmosphere of Venus: Pollack and Young (1975) were able to reproduce approximately the elevated surface temperature of Venus with a greenhouse model in which CO2, H20 vapor, and sulfuric acid aerosols were the major sources of infrared opacity. The water v a p o r mixing ratio in the lower atmosphere was set equal to 3 X 10-3 in these calculations. The above estimates of aH2O clearly indicate t h a t throughout most of the cloud almost all the water is present in the vapor phase rather t h a n in the cloud particles: we have earlier estimated t h a t the average mixing ratio of water in cloud material is about 2 X 10-6. A corollary of this conclusion is t h a t the acid concentration at all altitudes, except near the cloudtops, is determined b y the water vapor a m o u n t and not vice versa. Our results also imply t h a t there is a strong gradient in the total water content (water vapor plus aerosol water) throughout the cloud region: from known cloud and atmospheric properties in the top unit optical depth of the clouds (Hansen and Hovenier, 1974) we find t h a t the volume mixing ratio of cloud water in this region equals about 1 X 10 -6. A comparable mix-

CLOUDS OF VENUS ing ratio also applies to water vapor near the cloudtops (Fink et al., 1972). Thus, the volume mixing ratio of total water near the top of the clouds is about 2 X 10-6 , which is several orders of magnitude smaller than our estimates of an~o. The latter is equivalent to the mixing ratio of total water at the base of the clouds. We suggest that this large gradient in total water occurs partially as a result of photochemical processes, including photodissociation, which cause a substantial depletion of water vapor and hence total water in the upper parts of the clouds. A much smaller gradient of total H~SO4 may characterize the cloud region, since throughout all but the bottom few kilometers almost all the H2SO4 is present within the cloud particles and thus is much less susceptible to photochemical conversion processes. Detailed models that incorporate both photochemistry and aerosol growth will be needed to test these hypotheses. Finally, we consider the implications of our estimate of the acid concentration near the cloudtops. The nominal freezing point of an 84% solution is about 290°K. But, the temperature in the top unit optical depth of the clouds is about 50°K cooler (Young, 1973), where Hansen and Hovenier (1974) deduce that the particles are spherical. Conceivably liquid sulfuric acid could freeze into solid spheres, but, more likely, the particles are supercooled liquid spheres in this region of the atmosphere. As pointed out in the Introduction, sulfuric acid has a strong tendency to supercool. CONCLUSIONS We have analyzed our observed spectral reflectivity of Venus in the 1.2- to 3.9-um region (see Fig. 1) to determine a number of cloud parameters. These latter results were further examined on the basis of thermodynamic considerations. Our major conclusions are as follows: 1. We find strong confirmatory evidence

43

that the clouds are made of a concentrated sulfuric acid solution in their top unit optical depth. This conclusion is based on a comparison of theoretical reflectivity curves for HCI and H2SO, solutions with the data in the 2.9- to 3.9-urn spectral region (see Figs. 5 and 9). 2. Furthermore, the clouds are made of sulfuric acid down to an optical depth of at least 25. This statement follows from our analysis of the shape of the reflectivity spectrum in the 1.2- to 2.6-um region (see Fig. 8). 3. The acid concentration is 84 q - 2 % H2804 by weight in the top unit optical depth of the clouds. This parameter was determined from an analysis of the shape of the observed reflectivity curve in the 2.9- to 3.4-#m spectral region (see Fig. 3). 4. The optical depth of the clouds is 37.5 ± 12.5. These values were found from the absolute value of the reflectivity at 1.8 um and the shape of the refleetivity spectrum in the spectral regions of 1.2 to 1.4 and 2.2 to 2.6 #m (see Figs. 7 and 8). 5. The mean radius of the cloud particles in the top unit optical depth lies between 0.5 and 1.4 urn, a result consistent with the very good determination by Hansen and Hovenier (1974). These bounds were derived from the absolute value of the refleetivity at the 3.4-um wavelength (see Fig. 6). 6. Averaged over the cloud region, the volume mixing ratios of H2S04 and H20 contained in the cloud material are both about 2 X 10-6. These ratios were found from the inferred optical depth, acid concentration, and mean particle size. 7. The acid concentration equals 84 4- 6% at the cloud bottoms and the water vapor volume mixing ratio beneath the cloud bottoms lies between 6 X 10-4 and 10-2. These values were derived by means of vapor pressure arguments from the inferred H2804 mixing ratio in the 51- to 53-kin altitude region and the observed location of the cloud bottoms. They were further

44

POLLACK ET AL.

constrained by the upper b o u n d on auto set b y m i c r o w a v e d a t a . 8. C o m p a r i s o n of t h e t o t a l w a t e r c o n t e n t a t t h e cloud b a s e (6 X 10 -4 to 10 -2 b y v o l u m e ) a n d cloud t o p (--,2 X 10 -6) i m plies t h a t t o t a l w a t e r is n o t a c o n s e r v e d q u a n t i t y t h r o u g h o u t t h e clouds, as is commonly assumed. Consequently, the cloud m a s s is m u c h s m a l l e r t h a n t h e m a s s t y p i f y i n g m o d e l s c o n s t r u c t e d w i t h such a conservation constraint. Except near the t o p of t h e clouds, t h e w a t e r v a p o r conc e n t r a t i o n is n o t c o n t r o l l e d b y the sulfuric acid particles, b u t i n s t e a d b y o t h e r processes, such as p h o t o d i s s o c i a t i o n a n d v e r t i c a l transport. ACKNOWLEDGMENTS The authors are pleased to express their appreciation to the staff and the flight and ground crews of the Kuiper Airborne Observatory for their part in making the observations possible, to J. Gerdts for instrument construction, to C. HarloT for computing, ~nd to G. Busk for flight preparations of the instrument system. REFERENCES AVDUEVSKY,V. S., BORODIN,N. F., BURTSEV,V. P., MALKOV, YA. V., MAROV, M. YA., MOROZOV,S.

F., ROZHDESTVENSKII,M. K., ROMANOV, R. S., SOKOLOV, S. S., FOKIN, V. G., CHEREMUKHINA, Z. P., AND SHKIRINA, V. I. (1977). Automatic stations Venera 9 and 10-functioning of descent vehicles and measurement of atmospheric parameters. Cosmic Res. 14, 577-585. BURCH, D., GRYVNAK, D. A., AND PATTY, R. R. (1968). Absorption by CO2 between 3100 and 4100 cm-h Philco-Ford Report U-4132." GILLETT, F. D., MERRILL, K. M., ANDSTEIN, W. A. (1971). Observations of infrared radiation from cool stars. Astrophys. J. 154, 83-90. GMITRO, J. I., AND VERMEULEN, T. (1964). Vaporliquid equilibria for aqueous sulfuric acid. Amer. Inst. Chem. Eng. J. 10, 740-746. GNEDYKH,V. I., MoRoz, V. I., PARFENT'EV,SAN'KO, N. F., AND USTINOV, E. A. (1976). Venera-9 and Venera-10: Ilt-spectrophotometry of cloud layer onboard the orbiters and descenders. Presented at the 1976 COSPAR meeting, Philadelphia, Pa. GRYVNAK, D. A., PATTY,R. R., BUNCH,]~). E., AND MILLER, E. E. (1966). Absorption by CO~. between 1800-2850 cm-x (3.5-5.6 tim). Philco-Ford Report U-3857.

HACKWELL,J. H., ANDGEHRZ,R. D. (1974). Infrared photometry of high-luminosity supergiants earlier than M and the interstellar extinction law. Astrophys. J. 194, 49-56. HANSEN, J. E. (1969). Radiative transfer by doubling very thin layers. Astrophys. J. 155, 565573. HANSEN, J. E., ANn ARKING, A. (1971). Clouds of Venus: Evidence for their nature. Science 171, 669-672. HANSEN, J. E., AND HOVENIER, J. W. (1974). Interpretation of the polarization of Venus. J. Atmos. Sei. 31, 1137-1160. JOHNSON, H. L. (1962). Infrared stellar photometry. Astrophys. J. 135, 69-72. JOHNSON, H. L. (1965). The absolute calibration of the Arizona photometry. Comm. Lunar Planet. Lab. 3, 73-77. JOHNSON, H. L. (1966). Astronomical measurements in the infrared. Ann. Rev. Astron. Astrophys. 4, 193-206. KELDYSH, M. V. (1976). Venus exploration with the automatic stations Venera 9 and Venera 10. Presented at the 1976 COSPAR meeting, Philadelphia, Pa. KuIPER, G. P., AND FORBES, F. F. (1967). High altitude spectra from NASA CV-990 jet I : Venus, 1-2.5 microns, resolution 20 cin-1. Comm. Lunar Planet. Lab. 6, 177-188. LACIS, A. (1975). Cloud structure and heating rates in the atmosphere of Venus. J. Atmos. Sci. 32, 1107-1124. LEE, T. A. (1970). Photometry of high lmninosity M-type stars. Astrophys. J. 162, 217-238. LEwis, J. S. (1972). Composition of the Venus cloud tops in light of recent spectroscopic data. Astrophys. J. 171, L75-L79. MAROV, M. YA., BYVSHEV, B. V., ~'IANUII,OV,K. N., BARANOV,Ytr. P., KUZNETSOV,I. S., LEBEDEV, V. N., LYSTEV,V. E., MAKSIMOV,A. V. POPANDOPULA, G. U., RAZDOLIN,V. A., SANDIMIROV,V. A., ANn FROLOV, A. M. (1977). Nephelometric measurenlents by the Venera 9 and 10 spacecraft. Cosmic Res. 14, 637-642. MARTONCHIK, J. V. (1974). Sulfuric acid cloud interpretation of the infrared spectrum of Venus. Astrophys. J. 193, 495-501. MoRoz, V. I. (1967). Physics of Planets. NASA TT F-515. MOROZ, V. I., PARFENT'EV, N. A., SAN'KO, N. F., ZttEGULEV, V. S., ZASOVA, L. V., AND USTINOV, E. A. (1977). Preliminary results of narrow-band probing of the cloud layer of Venus in the 0.800.87-~ spectral region of the Venera 9 and 10 descent vehicles. Cosmic Res. 14, 649-661. NASA (1972). Models of the Venus Atmosphere. NASA SP-801I.

CLOUDS OF VENUS PALMER, K., ~_NDWILLIAMS,D. (1975). Optical constants of sulfuric acid : Applications to the clouds of Venus? Appl. Opt. 14, 208-219. PINKLE¥, L. W., AND WILLIAMS, D. (1976). The infrared optical constants of sulfuric acid at 250K. J. Opt. Soc. Amer. 65, 122-124. POLLACK, J. B. (1975). The rings of Saturn. Space Sci. Rev. 18, 3-93.

45

boundary conditions on the atmosphere and clouds of Venus. J. Atmos. Sci. 32, 1164-1176.

SCHILD, R., PETERSON, D. M., AND OKE, J. B.

(1971). Effective temperatures of B- and A-type stars. Astrophys. J. 156, 95-108. SILL, G. T. (1972). Sulfuric acid in the Venus clouds. Comm. Lunar Planet. Lab. 9, 191-198. SMITH, E. V. P., ANDGOTTLIEB,D. M. (1974). Solar flux and its variations. Space Sci. Rev. 16, 771POLLACK, J. B., ERICKSON, E. F., WITTEBORN,F. 802. C., CHACKERIAN,C., JR., SUMMERS, A. L., VAN CAMP, W., BALDWIN,B. J., AUGASON,G. C., AND STRECKER,D. W. (1976). Unpublished observations. CAROFF, L. J. (1974). Aircraft observations of VERHOFF, F. H., AND BANCHERO,J. T. (1972). A note on the equilibrium partial pressures of vapors Venus' near-infrared reflection spectrum : Implicaabove sulfuric acid solutions. Amer. Inst. Chem. tions for cloud composition. Icarus 23, 8-26. Eng. J. 18, 1265-1268. POLLACK, J. B., ERICKSON, E. F., GOORVITCH~D., BALDWIN, B. J., STRECKER, D. W., WITTEBORN$ WILLIAMS,D. (1973). Private communication quoted in Pollack et al. (1974). F. C., AND AUGASON,G. C. (1975). A determination of the composition of the Venus clouds from WILLIAMS, D. (1977). Private communication. WOFSV, S. (1974). Venus clouds models. In The aircraft observations in the near infrared. J. Atmosphere of Venus: Proceedings of a conference Atmos. Sci. 32, 1140-1150. October 15-17, 197~ (J. E. Hansen, Ed.). Goddard POLLACK, J. B., AND MORRISON, D. (1970). Venus: Institute for Space Studies, New York City. Determination of atmospheric parameters from YOUNG, A. T. (1973). Are the clouds of Venus the microwave spectrum. Icarus 12, 376-390. sulfuric acid? Icarus 18, 564-582. POLLACK,J. B., ANDYOUNG,R. (1975). Calculations YOUNG, A. T. (1974). Venus clouds: Structure and of the radiative and dynamical state of the Venus composition. Science 183, 407-409. atmosphere. J. Atmos. Sci. 32, 1025-1037. YOUNG, L. D. (1972). High resolution spectra of Rossow, W. B., AND SAGAN,C. (1975). Microwave Venus. Icarus 17, 632-658.