ICARUS 53, 10-17 (1983)

The Clouds of Venus: Sulfuric Acid by the Lead Chamber Process 1

GODFREY T. SILL Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721

Received January ! 1, 1982; revised October 11, 1982

The Pioneer Venus atmospheric probe provided new data on the clouds of Venus. A model consistent with this data involves SO2 being oxidized to H2SO4 by NOx in the presence of H:O. NOx also forms nitrosylsulfuric acid (NOHSO,) dissolved in the H2SO4 droplets. This acid solution, along with SO2 and perhaps NO2, can explain the uv and visible reflection spectrum of Venus. In the middle and lower clouds NOHSO4 forms solid particles.

The most dominant feature of the planet in density over the planet. The particle size Venus is its clouds. To the eye they seem to spectrometer measured three kinds of cover the planet in a uniform blanket. In cloud particles: mode 1, small, ubiquitous 1927-1928, Wright (1927) and Ross (1928) aerosols, which extended even below the made ultraviolet photographs of Venus and clouds to within 31 km of the surface; mode discovered dark patches in the clouds. As 2, liquid droplets of sulfuric acid, 2.0 Ixm in instrumental techniques developed, the re- diameter, which were seen in all three flectivity or albedo of Venus at various cloud layers; and mode 3, solid larger crys- wavelengths was measured (Irvine et al., tals of needle, plate, or dendritic shape, 1968), and the polarization properties of the which were restricted to the middle and clouds indicated the presence of liquid lower cloud layers. Apparently sulfuric droplets (Hansen and Arking, 1971; Cof- acid was not the only constituent of the Ve- feen and Hansen, 1972). In the early 1970s nus clouds. As a matter of fact, the mass of the cloud droplets were identified as sulfu- the solid crystals exceeded that of the sulfu- ric acid (Sill, 1972; Young, 1973), but the ric acid droplets. identity of the ultraviolet and blue absorber Any scheme to explain the clouds of Ve- in the atmosphere of Venus remained un- nus must address these new discoveries. known. The discovery of sulfur dioxide gas While not necessary, it would be useful if on Venus (Barker, 1979; Conway et al., the proposed mechanism involves the for- 1979; Stewart et al., 1979) marked SO2 for mation of a near-ultraviolet and blue ab- the parent of H2804 and provided good ab- sorbing substance. This absorber is concen- sorption in the uv between 2000 and 3200 trated in the upper cloud around 66 km /~, but another absorber was still required. (Esposito, 1980; Pollack et al., 1980), at a The Pioneer Venus and Venera atmo- lower altitude than where the SO2 is de- spheric probes revealed a complex struc- tected. The middle and lower cloud layers ture in the Venus clouds. Both the nephe- are mostly free of visible absorption (To- lometer (Ragent and Blamont, 1980) and masko et al., 1980). It would be satisfying if the particle size spectrometer (Knollenberg the solid particles in the clouds were chemi- and Hunten, 1980) of Pioneer Venus re- cally compatible with strong sulfuric acid vealed three distinct cloud layers: upper, solutions and perhaps even derived from middle, and lower, with the latter variable H2SO4. If all components of the clouds were condensates or reaction products of t Paper presented at "An International Conference volatile substances, so much the better. on the Venus Environment," Palo Alto, California, All the above properties of the Venus November 1-6, 1981. clouds can be explained if the formation of 10 0019-1035/83 $3.00 Copyright © 1983 by AcademicPress, Inc. All fightsof reproductionin any form reserved. SULFURIC ACID ON VENUS 11 sulfuric acid on Venus mimics the manufac- forming a more dilute sulfuric acid solution turing process called the "Lead Chamber" with dissolved nitrosylsulfuric acid. This process. so-called "nitrose" acid was not wasted. It Watson et al. (1979) have proposed was used at the very beginning of the pro- that nitrogen oxides may exist in the Venus cess, just before the first lead chamber. The atmosphere and may give rise to the nitrose acid was pumped to another tower formation of solid nitrosylsulfuric acid called the Glover tower where hot SO2 and (NOHSO4), a possible candidate for the air entered from the sulfur furnace. Here crystalline particles. In this case odd nitro- the NOx was recovered, and the sulfuric gen (i.e., N not in N2 molecules) should be acid was concentrated: present in parts per million mixing ratio. 2NOHSO4 + SO2 + 2H20--~ Especially interesting is the fact that solid NOHSO4 melts at 347°K, the temperature 3H2SO4 + 2NO. (5) near the border of the middle and lower Note that each mole of nitrosylsulfuric acid cloud levels, where the number of cloud removes a mole of water from the acid solu- particles rapidly decreases. tion as it forms H2SO4. The excess SO2, air, The production of sulfuric acid in the and the released NO then went to the first 19th and early 20th century was mostly by chamber of the process. the lead chamber process, so called be- This is the way a plant would operate cause the reactions occurred in a series of ideally. However, as Lunge (1903, p. 694) very large lead-lined vaults, as diagrammed warned in his extensive treatise, such a in Fig. 1. The oxidation of SO2 to sulfuric complex process can easily go astray if not acid involved nitrogen oxides as carrier cat- stringently monitored. In the first chamber, alysts for the reaction sequence for example, if not enough steam or water spray is injected, reaction (3) can be super- 2 NO + 02 --~ 2NO2, (1) seded by another reaction forming "cham- SO2 + NO2--~ SO3 + NO, (2) ber crystals": 2SO3 + H20 + NO + NO2--~ SO3 + H20 ~ H2SO4. (3) 2NOHSO4. (6) Nitric oxide, NO, was reconstituted in the These crystals of nitrosylsulfuric acid process. A properly operating lead cham- would occasionally coat the walls of the ber passed warm (70°C) SO2 gas, air, H20 lead chambers and even form in midair in vapor, and NOx (here, NO and/or NO2) into the shape of "feathery" crystals. the first chamber where a copious white NOHSO4 can be considered a derivative mist of H2SO4 droplets was formed. The of H2SO4 with the nitroso group, -NO, sub- chamber was not highly colored by NO2 stituting for one hydrogen. The melting and gas. As the SO2 gas was consumed in the decomposing point is often quoted as 74°C first couple of chambers, the ambient air (347°K) (Lunge, 1903, p. 215), but 85-87°C became more highly colored due to NO re- (358-60°K) (Elliot et al., 1926) and 130°C acting with surplus oxygen forming nitro- (Lunge, 1903, p. 216) are also mentioned. gen dioxide according to reaction (I). To The decomposition-melting point appears prevent the loss of NOx in the continuous to be dependent on contamination of air flow, the exiting gases passed into the NOHSO4 by H2SO4. The decomposition is Gay-Lussac tower where they were ab- the reverse of reaction (6). Other possible sorbed by sulfuric acid present in excess, decompositions are NO + NO2 + 2H2SO4 2NOHSO4--~ H2SO4 + SO3 + NO + NO2. 2NOHSO4 + H20, (4) (7) 12 GODFREY T. SILL

H2S04+ H20 + NOHS04 H20 H20 H2SO4 __it _jL_ Reactions /i', white l ' "" 1,2,3 droplets J to H~SO4 6 6 b 6 I white ~ ~eochon 1 Reoctionl_ _ _ Reaction6 chamber~ red orange to NOHS04 ory~ols [] gas

Reochon9 while //~// Hsome qn ~ A,r + .._J _ 3E ~,.., ~ qf~ droplets 2vv4 SO2 ~ ll

conc. H2SO4 \\ H2SO4 H2SO4 to Gay- Lussac Ndrose Acqd~ Tower to Glarer Tower

Glarer Chomber Chamber Goy'LussOC Tower 1 2 Tower

Temperature Altitude

225K-- S03 w 70 km H20 .oT[o] o HzSO 4 0F HzSO4 NO2 so2 'h ~-- NO~ Upper S03 j "-'NO HzS04 Cloud NO2 HzSO4*NOHSO 4 I /E? 0

286 K~ --56.5 km HzS04 NOHSO4

/,~O( NO~ HzSO4+NOHS04 H2S Middle

I-H20 ; Cloud H~0 S03~ S% ~$O= _ 345K~ 3 S03/~.H20 --50.5kin ,.--x _j SOz (~0(NI~OSO3)aO ~'-20~'~(c°nc)~HeS04 ~NO, HS04 Lower

FIG. l. Schematic diag,'am for the Lead Chamber process (top). (Conceptual model for the formation of the Venus clouds (bottom). Droplets of sulfuric acid and solutions of sulfuric acid and nitro- syisulfuric acid are shown as circles. Jagged shapes correspond to crystals of nitrosylsulfuric acid.

2NOHSO4----> (NO803)20 + H20. (8) (490°K) and boils at 360°C (633°K), with va- por pressure similar to that of concentrated The decomposition product of reaction (8) H2SO4. is nitrosylsulfuric anhydride (Lunge, 1903, Nitrosylsulfuric acid and its anhydride p. 216; Elliot et al., 1926). It melts at 217°C are both very soluble in liquid H2SO4 solu- SULFURIC ACID ON VENUS 13 tions, up to 50% by weight in H2504 of 98% layer on Venus contains SO2 and perhaps (2% H20) concentration. NOHSO4 is not some NO2 gas. Their absorption coeffi- stable in dilute H2SO4 (<40% H2SO4), de- cients (Thompson et al., 1963; Dixon, 1940; composing according to Hall and Blacet, 1952) are also plotted in Fig. 2, along with the spectral reflection, 2NOHSO4 + H20--) more properly, the spherical albedo of Ve- 2H2SO4 + NO + NO2. (9) nus (Travis, 1975; Barker et al., 1975). Where SO2 and NO2 have a low absorption The more concentrated the H2504, the coefficient (between 0.21 and 0.23 i~m and more stable the solution. Eighty-six percent between 0.31 and 0.33 ~m) NOHSO4 solu- H2SO4 with 1.4% of dissolved NOHSO4 has tion has considerable absorption. The trend a vapor pressure of NOx which is 0.02 that of the absorption coefficient of NOHSO4 of its vapor pressure of H20 (Bed, 1935). between 0.3 and 0.56 ~m seems quite com- In order to see if NOHSO4 could be a patible with the reflectivity of Venus in the possible uv- and blue-absorbing component near uv and visible. The NO2 contribution in Venus' upper cloud level, a solution of to Venus reflectivity is uncertain, but the 1.4% NOHSO4 in 86% H2SO4 was prepared spectral region between 0.4 and 0.56 ~m by dissolving NOC1 gas in concentrated can be depressed by the presence of NO2 H2SO4. NOHSO4 was readily formed and gas, perhaps at 0.02 ppm. The upper cloud HCI produced by the reaction degassed of Venus may correspond to that part of the readily from the concentrated H2SO4. The lead chamber process where NO and NO2 acid solution was diluted with H20 to 86% gases dissolve in liquid H2504 forming concentration. The NOHSO4 concentration NOHSO4-H2SO4 solutions, as in the Gay- was determined by titration with KMnO4. Lussac Tower reaction (4). The uv and visible absorption was mea- The middle cloud layer of Venus contains sured on a Beckman DK-2 spectrophotom- H2SO4 as well as the solid crystals. If the eter and the absorption coefficient was cal- solid is NOHSO4, it probably forms as in culated (Fig. 2). Not only does the solution the lead chamber according to reaction (3), absorb near-uv and blue light, but exposure with the reactants mostly available from the to this light darkens the solution after 1 day, decomposition that occurs at the bottom of turning it a yellow-brown color. Utilizing the middle cloud. This cloud layer is in a vapor pressures in Berl (1935), I calculate convectively unstable part of the Venus at- that such a solution has an equilibrium mosphere. If both liquid H2SO4 and solid pressure of NOx ranging from 1.0 x 10 -8 NOHSO4 are present, the H20 vapor pres- atm (0.3 ppm) at 225°K to 6.6 × 10 -7 atm sure is insufficient to form pure H2SO4, as (1.6 ppm) at 280°K, the temperature range happens in a badly managed lead chamber of the upper cloud. Mixtures of NO and process. If some H2804 co-condenses on NO2 around these values could therefore the NOHSO4 particles as a thin film, the form a dilute solution of NOHSO4 with the "feathery" particles or agglomerates are sulfuric acid of the Venus cloud droplets, subject to decomposition at the bottom of according to the reverse of reactions (7) and the cloud at 345°K. At the same time (9). NOHSO4 is formed within H2804 droplets, Since reaction (9) produces two gases on requiring higher concentrations of ambient decomposition or vaporization, the equilib- NOx for the higher temperature in the mid- rium partial pressure of the gases is deter- dle cloud layer. Since these clouds have lit- mined by the product of their partial pres- tle visible light absorption, the dominant ni- sures. If, for example, the NO2 is depleted, trogen oxide should be NO instead of NO2. it can be compensated by an increase in the The liquid drops are subject to evaporation partial pressure of NO. The upper cloud as well as decomposition according to the 14 GODFREY T. SILL

~1.0 , , i , I ' ' ' ' I r J i ~ I

> ,8

~ .6

"~ .4 o o ~-.2

-- 1.4% NOHS04 02 --- S02

103 T E "~1oo 2 wE i / ./i \ L) IO'

~°--~.IO0

~ l0-~ .io 162 id t

103 i i t I i i t i I L i i i L t i .2 .3 .4 .5 Wavelength (,u. m )

FIG. 2. Top: The spherical albedo of Venus, after Travis (1975) and Barker et al. (1975). The dashed line from 0.30 to 0.33 i~m is the value preferred by Barker. The solid line in this region is from Travis. Bottom: The absorption coefficients of SO2 and NOz gases and of a solution of 1.4% NOHSO4 dis- solved in 86% HzSO4. The value of the coefficient a (cm 1) of the solution is read from the left margin. The value of coefficient k (cm -~ amagat -~) for the gases is read from the right margin. The NOHSO4 solution was placed in a l-cm cell in a dual-beam Beckman DK-2 spectrophotometer. The reference path was a l-cm cell of pure 86% H2SO4. Hence only the absorption due to NOHSO4 solution was measured. The absorption by sulfuric acid alone was negligible. To obtain full uv coverage, the original NOHSO4 solution was diluted in three stages by 86% H2SO4 to a final strength 0.001 of the original, with Beer's law of absorption conserved. The absorption coefficient of more dilute or more concen- trated solutions would be linear multiples of the values in the graph, a and k are defined as I = loe -~ or I = loe -~.

Glover tower reaction (5) at the bottom of 2NO + 3803--> (NO803)20 + 502. (10) the middle cloud, in line with the observed increase of SO2 noted by Oyama et al. The anhydride crystals will evaporate along (1980). with concentrated (98%) H2SO4 at the bot- The mechanics of the lower cloud (345- tom of the lower cloud deck. Another feasi- 367°K) are less certain. The Glover tower ble option is that the lower cloud crystals reaction in the middle cloud would produce are nitrosylsulfuric acid not contaminated more concentrated H2804 at the expense of with H2SO4 liquid. These purer crystals NOHSO4 and H20, and would dry the am- melt at a higher temperature and decom- bient gas. H2SO4 droplets may be more con- pose less readily. The chemistry of the centrated here, and if the ambient has low lower cloud is less certain, in line with the H20 partial pressure, solid nitrosylsulfuric variable nature of this cloud (Ragent and anhydride may form according to this reac- Blamont, 1980). The existence of this layer tion (Elliot et al., 1926): may depend on a local chemical or physical SULFURIC ACID ON VENUS 15 effect, such as the local mixing ratio of whereas the gas chromatograph finds 502 H20, SO2, and SO 3. at levels of hundreds of ppm below the The identity of the smallest aerosols, clouds (Hoffman et al., 1980a). One cannot which persist in the Venus atmosphere simply dismiss chlorine as a possible cloud down to 31 km altitude (temperature constituent. This paper, however, concen- 482°K), where they suddenly disappear, re- trates on a possible sulfate constituent as an mains uncertain. The suggestion of Knol- alternative. lenberg and Hunten (1980) that these aero- What might be the possible abundance of sols may be the residual cores from the NOx in the Venus atmosphere? The mass evaporation of the larger particles is in line spectrometer apparently cannot give data with the experimental work of Elliot et al. of nitrogen compounds due to interference (1926), who heated nitrosylsulfuric acid effects from the various isotopes of carbon crystals way beyond the usual melting point and oxygen in CO2. Using a sample data of 74°C, up to 350°C. They observed for the record of Hoffman et al. (1980b), with an first 15 min copious NOx outgassing along assumed terrestrial abundance of isotopes with SO3 fumes. But after 30 min a pale of C and O, I was not able to find evidence yellow, very viscous oil remained. The at the ppm level of NO (masked by CISO), same oil remained after another sample was NOH (masked by CI8OH and 13CISO), NOz heated at 300°C for 9 hr. Analysis of the oil (masked by COlSO), or HONO (masked by showed some nitroso (-NO) groups still 13COISO), although there may be excess sig- present. Perhaps this "oil" may be the nal at mass 47 due to nitrous acid (HONO) small aerosol, at least in the lower part of (A. J. Watson, private communication). the Venus atmosphere. The gas chromatograph, as well, seems The discussion of nitrosylsulfuric acid unable to detect NO or NO2. One nitrogen has emphasized that crystalline NOHSO4 oxide species was capable of being de- could be the mode 3 particles discussed by tected, namely, N20, but only at the 10- Knollenberg and Hunten (1980). The same ppm level (Oyama et al., 1980). N20 was authors also discuss the possibility that the not found above this level. Since NO and/or solid mode 3 particles might be a compound NO2 is required to produce nitrosylsulfuric of chlorine, based on the findings of Surkov acid in the clouds, it would have been more et al. (1979) that the Venera 11 collected a satisfying to have some measurements of total of 2 mg m -3 of chlorine in the cloud NOx in the clouds, but this is not yet possi- particles. The chlorine abundance was ap- ble. If NO is produced by lightning near the proximately 10 times the sulfur abundance. ground (Scarf et al., 1981), it is difficult to If this result is not due to some instrumental assess its production rate. Watson et al. artifact, one has the difficult task of propos- (1979) estimated the production of NO from ing some mechanism which will produce Venera 11 measurements of lightning in the solid chloride particles in an environment clouds to be about 101° cm -2 sec -1, with rich in sulfuric acid. The fate of any chlo- ppm concentrations of NO requiring a ride salt attacked by concentrated sulfuric mean life of NOx in the Venus atmosphere acid is the formation of a sulfate and pro- of several thousand years. If one takes duction of HCI gas. Of course if the chlo- Scarf's (presentation at Venus Conference, ride has a greater absolute abundance than 1981) estimate of lightning frequency as the sulfuric acid, the solid chloride would only 0.07 of the Earth's frequency, then the survive the decomposition into HCI gas. So mean life of NOx in the Venus atmosphere far there is no data from the mass spectrom- would have to be hundreds of thousands of eter or gas chromatograph indicating any years. Since NO would react with surface chlorine species in the Venus atmosphere rocks to form oxidized species (especially more abundant than a few parts per million, Fe 3+ and sulfate, SO42-) such a long life 16 GODFREY T. SILL

seems unlikely. However, if the surface is photometry of Venus from 3067 to 5960 ~. J. At- already oxidized by 02 or CO2 such that mos. Sci. 32, 1205-1211. ferric iron and sulfate sulfur saturate the BARKER, E. S. (1979). Detection of SO2 in the UV spectrum of Venus. Geophys. Res. Lett. 6, 117-120. near-surface layers, NO may resist destruc- BERL, E. (1935). Studies of the Lead Chamber pro- tion by the crustal rocks. In that case, the cess. Trans. Amer. Inst. Chem. Eng. 31, 193-227. fate of NO is determined by photolytic COFFEEN, D., AND J. E. HANSEN (1972). Polarization processes in the upper atmosphere. De- studies of planetary atmospheres. In Planets, Stars More and Yung (1982) have estimated that and Nebulae (T. Gehrels, Ed.), pp. 518-581. Univ. of Arizona Press, Tucson. it is theoretically possible for total NOx to CONWAY, R. R., R. P. McCoY, AND C. A. BARTH obtain a mole fraction of 3 × l0 -8 in the (1979). IUE detection of sulfur dioxide in the atmo- Venus atmosphere, due to production by sphere of Venus. Geophys. Res. Lett. 6, 629-631. lightning. If the clouds themselves serve as DEMORE, W. B., AND Y. L. YUNG 0982). Catalytic a sink for NOx, then perhaps the local con- processes in the atmospheres of Earth and Venus. Science 217, 1209-1213. centration of NOx might exceed the value DIXON, J. K. (1940). The absorption coefficient of ni- determined by DeMore and Yung. trogen dioxide in the visible spectrum. J. Chem. Does odd nitrogen survive in the Lead Phys. 8, 157-160. Chamber process? While all the equations ELLIOT, G. A., L. L. KLEIST, F. J. WILKINS, AND H. that describe the process always involve W. WEBB (1926). Nitrosylsulphuric acid, Part 1. J. Chem. Soc. 129, 1219-1232. odd nitrogen like NO, NO2, or even HONO EsPOSlTO, L. W. (1980). Ultraviolet contrasts and the and HNO3, it is true that nitrogen com- absorbers near the Venus cloud tops. J. Geophys. pounds are usually added to the front end of Res. 85, 8151-8157. the process to make up for losses of NOx. HALL, T. C., AND F. E. BLACET (1952). Separation of However, since the whole process is con- the absorption spectra of NO2 and N.,O4 in the range ducted in an air flow-through process, 2400-5000,~. J. Chem. Phys. 20, 1745-1749. HANSEN, J. E., AND A. ARKING (1971). Clouds of Ve- open-ended to the atmosphere with its nus: Evidence for their nature. Science 171, 669- abundant N2, one cannot be sure whether 672. the odd nitrogen is lost physically by evap- HOFFMAN, J. H., V. I. OYAMA, AND U. VON ZAHN oration in the air stream or whether the odd (1980a). Measurements of the Venus lower atmo- sphere composition: A comparison of results. J. nitrogen is chemically converted to N2. Geophys. Res. 85, 7871-7881. However, nitric oxide, NO, is unusually HOFFMAN, J. H., R. R, HODGES, T. M. DONAHUE, stable to reduction to N2 at temperatures AND M. B. MCELROY (1980b). Composition of the below 1000°C. Venus lower atmosphere from the Pioneer Venus The morphology of the Venus clouds, as mass spectrometer. J. Geophys. Res. 85, 7882- 7890. diagrammed in Fig. 1, can be consistently IRVINE, W. M., T. SIMON, D. H. MENZEL, C. PIKOOS, explained in terms of reactions of H20, AND A. T. YOUNG (1968). Multicolor photoelectric SO2, SO3, NO, NO2, H2SO4, and NOHSO4. photometry of the brighter planets. II1. Observa- In short, the Venus clouds may mimic the tions from Boyden Observatory. Astron. J. 73, 807- Lead Chamber process for the manufacture 828. of sulfuric acid, but without the lead and KNOLLENBERG, R. G., AND D. M. HUNTEN (1980). The microphysics of the clouds of Venus. J. without the chambers. Geophys. Res. 85, 8039-8058. LUNGE, G. (1903). Sulphuric Acid and Alkali, Vol. 1, ACKNOWLEDGMENTS 3rd ed. Van Nostrand, New York. The author wishes to thank Donald Hunten and OYAMA, V. 1., G. C, CARLE, F. WOELLER, J. B. POL- Clark Chapman for reviewing the manuscript. This LACK, R. T. REYNOLDS, AND R. A. CRAIG (1980). work was supported by NASA Grant NAGW-67. Pioneer Venus gas chromatography of the lower at- mosphere of Venus. J. Geophys. Res. 85, 7891- 7902. REFERENCES POLLACK, J. B., O. B. TOON, R. C. WHITTEN, R. BARKER, E. S., J. H. WOODMAN, M. A. PERRY, B. A. BOESE, B. RAGENT, M. TOMASKO, L. ESPOSITO, L. HAPKE, AND R. NELSON (1975). Relative spectro- TRAVIS, AND D. WIEDMAN (1980). Distribution and SULFURIC ACID ON VENUS 17

source of the UV absorption in Venus' atmosphere. Study of the aerosol cloud layer of Venus. Sov. As- J. Geophys. Res. 83, 8141-8157. tron. A. J. Engl. Transl. 5, 1. RAGENT, B., AND J, BLAMONT (1980). The structure THOMPSON, B. A., P. HARTEK, AND R. R. REEVES of the clouds of Venus: Results of the Pioneer Venus (1963). Ultraviolet absorption coefficients of CO2, nephelometer. J. Geophys. Res. 85, 8089-8105. CO, 02, H20, N20, NH3, NO, SO2 and CH4 between Ross, F. E. (1928). Photographs of Venus. Astrophys. 1850 and 4000]~. J. Geophys. Res. 68, 6431-6436. J. 68, 57-92. TOMASKO, M. G., L. R. DOUSE, P. H. SMITH, AND A. SCARF, F. L., W. W. TAYLOR, C. T. RUSSELL, R. C. P. OOELL (1980). Measurements of the flux of sun- ELPHIC, AND J. G. LUHMAN (1981). Venus light- light in the atmosphere of Venus, J. Geophys. Res. ning: A review of Pioneer Orbiter whistler measure- 85, 8167-8197. ments. In Conference on the Venus Environment, TRAVlS, L. D. (1975). On the origin of ultraviolet con- Nov. 1-6, 1981, Palo Alto, Calif., p. 55. [Abstracts] trasts on Venus. J. Atmos. Sci. 32, 1190-1200. SILL, G. T. (1972). Sulfuric acid in the clouds of Ve- WATSON, A. J., T. M. DONAHUE, D. H. STEDMAN, R. nus. Commun. Lunar Planet. Lab. 9, 191-198. G. KNOLLENBERG, B. RAGENT, AND J. BLAMONT STEWART, A. I., D. E. ANDERSON, L. W. EsPOSITO, (1979). Oxides of nitrogen and the clouds of Venus. AND C. A. BARTH (1979). Ultraviolet spectroscopy Geophys. Res. Lett, 6, 743-746. of Venus: Initial results from Pioneer Venus Orbi- WRIGHT, W. H. (1927). Photographs of Venus made ter. Science 203, 777-779. by infrared and violet light. Publ. Astron. Soc. SURKOV, YU. A., F. F. KURNOZOV, V. K. KHRIS- Pacific 39~ 220, TIANOV, U. N. GURGANOV, V. N. GLAZOV, B. N. YOUNG, A. T. (1973). Are the clouds of Venus sulfuric KORCHNGANOV, AND A. G. DUPCHENKO (1979). acid? Icarus 18, 564-582.