On the occurrence of ferrous chloride in the clouds of Venus

On the Occurrence

of Ferrous

Chloride

in the Clouds of Venus

D. P. CRUIKSHANK Institute for Astronomy,

University of Hawaii, Honolulu, Hawaii 96822 AND

A. B. THOMSON _Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85721 Received January 5, 1971; revised April 6, 1971 In this paper we examine the observational basis for Kuiper’s (1969) conclusion that the upper cloud layers of Venus are composed of dihydrated ferrous chloride particles. Using new spectrophotometric observations of Venus, we find that there is no strong evidence for the Fe sf electronic band near 1 pm as deduced by Kuiper from the photometry of Venus by Irvine et al. (1968). Evidence for an absorption band on Venus near 0.5 pm is found to be valid, but nondefinitive in terms of the identification of ferrous iron. An absorption band near 0.39 pm appearing in the Venus photometric data of Irvine et al. (1968) is judged to be unreliable as an indicator of chemical composition because of time-dependent variations in the violet and ultraviolet clouds and spectrum of Venus. In conclusion, we find that the evidence for dihydrated ferrous chloride in the atmosphere of Venus is inconclusive.

INTRODUCTION Twenty-one decades have elapsed since the discovery of the atmosphere of Venus by M. V. Lomonosov during a solar transit in 1761. A century of spectroscopic studies, beginning with the visual work in the 187Os, while providing the basic information we have on the composition, temperature, and pressure of the upper atmosphere, have failed to give a firm answer to the fundamental question of the composition of the clouds of Venus. Some of the multifarious hypotheses that have emerged in recent years are reviewed by Koenig et al. (1967). Even three direct-entry space probes have not given a definitive solution to the problem of the cloud composition, though some Soviet scientists favor HZ0 ice cloud interpretation (Vinogradov et al., 197 1) (see also ref. list) from the chemical composition of the Venus atmosphere determined by Veneras 4,5, and 6. While water vapor is definitely observed in the atmosphere above some mean reflecting level in the clouds, and while the observed amount is known to fluctuate 497

(Schorn et al., 1969), there is no general agreement on the possible formation of clouds from the known amounts of water vapor. In the midst of the persistent disagreement on this matter, Kuiper (1969) has proposed that the clouds of Venus are a result of a halide based meteorology in contrast to the Earth’s water-based meteorology. He proposes that the clouds of Venus are composed of micron-sized particles of partially hydrated ferrous chloride, probably FeCl, - 2Hz0. His conclusion is based on the detailed coincidence of the curves of the spherical albedo of Venus and the rellectivity of partially hydrated FeCI, between about 0.22 and 3.5 pm. In this paper we propose to examine certain aspects of the coincidence of the two albedo curves. In Fig. 1 we show a composite of the two curves presented separately by Kuiper (1969) in his Figs. 2 and 6. The upper curve (Venus) was constructed by Kuiper from rocket observations (0.23-0.35 pm), narrow-band photometry by Irvine et al.

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k (.~) FIG. 1. U p p e r c u r v e : spherical albedo of Venus according to K u i p e r (1969). The d a t a points shortward of 1.06 ~ m are from I r v i n e et al. (1968). L o w e r c u r v e : albedo of d i h y d r a t e d ferrous chloride m e a s u r e d in laboratory. L e t t e r s a and b denote " e e n t r e s of g r a v i t y " of t h e depression in t h e two curves. Significance of c explained in t e x t .

(1968) (0.35-1.1 ]zm), Kuiper's own spectra (1.1-4.0 /~m), and various additional sources from 2.2 to 4.0 ~m. We have included an additional point at 0.315 ~m given b y Irvine et al. (1968) but omitted b y Kuiper (1969) (open circle). The matter we consider in this section of the present paper is the presence of the small dip in the Venus curve, amounting to about 6%, centered near 1.0 ~m. A band centered near this wavelength is seen in many iron-bearing compounds including certain natural terrestrial rocks (e.g., Adams, 1968), lunar rocks in the laboratory (Adams and Jones, 1970), and the Moon as observed from the Earth (e.g., Cruikshank, 1969). The band is caused b y electronic transitions in the unfilled d-orbital in ferrous iron (Fee+), and its exact position depends on the crystal coordination state of the ion in a given compound and on the particular anion. Vertical dashed lines in Fig. 1 mark the "center of gravity" of the bands in the spectra of Venus and ferrous chloride. There is a discrepancy of about 0.08 ~m in matching of the bands in the two spectra, though this could be interpreted as a temperature effect, since this band's position is known to be slightly temperature sensitive {Winter, 1968). As Kuiper ~1969, p. 244) pointed out, there are two

additional d - d transitions observed in ferrous iron, at 0.35 and 0.48 ~m. We consider these below. From the points given b y Irvine et al. (1968) alone, one would not infer that the absorption band at 1 ~m exists, especially when the errors in the albedo determination at each wavelength are considered. A photometrically derived point at 1.2 ~m would have shown the position of the continuum and thereby establish the presence or absence of a band. OBSERVATIONS

To test for the presence of the band at 1/zm reported b y Kuiper (1969), we observed the spectra of Venus and the Sun with a scanning spectrometer in the wavelength interval 0.7-1.3 ~m. We used the spectrometer described by Kuiper et al. (1962) on the 61-in. telescope of the Lunar and Planetary Laboratory for the spectrum of Venus, and the same spectrometer without the telescope for recording the solar spectrum reflected from a surface of Eastman K o d a k barium sulfate paint. The spectra obtained on May 15, 1970 are shown in Fig. 2. The spectrum of Venus in Fig. 2 is a hand-drawn copy of the best single spectrum with small sections added from other spectra obtained within a few

499

CLOUDS OF VENUS

minutes. Telluric absorptions are indicated, and it can be seen from the strength of the water vapor bands at 0.91 and 0.95 /zm t h a t the airmasses of the Sun and Venus were similar. The computed airmasses are 1.08 and 1.01, respectively. The spectra in Fig. 1 have sufficiently high resolution to permit the continua to be accurately drawn in to cover gaps left by strong atmospheric absorptions (dashed lines). The ratio of the brightness of Venus to t h a t of the Sun was measured from the spectra at intervals of 0.05/Lm. The ratios were then multiplied by a constant factor to allow for the different slit width used for the solar spectrum and for the reflectivity of the barium sulfate surface. In order to reduce the resulting color curve of Venus/ Sun to an absolute color curve, we used the solar color data of Arvesen et al. (1969). The Bond albedo of Venus was then obtained by normalizing our spectral curve of the 1.063/zm point of Irvine et al. (1968). I

I

A similar technique was used by Kuiper (1969). Figure 3 shows the Bond albedo curve of Venus determined from our spectra between 0.75 and 1.30/~m. The extension to 0.70/~m was made by extrapolation of the spectrum of Venus. I t can be seen t h a t within the error of the points on our albedo curve in Fig. 3, there is no perceptible absorption in this spectral region. In Fig. 2 we have sketched the approximate filter-detector response curves of two of the filters and photomultiplier used in the photometric study by Irvine et al. (1968) as given in an earlier paper by Young and Irvine (1967). I t can be seen t h a t the broadband filter centered at 1.06 /~m includes a region of COs absorption bands in the Venus spectrum. Since the intensity of Venusian atmospheric absorption varies with phase angle (e.g., Moroz, 1968), we searched for a systematic error in the 1.06 /~m Bond albedo given by Irvine et al.

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(1968). However, inspection of the photometric data for this wavelength given in the paper by Irvine et al. (1968) revealed no aspect of the phase curve which appears to be a result of variation in strength of the relatively weak CO2 bands near 1.05/zm. We therefore find no reason to question the photometrically-determined Bond albedo curve from 0.70 to 1.06/~m; only the extrapolation to about 1.3 /~m with the resulting absorption band shown in Fig. 1 is considered unreliable. The Spectral Region Near 0.5 I~m I t is a curious fact t h a t no reliable spectrophotometric data have been published for Venus, the third brightest object in the sky, in the visual part of the spectrum. While Glushneva (1969) has presented ultraviolet reflectivity curves from photographic spectrophotometry, the most easily observed part of the spectrum has been neglected. Thus in examining the region near 0.5 vm in the spectrum of Venus we are obliged to use the spherical albedo curve determined by Irvine et al. (1968). While this curve has high photometric precision, its wavelength resolution is inadequate for a thorough description of the planet's spectral brightness distribution. Still, as can be seen in Figs. 1 and 5, Irvine's d a t a do show certain spectral features in the visual and violet regions. In this section we consider the albedo decrease near 0.5tzm.

Kuiper (1969) has compared the depression near 0.5 vm with a similar one in the spectrum of ferrous chloride. The albedo curves of Venus and dihydrated ferrous chloride from 0.5 to 0.9 ~m are shown in Fig. 4. As before, the ferrous chloride data are taken from Kuiper's paper. The correspondence between the two curves near 0.5 /~m is quite close. We have plotted on the same diagram the spectral reflectivity curves (not true spherical albedos) of three naturally-occurring sands. The data for the sands came from

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CLOUDS

different sources. Curve c was obtained from Krinov (1947), who measured the spectral reflectivity of m a n y natural objects, including this barkhan sand deposit in the eastern part of the K a r a - K u m desert in Turkmen, U.S.S.R. The second sand curve (d) was published by Ashburn and Weldon (1956) and comes from the Mojave Desert. Curve e is taken from Romanova (1964, p. 23) who observed in westel~ Turkmen near the city of NebitDag (about 800 km from the dunes observed by Krinov). All three sands show an absorption in the albedo curve near 0.5 ~m. Unfortunately, a chemical analysis is available for only one of the samples, t h a t o£ Romanova (1964), pp. 89, 92-93). Her analysis of the dark fraction of the sand (excluding the pure silica) showed t h a t it consisted of over 6% FeeOs (containing ferric iron). Other minerals containing ferrous iron were present in very small amounts. Ferric (Fe 3+) iron exhibits absorption bands at or near 0.5 ~m, as do several other cations. Reference is made to an extensive catalog of cationic spectra in Tables of

Spectroplwtometric Absorption Data of Compounds Used for the Colorimetric Determination of Elements, which give absorption spectra of ions in solution. That the feature near 0.5 ~m is not uniquely attributable to Fe 2+ is shown by the wide variety of spectral bands of transition metal cations in 3d complexes reported by Ferguson (1970). A more complete compilation of spectra of crystals containing transition metal ions in all eleven electron configurations is given by Hush and Hobbs (1968). While we do not suggest t h a t the 0.5 m absorption on Venus is directly related to t h a t in terrestrial sands, we believe t h a t these data indicate t h a t the presence of an absorption in the Venus spectrum at this wavelength is not uniquely related to the presence of dihydrated ferrous chloride in the planet's atmosphere.

The Ultraviolet Albedo Curve The ultraviolet portion of the spherical albedo curve of Venus is shown in Fig. 5A with the estimated error bars indicating a uncertainty of about 4% in each point, as

OF VENUS

501

noted b y Irvine el al. (1968). The most significant feature is the small depression, amounting to about 4%, at the 3926 A point. This has been interpreted by Kuiper (1969) in his theory of the composition of the clouds of Venus as an electronic transition in ferrous iron. M o r e precisely, the ferrous iron transition in ferrous chloride is expected to occur at 3500 A, but since the Venus curve was obtained from filter photometry instead of spectrophotometry, the spectral resolution is low. In Fig. 5A the widely dashed segment of the curve shows the appearance of the albedo curve if the 3926 /I~ point were to be moved upward by 0.04, or to a position just beyond the upper extent of the error bar associated with this point. We have little I

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5 02

D . P . CRUIKSHANK AND A. B. THOMSON

assurance of the reality of spectral features than 4200 A. Further evidence for the ultrathat are of the same order of size as the violet variability is seen in the color curves error bars on the defining points. in Fig. 5B taken from the data of Irvine et We have studied the filter transmission al. (1968) for three different phase angles. curves and the color-magnitude trans- I t is insufficient .to assume only that the formations for possible systematic effects phase integral is strongly dependent on that could cause a depression of the wavelength in order to explain the wide 3926 A point. The filter used for this point variation evident in Irvine's curves. includes the K line in the solar spectrum, Variable broadband absorptions are necesb u t because the photometry was calibrated sary to understand the changes in the using observations of main sequence G ultraviolet reflectivity of Venus. Based on stars, systematic variations in the K line the data presented here, we question the are not expected. We found no reason to uniqueness of the ultraviolet portion of the suspect that the solar intensity at this spherical albedo curve for Venus generated point was overestimated (which would from the photometric data of Irvine et al. result in the observed depression in the (1968). Inspection of the color curves in Fig. 5 lead us to suspect the 3926 and 4155 Venus albedo). While there is no clear evidence for a A points most strongly of random variasystematic error in the observing and tions. reduction techniques, it appears probable CONCLUSION that random variations in the ultraviolet We have examined each of three reported brightness of the planet itself could cause an uncertainty in the true ultraviolet absorption bands in the spherical albedo albedo. I t is well known that variable curve of Venus and have found that one markings which appear at wavelengths probably does not exist, and that the shorter than 3900 Asignificantly alter the second m a y exist b u t can be explained by appearance of the planet on a day to day several hypotheses of which dihydrated basis. Examples of the variations can be ferrous chloride is only one. A possible seen in the excellent photographs published third band may result from time-dependent b y Kuiper et al. (1969). The dark markings variations in the atmosphere of Venus and frequently cover a large fraction of the thus is of dubious value as a diagnostic illuminated area of the planet. Their spectral feature. The interpretation of any intensity and number m a y vary in a matter one band as due to ferrous chloride would be greatly strengthened b y the unequivocal of days and from year to year. The effect of the variable ultraviolet identification of a second band, and even markings on the albedo of the planet has more so b y a third. From our present been discussed b y Glushneva (1969) who analysis, we submit that only one of the refers to earlier work b y Polozhentseva three spectral features discussed b y Kuiper (1964). A strong correlation between the (1969) is likely to be caused b y ferrous iron, appearance of ultraviolet markings and and the identification is not unique. Perhaps the major factor in favor of variations in the ultraviolet spectral refiectivity was found. In Fig. 5C and D we Kuiper's hypothesis is the general simihave reporduced two color curves (the larity in shape of the albedo curves of ratio Lvenus/Isu n v s . wavelength) of Venus Venus and dihydrated ferrous chloride determined b y Glushneva (1965, 1969). between 0.2 and 3/xm. There is a notable The data given b y Glushneva were reduced exception near 1.8/~m (marked c in Fig. 1) b y her using the color of Sun determined b u t this may be another error in the interb y Johnson (1954), b u t we have corrected polated Venus curve. None of the many them to the solar data published b y Arvesen other chemical compounds examined b y et al. (1969). The curves do not.represent • .Kuiper m a t c h e d . t h e gensral_form o f the true geometric albedoes. We note here the Venus curve even approximately. Considering the evidence for and against considerable variability of the details of the curves, especially at wavelengths shorter ferrous chloride in terms of the absorption

CLOUDS OF VENUS

bands and the total shape of the albedo curves of Venus and the chemical, we conclude that the identification of particles of dihydrated ferrous chloride as the opaque material in the clouds of Venus must remain conjectural. ACKNOWLEDGMENTS The new observations reported here were made when the first author was at the Lunar and P l a n e t a r y L a b o r a t o r y supported b y I~ASA grant NsG- 161-61, and the investigation was completed after he moved to the I n s t i t u t e for Astronomy, University of Hawaii, where he was supported b y NASA grant N G R 12-001-083. W e are grateful to Drs. Theodore Simon, W. M. Irvine, and David Morrison for discussions and correspondence on the p h o t o m e t r y of Venus. REFERENCES ADAMS,J. B. (1968). Lunar and Martian surfaces: petrologic significance of absorption bands in the near infrared. Science 159, 1453-1455. ADAMS, J. B., AI~DJONES, R. L. (1970). Spectral reflectivity of lunar samples. Science 167, 737-739. ARVESEI% J. C., GRIFFII~, R. N., Jr., Am) PEARSON, B. D., JR. (1969). Determination of extraterrestrial solar spectral irradiance from a research aircraft. Appl. Opt. 8, 2215-2232. ASHBIrRI~, E. V., AND WELDON, R. G. (1956). Spectral diffuse reflectance of desert surfaces. J. Opt. Soc. Amcr. 46, 583-586. CRUIKSHANK, D. P. (1969). Moon: infrared studies of surface composition. Science 166, 215-218. FERGIISO~¢, J. (1970). Spectroscopy of 3d complexes. I n "Progress in Inorganic Chemistry," Vol. 12 (S. J. Lippard, Ed.), pp. 159-293. Wiley, New York. GLUSHZe~VA, I. N. (1965). Wavelength dependence of the albedo of Venus and J u p i t e r in the ultraviolet. Soy. A s t r o n . - - A J 8, 573-575. GLVSH~EVA, I. N. (1969). A new determination of the wavelength dependence of Venus' albedo in the ultraviolet region. Soy. Astron.-A J 13,162-165. HUSH, N. S., AND HOBBS, R. J. M. (1968). Absorption spectra of crystals containing transition metal ions. I n "Progress in Inorganic Chemistry," Vol. 10 (F. A. Cotton, Ed.), pp. 259-486. Wiley, New York. International Union of Pure and Applied Chemistry. (1963). "Tables of Spectrophotometric Absorption D a t a of Compounds Used for the Colorimetric Determination of Elements." 625 pp. Butterworths, London.

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IRVINg., W. M. (1968). Monochromatic phase curves and albedoes for Venus. J. Atmos. Sci. 25, 610-616. ~tVINE, W. M., SIMON, T., ME1WZEL, D. H., PIKOOS, C., AND YOUNG, A. T. (1968). Multicolor photoelectric p h o t o m e t r y of the brighter planets. H I Observations from Boyden Observatory. Astron. J. 73, 807-828. J o ~ s o ~ , F. S. (1954). The solar constant. J. Meteorol. 11, 431-439. KOENIG, L. R., MURRAY, F. W., MICHAUX, C. M., AND HYATT, H. A. (1967). H a n d b o o k of the physical properties of the planet Venus. NASA SP-3029. KRr~ov, E. L. (1947). "Spectral Reflection Properties of Natural Formations." A c a d e m y of Sciences, Moscow, 270 pp. (in Russian). (Available in English in Nat'l. Research Council of Canada Technical Translation TT-439, Ottawa, 1953.) KUIPER, G. P. (1969). Identiflcatoin of the Venus cloud layers. Commun. Lunar Planet. Lab. 6, 229-250. KUIPER, O. P., FOU~TAr~, J. W., LAWSON,S. M., AND HARTMAN, W. K. (1969). Venus photographs. Commun. Lunar Planet. Lab. 6, 251-274. KUIPER, G. P., GORANSOI~,R. B., BINDER, A. B., A~D JOHnSOn, H. L. (1962). A n infrared stellar spectrometer. Commun. Lunar Planet. Lab. 1,119-127. MOROZ, V. I. (1968). The CO2 bands and some optical properties of the atmosphere of Venus. Sov. A s t r o n . - - A J 11,653-661. POLOZHENTSEVA,r . A. (1964). Izv. Glav. Astron. Obs. Pulkove. 23 (5), No. 175, 75. ROMA~IOVA, M. A. (1964). "Air Survey of Sand Deposits b y Spectral Luminance." Consultants Bureau, New York, 158 pp. (translated from Russian). SCHORN, 1~. A., BARKER, E., GRAY, L. D., AND MOORE, R. (1969). High-dispersion spectroscopic studies of Venus, I I : The water-vapor variation. Icarus 10, 98-104. VINOGRADOV,A. P., SVRKOV,YU. A., A_~DREICHIKOV, B. M., KALINKINA, 0. M., AND GRECHISCHEVA, I. M. (1971). The chemical composition of the atmosphere of Venus (1971). I n " P l a n e t a r y Atmospheres", (C. Sagan, T. C. Owcn, and H. J. Smith, eds.), pp. 3-16. Reidel, Holland. WX~ER, G. (1968). The effect of temperature on the electronic spectra of octahedral iron (II) complexes. Austral. J. Chem. 21, 2859-2864. YounG, A. T., A~-D IRVI~rE, W. M. (1967). Multicolor photoelectric p h o t o m e t r y of the brighter planets. I. Program and procedure. Astron. J. 72, 945-950.