Liquid content of the lower clouds of venus as determined from mariner 10 radio occultation

S-band (13.06-cm) and X-band (3.56-cm) radio occulation data obtained during the flyby of Venus by Mariner 10 on February 5, 1974 were analyzed to obtain the effects of dispersive microwave absorption by the clouds of Venus. The received power profiles were first corrected for the effects of refraction in the atmosphere of Venus, programmed changes in the pointing direction of the high-gain antenna, and limit-cycle motion of the spacecraft attitude control system. The resulting excess attenuation profiles presumbaly due to cloud absorption have been inverted discretely to obtain profiles of absorption coefficient at the two wavelenghts. The ratios of the absorptivities are consistent with a sulfuric acid-water mixture as the constituent of the absorbing clouds, having a sulfuric acid concentration of 75 ± 25%. Three absorption peaks are evident in the profiles at altitudes of 68, 60, and 48 km. With a sulfuric acid concentration of 75%, the upper cloud has a peak liquid content of 0.08 g/m³, and an integrated content of 0.024 g/cm², which corresponds roughly to terrestrial stratus or altostratus clouds. The major absorption layer has a peak of 1.1 g/m³ at an altitude of 48 km, with an integrated content of 0.5 g/cm², similar to that of terrestrial cumulus and cumulonimbus clouds. The absorption ratios for the middle cloud at 60 km are not consistent with a sulfuric acid-water mixture.     

Net global thermal emission from the Venusian atmosphere

Improved calculations of net emission from the northern hemisphere of Venus are presented. These are based on temperature profiles, water vapor mixing ratio profiles, and cloud models retrieved in 120 solar-fixed latitude-longitude bins from infrared measurements in six spectral channels made over a period of 72 days by the orbiter infrared radiometer (OIR) instrument of the Pioneer Venus mission. Only carbon dioxide, sulfuric acid cloud, and water vapor are considered as significant sources of atmospheric opacity, and the role of the latter component is found to be minor. The sensitivity of the calculations to extreme alternative cloud models, measurement errors, and calibration errors is also discussed. Net emission is found to be only weakly dependent on latitude and longitude during the period of observation with the exception of the high-latitude polar collar region, where emission is low. Mean net emission from the northern hemisphere is 157.0 ± 6.9 W.m^?2, corresponding to an equivalent temperature of 229.4 ± 2.5°K. If this figure is characteristic of the whole planet and if thermal balance is assumed, the bolometric albedo of Venus is 0.762 ± 0.011. This value is consistent with the latest estimates within experimental error.     

Laboratory measurements of the microwave opacity and vapor pressure of sulfuric acid vapor under simulated conditions for the middle atmosphere of Venus

Microwave absorption observed in the 35- to 48-km-altitude region of the Venus atmosphere has been attributed to the presence of gaseous sulfuric acid (H₂SO₄) in that region. This has motivated the laboratory measurement of the microwave absorption at 13.4- and 3.6-cm wavelengths from gaseous H₂SO₄ in a CO₂ atmosphere under simulated conditions for that region. As part of the same experiments, upper limits on the saturation vapor pressure of gaseous H₂SO₄ have also been determined. The measurements for microwave absorption have been made in the 1- to 6-atm pressure range, with temperatures in the 500 to 575°K range. Using a theoretically derived temperature dependence, the best-fit expression for absorption from gaseous H₂SO₄ in a CO₂ atmosphere at the 13.4-cm wavelength is $\text{9.0 × 10}{}^9\text{q(P)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{T}{}^{\mathrm{?3}}\text{(}\text{dB km}{}^{\mathrm{?1}}\text{),}\text{where}\text{q}$ is the H₂SO₄ number mixing ratio, P is the pressure in atmospheres, and T is the temperature in degrees Kelvins. The best-fit expression for absorption at the 3.6-cm wavelength is 4.52 × 10¹⁰q(P)^0.85T^?3 (dB km^?1). The inferred H₂SO₄ vapor pressure above liquid H₂SO₄ corresponds to ln p = 8.84 ? 7220/t where p is the H₂SO₄ vapor pressure (in atm) and T is the temperature in degrees Kelvins. These results suggest that abundances of gaseous H₂SO₄ on the order of 15 to 30 ppm could account for the microwave absorption observed by radio occultation experiments at 13.3- and 3.6-cm wavelengths. They also suggest that such abundances would correspond to saturation vapor pressure existing at or above the 46- to 48-km range, which correlates with the observed cloud base. It is suggested that future measurements of absorption in the 1- to 3-cm wavelength range will provide additional tools for monitoring variations in H₂SO₄ abundance via radio occultation and radio astronomical observations.     

A bulk cloud parameterization in a Venus General Circulation Model

A condensing cloud parameterization is included in a super-rotating Venus General Circulation Model. A parameterization including condensation, evaporation and sedimentation of mono-modal sulfuric acid cloud particles is described. Saturation vapor pressure of sulfuric acid vapor is used to determine cloud formation through instantaneous condensation and destruction through evaporation, while pressure dependent viscosity of a carbon dioxide atmosphere is used to determine sedimentation rates assuming particles fall at their terminal Stokes velocity. Modifications are described to account for the large range of the Reynolds number seen in the Venus atmosphere.Two GCM experiments initialized with 10 ppm-equivalent of sulfuric acid are integrated for 30 Earth years and the results are discussed with reference to “Y” shaped cloud structures observed on Venus. The GCM is able to produce an analog of the “Y” shaped cloud structure through dynamical processes alone, with contributions from the mean westward wind, the equatorial Kelvin wave, and the mid-latitude/polar Mixed Rossby/Gravity waves. The cloud top height in the GCM decreases from equator to pole and latitudinal gradients of cloud top height are comparable to those observed by Pioneer Venus and Venus Express, and those produced in more complex microphysical models of the sulfur cycle on Venus. Differences between the modeled cloud structures and observations are described and dynamical explanations are suggested for the most prominent differences.     

A reanalysis of Venus winds at two cloud levels from Galileo SSI images

The global circulation of the Venus atmosphere is characterized at cloud level by a zonal super rotation studied over the years with data from a battery of spacecrafts: orbiters, balloons and probes. Among them, the Galileo spacecraft monitored the Venus atmosphere in a flyby in February 1990 in its route toward Jupiter. Since the flyby was almost equatorial, published analysis of zonal winds obtained from displacements of cloud elements on images obtained by the SSI camera [Belton, M.J.S., and 20 colleagues, 1991. Science 253, 1531-1536] stop at latitudes 50° north and south. In this paper we present new results on Venus winds based on a reanalysis of an extended set of images obtained at two wavelengths, 418 nm (violet) and 986 nm (near infrared), that sense different altitude levels in the upper cloud. Our main result is that we have been able to extend the zonal wind profile up to the polar latitudes: 70° N and 70° S at 418 nm and 70° N at 986 nm. Binned and smoothed profiles are given in tabular form. We show that the zonal winds drop in their velocity poleward of latitudes 45° N and 50° S where an intense meridional wind shear develops at the two cloud levels. Our data confirm the magnitude of this shear, retrieved previously from radio occultation data, but disagrees with it in the latitudinal location of the sheared region. The new wind data can be used to recalibrate the zonal winds retrieved from the previous measurements of the temperature field and the cyclostrophic balance assumption. The meridional profiles of the zonal winds at the two cloud levels are used to assess the vertical wind shear in the upper cloud layer as a function of latitude and locate the most unstable region.     

Venus cloud properties: Infrared opacity and mass mixing ratio

By using the Mariner 5 temperature profile and a homogeneous cloud model, and assuming that CO₂ and cloud particles are the only opacity sources, the wavelength dependence of the Venus cloud opacity is infrared from the infrared spectrum of the planet between 450 and 1250 cm^?1. Justification for applying the homogeneous cloud model is found in the fact that numerous polarization and infrared data are mutually consistent within the framework of such a model; on the other hand, dense cloud models are not satisfactory.Volume extinction coefficients varying from 0.5 × 10^?5 to 1.5 × 10^?5 cm^?1, depending on the wavelength, are determined at the tropopause level of 6110 km. By using all available data, a cloud mass mixing ratio of approximately 5 × 10^?6 and a particle concentration of about 900 particles cm^?3 at this level are also inferred. The derived cloud opacity compares favorably with that expected for a haze of droplets of a 75% aqueous solution of sulfuric acid.     

Comment on absorbing regions in the atmosphere of Venus as measured by radio occultation

Two recent papers, one by A.J. Kliore, C. Elachi, I.R. Patel, and J.B. Cimeno, Icarus37, 51-2- 72, 1979, and one by B. Lipa and G.L. Tyler, Icarus39, 192–208, 1979, reach fundamentally different conclusions concerning microwave absorption in the atmosphere of Venus, even though they are based on the same Mariner 10 radio occultation data. The Lipa and Tyler results are in general agreement with earlier Mariner 5 measurements analyzed by G. Fjeldbo, A.J. Kliore, and V.R. Eshleman, Astron. J.76, 123–140, 1971. We find that in the Kliore et al. treatment: (1) the effects of measurements and analysis uncertainties in the derived values of absorption are underestimated; (2) an incorrect formula is used for computation of the refractive effects needed to determine the absorption; (3) detailed features of a derived profile of absorption would have been created in an optically thin region by known motions of the spacecraft antenna, if its axial direction were biased about 0.5° from the computed directions; and (4) this particular angular bias is consistent with other available information about an apparent residual difference between true and reconstructed antenna pointing directions. We conclude that: (1) there is no credible evidence for measurable microwave absorption in the atmosphere of Venus at heights greater than 55 km for any of the wavelengths that have been used in radio occultation experiments, even though Kliore et al. indicate that there are significant amounts up to at least 70 km for both Mariner 10 wavelengths (13 and 3.6 cm); (2) absorption in the region 35 to 50 km has been reasonably well determined from the two concordant Mariner 5 and 10 analyses, but only at one wavelength (13 cm); and (3) improved instrumentation and careful planning and analysis will be required for the radio occultation technique to realize its potential for the study of absorbing regions in the atmospheres of Venus and the major planets.     

SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere

Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft.A UV spectrometer (118–320 nm, resolution 1.5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO₂, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, γ and δ bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO₂, temperature, SO₂, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (∼60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO⁺, and CO₂⁺, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere.The SPICAV VIS-IR sensor (0.7–1.7 μm, resolution 0.5–1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H₂O, CO₂, and aerosols.The SOIR spectrometer is a new solar occultation IR spectrometer in the range λ=2.2–4.3 μm, with a spectral resolution λ/Δλ>15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H₂O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO₂ and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents.     

Chemical composition of venus clouds

From estimates of drying effect in the cloud layer, data of the Venera 14 X-ray fluorescent spectroscopy, and evaluation of photochemical production of sulfuric acid, it follows that sulfuric acid and/or products of its further conversion should constitute not only the Mode 2 particles but most of the Mode 3 particles as well. The eddy mixing coefficient equals 2 × 10⁴ cm² s^?1 in the cloud layer. The presence of ferric chloride in the cloud layer is indicated by the Venus u.v. absorption spectrum in the range of 3200–5000 Å, by the Venera 12 X-ray fluorescent spectrum, by the coincidence of the calculated FeCl₃ condensate density profile and that of the Mode 1 in the middle and lower cloud layer, as well as by the upward flux of FeCl₃ from the middle cloud layer which provides the necessary concentration of FeCl₃ in H₂SO₄ solution. FeCl₃ as the second absorber explains the localization of absorption in the upper cloud layer due to the FeCl₃ conversion to ferric sulfate near the boundary between the upper and middle cloud layers. Other possible absorbers such as sulfur, ammonium pyrosulfite, nitrosylsulfuric acid, etc. are discussed.     

Venusian middle-atmospheric dynamics in the presence of a strong planetary-scale 5.5-day wave

The middle atmospheric dynamics on Venus are investigated using a middle atmosphere general circulation model. The magnitude of the superrotation is sensitive to the amplitude of the planetary-scale waves. In particular, the critical level absorptions of the forced planetary-scale waves might contribute to the maintenance of the superrotation near the cloud base. In the case of strong 5.5-day wave forcing, the superrotation with zonal wind speed higher than 100 m s^?1 is maintained by the forced wave. Four-day and 5.5-day waves are found near the equatorial cloud top and base, respectively. The planetary-scale waves have a Y-shaped pattern maintained by the amplitude modulation in the presence of strong thermal tides.The polar hot dipole is unstable and its dynamical behavior is complex near the cloud top in this model. The dipole merges into a monopole or breaks up into a tripole when the divergent eddies with high zonal wavenumbers are predominant in the hot dipole region. A cold collar is partly enhanced by a cold phase of slowly propagating waves with zonal wavenumber 1. Although such a complex dipole behavior has not been observed yet, it is likely to occur under a dynamical condition similar to the present simulation. Thus, the dynamical approach using a general circulation model might be useful for analyzing Venus Express and ground-based observation data.     

The microwave absorption of SO2 in the Venus atmosphere

Sulfur dioxide has a strong and complex rotational spectrum in the microwave and far infrared regions. The microwave absorption due to SO₂ in a CO₂ mixture is calculated for conditions applicable to the Venus atmosphere. It is shown that at the concentrations detected by Pioneer-Venus in situ measurements, SO₂ may be expected to contribute significantly to the microwave opacity of the Venus atmosphere. In particular, SO₂ might provide the major source of opacity in the atmospheric region immediately below the main sulfuric acid cloud deck. The spectrum is largely nonresonant at the pressures where SO₂ is expected to occur, however.     

Thermal structure of the atmosphere of Venus from Pioneer Venus radio occultations

Eighty-seven measurements of the thermal structure in the atmosphere of Venus between the altitudes of about 40 and 85 km were derived from Pioneer Venus Orbiter radio occultation data taken during four occultation seasons from December 1978 to October 1981. These measurements cover latitudes from ?68 to 88° and solar zenith angles of 8 to 166°. The results indicate that the characteristics of the thermal structure in both the troposphere and stratosphere regions are dependent predominantly on the latitude and only weakly on solar illumination conditions. In particular, the circumpolar collar cloud region in the northern hemisphere (latitude 55 to 77°) displays the most dramatic changes in structure, including the appearance of a large inversion, having an average magnitude of about 18°K and a maximum of about 33°K. Also in this region, the tropopause altitude rises by about 4.8 km above its value at low latitudes, the tropopause temperature drops by about 60°K, and the pressure at the tropopause decreases by an average of about 240 mbar. These changes in the collar region are correlated with observations of increased turbulence and greater amplitude of thermal waves in the region, which is located where the persistent circulation pattern in the Venus atmosphere changes from zonally symmetric retrograde rotation to a hemispherical circumpolar vortex. It was shown that the large zonal winds associated with this circulation pattern are not likely to produce distortions in the atmosphere of a magnitude that could lead to temperature errors of the order of the mesosphere inversions observed in the collar region, but under certain circumstances zonal wind distortion could cause errors of 3–4°K.     

Aircraft observations of Venus' near-infrared reflection spectrum: Implications for cloud composition

We have obtained measurements of Venus' reflection spectrum in the 1.2 to 4.1-μm spectral region from a NASA-Ames operated Lear jet. This was accomplished by observing both Venus and the sun with a spectrometer that contained a circular, variable interference filter, whose effective spectral resolution was 2%. The aircraft results were compared with computer generated spectra of a number of cloud candidates. The only substance which gave an acceptable match to the profile of Venus' strong 3-μm absorption feature, was a water solution of sulfuric acid, that had a concentration of 75% or more H₂SO₄ by weight. However, our spectra also show a modest decline in reflectivity from 2.3 μm towards 1.2-μm wavekength, which is inconsistent with the flat spectrum of sulfuric acid in this spectral region. We hypothesize that this decline is due to impurities in the sulfuric acid droplets.We also compared our list of cloud candidates with several other observed properties of the Venus clouds. While this comparison does not provide as unique an answer as did our analysis of the 3-μm band, we find that, in agreement with the results of Young (1973) and Sill (1973), concentrated sulfuric acid solutions are compatible with these additional observed properties of the Venus clouds. We conclude that the visible cloud layer of Venus is composed of sulfuric acid solution droplets, whose concentration is 75% H₂SO₄, or greater, by weight.     

High-resolution photography of the solar chromosphere

High-resolution filtergrams of an active region loop taken at seven wavelengths in Hα have been used to derive the contrast at eleven locations along its length as a function of wavelength. With an appropriate choice of parameters, theoretical curves calculated on the basis of the ‘cloud” model give a reasonable fit to the observed contrast profiles. The inferred line-of-sight components of the mass velocity range from 27 km s^?1 upward to 78 km s^?1 downward. However, more accurate profiles and a more rigorous theory are needed to confirm the validity of this application of the cloud model.     

Relationships between the equatorial electrojet and polar magnetic variations

On moderately disturbed days when substorms occur frequently, the quiet day daily variation in the polar region (S_q^p) is enhanced. On such days, however, the quiet day variation along the dip equator appears to be suppressed, as well as being superposed with ‘fluctuations’.It is suggested that the enhancement of S_q^p is related to a partial suppression of the equatorial electrojet. The asymmetric ring current also causes an apparent suppression of the electrojet.On the other hand, the substorm-associated electric field which drives the eastward current in the auroral and subauroral zone (causing positive bays) in the afternoon sector appears to enhance the equatorial electrojet.Thus, magnetic variations along the dip equator are influenced by a number of processes in the magnetosphere.     

Microwave Remote Sensing of the Temperature and Distribution of Sulfur Compounds in the Lower Atmosphere of Venus

A multi-wavelength radio frequency observation of Venus was performed on April 5, 1996, with the Very Large Array to investigate potential variations in the vertical and horizontal distribution of temperature and the sulfur compounds sulfur dioxide (SO₂) and sulfuric acid vapor (H₂SO₄(g)) in the atmosphere of the planet. Brightness temperature maps were produced which feature significantly darkened polar regions compared to the brighter low-latitude regions at both observed frequencies. This is the first time such polar features have been seen unambiguously in radio wavelength observations of Venus. The limb-darkening displayed in the maps helps to constrain the vertical profile of H₂SO₄(g), temperature, and to some degree SO₂. The maps were interpreted by applying a retrieval algorithm to produce vertical profiles of temperature and abundance of H₂SO₄(g) given an assumed sub-cloud abundance of SO₂. The results indicate a substantially higher abundance of H₂SO₄(g) at high latitudes (above 45°) than in the low-latitude regions. The retrieved temperature profiles are up to 25 K warmer than the profile obtained by the Pioneer Venus sounder probe at altitudes below 40 km (depending on location and assumed SO₂ abundance). For 150 ppm of SO₂, it is more consistent with the temperature profile obtained by Mariner 5, extrapolated to the surface via a dry adiabat. The profiles obtained for H₂SO₄(g) at high latitudes are consistent with those derived from the Magellan radio occultation experiments, peaking at around 8 ppm at an altitude of 46 km and decaying rapidly away from that altitude. At low latitudes, no significant H₂SO₄(g) is observed, regardless of the assumed SO₂ content. This is well below that measured by Mariner 10 (Lipa and Tyler 1979, Icarus39, 192-208), which peaked at ∼14 ppm near 47 km. Our results favor ≤100 ppm of SO₂ at low latitudes and ≤50 ppm in polar regions. The low-latitude value is statistically consistent with the results of Bézard et al. (1983, Geophs. Res. Lett.20, 1587-1590), who found that a sub-cloud SO₂ abundance of 130±40 ppm best matched their observations in the near-IR. The retrieved temperature profile and higher abundance of H₂SO₄(g) in polar regions are consistent with a strong equatorial-to-polar, cloud-level flow due to a Hadley cell in the atmosphere of Venus.     

Turbulence in planetary occultations: II. Effects on atmospheric profiles derived from Doppler measurements

Turbulence in planetary atmospheres leads to both fluctuating and systematic errors in atmospheric profiles derived from Doppler measurements during radio occultation. If the upper atmospheres of Venus and Jupiter are about as turbulent as the earth's troposphere, we deduce rms fractional errors in temperature and pressure of less than ～ 10^?2 for the Mariner 10 and Pioneer 10/11 occultation experiments. Fractional systematic errors are typically of the order of 10^?6. These estimates depende rather weakly on quantities characterizing the atmosphere and the occultation, and it is conjectured that turbulence-induced errors in atmospheric profiles derived from Doppler measurements are always very small in the weak scattering limit     

Evidence for high-altitude haze thickening on the dark side of Venus from 10-micron heterodyne spectroscopy of CO2

The 10.86-μm P(44) and 10.33-μm R(8) lines of ¹²C¹⁶O₂ were observed on Venus with an infrared heterodyne spectrometer. The spectral resolution equals the Doppler half-width and the line profiles are fully resolved. The P(44) line was observed in June 1979 on the day side of the planet. The P(44) line core appears in absorption; the nonthermal core emission, which is present at low J values, is negligible at J = 44. Modeling of the line profile indicates that a discrete, optically thick, cloud deck occurs at 45 mbar pressure, in essential agreement with current understanding of the Venusian cloud structure. The 10.33-μm R(8) line was observed in April 1980 at a variety of positions on the day side, and at a single position on the night side. The strong nonthermal core emission which appears on the day side for this line is not present on the night side, where the line core appears in absorption. This behavior is consistent with a solar radiative pump as an excitation mechanism for the nonthermal emission. Modeling of the R(8) night-side profile indicates that substantial high-altitude haze occurs above the cloud tops, in the region from 15 to 35 mbar pressure. Comparing the modeling for the R(8) line to the P(44) line we find that the variation in the mass of the high-altitude haze was greater than a factor of 2.     

Sulfuric acid vapor and other cloud-related gases in the Venus atmosphere: Abundances inferred from observed radio opacity

Laboratory measurements of the microwave opacity of gaseous sulfuric acid under Venus atmospheric conditions indicate that it is an exceptionally strong absorber. They also suggest that its absorptivity has a surprisingly weak dependence on radio frequency, as compared with other common gaseous absorbers. Initial theoretical studies also indicate a large absorptivity and weak frequency dependence, although the measured opacity is several times the computed value, presumably due to deviations from Van Vleck-Weisskopf theory for pressures near and above about 1 atm. The absorbing characteristics of sulfuric acid vapor appear to reconcile what had been thought to be an inconsistency among measurements and deductions concerning the constituents of the atmosphere of Venus, and radio occultation, radar reflection, and radio emission measurements of its opacity. These and previous laboratory measurements of sulfur dioxide, water vapor, and carbon dioxide are used to model relative contributions to opacity as a function of height, in a way that is consistent with observations of the constituents and absorbing properties of the atmosphere. We conclude that sulfuric acid vapor is likely to be the principal microwave absorber in the 30- to 50-km-altitude range of the middle atmosphere of Venus. It would need to have a mixing ratio there of about 35 to 90 ppm if it were the sole absorber. Carbon dioxide, the predominant atmospheric gas, is the main absorber below about 30 km, while sulfur dioxide is an important but secondary absorber in both regions. Water vapor and cloud particulates appear to be only minor contributors to the total opacity. While gaseous sulfuric acid has not been directly measured in any of the in situ probe experiments (due to particular instrumental limitations), its presence at an abundance of the deduced order of magnitude is implied by these and other observations. We suggest that improved radio occultation measurements, in conjuction with high-resolution microwave emission observations and more detailed laboratory studies, could provide important data for investigating the sulfur compound chemistry in the atmosphere of Venus, and that the techniques and results may have application to the study of atmospheric conditions associated with acid rain on Earth.     

An improved Venus cloud model

A simple radiative-transfer theory that allows for the change in the absorptions of sulfur and carbon dioxide with depth in the atmosphere of Venus can account simultaneously for (1) the spectral reflectance of Venus; (2) the wavelength dependence of contrast in uv cloud features; (3) the CO₂ line profile; (4) the change in slope of the curve of growth from the 7820- to the 10488-Å CO₂ bands; and (5) the rotational temperature near 246°K found for all CO₂ bands. The model cloud consists of 1-μm sulfuric-acid particles, which are well mixed between about 64 km and the 49-km cloud base found by Veneras 9 and 10, plus an overlapping cloud of much larger sulfur particles that extends down to the 35-km cloud base found by Venera 8. The mixing ratios (by number of molecules) below about 64 km are: H₂O, 2 × 10^?4; H₂SO₄, 10^?5; and sulfur, 10^?4. Although the cloud contains an order of magnitude more sulfur than sulfuric acid, the sulfur particles are an order of magnitude larger, and so have only about 1% of the number density of the acid droplets. The “black-white” radiative-transfer model assumes perfectly conservative scattering above the level (which depends on wavelength) where an absorber becomes “black” due to the local temperature and pressure. So-called homogeneous scattering models are inherently self-contradictory, and are inapplicable to planetary atmospheres; the vertical inhomogeneity is an essential feature that must be modeled correctly. The pressure of CO₂ line formation is about half the pressure in the region where uv markings occur.