The chemical composition sensors on the Venera-4, -5, and -6 spacecraft are described. The mixing ratio by volume of carbon dioxide is determined to be 97 (+3, −4)%; nitrogen, less than 2%, oxygen, less than about 0.1%; and water vapor at the 25 °C temperature level, on the order of 10 mg/l, an amount large enough to imply that the clouds of Venus are composed of condensed water.

The Lyman alpha measurements made on the sunlit side of Venus by Mariner 5 showed that the distribution of the radiating hydrogen atoms was governed by two scale heights. A revised model is presented in which the inner or cold distribution is attributed to hydrogen atoms in thermal equilibrium with the thermosphere and the outer or hot distribution is due to hydrogen atoms produced by the photodissociation of molecular hydrogen. The consequence of this interpretation is that the temperature of the dayside thermosphere of Venus is 325 K.

Theoretical modeling of the daytime Venus ionosphere can be used to augment the measurements of Mariner V made during the 1967 fly-by mission of Venus. The models discussed here are obtained by solving the equations of heat conduction for the electron, ion, and neutral gases along with the momentum and chemical equations for the charged particle densities [1, 2, 3]. When the model is brought into conformity with as much of the data as is possible, constraints can be placed on some of the unknown parameters such as the electron and ion temperatures, and the strength of the magnetic field in the topside Venus ionosphere.

The microwave spectrum of Venus is compared with the structure and composition of the Venus atmosphere as determined by Veneras 4, 5, and 6 and Mariner V. The results are consistent with a radar radius of 6049.5 ± 3 km, surface pressures of 95 ± 20 atm, and a surface temperature of 770 ± 25 K, as well as a water vapor volume mixing ratio of 0.65 ± 0.35%.

The total vertical absorption of mm and cm radiowaves in the Venus atmosphere is calculated from direct measurements of temperature, pressure and chemical composition, provided by the automatic interplanetary stations Venera-4, Venera-5 and Venera-6. The results are in good agreement with radar measurements of Venus and with extrapolations of direct measurements of temperature and pressure. The data fit a model with surface pressure 90 atm and a water vapor content of a few milligrams per liter. Vertical radiowave attenuations at various wavelengths are computed.

Mariner V and the three successful Venera probes have shown the density of the atmosphere of Venus to be so high that critical refraction occurs at an altitude near 35 km. Somewhat bizarre optical effects are to be expected in such an atmosphere. Using a spherically symmetric model based upon the measurements of the refractivity at the top of the atmosphere by the Mariner V S-band occultation experiment, the Venera 4 data in its altitude range, and adiabatic extrapolation to the surface, calculations of a variety of ray paths have been made with a double-precision computer program. Phenomena discussed include the magnification of the planet, the elevation of the horizon, the apparent motion and distortion of the sun, and the possibility of ‘seeing’ completely around the planet.

An approximate, analytic theory has been developed for the formation of spectral absorption lines in a hazy atmosphere that scatters isotropically and has a homogeneous mixture of scattering and absorbing matter. The behavior – i.e., the curve of growth and the dependence of equivalent width on incident and emergent angles – has been examined for various possible situations, with emphasis on the physical reasons for a particular behavior. It is emphasized that two ratios – essentially the line and continuum absorption coefficients relative to the scattering coefficient – are important in any quantitative theory of the curve of growth and phase variation of the absorption spectrum.

In attempting to resolve the CO2 band at 7159 Å identified in the spectrum of Venus by Spinrad (1962), we discovered a new band at 7105 Å. Both bands are fully resolved on our spectra and tentative J numbers have been assigned to the rotational lines. In both cases, the P branch appears to be anomalously weak. The 7159 Å band has been observed in the laboratory (unresolved) by Herzberg and Herzberg (1953) at path lengths on the order of 55 km atm. To our knowledge, the 7105 Å band has not been recorded previously. Following the predictions of Herman (1948) the two bands are given the assignments 5v3 + 2v2 + v1 (7159 Å) and 5v3 + 2v1 (7105 Å). An unsuccessful attempt was made to find the third member of this triad, 5v3 + 4v2. In view of the very large amount of carbon dioxide apparently required to produce these absorptions, further study in the laboratory and at the telescope is encouraged.

The Ã1B2 state of CO2, identified by Dixon (1963) as the upper level of the ‘carbon monoxide flame bands’, must be of importance in the upper atmospheres of Venus and Mars. New calculations of the high vibrational levels of the ground state, which lead to improved fits of the observed vibration-rotation bands, confirm Dixon's analysis, except that the numbering must be lowered by two, and fix the energy of the v = 0, K = 0 level of 1B2 at 45210 ± 10 cm−1 = 5.605 eV.

The thermal emission spectra of Venus, Mars, Jupiter, and the moon were observed at the coude focus of the McDonald Observatory 107-inch telescope in the 400–1400 cm−1 spectral range with spectral resolutions of 0.3–0.7 cm−1. A preliminary interpretation of the Venus/lunar ratio spectrum allows identification of four upper state CO2 bands in the Venusian atmosphere at 791, 828, 865, and 961 cm−1 and confirms previous observations of the broad absorption-like depression around 890 cm−1. The rotational structure of the 791 and 961 cm−1 bands is well developed at this spectral resolution.

Limb darkening measurements of the thermal radiation from Venus at 5, 9, 11, 13 and 18 μ are presented. This data, produced by deconvolving the observations with a measured instrumental response function, indicates a complex atmospheric structure which will require careful atmospheric modeling to fully interpret.

It is proposed that Venus may have polar seas which are acidic and thus cannot precipitate calcium carbonate. This leaves the carbon dioxide in the atmosphere. The argument is that the great equatorial land masses always have been too hot for liquid water and thus could not be weathered to give the sea salts necessary to form the precipitate. The action of steam on rocks is to liberate acids which are volatile and would dissolve in the polar seas. The volcanic vapors issuing in the early times consisting mainly of water and carbon dioxide would have begun polar seas at once since the expected equatorial (black body) surface temperature of the bare planet is too high (464 K) due to proximity of the sun. The accumulation of carbon dioxide in the atmosphere would have ensured the continued increase of the temperature due to the greenhouse effect. On earth, on the contrary, condensation over most of the planetary surface probably was possible from the beginning. Liquid water, ice-weathering, and river transport of salts to the seas all probably occurred from the beginning.

As the pressure at the surface probably approximates 100 atm (Venera 5 and 6) we can expect the polar seas to be below the boiling point although possibly hot. An isothermal layer of some thickness is naturally established over liquid water heated by infrared from above. Evaporation and condensation to form rain constitutes an efficient heat transport mechanism. Such a layer naturally would move toward lower latitude carrying moisture which then will rise and eventually move poleward in the high atmosphere causing rain and possibly the planet wide cloud cover. The atmosphere containing volatiles such as hydrochloric and hydrofluoric and sulfurous and sulfuric acids as well as carbon dioxide will form clouds which might be expected to consist of concentrated acid solutions. The main rain over the poles probably falls from altitudes well below the cloud top seen from earth. It is possible that the Venus clouds seen from earth are non aqueous just as our stratosphere carries dust clouds apparently of ammonium sulfate. At the moment it is very difficult to decide between these alternatives.

In the more polar regions the seas might conceivably be as cool as 50 °C.

Experiments are described with algae grown in a new environment of pure CO2 under pressure and in an acidic nutrient medium at elevated temperatures. One species found in hot-springs was observed to grow. If the planet Venus has acidic polar seas as we suggest, they may harbor photosynthetic life.

Contrast of the Venus cloud patterns is measured by photoelectric scans using a sequence of narrow-band interference filters. The contrast appears constant in the range 3100–3600 Å, drops to the limit of detectability at about 4100 Å, and is absent at longer wavelengths.

A comparison of a wide range of Venus observations with the ultraviolet, visible, and infrared reflection spectra and thermodynamic properties of a variety of candidate materials indicates that the principal constituent of the Venus clouds is partially hydrated FeCl2.

Spectroscopic evidence is strong that the clouds of Venus are composed of ferrous chloride hydrate. At the surface of the planet it appears as though the vapor pressure of ferrous chloride may be quite low (10−6 atm), especially if such vapor is allowed to come to equilibrium with the other constituents of the Venus atmosphere. The formation of magnetite (Fe3O4) may be favored, and the spectrum of magnetite is shown in relation to the spectrum of the Venus cloud cover. The presence of ferrous chloride in the upper atmosphere may be due to a non-equilibrium condition.

The spectrum of volatiles, such as the halides of mercury, are considered vs the albedo of Venus. The presence of these volatiles seems incompatible with the spectroscopic evidence. It is suggested that the planet has been depleted of its volatiles, as it has been depleted of its water.

Venus is the closest planet. Its surface has never been seen at optical frequencies; nevertheless we now know with at least fair reliability, and in some cases with remarkable accuracy, its surface temperature and pressure, its atmospheric structure, its period of rotation, the obliquity of its rotation axis, the mean surface dielectric constant, its ionospheric structure, and even a little about its surface topography. And yet the clouds of Venus, visible to the naked eye and known to be clouds since the time of Lomonsov, continue to elude our efforts to understand them comprehensively. Not only do we disagree on the chemical composition of the clouds, but it is not even settled whether they are condensation clouds or non-condensable aerosols. And yet there is a very wide variety of relevant data on the clouds. Indeed, the ratio of potentially diagnostic data points to mutually exclusive hypotheses is of the order unity.

The contrast between light and dark areas on Mars has been the subject of two photoelectric studies by Younkin (1966) and McCord (1969), both of which were somewhat limited in scope. Younkin's data are based on one night's observations. McCord had a more suitable instrument for making a comparison between two areas on Mars, but still used only six nights. The agreement between these two studies is excellent. However, there is ample evidence from photographs that there are short-term changes in the contrast and brightness of Martian surface features, especially in the case of blue clearing. Visual observations are unsuitable for investigating these changes, due to the limited wavelength range of the eye's sensitivity, as well as the poor accuracy of the eye as a photometric instrument. Even photography is of limited usefulness when high accuracy is desired. Consequently, a program of continuous photoelectric observation of Mars was set up for the 1969 opposition. The technique of area scanning was chosen as the best method of recording the contrast of Martian surface features.