From its size (equatorial ray of 6052 km) and its mass (0,815 times mass of the Earth), like its orbit (0,72 astronomical unit), Venus is the planet sister of the Earth. However, certain characteristics make these two planets of the very different celestial bodies. The year on Venus lasts 224,7 J, that is to say approximately 7 terrestrial months and half. Far from tilted on the plan of its orbit (2.6°), this one being of more than low eccentricity, Venus do not present significant seasonal phenomena. Single case in the solar system, the alternation of the days and the nights is done on Venus according to a 243 days cycle terrestrial, that is to say approximately 8 months, in the retrograde direction (sun rising to the west). Very massive atmosphere, induced a pressure on the ground nearly one hundred times higher than that Earth's atmosphere. Essentially made up of carbon dioxide (CO2), it is characterized moreover by a thick layer of clouds, made up of fine droplets of sulphuric acid, divided into layers stable and laminated between 45 km and 70 km of altitude.

The atmosphere

The Venus planet has the most massive atmosphere of telluric planets of the solar system. The pressure on the ground reaches 90 to 95 bars, that is to say nearly 100 times the terrestrial atmospheric pressure, equivalent to the pressure which an underwater machine with a depth of almost 1000 meters undergoes. From the point of view of the chemical components, the carbon dioxide (CO2, 96,5 %) and nitrogenizes it (N2, 3,5%) represent alone more than 99,9% of the composition. It should be noted that taking into account the quantity of gas nearly one hundred times higher than that of the Earth's atmosphere, there is in absolute value an appreciably equal quantity of nitrogen molecules in the atmosphere of Venus (3,5 % in proportion) that on Earth (78,0 % in proportion). To the middle of the XVIIIe century, at the time of the observation of the Venus transit of 1761, the astronomer Mr. V Lomonossov reported the presence of a halation which it allotted to the existence of an atmosphere around Venus. Since the XIXe century, dark spots or marks, generally in the equatorial part of the crescent, when the phase and the angular dimension of planet allow it, were drawn then photographed. It is into 1932 that CO2 was identified for the first time by the observation of the absorption bands of the infra-red close relation, around 0,8 mm, in the reflected solar spectrum, by Adams and Dunham.

Detailed composition. - other components that CO2 (CO, HCl, HF) were highlighted with the development of the instrumental techniques after the second world war, also by infra-red spectroscopy, but it is only in 1967, with the module of descent of the Soviet probe Venera-4, that the concentrations of the components could for the first time measured in-situ. The analysis by gas chromatography, starting from the automatic mission Venera-11 (1978), revealed new minor molecular components such as H2, O2, Kr, H2O, H2S and COS. The presence of sulphur dioxide (SO2) is established in 1979 by the observation with average spectral resolution in the ultraviolet close relation since the Earth. Although in small quantities, this very reactive gas is an essential element of the chemistry of the atmosphere of Venus. Steam was detected in extremely small quantities (approximately 30 parts per million or ppm, 1 ppm = 0,001 %), which makes of Venus the driest planet of the solar system. The reports/ratios of mixture of CO, H2O and SO2 vary in an important way with altitude and represent the reactions of chemical balance between the various components.

Table 1. – Compared physical Characteristics and principal components of the atmosphere for Venus, Earth and Mars (0,0001 % = 1 ppm).

Fig. 1 – General Aspect of the Venus planet illuminated by the sun. Only the higher part of the thick vapour cloud, made up of fine droplets of sulphuric acid, is observed at an altitude of approximately 70 km. This one is moving constant of in west, is involved with the whole of the atmosphere by a mechanism of super-rotation in 4,2 jours.La photography above is taken since orbiting it of the probe Pioneer-Venus on February 26, 1979, at a distance of approximately 65 000 km (NASA/NSSDC).

Albedo. The pale yellow Venus color, visible with the naked eye like on photography of figure 1, results from a relative absence of sunlight considered in the blue-violet part of the spectrum. Spectroscopic measurements of the Venus albedo, i.e. reflected sunlight, reveal an absorption by the atmosphere of Venus in all the area extending from 200 to approximately 350 Nm (1 Nm = 0,001 micron = 10-9 m). From 200 to 320 Nm, the problem was solved in 1979 by the discovery and the identification of the SO2, a very active gas chemically in the atmosphere and on the Venus surface. In the part 320-350 Nm, uncertainty still remain on the cause of this absorption of light radiation, because none the chemical components detected and identified to date absorbs the radiation in this spectral field to a significant degree, or do not have the concentration suffusante to do it. They could be products resulting from polymerization from sulphur in acid medium, dissolved in the droplets of H2SO4, or even of solid particles in suspension in the layers of clouds.

Table 2. – Compared Characteristics of temperature of the atmosphere of Venus, Earth and Mars.

Temperature. - The very high Venus temperature (740 K, are approximately 460 °C) was highlighted in the years 1950 by measurements in centimetric waves since the terrestrial ground, with a wavelength of 3,15 cm. This exceptional temperature does not result directly from the proximity of the sun; on the contrary, because of thick vapour cloud which reflects approximately 65 % of the incidental light, flow Net of solar energy on the level of the ground, which one names the solar constant, is lower than that received by the Earth (see Table 3). This temperature is the consequence of an effect of greenhouse resulting not from the component but carbon dioxide(CO 2) in very small relative quantities such as SO2 and H2O Indeed, in the infra-red range corresponding to the maximum of emission thermal for a body at the temperature of the surface and lower atmosphere of Venus, CO2 present of the very broad windows of transmission which cannot trap the infra-red radiation effectively. On the other hand, SO2 and H2O, although in very small quantities, absorb radiations in this field wavelengths, just like also do it the fine particles of sulphuric acid which constitute the clouds. The addition of temperature due to the effect of greenhouse is of 35 Kelvins for the Earth, it reaches more than 500 Kelvins on Venus.

Fig. 2. – Variation in the temperature of the atmosphere of Venus according to altitude, obtained in-situ at the time of the descent in the atmosphere of four automatic probes during the mission Pioneer-Venus in 1979 (milked continuous). On right-hand side, the relative average density of particles of the fogs and clouds according to altitude is represented, revealing several distinct layers. The clouds, located at an altitude included/understood into 45 and 70 km, consist of fine droplets of sulphuric acid in aqueous solution, made up to 75% of sulphuric acid (H2SO4) and to 25 % of water (H2O). Their diameter ranging between some dizièmes of mm and ten mm (1 mm = 10-3 mm). The lower atmosphere of Venus does not receive sunlight with the wavelengths lower than 400 Nm. In visible light, it is hardly 5% of the sunlight which reaches surface.

Constitution and formation of the clouds. - The clouds, located at an altitude included/understood into 45 and 70 km, consist of fine droplets of sulphuric acid in aqueous solution, constituted to 75% of sulphuric acid (H2SO4) and to 25 % of water (H2O). Their diameter ranging between some dizièmes of mm and ten mm (1 mm = 10-3 mm), the majority of these particles has a diameter of 0.2 mm or approximately 1 mm. An experiment on board large probe of descent of Pioneer Venus showed that five areas of clouds or fogs consist of particles of composition and optical and physical properties different. Higher fog (upper haze area, 70 km? Z? 90 km) have an average optical thickness from 0,05 to 1,0. At the top of the roadbase of the clouds (upper cloud area, 56.5? Z? 70 km) appear the droplets of H2SO4. The transition course (middle cloud area, 50.5? Z? 56.5 km) and the sub-base (lower cloud area, 47.5? Z? 50.5 km) are characterized by the presence of larger particles being able to reach several microns in diameter. All these particles in liquid phase are formed at very high altitude, on the level of summon roadbase of the clouds, where the ultraviolet radiation of the sun acts by photolysis on the atmospheric components. In particular, gas SO2 forms SO3 while reacting with O, product of the photolysis of CO2, then finally H2SO4 starting from H2O, which passes in the liquid state because of the pressure partial of the sulphur gas species in surrounding gas. Conversely, in the lower atmosphere, one attends the decomposition of the fine droplets of sulphuric acid H2SO4: migrating through the laminated structure of the clouds at the low speed from approximately 1 mm s-1, they are vaporized when they reach the hotter layers of the atmosphere at the base of the sub-base of the clouds, around 40 km of altitude.

Venusian airships. - Among the most daring missions of exploration of the atmosphere of Venus the installation appears of Venusian airships deployed at the time of the Véga mission June the 11, and 15 1985. The balloons, of a diameter of 3,4 meters were inserted into four days of interval during the phase of descent towards the ground of the modules of landing. Each one supported 25 kg of nacelle. 5 kg of instrumentation floated with 12 meters below the balloon at an altitude ranging between 50 and 55 km, in the intermediate area of the clouds towards 0.6 bar, throughout one 47 hour (the lifespan of the batteries was 60 hours). The displacement of the balloons was followed by interferometry to very broad base VLBI. After 48 hours the balloons passed on the side day then disappeared because of the solar radiative heating (dilation then rupture of the envelope). They have on the whole traversed 100 degrees of longitude on 360. One derived in the neighbourhoods from 7° N and the other towards 7° S. The instruments on board measured the temperature, the pressure, the vertical speed of the wind, atmospheric opacity (density and average size of the aerosols), the level of illumination as well as the detection of flashes. During their vertical excursion from 2 to 3 km of amplitude, they could measure variations in temperature close to the adiabatic gradient. The maximum ones of measured temperature differed from 6,5 K between the two balloons.

Atmospheric circulation and winds. The general rotation of the atmosphere as a whole, called super-rrotation, is carried out of west in is, i.e. in the same retrograde direction as the rotation of planet itself, according to one 4,2 days period, but at a speed more than 50 times higher. The movement of super-rotation starts around 10 km of altitude, develops regularly up to 65 km, where it reaches a speed at the equator of about 540 km/h, to decrease and cancel themselves around 95 km. The mechanism which produces and maintains super-rotation is not entirely included/understood but it would imply for a significant share the effect of tide thermal exerted by the sun on a part of the atmosphere exposed to the solar radiation. With this general atmospheric circulation, parallel at the equator of planet, other components are added at low speed (D ...) who could show characteristics close to the cells of Hadley, along the meridian lines, with upswing close of the equator produced by the heating due to the solar radiation.