The mystery below the clouds
Venus is the brightest object in the sky, after the Sun and the Moon. Like Mercury, Venus can be generally seen in the morning or in the evening sky, in the Northern Hemisphere.
The diameter of Venus is about 12,000 km, similar to that of the Earth. Venus is covered by clouds, such that its surface is nowhere visible, indeed only featureless yellowish cloud tops can be seen. The rotation period remained for long unknown. In 1962, radar measurements revealed that the rotation period is 243 days in a retrograde direction, which means that it is opposite to the rotation of other planets and the axis of rotation is almost perpendicular to the orbital plane.
The temperature at the cloud tops is about 250 K (-23 degrees Celsius). The surface temperature as measured at the end of the 1950’s turned out to be 750 K, well above the melting point of lead. The reason for this is the greenhouse effect. The outgoing infrared radiation is blocked by atmospheric carbon dioxide, the main component of the atmosphere. The chemical composition of the Venusian atmosphere was known prior to the Space Age. Spectroscopic observations revealed CO2, and some clues to the cloud composition were obtained from polarimetric observations. Clouds are mainly formed by sulphuric acid H2SO4 as confirmed also by spacecraft observations.
Venus’ atmosphere is very dry: the amount of water vapor is only 1 millionth with respect to the Earth’s atmosphere and a possible explanation is that, due to solar UV radiation, the water has dissociated to hydrogen and oxygen in the upper layers of the atmosphere, and the former escaped into interplanetary space.
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