The atmosphere

Many of the probes sent to Venus have provided information about its atmosphere. The clouds of Venus conceal a hostile atmosphere that reaches a height of about 250 km. Most of the atmosphere is concentrated within 30 km of the surface.

The first probe to be placed directly into the atmosphere and to return data was Venera 4 in 1967. It showed that the atmosphere was 90-95 per cent carbon dioxide. Mariner 5 arrived at Venus one day after Venera 4, and passed within 3900 km of the planet's surface - it also found an atmosphere dominted by carbon dioxide. Veneras 5 and 6 reported an atmosphere of 93-97 per cent carbon dioxide, 2-5 per cent nitrogen, and less than 4 per cent oxygen. These two probes returned data to within 26 km and 11 km of the surface respectively before being crushed by the high atmospheric pressure.

From 1978 to 1988, the amount of sulfur dioxide in the atmosphere decreased by 10 per cent. The reason for this decrease may have been a decrease in volcanic activity during this period.

In 1978, the Pioneer Venus 2 probe detected a fine haze in the atmosphere at a height of 70-90 km. Between 10-50 km there was some atmospheric convection, and below 30 km the atmosphere was clear.

Unlike the clouds on Earth, which appear white from above, the cloud tops on Venus appear yellowish or yellow-orange. These colours are thought to be caused by sulfur and sulfur compounds in the atmosphere. Evidence suggests these compounds have originated from volcanic activity.

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