Venus

While Venus is often referred to as Earth's twin on the basis that the two planets have similar physical characteristics (radius, mass, density, composition, etc), it is far from being Earth's twin when atmospheric characteristics are compared.

The many spacecraft that have flown past, or landed on, the Venusian surface have found that the uppermost cloud tops, which obscure Earth-based observers' view of the planet, are about 40 mi (65 km) above the surface. For comparison, on Earth, the highest clouds are about 10 mi (16 km high.) Observations taken of ultraviolet wavelengths reveal that the upper Venusian clouds follow a jet stream-like pattern and circle the planet once every four days, or so. The circulation velocity of the upper cloud deck is much greater than the rotation rate of the planet, and it is believed that this is the result of extensive atmospheric convection driven by solar heating.

The upper cloud deck is about 3 mi (5 km) thick. At about 31 mi (50 km) altitude there is a second much more dense cloud deck. Below about 18 mi (30 km) in altitude the Venusian atmosphere is clear of clouds. The upper cloud deck has been found to contain substantial amounts of sulfur, which give the clouds their dark yellow to yellow-orange color. The lower cloud deck has been found to contain large concentrations of sulfur dioxide, hydrogen sulfide compounds, and droplets of sulfuric acid. It has been suggested that the presence of atmospheric sulfides is indicative of very recent volcanic activity on the planet's surface.