The Pioneer Venus spacecraft took this image of Venus in 1979 using a filter that only allowed ultraviolet light to reach the camera. The visible surface of Venus (which is almost Earth’s twin in size) is actually the top of a cloud layer that completely covers the planet. The clouds are composed of droplets of sulfuric acid in a carbon dioxide atmosphere with a surface pressure over 90 times greater than the pressure at sea level on Earth. The dense atmosphere produces an extreme greenhouse effect by trapping heat radiated by the planets, causing the surface temperature to be near 900°F. The clouds on Venus rotate in a retrograde (opposite to Earth’s rotation) direction once in four days. Earth-based radar measurements show that the solid surface of Venus also rotates in a retrograde direction, requiring 243 days for one rotation.