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Venus – Earth’s Evil (?) Twin Highlights The second planet from the Sun, Venus is Earth's “sister" planet, comparable in size and mass. Unlike Earth, Venus has unusual rotation properties: One Venusian rotation period takes 243 Earth days. Venus rotates in a retrograde sense, i.e., backwards, compared to the other planets. A day on Venus (noon to noon) lasts 117 Earth days. Venus possesses a dense atmosphere and is completely enshrouded in clouds. The surface is NOT visible from above, except using cloudpenetrating radar. Nothing was known about the Venusian surface prior to the space age. The atmosphere is about 90 times more massive than Earth's and contains 96% carbon dioxide. Venusian clouds are composed of sulfuric acid, and lie at a altitude between 50 and 60 kilometers above the surface A runaway greenhouse effect has occurred on Venus. Carbon dioxide in the atmosphere traps infrared radiation, powering this process. Long ago the water present early in Venusian history boiled away. Due to the greenhouse effect in its dense atmosphere, Venus' mean surface temperature is about 730 K (900 F). The dense atmosphere insures that the entire surface has about the same temperature The surface has been mapped using radar, both from Earth and using spacecraft placed into orbit around Venus. Although many volcanic features are found on the surface, there is no evidence for wide-scale plate tectonics. Venus is a hot, dry, and rocky world. Due to its slow rotation, Venus does not possess a planetary magnetic field. Although no “smoking volcano” has yet been pinpointed, evidence suggests that volcanic activity continues at present, and is the dominant process shaping the Venusian surface today.