Although similar to the Earth in size and mass, Venus ' slightly closer orbit to the Sun create for it a much thicker atmosphere and a much hotter surface. The thick atmosphere was photographed above in ultraviolet light in 1979 by the Pioneer Venus Orbiter . Whether or not Venus has a moon was the center of a great controversy in the 1700s and 1800s. Today we know Venus has no natural satellites. Venus 's extremely uncomfortable climate was likely caused by a runaway greenhouse effect .
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