Before the 1960s, scientists thought that the climate of Venus might be similar to Earth. It has clouds, and here Earth, clouds mean rain, water, oceans and even life. But microwave observations of Venus showed that its surface must be incredibly hot, too hot for liquid water to exist. And spacecraft visiting the planet in the 1960s and 70s confirmed that the clouds of Venus are made up almost entirely of carbon dioxide; a potent greenhouse gas keeping the planet so hot.

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