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# 6.7 Venus

Venus is often known as Earth's sister planet, because it’s the planet closest to us in space and the most similar in size. Its diameter is 95% of Earth's, and its mass is 82% of Earth's. Both planets have atmospheres and whitish clouds. But at that point, the resemblance ends.

Venus as seen by the Hubble Space Telescope (false color image). Click here for original source URL.

If you view Venus through a backyard telescope, you can see the disk of Venus go through phases, but you won’t see any surface details. The entire surface is hidden by nearly featureless clouds. For many years, astronomers assumed that these were clouds of water vapor, like the Earth's clouds. Science fiction writers pictured the Venusian surface as a swamp with perpetual rain, or as a rainforest with giant tropical plants and dinosaurs. In 1932, data replaced speculation. Astronomers from the Mt. Wilson observatory in California were able to identify features in the spectrum of Venus’ atmosphere. Surprisingly, the gas was not water vapor, oxygen, or any of the other gases that are important in the Earth's atmosphere, but carbon dioxide. Venus' atmosphere is about 96% CO2 by volume, with nitrogen the largest trace constituent.

In the 1960s, scientists learned another surprising fact about Venus when they first detected its thermal infrared radiation. They applied Wein’s law to calculate the temperature of Venus. The temperature was not slightly warmer than Earth, as you might expect for a cloudy planet 72% as far from the Sun as Earth, but a hellish 750 K (891 °F), which is even hotter than Mercury! Moreover, because the atmosphere is so insulating, the temperature is about the same on the night side as on the daytime side. Soon after this discovery, astronomers used Earth-based telescopes for spectroscopic studies of the clouds, and they found that the clouds are not composed of water droplets. They are made primarily of sulfuric acid (H2SO4) droplets! With its acid rain and blazing temperatures, Venus differs markedly from the tropical paradise envisioned by science fiction writers. The planet’s eponym is the ultimate irony: Venus is the Roman goddess of love and beauty.

Astronomer Carl Sagan made his scientific name by correctly deducing that the dense CO2 of Venus' atmosphere might create a very strong greenhouse effect — much stronger than the one we worry about on Earth. This work was one of the earliest and most noteworthy of Sagan's scientific contributions. He became known to many people through his &quo;Cosmos&quo; book and TV programs, and through his work as a spokesperson and popularize of science. It’s a rare and happy occurrence when a scientist has the enthusiasm and ability to convey his or her subject to a wider audience.

Composite radar map of Venus from Magellan. Click here for original source URL.