Carl Sagan on Venus

“Some scientists believe that until about 500 million years ago the Venus surface was almost entirely devoid of landforms. Streams and oceans of molten rock were relentlessly pouring out of the interior filling in and covering over any relief that had managed to form.”

“Those who are skeptical about carbon dioxide greenhouse warming might profitably note the massive greenhouse effect on Venus. No one proposes that Venus's greenhouse effect derives from imprudent Venusians who burned too much coal, drove fuel-inefficient autos, and cut down their forests.”

“Venus has about 90 times more air than Earth. It isn't mainly oxygen and nitrogen as here — it's carbon dioxide. But carbon dioxide doesn't absorb visible light either. What would the sky look like from the surface of Venus if Venus had no clouds? With so much atmosphere in the way, not only are violet and blue waves scattered, but all other colours as well — green, yellow, orange, red. The air is so thick, though, that hardly any blue light makes it to the ground; it’s scattered back to space by successive bounces higher up. Thus the light that does reach the ground should be strongly reddened — like an Earth sunset all over the sky. Further sulfur in the high clouds will strain the sky yellow. Pictures taken by the Soviet Venera landers confirm that the skies of Venus are a kind of yellow-orange.”