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Venus and the Runaway Greenhouse EffectWeek of August 4, 1997
However, modern science burst yet another bubble. The atmosphere of Venus was found to be mostly carbon dioxide (CO2), and it was Carl Sagan himself who proposed that the surface of Venus was hotter then we had guessed before-- carbon dioxide is now known to be a very efficient greenhouse gas. The surface of Venus is at 900 degrees! That's hot enough to melt zinc, if there were any sitting around on the surface. Worse, the atmospheric pressure on the surface of the planet is truly tremendous: 90 times that of Earth! Even worse than that, those beautiful clouds were found to be not water, but sulfuric acid! Ouch.
Oh, one more thing: a surprise awaited astronomers when they calculated the amount of CO2 in Venus' atmosphere: it has the same amount as on the Earth! However, the Earth stores its CO2 in its rocks and water, and not in its air. Some astronomers believe that Venus was much the same as the Earth, but its closer distance to the Sun meant it couldn't hold the CO2 in its rocks, so it all went into the atmosphere. This caused a runaway heating cycle, and we have the modern Venus: hostile, hot, lifeless. Could this happen to the Earth? No one knows.
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