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Venus and the Runaway Greenhouse Effect

Week of August 4, 1997

image of Venus with clouds Right now as I write this, the planet Venus shines brightly in the evening sky, getting higher with each passing day. It shines so brightly, third only to the Sun and the Moon, that it is no wonder the ancients named it after their goddess of beauty (or was it the other way around? Hmmmm...). Legends have always abounded about our sister planet. Actually, when astronomers were first able to start measuring properties of Venus, it did look very much like a twin of Earth. It has nearly the same diameter and mass. As you can see in the picture, it has air and weather, and even clouds. Before we were able to analyze those clouds, people thought the surface of Venus would be very Earthlike, though hotter due to its proximity with the Sun.

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.

image of Venus' surface So instead of a lush jungle filled with exotic animals and odd human-like inhabitants, all we really find on the surface are rocks baked by broiling heat, squashed by huge pressures, and, the final indignity, rained on by sulfuric acid. Those are some flat, hot, unhappy rocks.

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.



©2008 Phil Plait. All Rights Reserved.

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