Astronomers figured out long ago that Venus must be covered with clouds because it is so brilliant in the sky. The clouds made it impossible to see Venus' surface from Earth at all. Many people thought that the clouds must mean Venus was very warm and wet, possibly with life.
Earth-based radio observations in the 1950's, and spacecraft observations beginning with Mariner II in 1962, showed that Venus is far too hot for water or life. Its atmosphere is 90 times as dense as Earth's, and the pressure on the surface is about equal to half a mile deep in the oceans on Earth. The atmosphere is so dense that even if there were no clouds at all, we could not see the surface because the atmosphere would scatter light too much, like trying to see through frosted glass. The atmosphere is carbon dioxide, which traps heat from the sun, raising the temperature to 900 degrees F. The clouds are fine droplets of sulfuric acid, possibly erupted from volcanoes.
Even though Venus is almost a twin to the Earth in size, it is different in almost every other way. Nothing at all was known of the surface of Venus until Russian and American spacecraft mapped the surface with radar. How do we measure elevations on a planet with no seas? We use the average diameter of the planet as zero elevation. Venus is much smoother than Earth. The average elevation on earth is about 3 kilometers below sea level, but most of the earth is several kilometers higher (the continents) or lower (the oceans) than that. On Venus, most of the surface is within a kilometer of average.
One of the surprises (and disappointments) of mapping Venus is that Venus seems to lack plate tectonics. There is nothing on the topographic maps of Venus that looks like mid-ocean ridges or ocean trenches. Venus has many volcanoes and two large plateaus, one of which, Aphrodite Terra, is shown here. It lies in equatorial latitudes and is about the size of South America. Venus' south pole is indicated with a cross near the bottom of the picture.
Places on planets are named using Latin geographic terms ("terra" is Latin for "land" and is used for large plateaus). Since Venus was a goddess, features on Venus have female names. Aphrodite was the Greek goddess corresponding to Venus.
The large circular arc on the southeast edge of Aphrodite Terra looks like it might be a giant crater, but it is not. The most likely explanation is that it is something like a hot spot in terrestrial plate tectonics, a place where magma rises to the surface and erupts repeatedly. This is the largest of many such circular features, called coronae (from the Latin word for crown), which have been found so far only on Venus. This one is called Artemis Corona (Artemis was the Greek goddess of childbirth, fertility, and hunting).
Created 21 August 2009, Last Update 20 January 2010
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