Moon, Venus, and Antares
The one constant thing about the planets is that they're always changing. The surface of Earth has been reshaped by the motions of its continents, for example. Mars was once much warmer and wetter than it is today. And Venus was probably much cooler and wetter.
Today, Venus is the hottest planet in the solar system. Its thick atmosphere consists mostly of carbon dioxide, which traps the Sun's heat. But there's evidence that in the past, the atmosphere contained fair amounts of water.
Today, the total amount of water vapor in Venus's atmosphere is barely enough to get a sponge wet. If you squeezed out all the water, there'd be enough to cover the surface in a layer about an inch thick. By contrast, there's enough water on Earth to make a layer about two miles thick.
Scientists are still trying to figure out why Venus lost its water. But they do know how at least some of it escaped: it's gone with the wind -- the solar wind, that is.
A spacecraft called Venus Express has detected hydrogen and oxygen escaping into space from the top of the atmosphere. Solar radiation splits water molecules apart, and the solar wind whisks the parts off into space -- a process that's still going on.
Venus stands a little above the Moon after sunset this evening. It's the brilliant "evening star." The true star Antares is just to the right of the Moon. They're quite low in the southwest at sunset, so you need to look quickly to see them.
Script by Damond Benningfield, Copyright 2008
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