The crescent Moon and the planet Venus continue their show in the western sky this evening. Venus is the brilliant “evening star,” and is below the Moon as twilight begins to fade.
Today, Venus and the Moon are both dry. The Moon has some frozen water mixed with its powdery dirt, and some large chunks of ice at its poles, but it’s still far dryer than any desert here on Earth. And Venus has just a trace of water vapor in its dense, hot atmosphere.
But Venus once had much more water than it does today — perhaps enough to fill oceans.
The evidence of a wetter Venus is found in the planet’s upper atmosphere, where hydrogen is escaping into space. The hydrogen comes from water molecules that have been split apart by ultraviolet energy from the Sun. The rate at which Venus is losing hydrogen indicates that it must have had a lot more water in its past.
In addition, the ratio of normal hydrogen to “heavy” hydrogen — a form that Venus is more likely to hold on to — also suggests that Venus must have had more water in the past.
Just how much is uncertain, though. Some scientists say that oceans could have covered much of Venus during the planet’s early history. As Venus got hotter, the water evaporated and climbed into the upper atmosphere. There, it was split apart, with the hydrogen escaping into space, and the oxygen binding with other elements.
Future missions to Venus may solve the mystery of the planet’s watery past.
Script by Damond Benningfield, Copyright 2011
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