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Venus and Spica

September 5, 2013

The brilliant object in the western sky after sunset tonight is the planet Venus, the “evening star.” And the true star Spica, the leading light of Virgo, stands quite close to the lower left of Venus.

Venus is closer to the Sun than Earth is. Partly because of that proximity, its surface temperature is a blistering 860 degrees Fahrenheit. But billions of years ago, the Sun was fainter than it is now, so some scientists have suggested that Venus was once milder. Its surface might even have been covered with water.

Recently, though, a Japanese study suggested that Venus was probably born hellish.

The study modeled the early Earth and Venus. Both planets probably were born with hot, molten surfaces — the result of a constant bombardment by giant space rocks — and thick, steamy atmospheres. Because Earth was farther from the Sun, its surface cooled and solidified after just a few million years. Then rain fell and created the oceans, where the water was safe: Sunlight couldn’t break the water molecules apart.

In contrast, the new work suggests that because Venus was born closer to the Sun, its surface remained hot and molten for up to a hundred million years. If any rain fell during that time, it vaporized as it hit the ground. So Venus’s atmosphere stayed steamy. Sunlight at the top of the atmosphere broke the molecules of water vapor into hydrogen and oxygen. The lightweight hydrogen escaped into space, while the oxygen joined with elements on the surface to make rocks — leaving Venus dry and barren.


Script by Ken Croswell, Copyright 2013

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