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Moon and Venus

December 2, 2016

The Moon and the planet Venus form a beautiful pairing early tonight. Venus is the brilliant “evening star,” not far to the left of the Moon. They set by about eight o’clock.

Venus looks so bright in part because its atmosphere is topped by an unbroken layer of clouds. The clouds reflect most of the sunlight that strikes them back into space.

Those clouds do something that doesn’t happen on Earth or any other planet in the solar system. They race around Venus much faster than the planet spins on its axis.

Venus takes about 243 Earth days to complete one turn. But the clouds at the top of its atmosphere whip around the planet once every four days — an effect called super-rotation. That means the clouds can complete 60 trips around the planet for every Venusian day. On Earth and the other planets, the clouds rarely move any faster than the planet’s rotation rate.

The cause of the super-rotation is a mystery. Even though planetary scientists have been studying it for decades, they don’t have a good explanation. They do have some ideas, though.

One says that the atmosphere above the clouds interacts with the solar wind — a stream of charged particles from the Sun. That creates waves in the atmosphere, which carry momentum down to the cloudtops, about 40 miles above the surface. That gives the clouds a big push.

But another idea says the super-rotation is driven by pressure from the Sun’s light — revving up Venus’s high-speed clouds.


Script by Damond Benningfield


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