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Why do planets farthest from sun have highest winds? Team closes in on answer

The planets beyond Mars exhibit the highest winds speeds of any other planets in the solar system. It's a puzzle, because less energy from the sun is available there to drive higher winds. 

By Staff writer / May 17, 2013

This image of Neptune was taken during the August 16-17, 1989, period as Voyager 2 photographed the planet almost continuously, but had no way to measure the winds or how deep they reach into the atmosphere. Now, scientists have turned to subtle changes in the planet's gravity for clues.



Astronomers have long marveled that the fastest wind speeds in the solar system have been clocked on the planets farthest from the sun.

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Now, they may be a step closer to figuring out the energy source that drives these mighty winds.

In a new study, a team of scientists from Israel and the US finds that on Uranus and Neptune the winds appear to be confined to the top 680 miles of the atmosphere – and may actually involve a thinner layer than that.

The results not only reveal new information about Uranus and Neptune, the researchers say. They also provide insights into the mechanisms driving the atmospheres of planets orbiting other stars, says William Hubbard, a researcher at the University of Arizona's Lunar and Planetary Laboratory and a member of the team reporting the results in Thursday's issue of the journal Nature.

Up to now, researchers have posited two possible sources: processes confined to the top layer of the atmosphere or heat welling up from deep in the planets' interiors. Both planets emit more heat than they receive from the sun, with Neptune radiating twice as much. And while 680 miles of atmosphere seems towering by Earthly standards, it's only skin deep for Uranus and Neptune.

The winds in the planets' wide equatorial jet streams rip along at speeds of up to 450 miles an hour on Uranus and as high as 1,300 miles an hour on more-distant Neptune. Still, the flows "seem to be rather shallow, so the amount of energy that has to be supplied to keep them going is much less than might have been thought," Dr. Hubbard says.

The planets beyond Mars exhibit the highest winds speeds of any other planets in the solar system. Yet from Jupiter on out, wind speeds increase with distance, even though less energy is available from the sun to drive atmospheric circulation at each orbit along the way.

The reasons for this trend "are not well understood, actually," says Adam Showman, also with the Lunar and Planetary Laboratory and a member of the study's team. But the prime suspect is atmospheric drag, or rather, the lack of it.

The outer planets' atmospheres behave more like liquids deep in their interiors, so there is virtually no surface roughness to act as a drag on winds, as there is on Earth. And as the distance between a planet and the sun increases, there is less solar energy to impart turbulence to the atmosphere, which also acts as a drag.


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