Why Does Slow-Turning Venus Have Such A Fast-Moving Atmosphere?

A computer simulated image of Venus. NASA/JPL

As the Earth turns it pulls the air with it, rotating the atmosphere at a rate somewhat slower than the rock beneath. The same is true for gas giants like Jupiter, but Venus is an anomaly. Its thick atmosphere doesn't just rotate a little faster than the planet – it spins up to 60 times as fast. Data from the Akatsuki spacecraft may finally have allowed astronomers to explain this mystery, which has been confusing them since the first interplanetary spacecraft revealed it.

Venus – knowns as Earth's hellish twin – takes 243 days to turn upon its axis, making it the only planet whose day is longer than its year. Consequently, it has minimal angular momentum to transfer to its atmosphere. Yet instead of standing still, its hurricane-strength winds increase the higher one goes. Seventy kilometers (45 miles) from the surface, it takes just four days for cloud tops to circle the planet. The speed is particularly remarkable because the winds have to work against friction with the planet's surface.

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Dr Takeshi Horinouchi of Hokkaido University used the ultraviolet and infrared cameras on the Japan Aerospace Exploration Agency's (JAXA) spacecraft Akatsuki to track the movement of Venus's sulfuric acid clouds in greater detail than ever before. He also noticed the temperature gradient between the equator and the poles is much less than would be expected, indicating cross-latitude mixing.

"Since such circulation should alter the wind distribution and weaken the super-rotation peak, it also implies there is another mechanism which reinforces and maintains the observed wind distribution," Horinouchi said in a statement

In Science, Horinouchi and co-authors explain that heat from the Sun warms the day side of Venus, particularly the tropical zone. This creates a thermal tide from this hotzone both to the night side and to higher latitudes.

Model of how Venus' atmosphere maintains its super-rotation. The thermal tide (red) towards the equatorial top enforces the westward super-rotation. The atmosphere is controlled by a dual circulation system: the meridional (vertical) circulation (white) that slowly transports heat towards the poles and the (yellow) super-rotation that rapidly transports heat towards the planet's nightside.: Planet-C project team

The idea is not new, but has been one of several explanations. Others have proposed turbulence and other atmospheric waves contribute to the global winds, but Horinouchi says the opposite – these effects marginally slow down winds that would be even faster otherwise.

"Our study could help better understand atmospheric systems on tidally-locked exo-planets whose one side always facing the central stars, which is similar to Venus having a very long solar day," Horinouchi said. Some of these planets, in orbit around much fainter stars than the Sun, might have narrow habitable zones where their day and night sides meet, but whether this is possible depends in part on how heat is redistributed from the daylight side.

Were it not for the speed of Venus' winds the side that faced the Sun would reach even more hellish temperatures during the long day, while the night would be relatively cool. That might assist exploration as space probes could survive a little longer on the night side, but we'd still need to deal with the phenomenal atmospheric pressure and extreme acidity.

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