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Venus spins slowly, but its clouds move fast.

Image: JAXA/ISAS/DARTS/Kevin M. Gill

If you think the days feel long, be glad you don’t live on Venus. Our inner neighbour rotates only once every 243 Earth days, but you wouldn’t be able to tell this by looking at it. Venus is cloaked in thick clouds that are whipped around the planet by wind. These clouds complete a lap of Venus in only four Earth days. So what’s driving this extreme wind?

New clues about Venus have come from the space probe Akatsuki, launched by the Japanese space agency JAXA. Since 2015, the probe has been providing up-to-date science while orbiting Venus. And that includes precise measurements of the top layer of Venus’ clouds.

Back here on Earth, scientists got to work with the new data. They tracked the clouds on Venus to work out exactly how they were moving. Then they made a computer simulation of Venus’ atmosphere and used their cloud readings to keep it realistic.

Looking at the model, the team found one thing that made the cloud tops race. The heat of the Sun, even warmer than on Earth, makes the air expand in a process known as a thermal tide. This hot air blows west to the night side of the planet where it cools down again. And in the process, this wind keeps the dark side warm, even through its months-long night.

Sadly, humans are unlikely to visit Venus any time soon. The atmosphere is extremely toxic. Plus, the air pressure is strong enough to crush submarines designed to withstand the high pressures of Earth’s oceans. And Venus is extremely hot, over 400 degrees Celsius, even at night. But this research can also tell us something about far more distant planets outside our solar system, known as exo-planets.

“Our study could help better understand atmospheric systems on tidally-locked exo-planets,” says Takeshi Horinouchi, one of the scientists behind the project. Especially for exo-planets where one side faces the central star, in a long solar day like Venus.

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