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For years, scientists have wondered why Venus’ atmosphere rotates so much faster than its surface. The answer, according to a recent study by a team of scientists led by Takeshi Horinouchi (Hokkaido University in Sapporo, Japan), lies in its thermal tides.
It takes Venus 243 Earth days to complete a rotation about its axis. However, its thick toxic atmosphere takes only 4 days to circumnavigate the planet, making it 60 times faster. This effect, while extreme in the case of Venus, is called superrotation and is most pronounced at the upper level of its troposphere. However, this type of sustained super rotation requires that a constant angular momentum source be introduced into the system to overcome surface friction.
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Horinouchi and his team were able to map the winds using ultraviolet images of the Venus clouds provided by the Japanese space probe Akatsuki (in orbit around Venus since 2015). They then enter this data into a computer model of planetary transport of atmospheric angular momentum. The results showed that thermal tides (atmospheric pressure differentials driven by solar heating at the equator) were the main contributors to angular momentum.
Sustained and stable super rotation could be key to the habitability of those exoplanets that are blocked by the tides to their host stars, so it could balance the otherwise large temperature differences between the light and dark sides.
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