Why does it take Venus 243 days to rotate, but the atmosphere only takes 4 days to revolve around Venus?

[Mobile software: Bo Ke Yuan] Images from the Akatsuki spacecraft, a Venus probe, reveal what makes Venus' atmosphere rotate faster than the planet itself. An international research team led by Takeshi Horinouchi of Hokkaido University revealed that this "super rotation" is maintained near the equator by atmospheric tidal waves formed by solar heating during the day and cooling at night. However, closer to the poles, atmospheric turbulence and other types of waves have a more obvious effect, and their research results were published in the journal Science.

Venus rotates very slowly, taking 243 Earth days to rotate once on its axis. Despite its very slow rotation, Venus' atmosphere whirls westward 60 times faster than Venus itself rotates. This super-rotation increases with altitude, taking only four Earth days to rotate around the entire planet toward the cloud tops. The fast-moving atmosphere transports heat from the dayside (diurnal) hemisphere of Venus to the nightside (nocturnal) hemisphere, thereby reducing the temperature difference between the two hemispheres.

Illustration: Venus - Computer simulated global view centered on 90 degrees east longitude (NASA/JPL).

However, the mechanism of super-rotation formation and maintenance has been a long-standing mystery since its discovery in the 1960s. Horinouchi and colleagues from the Institute of Space and Astronautical Science (IAS, JAXA) and other institutes have developed a new, high-precision method to track clouds and derive wind speeds from images provided by ultraviolet and infrared cameras on the Akatsuki spacecraft, which began orbiting Venus in December 2015. This allowed scientists to estimate the contribution of atmospheric waves and turbulence to super-rotation.

The team first noticed that the atmospheric temperature difference between low and high latitudes was so small that it could not be explained without a circulation between the latitudes. Since such a circulation should change the distribution of winds and weaken the superrotation peak, it also means that there is another mechanism to strengthen and maintain the observed wind distribution. Further analysis showed that this maintenance is supported by thermal tides (a type of atmospheric wave excited by the contrast in solar heating between the dayside and nightside) providing acceleration at low latitudes. Earlier studies have suggested that atmospheric turbulence and fluctuations other than thermal tides may provide acceleration.

Illustration: The system that maintains the superrotation of Venus' atmosphere (yellow). Thermal tides at the equatorial tops (red) enhance the westward superrotation. The atmosphere is controlled by a dual circulation system: the meridional (vertical) circulation (white) that slowly transports heat to the poles, and the superrotation that rapidly transports heat to the planet's night side.

However, the current study shows that while they play an important role at mid- and high-latitudes, they play the opposite role at low latitudes, acting as a weak decelerator of superrotation. The findings shed light on the factors that sustain superrotation, while proposing a dual circulation system that effectively transports heat around Venus globally: the meridional circulation that slowly transports heat to the poles, and the superrotation that rapidly transports heat to the night side of the planet. This research could help better understand atmospheric systems on tidally locked exoplanets, planets that always have one side facing the central star, similar to Venus with its very long solar day.

Bo Ke Yuan | Research/From: Hokkaido University

Reference journal: Science

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