Super-rotation and Venus’ atmosphere

Scientists tracking the thick clouds of Venus’ rapidly rotating atmosphere say they have gained new insights into the forces that drive atmospheric super-rotation – a phenomenon in which an atmosphere rotates much more quickly than the solid planetary body below. 

Using observations from the JAXA spacecraft Akatsuki, which has been orbiting Venus since 2015, they suggest super-rotation is maintained by a combination of solar heating-driven thermal tides, planetary waves and atmospheric turbulence. 

The work by a team led by Takeshi Horinouchi from Japan’s Hokkaido University is described in a paper in the journal Science.

Venus moves slowly – its surface takes 243 Earth days to complete one rotation – but its atmosphere spins at nearly 60 times that speed, whipping around the planet in 96 hours. 

This super-rotation increases with altitude, taking only four days to circulate around the entire planet towards the top of the cloud cover. Heat is transported from the planet’s dayside to nightside, reducing the temperature differences between the two hemispheres.

Horinouchi and colleagues note that for this phenomenon to occur, a continuous redistribution of angular momentum is needed to overcome friction with the planet’s surface, although neither the source of this momentum nor how it’s maintained are known. 

They report that by using ultraviolet images and thermal infrared measurements taken by Akatsuki, they tracked the motion of clouds and used them to map Venus’ winds, which provided a consistent picture of its angular momentum balance at the cloud-top level. 

They could then estimate the atmospheric forces sustaining the planet’s super-rotating atmosphere. Their results suggest the required angular momentum is provided through thermal tides, driven by solar heating near the planet’s equator, and is opposed by planetary-scale waves (called Rossby waves) and large-scale atmospheric turbulence. 

In a complementary Perspective in the journal, Sebastion Lebonnois from Laboratoire de Meteorologie Dynamique, Paris, says the research provides “an important piece of the super-rotation puzzle” but suggests “the question of whether their analysis presents a complete picture of the angular momentum balance may still be open”. 

“The observation and analysis focus on only one level of the thick atmosphere of Venus,” he writes. “The possibility remains that the multiple wave activities and their impact on this very sensitive balance may be different at other levels within the 20-km-thick cloud layer.”

Full caption

In the schematic illustration above, in the cloud layer of Venus a vertical and north-south circulation, called the meridional circulation, exists (white arrows) to transport heat from low latitudes to high latitudes, because sunlight is absorbed more at low latitudes. 

This circulation also transports the angular momentum, which corresponds to the strength of the circulating winds (yellow arrows), to decelerate the super-rotation. The deceleration is compensated by the acceleration by the thermal tide, which transports angular momentum both horizontally and vertically (red arrows). 

Other waves and turbulence work oppositely but weakly at low latitudes (blue allows), while they play an important role at mid-latitudes (pale blue arrows; to transport the angular momentum to shortcut the meridional circulation). 

The combination of these effects manifests a system to effectively transport heat across the globe by the combination of the slow poleward heat transport by the meridional circulation and the fast heat transport to the night-side by the meridional circulation. Such a dual circulation system might present on tidally locked exo-planets to reduce temperature differences over them.