By tracking the thick clouds of Venus' rapidly rotating atmosphere, researchers have gained new insight into the dynamic forces that drive atmospheric super-rotation - a little-understood phenomenon in which an atmosphere rotates much faster than the solid planetary body below. Based on observations from the Japan Aerospace Exploration Agency (JAXA) spacecraft Akatsuki, which has been orbiting Venus since 2015, the study suggests that super-rotation is maintained by a combination of solar heating-driven thermal tides, planetary waves and atmospheric turbulence. "Among the intriguing mysteries that remain for planetary atmospheres, the phenomenon of super-rotation is still a teasing problem," writes Sebastion Lebonnois in a related Perspective. Compared to Earth, the rotation of Venus is slow - its surface takes 243 Earth days to complete one rotation. However, the Venusian atmosphere spins nearly 60 times faster, whipping around the planet once every 96 hours. 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. Using ultraviolet images and thermal infrared measurements taken by Akatsuki, Takeshi Horinouchi and colleagues tracked the motion of clouds and used them to map the planet's winds, which provided the authors with a consistent picture of Venus' angular momentum balance at the cloud-top level. These data allowed Horinouchi et al. to estimate the atmospheric forces sustaining the planet's super-rotating atmosphere. Their results show 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. "Horinouchi et al. provide an important piece of the super-rotation puzzle, that can offer a strong constraint on numerical simulations of the Venusian atmosphere," Lebonnois writes.
