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Mystery of Venus’ atmosphere

Date: 28 April 2020 Tags: Space

Issue

The mystery behind why the atmosphere of Venus spins much faster than the planet's surface may finally be solved, a new study finds.

 

Background

Compared to Earth, Venus twirls at a leisurely pace on its axis, with its surface taking 243 Earth days to complete one rotation. However, the hot, deadly atmosphere of Venus spins nearly 60 times faster than its surface, whirling around the planet once every 96 hours, an effect known as super-rotation.

 

Details

  • This mysterious phenomenon of atmospheric super-rotation is seen not just on Venus, but also on Saturn's largest moon, Titan.

  • Previous research suggested that in order for this super-rotation to occur, the atmosphere of Venus has to possess enough angular momentum to overcome friction with the planet's surface. However, it was uncertain what exactly the source of this angular momentum was.

  • Scientists analyzed data from the Japanese space probe Akatsuki, which has orbited Venus since 2015. They focused on super-rotation in the cloud layer of Venus, where the rotation speed is highest, reaching about 245 mph (395 km/h) in the region surrounding the equator.

  • Based on ultraviolet images and thermal infrared data from Akatsuki, the scientists developed a way to track Venusian cloud motions in order to map the planet's winds and the way heat circulated in the atmosphere.

  • This helped give the researchers a picture of how angular momentum was distributed across the cloud-top level, located at about 42 miles (70 kilometers) in altitude. This in turn helped them estimate the forces sustaining the super-rotating atmosphere.

  • The scientists discovered the Venusian atmosphere received angular momentum though thermal tides, which are variations in atmospheric pressure driven by solar heating near the planet's equator.

  • They also found planetary-scale waves in the atmosphere as well as large-scale atmospheric turbulence worked against this effect from thermal tides.

  • These findings may shed light on the habitability of tidally locked exoplanets, worlds that each have one dark side that always faces away from its star and one side that is bathed in constant sunlight. 

  • One might expect the daysides of these exoplanets would be hot and the nightsides would be cold. However, super-rotation could balance the temperature differences between daysides and nightsides.