Solved: mystery of Venusian atmosphere’s weird rotation
The mystery behind why the atmosphere of Venus spins much faster than the planet’s surface may finally be solved. The finding might help shed light on how habitable distant exoplanets, worlds beyond our Solar System, may actually be.
Compared to Earth’s 24-hour spin, Venus twirls at a leisurely pace on its axis, with its surface taking 243 Earth days to complete one rotation. However, the hot, deadly carbon-dioxide atmosphere of
Venus spins nearly 60-times faster than its surface, whirling around the planet once every 96 hours in an effect known as super-rotation.
Previous research suggests that in order for this super-rotation to occur the atmosphere of Venus has to possess enough angular momentum – the amount of momentum a body has because of its spin – to overcome friction with the planet’s surface. However, it was uncertain what exactly the source of this angular momentum was.
To shed light on this mystery, scientists from Hokkaido University in Sapporo analysed data from the Japanese space probe Akatsuki, which has orbited Venus since 2015. They focused on superrotation in the cloud layer of Venus, where the rotation speed is highest, reaching about 395 kilometres (245 miles) per hour 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 to give the researchers a picture of how angular momentum was distributed across the cloud-top level, located at about 70 kilometres (42 miles) in altitude. This in turn helped them estimate the forces sustaining the super-rotating atmosphere.
The scientists discovered that 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.