Atmospheric tidal waves maintain Venus’ super-rotation

Posted: April 24, 2020 by oldbrew in atmosphere, research, solar system dynamics, Thermodynamics, waves, wind


Super-rotation of its thick atmosphere, that is. The researchers believe their findings could also be a model for tidally-locked exoplanets with atmospheres.

An international research team led by Takeshi Horinouchi of Hokkaido University has revealed that this ‘super-rotation’ is maintained near the equator by atmospheric tidal waves formed from solar heating on the planet’s dayside and cooling on its nightside.

Closer to the poles, however, atmospheric turbulence and other kinds of waves have a more pronounced effect. The study was published online in Science on April 23.

Venus rotates very slowly, taking 243 Earth days to rotate once around its axis. Despite this very slow rotation, Venus’ atmosphere rotates westward 60 times faster than its planetary rotation.

This super-rotation increases with altitude, taking only four Earth days to circulate around the entire planet towards the top of the cloud cover. The fast-moving atmosphere transports heat from the planet’s dayside to nightside, reducing the temperature differences between the two hemispheres.

“Since the super-rotation was discovered in the 1960s, however, the mechanism behind its forming and maintenance has been a long-standing mystery,” says Horinouchi.

Horinouchi and his colleagues from the Institute of Space and Astronautical Science (ISAS, JAXA) and other institutes developed a new, highly precise method to track clouds and derive wind velocities from images provided by ultraviolet and infrared cameras on the Akatsuki spacecraft, which began its orbit of Venus in December 2015.

This allowed them to estimate the contributions of atmospheric waves and turbulence to the super-rotation.

The group first noticed that atmospheric temperature differences between low and high latitudes are so small it cannot be explained without a circulation across latitudes.

“Since such circulation should alter the wind distribution and weaken the super-rotation peak, it also implies there is another mechanism which reinforces and maintains the observed wind distribution,” Horinouchi explained.

Further analyses revealed that the maintenance is sustained by the thermal tide — an atmospheric wave excited by the solar heating contrast between the dayside and the nightside — which provides the acceleration at low latitudes.

Earlier studies proposed that atmospheric turbulence and the waves other than the thermal tide may provide the acceleration.

However, the current study showed that they work oppositely to weakly decelerate the super-rotation at low latitude, even though they play an important role at mid- to high latitudes.

Their findings uncovered the factors that maintain the super-rotation while suggesting a dual circulation system that effectively transports heat across the globe: the meridional circulation that slowly transports heat towards the poles and the super-rotation that rapidly transports heat towards the planet’s nightside.

Full article here.
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AAAS Science : How waves and turbulence maintain the super-rotation of Venus’ atmosphere – Abstract

Explaining super-rotation on Venus

The solid surface of Venus rotates very slowly, once every 243 days, but its thick atmosphere circles the planet in just 4 days. This phenomenon, known as super-rotation, requires a continuous input of angular momentum, from an unknown source, to overcome friction with the surface. Horinouchi et al. mapped the planet’s winds using ultraviolet observations of Venus’ clouds from the orbiting Akatsuki spacecraft (see the Perspective by Lebonnois). They incorporated these data into a global model of angular momentum transport in the atmosphere, finding that the super-rotation is maintained through thermal tides driven by solar heating.

  1. oldbrew says:

    Technical graphic…

    [Click on image to enlarge]

  2. tallbloke says:

    I wonder if the polar regions circulate slower than the equator at the top of the atmosphere, like the Sun’s plasma does.

  3. oldbrew says:

    Not sure TB.

    Swansong experiment sheds light on Venus’ polar atmosphere [2016]

    Additionally, the polar region was found to be dominated by strong atmospheric waves, a phenomenon thought to be key in shaping planetary atmospheres — including our own.