Venus is unique—almost—in our Solar System because it’s what’s known as a “super-rotator.” That means that Venus’ atmosphere rotates faster than the planet itself. Only Saturn’s moon Titan has the same characteristic.
Scientists have been trying to figure out what causes this super-rotation, and now an international team of researchers might have figured it out.
On Venus, the winds can move up to 60 times faster than the planet itself, and though the planet takes 243 days to rotate, the atmosphere only takes four days to circle the planet. For comparison, Earth’s atmosphere moves at between 10% to 20% the speed of the planet. Scientists have known since the 1960s that Venus is a super-rotator, but haven’t been able to figure out why.
In 2016, researchers found a large stationary gravity wave structure in Venus’ atmosphere. The bow-shaped structure stretched for 10,000 km (6200 miles) across Venus’ cloud tops. It remained stationary relative to the surface of the planet, while the atmosphere maintained its super-rotation.
In 2018, scientists published a paper showing what role the massive wave played in the planet’s super-rotation. The gravity wave on Venus is so huge because the atmosphere only moves in one direction, while on Earth, for example, more variable winds don’t create such massive wave structures. The 2018 paper showed that the huge wave tugged on the planet, altering its rotation rate, but didn’t explain Venus’ super-rotation.
“Since the super-rotation was discovered in the 1960s, however, the mechanism behind its forming and maintenance has been a long-standing mystery,” says Takeshi Horinouchi, lead author of the new study.
This new study says that there’s more going on in Venus’ atmosphere, and that the super-rotation is related to not only atmospheric tidal waves, but to other features as well.
The new study is titled “How waves and turbulence maintain the super-rotation of Venus’ atmosphere.” The lead author is Takeshi Horinouchi of Hokkaido University in Japan. The study is published in the journal Science.
In broad terms, the study shows two contributing factors to Venus’ super-rotation.
At the equator, solar heating creates atmospheric tidal waves on the day side. On the night side, cooling creates the same waves. But at the poles, something else is happening. A press release says, “Closer to the poles, however, atmospheric turbulence and other kinds of waves have a more pronounced effect.”
The new study is based on data from Japan’s Akatsuki spacecraft. The spacecraft is in a large elliptical orbit around Venus. Akatsuki complements the ESA’s Venus Express orbiter, which was in a polar orbit from 2006 to 2014. Together, the pair of spacecraft have made an immense contribution to our understanding of Venus.
The Akatsuki spacecraft carries five imaging cameras: three infrared, one ultraviolet, and one visible light camera. Horinouchi and his colleagues used ultraviolet and infrared images from the spacecraft to develop a precise method of tracking clouds. The cloud tracking led to accurate measurement of wind velocities. From there, the team estimated what contribution the atmospheric waves and the turbulence made to Venus’ super-rotation.
The first thing they noticed was temperature variations. There were atmospheric temperature variations between altitudes that couldn’t be explained, unless there was atmospheric circulation across latitudes.
In the press release, Horinouchi said, “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.”
What was the other mechanism?
After more analysis of the data, and more modelling, the team came up with something else: the thermal tide. The American Meteorological Society describes a thermal tide ss “A variation in atmospheric pressure due to the diurnal differential heating of the atmosphere by the sun.” Horinouchi and his colleagues say that the thermal tide is responsible for the wind at low latitudes.
That’s in contrast to earlier studies, which showed that the thermal tides played no role. This study showed that thermal tides play a role in acceleration at mid and high-latitudes, while having a small deceleration effect at low latitudes.
So, the team has uncovered some important evidence that helps explain Venus’ unusual atmosphere. Not only does their work show how the super-rotation is maintained, it shows how heat is transported around the planet. Circulation along meridians slowly moves heat towards Venus’ poles, while super-rotation moves heat from the day-side to the night-side.
Like so much of planetary science, it not only explains the actual planet being studied, but could help scientists understand the increasing number of discovered exoplanets.
“Our study could help better understand atmospheric systems on tidally-locked exo-planets whose one side always facing the central stars, which is similar to Venus having a very long solar day,” Horinouchi added.