Vast atmospheric waves on Venus are caused by largest known ‘hydraulic jump’

The mysterious origin of an impressive cloud disturbance on Venus has now been revealed by a team including the University of Tokyo. Researchers used numerical models to show that an enormous 6,000-kilometer-wide atmospheric wave front, which circumnavigates the planet for days at a time, is caused by a large “hydraulic jump.” This is when a fluid abruptly slows down, changing from shallow and fast to deep and slow.

How Venus’ thick clouds behave

On Venus, a sudden change in airflow in the lower cloud region is coupled with the creation of a strong updraft, forcing sulfuric acid vapor higher into the atmosphere where it condenses into a massive line of cloud. The work is published in the Journal of Geophysical Research: Planets.

Future planetary studies can consider the potential impacts of this process, and what it might mean for any exploratory missions.

A grim, gray day may spoil weekend plans now and then, but on Venus, it’s cloudy all day every day with a chance of sulfuric acid showers. On the bright side, Venus’ constant thick cloud cover provides an excellent opportunity for us to study patterns and processes that would be difficult to spot on planets where clouds are more sparse or intermittent, like here on Earth.

A key feature of Venusian clouds is that they “superrotate,” moving about 60 times faster than the planet turns. We now know that superrotation also occurs elsewhere, including on Mars, our sun, and even Earth’s upper atmosphere. In 2016, images from Japan’s Akatsuki Venus orbiter also revealed that an enormous atmospheric wave—sometimes 6,000 km wide—repeatedly sweeps around the planet’s equator.

“We identified the phenomena, but for years we couldn’t understand it,” said Professor Takeshi Imamura from the Graduate School of Frontier Sciences at the University of Tokyo. “However, thanks to this research, we’re now able to show that this cloud disruption is caused by the largest known hydraulic jump in the solar system.”

Unpacking the giant hydraulic jump

We can see a hydraulic jump in action in the humble kitchen sink. As water from the tap hits the basin, it appears fast and shallow at first, but suddenly slows and becomes deeper as it spreads.

The hydraulic jump on Venus occurs when an eastward-moving atmospheric wave (called a Kelvin wave) in the lower to middle cloud region suddenly becomes unstable. Wind speed as seen from the atmospheric wave abruptly slows down and a strong localized updraft is created, which carries sulfuric acid vapor higher into the atmosphere. The droplets condense into clouds which trail behind, causing the massive wave front which can be seen sweeping around the planet.

“Venus has three distinct cloud layers, and the dynamics of the lower and middle layers are not so well understood,” said Imamura. “Our discovery of a hydraulic jump on Venus connecting a very large-scale horizontal process with a strong localized vertical wave is unexpected, as in fluid dynamics these are usually disconnected.”

Modeling Venus and looking ahead

The hydraulic jump was simulated using a fluid dynamic model (a numerical analysis which simulates how gas or liquids flow), and the cloud formation studied using a microphysical box model (which follows the behavior of an example section of air as it moves through the atmosphere). As well as simulating the same cloud disturbance, the team also found that this process helps to maintain the superrotation of Venus’ atmosphere.

“Up until now, we used a global circulation model (GCM) for Venus that is similar to Earth’s, but this model doesn’t include the hydraulic jump which we have now identified,” explained Imamura. “Our next step will be to test this discovery within a more inclusive climate model that includes other atmospheric processes. We will face some challenges due to the huge amount of processing power required to run such simulations. Even with modern supercomputers, it isn’t easy.”

Although this is the first observation of a hydraulic jump of this scale on another planet, the physics behind it may also occur on other celestial bodies. “Under some circumstances, Mars’ atmosphere may also have the right conditions for a hydraulic jump,” mentioned Imamura. Creating more accurate models of atmospheric conditions will aid in the success of future missions to Mars, as well as wider space exploration.

Who’s behind this story?

Lisa Lock

BA art history, MA material culture. Former museum editor, paramedic, and transplant coordinator. Editing for Science X since 2021.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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