In 2016, a Japanese Venus orbiter repeatedly spotted colossal waves of acidic clouds sweeping through the planet’s atmosphere. For about a decade, astronomers failed to reconcile their observations with existing models. But an unexpected connection finally offers an answer.
In a recent study in the Journal of Geophysical Research: Planets, an international research team describes how a large “hydraulic jump” forces sulfuric acid vapor higher into the atmosphere, where it aggregates into a massive acid cloud. These fronts can reach approximately 3,728 miles (6,000 kilometers) and persist for an extended period of time. As a result, the team believes this hydraulic jump also sustains planetary-scale atmospheric phenomena on Venus, such as its unusually fast winds.
“With this research, we are now able to show that this cloud disruption is caused by the largest known hydraulic jump in the solar system,” Takeshi Imamura, first author of the study and a planetary scientist at the University of Tokyo in Japan, said in a statement.
Same, but not really
In terms of size, mass, density and volume, Venus bears an uncanny resemblance to Earth. But the similarities end there. Notably, Venus’ dense atmosphere and extreme temperatures make the planet exceptionally difficult to study, even with orbiters like Akatsuki observing safely from its orbital perch.
Of course, this hasn’t deterred researchers from exploiting every available opportunity to extract useful data from Venus. For example, because Venus has such thick cloud cover, it is an “excellent” target for studying atmospheric patterns that would not be as evident where clouds are sparser, such as on Earth, according to the release.
A cloudy sandwich
According to the study, the Venusian atmosphere can be divided into three layers of sulfuric acid clouds. Massive winds called “superrotation” cause these clouds to circulate around the planet at blinding speeds, about 60 times faster than Venus’s own rotation. These bursts also regulate the planet’s radiative energy balance as well as atmospheric chemistry and dynamics, the paper adds. For obvious reasons, Venus probes – and therefore scientists – have found it easier to study the upper clouds, but the lower and middle layers have proven difficult to study, Imamura explained.
Atmospheric models could explain little, as Imamura discovered in 2016, when Akatsuki reported the first images of repeating, sweeping cloud waves propagating around Venus’ atmosphere. “We identified the phenomenon, but for years we couldn’t understand it,” Imamura said.
Previous surveys conducted by ESA’s Venus Express between 2006 and 2022 also confirmed similar observations. Additionally, a review of the literature indicated that this cloud feature has been recurring on Venus since at least 1983, meaning that, for whatever reason, astronomers have not or have been unable to identify the cause of this phenomenon.
The cosmic kitchen sink
Imamura and his colleagues tested the hypothesis that a gigantic hydraulic jump was at the origin of this cloud wave. Hydraulic jumps are surprisingly commonplace phenomena, even on Earth. In fact, you can observe one right next to your kitchen sink. Let the water flow and you will see that as the water column hits the sink, it forms a smooth inner circle of shallow, fast-moving water, with deeper, slower ripples of water at the margins of the circle.
Something similar happens on Venus when an eastward atmospheric wave in the lower to middle cloud region becomes unstable. This “shock,” as the team puts it in the paper, forces air to rise sharply along a front. This sudden movement carries the sulfuric acid vapor higher and higher until it eventually condenses into clouds that encircle the entire planet. The team’s numerical simulation also suggests that similar processes help maintain the superrotation of Venus’ atmosphere.
Beyond our planet’s twin
In addition to solving a decades-old mystery, the results could inform planning for future space missions, not just to Venus, the team said. For example, recent research has confirmed that superrotation occurs on Mars, on the Sun, and even in Earth’s atmosphere. This will be essential as humanity seeks to expand its presence in space, as accounting for weather conditions is vital for the protection of astronauts and spacecraft. The research may rely on simulations, but every detail counts when exploring the unknown.
“Our next step will be to test this finding within the framework of a more inclusive climate model that includes other atmospheric processes,” Imamura said. “Under certain circumstances, Mars’ atmosphere may also present the ideal conditions for a hydraulic jump.”
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