Some Storm: Atmosphere on Venus Spins Faster Than Surface

It takes 243 Earth days for Venus to rotate, meaning a Venusian day is two-thirds of an Earth year long. But its atmosphere spins around the planet in just four Earth days.

The hemispheric view of Venus, as revealed by more than a decade of radar investigations. (NASA photo)

(CN) — Astronomers peering into thick, rapidly rotating clouds in Venus’ atmosphere have gained an understanding of the energy powering “super rotation,” a phenomenon in which a planetary body’s surface spins at a slower rate than the heavens above it, according to a study released Thursday.

The second planet from the sun has a thick atmosphere that traps heat and creates a greenhouse effect that makes it the hottest planet in our solar system.

Venus also spins slowly in the opposite direction that most planets do. Surface rotation on Earth’s planetary neighbor is slow, requiring 243 Earth days to complete one rotation.

But the Venusian atmosphere spins at a rate that is nearly 60 times faster, whipping around the planet once every four Earth days.

Data from the Japan Aerospace Exploration Agency (JAXA) spacecraft Akatsuki, which has been orbiting Venus since 2015, played a key role in helping the research team understand atmospheric super rotation.

The Akatsuki craft captured ultraviolet images and thermal infrared measurements of the planet’s clouds, providing a team of Japanese and U.S.-based researchers with a detailed map of planetary wind patterns.

An analysis of the observations indicate super rotation is caused by a combination of planetary waves, atmospheric turbulence and the thermal tides, according to the study published Thursday in the journal Science.

The moonless planet’s wind map helped researchers understand a force at the cloud-top level of Venus’ atmosphere known as angular momentum balance.

A continuous redistribution of angular momentum helps the planet overcome friction with the planet’s surface, leading to the super rotation phenomenon.  

“A thermally induced latitudinal-vertical circulation acts to homogenize the distribution of the angular momentum around the rotational axis,” the study states. “Maintaining the super-rotation requires this to be counteracted by atmospheric waves and turbulence.”

Both the force behind angular momentum and the energy that maintains it remain mysteries, the scientists said in a statement.

But observations did reveal thermal tides play a role in generating portions of the angular momentum.

Tides are driven by solar heating near the planet’s equator and crash dramatically against powerful planetary-scale waves — also called Rossby waves — and large-scale atmospheric turbulence.

Study lead author Takeshi Horinouchi of Hokkaido University and colleagues said super rotation may also occur in “tidally locked” exoplanets which remain heated only on one side since they always face the same way toward their host star.

“Zonal (east-west) flow around the rotation axis, including [super rotation], can transport heat from the dayside to the nightside of those exoplanets,” the study states.

Sebastion Lebonnois, a scientists not affiliated with the study, said in a statement the dynamic forces driving the atmospheric super rotation anomaly are not well-understood yet by scientists.

“Among the intriguing mysteries that remain for planetary atmospheres, the phenomenon of super-rotation is still a teasing problem,” Lebonnois said. “Horinouchi et al. provide an important piece of the super-rotation puzzle, that can offer a strong constraint on numerical simulations of the Venusian atmosphere.”

Lebonnois, a researcher at Laboratoire de Météorologie Dynamique in Paris, France, published a related study titled “Super-rotating the Venusian atmosphere.”

Researchers did not immediately respond to a request for further comment on the study.

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