Venus is wrapped in thick clouds of sulfuric acid that swirl around the planet at 225 mph, but not everything in the atmosphere is moving.

On Dec. 7, 2015, during its very first orbit around Venus, Japan's Akatsuki spacecraft discovered a mammoth, bow-shaped structure in the upper atmosphere that remained oddly fixed over a mountainous region known as the western highlands of Aphrodite.

The structure, which stretched for 6,200 miles across the sky, remained fixed in place until Dec. 11. Five days later, when Akatsuki's longwave infrared camera was next available to make observations, the bright region was gone.

"We believe it is generated by a gravity wave," physicist Makoto Taguchi, with Rikkyo University in Tokyo, wrote in an email to Seeker. "We examined every possibility, such as a thermal tide or an instrumental error, but all of them except for a gravity wave were excluded."

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Like on Earth, gravity waves on Venus could be triggered by disturbances in stable layers of the atmosphere, such as the flow of wind over a mountain range. The effect is similar to ripples that are caused by dropping a stone into a pool of water.

Gravity waves are not to be confused with the recently detected cosmological phenomenon known as gravitational waves, which are caused by the bending of space and time due to extremely massive objects, such as colliding black holes.

Taguchi and colleagues, writing in this week's Nature Geoscience, said they believe Venus' mountains caused a gravity wave to form in the lower atmosphere. The bow-shaped structure, which was hotter than surrounding atmosphere, then rose up to an altitude of about 40 miles.

Computer models show a gravity wave on Venus could propagate to such an enormous size and altitude, but more work is needed to understand the planet's near-surface conditions, the scientists said.

"We suggest that winds in the deep atmosphere may be spatially or temporally more variable than previously thought," the paper said.

Additional observations with Akatsuki, which is expected to remain operational until mid-2018, are underway.

Image: An infrared view of Venus' disk, showing structure in the planet's atmsophere. Credit: JAXA/Planet-C

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