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Akatsuki spies massive wave on Venus.

A MONSTER WAVE recently roiled Venus's atmosphere, forming a planet-spanning, bow-shaped feature. Japan's Akatsuki orbiter noted this disturbance in late December 2015 and early January 2016, shortly after the craft's arrival, Tetsuya Fukuhara (Rikkyo University, Japan) and colleagues report in the February Nature Geoscience.

When Akatsuki looked back at the region later in 2016, the wave had, for the most part, vanished.

The feature spanned the Venusian cloudtops from the northern to southern hemisphere, extending more than 10,000 km (6,200 miles). It appeared near the evening terminator on the dayside and was embedded in the interface between the upper troposphere and lower stratosphere. Although the cloudtops whip along at 100 meters per second (200 mph)--much faster than the slow-moving surface of the planet below --the curious structure seemed to stay in lockstep with the planet's rotation, suggesting a complex interplay between the surface and the atmosphere.

The team's computer models suggest that air flowing over mountainous terrain produced a gravity wave that then propagated upward to the cloudtops, where the large bow wave was seen. (A gravity wave is an undulation triggered in a fluid--such as the atmosphere--by the interaction of gravity and other forces.) The wave's longitude corresponded with the western slope of Aphrodite Terra, the largest of Venus's three continent-size highlands, whose surface area is comparable to Africa's.

We see similar gravity-wave phenomena here on Earth, and NASA's New Horizons spacecraft chronicled evidence for gravity waves in the atmosphere of Pluto during its historic 2015 flyby.

Caption: Infrared image of Venus's gravity wave. The dashed line marks the day-night terminator.

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Title Annotation:SOLAR SYSTEM
Author:Dickinson, David
Publication:Sky & Telescope
Article Type:Brief article
Date:May 1, 2017
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