Wave on Venus
The bow-shaped feature spans the Venusian cloudtops from hemisphere to hemisphere, more than 6,200 miles (10,000 km) long. Although the cloud tops whip along at 100 meters per second (200 mph)—much faster than the slow-moving surface of the planet below—the curious structure seems to stay in lockstep with the rotation of the planet, suggesting a complex (and previously unsuspected) interplay between the mountainous surface and the sulfurous cloudtops. The structure appeared near the evening terminator on the daytime side of Venus. It’s unclear at this point just how common such a wave is. When Akatsuki looked back at the region later in 2016, the wave had, for the most part, vanished.
A Solar System oddball, Venus rotates retrograde (backward) once every 243 Earth days, longer than its 225 day orbital path around the Sun—its day is longer than its year. You could outrun sunrise and sunset on Venus if, of course, you could find a way to avoid getting simultaneously fried by the 864° Fahrenheit (462° Celsius) surface temperatures and crushed under an atmospheric pressure more than 90 times that of Earth’s at sea level. The atmosphere consists almost entirely of carbon dioxide, with some nitrogen and sulfur dioxide.
The wave that Akatsuki detected is embedded in the cloud interface layer between the upper troposphere and the lower stratosphere. Computer models run by the team, led by Makoto Taguchi (Rikkyo University, Tokyo), suggest that air flowing over the mountainous terrain produces a gravity wave that then propagates upward to the cloud tops, where the large bow wave is seen.
We see similar gravity wave phenomena here on the Earth, though not on such a large scale. NASA’s New Horizons spacecraft also chronicled evidence for gravity waves in the atmosphere of Pluto during its historic 2015 flyby.
It’s worth noting that gravity waves propagated through a planetary atmosphere are a distinct and separate phenomenon from gravitational waves, ripples in spacetime produced when black holes merge (among other events) detected by LIGO.