In December 2015, Japan’s Akatsuki Venus Climate Orbiter finally started beaming back images of Venus. Its epic journey included wandering off course around the sun for half a decade before entering orbit around the solar system's second planet. But the data so far has been worth the wait. During its first month of orbit the craft caught images of a large, stationary bow-shaped wave in the upper atmosphere of the planet.
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.
Tetsuya Fukuhara from Rikkyo University in Tokyo and colleagues in Japan reveal that this gravity wave was generated between two layers of different density, perhaps thanks to mountains on the Venusian surface. But if that is the case, they posit in Nature Geoscience, the winds in the deep atmosphere must shift differently than planetary scientists first thought.
“Some researchers have imagined that a gravity wave excited in the lower atmosphere may reach the upper cloud deck or higher in the Venus atmosphere, but no direct evidence of that has been found before,” Makoto Taguchi of Rikkyo University in Tokyo and coauthor of the study tells Botkin-Kowacki. “This is the first evidence of gravity wave propagation from the lower atmosphere to the middle atmosphere. This means that conditions of the lower atmosphere may affect the dynamics of the higher atmosphere by momentum transfer of the gravity waves.”
The researchers hope that detecting events in the upper atmosphere of Venus will help them figure out what is happening in the lower and middle atmosphere, where most sensors can’t penetrate.
But not everyone thinks that gravity waves are the main cause for the atmospheric structure. “It can’t be as simple as surface winds flowing over mountains, because the feature has been seen only in the late afternoon on Venus,” Gerald Schubert, a geophysicist at UCLA tells Ellis. Time of day should not impact the formation of gravity waves. That’s just one thing researchers want to answer in the next phase of their study. They are hoping that the structure or something similar will reappear to give them more data to work with.
The Japanese Space Agency's Akatsuki probe was launched on May 20, 2010 to explore Venus' climate. It reached its destination less than seven months later but technical troubles meant it failed to pop into orbit. Only five years later was it maneuvered into an elliptical orbit – and then its Longwave Infrared Camera saw the wave.