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.
Researchers were initially flummoxed by the chevron-shaped formation in the clouds, which stretched 6,200 miles, almost connecting the planet's poles. It appeared for just a few days before disappearing, reports Eva Botkin-Kowacki at the The Christian Science Monitor. Now, a new paper in the journal Nature Geoscience suggests that the feature was the result of gravity waves.
Unlike the wrinkle-in-spacetime gravitational waves (which were hypothesized by Einstein and detected by LIGO last year), gravity waves happen when fast-moving air travels over bumpy surfaces like mountains, explains Emma Grey Ellis at Wired. The interaction between the air molecules trying to float up and gravity, which pulls them back down, creates these tugging gravity waves. In mountainous areas on Earth, Ellis reports, the waves can extend all the way into the atmosphere. That’s the process the researchers believe is happening on Venus.
Venus is shrouded in thick clouds of sulphuric acid reaching from the surface all the way to its outer atmosphere, writes Andrew Coates at The Conversation. And the surface of the planet is hot enough to melt lead. While it takes 243 Earth days to make one spin around its axis, its atmosphere has a “super rotation,” requiring only two weeks to whirl around, leading to hurricane-force winds.
The wave appeared above an area known as Aphrodite Terra, which is about the size of Africa and stands up to three-miles tall above the planet's surface. Fast-moving atmosphere blowing over Aphrodite Terra could have created such a wave and wrinkle in the atmosphere, Coates explains.
“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.