Venus is a strange planet. Its runaway greenhouse effect makes it the hottest planet in our solar system, yet it rotates so slowly that a day on Venus is longer than a year.
Regardless of the crawling surface speed, its upper atmosphere whizzes around at a blistering 100 metres per second, encasing the hellish rock in a blur of sulfuric acid clouds. In such an environment, you would think that nothing can stand still.
However, astronomers have reported the sighting of a bright, stationary bow-shaped structure in those very clouds, forming one of the largest waves ever seen in the solar system.
Recorded by the Japanese meteorological satellite Akatsuki in December 2015, the huge pressure wave spanned 10,000 kilometres across both hemispheres of Venus, almost from pole to pole.
The satellite captured this vision shortly after entering the planet’s orbit, and the wave appeared to stand still for several days.
Chart shows brightness temperature and UV brightness of the Venus disk, captured by the Akatsuki probe.
IMAGE SUPPLIED BY JAXA, © PLANET-C
Researchers at Japan’s Aerospace Exploration Agency (JAXA) have now analysed data beamed back by the Akatsuki probe and published their report in Nature Geoscience today.
They think the mysterious arc was formed by a huge atmospheric gravity wave (not to be confused with gravitational waves). Gravity waves happen on any rocky planet with an atmosphere—as winds blow over uneven surface features, such as mountains, the air particles rush upwards while gravity pulls them downwards, resulting in waves.
As planetary scientist Dr Colin Wilson explained to BBC News, you can also see this phenomenon in liquids.
"If you have a stream and it's flowing over a rock, you get the gravity waves propagating upwards through the water,” he says. “At the surface of the stream, you will see it as changes in height.”
Atmospheric gravity waves on a much smaller scale happen on Earth all the time, and are an important feature in climate modelling. Before Venus flashed its giant sideways smile, scientists didn’t think gravity waves could reach so high up in the atmosphere.
We also don’t really know why the top layer spins quite so fast, which makes it tricky to observe interactions between various atmospheric layers on Venus. Studying the newfound wave could help planetary scientists figure things out.
“The feature probably arising at least in part by gravity waves will tell us more about the role they play in the atmospheric dynamics,” geophysicist Dr Gerald Schubert told Wired.
“More elaborate modelling is left for future studies,” the authors write in the paper. “Since gravity waves transfer momentum, mountain-induced waves may be important for the climatology of Venus.”modelling is left for future studies,” the authors write in the paper. “Since gravity waves transfer momentum, mountain-induced waves may be important for the climatology of Venus.”
Header image: The wave as it was captured by Akatsuki probe in December 2015. IMAGE SUPPLIED BY JAXA, © PLANET-C