VENUS is known both as the “planet of love” and the Earth’s “evil twin”. And although research suggests its environment is more hellish than romantic, there's a lot we don't know about our celestial neighbour.
Now Japanese scientists have made a surprising discovery: an enormous, bow-shaped feature in the planet's cloud region which seems fixed to the slowly rotating planet. Clouds around it, on the other hand, whizz by. So what is it?
Before the space age, it was thought Venus would be similar to Earth. The expectation in science fiction was that the planet may support life, with thick vegetation under water-rich clouds. But Venus is lifeless and very different from our planet – and the clouds are sulphuric acid.
It has the hottest planetary surface in the solar system, a thick atmosphere and no protective magnetic field. Its rotation is slow and it has hurricane-force winds and strange vortices near the poles.
Although it may have had surface water, this evaporated, leading to a greenhouse effect in which the atmosphere got thicker, the surface got hotter, more water evaporated into the atmosphere and so forth. Volcanism continued pumping gases into the atmosphere, building up the atmospheric pressure. The atmosphere of Venus now is principally carbon dioxide, which is why the surface is extremely hot.
Early missions, including Mariner, Venera and Pioneer Venus, determined the composition of the clouds and measured the atmospheric structure. The Russian Venera landers, the only craft so far to have landed in the harsh environment, showed images of lava plains and volcanic terrain.
Later the Magellan mission, which used radar to peer under the clouds, allowed mapping of the volcanoes and lava channels – revealing a young surface with few craters. This shows that the planet was resurfaced by volcanic activity about 500 million years ago. More recently, Venus Express has shown possible signs of some volcanism within the last 100 to 10 000 years.
The super-rotation of Venus’s atmosphere makes it very different from Earth. At the cloud level of 50km-65km, where the atmospheric pressure varies between the Earth’s surface pressure to 10 percent of that, the speed of rotation is about 60 times the speed of the planet’s rotation. Earth’s fastest winds are only 10 percent to 20 percent of the planet’s rotation speed. Pioneer Venus showed the high speed reduces through the lower atmosphere, eventually rotating with the planet at the surface.
Perplexing planet
Enter the Japanese spacecraft Akatsuki, which was launched on May 20, 2010. The spacecraft is designed to study the structure and activity of Venus's atmosphere. It was successfully inserted into orbit in 2015. This, with the first results, was a huge achievement.
The new study reporting the discovery of the bow-shaped structure, just published in Nature Geoscience, is the most recent result of the mission. The wave was caught by Akatsuki’s imaging instruments, looking in the infrared and ultraviolet parts of the electromagnetic spectrum. The astronomers analysing the data noted that the structure extended 10 000km through the Venus cloud tops and persisted for a few days, then suddenly disappeared.
The shape seems tied to the slowly rotating terrain below, particularly a high region called Aphrodite Terra, which is up to 5km high and the size of Africa near the equator. The structure persists in the rapidly moving, super-rotating winds at cloud level – like the water flow around a submerged stone in a stream.
The researchers suggest a stationary “gravity wave” in the atmosphere might cause the effect. Gravity waves are generated at the boundary between the atmosphere and a surface, or between horizontal layers in the atmosphere, when the force of gravity opposes buoyancy. Gravity waves on a larger scale can be seen in the upper atmosphere between layers.
Although smaller scale gravity waves have been seen near to ground level on Venus before, the scale of this new feature seems to be extremely large, probably the largest in the solar system. It is unclear whether it is even possible for gravity waves to cause such a big effect.
The discovery illustrates that, although we can explain some of the features of the thick, fast Venus atmosphere, it appears that low-altitude atmospheric dynamics are not fully understood yet. But we are slowly uncovering the planet’s secrets and the latest study is certainly making waves.
l Coates is a professor of physics, deputy director (solar system) at the Mullard Space Science Laboratory, University College London. This article first appeared in The Conversation.