The Forces Behind Venus’ Super-Rotating Atmosphere
Earth’s sister planet spins slowly, but its atmosphere whips around at high speeds
/https://tf-cmsv2-smithsonianmag-media.s3.amazonaws.com/filer/28/44/28444db2-09c1-4c30-bb1b-6f55894d9cdb/2020_apr27_venus.jpg)
Venus is Earth’s sister planet, similar in size and history, and our closest planetary neighbor in the solar system. It’s also like an evil twin, with a surface hot enough to melt lead covered with thick, sulfuric acid clouds. Venus spins on its axis in the opposite direction as most planets in the solar system, and it takes its time to rotate—one Venusian day lasts 243 Earth days.
That is, if you’re measuring the planet’s rocky surface. Its atmosphere, however, moves about 60 times faster. Powered by constant, hurricane-force winds, Venus’ clouds can lap the planet in just four Earth days. This odd phenomenon is known as super-rotation, and within our solar system it’s only seen on Venus and Saturn’s largest moon, Titan, and the upper reaches of Earth’s atmosphere.
Now, researchers have analyzed imagery taken by Japan’s Akatsuki spacecraft, which has been orbiting Venus since 2015, to figure out where the energy for those winds comes from and how the extreme weather has stuck around for so long. According to the paper, published on April 24 in the journal Science, the super-rotation seems to be driven by heat from the sun.
On Earth, gravity holds the atmosphere down near the surface, and friction keeps the air rotating at about the same rate as the planet. For Venus’ atmosphere to super-rotate, it has to overcome the forces of planet-wide friction. The researchers used ultraviolet images and thermal measurements of the top of Venus’ clouds, taken by the Akatsuki spacecraft, and followed the ways that clouds moved around the upper atmosphere.
"Personally, our success at doing so was my biggest surprise," Hokkaido University planetary scientist and lead author of the paper Takeshi Horinouchi tells Charles Choi of Space.
The clouds moved fastest around Venus' equator, where the heat from the sun is the most intense. The heat causes the atmosphere on the sunny side of the planet to expand and lose pressure, called a thermal tide. The hot air races westward, toward the dark side of the planet, where it’s cooler. The clouds also moved from the equator toward either of the planet’s poles, distributing the momentum needed for fast super-rotation.
"There was a suggestion that thermal tides might be contributing to the acceleration behind super-rotation,” Horinouchi tells Space. “But I think the mainstream assumption was different, so this was a surprise."
While the solar powered thermal tides cause Venus’ super-rotation to speed up, there are also a few forces counteracting them. Slow-moving planetary waves that occur on any spinning planet covered in liquid or gas, including Earth, as well as smaller scale atmospheric turbulence, act against the thermal tides and slow down the wind at Venus’ equator, according to the new model.
“Horinouchi et al. provide an important piece of the super-rotation puzzle that can offer a strong constraint on numerical simulations of the Venusian atmosphere,” Sebastien Lebonnois, a planetary scientist at Sorbonne University who wasn’t involved in the research, writes in a commentary also published in Science. “However, the question of whether their analysis presents a complete picture of the angular momentum balance may still be open.”
Lebonnois points out that Horinouchi’s team focused on the top cloud layer, but Venus’ clouds are about 12 miles thick, leaving the possibility of different situations at deeper layers of the atmosphere.
But the new findings could have implications beyond our solar system. A super-rotating atmosphere distributes heat across the planet even if it’s turning very slowly, so the dark side of the planet isn’t necessarily as cold as might be expected.
"Our study could help better understand atmospheric systems on tidally-locked exo-planets whose one side always facing the central stars, which is similar to Venus having a very long solar day,” Horinouchi says in a statement.