ESA’s spacecraft probed Venus’ nightside for the first time. After astronomers reviewed the data, they were surprised to find unusual cloud formations that shouldn’t be there according to computer models.

This mosaic illustrates the atmospheric super-rotation at the upper clouds of Venus. Credit: ESA, JAXA, J. Peralta and R. Hueso.

This mosaic illustrates the atmospheric super-rotation at the upper clouds of Venus. Credit: ESA, JAXA, J. Peralta and R. Hueso.

Earth’s hellish twin

If there’s a hell in our solar system, it’s on Venus. The brightest object in the night sky after the moon has, for a very long time, conjured the imagination of scientists, artists, and free-thinking folk around the world. Shrouded in a thick haze, people imagined that under this blanket lies a hot jungle world, maybe teeming with life. In 1962, the U.S.-launched Mariner 2 space probe found something totally different, though.

Venus has an average surface temperature of 460°C, hotter than the surface of Mercury despite being considerably farther away than the sun. The leading explanation right now is that Venus’ enormous heat is trapped in the atmosphere due to a runaway greenhouse effect (some speculate something similar might happen to Earth).

These clouds are so thick that they reflect 90% of the sunlight that hits the planet, making Venus a very dark and gloomy world despite it being closer to the sun than Earth.

Slow planet, fast winds

Scientists at the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA) are more interested in Venus’ winds than its clouds. Venus is the slowest-turning body in the solar system completing a single revolution around its own axis in 234 Earth days. Winds on the planet, however, are 60 times faster than its rotation which has always intrigued scientists. This phenomenon, known as ‘super-rotation’, is more pronounced in the upper atmosphere where the winds push and drag clouds.

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To learn more, scientists turned to ESA’s Venus Express which revealed that the nightside of the planet behaves radically different from the dayside of the planet, which is facing the sun. Writing in Nature Astronomy, researchers report finding unexpected and previously-unseen cloud types, morphologies, and dynamics.

“This is the first time we’ve been able to characterise how the atmosphere circulates on the night side of Venus on a global scale,” says Javier Peralta of the Japan Aerospace Exploration Agency (JAXA), Japan, and lead author of the new study.

High velocity filaments seen on the night side upper clouds of Venus, as seen through the eye of the VIRTIS instrument on Venus Express. Credit: ESA, S. Naito, R. Hueso and J. Peralta.

High velocity filaments seen on the night side upper clouds of Venus, as seen through the eye of the VIRTIS instrument on Venus Express. Credit: ESA, S. Naito, R. Hueso and J. Peralta.

“While the atmospheric circulation on the planet’s dayside has been extensively explored, there was still much to discover about the night side. We found that the cloud patterns there are different to those on the dayside, and influenced by Venus’ topography.”

Super-rotation has always been perplexing because scientists back on Earth were never able to reproduce it in their computer models. With the help of the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on ESA’s Venus Express spacecraft, they could observe the clouds on the dark side of the planet in the infrared. This revealed phenomena on Venus’ nightside that have never before been seen on the dayside.

The main finding is that the super-rotation seems to be more irregular and chaotic on the night side. That’s contrary to the weather models of Venus compiled so far that predicted super-rotation occurs in much the same way on Venus’ night side as on its dayside.

“It was an exciting moment when we realised that some of the cloud features in the VIRTIS images didn’t move along with the atmosphere,” said Peralta in a statement.

What’s more, the nightside seems to produce large, wavy, and irregular clouds in filament-like patterns that were never seen before on the sunny side. Scientists think these cloud formations are made by unmoving phenomena known as stationary waves.

“Stationary waves are probably what we’d call gravity waves–in other words, rising waves generated lower in Venus’ atmosphere that appear not to move with the planet’s rotation,” says co-author Agustin Sánchez-Lavega of University del País Vasco in Bilbao, Spain.

“These waves are concentrated over steep, mountainous areas of Venus; this suggests that the planet’s topography is affecting what happens way up above in the clouds.”

New types of cloud morphology on Venus. Credit: ESA, NASA, J. Peralta and R. Hueso

New types of cloud morphology on Venus. Credit: ESA, NASA, J. Peralta and R. Hueso

Strangely, these sort of clouds weren’t found in the lower atmospheric levels. Topography might be involved, which can only mean our climate and weather models of Venus need a revamp.

“This study challenges our current understanding of climate modelling and, specifically, the super-rotation, which is a key phenomenon seen at Venus,” said Håkan Svedhem, ESA Project Scientist for Venus Express.

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