Venus’ mysterious night side revealed – Astronomy Now Online

 In Science
Atmospheric super-rotation at the upper clouds of Venus. Credit: ESA, JAXA, J. Peralta and R. Hueso

Scientists have used ESA’s Venus Express to characterise the wind and upper cloud patterns on the night side of Venus for the first time-with surprising results.

The study shows that atmosphere on Venus’ night side behaves very differently to that on the side of the planet facing the Sun (the ‘dayside’), exhibiting unexpected and previously-unseen cloud types, morphologies, and dynamics — some of which appear to be connected to features on the planet’s surface.

“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 published in the journal Nature Astronomy. “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.”

Venus’ atmosphere is dominated by strong winds that whirl around the planet far faster than Venus itself rotates. This phenomenon, known as ‘super-rotation,’ sees Venusian winds rotating up to 60 times faster than the planet below, pushing and dragging along clouds within the atmosphere as they go. These clouds travel fastest at the upper cloud level, some 65 to 72 km above the surface.

“We’ve spent decades studying these super-rotating winds by tracking how the upper clouds move on Venus’ dayside — these are clearly visible in images acquired in ultraviolet light,” explains Peralta. “However, our models of Venus remain unable to reproduce this super-rotation, which clearly indicates that we might be missing some pieces of this puzzle.

“We focused on the night side because it had been poorly explored; we can see the upper clouds on the planet’s night side via their thermal emission, but it’s been difficult to observe them properly because the contrast in our infrared images was too low to pick up enough detail.”

Mysterious fast filaments seen on the night side upper clouds of Venus, observed with the VIRTIS instrument on Venus Express. Credit: ESA, S. Naito, R. Hueso and J. Peralta

The team used the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on ESA’s Venus Express spacecraft to observe the clouds in the infrared. “VIRTIS enabled us to see these clouds properly for the first time, allowing us to explore what previous teams could not — and we discovered unexpected and surprising results,” adds Peralta.

Rather than capturing single images, VIRTIS gathered a ‘cube’ of hundreds of images of Venus acquired simultaneously at different wavelengths. This allowed the team to combine numerous images to improve the visibility of the clouds, and see them at unprecedented quality. The VIRTIS images thus reveal phenomena on Venus’ night side that have never before been seen on the dayside.

The best models for how Venus’ atmosphere behaves and circulates, known as Global Circulation Models (GCMs), predict super-rotation to occur in much the same way on Venus’ night side as on its dayside. However, this research by Peralta and his colleagues contradicts these models.

Instead, the super-rotation seems to be more irregular and chaotic on the night side. Night side upper clouds form different shapes and morphologies than those found elsewhere-large, wavy, patchy, irregular, and filament-like patterns, many of which are unseen in dayside images — and are dominated by unmoving phenomena known as stationary waves.

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