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Venus’ mysterious night side revealed

Researchers using Venus Express spacecraft data report “stationary waves” and slowly moving features in the planet’s upper clouds, during Venus’ long and mysterious night.

Planet Venus through an earthly telescope, showing us both its day and night sides, via Damian Peach’s View of the Solar System.

The planet Venus has the slowest rotation of any major planet in our solar sytsem. It spins only once every 243 Earth days. So “night” or “day” on the planet last a very long time, and, as you might imagine, the charcteristics of Venus’ night and days side undergo differences, as a result. The European Space Agency (ESA) said late last week (September 14, 2017) that scientists have used data from the Venus Express spacecraft – which arrived at Venus in April 2006 and orbited the planet until late 2014 – to characterize the wind and upper cloud patterns on the night side of Venus for the first time. They said the results were “surprising.”

The study showed that the atmosphere on Venus’ night side exhibits unexpected and previously unseen cloud types, morphologies (structures), and dynamics – some of which appear to be connected to features on the planet’s surface. Javier Peralta of the Japan Aerospace Exploration Agency (JAXA), Japan, is lead author of the new study published in the peer-reviewed journal Nature Astronomy, said in a statement:

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. 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.

Since the 1960s, it’s been known that the winds on Venus blow faster than the planet rotates. Scientists call this super-rotation. This mosaic illustrates the atmospheric super-rotation at the upper clouds of Venus. While the super-rotation is present in both day and night sides of Venus, it seems more uniform in the day, while in the night this seems to become more irregular and unpredictable. Image via ESA/ S. Naito/ R. Hueso and J. Peralta.

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, about 40 miles (65 km) above the surface. Peralta explained:

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. 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.

The team used the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on ESA’s Venus Express spacecraft to observe the clouds in the infrared. It 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.

Mysterious fast filaments seen on the night side upper clouds of Venus. Image via ESA Venus Express/ S. Naito/ R. Hueso and J. Peralta.

Popular models for how the atmospheres of planets – like Venus or Earth – behave and circulate are the Global Circulation Models (GCMs). They predict super-rotation to occur in much the same way on Venus’ night side as on its dayside. However, the new research by Peralta and his colleagues contradicts these models.

Instead, the super-rotation seems to be more irregular and chaotic on the night side, according to these scientists. They say that night side upper clouds form different shapes and morphologies than those found elsewhere on Venus. They found large, wavy, patchy, irregular, and filament-like patterns, many of which are unseen in dayside images.

What’s more, the night side clouds are dominated by unmoving phenomena known as standing waves, or stationary waves. Co-author Agustin Sánchez-Lavega of University del País Vasco in Bilbao, Spain, explained:

Stationary waves are probably what we’d call gravity waves. In other words, they are rising waves generated lower in Venus’ atmosphere that appear not to move with the planet’s rotation. 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.

Stationary waves in clouds on Venus. Image via ESA/ VIRTIS/ J. Peralta and R. Hueso.

Stationary waves in clouds on Venus. Image via ESA/ VIRTIS/ J. Peralta and R. Hueso.

Peralta said:

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. We had a long debate about whether the results were real–until we realised that another team, led by co-author Dr. Kouyama, had also independently discovered stationary clouds on the night side using NASA’s Infrared Telescope Facility (IRTF) in Hawaii! Our findings were confirmed when JAXA’s Akatsuki spacecraft was inserted into orbit around Venus and immediately spotted the biggest stationary wave ever observed in the solar system on Venus’ dayside.

These researchers said that the effect of a planet’s surface features on its atmospheric circulation remains unclear among climate modelers. Håkan Svedhem, ESA Project Scientist for Venus Express, commented:

This study challenges our current understanding of climate modeling and, specifically, the super-rotation, which is a key phenomenon seen at Venus.

Read more about this study from ESA

These panels show examples of new types of cloud morphology discovered on the night side of Venus thanks to ESA’s Venus Express and NASA’s infrared telescope IRTF. Top row, from left to right: stationary waves observed by Venus Express, “net” patterns observed with IRTF. Bottom row: mysterious filaments (left) and dynamical instabilities (right) observed by Venus Express. Image via ESA/ VIRTIS/ J. Peralta and R. Hueso.

Bottom line: Researchers using Venus Express spacecraft data report “stationary waves” and slowly moving features in the planet’s upper clouds, during Venus’ long and mysterious night.

Deborah Byrd