A single day on Venus is longer than a year on the planet; it takes 243 Earth days to rotate on its axis, but only 224 to orbit the Sun. While this may be a useful fact to bring out at a dinner party, it also gives rise to some mysterious features of the planet which we’re only just starting to understand.
The planet rotates so slowly, for example, that the side facing the Sun receives much more light than the side facing away, meaning the planet can be split into its ‘day’ and ‘night’ sides. Now, for the first time, the wind and cloud patterns on the mysterious night side of Venus have been explored using the European Space Agency’s Venus Express spacecraft.
The study, published in the journal Nature Astronomy, found the atmosphere on Venus’ night side behaves extremely differently to that on the day side. Specifically, the researchers discovered unexpected types of clouds that have never been seen before.
“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,” said Javier Peralta, from the Japan Aerospace Exploration Agency (JAXA), Japan, and lead author of the new study. “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.”
The atmosphere of Venus is dominated by winds that swirl up to 60 times faster than the planet rotates, which is called ‘super-rotation’. The winds push and drag clouds within the atmosphere as they go, and the clouds travel fastest at the upper cloud level.
“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,” said Peralta. “However, our models of Venus remain unable to reproduce this super-rotation, which clearly indicates 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.”
To combat this, the researchers used the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express to study the clouds in 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,” said Peralta.
The super-rotation is more irregular and chaotic on the night side, the authors found. The behaviour of the clouds is also linked to changes on the surface of the planet.
“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.
“It demonstrates the power of combining data from multiple different sources–in this case, remote sensing and radio-science data from Venus Express’ VIRTIS and VeRa, complemented by ground-based observations from IRTF’s SpeX. This is a significant result for VIRTIS and for Venus Express, and is very important for our knowledge of Venus as a whole.”
Images: NASA
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