Our sister planet Venus might be completely covered in a dense layer of cloud, but that hasn’t stopped astronomers observing what the steamy hot planet looks like on the surface.
Until now, those gazing upon the Evening Star could only imagine what its topography would look like due to solid clouds blocking us from seeing the view below.
But using observations from the European Space Agency's Venus Express satellite, scientists have, for the first time, discovered how weather patterns seen in the planets 20km-thick cloud covering are directly linked to the physical features of its terrain. They can, therefore, tell us a lot about what it might look like below.
Published in the Journal of Geographical Research, the findings were put together by exploring three aspects of the planet's cloudy weather: how quickly winds on Venus circulate, how much water is locked up within the clouds, and how bright these clouds are across the spectrum (specifically in ultraviolet light).
“Our results showed that all of these aspects – the winds, the water content, and the cloud composition – are somehow connected to the properties of Venus' surface itself," said lead author of the new Venus Express study, Jean-Loup Bertaux. "We used observations from Venus Express spanning a period of six years, from 2006 to 2012, which allowed us to study the planet's longer-term weather patterns."
Although Venus is very dry by Earth standards, its atmosphere does contain some water in the form of vapour, particularly beneath its cloud layer. Bertaux and colleagues studied Venus' cloud-tops in the infrared part of the spectrum, allowing them to pick up on the absorption of sunlight by water vapour and detect how much was present in each location at cloud-top level (70km altitude).
They found one particular area of cloud, near Venus' equator, to be hoarding more water vapour than its surroundings. This 'damp' region was located just above a 4500-metre-altitude mountain range named Aphrodite Terra.
According to the scientists, this phenomenon appears to be caused by water-rich air from the lower atmosphere being forced upwards above the Aphrodite Terra mountains, leading researchers to nickname this feature the 'fountain of Aphrodite'.
"This 'fountain' was locked up within a swirl of clouds that were flowing downstream, moving from east to west across Venus," said the paper’s co-author Wojciech Markiewicz. "Our first question was, 'Why?' Why is all this water locked up in this one spot?"
The scientists then used Venus Express to observe the clouds in ultraviolet light, and to track their speeds. They found the clouds downstream of the 'fountain' to reflect less ultraviolet light than elsewhere, and the winds above the mountainous Aphrodite Terra region to be some 18 per cent slower than in surrounding regions.
Bertaux and his team proposed that these three factors could only be explained by one single mechanism – Venus' thick atmosphere.
"When winds push their way slowly across the mountainous slopes on the surface they generate something known as gravity waves," added Bertaux. "Despite the name, these have nothing to do with gravitational waves, which are ripples in space-time – instead, gravity waves are an atmospheric phenomenon we often see in mountainous parts of Earth's surface.
“Crudely speaking, they form when air ripples over bumpy surfaces. The waves then propagate vertically upwards, growing larger and larger in amplitude until they break just below the cloud-top, like sea waves on a shoreline."
As the waves break, they push back against the fast-moving high-altitude winds and slow them down, meaning winds above Venus' Aphrodite highlands are persistently slower than elsewhere.
However, these winds re-accelerate to their usual speeds downstream of Aphrodite Terra – and this motion acts as an air pump. The wind circulation creates an upwards motion in Venus' atmosphere that carries water-rich air and ultraviolet-dark material up from below the cloud-tops, bringing it to the surface of the cloud layer and creating both the observed 'fountain' and an extended downwind plume of vapour.
"We've known for decades that Venus' atmosphere contains a mysterious ultraviolet absorber, but we still don't know its identity," said Bertaux. "This finding helps us understand a bit more about it and its behaviour – for example, that it's produced beneath the cloud-tops, and that ultraviolet-dark material is forced upwards through Venus' cloud-tops by wind circulation."
As well as helping us understand more about Venus, the scientists said their findings of surface topography affecting atmospheric circulation is an achievement not only for our understanding of planetary super-rotation, but of climate in general.
This article was originally published by WIRED UK
