Using data from ESA’s Venus Express spacecraft, European planetary researchers have shown how weather patterns seen in Venus’ cloud layers are directly linked to the topography of the surface below.
Venus in real colors, processed from Mariner 10 images. Image credit: Mattias Malmer / NASA.
Venus is famously hot. The average temperature on the Venusian surface is 864 degrees Fahrenheit (462 degrees Celsius).
The climate at the surface is oppressive; as well as being hot, the surface environment is dimly lit. Ground-level winds are slow, pushing their way across the planet at speeds of only 3.2 ft/sec (1 m/sec).
However, that is not what we see when we observe our sister planet from above. Instead, we spy a smooth, bright covering of cloud.
This cloud forms a 12-mile (20 km) thick layer that sits 31-43 miles (50 – 70 km) above the surface and is far colder than below, with typical temperatures of about minus 94 degrees Fahrenheit (minus 70 degrees Celsius) – similar to temperatures found at the cloud-tops of Earth.
The upper cloud layer also hosts more extreme weather, with winds that blow hundreds of times faster than those on the surface.
While these clouds have traditionally blocked our view of Venus’ surface, meaning we can only peer beneath using radar or infrared light, they may actually hold the key to exploring some of Venus’ secrets.
Researchers suspected the weather patterns rippling across the cloud-tops to be influenced by the topography of the terrain below. They have found hints of this in the past, but did not have a complete picture of how this may work – until now.
Using Venus Express observations, a team of scientists from Europe has now greatly improved our climate map of Venus by exploring three aspects of the planet’s cloudy weather: (i) how quickly winds on Venus circulate; (ii) how much water is locked up within the clouds; and (iii) how bright these clouds are across the spectrum.
“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 team leader Dr. Jean-Loup Bertaux, from the Laboratoire Atmosphères, Milieux, Observations Spatiales in France.
“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.”
The team studied Venus’ cloud-tops in the infrared part of the spectrum, allowing them to pick up on the absorption of sunlight by water vapor and detect how much was present in each location at cloud-top level (43 miles altitude).
They found one particular area of cloud, near Venus’ equator, to be hoarding more water vapor than its surroundings.
This region was located just above a 2.8-mile (4.5 km) altitude mountain range named Aphrodite Terra.
This phenomenon appears to be caused by water-rich air from the lower atmosphere being forced upwards above the Aphrodite Terra mountains, leading the team to nickname this feature the ‘Fountain of Aphrodite.’
Schematic illustration of the proposed behavior of gravity waves in the vicinity of mountainous terrain on Venus. Winds pushing their way slowly across the mountainous slopes on the surface generate gravity waves – an atmospheric phenomenon also often seen in mountainous parts of Earth’s surface. These waves 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. The background is an artist’s impression of the Venusian surface beneath the cloud tops. Image credit: ESA.
“The fountain was locked up within a swirl of clouds that were flowing downstream, moving from east to west across Venus,” said team member Dr. Wojciech Markiewicz, from the Max-Planck Institute for Solar System Research in Germany.
“Our first question was ‘why is all this water locked up in this one spot?”
The researchers also observed the clouds in UV (ultraviolet) light and tracked their speeds. They found the clouds downstream of the ‘fountain’ to reflect less UV light than elsewhere, and the winds above the mountainous Aphrodite Terra region to be 18% slower than in surrounding regions.
“All three of these factors can be explained by one single mechanism caused by Venus’ thick atmosphere,” Dr. Bertaux said.
“When winds push their way slowly across the mountainous slopes on the surface they generate something known as atmospheric gravity waves.”
“The waves form when air ripples over bumpy surfaces. They 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 that 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 UV-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 vapor.
“We’ve known for decades that Venus’ atmosphere contains a mysterious UV absorber, but we still don’t know its identity,” Dr. Bertaux said.
“This finding helps us understand a bit more about it and its behavior – for example, that it’s produced beneath the cloud-tops, and that UV-dark material is forced upwards through Venus’ cloud-tops by wind circulation.”
Scientists already suspected that there were ascending motions in Venus’ atmosphere all along the equator, caused by the higher levels of solar heating.
This finding reveals that the amount of water and UV-dark material found in Venus’ clouds is also strongly enhanced at particular places around the planet’s equator.
The team’s results were published online June 30 in the Journal of Geophysical Research: Planets.
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Jean-Loup Bertaux et al. 2016. Influence of Venus topography on the zonal wind and UV albedo at cloud top level: The role of stationary gravity waves. Journal of Geophysical Research: Planets 121 (6): 1087-1101; doi: 10.1002/2015JE004958
