ESA Science: The Shifting Southern Hemisphere of Venus

New analysis of images taken by ESA's Venus Express orbiter has revealed surprising details about the remarkable, shape-shifting collar of clouds that swirls around the planet's South Pole.

This fast-moving feature is all the more surprising since its centre of rotation is typically offset from the geographical pole.

The results of this study are published online in Science Express.

Several planets in the Solar System, including Earth, have been found to possess hurricane-like polar vortices, where clouds and winds rotate rapidly around the poles. Some of these take on strange shapes, such as the hexagonal structure on Saturn, but none of them are as variable or unstable as the southern polar vortex on Venus.

Scientists have known about the presence of swirling clouds around the poles of Venus since they were first imaged by Mariner 10 in 1974. At the same time, it was discovered that Venus' upper winds sweep westwards around the planet in only four days, 60 times faster than the rotation of the solid surface of the planet - a phenomenon known as superrotation.

Thermal infrared imagery from the Pioneer Venus spacecraft subsequently revealed an enormous depression in the cloud blanket at the North Pole. This relatively warm polar 'hole' was thought to be caused by downward movement of gases, rather like water flowing down a drain. However, detailed examination of the thick clouds and dense atmosphere over the South Pole had to wait until the arrival of Venus Express in April 2006.

During its first orbit around the planet, multi-wavelength observations confirmed for the first time the presence of a huge 'double-eye' atmospheric vortex at the planet's South Pole. Some 2000 km across, it was comparable to the structure that had previously been detected at the North Pole.

Since then, high-resolution infrared measurements obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument on Venus Express have revealed that the southern vortex is far more complex than previously believed. The VIRTIS images, taken at wavelengths of 3.8 and 5.0 microns, are ideal for tracking polar features on both the day and night sides of the planet, probing the polar cloud layer at an altitude of about 65 km.

The new observations, reported this week in the journal Science on the Science Express website, show that the vortex has a highly variable shape and internal structure. Images show that its morphology is constantly changing on timescales of less than 24 hours, as a result of differential rotation.

"The southern vortex is very dynamic compared with a hurricane on Earth, which remains stable for several days," said Hakan Svedhem, ESA's Venus Express Project Scientist. "It can take almost any shape, so although it often looks like an 'S' or figure 8, it may become completely irregular, even chaotic, in appearance."

The rapid shape changes indicate complex weather patterns, which are strongly influenced by the fact that the centre of the vortex does not coincide with the geographical pole.