Paris, March 14 : Venus Express scientists that have been studying the planet, have been puzzled by an enormous structure with a central part that looks like the eye of a hurricane, which morphs and changes shape within a matter of days.
Discovered in 1974 by the Mariner 10 spacecraft, the eye of the hurricane is at the centre of a 2000 km-wide vortex at the south pole of Venus.
Scientists have been studying the structure in the thermal infrared, the wavelength range which reveals the temperature at the cloud-tops.
Seen in this wavelength, the core of the vortex appears very bright, probably indicating that a lot of atmospheric gases are moving downward in the region, which creates a depression at the cloud-tops, making the region hotter.
"Simply put, the enormous vortex is similar to what you might see in your bathtub once you have pulled out the plug" said Giuseppe Piccioni, co-Principal Investigator for the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express.
An image taken on 26 February 2007 shows the 'classic' dipole shape at the centre of the vortex, similar to that which has been observed previously.
But an image taken a mere 24 hours earlier shows the centre of the vortex to be almost circular, indicating that the shape of this feature can change very fast. At other times, it is typically oval.
The dynamic nature of the vortex can be seen clearly in a video. It shows that the vortex is very complex, with atmospheric gases flowing in different directions at different altitudes.
But, scientists are not sure what actually creates the vortex.
According to Colin Wilson from the University of Oxford, "One explanation is that atmospheric gases heated by the Sun at the equator, rise and then move poleward. In the polar regions, they converge and sinks again. As the gases moves towards the poles, they are deflected sideways because of the planet's rotation."
The dynamic nature of this vortex is similar to behaviour observed in other vortices on Earth, including those observed at the centre of hurricanes.
Investigators will keep a close watch on the polar region and its variability, in order to gain a better understanding of how it works.