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Venus polar vortex
all shook up

Posted: 23 September 2010

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New animations created from ESA's Venus Express mission show striking changes in the vortex that rides around the planet's south pole, its double-eyed feature currently absent.

Three-dimensional perspective view of the south polar vortex in its present state (left) and in a dipole configuration (right). Image: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA.

Since Venus Express' arrival at our neighbouring planet in 2006, the VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer) instrument has been collecting data on the temperature of the planet's atmosphere and cloud decks, finding that the polar region's dynamics follow rules of their own.

A north polar vortex was observed as early as the 1979 Pioneer Venus mission, but Venus Express soon found its double-eyed southern hemisphere twin. Covering a diameter of around 3,000 kilometres, it rotates almost like a solid body. As the Venus Express mission progressed, however, a variety of vortex shapes have been seen, with the double-eye feature now having apparently disappeared.

"We had ironically observed it in a dipole configuration right at the beginning of the mission, but we soon discovered that this was just a coincidence, since the dipole in reality is not a stable feature on Venus but just one shape among others," says Giuseppe Piccioni.

Click here for animation. Dynamics within the south polar vortex are extremely complex this animation shows atmospheric flow in different directions and at different speeds. Image: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. of Oxford.

In addition, by tracking clouds the team were able to measure wind speeds of its 'super-rotating' atmosphere, which speeds around the planet 60 times faster than the planet spins on its own axis. Employing the different wavelengths of Venus Express' instruments enabled the scientists to explore phenomena at different altitude levels.

"We found a significant vertical shear, or change of winds with height, at low latitudes, with winds doubling from the lower clouds to the cloud tops," says Piccioni. "However, the shear disappeared at higher latitudes, in combination with a decreasing wind speed toward the pole." The ring surrounding the polar region, known as the cold collar, acts as a barrier separating the two rotation zones.

Venus Express will soon be joined by the Japanese mission Akatsuki (also known as Planet-C) and both will continue to decipher the mysteries of Venus' dynamic atmosphere.

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