A gigantic hurricane with two calm, dark eyes was discovered at Venus’s north pole by the Pioneer Venus spacecraft in 1979. This double-eyed feature, dubbed the “dipole of Venus,” was thought to form when warm air from the planet’s equator rose and traveled toward the pole, where it cooled and sank to form a deep, swirling atmospheric pit.

For decades, astronomers expected to find a similar vortex at Venus’s south pole. While Venus itself rotates slowly, just once every 117 Earth days, its atmosphere whips around the planet once every 4 Earth days. This “super-rotating” atmosphere ought to form massive storms at both poles, astronomers reasoned.

So when Venus Express saw what looked like a double vortex at the south pole shortly after arriving at Venus in April 2006, scientists took it as confirmation that these whorls were stable and permanent. But as they watched, the vortices shifted and morphed.

“We had ironically observed it in a dipole configuration right at the beginning of the mission,” said astronomer Giuseppe Piccioni of the Italian National Institute for Astrophysics in Rome. “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.” Piccioni presented his results at the European Planetary Science Congress in Rome on September 23.

Data from the VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer) instrument on Venus Express also showed that the dynamics at Venus’s poles are quite different from the rest of the planet. Near the equator, the wind speeds vary greatly with altitude, with wind speeds doubling from the lower clouds to the cloud tops, Piccioni said. By contrast, the 1800-mile-wide polar vortex rotates almost like a solid body. These two zones of rotation are separated by a ring of cold air called a “cold collar” surrounding the polar region.

Images: 1) 3D perspective views of Venus’s south polar vortex as is is now (left) and as it was when it displayed the dipole configuration (right). The y-axis plots the temperature of the clouds’ top correlated with altitude. 2) Four views of the south polar vortex as recorded by VIRTIS. Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA

See Also:

• Planetwide Smog Ebbs and Flows Across Venus
• An Up-Close View of Evening on Venus
• Venus Orbiter Finds Potential Active Volcanoes
• Sending a Probe to See Shapes in Venusian Clouds

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Authors: Lisa Grossman

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