Double eye of Venus polar vortex disappears

Using data from ESA’s Venus Express spacecraft, a new animation has shown that the double eye of the giant vortex at Venus’s South pole has disappeared.

Dr Giuseppe Piccioni presented the results of a study that shows the complex, variable dynamics at the Venusian south pole.

Orbiting around Venus since April 11, 2006, the ESA mission Venus Express is providing an extensive and unique dataset of great scientific importance, spanning from the surface to the atmosphere and its interaction with the solar wind.

The VIRTIS (Visible and InfraRed Thermal Imaging Spectrometre) instrument studies Venus from orbit through atmospheric ‘windows’, which are transparent at certain infrared wavelengths and thus able to transmit the thermal radiance from very deep regions in the Venusian atmosphere.

It also provides information about temperature of the atmosphere and the cloud tops, from which it is possible to study the dynamics and in particular the polar vortex.

In 1979, the Pioneer Venus mission observed a similar polar vortex with two apparent centers of rotation in the Venusian northern hemisphere. This was labeled the dipole of Venus.

The VIRTIS instrument, right at the beginning of the Venus Express mission, observed a very similar shape in the southern hemisphere.

This discovery revealed a north-south symmetry on Venus and, at a first glance, confirmed the stability of the dipole. However, in the course of the mission, systematic observations with VIRTIS showed a large number of different shapes of the vortex, with complex configurations and not well identified stable features.

“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,” said Piccioni.

Piccioni and colleagues also tracked the clouds in the Venusian atmosphere in order to measure the wind speeds of the significant atmospheric ‘super-rotation’ rotating 60 times faster than planet underneath.

Observing at different wavelengths, they were able to probe different altitude levels within the atmosphere.

“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. However, the shear disappeared at higher latitudes, in combination with a decreasing wind speed toward the pole,” said Piccioni.

In fact, the polar region of Venus has peculiar dynamics, quite different than the rest of the planet. A permanent giant vortex, extending more than 3000km, dominates which, on average, rotates almost like a solid body.

This is quite contrary to the vertical shear in the mid-to-low latitudes, observed by Dr Piccioni’s team.

The ring surrounding the polar region, known as cold collar, acts as a barrier separating the two rotation zones.

The study was presented at the European Planetary Science Congress.