The many faces of the Venus' polar vortex

A new animation using data from ESA's Venus Express spacecraft shows that the double eye of the giant vortex at Venus's South pole has disappeared. Results of a study that shows the complex, variable dynamics at the Venusian south pole will be presented by Dr Giuseppe Piccioni at the European Planetary Science Congress, on Thursday 23rd September.

Orbiting around Venus since April 11 2006, the ESA mission Venus Express, in particular the VIRTIS (Visible and InfraRed Thermal Imaging Spectrometer) instrument on board, 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. VIRTIS is perfectly suitable to study Venus from orbit, in particular through the so called infrared atmospheric windows (very narrow holes at selected wavelengths almost transparent and thus able to transmit the thermal radiance from very deep regions into the Venusian atmosphere). Also, it provides information about temperature of the atmosphere and the clouds' top, from which it is possible to study its dynamics and in particular the polar vortex.

The Pioneer Venus mission observed for the first time in 1980 the elliptical shape of a polar vortex with two apparent centres of rotation, in the Venusian northern hemisphere labelling it the dipole of Venus. The VIRTIS instrument, right at the beginning of the Venus Express mission, observed for the first time 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 after many years since 1980. However, in the course of the mission, systematic observations with VIRTIS showed a large number of different shapes of the vortex, complex configurations with a not well identified stable feature. 'We had ironically observed it in a dipole configuration right at the beginning of the mission. But we soon discovered that this was an evil coincidence, since the dipole in reality is not a stable feature on Venus but just one shape among others,' says Dr Piccioni.

Dr 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 its solid body. Measuring the solar light as is reflected or transmitted at different wavelengths they were able to probe different altitude levels within the atmosphere. 'We found a significant vertical shear (change of winds with height) at low latitudes, with winds doubling from the lower clouds to the clouds' top,' says Dr Piccioni. 'However, the shear disappeared at higher latitudes, in combination with a decreasing wind speed toward the pole' he adds.

In fact, the polar region of Venus is known for its very peculiar dynamics, quite different than the rest of the planet. A permanent giant vortex, extending more than 3000 km, dominates its dynamics with, on average, an almost solid body rotation. 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 real barrier of separation between the two rotation zones.

Starting from this December, Venus Express will not be alone any more orbiting around Venus, because the Japanese mission Planet-C, launched last May, will join it into the adventure of exploring the mysterious sister planet.