A chaotic long-lived vortex at the southern pole of Venus

Polar vortices are common in the atmospheres of rapidly rotating planets(1-4). On Earth and Mars, vortices are generated by surface temperature gradients and their strength is modulated by the seasonal insolation cycle(1-3). Slowly rotating Venus lacks pronounced seasonal forcing, but vortices are known to occur at both poles, in an atmosphere that rotates faster than the planet itself(5-8). Here we report observations of cloud motions at altitudes of 42 and 63 km above Venus's south pole using infrared images from the VIRTIS instrument onboard the Venus Express spacecraft. We find that the south polar vortex is a long-lived but unpredictable feature. Within the two cloud layers sampled, the centres of rotation of the vortex are rarely aligned vertically and both wander erratically around the pole with velocities of up to 16ms(-1). At the two horizontal levels, the observed cloud morphologies do not correlate with the vorticity of the wind field and change continuously, and vertical and meridional wind shears are also highly variable. We conclude that Venus's south polar vortex is a continuously evolving structure that is at least 20 km high, extending through a quasi-convective turbulent region.