Artist’s impression of the change in observed atmospheric effects before, during and after equinox in 2009. The Titan globes also provide an impression of the detached haze layer that extends all around the moon (blue).
During the first years of Cassini’s exploration of the Saturnian system, Titan sported a ‘hood’ of dense gaseous haze (white) in a vortex above its north pole, along with a high-altitude ‘hot spot’ (red). During this time the north pole was pointed away from the Sun.
At equinox both hemispheres received equal heating from the Sun. Afterwards, the north pole tilted towards the Sun, signalling the arrival of spring, while the southern hemisphere tilted away from the Sun and moved into autumn.
After equinox and until 2011 there was still a significant build up of trace gases over the north pole, but the vortex and hot spot had almost disappeared. Instead, similar features began developing at the south pole, which are still present today.
These observations are interpreted as a large-scale reversal in the single pole-to-pole atmospheric circulation cell of Titan immediately after equinox, with an upwelling of gases in the summer hemisphere and a corresponding downwelling in the winter hemisphere.