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Titan's middle atmosphere is currently experiencing a rapid change of seasonafter northern spring arrived in 2009. A large cloud was observed for the firsttime above Titan's southern pole in May 2012, at an altitude of 300 km. Thisaltitude previously showed a temperature maximum and condensation was notexpected for any of Titan's atmospheric gases. Here we show that this cloud iscomposed of micron-sized hydrogen cyanide (HCN) ice particles. The presence ofHCN particles at this altitude, together with new temperature determinationsfrom mid-infrared observations, indicate a very dramatic cooling of Titan'satmosphere inside the winter polar vortex in early 2012. Such a cooling iscompletely contrary to previously measured high-altitude warming in the polarvortex, and temperatures are a hundred degrees colder than predicted bycirculation models. Besides elucidating the nature of Titan's mysterious polarcloud, these results thus show that post-equinox cooling at the winter pole ismuch more efficient than previously thought.

HCN ice in Titan's high-altitude southern polar cloud