  Titan's middle atmosphere is currently experiencing a rapid change of season
after northern spring arrived in 2009. A large cloud was observed for the first
time above Titan's southern pole in May 2012, at an altitude of 300 km. This
altitude previously showed a temperature maximum and condensation was not
expected for any of Titan's atmospheric gases. Here we show that this cloud is
composed of micron-sized hydrogen cyanide (HCN) ice particles. The presence of
HCN particles at this altitude, together with new temperature determinations
from mid-infrared observations, indicate a very dramatic cooling of Titan's
atmosphere inside the winter polar vortex in early 2012. Such a cooling is
completely contrary to previously measured high-altitude warming in the polar
vortex, and temperatures are a hundred degrees colder than predicted by
circulation models. Besides elucidating the nature of Titan's mysterious polar
cloud, these results thus show that post-equinox cooling at the winter pole is
much more efficient than previously thought.
