title: HCN ice in Titan's high-altitude southern polar cloud.
abstract: Titan's middle atmosphere is currently experiencing a rapid change of season after northern spring arrived in 2009 (refs 1, 2). A large cloud was observed for the first time above Titan's southern pole in May 2012, at an altitude of 300 kilometres. A temperature maximum was previously observed there, and condensation was not expected for any of Titan's atmospheric gases. Here we report that this cloud is composed of micrometre-sized particles of frozen hydrogen cyanide (HCN ice). The presence of HCN particles at this altitude, together with temperature determinations from mid-infrared observations, indicate a dramatic cooling of Titan's atmosphere inside the winter polar vortex in early 2012. Such cooling is in contrast to previously measured high-altitude warming in the polar vortex, and temperatures are a hundred degrees colder than predicted by circulation models. These results show that post-equinox cooling at the winter pole of Titan is much more efficient than previously thought.
authors.affiliation: 1] Leiden Observatory, Leiden University, Postbus 9513, 2300 RA, Leiden, The Netherlands [2] SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands.
authors.forename: Remco J
authors.initials: RJ
authors.lastname: de Kok
authors.affiliation: School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen's Road, Bristol BS8 1RJ, UK.
authors.forename: Nicholas A
authors.initials: NA
authors.lastname: Teanby
authors.affiliation: LESIA-Observatoire de Paris, CNRS, UPMC Université Paris 06, Université Paris-Diderot, 5 place Jules Janssen, F-92195 Meudon, France.
authors.forename: Luca
authors.initials: L
authors.lastname: Maltagliati
authors.affiliation: Atmospheric, Oceanic and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK.
authors.forename: Patrick G J
authors.initials: PG
authors.lastname: Irwin
authors.affiliation: LESIA-Observatoire de Paris, CNRS, UPMC Université Paris 06, Université Paris-Diderot, 5 place Jules Janssen, F-92195 Meudon, France.
authors.forename: Sandrine
authors.initials: S
authors.lastname: Vinatier