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

TL;DR

This paper reports the discovery of hydrogen cyanide ice clouds at Titan's south pole, revealing unexpected rapid cooling of the atmosphere and challenging existing circulation models.

Contribution

It provides the first evidence of HCN ice clouds at high altitude and demonstrates more efficient post-equinox cooling than previously modeled.

Findings

Detection of HCN ice particles at 300 km altitude

Significant cooling of Titan's atmosphere in early 2012

Temperatures much colder than circulation model predictions

Abstract

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.