Condensation in Titan's stratosphere during polar winter

In Titan's north polar region stratospheric clouds are expected to form due to a combination of low temperatures and downward motion of volatile-enriched air. Here we investigate possible sources of stratospheric clouds at Titan's pole using data from the Cassini Composite Infrared Spectrometer and a simple condensation model. An upper limit for C4N2 gas was determined to be 9×10−9, which is less than required to make the C4N2 cloud at the Voyager epoch. Hence, the presence of this cloud after equinox remains a mystery. The largest cloud seen in far-infrared spectra has a feature around 220 cm−1 and is located around an altitude of 140 km. The upper limit for propionitrile (C2H5CN) gas shows that the feature around 220 cm−1 is probably not due to pure propionitrile ice. Instead, our model calculations show that HCN should cause by far the largest cloud around 140 km. We therefore propose that HCN ice plays an important role in the formation of the massive polar cloud, because of the unavailability of sufficient condensable gas other than HCN to produce a strong enough condensate feature. However, the signature at 220 cm−1 is not consistent with that of pure HCN ice at 172 cm−1 and mixing of HCN ice with other ices, or chemical alteration of HCN ice might mask the HCN ice signature.