HCN-C₆H₆ co-condensed ice cloud in Titan’s stratosphere: Laboratory study of the CIRS-observed HASP cloud

Abstract

For thirteen years, the Cassini mission to explore Saturn and its icy moons has provided a large repository of data on Titan’s atmosphere. Ice clouds have been repeatedly observed in Titan's stratosphere by the Cassini Composite InfraRed Spectrometer (CIRS). Pure nitrile ice clouds have been identified but the chemical composition of other observed ice clouds is still undetermined. We propose cocondensation to be a formation mechanism of these other clouds. With the aim to identify the HighAltitude South Polar (HASP) ice cloud, the most recently observed cloud during the early Titan’s southern winter, we have conducted laboratory thin ice film spectroscopy of a series of organic mixed ices and found that a C6H6-HCN co-condensed ice, enriched in benzene, is the best chemical candidate for the HASP cloud.