| Summary: | International audience Revealed by the Cassini and Voyager Missions, a plethora of volatile species are formed in Titan's upper atmosphere from the initial N 2 /CH 4 (~98/2%) composition. As they precipitate, most of the volatiles condense in the colder lower atmosphere where they can form icy clouds (e.g. C 4 N 2 , HCN, C 2 H 6 ) which have been detected above the poles. HCN is the most abundant nitrile in Titan's atmosphere and suspected to condensate in Titan's lower stratosphere (typically <100 km). Micron-sized HCN ice particles were also observed above the south pole at high altitudes (300 km). This cloud is thought to have been formed in the post-equinox winter polar vortex. In the north polar regions, HCN is also a likely prominent contributor to the haystack spectral signature seen at 221 cm -1 . These stratospheric ices may contribute as condensation nuclei for ices deeper down in the troposphere. Furthermore, C 4 H 2 , a simple alkyne formed by the chemistry and relatively abundant in the stratosphere, condenses near 75 km, lower than HCN. C 4 H 2 can also absorb the lesser energetic photos at these low altitudes, forming the radical C 4 H which then reacts with CH 4 , causing the consumption of methane. Consequently, the loss mechanism for C 4 H 2 is photo-chemistry. As soon as C 4 H 2 condenses, small molecule and complex accretion with HCN may occur. The reactive state that these ices may undergo with long-UV radiation after they form is still largely unknown. We explore these conditions by studying HCN-C 4 H 2 ice mixtures in the laboratory, by using the Titan Organic Aerosol Spectroscopy and chemisTry (TOAST) setup at JPL's Ice Spectroscopy Laboratory (ISL). These ices are then irradiated at long-UV wavelengths pertaining to these low-altitude regions at low-controlled temperatures. The residue is analyzed using long-IR absorption. Our results show a solid-state HCN consumption due to irradiation, to which C 4 H 2 acts as a catalyst, indicating HCN ice particle ageing in the troposphere may ...
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