Solid-state photochemistry as a formation mechanism for Titan's stratospheric C_4N_2 ice clouds

We propose that C_4N_2 ice clouds observed in Titan's springtime polar stratosphere arise due to solid-state photochemistry occurring within extant ice cloud particles of HCN-HC_3N mixtures. This formation process resembles the halogen-induced ice particle surface chemistry that leads to condensed nitric acid trihydrate (NAT) particles and ozone depletion in Earth's polar stratosphere. As our analysis of the Cassini Composite Infrared Spectrometer 478 cm^(−1) ice emission feature demonstrates, this solid-state photochemistry mechanism eliminates the need for the relatively high C_4N_2 saturation vapor pressures required (even though they are not observed) when the ice is produced through the usual procedure of direct condensation from the vapor.